# Exhaust Thrust



## Zipper730 (Mar 24, 2018)

I'm curious as to the following

When did people begin to devise ways to increase the amount of exhaust thrust produced by a piston engine (at either the level of research scientist, design engineer, or military personnel)?
When was it first realized that, at approximately 350 mph (at either the level of research scientist, engineer, or military personnel), one pound of thrust became roughly equivalent to 1 horsepower?
In the various countries of the world such as the UK, France, Germany, USA, Russia/USSR?


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## swampyankee (Mar 24, 2018)

Zipper730 said:


> I'm curious as to the following
> 
> When did people begin to devise ways to increase the amount of exhaust thrust produced by a piston engine (at either the level of research scientist, design engineer, or military personnel)?
> When was it first realized that, at approximately 350 mph (at either the level of research scientist, engineer, or military personnel), one pound of thrust became roughly equivalent to 1 horsepower?
> In the various countries of the world such as the UK, France, Germany, USA, Russia/USSR?




Probably as soon as they noticed power is equal to force times speed, in the mid-19th-century.

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## Zipper730 (Mar 24, 2018)

swampyankee said:


> Probably as soon as they noticed power is equal to force times speed, in the mid-19th-century.


You'd think that'd been the case, but it seemed it took some time before they started fiddling with the exhaust pipes!


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## pbehn (Mar 24, 2018)

Zipper730 said:


> You'd think that'd been the case, but it seemed it took some time before they started fiddling with the exhaust pipes!


Even before the war Meredith and his "effect" was attempting to get "thrust" from radiator cooling, so exhaust thrust must have been before that. If you look at early Hurricane Spitfire exhausts, they are obviously designed for exhaust thrust but also have a two into one collector which I would think was to help scavenging of the cylinders, later models just had single outlets for each cylinder (not each port). To maximise the effect needs a lot of knowledge and wind tunnel research. However for the British the vast majority of engines were used at night so flare suppression was probably more important than thrust.


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## swampyankee (Mar 24, 2018)

swampyankee said:


> Probably as soon as they noticed power is equal to force times speed, in the mid-19th century





Zipper730 said:


> You'd think that'd been the case, but it seemed it took some time before they started fiddling with the exhaust pipes!




Possibly because getting thrust from engine exhaust is only really possible with highly supercharged engines. Also, propulsive efficiency -- the amount of power required to produce thrust -- means that if it's practical to put the power into the propeller, there's more thrust than just using the exhaust pipes as rocket nozzles. Thrust is mass flow rate times change in velocity, but the power needed to produce that thrust is mass flow rate times the velocity change squared. 

This is why turbocompounding worked, and why turbofans keep increasing bypass ratio.

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## pbehn (Mar 24, 2018)

swampyankee said:


> Possibly because getting thrust from engine exhaust is only really possible with highly supercharged engines. Also, propulsive efficiency -- the amount of power required to produce thrust -- means that if it's practical to put the power into the propeller, there's more thrust than just using the exhaust pipes as rocket nozzles. Thrust is mass flow rate times change in velocity, but the power needed to produce that thrust is mass flow rate times the velocity change squared.
> 
> This is why turbocompounding worked, and why turbofans keep increasing bypass ratio.


Also any attempt to increase thrust by narrowing the exhaust will just change the scavenging, the pressure is provided by the piston so any power it provides is automatically less power to the propeller, it isn't completely "free".

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## swampyankee (Mar 24, 2018)

pbehn said:


> Also any attempt to increase thrust by narrowing the exhaust will just change the change the scavenging, the pressure is provided by the piston so any power it provides is automatically less power to the propeller, it isn't completely "free".



Certainly, that is true.

There is a lot of system engineering going on here: increased inlet pressure from supercharging increased power, but the exhaust was then much above ambient pressure because the expansion ratio of the engine is fixed by the compression ratio, unless they try for a Miller cycle, so the work that went into compressing the inlet air is lost. _That _was noticed pretty quickly, but only one aircraft engine with power recovery turbines made it to service, the turbo-compounded Wright Cyclone.

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## Zipper730 (Mar 24, 2018)

When you say scavenging, do you mean that the ducting makes back pressure on the expansion of the engine itself, causing it to be unable to explode all the fuel-air out


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## pbehn (Mar 24, 2018)

Zipper730 said:


> When you say scavenging, do you mean that the ducting makes back pressure on the expansion of the engine itself, causing it to be unable to explode all the fuel-air out



It is a sort of all encompassing word used to describe the replacing of burnt fuel/air with fresh fuel air mixture. There are all sorts of things that improve it or make it worse. Things like back pressure and gas flow are important. I know next to nothing about supercharged engines, but I did race two strokes and in some ways they behave like supercharged. Even things like harmonics come into it, which is possibly why it is called tuning to start with. Along with the basic things like pressure and temperature there are other things to consider like the exhaust and inlet valves being open at the same time, on a two stroke back pressure and harmonic shock waves stop all the charge shooting straight out of the exhaust increasing actual compression ratio and decreasing consumption. Some four stroke twin racers used Siamese pipes, the early Spitfire Hurricane exhausts look like a similar set up.

Scavenging (automotive) - Wikipedia

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## Shortround6 (Mar 24, 2018)

There are all sorts of tricks used on non supercharged engines to increase the gas flow. Most aren't worth the trouble on a supercharged airplane engine.

For one thing an engine at 20,000 ft has a LOT less air pressure trying to keep exhaust gases inside the exhaust pipe/cylinder than an engine at sea level 
another thing is that whatever "pulses" you get in an intake manifold between the inlet to the carb and the cylinder/piston probably don't make it past the supercharger. It is also a LOT easier to set-up and work with those pulses when you used one carburetor throat per cylinder. Multiple cylinders per intake tract and/or per exhaust pipe tend to damp out the pulses or overlap at the wrong times.
Best design for exhaust thrust is a "pipe" as short as possible with one 90 degree bend and an outlet sized to let out the most gas as fast as possible consistent with keeping the exhaust gas velocity high. When P-40s were allowed to use WEP power settings they had to hacksaw off part of the exhaust pipe nozzle in order to accommodate the increased mass flow. Too small a nozzle prevented all the exhaust gas from exiting the cylinder.

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## pbehn (Mar 24, 2018)

Shortround6 said:


> There are all sorts of tricks used on non supercharged engines to increase the gas flow. Most aren't worth the trouble on a supercharged airplane engine.
> 
> For one thing an engine at 20,000 ft has a LOT less air pressure trying to keep exhaust gases inside the exhaust pipe/cylinder than an engine at sea level
> another thing is that whatever "pulses" you get in an intake manifold between the inlet to the carb and the cylinder/piston probably don't make it past the supercharger. It is also a LOT easier to set-up and work with those pulses when you used one carburetor throat per cylinder. Multiple cylinders per intake tract and/or per exhaust pipe tend to damp out the pulses or overlap at the wrong times.
> Best design for exhaust thrust is a "pipe" as short as possible with one 90 degree bend and an outlet sized to let out the most gas as fast as possible consistent with keeping the exhaust gas velocity high. When P-40s were allowed to use WEP power settings they had to hacksaw off part of the exhaust pipe nozzle in order to accommodate the increased mass flow. Too small a nozzle prevented all the exhaust gas from exiting the cylinder.



Some of the reasons for exhaust set ups on racing four strokes are not to do with maximum power but spreading the power band. Maximum power is always achieved with straight through exhausts with no connection between them. I was thinking that the early exhausts on Merlins were to do with having twin blade fixed pitch props and later variable coarse/fine pitch props. I believe a fixed pitch prop on a stationary aircraft has a limit to how fast it can spin before it starts to transmit less not more power and on a fixed pitch plane prop speed and engine speed are fixed to each other.

Just an idea, RR were no mugs, there must be some reason for those early exhaust cans.


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## Shortround6 (Mar 24, 2018)

Just going from the tests at Spitfire performance it appears that the fixed pitch prop planes were flown off the ground and climbed at very much reduced RPM until aircraft speed built up to keep from stalling the props. 

While not 'designed" as night fighters there was a requirement that the Hurricane and Spitfire at least operate by night and they had landing lights and flares they could drop. An exhaust system that minimized the glare/flames from the the exhaust may have been thought to be a good thing?

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## pbehn (Mar 24, 2018)

Shortround6 said:


> 1 Just going from the tests at Spitfire performance it appears that the fixed pitch prop planes were flown off the ground and climbed at very much reduced RPM until aircraft speed built up to keep from stalling the props.
> 
> 2 While not 'designed" as night fighters there was a requirement that the Hurricane and Spitfire at least operate by night and they had landing lights and flares they could drop. An exhaust system that minimized the glare/flames from the the exhaust may have been thought to be a good thing?



1) I suspect the reason for the early exhaust "cans" is linked to take off performance, that's the problem with todays world, you just cant find a Rolls Royce engineer when you want one.

2) Of the two the Hurricane was by far the better "night fighter" not because it was any good, it wasn't. But the pilot wasn't blinded by the exhausts as he was in a Spitfire, the lower seat position had drawbacks, and the wide track of the Hurricane undercarriage was a help at night.


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## swampyankee (Mar 24, 2018)

Shortround6 said:


> Just going from the tests at Spitfire performance it appears that the fixed pitch prop planes were flown off the ground and climbed at very much reduced RPM until aircraft speed built up to keep from stalling the props.
> 
> While not 'designed" as night fighters there was a requirement that the Hurricane and Spitfire at least operate by night and they had landing lights and flares they could drop. An exhaust system that minimized the glare/flames from the the exhaust may have been thought to be a good thing?




Most constant speed propellers have a significant portion of their blades stalled during the takeoff run; this is one of the constraints on blade root thickness and chord. With a fixed pitch propeller, the problem is that engine power is (roughly) linear with RPM, but the propeller power load is roughly proportional to the cube of the RPM; it's not that the propeller will stall, it's that the engine can't turn the prop any faster. Setting the pitch of a fixed pitch prop is a non-trivial problem: if the propeller is pitched so that all engine power is available at maximum speed, the engine won't be able to produce enough power to give the best rate of climb; conversely, pitching the prop for climb will mean the engine will overspeed before maximum speed is reached. 

I don't understand why the RAF used a fixed-pitch prop on the Spitfire; contemporaries from other countries, _i.e._, the US, were already using CP props.


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## pbehn (Mar 24, 2018)

swampyankee said:


> Most constant speed propellers have a significant portion of their blades stalled during the takeoff run; this is one of the constraints on blade root thickness and chord. With a fixed pitch propeller, the problem is that engine power is (roughly) linear with RPM, but the propeller power load is roughly proportional to the cube of the RPM; it's not that the propeller will stall, it's that the engine can't turn the prop any faster. Setting the pitch of a fixed pitch prop is a non-trivial problem: if the propeller is pitched so that all engine power is available at maximum speed, the engine won't be able to produce enough power to give the best rate of climb; conversely, pitching the prop for climb will mean the engine will overspeed before maximum speed is reached.
> 
> I don't understand why the RAF used a fixed-pitch prop on the Spitfire; contemporaries from other countries, _i.e._, the US, were already using CP props.


I thought there is also an issue of speed differential. A Spitfire on the ground is parked in stationary air, like trying to pull away in a car in top gear.
When discussing contemporaries of the Spitfire in the USA which do you mean? The P40 had its first flight after the Spitfire entered service. Much is to do with cost, when war was actually declared all sorts of things appeared very quickly, one of them was variable and then constant speed props.


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## Shortround6 (Mar 24, 2018)

On the Hurricane and Spitfire the Props _may_ have been near stalling. The fixed pitch was set for over 350mph on the Spitfire.
The Merlin III was restricted to 2600rpm on the climb in the early versions (for 30 minutes?) 
yet the climb chart 
Spitfire Mk I K.9787 Trials Report

shows 
sea level........2095rpm
1000ft...........2139rpm
2000ft...........2165rpm
3000ft...........2200rpm
5000ft...........2270rpm
6500ft...........2320rpm
10,000ft........2440rpm

at no altitude does the engine rpm ever exceed 2475rpm in climb while the level speeds are done at 3000rpm. 

There are several notes in this report relating to two different exhaust manifolds and night flying. 

I would note that on the test of the DH two pitch prop engine rpm were 2750rpm at 1000ft (fine pitch?) and at 2000ft both 2850 and 2080 rpm are listed and form 2000ft on up no rpm higher than 2450 is listed. 

The test with the constant speed prop shows 2600rpm being used at all altitudes.

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## pbehn (Mar 24, 2018)

Shortround6 said:


> On the Hurricane and Spitfire the Props _may_ have been near stalling. The fixed pitch was set for over 350mph on the Spitfire.
> The Merlin III was restricted to 2600rpm on the climb in the early versions (for 30 minutes?)
> yet the climb chart
> Spitfire Mk I K.9787 Trials Report
> ...


Great info S/R those exhausts have puzzled me for decades.


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## Shortround6 (Mar 24, 2018)

pbehn said:


> When discussing contemporaries of the Spitfire in the USA which do you mean? The P40 had its first flight after the Spitfire entered service. Much is to do with cost, when war was actually declared all sorts of things appeared very quickly, one of them was variable and then constant speed props.



For some reason the Air Ministry thought variable pitch or constant speed propellers were a passing fad 
They had tested variable pitch props back 20s and found them wanting and never changed their minds despite doing little, if any, new testing and despite their adoption by both foreign air forces and dozens of air lines (not dozens of airliners) around the world. British bombers were lucky they got two pitch props before WW II (or even in the early years).
Please note that both Roy Fedden and the people at Rolls-Royce were dismayed with this state of affairs and joined together to from ROTOL despite have few, if any firm orders for such propellers. Bristols board of directors were not happy as they figured that DH had the variable pitch market in England sewed up (or had the capacity to produce all the propellers the air ministry wanted)
In 1939 somewhere near 20 different airlines around the world (mostly US but some foreign) were using not only constant speed but fully feathering props on their multi-engine airliners.


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## pbehn (Mar 24, 2018)

Shortround6 said:


> For some reason the Air Ministry thought variable pitch or constant speed propellers were a passing fad
> They had tested variable pitch props back 20s and found them wanting and never changed their minds despite doing little, if any, new testing and despite their adoption by both foreign air forces and dozens of air lines (not dozens of airliners) around the world. British bombers were lucky they got two pitch props before WW II (or even in the early years).
> Please note that both Roy Fedden and the people at Rolls-Royce were ed with this state of affairs and joined together to from ROTOL despite have few, if any firm orders for such propellers. Bristols board of directors were not happy as they figured that DH had the variable pitch market in England sewed up (or had the capacity to produce all the propellers the air ministry wanted)
> In 1939 somewhere near 20 different airlines around the world (mostly US but some foreign) were using not only constant speed but fully feathering props on their multi-engine airliners.


This post S/R sums up perfectly the operation of any branch of the British civil service at almost any time in history from the 1700s to present day, the declaration of war and a national government sort of concentrated minds. I read here years ago that civilian airliners used variable pitch props, but they were "commercial chaps" only interested on making money, the RAF was not a commercial enterprise so why would they need new and expensive propellers? With a fixed prop. the Spitfire satisfied the specification so why spend more money, (even more money in civil service speak) on a toy for a war that probably wont happen.


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## Shortround6 (Mar 24, 2018)

I would say that in the 1920s (or even very early 30s) that puting variable pitch props on an underpowered fixed landing gear biplane was unlikely to show any real performance advantage and an increase in both weight and costs (both purchase and maintenance). 
A plane that took off and landed at 50mph and cruised at 150 mph or under can get away with a fixed pitch prop. One that takes off at 70 and tops out at 280-350mph can't. it was the ability to use near full power from the engine instead of 2/3s power that shortened up the take-off run so dramiticaly on the British fighters. And significantly improved the rate of climb. 
However once you start buying all metal, retractable landing gear monoplanes the advantage of better propellers should have been obvious. 
The much improved single engine ceiling of the twin engine planes that used them (and the much better safety record) should have attracted somebodies attention. WHile a dead engine on a single engine plane means a crash no matter what kind of propeller putting a two pitch propeller into course pitch and appling a propeller brake to stop the prop from windmilling and further damaging the dead engine came nowhere near the reduction in drag the fully feathering prop did.


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## swampyankee (Mar 24, 2018)

pbehn said:


> I thought there is also an issue of speed differential. A Spitfire on the ground is parked in stationary air, like trying to pull away in a car in top gear.
> When discussing contemporaries of the Spitfire in the USA which do you mean? The P40 had its first flight after the Spitfire entered service. Much is to do with cost, when war was actually declared all sorts of things appeared very quickly, one of them was variable and then constant speed props.



The P-36 had a constant speed propeller. The F3F had a controllable pitch propeller. HSD introduced the Hydramatic in 1938; it was constant speed, but it was proceeded by propellers with in-flight controllable pitch.


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## pbehn (Mar 24, 2018)

Shortround6 said:


> I would say that in the 1920s (or even very early 30s) that puting variable pitch props on an underpowered fixed landing gear biplane was unlikely to show any real performance advantage and an increase in both weight and costs (both purchase and maintenance).
> A plane that took off and landed at 50mph and cruised at 150 mph or under can get away with a fixed pitch prop. One that takes off at 70 and tops out at 280-350mph can't. it was the ability to use near full power from the engine instead of 2/3s power that shortened up the take-off run so dramiticaly on the British fighters. And significantly improved the rate of climb.
> However once you start buying all metal, retractable landing gear monoplanes the advantage of better propellers should have been obvious.
> The much improved single engine ceiling of the twin engine planes that used them (and the much better safety record) should have attracted somebodies attention. WHile a dead engine on a single engine plane means a crash no matter what kind of propeller putting a two pitch propeller into course pitch and appling a propeller brake to stop the prop from windmilling and further damaging the dead engine came nowhere near the reduction in drag the fully feathering prop did.


I agree S/R but the people making these decisions were generally old guys who had grown up without any airplanes at all, in the 1920s who actually knew anything about aircraft apart from the people who made them?

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## pbehn (Mar 24, 2018)

swampyankee said:


> The P-36 had a constant speed propeller. The F3F had a controllable pitch propeller. HSD introduced the Hydramatic in 1938; it was constant speed, but it was proceeded by propellers with in-flight controllable pitch.


But if you are a civil servant counting beans the P-36 is no argument for fancy-dan propellers the Spitfire was a better plane. I agree with you completely I am just saying what is obvious now wasn't obvious then. Companies saying "you must buy this, it is a new wonder" were commonplace.


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## Shortround6 (Mar 24, 2018)

The thing is there doesn't seem to be a record of any trial or experiment in the mid to late 30s. There very well may have been but none is showing up in popular literature. 
The DH Comet of 1934 used a crude form of two pitch propeller. Or elegant for a racing plane. Air pressure in a pumped up bladder held the blades in fine pitch and a small disc on the front of the prop hub, when airspeed was high enough, had enough force to move back and release a valve allowing the blades to go to coarse pitch. Prop was reset to fine pitch after landing. Light weight but not very flexible in operation.
Plenty of DC-2s, DC-3s, Lockheed Electras (models 10, 12 and 14) had flown through Britain or been based in Britain for a few demonstration flights to have been flown had anybody wanted to.

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## swampyankee (Mar 24, 2018)

pbehn said:


> But if you are a civil servant counting beans the P-36 is no argument for fancy-dan propellers the Spitfire was a better plane. I agree with you completely I am just saying what is obvious now wasn't obvious then. Companies saying "you must buy this, it is a new wonder" were commonplace.




Are commonplace. The interest of companies does not necessarily coincide with the interest of the country.

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## swampyankee (Mar 24, 2018)

pbehn said:


> But if you are a civil servant counting beans the P-36 is no argument for fancy-dan propellers the Spitfire was a better plane. I agree with you completely I am just saying what is obvious now wasn't obvious then. Companies saying "you must buy this, it is a new wonder" were commonplace.



The first controllable pitch propeller was introduced by Hamilton Standard in 1934; their first constant speed propeller in 1935 (Hamilton Standard Hydromatic Propeller History). The RAF -- or Supermarine -- didn't use them on the Spitfire by choice, not by lack of availability.

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## XBe02Drvr (Mar 25, 2018)

swampyankee said:


> The RAF -- or Supermarine -- didn't use them on the Spitfire by choice, not by lack of availability.


"We've got a world-beater here, just the way it is; why load it down with something complicated, heavy, and expensive that Government doesn't want and won't pay for? Besides, then we'd have to drill ports and galleries in all of our splineshafts and redo the nose cases on our Merlins to provide a mounting pad for the governor. Just doesn't make sense, old boy."
Cheerio chaps,
Wes

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## Elmas (Mar 25, 2018)

Elica a passo variabile

AlfaSport Club

In Italy research about constant speed propellers started well before 1930, by Alfa Romeo and Piaggio.

This propellers had an electric movement that sometimes gave problems (the prototipe of RE 2001 was lost for a propeller failure).
Ing. Trojani, designer of A.U.T. 18 in late ‘30s, in his memories recalls that he wanted to test an Hamilton Standard on this plane, but Fiat, who had the license, never put this propeller in production and to import the originals from U.S.A. and pay it in dollars would have been by far too expensive.
I think that bean counters are widespread in the world over.

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## pbehn (Mar 25, 2018)

Looking at pictures of Merlin exhausts there are a huge variety of shapes. Mosquito exhausts point slightly down for obvious reasons, the radiator inlet is behind on one side of the engine and a wooden wing is behind on the other. On spitfires there seem to be a huge number of different types many basically similar but not exactly the same. Problem is how to tell what was actually used in 1939/45.


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## Benjdragon (Mar 29, 2018)

Zipper730 said:


> I'm curious as to the following
> 
> When did people begin to devise ways to increase the amount of exhaust thrust produced by a piston engine (at either the level of research scientist, design engineer, or military personnel)?
> When was it first realized that, at approximately 350 mph (at either the level of research scientist, engineer, or military personnel), one pound of thrust became roughly equivalent to 1 horsepower?
> In the various countries of the world such as the UK, France, Germany, USA, Russia/USSR?


The one aircraft that I know was designed to use exhaust thrust was the XP-67. It was supposed to get a significant amount of its total thrust from the exhaust.


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## Vintageaeroengines (Mar 29, 2018)

swampyankee said:


> Probably
> 
> Back in 1981 we used the NACA data to design the exhaust stacks for the air racer Dago Red.
> The formula inputs were boost pressure
> ...


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## mikemike (Mar 30, 2018)

What I know offhand is that the early, Jumo210-powered versions of the Bf109 had flush exhausts. Starting with the E model (DB601) they used ejector exhausts, originally because the hot exhaust gases were damaging the aircraft skin. I imagine someone cottoned on fairly quickly to the additional thrust produced by these exhaust stubs. That would probably have been about 1938. In later years, it was reckoned that on high-flying '109s the exhaust gases contributed about a third of the total thrust produced by the engine package; the effect is of course more pronounced the higher the aircraft flies. 

The DVL (the German aeronautical research body) produced a theoretical comparison early in the war between two engines that were identical apart from the supercharging system, one with a mechanically-driven supercharger and nozzle exhausts, the other with a turbocharger. The conclusion was that the turbo was superior only at low speeds (200-250 mph) and at high altitudes (above 30k feet); in addition the mechanical supercharger had no turbo lag.


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## pbehn (Mar 31, 2018)

The exhaust manifolds on Spitfires were used for a gun heating system, finally saw a picture that shows the exit pipe.


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## wuzak (Mar 31, 2018)

pbehn said:


> Looking at pictures of Merlin exhausts there are a huge variety of shapes. Mosquito exhausts point slightly down for obvious reasons, the radiator inlet is behind on one side of the engine and a wooden wing is behind on the other. On spitfires there seem to be a huge number of different types many basically similar but not exactly the same. Problem is how to tell what was actually used in 1939/45.



The original Mosquito pipes had the 6 cylinders on each side connect to a single exhaust pipe:






Then they went to the saxophone exhaust - the six outlets went into a common exhaust with two outlets.





This Merlin has the saxophone exhausts


Or the exhaust stubs, which had the rear two cylinders siamesed so that there were 5 exhaust outlets per side. This was, of course, to clear the radiators.






5 stub exhaust from NZ Civil Aircraft: ZK-FHC First Engine Run at AMZ 17-8-16

The 2 stage Merlins were longer, which enabled 6 stub exhausts to be fitted.

EDIT: Inserted picture of saxophone exhausts and moved the picture of ejector exhausts.

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## wuzak (Apr 1, 2018)

The drawings for the above post came from this thread.


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## DarrenW (Apr 8, 2018)

This is an interesting topic for me as I have never given this concept much thought until only recently. And while I can appreciate the theory behind the residual thrust benefit delivered by an engine's exhaust system, I'm still unsure about how much attention it received during the design and engineering phase. Was the amount of attainable exhaust thrust of the typical WWII aero engine actually high enough that it became a serious design consideration, or was the added thrust more of a residual benefit that for the most part became apparent only after all other engineering factors were hammered out?

From what I've gleaned from casual internet and forum searches this thrust became more and more apparent as altitudes increased, where the air was thinner and overall airframe drag was at it's lowest level. For the most part, did liquid-cooled engines produce more of this thrust than a typical air-cooled radial, or were they mostly the same?


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## swampyankee (Apr 8, 2018)

The engineers at the time realized that a highly supercharged engine would have high exhaust pressure; they also knew that capturing that energy to feed into the propeller was better than using that energy as a low performance rocket, but it was, alas, not practical to use blowdown turbines on WWII fighters. Using the energy for a bit of thrust was getting some use out of it. 

The thrust was because of mechanical supercharging; it’s effectively an engine with a much higher compression ratio than expansion ratio.

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## DarrenW (Apr 8, 2018)

Oh so the additional thrust at attitude had more to do with the effects of supercharging than anything else. Definitely missed that part. Thanks swampyankee for that realization. Knowing that were their any poorly designed exhausts which could have been easily modified to produce more of this thrust, without losing much if any exhaust efficiency? Through my readings the F4U comes to mind as a possible candidate.


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## Shortround6 (Apr 8, 2018)

You have two basic components. Mass X velocity of the gas stream. 
This governs the the thrust. 
However thrust as power varies with the speed of the aircraft and exhaust gas velocity can be quite high depending on engine/ pressure in the cylinders and air pressure at the nozzle it could be around 1900fps or 1300mph. I will get back to that.

Mass is the amount of air, gasoline/fuel and anything else (water/alcohol) exiting the exhaust pipes. This varies with the altitude of the engine. It is going to be the greatest at the critical altitude/s of the engine. Engine is part throttled below those heights and engine is getting less air above those heights.

Exhaust gas velocity changes with height, the less pressure at the outside of exhaust nozzle the higher the exhaust gas velocity, assuming the same pressure in the cylinder or exhaust passage leading to the exhaust nozzle. Should the the pressure in the cylinder/exhaust passage drop enough compared to the outside air pressure the velocity will go down. This is a minor consideration as it may take somewhere around 15,000ft above the critical altitude of the engine for this to start happening. 

Data I have is from a RR Merlin XX engine so the principles should be good, exact results may vary 

It was tested at full throttle in high supercharger gear only.

Now at less than full throttle, say cruise speed, two things are going to happen to the engine, The mass goes down, it some cases way down, and exhaust velocity may drop a bit. Peak pressure in the cylinder may drop and the pressure in cylinder when the exhaust valve is open may also drop (but not as much) and that pressure is the important (but unknown) one. 

A 3rd thing happens with the airplane, since the HP of the same amount of thrust is dependent on the speed of the aircraft/vehicle a slower moving plane gets less benefit from the same amount of thrust. This also affects a plane while climbing, it gets a lot less boost from exhaust thrust than a plane in high speed level flight. 

They found, rather quickly, that more than 3 cylinders sharing the same exhaust pipe/nozzle didn't work very well at all and any arrangement of off less than one cylinder per pipe/nozzle was less than ideal. 

It actually doesn't matter if the engine is liquid cooled or air cooled. A V-12 is much easier to arrange the exhaust system on 
A Wright R-2600 with exhaust thrust 




14 separate exhaust outlets can create a lot more drag than the two rows on the V-12.

Since what matters is the pressure in the cylinder/exhaust passage an air cooled engine operating at high boost will have more pressure than a V-12 operating at low boost (German and Russian engines) 

Please note that the longer the exhaust pipe and the more bends you put in it the lower the velocity will be at the end. 

I hope that helps with a general overview.

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## DarrenW (Apr 8, 2018)

It does help a lot. Thank you  Can you think of any particular exhaust configurations of wartime aircraft which you would classify as capitalizing the most from this concept?


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## Shortround6 (Apr 8, 2018)

I would think a good many of the V-12s, but obviously an engine running at 18lbs boost (66-67in) of manifold pressure going into the cylinders is going to have higher pressure leaving the cylinders than an engine running at 40-44inches (5-7lb boost).

The only radials that I know of in WW II to use individual stacks were the R-2600s on _some _A-20s and B-25s. I could well have over looked some.
The later Zeros used paired stacks, 4 on one side and 3 on the other? WHich leads to difficulties on the top and bottom cylinders having much longer paths than the side cylinders. The FW 190 used side exhausts with most paired 





but since 8 doesn't go into 14 evenly two cylinders had their own pipes? two cylinders shared two pipes? 

F4Us used 6 exhaust pipes but the pre-4 had some rather long pipes. 

and then





We can cover basics but when it comes to getting specific about any one installation/benefits it gets a lot harder as we are guessing at many things that they knew from measurements. By late in the war they were not designing exhaust systems by guessing or by what looked good or trying to fit it into space left over.

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## DarrenW (Apr 9, 2018)

Shortround6 said:


> I would think a good many of the V-12s, but obviously an engine running at 18lbs boost (66-67in) of manifold pressure going into the cylinders is going to have higher pressure leaving the cylinders than an engine running at 40-44inches (5-7lb boost).
> 
> The only radials that I know of in WW II to use individual stacks were the R-2600s on _some _A-20s and B-25s. I could well have over looked some.
> The later Zeros used paired stacks, 4 on one side and 3 on the other? WHich leads to difficulties on the top and bottom cylinders having much longer paths than the side cylinders. The FW 190 used side exhausts with most paired
> ...



I can appreciate the more difficult task of designing an efficient exhaust system for a radial engine and how by it's nature the V-12 intrinsically allowed for a much simpler and straightforward exhaust arrangement. Thanks for explaining the basics to me.


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## swampyankee (Apr 9, 2018)

One other issue is that some radial engines had the exhaust valves on the front side of the cylinders. 

I think it also can't be mentioned enough: using excess pressure from the exhaust system to provide thrust is much less efficient than using that pressure to increase the power passed to the propeller; the reason it was not done was because to do so required blow-down turbines, which added a level of complexity, weight, and volume to the engine installation. Turbocompounding is used in large engines today; it's just not used on engines where fuel economy is not a driving consideration.


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## Zipper730 (Apr 9, 2018)

Swampyankee said:


> Possibly because getting thrust from engine exhaust is only really possible with highly supercharged engines.


Meaning the pressure of the exhaust is higher, and as the air gets thinner, more thrust can be produced which helps out?



pbehn said:


> It is a sort of all encompassing word used to describe the replacing of burnt fuel/air with fresh fuel air mixture.


Looking at the wikipedia entry, it seems to be the ability to properly breathe out all the air in the engine before breathing in new air for combustion.


> there are other things to consider like the exhaust and inlet valves being open at the same time


That sounds like it wouldn't expand all the air out right...


> or on a two stroke back pressure and harmonic shock waves stop all the charge shooting straight out of the exhaust increasing actual compression ratio and decreasing consumption.


I'd have figured stopping the charge shooting out would just make the engine perform badly, though I'm surprised you'd see shockwaves form in the engine.



Shortround6 said:


> There are all sorts of tricks used on non supercharged engines to increase the gas flow.


Thrust?


> For one thing an engine at 20,000 ft has a LOT less air pressure trying to keep exhaust gases inside the exhaust pipe/cylinder than an engine at sea level


Which produces an effect similar to a rocket...


> another thing is that whatever "pulses" you get in an intake manifold between the inlet to the carb and the cylinder/piston


Does this have to do with the pistons moving?


> It is also a LOT easier to set-up and work with those pulses when you used one carburetor throat per cylinder.


So basically the carburetor splits up to feed each cylinder?


> Best design for exhaust thrust is a "pipe" as short as possible with one 90 degree bend and an outlet sized to let out the most gas as fast as possible consistent with keeping the exhaust gas velocity high.


The 90-degree bend is to avoid an excessively long duct?


> When P-40s were allowed to use WEP power settings they had to hacksaw off part of the exhaust pipe nozzle in order to accommodate the increased mass flow. Too small a nozzle prevented all the exhaust gas from exiting the cylinder.[/QUOTE





Benjdragon said:


> The one aircraft that I know was designed to use exhaust thrust was the XP-67.


That's not exactly right: It was a turbocharged engine that was supposed to get more thrust than usual turbochargers.

As I understand it, turbochargers get little thrust out of their exhaust because of the turbine and the fact that in many cases the exhaust tends to go downward not straight back. This system had the exhaust go right out piston, through the turbo, and right out the back.


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## Shortround6 (Apr 9, 2018)

Well, there is efficiency and there is efficiency. 
For a fighter (or small high performance plane) the exhaust manifolds/stacks on a V-12 engine can weigh 30-40lbs and provide, under optimum conditions around 10-11% more power over and above what is going to the prop. 
less than optimum conditions means a lot less. 
However the blown down turbine and drive set-up are going to add several hundred pounds and several cubic feet of space (per engine). 




Granted this engine has the blowdown turbine grafted onto the auxiliary stage supercharger which adds some bulk.
It also doesn't have any exhaust duct for the gases after they leave the turbine. 

At what point does the extra power/fuel economy balance out over the engine without the blowdown turbine?
As in just add 20-30 gallons to the fuel capacity of the single engine fighter and get another 30 minutes at cruising speed? 
The Wright turbo compound




Please notice that the supercharger and accessories section has been moved back away from the power section (cylinders/crankcase) to allow room for the turbines and drive system. And system it is as there has to be a variable speed drive from the turbines to the crankshaft to match the relative speeds at different power settings/altitudes.


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## pbehn (Apr 9, 2018)

Zipper730 said:


> That sounds like it wouldn't expand all the air out right...
> I'd have figured stopping the charge shooting out would just make the engine perform badly, though I'm surprised you'd see shockwaves form in the engine.
> .


In a supercharged engine I would think it is more likely to blow unburned fuel straight out of the exhaust. In order to have a longer duration with the valves fully open the inlet starts opening before the exhaust is closed. 

I was talking about a two stroke, if ever you have run one without the exhaust on it is a very painful and loud experience, here is a schematic.
www.instructables.com/id/Tuning-Two-Stroke-Engines/


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## Zipper730 (Apr 9, 2018)

Shortround6 said:


> Well, there is efficiency and there is efficiency.
> For a fighter (or small high performance plane) the exhaust manifolds/stacks on a V-12 engine can weigh 30-40lbs and provide, under optimum conditions around 10-11% more power over and above what is going to the prop.


So, if the prop is producing 3000 pounds of thrust, the exhaust produces 300-330?



pbehn said:


> In a supercharged engine I would think it is more likely to blow unburned fuel straight out of the exhaust.


You mean the inlet valve opening before the outlet valve?


> In order to have a longer duration with the valves fully open the inlet starts opening before the exhaust is closed.


Do you mean more time for everything to burn right? 


> I was talking about a two stroke, if ever you have run one without the exhaust on it is a very painful and loud experience, here is a schematic.
> www.instructables.com/id/Tuning-Two-Stroke-Engines/


So the back pressure is used to deliberately hold the exploding fuel in the combustion chamber to make it burn all the way?


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## Zipper730 (Apr 9, 2018)

pbehn said:


> I was thinking that the early exhausts on Merlins were to do with having twin blade fixed pitch props and later variable coarse/fine pitch props.


So the Spitfire went from fixed pitch, to twin-pitch, to variable pitch?



Shortround6 said:


> Just going from the tests at Spitfire performance it appears that the fixed pitch prop planes were flown off the ground and climbed at very much reduced RPM until aircraft speed built up to keep from stalling the props.


Why would spinning the blades slower reduce the odds of stalling the props? I figure spinning them faster would move more air over them and keep them lifting...

I just thought the climb performance on the Spitfire was poor early on because the blade was geared for higher true airspeeds.



swampyankee said:


> Most constant speed propellers have a significant portion of their blades stalled during the takeoff run


I never knew that


> With a fixed pitch propeller, the problem is that engine power is (roughly) linear with RPM, but the propeller power load is roughly proportional to the cube of the RPM


Power load? Do you mean wing-loading?


> it's not that the propeller will stall, it's that the engine can't turn the prop any faster.


I'm wondering if you're talking about a dynamic similar to a feathering prop, whereby air on either side of the prop tend to hold it in place? The high pitch prop geared for high speed would be unable to spin fast because of this?


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## Zipper730 (Apr 9, 2018)

Shortround6 said:


> For some reason the Air Ministry thought variable pitch or constant speed propellers were a passing fad
> They had tested variable pitch props back 20s and found them wanting


Which was because earlier planes had more drag and a narrower range of speeds to operate across



pbehn said:


> I read here years ago that civilian airliners used variable pitch props, but they were "commercial chaps" only interested on making money, the RAF was not a commercial enterprise so why would they need new and expensive propellers?


Well efficiency has use for military enterprises too, more thrust per horsepower means they can climb, accelerate, and hold power in turns...


> With a fixed prop. the Spitfire satisfied the specification so why spend more money, (even more money in civil service speak) on a toy for a war that probably wont happen.


And since they were older people who lived in an area prior to powered aircraft...

It's interesting how the U.K. and U.S. both had ways they were slow to adopt newer ideas

The U.K. with variable pitch props
The U.S. with jet-engines



> Companies saying "you must buy this, it is a new wonder" were commonplace.


The issue is to figure out how to identify which is nonsense and ignore it, then identify things that might work and test them out to see if they do.


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## pbehn (Apr 9, 2018)

Zipper730 said:


> 1 Do you mean more time for everything to burn right?
> 2 So the back pressure is used to deliberately hold the exploding fuel in the combustion chamber to make it burn all the way?


1 No, the valves are working at their best when opened fully, but they are opened by a cam. To get to the fully opened position and back to fully closed takes time so they sometimes overlap, the exhaust is still closing when the inlet starts opening, but behind the inlet valve the air is pressurised by the supercharger.

2 No, this is a two stroke engine, the power stroke provides power until just after the piston moves down and uncovers the exhaust port, slightly further down the stroke the inlet ports are opened so the cylinder is emptying the spent charge and filling with the new charge at the same time. These shock waves are not the same as back pressure, they stop the charge shooting straight from the inlet and out of the exhaust as you can see on the schematic.

This is harmonics, a highly tuned two stroke runs and sounds like a bag of hammers at low revs, when it gets into the power band, torque and power rise sharply, then just as quickly it runs out of the power band, there is no point at all in over revving a two stroke, nothing happens because the engine cant breathe at all.

I once saw Barry Sheene practicing at Brands Hatch and he was the only one with an engine running, you could hear the bike go into the power band as he left the pits, like a musician hitting the right notes after hitting a lot of wrong ones.


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## Shortround6 (Apr 9, 2018)

Zipper730 said:


> So, if the prop is producing 3000 pounds of thrust, the exhaust produces 300-330?




For the example I have, the Merlin XX in a Hurricane, the engine had 1126hp going to the prop at 20,000ft, the exhaust *HP* (not thrust) was calculated at 126.8. This is with an aircraft speed of 340mph and using 50.67in of manifold pressure. 
At the same altitude but using 48.24in of manifold pressure aircraft speed dropped to 335mph, prop hp was 1073. Exhaust *HP* dropped to 113.0

Three reasons. 
Mass flow (air plus gasoline) had dropped from 151lb/min to 144.0lb/min
exhaust gas velocity had dropped from 1788fpm to 1695fpm.
The aircraft was moving slightly slower.

At 15,000ft the Exhaust HP was only 86.5 compared to the prop hp of 1048. Mass flow was 140.5lb/min but exhaust velocity had dropped to 1395fpm and the plane was doing 325mph. 

Explanations of car/motorcycle intake exhaust for none supercharged engines will have to wait. 
or google like
Intake Tuning | Intake Runner Lengths | Engine Tuning

Please note that such tricks are useless on supercharged engines and also useless at the rpms that aircraft engines operate at.

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## Zipper730 (Apr 9, 2018)

Shortround6 said:


> For the example I have, the Merlin XX in a Hurricane, the engine had 1126hp going to the prop at 20,000ft, the exhaust *HP* (not thrust) was calculated at 126.8. This is with an aircraft speed of 340mph and using 50.67in of manifold pressure.


Exhaust horsepower is 11.26% of the engine horsepower at 50.67"


> At the same altitude but using 48.24in of manifold pressure aircraft speed dropped to 335mph, prop hp was 1073. Exhaust *HP* dropped to 113.0


10.53% of engine horsepower, a reduction of 0.73% of engine horsepower with a manifold pressure decrease of 4.796%


> Three reasons.
> Mass flow (air plus gasoline) had dropped from 151lb/min to 144.0lb/min
> exhaust gas velocity had dropped from 1788fpm to 1695fpm.


Sucking in and blowing out less


> The aircraft was moving slightly slower.


Ram compression effects?


> At 15,000ft the Exhaust HP was only 86.5 compared to the prop hp of 1048. Mass flow was 140.5lb/min but exhaust velocity had dropped to 1395fpm and the plane was doing 325mph.


8.25% of engine horsepower.


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## wuzak (Apr 9, 2018)

Zipper730 said:


> Meaning the pressure of the exhaust is higher, and as the air gets thinner, more thrust can be produced which helps out?



It is mass flow rate which gives the thrust.

I read an article some time ago explaining that in some cases the Merlins used by Reno racers had pushed the boost pressure up so high that they were no longer getting any extra power at the prop (more power in the main section was cancelled by the extra power required to drive the supercharger). The reason that they would do such a thing is that the extra mass flow creates greater thrust effect. And though they are at relatively low altitude, they are flying at 500mph or above.


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## wuzak (Apr 9, 2018)

Interestingly, airframe manufacturers in the US were often responsible for the exhaust systems.

Allison, for example, supplied the engine with flanges, to which the exhaust pipes or stubs would be welded. 

In the UK a lot of the exhaust was done by the manufacturer, such as Rolls-Royce. That is why the Hurricane, Spitfire and some other Merlin powered aircraft had the same style ejector stubs early on.


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## Shortround6 (Apr 9, 2018)

Zipper730 said:


> Exhaust horsepower is 11.26% of the engine horsepower at 50.67"
> 10.53% of engine horsepower, a reduction of 0.73% of engine horsepower with a manifold pressure decrease of 4.796%
> Sucking in and blowing out less
> Ram compression effects?
> 8.25% of engine horsepower.




The formula for figuring thrust to horsepower is force X speed of the aircraft divided by 375. Just make sure you get all the measurements sorted out as to seconds, minutes etc. 
But obviously the HP changes with speed. At 375mph the force equals HP. At 340mph the same force is multiplied by 0.9066.
At 335mph it is 0.8933. 
At 200mph (say climbing) it is 0.5333 of the force. 

We know it exists, but trying to figure out the actual benefit for individual aircraft and different flight conditions needs a lot of information.


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## Zipper730 (Apr 10, 2018)

Shortround6 said:


> The formula for figuring thrust to horsepower is force X speed of the aircraft divided by 375.


I assume this doesn't factor in ram compression?


> Just make sure you get all the measurements sorted out as to seconds, minutes etc.


You mean airflow requirements?


> But obviously the HP changes with speed.


So this comes out to

150 mph: 1 lb/thrust = 0.4 HP
200 mph: 1 lb/thrust = 0.5333 HP
250 mph: 1 lb/thrust = 0.6667 HP
300 mph: 1 lb/thrust = 0.8 HP
325 mph: 1 lb/thrust = 0.8333 HP
350 mph: 1 lb/thrust = 0.9333 HP
375 mph: 1 lb/thrust = 1 HP
400 mph: 1 lb/thrust = 1.0667 HP
425 mph: 1 lb/thrust = 1.1333 HP
450 mph: 1 lb/thrust = 1.2 HP
475 mph: 1 lb/thrust = 1.2667 HP
500 mph: 1 lb/thrust = 1.3333 HP
525 mph: 1 lb/thrust = 1.4 HP
550 mph: 1 lb/thrust = 1.4667 HP
575 mph: 1 lb/thrust = 1.5333 HP
600 mph: 1 lb/thrust = 1.6 HP
When was his realized?


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## wuzak (Apr 10, 2018)

Zipper730 said:


> When was his realized?



When Newton published his laws of motion?


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## swampyankee (Apr 12, 2018)

Zipper730 said:


> I assume this doesn't factor in ram compression?
> You mean airflow requirements?



It does; ram compression is buried in the thrust calculation: thrust is mass flow rate times change in velocity plus pressure times area at the nozzle exit; the latter is almost certainly negligible. Exhaust mass flow is going to be intake air flow plus fuel flow minus any system leakages, which one would assume to be negligible.



Zipper730 said:


> So this comes out to
> 
> 150 mph: 1 lb/thrust = 0.4 HP
> 200 mph: 1 lb/thrust = 0.5333 HP
> ...




No later than the mid-19th Century, when people started designing locomotives for pulling stuff and including specifications for tractive force at a given speed.


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## Zipper730 (Apr 13, 2018)

wuzak said:


> When Newton published his laws of motion?


Intuitively that makes perfect sense, but compressibility effects were known since the 1700-1800's on some level, but rarely factored into early aircraft designs because they flew nowhere near fast enough to cause trouble (except the props).

I'm wondering if something like that applied to attempting to extract exhaust thrust from the engine system, or at the very least not really pursuing it in any degree because of the fact that

Most early engines didn't produce all that much horsepower, so the amount of thrust to be gained by a creative exhaust pipe set-up wouldn't have been worth it
Most early engines didn't have particularly high manifold pressures, so it would have interfered with scavenging
Most early engines were naturally aspirated or used minimal supercharging, and were not capable of propelling an aircraft high enough to make use of it
Most early aircraft were not capable of flying fast enough to make use of such a thing



swampyankee said:


> It does


Okay


> thrust is mass flow rate times change in velocity plus pressure times area at the nozzle exit


I'm confused about something. Some things increase to the square of velocity, others are proportional.

I keep mixing things up


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## swampyankee (Apr 13, 2018)

Zipper730 said:


> Intuitively that makes perfect sense, but compressibility effects were known since the 1700-1800's on some level, but rarely factored into early aircraft designs because they flew nowhere near fast enough to cause trouble (except the props).
> 
> I'm wondering if something like that applied to attempting to extract exhaust thrust from the engine system, or at the very least not really pursuing it in any degree because of the fact that
> 
> ...




One way to sort out whether velocity is squared or not is to sort out the units. 

Mass flow is mass per unit time, velocity is distance per unit time; multiplying these gives (mass times distance) divided by (time squared), which is the mix of units for force. This is easier to explain in SI, partly because the US Customary system uses the same term, pound, for mass and force. 

Work and energy are force times distance (torque is a vector, and torque and energy are not interchangeable!); both have the units of joules, or (mass times [distance squared]) divided by (time squared). It's measured in joules. Power is energy per unit time, so its units are joules per second, or watts. Dimensional analysis is a useful technical skill.


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## Shortround6 (Apr 13, 2018)

Zipper730 said:


> Intuitively that makes perfect sense, but compressibility effects were known since the 1700-1800's on some level, but rarely factored into early aircraft designs because they flew nowhere near fast enough to cause trouble (except the props).
> 
> I'm wondering if something like that applied to attempting to extract exhaust thrust from the engine system, or at the very least not really pursuing it in any degree because of the fact that
> 
> ...




You pretty much have it, go back to your chart on hp and Look at the 100-200mph range, then figure that superchargers were actually pretty rare until the early 1930s so cylinder pressure was low and that 500-600hp for a an engine was pretty hot stuff, racing engines aside. 



> Okay
> I'm confused about something. Some things increase to the square of velocity, others are proportional.
> 
> I keep mixing things up



mass times velocity gives you momentum, Mass times velocity squared gives you energy. 

we can break it down a bit by looking at a rifle. Bullet weight times velocity squared gives us the energy of the bullet (it's ability to do work, like punch through an obstacle) but when figuring recoil we take the weight of the bullet times velocity (NOT squared) plus the weight of the propellent times the escape velocity of the gas at the muzzle and then divide by the weight of the rifle to give us the _velocity _of the rifle moving backwards. _Then _we use the mass/weight of the rifle times it's recoil velocity squared to get the _recoil energy. _

Drag gets a bit stranger


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## Zipper730 (Apr 13, 2018)

swampyankee said:


> One way to sort out whether velocity is squared or not is to sort out the units.


Mass flow = mass/time; Velocity = distance/time; multiplied mass-distance/time^2?


> Work and energy are force times distance (torque is a vector, and torque and energy are not interchangeable!); both have the units of joules, or (mass times [distance squared]) divided by (time squared). It's measured in joules. Power is energy per unit time, so its units are joules per second, or watts. Dimensional analysis is a useful technical skill.


What's a vector? As for joules, you could call it a newton-second right?

A newton is (kg)/(m/s^2), so a joule is [(kg)/(m/s^2)]*(second), which is kg*s^2/m (provided I can take the reciprocal, reverse it and multiply it)



Shortround6 said:


> You pretty much have it, go back to your chart on hp and Look at the 100-200 mph range, then figure that superchargers were actually pretty rare until the early 1930s so cylinder pressure was low and that 500-600hp for a an engine was pretty hot stuff, racing engines aside.


Provided you have a top-speed of 150 mph, 500-600 horsepower, and 2.5 pounds of thrust per horsepower from the propeller, that comes out to a propeller thrust of 1250-1500 pounds, an exhaust thrust of 50-66 pounds?


> Bullet weight times velocity squared gives us the energy of the bullet


Yup


> when figuring recoil we take the weight of the bullet times velocity (NOT squared) plus the weight of the propellent times the escape velocity of the gas at the muzzle and then divide by the weight of the rifle to give us the _velocity _of the rifle moving backwards. _Then _we use the mass/weight of the rifle times it's recoil velocity squared to get the _recoil energy._


So you factor the force of the bullet shoving the rifle back and then the mass of the rifle times the velocity it goes flying back squared


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## Shortround6 (Apr 13, 2018)

Zipper730 said:


> Provided you have a top-speed of 150 mph, 500-600 horsepower, and 2.5 pounds of thrust per horsepower from the propeller, that comes out to a propeller thrust of 1250-1500 pounds, an exhaust thrust of 50-66 pounds?


Probably not even that. The Merlin was making 1362hp at the crankshaft (but not including friction) but 236 hp was being used to drive the supercharger so while 1126hp went to the prop you had the air/fuel mass needed for 1460-1500hp (including friction. rough guess) going out the exhaust. 

1920s engine without supercharger only made 500-600hp near sea level. It was smaller in displacement (mostly) and turned fewer rpm. Approximately 80% of the friction on an engine comes from the pistons/piston rings and friction goes up with the square of the speed. A 2400 rpm engine has about 64% of the friction of a 3000rpm engine with everything else being equal (cylinder sizes, bearing sizes, etc).
With no supercharger such an engine might only be using 1/3 of the fuel/air a 1100hp engine is at 20,000ft 

It is certainly going have a much different exhaust thrust ratio ( lower compression plus no boost in the intake fighting higher back pressure if flying lower) than a 1940 engine. 

They were much more interested in keeping the exhaust out of the cockpit than getting thrust from it.
They also found that some of the engines tended to warp valves using short stacks. Low rpm engines had a longer time between exhaust pulses and cold air could be drawn in hitting the valve stem between exhaust pulses. 1920s valves were not what 1940s valves were nor was coolingo f the whole exhaust valve area.


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## swampyankee (Apr 13, 2018)

Joules are newton-meters, that is kg-m^2/s^2. Foot-pounds is also a unit of energy. Confusingly, torque is also measured in foot-pounds, which is part of why I think there is an inane argument about torque vs power: people conflate energy and power all the time, and the units make it easy to conflate energy and torque.

A vector is a quantity with an associated direction: going 4 meters north is a vector, as there's a quantity (4 meters) with a direction (north). Everybody uses them all the time. 

Recoil is conservation of momentum: the mass times velocity of the bullet (plus the mass times velocity of the gun gas) is equal and opposite to the mass times velocity of the recoiling gun; this is one of the basic laws of physics, the law of conservation of momentum. A ballistic pendulum, which used to be used to measure muzzle velocity, relies on this law and the law of conservation of energy. That energy is (1/2)mv^2 and momentum is mv is why a high-velocity, light-weight bullet may produce less recoil than a heavy, slow bullet with the same energy: if you cut the mass in half and double the velocity, the momentum (recoil) is the same but the muzzle energy is twice as much.

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## wuzak (Apr 13, 2018)

Zipper730 said:


> A newton is (kg)/(m/s^2), so a joule is [(kg)/(m/s^2)]*(second), which is kg*s^2/m (provided I can take the reciprocal, reverse it and multiply it)



wtf!

F = m * a
N = kg*m/s^2 

Work (energy) = F * displacement
J = N * m = kg*m/s^2*m = kg*m^2/s^2


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## wuzak (Apr 13, 2018)

swampyankee said:


> A vector is a quantity with an associated direction: going 4 meters north is a vector, as there's a quantity (4 meters) with a direction (north). Everybody uses them all the time.



Velocity is a vector.
Speed is a scalar (quantity without direction).


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## wuzak (Apr 13, 2018)

swampyankee said:


> Joules are newton-meters, that is kg-m^2/s^2. Foot-pounds is also a unit of energy. Confusingly, torque is also measured in foot-pounds, which is part of why I think there is an inane argument about torque vs power: people conflate energy and power all the time, and the units make it easy to conflate energy and torque.



In SI units energy is J and torque is Nm. While they are dimensionally the same, they are not. Completely different concepts.

Torque is the rotational equivalent of force.
Power = Torque * angular velocity

is equivalent to
Power = Force * Velocity

Edit: Isn't one foot-pounds and the other pound-feet?


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## wuzak (Apr 13, 2018)

Indeed:

Foot-pounds and pound-feet—what’s the difference?

foot-pound is unit of work.
pound-foot is unit of torque.


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## Zipper730 (Apr 22, 2018)

wuzak said:


> wtf!


when dividing a fraction, you could treat it as multiplying the reciprocal, so kg/(m/s^2) could be treated as kg(s^2/m) though I might very well have screwed some things up as that looks like it'd come out to kg*s^2 / kg/m


> Velocity is a vector.


A distance covered over a period of time in a given direction?


> Speed is a scalar (quantity without direction).


Velocity without a specified direction?


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## pbehn (Apr 22, 2018)

Zipper730 said:


> when dividing a fraction, you could treat it as multiplying the reciprocal, so kg/(m/s^2) could be treated as kg(s^2/m) though I might very well have screwed some things up as that looks like it'd come out to kg*s^2 / kg/m
> A distance covered over a period of time in a given direction?
> Velocity without a specified direction?


I am getting a bit nervous about flying this plane you are building Zipper, do you know any other pilots?


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## Zipper730 (Apr 22, 2018)

Fortunately, I'm not planning on building any planes


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## XBe02Drvr (Apr 22, 2018)

Zipper730 said:


> A distance covered over a period of time in a given direction?


Not quite. This sounds like nitpickin' semantics, but think of it as a linear rate of change of position in a specific direction. It's a rate, not a distance.
Cheers,
Wes

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## pbehn (Apr 22, 2018)

XBe02Drvr said:


> Not quite. This sounds like nitpickin' semantics, but think of it as a linear rate of change of position in a specific direction. It's a rate, not a distance.
> Cheers,
> Wes


That is the difference between theoretical mathematics and real life situations. In college I had lots of theoretical problems concerning speed, velocity etc. Almost all of them concerned a constant speed or velocity or a change in accordance with another mathematical formula. In life of course very few things behave so uniformly. I remember several problems regarding bullets, but this was always as if in a vacuum, only gravity was considered not air resistance.

A police speed trap using time between distance may record an average speed of 70MPH which can mean the car travelled at a constant 70MPH ......or went for a 180MPH blast then stopped for a cigarette.


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## XBe02Drvr (Apr 22, 2018)

pbehn said:


> A police speed trap using time between distance may record an average speed of 70MPH which can mean the car travelled at a constant 70MPH ......or went for a 180MPH blast then stopped for a cigarette.


Ah, the wondrous VASCAR speed trap system! Boy does that bring back memories. Only works well if you can hide the cruiser or if you have an aircraft. In the Keys it was hard to find a place to hide a cruiser, and they didn't have aircraft, so as soon as you spot the cruiser, get on the brakes and they can't touch you. Years: 4, Tickets: Zero.
Cheers,
Wes


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