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?

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.

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!

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.

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.

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".

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.

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

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

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.

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.

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?

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.

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.

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.

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.

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

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.

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

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.

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 disgusted 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.

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.

Exhaust Thrust

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