Exhaust Thrust

[swampyankee]

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.

 

[pbehn]

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?

 

[pbehn]

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.

 

[Shortround6]

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.

 

[swampyankee]

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.

 

[swampyankee]

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.

 

[XBe02Drvr]

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

 

[Elmas]

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.

 

[pbehn]

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.

 

[Benjdragon]

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.

 

[Vintageaeroengines]

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
RPM
Back pressure of altitude
Cylinder volume per pipe
Results were the diameter of the exhaust opening.
That gave the best increase in thrust vs back pressure loss of HP
All from the 1940's
Mike

 

[mikemike]

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.

 

[pbehn]

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

 

[wuzak]

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.

 

[wuzak]

The drawings for the above post came from this thread.

 

[DarrenW]

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?

 

[swampyankee]

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.

 

[DarrenW]

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.

 

[Shortround6]

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.

 

[DarrenW]

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?