Why were turboprop fighters never considered?

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Clay_Allison

Staff Sergeant
1,154
7
Dec 24, 2008
I was thinking about (and reading threads about) various desperate German attempts to gain an edge in the skies that would keep the war from tipping violently against them. I was wondering why they never seemed to consider using a turboprop-driven fighter. A turbine engine could potentially deliver a big boost in power, provide additional thrust for more speed, and be incorporated into an existing fighter. I don't know enough to say, (that's why I'm asking) but a 2000 horsepower Bf-109 might be possible, and might just fly like a bat out of hell.
 
In order to have a working turboprop you need a working jet engine. Then you need a gear box to reduce the shaft rpm down to an rpm that the propeller can use about a 5 or 6 to 1 reduction. Then you need extra turbine sections to turn the exhaust thrust into shaft HP.
THis last was probably a bigger problem for the Germans. Doubling the amount of tubine blades per engine over a straight jet was probably not something they wanted to get into given their shortage of nichol and chrome.
I beleive there was a Hungarian (?) who had a protoype engine running on a test stand in 1940(?)
 
The propeller disc would run into the same difficulties as it was beginning to encounter with the final-generation piston engines.
 
Something like this Clay.

FW.281



The prop was driven by a shaft from the front of the jet engine through a gearbox.
 
Clay - turboprops in warplanes, especially single-engined warplanes, are not simple engineering. Post-war USA: consider the number of designs - from Douglas SkyShark (SkyRaider replacement) to VTOL's, to flying boats that ALL were based on a reliable Allison turboprop engine that NEVER ARRIVED. In the UK they were more successful - the RN using several turboprop machines like the Westland Wyvern and the Gannet. But turboprops without reliable turbines is just hard luck...

Oddly - the Soviets are by far the most committed users of turboprops - counter rotating - for miliary AC and helicopters.

So - think of all the vulnerabilities of the Me-262 (turbine wise). How will putting a gearbox and a prop on THAT (or similar) turbine provide any performance benefits over - say - a Ta-152?

Just wondering

MM
 
Back in the 1940s and 50s they sometimes listed turbo props with ESHP (estimated shaft horse power) which was made up from the shaft HP to the propeller plus the estimated HP of the residual thrust of the jet exhaust. Since the conversion of thrust to HP varies with the speed of the aircraft and the altitude this figure is less useful to us armchair type engineers than to real engineers

But looking at the figures does show that some companies got their turbine section to "capture" a higher percentage of the power in the jet exhaust for the propellor than other companies.

Figureing fuel economy based on the ESHP gave more favorable numbers than just using the shaft horse power too.
 
If I was making a turbo-powered warbird replica, I'd rather have a turboshaft (helicopter) engine that gives more shaft horsepower and forget about thrust.
 
The amont of thrust produced from a turboshaft engine is minimal - its the torque applied to the propeller shaft that will determine power, and of course an effective propeller/ airframe combination that will develop the most power or efficency, depending on application.
 
The 'rule of thumb' for relationship between E.S.H.P and Direct H.P. is

ESHP=DirectHP + (net Thrust)*(Velocity in Kts)/(326*.80)

The .8 efficiency factor is strictly an empirical average for propeller efficiency of various props designed for achievable speeds of the airframe/turboprop system > .8+M..

WWII prop efficiencies were near .85 at .5M but dropped rapidly to .7 at .65M and .6 at .75M (where they hit the 'wall').

The modern turbo prop efficiency is .92+ at .5M and .8+ at .9M - a HUGE improvement and essential for high speed turbo props.

It is interesting to note the evolution from thick paddle blade props designed for .5M+ (P-51/47/Me 109/Spit) to a rectangular/thinner to a tapered and very thin square tip prop as top speeds closed on .8-.9M.
 
I am just telling you what was listed in books of the time.

You can tell how much was really thought of the exhaust thrust by looking at the Exhaust pipes of some of the turbo powered aircraft.

File:T-34C Turbo Mentor.jpg - Wikipedia, the free encyclopedia

File:Wyvern-2.jpg - Wikipedia, the free encyclopedia

Piper PA-48 Enforcer - close support aircraft

for the exhaust thrust to any real good it has to be directed rearward and not just dumped out the side of the aircraft.
Also note that a higher speed aircraft at high altitude (turbo-prop busness plane) will get much more benifit than a low speed-low altitude aircraft (crop duster) from the exhaust of the same engine.
 

Actually, you know why the exhaust is so big? Cooling. A narrower exhaust would cause turbine inlet temps and exhaust temps to go up but you would have more thrust from the engine, as if you put your thyumb over a garden hose.You would also probably have higher fuel consumption as well.

Here's a pic of a Twin Otter I work with - you could see the size of the exhaust, the same at the PT-6 T-34.

Bottom line, with just about any turboprop you're only getting about 10% power from the turbine exhaust. 90% will come from the props.
 

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FLYBOYJ said:
Bottom line, with just about any turboprop you're only getting about 10% power from the turbine exhaust. 90% will come from the props.
Click to expand...

thank you for the picture.

You are right about the percentage of power but maybe back in the 50's it made the fuel consumption figures look about 10% better in the sales brochure if they figured in the exhaust thrust as power

Especially if some of the Piston engine guys were touting exhaust thrust augmenters.

like: File:C-FUKN-Northway-Aviation-DHC-3-Otter-2.jpg - Wikipedia, the free encyclopedia

or:

File:Convair 240 HB-IRV Swiss Air Lines Ringway 25.03.50.jpg - Wikipedia, the free encyclopedia
 
Shortround6 said:
[
Also note that a higher speed aircraft at high altitude (turbo-prop busness plane) will get much more benifit than a low speed-low altitude aircraft (crop duster) from the exhaust of the same engine.
Click to expand...

The reason for relative change between disk thrust to exhaust thrust is that the rate of change for net thrust from the prop disk as it approaches max speed at high altitude drops rapidly while the exhaust thrust is 'steady'....

At high cruising speed well below the threshold of max speed the exhaust thrust is minor for a turbo prop at any altitude
 
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