Vought XF5U "Flying Pancake

chuter · Jan 31, 2019

Yep, in Post 38 third photo we see the left hand ground run propeller's Hamilton Standard decals facing aft showing the counterclockwise turning blade is thrusting forward (there was no left hand turning version of this propeller). Right hand prop is normal in direction and function. The Pancake's complex rotor drive system (both rotors were tied together - couldn't turn separately - and either engine could be clutched out) was never close to being airworthy and doomed the project.

Also, it looks like World of Warplanes has wrongly modeled the aircraft with Hamilton Standard props which were strictly ground run clubs.

Gnomey · Feb 3, 2019

Nice shots guys!

johnbr · Jun 30, 2019

643 Test 160 - XF5U-1 (Stability) - NasaCRgis

Zipper730 · Jul 1, 2019

chuter said:
Yep, in Post 38 third photo we see the left hand ground run propeller's Hamilton Standard decals facing aft showing the counterclockwise turning blade is thrusting forward (there was no left hand turning version of this propeller).

So they would have had to make such a propeller?

The Pancake's complex rotor drive system (both rotors were tied together - couldn't turn separately - and either engine could be clutched out) was never close to being airworthy and doomed the project.

I'm curious if it would be possible to find a picture of the power-train?

chuter · Jul 5, 2019

Zipper730 said:
So they would have had to make such a propeller?
I'm curious if it would be possible to find a picture of the power-train?

If they were going to fly the aircraft with standard props, yes, but no, the plane was designed to fly with the complex rotor system seen in other photos, the Hamilton Standard props (of which there was no left hand turning model) were used strictly for ground running to avoid putting needless hours of wear and tear on the expensive bespoke rotors.

I don't recall seeing photos clearly showing the entirety of the drive system but there are diagrams and drawings floating about.

As an aside the V-22 Osprey had the same dual engine/single drive requirement that the Pancake had and I think we all can recall the drawn out development issues (and crashes) that were experienced by that (mandated) program.

Gnomey · Jul 8, 2019

Good shots!

swampyankee · Jul 8, 2019

chuter said:
If they were going to fly the aircraft with standard props, yes, but no, the plane was designed to fly with the complex rotor system seen in other photos, the Hamilton Standard props (of which there was no left hand turning model) were used strictly for ground running to avoid putting needless hours of wear and tear on the expensive bespoke rotors.

I don't recall seeing photos clearly showing the entirety of the drive system but there are diagrams and drawings floating about.

As an aside the V-22 Osprey had the same dual engine/single drive requirement that the Pancake had and I think we all can recall the drawn out development issues (and crashes) that were experienced by that (mandated) program.

Boeing-Vertol had decades of experience with that sort of transmission on its helicopters when it was building the V-22. Vought didn't, nor was the process of designing and producing high power bevel gears as well known.

There were a lot of technical issues with the XF5U, the cross-shafting and need for flapping and lead-lag hinges on the propellers being two.

nuuumannn · Jul 8, 2019

wuzak said:
You could have the power turbine in line with the propellers, while the main part of the engine remains where it is.

Dunno how that's gonna work. The power turbine is driven by the exhaust gases, so it has to be in line with the combustor section.

nuuumannn · Jul 8, 2019

chuter said:
As an aside the V-22 Osprey had the same dual engine/single drive requirement that the Pancake had and I think we all can recall the drawn out development issues (and crashes) that were experienced by that (mandated) program.

Although the V-22 programme was complex and the rotor system might have caused delays in construction and testing, I don't believe any of the frame losses the Osprey has suffered have been as a result of the complexities of the 'dual engine/single drive system'. Take a look here:

Accidents and incidents involving the V-22 Osprey - Wikipedia

wuzak · Jul 8, 2019

nuuumannn said:
Dunno how that's gonna work. The power turbine is driven by the exhaust gases, so it has to be in line with the combustor section.

No, it doesn't have to be in line with combustor.

SZR: Z32 Turbocharger based Gas Turbine, MK2.5

GR-1 Turbojet Project 2/21/04

A power turbine does not have to be connected to the gas generator mechanically. This makes it a free power turbine, and is used to drive something - a generator, a prop, etc.

The gas generator is a gas turbine, with compressor driven by its own turbine.

nuuumannn · Jul 8, 2019

wuzak said:
A power turbine does not have to be connected to the gas generator mechanically. This makes it a free power turbine, and is used to drive something - a generator, a prop, etc.

Yeah, I know this, I work on gas turbines. The free turbine is still in line with the combustor though, through concentric shafts. It, as you say is not mechanically linked to the compressor section, but it still needs that power from the exhaust, so in a gas turbine that's fitted to an aircraft, not a car turbocharger, has to be in line with the combustor. In, say the PW100 series engines, which I work on, the LP and HP compressors have their own turbine, but the power turbine is separate to them in that it is connected via the reduction gearbox to the propeller, but the turbines are driven by exhaust gases, which means it needs to be in line with the combustor.

nuuumannn · Jul 8, 2019

Here's a short clip on the PT-6; you'll notice for one thing that the engine is reverse flow, the intake is at the rear of the engine and the air flow moves forward through centrifugal compressors to the combustion section, after which the turbines are driven by the combusted gases. The power turbine is the foremost connected to the drive shaft that turns the prop, via an RGB:

wuzak · Jul 8, 2019

That's the way it is generally done, but it doesn't have to be.

And it is logical for most applications.

nuuumannn · Jul 8, 2019

wuzak said:
That's the way it is generally done, but it doesn't have to be.

Not in an aircraft engine. The turbine relies of the energy from the combusted gases. The further you move the turbine away from that heat source, the greater amount of effort to produce a given amount of power, which is just inefficient. In an application like an aircraft engine, it would be foolhardy to do it any other way. Heat is the key, that is why in pure jet engines you have guide vanes and turbine blades that regularly operate in temps greater than their base metal melting points (this is done through efficient cooling).

You need to read the theory behind the operation of the turbine section and the extraction of energy from combusted gases to understand why it doesn't make sense to put the turbine away from the combustor. In aircraft engines and industrial sized gas turbine engines, that's the way it is.

Zipper730 · Jul 8, 2019

Why would you have the exhaust flow backwards? I figure you would want to have the airflow going as straight aft as possible with the minimum number of directional changes.

nuuumannn · Jul 8, 2019

Zipper730 said:
Why would you have the exhaust flow backwards?

It's just another means of doing the same thing, it creates a compact and light engine arrangement. Note also that the compressor section is also centrifugal flow, again, in the PT-6's case for compactness. An axial flow compressor takes up space. In most (not all) small helicopter and aeroplane turbines the compressor is centrifugal. In the PT-6 the coupling between the turbine and RGB/prop is short, because in every gas turbine there are mechanical inefficiencies in the extraction of energy from the combustor section by the turbine (this is why the turbine needs very hot gases, to maximise efficiency - high temps = high efficiency), so with a reverse flow engine the length of shafts etc can be kept short, which produces a compact yet efficient engine. Note also that the combustion chamber is designed so the gases double back on themselves before they exit the combustor section.

PT-6s are extraordinarily efficient for their size and have enormous application. Take a look at this page, which gives you a list of the applications of this engine.

Pratt & Whitney Canada PT6 - Wikipedia

nuuumannn · Jul 8, 2019

Zipper730 said:
I figure you would want to have the airflow going as straight aft as possible with the minimum number of directional changes.

In a gas turbine the air doesn't flow straight. It is constantly changing direction to get the most efficiency out of it. In the compressor section and in the turbine section the air is swirled and its direction changed with the use of stators and guide vanes to move that air perpendicular to the linear mechanics of the engine, then back again over the rotor sections of the compressor and turbines.

There's plenty of links to this stuff on the net.

Zipper730 · Jul 8, 2019

nuuumannn said:
In a gas turbine the air doesn't flow straight. It is constantly changing direction to get the most efficiency out of it. In the compressor section and in the turbine section the air is swirled and its direction changed with the use of stators and guide vanes to move that air perpendicular to the linear mechanics of the engine, then back again over the rotor sections of the compressor and turbines.

That's why I said "with the minimum number of directional changes".

nuuumannn · Jul 8, 2019

Zipper730 said:
That's why I said "with the minimum number of directional changes".

Even then, that's not accurate.

wuzak · Jul 9, 2019

nuuumannn said:
Not in an aircraft engine. The turbine relies of the energy from the combusted gases. The further you move the turbine away from that heat source, the greater amount of effort to produce a given amount of power, which is just inefficient. In an application like an aircraft engine, it would be foolhardy to do it any other way. Heat is the key, that is why in pure jet engines you have guide vanes and turbine blades that regularly operate in temps greater than their base metal melting points (this is done through efficient cooling).

You need to read the theory behind the operation of the turbine section and the extraction of energy from combusted gases to understand why it doesn't make sense to put the turbine away from the combustor. In aircraft engines and industrial sized gas turbine engines, that's the way it is.

For an installation like I suggested for the XF5U the question is whether the losses are greater in using an offset power turbine or using an in-line power turbine with bevel drive to the props.

However, I believe it was pointed out earlier in the thread that the geared solution includes connecting one side to the other for redundancy, which would not work with the offset turbine.

Vought XF5U "Flying Pancake

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