Props in pylons next to aft fuselage?

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karoliina.t.salminen

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Hi

CriCri is an example of props in pylons. However, what about pylons in aft fuselage?
TT62 Alekto is example of that. It is unclear why it failed, was it due to mechanical problem or aerodynamic problem? Will the pylons function as additional unwanted tails with moment arm clearly aft from CG if placed like this? Why the CriCri works but there are no examples of pylons in aft fuselage (projects that would not have failed), at least that I would know of?
In electric airplane, it would be easy to fit motors in such pylons and it would not require complex shafts and belt drives but the motors would easily fit similar pods than Alekto has. Was Alekto a pylon aeodynamics failure or Thielert + associated mechanics failure (why they abandoned this config)?
They obviously went to bankrupt after getting to redesign. But why was it redesigned?
Would such placement work in electric plane actually without problems?

Best Regards,
Kaoliina
 

Himat

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There are several aircraft with twin turbojet or turbofan engines on pylons on the aft fuselage. Aerodynamic twin propellers on pylons placed aft on the fuselage should work if turbofans do. I suspect that the question of weight and balance have something to do with the few airplanes with such a configuration.

An exemample of such an installation on a light twin is the Beriev 103.
beriev103.jpg
 

Jan Carlsson

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The closeness to the fuselage and or wing effect the relative wind into the prop disk, so different airspeed into the blade depending on if it close to fuselage or opposit.
 
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Dana

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Propellers close in front of the tail could be a destabilizing influence when the prop is idling or windmilling.

-Dana

Don't ever think you know what's right for the other person. He might start thinking he knows what's right for you.
 

Detego

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The design goal of the electric, propfan pusher is for lower Cd, less noise on the ground, less noise
in the air, providing for a comfortable aircraft interior environment.

The engine requirements would consist of two electric motors, in tandem on a single shaft; turning
two sets of propfans.

The pods will function as the motors housing and heat-sink.


unductedpropfan-PIREP.gif
 

Himat

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Dual rotating props is noisier then single rotating props.
How?
Given equal input power and equal propeller efficiency, the same amount of energy should be converted to noise either one or two propellers are used. And with the same amount of acoustic energy, the noise level should be equal.
 

Himat

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I did find another example of twin engines mounted on the aft fuselage. The BAE Mantis UAV. I do think I have seen pictures of a few others airplanes too, but can’t remember any names or details. One observation about “unconventional” layouts of airplanes, quite often it is easier to find an UAV example than a manned aircraft example. At least to find examples of “rear” mounted engine aircraft. This might be because weight and balance considerations work out a little different on an UAV. Less the fuel, the weight of the “payload” on an UAV stays much the same on each flight.
BAE_Mantis.jpg
 

karoliina.t.salminen

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The closeness to the fuselage and or wing effect the relative wind into the prop disk, so different airspeed into the blade depending on if it close to fuselage or opposit.
This was what I was thinking also. How would it affect dynamic stability? I don't know, but I would like to hear if this effect is significant.

What should the pylon incidence angle designed to be? Should it produce or not produce any lift?
 
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BBerson

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There is a down wash around the aft fuselage. I suppose you could angle the prop axis up some to align with airflow.
The uneven airflow will still remain worse than tractor prop, of course. This tends to shorten prop life. A good high damped and fatigue resistant prop is needed, I think. Prop failure in this case can destroy aft fuselage .
 

Detego

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Higher loads maybe expected on the propeller living in the changing air flow field and wake, created
by the body and flying surfaces in front. Depending on the severity of these loads this may increase
the structural needs of the aircraft, reducing propeller efficiency.
 

Jan Carlsson

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How?
Given equal input power and equal propeller efficiency, the same amount of energy should be converted to noise either one or two propellers are used. And with the same amount of acoustic energy, the noise level should be equal.
The rear prop is facing the vortex disk from the front propeller, same on Penta's Duo-Prop, more noise, more vibrations, but better efficiency, pulling straight.
 

Jan Carlsson

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Higher loads maybe expected on the propeller living in the changing air flow field and wake, created
by the body and flying surfaces in front. Depending on the severity of these loads this may increase
the structural needs of the aircraft, reducing propeller efficiency.
A pusher more so then a tractor, but even a tractor prop see different airspeeds around the clock.
Something I have been noticed is that I have never read about lost props as much as from pirep's from vari-long eaze pilots!
 

Dan Thomas

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The rear prop is facing the vortex disk from the front propeller, same on Penta's Duo-Prop, more noise, more vibrations, but better efficiency, pulling straight.
The Cessna 336/337 is a classic example of a push/pull twin that creates more noise than other twins, and it's due to the rear prop flying in the turbulence off the front prop. Get those propellers closer and the noise gets worse.

Dan
 

Himat

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Hi

CriCri is an example of props in pylons. However, what about pylons in aft fuselage?
TT62 Alekto is example of that. It is unclear why it failed, was it due to mechanical problem or aerodynamic problem? Will the pylons function as additional unwanted tails with moment arm clearly aft from CG if placed like this? Why the CriCri works but there are no examples of pylons in aft fuselage (projects that would not have failed), at least that I would know of?
In electric airplane, it would be easy to fit motors in such pylons and it would not require complex shafts and belt drives but the motors would easily fit similar pods than Alekto has. Was Alekto a pylon aeodynamics failure or Thielert + associated mechanics failure (why they abandoned this config)?
They obviously went to bankrupt after getting to redesign. But why was it redesigned?
Would such placement work in electric plane actually without problems?

Best Regards,
Kaoliina
If Wikipedia is correct, HPA TT62, the failure of the HPA TT62 Alekto was due to lower than expected performance. The cause of less than predicted performance is in the Wikipedia described as unexpected interference between wing, propeller, pylons and elevator.

Do the pylons function as additional unwanted tails?
Weight wise, yes probably. On the other hand, aft mounted propellers are in aerodynamic text books described as stabilizing. This may or may not pose a problem. With a tractor propeller in the nose the airplane will usually gain stability with a stopped engine and feathered propeller. The same condition with a tail mounted propeller and the aircraft might lose some stability.

I did find two examples of aircraft with this configuration. The Beriev is certified, but I don’t know its flying qualities.

If such a placement of the motors would work in an electric airplane? I would say the configuration could be made to work. But there is less experience to draw on doing the design and probably a few more and different pitfalls to hit than with a conventional configuration. If considering this pacement of the engines/motors, what advantage versus a conventional configuration do you see?
 

BBerson

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If the motors are light enough, you could mount two motors on the front of the horizontal tail, and eliminate the pylons.
The Rutan Bibpod does this.

Or, the pylons could blend into the horizontal tail, making a single wide chord surface.
 

karoliina.t.salminen

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Himat, I remember reading a study (some AIAA paper possbily, can't remember, seen so many of them) of optimal propeller placement, which is the least compromise between prop efficiency and drag. Optimal was a bit above the wing aft section of the wing chord, because it lowered drag, increased lift and had prop efficiency not degraded. The challenges of this position were seen structural/mechanical. But with small electric motors that is not the case unlike it would be with a internal combustion engine that has too large footprint and weight to fit to this said pylon mounted pod.
 
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