Pusher Prop Behind Flaps

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DaveK

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I would think that you wouldn't want the prop mounted behind the flapped portion of a wing due to the large wake that would be ingested. But I see that several aircraft use this configuration including the Avanti. Did some searching and didn't find much on the subject. Anyone have something more than a hunch on whether this is truly something to avoid or not?
 

wsimpso1

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Simply put, the prop on pusher aircraft has to be designed to be relatively insensitive to varying AOA because even in cruise, the wake of the wing is going through the prop. That is why you see so many blades on props on Starship, Avanti. Even a lot of Cozy, LongEze, etc have three blade props. While I have not perused the operating manual of the Avanti, I would bet that the primary use of flaps is to add drag and Cl for landing, when the power is pulled back anyway.

I heard a long time ago that Burt abandoned pushers and went tractor because pushers will always have a significant loss in efficiency due to the varying AOA.

Billski
 

DaveK

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That's about what I expected to hear. I would imagine power on stall with full flaps would be an issue. Could be very hard on a PSRU as well.
 

karoliina.t.salminen

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Why? I think it would be unpleasant for the prop, but for stall behavior, why it would be worse than a tractor? I think tractor twin accelerates air on the wing root, which makes it stall potentially later than the tip. Which can lead to tip stall and spin. In case of pusher behind the wing, the prop does not affect how the wing stalls (except for maybe some suction in the wake) but I would guess (with common sense without CFD analysis) that the tractor twin will promote more tip stall than the pusher twin would.

There is example of this type aircraft in conventional layout: Polish EM-11 Orka.
MARGANSKI

If you look the pictures, the flap opens on both sides of the engine nacelle, and the prop is completely behind the flap.
 

wsimpso1

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The wing will give one and two cycles per rev to each blade, which becomes two and four per rev on a two blade prop. Now if you have a four cylinder engine, the biggest forcing functions are two and four per rev. Hmmm, seems to this powertrain dynamics engineer that we are talking about the same frequency as we already must have under control anyway as well as the possibility of eight per rev.

Billski
 

karoliina.t.salminen

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What's the problem for having 3 or more blades like pushers prefer to have? We have 2 blades in Diamond, but I don't think there is much difference with 3 blades.
"less blades better" seems to be valid only on static thrust which is not important for aircraft that travels at meaninful speed. The next blade is not exactly behind
the previous blade because the downwash gets deflected and the plane moves forward during the blade rotation.
 

wsimpso1

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More blades does mean higher frequency forcing function on the powertrain - With a three-blade prop, you will have 1 and 2 per rev per blade, which means 3 and 6 per rev from blade passage through the wing wake, plus 2 and 4 per rev from engine (four cylinder). This is all assuming 1:1 engine and prop speeds - multiply the engine orders by the reduction ratio to get engine pulses per prop rev. With direct drive conventional airplane engines and only short prop extensions, this has proven OK in thousands of EZ's and derivatives. But if you put in a driveshaft you can have trouble from these sources - I know, I have helped out in fixing a system. PSRU's have the potential for fuss too. Over the engine operating range, you have to drive resonance frequencies away from all of these orders. The more orders you have giving excitation, the more orders you have to find ways to push out of range.

Billski
 

Dan Thomas

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Why? I think it would be unpleasant for the prop, but for stall behavior, why it would be worse than a tractor? I think tractor twin accelerates air on the wing root, which makes it stall potentially later than the tip. Which can lead to tip stall and spin. In case of pusher behind the wing, the prop does not affect how the wing stalls (except for maybe some suction in the wake) but I would guess (with common sense without CFD analysis) that the tractor twin will promote more tip stall than the pusher twin would.
.
Don't be misled by airspeed over the wing. It's angle of attack that determines stall, not airspeed. We get trained to watch for "stall speed," but that's at 1G. We can stall at a higher airspeed if the loading is higher (steep turn, sharp pull-up, etc) because we exceeded the stall angle of attack. The propeller's blast doesn't change the AoA over the flapped portion of the wing significantly, and in fact the flaps-down position increases the AoA at that area and brings the stall at the root on sooner, not later. The increased camber with flaps down mitigates this, though.

Further, most aircraft are designed to stall at the roots first in any configuration or maneuver, and most modern light aircraft never do get the wingtips into a stall. Just because one wing falls off when we get to the stall doesn't mean the tip has stalled; it just means that the stall pattern between the left and right wings is uneven and the dropped wing has a little more stalled area and is unable to keep the nose up. You can prove it in most GA idiot-proof airplanes by lifting the dropping wing with aileron; if the tip was stalled the drop would get worse as the downgoing aileron increased the AoA on the outboard area.


Dan
 
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karoliina.t.salminen

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Airspeed has importance because it is not angle of attack, but relative angle of attack to the motion of the air. The movement through air affects the angle of attack the wing sees. It is not absolute angle of attack. Accelerating air on front of the wing will decrease the relative angle of attack of the wing portion that is affected by the propwash. it could be negligible but fact is that increasing airspeed will lower some of the angle of attack the wing portion sees, and it will stall a bit (or more than a bit) later depending how large the effect is (that I don't know). It is a bit misguiding to say "stall speed" and it is equally misguiding to say "stall angle of attack". One can have zero airspeed at low angle of attack (and the plane surely is not flying), and one can have nose pointing up to the sky and the wing is not stalled. It is all relative. I am sure you know this, but I think you could be also interpreted by somebody else that you would actually mean absolute angle of attack which would mean that for example flying loops would be impossibility.. G-consideration is irrelevant for consideration of propwash to relative angle of attack of wing, because the prop wash occurs with or without the g loading. It even occurs at zero g if engines are running.
 

Dan Thomas

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Airspeed has importance because it is not angle of attack, but relative angle of attack to the motion of the air. The movement through air affects the angle of attack the wing sees. It is not absolute angle of attack. Accelerating air on front of the wing will decrease the relative angle of attack of the wing portion that is affected by the propwash. it could be negligible but fact is that increasing airspeed will lower some of the angle of attack the wing portion sees, and it will stall a bit (or more than a bit) later depending how large the effect is (that I don't know). It is a bit misguiding to say "stall speed" and it is equally misguiding to say "stall angle of attack". One can have zero airspeed at low angle of attack (and the plane surely is not flying), and one can have nose pointing up to the sky and the wing is not stalled. It is all relative. I am sure you know this, but I think you could be also interpreted by somebody else that you would actually mean absolute angle of attack which would mean that for example flying loops would be impossibility.. G-consideration is irrelevant for consideration of propwash to relative angle of attack of wing, because the prop wash occurs with or without the g loading. It even occurs at zero g if engines are running.
The propwash is spiralling and strikes the left and right wings at different angles. It often causes the left wing to stall first if done at high power. The only time propwash will have any significant effect on stall will be a low speed.

Edit: the area of flaps affected by propwash is also quite small. On a Cessna 172, for example, the propeller is 76" in diameter; a bit over six feet. The column of air off the prop narrows a bit, probably down to six feet, due to the lower pressure inside a moving column. The cabin is around 40" wide; the 72" propeller wash therefore affects about 16" of flap on each side. The flaps are 7 feet or so long, so the propwash's effect on flaps is minimal. On the Cessnas in our fleet, the bigger hassles come from fatigue of the aluminum flap skins and cove skins; the pulsation of the propwash when the flaps are down vibrates these things on the inboard areas and cracks them around the rivets.


Dan
 
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Aircar

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The cyclic loading behind flaps certainly is significant in terms of loading and noise and it is true that Burt Rutan has 'seen the light' on wing mounted/fuselage behind props --I just took del;ivery of my copy of "Burt Rutan's race to space" (highly recommended) in which he
'disowns' pushers because of the wing wake issue and unavoidable noise .

This drawback does not have to apply to remote driven propellers or props aft of a 'passive' surface such as a fin --not having a big blunt body with cooling air exit and wing wake involved removes the objections to pushers (see "Australian Opal" thread for a 1974 -pre VariEze -- solution.) I have the wartime reports on the tail pusher XB 42 (Mixmaster")
which includes detailed wake pressure surveys and the influence of flaps and bomb bay doors is very large even so far aft . The B 36 shed props in flight at low speed due to the a ssymetry (Paul Bikle related being a flight test engineer when one prop separated and sliced though the fuselage ) Amphibians with pusher props are notorious for noise as is the rear seat on a Varieze/long etc --they are very distinctive from below even at great height (the Piaggio gull wing pushers could be heard when too high to see ..)
 
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