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Turd Ferguson

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There's no problems that can't be overcome. Lots of successful airplanes have pusher propellers. Some canard pusher planes where the prop passes close to the trailing edge of the wing have an issue because the prop chops through the wing wake but again, lots of successful airplanes seem to do okay with it.
 

Topaz

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+1 There are issues, but not necessarily problems.

A pusher prop that is relatively close to an upstream flying surface is going to make more noise and vibration, and be somewhat less efficient than the equivalent tractor prop that sees a clean inflow. For example, there's a Piaggio Avanti that flies into SNA almost every day, and I happen to live near the outer part of the approach, near the usual turn to final. I don't even need to look outside to know the Avanti just flew in - the sound of those five-bladed props turning in the wing wake is extremely distinctive.

All of these effects can be mitigated to some degree by moving the prop aft of the flying surface. A full prop diameter would be nice, but is rarely practical.
 

Norman

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Puting a propeller close behind a control surface usually degrades the performance of both. The suction of the prop can cause separation from the control surfaces and the disterbed inflow to the prop will cause vibration and noise. There will also be a lose of thrust do to the speed difference of parts of the inflow.
 

Himat

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Ok. I recall someone mentioning a problem with a pusher prop directly behind the tail, but i can't remember it. Something like this :https://en.m.wikipedia.org/wiki/File:CirrusVK-30N94CM01.jpg
Norman and Topaz have listed two of the issues and the noise may be linked to inflow problem.
As for efficiency, the jury is out. It probably depends most on the design particulars.

Some other issues:
- FOD. Rear mounted propellers are more vulnerable to foreign object damage.
- Packaging of the airplane that often dictates long a driveshaft with associated design challenges.

A side note, one of the most numerous commercial built light single engine piston airplanes built the later years do have a pusher prop on its Rotax engine.
 

RPM314

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Someone on this forum linked to a TEDx talk where they talked about boundary layer ingesting jet engines, so having the prop way at the back should cancel out lost energy in the same way.
I'm not actually trying to input, I'm just wondering if a tractor does the same thing, but in reverse order (ingesters take slowed down air and accelerate it, tractors accelerate then the fuse slows it down). Anyone know?
 

blane.c

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One advantage to a pusher prop is that when/if something goes wrong with the engine the problem does not end up all over the windscreen also depending on design of course, smoke and or fumes could be less likely to make it into the cockpit. Oil on the windscreen and a cockpit full of smoke from the failed engine were contributing factors to a friend of mine's death. This for me is a strong persuader for the pusher configuration.
 

autoreply

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The Avanti has very good inflow and would be almost silent if it wasn't for the exhaust gasses passing through the prop.

Two enemies; unaligned flow and boundary layers.

Boundary layers are highly dependent on the surface in front of the prop (laminar or turbulent), Re number and how much lift it's making. All affect the boundary layer thickness which drives how much disruptive wake momentum your prop has to chop through.

Misalignment can be either local or global. Both cause the prop to be less efficient.

It's pretty hard to design a good pusher prop. If you do however, it'll be extremely efficient. The proverse pressure gradient will thin out the boundary layer and reduce drag. Doing that with a nice flow through the prop is the hard part.
 

Jay Kempf

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Noise and the big view of the glider style front end are the big functional advantages as well some stability from the pusher thrust vector being behind the NP. Getting the flow into the prop sorted out is one of the trickiest configuration problems in design. Get it right and get some airframe efficiency. Get it wrong and it won't be as good as the best tractor designs. Two things you don't want going into the prop; wing downwash and/or any other large sources of turbulence. If you do that properly you get the benefit of maintaining more square footage of laminar flow on the wetted areas up stream of the prop and the square footage just in front of the prop will be helped by the prop.

If you are shedding any vortexes into the prop disc you lose. Which means no matter what you choose for a configuration wing/fuselage junctions are going to be a headache for you. You have to get that right first before moving to smaller details. One thing most people miss when starting to look at all these sorts of issues leading to design solutions is that you have to make all this stuff work in more than one flight regime. You have to keep flows organized at multiple angles of attack, power settings, velocities, gear out, flaps, etc... if you want to have efficiency gains in all. Or at least you have to understand the tradeoffs to achieve your goals. I am actually doing some flow analysis right now on a configuration that has two critical design points, one not very high above stall, clean no power (thermaling) and one very fast with almost no AOA under power (high speed cruise) and it is a huge headache. But get it right and you have something interesting. You don't just get those things right by guessing and scribbling what it will look like. You get it right by compromising to 12 decimal places.
 

Riggerrob

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One problem can be poor control authority when the engine is idling. The propeller acts like a giant sheet of plywood ... Er ... speed brake ruining smooth airflow across the tail surfaces.
The worst possible configuration is a close-coupled delta wing with the propeller immediately behind the tail surfaces. Short-coupled airplanes are always sensitive in pitch. Combine this with some balance compromises with the undercarriage, and the airplane becomes overly sensitive during take-off. Back during the 1960s, Sport Aviation magazine featured a photo of a pretty, close-coupled delta with a pusher propeller. Because of ground balance issues, the main wheels were farther aft than ideal. During the first test-flight, the pilot found that he could not lift the nosewheel off the runway, even pulling hard on the control stick. So he pulled the throttle to idle. With thrust no longer pushing the nose wheel onto the runway, the nose pitched up vigorously. The airplane lept off the runway and did the world's smallest inside loop, killing the pilot.

Short-coupled airplanes with tractor propellers immediately in front of the tail surfaces are not much better. Remember all the pitch problems suffered by SeaWinds and all the crashed prototypes. SeaWind adds the classic seaplane problem
of a high thrust-line. High thrust-lines create massive pitch changes with different throttle settings .
That is why delta pushers are rare, but delta tractors (Dyke Delta, Verhees Delta, etc.) are comparatively tame to fly.
 
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Norman

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That is why delta pushers are rare, but delta tractors (Dyke Delta, Verhees Delta, etc.) are comparatively tame to fly.
Yep, putting it on the nose gets around a lot of problems as long as you remember to increase the size of all the stabilizing surfaces to remove the negative effects of the propeller side forces.
 

Himat

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...
That is why delta pushers are rare, but delta tractors (Dyke Delta, Verhees Delta, etc.) are comparatively tame to fly.
I am not quite sure about that. At least not if include un manned vehicles.
The one in the picture have been built and operated in numbers, as has other similar vehicles:
 

Topaz

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I am not quite sure about that. At least not if include un manned vehicles.
The one in the picture have been built and operated in numbers, as has other similar vehicles: ...
+1. Stability is stability. The prop is either destabilizing (tractor) or stabilizing (pusher), and the location of the allowable CG range should allow for the location of the propeller, and both power-on and power-off conditions. There is nothing inherent about a delta or other tailless design that makes any of this any different than any other airplane.

Tailless designs may have a relatively low control power, depending on the exact configuration, which just means the allowable CG range is that much narrower.
 
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