Pusher verses tractor propeller

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Riggerrob

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Dear Norman,
To summarize ...
The worst vibrations occur when you hang a 4-bladed propeller close behind a cruciform tail .. directly aligned with the mean aerodynamic chords of both rudder and elevator.
Almost as bad is hanging a 2-bladed propeller directly behind a vertical fin.

OTOH displacing the thrust-line (prop hub) away from the center-line or chord-line smooths vibrations because the entire blade does hit the wing wake at the same time ... rather, the prop blade is gradually introduced to the wing wake.

Apparently much of the noise generated by a Beechcraft Starship or Piaggio Avanti is the sound of the propeller "cutting" engine exhaust.

This logic encourages us to hang pusher propellers slightly off-center ... to reduce vibrations.
 

Norman

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The worst vibrations occur when you hang a 4-bladed propeller close behind a cruciform tail .. directly aligned with the mean aerodynamic chords of both rudder and elevator.
Yeah prop behind tail is pretty bad. Each lifting surface produces a shear layer and crossing that shear layer is the source of the AoA change that turns part of your horsepower into vibration and noise. blowing the exause into the prop makes its own special sound but doesn't necessarily affect thrust although it does get the prop pretty dirty and can burn it.


OTOH displacing the thrust-line (prop hub) away from the center-line or chord-line smooths vibrations because the entire blade does [not] hit the wing wake at the same time ... rather, the prop blade is gradually introduced to the wing wake.
That's what the drawing I posted in #25 was showing. Gyroscopic precession also causes some vibration because a 2 bladed prop has one inertial axis empty (that's what dynamic balancing is supposed to address), this is where any number greater than 2 is a hard rule that you can't invalidate by puting a fin in front of it although weights on the flywheel can help by providing that missing axis.
 

rotax618

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Tall tailed Autogyros are an anomal, the don’t seem to suffer excessive vibration even though the tail, both vertical and horizontal is very close to the prop.
 

Kyle Boatright

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Tall tailed Autogyros are an anomal, the don’t seem to suffer excessive vibration even though the tail, both vertical and horizontal is very close to the prop.
Most of the problem comes from having the prop downwind of the tail (or other surface), not upwind like on a Gyro.
 

blane.c

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I have first hand experience in solving the problems of airflow into a pusher prop. My Mk1 Boorabee’s test flying went very well, it was stable with pleasantly light control inputs, although it wasn’t obvious, if you observed the wingtip against the horizon there was an small oscillation in yaw.
The oscillation was quite unusual in that it wasn’t a regular frequency or amplitude. I posed the problem to some graduate aeronautical students I met at an airshow I attended. Their unanimous advice was that the fin needed to be larger. I did build two prototypes so I increased the size of the fin on the second aircraft by adding a large strake to the fin LE. The increased fin also increased the oscillation in both frequency and amplitude.
I won’t bore you with all of my experiments, sufficient to say they were extensive.
The solution was the placement of vortex generators down the side of the fuselage pod. Remember this was nearly 30years ago when the use of vortex generators wasn‘t common especially on “ultralights”.
The vortex generators had a profound effect on the aircraft, they not only stopped the oscillation making the aircraft rock solid in the yaw plane they also quietened the prop and required the removal of nearly 2deg in pitch for an increase of 10kts to the cruise.View attachment 105460View attachment 105461
There are so many vortex generators on the empennage of some of the 737's they look like porcupines.
 

Aesquire

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One early attempt to power hang gliders used an engine & prop mounted above the wing to the unfaired tube kingpost. 2 bladed prop. The vibration from the prop passing the disturbed flow was heavy.

Most here would take one look & predict that.

There was also thrust line issues on a weight shift craft, not relevant to this thread.
 

Dusan

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The pusher prop theoretically ingest and re-energize the boundary layer so overall the plane should be more efficient, but at the same time the prop needs to cope with dirty air coming in, so the prop efficiency and noise suffers. Eventually the design - as all engineering is a compromise. Not to forget a pusher tail coefficients can be smaller so less drag on tail.
 

Norman

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all engineering is a compromise. Not to forget a pusher tail coefficients can be smaller so less drag on tail.
Some effects exist but shouldn't be exploited. Reducing the size of the empennage because of the stability contribution of a pusher prop is dangerous because most planes have just enough fin to meet the power-on stability requirements and no more. What happens to a pusher with an undersized tail when the engine quits?
 
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Riggerrob

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Some effects exist but shouldn't be exploited. Reducing the size of the empennage because of the stability contribution of a pusher prop is dangerous because most planes have just enough fin the power-on stability requirements ans no more. What happens to a pusher with an undersized tail when the engine quits?
Another problem is that a free-wheeling pusher propeller can "mask" tail control surfaces if it is too close to them: Seawind and Deltaire.

Also consider that pusher propellers only contribute to directional stability if they are mounted on long tail moment arms like Molt Taylor's IMP series.
 

Norman

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Another problem is that a free-wheeling pusher propeller can "mask" tail control surfaces if it is too close to them: Seawind and Deltaire.
Yeah... A free-wheeling propeller is like a parachute.

Also consider that pusher propellers only contribute to directional stability if they are mounted on long tail moment arms like Molt Taylor's IMP series.
Just like fins. {Gasp} blades are fins.
 

Norman

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Aft mounted tractor?


BJC
Self cooling pusher... Probably. What differentiates pushers from tractors is the position of the prop relative to the CG not to the engine. The Seawind's prop looks like it's a few inches aft of the CG so it's a pusher although the arm is so short that the in-plane forces of the propeller would be negligible. The effect of the prop blast on the horizontal stabilizer is huge though, and in fact the only thing that allows airplanes with pylon mounted engines to stay in trim with changes in power settings. Think I'm wrong about that? Find one example of a pylon engined airplane with the H stab outside the prop blast that isn't miserable to fly, if it flys at all.
 

BJC

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What differentiates pushers from tractors is the position of the prop relative to the CG not to the engine.
Norman:

Can you provide sn attribution for that definition? I haven’t found a widely accepted definition, other than the propeller placement relative to the engine. But would appreciate one.

(See, also the thread where someone referred to a swept wing as one with a mean chord line that is angled forward. Example: Edge 540, with a straight leading edge at 90 degrees, but with a mean chord line angled forward. I choose to define sweep relative to the LE, since the aero effects of sweep are a function of LE angle, not mean chord angle. But still looking for a widely agreed to definition.)

Thanks,


BJC
 

Norman

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Norman:

Can you provide [a]n attribution for that definition? I haven’t found a widely accepted definition, other than the propeller placement relative to the engine. But would appreciate one.
You have a force and a load. The propeller generates the force. The engine is part of the load but not the whole thing. The propeller is either pushing or pulling on the CG and depending on its distance from the CG and NP can also produce moments about the CG. These moments and the the fuselage drag/prop efficiency are the only meaningful difference between pushers and tractors. Inflight the engine is just a big chunk of mass that spins the prop and helps determine where the CG is but does not itself produce thrust or affect stability.

(See, also the thread where someone referred to a swept wing as one with a mean chord line that is angled forward. Example: Edge 540, with a straight leading edge at 90 degrees, but with a mean chord line angled forward. I choose to define sweep relative to the LE, since the aero effects of sweep are a function of LE angle, not mean chord angle. But still looking for a widely agreed to definition.)
Sweep is normally the sweep of the 1/4 chord line so, yeah, a tapered wing with a straight leading edge is technically swept forward. What's important here is not the actual geometry of the wing but the pressure distribution. Each chordwise strip has its own pressure distribution and that's dependent on the airfoil. Now if you take all these thin strips and draw lines between points of equal pressure you have an isobar chart of your pressure distribution. The sweep of the isobars is never as simple as one line on the wing but 1/4c is pretty close if the wing isn't tapered or swept. If the wing is swept significantly (say more than 10 degrees) then the sweep of the isobars gets pretty messy but for the most part they don't follow the leading edge either.

Imagine you are watching 3 slugs of air flow over a wing. If it weren't swept and had a constant cross section and infinite span (a 2D airfoil) they would all have the same pressure and the wall streamlines (what an oil flow test shows) they would all stay straight and parallel. Now yaw that infinite wing.. Our 3 slugs now hit the stagnation point at different times. The slug in the middle sees different pressure on either side and as slugs of infinitely elastic fluids do he turns toward the low pressure (or away from the high pressure if that's a clearer perspective). Right at the stagnation point on an aft swept wing his inboard neighbor shoves him toward the tip hard but a few microns downstream he'll start turning toward the root which will bring him back to chordwise flow some percentage of c downstream of the leading edge (chordwise in this context is perpendicular to the 1/4c line). All 3 slugs will follow that line pretty much straight until they get fairly close to the trailing edge then they get another hard shove toward the tip from the trailing edge stagnation. Things get even more messed up in the area about 1*MAC from the tip.
 

Dusan

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Reducing the size of the empennage because of the stability contribution of a pusher prop is dangerous because most planes have just enough fin to meet the power-on stability requirements and no more.
Most planes are designed with oversize empennage to cope with propeller de-stabilizing effects, rotating stream flow-fuselage interactions, and so on. If the prop is mounted behind the fuse, the effects are much smaller, the empennage can be sized similarly to what is customary for gliders. The aft prop, rotating or not, is only adding a stabilizing effect.

The engine is part of the load but not the whole thing. The propeller is either pushing or pulling on the CG and depending on its distance from the CG and NP can also produce moments about the CG.
As the propeller forces are transmitted to the airframe through the engine, is important to the engine to be defined (or designed/tested) as pusher/tractor.
 

BJC

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These moments and the the fuselage drag/prop efficiency are the only meaningful difference between pushers and tractors.
Thanks, but I have had no disagreement with that. However, I still am looking for a text or standard that defines a pusher verses a tractor in those terms. Everything that I have seen (so far) defines the configuration in relationship to the engine. Therefore my request for an attribution.
Sweep is normally the sweep of the 1/4 chord line
Ditto.


BJC
 

Norman

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Most planes are designed with oversize empennage to cope with propeller de-stabilizing effects, rotating stream flow-fuselage interactions, and so on. If the prop is mounted behind the fuse, the effects are much smaller, the empennage can be sized similarly to what is customary for gliders. The aft prop, rotating or not, is only adding a stabilizing effect.
Are you saying that power-on and power-off are only relevant if the propeller is up front? Any system that produces thrust by accelerating a fluid also produces in-plane forces at the point where the fluid is deflected when the thrust line is not parallel to the direction of the free stream, thus propellers produce side forces at the prop disk and jets produce side forces on the inlet lip (that's probably why you don't see inlets on the nose anymore although long ducts are also not desirable).

As the propeller forces are transmitted to the airframe through the engine, is important to the engine to be defined (or designed/tested) as pusher/tractor.
That's a mechanical issue isn't it? Are you implying that the position of the impeller disk relative to the CG is unimportant?
 

Norman

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Thanks, but I have had no disagreement with that. However, I still am looking for a text or standard that defines a pusher verses a tractor in those terms.
I'm not going to dig through my library but I did look it up in Raymer. He agrees with you, so there's one. I don't care though because the position of the engine and how it's connected to the propeller has nothing to do with how are airplane flys. If you want to define it as a purely mechanical issue then use the engine. If you want to consider it from the aerodynamic perspective then use the relative positions of the impeller and center of mass.
 

Riggerrob

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Backing Norman on this definition ...
If the propeller is mounted forward of the centre of gravity, it is a tractr. Most tractors have both the engine(s) and propeller(s) mounted on the front of the fuselage or leading edge of wings (e.g. Beechcraft Bonanza and King Air).

If the propeller is mounted aft of the centre of gravity, then it is a pusher (e.g. BD-5, Avanti, Taylor IMP, etc. Because these props are mounted aft of tail surfaces, they help with directional and pitch stability

Propeller mounting relative to the engine is a less precise definition. For example, Seawind propeller is mounted forward of the engine, which is mounted on the leading edge of the vertical fin. But the engine mount extends so far forward, that the prop is only a short distance behind the C.ofG, ergo it is a pusher ... by a short distance.
Similarly a few experimental prop-fans have been built on nacelles that extend from the rear fuselage (ala. DC-9 airliner) making them pushers. Even if the fan is bolted to the front end of the nacelle, they are still pushers because the prop/fan is aft of the C.ofG.

Even more confusing is the Edgley Optica where the propeller is almost at the C.ofG. but the engine is mounted aft of the prop.

Equally confusing are light, single-engined flying boats with engines mounted high on pylons near the C.ofG. (e.g. Lake Buccaneer) Volmer Sportsmen amphibians have been built with both tractor and pusher propellers even thought both engines are located directly above the C.ofG. so Sportsman propellers are only "tractor" or "pusher" relative to the engine. These props contribute little to directional stability, but can seriously mess with pitch stability when changing throttle settings.

Finally, Dornier built several large flying boats with both tractor and pusher propellers. Often they mounted two engines in the same pylon/nacelle, with one tractor prop protruding out in front of the wings' leading edge, with the second prop "pushing" out the back. Since both Dornier props are close to the C.ofG they have little affect on directional stability.

From the engine's perspective, tractor or pusher defines where to mount thrust bearings (usually ball-bearings) and flow of cooling air.
 

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