Facet Opel

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Dennis K

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May 23, 2014
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17
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Portsmouth, NH USA
Michel Mangenot (seller of kit of Pelican) told me that it was due to the prop being too close. Or ...was it JC Debreyer himself?? I had a few letters from him.
I can't vouch for the vibration because my only experience with pushers is with models but the prop close to the trailing edge sure makes a racket, even more so if in a slot.
 

Victor Bravo

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Jul 30, 2014
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KWHP, Los Angeles CA, USA
IMHO there is some chance that clever experimentation and placement of VG's on the fuselage and wing root could possibly mitigate that vibration, by reducing or eliminating "dead spots" in the in-flow to the propeller. However, almost all propeller driven pusher aircraft make some amount of extra noise. You can tell when a Long-EZ flies overhead without even looking :)
 

DaveK

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Apr 21, 2007
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354
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Northern California
A three blade prop can help with vibration, but offsetting the prop from the trailing edge is important. Wind turbines have similiar vibration issues, having two blades causes a rocking motion in the plane of the prop every time a blade went through the wake of the tower. Three blades considerable reduces the effect.
And Victor Bravo is right a Long-EZ, Avanti, or Cessna 377 are each recognizable just by the sound of the pusher props.
 

Victor Bravo

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KWHP, Los Angeles CA, USA
Many years ago there was a Piaggio Royal Gull that used to be based at Van Nuys airport, and as a 7th grade schoolkid I could always hear the unique sound of those twin pusher props when that airplane flew over. I never knew what it was until many years later, I saw the airplane in a dilapidated state at Van Nuys sinking into the asphalt. But it remains an indelibe childhood memory from 45 years ago, the sound of that airplane, and seeing that it didn't look like any other airplane.

Here is a photo of what I believe is the very airplane I used to see as a child:

 
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Jay Kempf

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Interesting concept. Never heard of it being tested. Makes perfect sense that if the sheet of boundary layer was coming through the prop some how spread out in time or intensity it might change the noise. But if this was a low wing plane it wouldn't have a wing boundary layer in the prop at all. Long pusher nacelles seem to not have the issue. Short ones do. I think that is because the wing boundary layer is a thin active turbulent vortex sheet and the prop blade is just smacking it each 1 rev. Well organized fuselage boundary layers only hit the prop in an annulus around the spinner so less important as it is a constant ring and well in on the slow part of the prop where there is a lot of pitch.

Just too bad there isn't a way to test on the ground or at scale. Maybe gross effects could be tested on models but the frequencies would be all wrong. Relative intensity might be able to be quantified and changed.

Wonder if Barnaby has ever tested anything like this?
 
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poormansairforce

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Mar 28, 2017
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Just an Ohioan
Interesting concept. Never heard of it being tested. Makes perfect sense that if the sheet of boundary layer was coming through the prop some how spread out in time or intensity it might change the noise. But if this was a low wing plane it wouldn't have a wing boundary layer in the prop at all. Long pusher nacelles seem to not have the issue. Short ones do. I think that is because the wing boundary layer is a thin active turbulent vortex sheet and the prop blade is just smacking it each 1 rev. Well organized fuselage boundary layers only hit the prop in an annulus around the spinner so less important as it is a constant ring and well in on the slow part of the prop where there is a lot of pitch.

Just too bad there isn't a way to test on the ground or at scale. Maybe gross effects could be tested on models but the frequencies would be all wrong. Relative intensity might be able to be quantified and changed.

Wonder if Barnaby has ever tested anything like this?
Old RC trick...
 

Jay Kempf

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Very cool. What's really impressive is how there is no change in the noise until very close either way.
 

poormansairforce

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Mar 28, 2017
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Just an Ohioan
It's the prop speed that affects the noise so further away at the tips. The angle doesn't really play a part other than set the necessary distance due to speed. It also makes the prop slot jet look less 'holey'.😄
 

Norman

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Nov 28, 2003
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Location
Grand Junction, Colorado
Prop vibration has two components:

Velocity and/or AoA differential across the prop disk.
and
Gyroscopic precession AKA intermediate axis theorem.

Both tractors and pushers experience AoA differences around the disk due to pitch and yaw but the velocity into the disk is constant for tractors. This is the source of P-factor. The velocity into the disk of a pusher usually isn't uniform around the disk unless you can figure out how to get the propeller hub at least one prop radius above or below the wing trailing edge. The speed differential between the air flowing above the wing and that flowing below it is not just in the boundary layer. The air above the wing is accelerated several chord lengths away from the wing. Cutting through the wing wake causes noise and vibration.

The inertia of a spinning T-shaped object (like a 2 blade prop attached to a prop shaft) will tend to try to flip ends every couple of revolutions. Any number of blades greater than 2 will tend to damp this source of vibration because the mass is distributed in a plane rather than an line.

 

erkki67

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Feb 18, 2010
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Romont / Fribourg / Switzerland
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