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Pusher verses tractor propeller

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Topaz

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If the prop is unaffected by the air-frame then there is no differentiation between a tractor and a pusher propeller. So the discussion is really about the air-frame. Now is the prop in the wake of the air-frame more efficient than the fuselage in the wake of the prop? Again this is dependent mainly on the air-frame and there is not a one answer fits all. Both can be made to work.

While there are many exceptions in the real world and there are many practical design details that need to taken into account, a pusher prop can be slightly more efficient. Think aircraft like the Vmax probe, Taylor Mini-Imp and Piaggio Avanti. The first had other practicality issues.

Mid wing is also slightly more efficient but at times you do not want the spar right in the middle of the people or engine.
^ This post. You'll never see a "general" answer to the pusher-tractor question, because the final answer is completely airframe-specific.

About the only thing you can say definitively on this question is that it's much harder to design an "equivalently clean" pusher aircraft than a tractor one. There will be more compromises in other areas (notably landing gear and engine cooling) for the pusher, which may or may not be "worth it" for the possible gain in cruise performance over the tractor design.

Is a pusher more likely to see a reduction in climb performance when "dirty" when compared to an "equivalent" tractor design? Yes. It's my opinion that if the climb performance of the aircraft is so marginal that this becomes a genuine safety factor in comparing pusher or tractor props in the design phase, then the project has much larger issues to deal with than which end to install the prop.
 

Ken Leonard

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One quick remark about Vigilant's comment: I have the same question to the FAA, because twin-engine aircraft that do not have acceptable performance to fly one-engine-out can be more dangerous than single engine aircraft: trying to fly a poor performance twin engine aircraft with a failed engine may end up in an upset, that is, an out-of-control abnormal attitude if speed is allowed to drop below VMC, while single engine aircraft with a failed engine do not end up in an out-of-control situation. As for pusher or tractor, aeromomentum is right: it depends on propeller location, on fuselage shape, etc. One thing is true, though: pusher propeller are in a disturbed airflow field, which may result in vibration, noise, etc., and - more important than those things - that disturbed field - if for example the upper portion of the propeller disk encouters lower pressure airflow than the lower portion such as with the Vary/Long Eze, beware of metal propellers, because that pressure difference means cyclic loads and may eat up the propeller fatigue life!
Your comment re: low pressure zone causing props to be damaged may be true in high power applications but has not proven so in the Searey fleet - admittedly, 115 hp max. The pusher Searey prop is composite and damage is limited to water ingestion. 90%+ of Seareys don’t have a cowling and the air IS disturbed, but the props last for over 1000 hours if you can avoid massive or repeated water ingestion.
 

Topaz

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There's an Avanti that, until this year, was in and out of SNA almost every day. Very distinctive sound as it flew over, even with the power reduced for approach.
 

Norman

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I assume the rear prop does what the jet engine does...accelerates the slowed down air flow...thus the thrust effect of the accelerated air flow is greater.
That's only a valid statement for axisymmetric bodies such as torpedoes and submarines where the fuselage boundary layer is a small disk in the middle of the prop disk, or when the body is huge in relation to prop diameter as on a ship where the propeller is completely immersed in the boundary layer. If you put a lifting surface (wing or fin with a deflected control surface) in front of the prop that causes the blades to see a different AoA at different parts of the prop disk. It's the rapid change in AoA that turns some of your horsepower (as much as 15% on an average fuselage) into vibration and noise. A rule of thumb when designing a pusher is to keep everything except the shaft housing at least 1 propeller disk radius away from the prop hub. Also any number of blades larger than 2 will vibrate less than a 2 blade prop.
prop-noise.JPG
 

Speedboat100

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Celera 500 shows us that we are nowhere even close to the optimal aerodynamic lay outs in presents airplanes.
 

Vigilant1

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Celera 500 shows us that we are nowhere even close to the optimal aerodynamic lay outs in presents airplanes.
Maybe wait until we see some verified numbers before we start gushing. Not just aerodynamic performance (which is important, and the jury is still out on Celera 500), but initial costs, operating costs, reliability, etc.
Marrying an unconventional new design to an unconventional new engine has been tried by many, but normally does not end well.
If performance projections, novelty, big development expenditures, and PR equalled reality, we'd all be flying around in Moller aircars.
 

Speedboat100

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Maybe wait until we see some verified numbers before we start gushing. Not just aerodynamic performance (which is important, and the jury is still out on Celera 500), but initial costs, operating costs, reliability, etc.
Marrying an unconventional new design to an unconventional new engine has been tried by many, but normally does not end well.
If performance projections, novelty, big development expenditures, and PR equalled reality, we'd all be flying around in Moller aircars.

May I still have my own opinion ?
 

Speedboat100

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Absolutely. Does it have any basis in demonstrated fact, or do you just like the look of that whale and the estimated performance cited by its cheering fans?

I like to keep the dream alive that electric aeroplane can one day be as capable as a piston plane.
 

Norman

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I like to keep the dream alive that electric aeroplane can one day be as capable as a piston plane.
It's not electric, it's a diesel. A laminar fuselage sounds great but somebody is going to have to climb up onto that turkey and wash it before every flight. Laminar flow doesn't like bugs, or scratches, or windshield frames, or the gaps around gear doors.
 

autoreply

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And a little robot tape dispenser to seal the gear doors after you pull them up.
Suction works just fine in sailplanes (canopy gap due to CTE can exceed 1 mm/0.04") and should work here too. As long as you maintain a lower pressure in the gear well and the geometry is right, you'll restore laminar flow behind it ;-)
 

Norman

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Suction works just fine in sailplanes (canopy gap due to CTE can exceed 1 mm/0.04") and should work here too. As long as you maintain a lower pressure in the gear well and the geometry is right, you'll restore laminar flow behind it ;-)
Ah yes. One of the essential pillars of modern civilization... indoor plumbing.
 

rotax618

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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.1DAD4EB4-2A8D-4820-B422-68D21A7D2802.jpegC8FBE72F-DECF-4536-93B6-83472905D71E.jpeg
 
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