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Pusher verses tractor propeller

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BJC

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I have a question about the difference between a pusher configuration and a tractor configuration with the same HP on airframes with equal clean configuration drag.

Assumption: Inflow to a tractor propeller is mostly unaffected by the airframe. For example, flap deployment, elevator deflection and retractible landing gear position all redirect some of the airflow, thus increasing drag. That slows the aircraft, but does little to affect the efficiency of the propeller. In contrast, a pusher aircraft, with the same HP, has disturbed propeller inflow that, when the gear is extended, or the control surfaces deflected, or the angle of attack changed significantly, affects the inflow much more than the tractor configuration.

A random data point: the prototype Twin Velocity was able to maintain level flight on one 160 HP engine with the landing gear retracted, but was unable to do so with the gear extended.

Does anyone have any data that might be used to quantify the difference in thrust available between otherwise equal clean configuration drag aircraft with a pusher propeller verses a tractor propeller?

Thanks,


BJC
 

TarDevil

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A random data point: the prototype Twin Velocity was able to maintain level flight on one 160 HP engine with the landing gear retracted, but was unable to do so with the gear extended.
Several twin airplanes were manufactured and sold with 160 hp engines... not sure how comparable they would be to Velocity. I got my multi in a Twin Comanche. I was able to fly the pattern on one engine, but can't remember trying to climb on one with gear down.
 
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Vigilant1

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BJC,
Sorry, you asked for data, and I have no numbers to add to the timeless tractor vs pusher debate. But:
A random data point: the prototype Twin Velocity was able to maintain level flight on one 160 HP engine with the landing gear retracted, but was unable to do so with the gear extended.
As you qualified it: a data point. Could the plane's inability to climb be strictly the result of aero drag and not related to prop efficiency? I'd think that a gear leg upstream of a pusher prop might lead to less total impact on performance than a gear leg hanging into the high speed prop wash of a tractor prop.

Mark
 

Vigilant1

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I think the Cessna 337 climbs better and goes faster on the rear engine than the front if I recall correctly.
That's true (Pops has some Skymaster time and says the same thing). The Cessna design team members say that this shouldn't be attributed to differences in prop efficiency between the front and rear engines, but due to parasite drag differences. With the rear prop running, the low pressure zone in front of the prop disk helps keep flow smooth over the rear cowling. With the rear engine stopped, flow back there is a lot more turbulent and performance suffers, even at climb speeds.
 

BJC

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.... Could the plane's inability to climb be strictly the result of aero drag and not related to prop efficiency? I'd think that a gear leg upstream of a pusher prop might lead to less total impact on performance than a gear leg hanging into the high speed prop wash of a tractor prop.
That is my point of interest, and the answer might well be different for different airframes.

It was a bit of a suprise to me to learn about the Twin Velocity, which, according to Velocity, “That’s not a problem, because you descend to land, and you don’t put the gear down until you are ready to land.”


BJC
 

Speedboat100

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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.

Cooling and stability issues may arise.
 

Hot Wings

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That is my point of interest, and the answer might well be different for different airframes.
I suspect that this variable may have as much or influence than the location of the prop.
A very clean airframe probably having a better chance of matching performance with the prop on either end compared to an aerodynamically poor airframe.

The break even point may be well into the 'clean' range of the bell curve.
 

TFF

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There was a more loss in efficiency having disturbed air at the inlet than after it has been accelerated. Most helicopters that have the main rotor turn the American direction will have left facing tail rotors. There is less loss blowing air across the tail guards and boom than trying to draw air around them. I converted an old Enstrom that originally had right facing tailrotor to the now standard production left. You could barely hold it straight in a hover as designed in the 60s. Only one I have physically seen. Most were converted the minute the factory flipped it the other direction about 1975. Of course the Fenestron tail eliminates obstructions as much as you can.

I believe certified twin aircraft have to climb on one engine at 100 ft a minute. Twin pilots know some out there had to have been certified on a freezing day to get it to climb. Still it sounds like you don’t want to loose an engine at rotation in a loaded Velocity.
 
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Jay Kempf

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That is my point of interest, and the answer might well be different for different airframes.
It was a bit of a suprise to me to learn about the Twin Velocity, which, according to Velocity, “That’s not a problem, because you descend to land, and you don’t put the gear down until you are ready to land.”
BJC
Yeah, well, even the guys at Velocity know that you have to climb out of an aborted landing... so that statement is worthless. Powering through the landing condition to adequate climb at whatever DA you are operating at it a feature that ALL good airplanes have. Has to be adequate cause you can't just start dumping flaps while you are making that decision. Seems a good design point to work toward would be to be able to take off, climb and go around the pattern with flaps stuck in landing position. If designing and airplane and you didn't provide this basic margin you ain't done designing. Some aircraft sequence the biggest door shut after extending the gear for this reason just to keep the drag reasonable.
 
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Protech Racing

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It come down to square inches of involvement . The tractor blows high velocity air on a large percentage of the AC. This misdirects the Vectors against the AC. The pusher blows less High velocity air onto itself . The vectors are unhindered as the thrust cone blows clean to the rear . Thus the pusher can cruise at a reduced power setting at the same V.

The flip side is that the tractor blows lots of air unto the wing and thus can make lift at very low ground speed. The pusher makes lift only by a little tiny bit of vacuum of the rear edge of the wing but mostly from ground speed.
 

Riggerrob

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A close comparison can be made with Volmer Sportsman amateur-built light seaplane.
Volmer Jensen built the prototype with a pusher propeller mounted on top of a pylon. A few later builders installed tractor propellers which improved rate of climb by a few hundred feet per minute.
Designer Volmer Jensen always discouraged installing tractor propellers because the propeller spun immediately behind the open canopy. Volmer did not want to risk his customers flying "hands free."
Same risk on the Smith/Piper Aerostar light Twin because its propellers spin immediately aft of the pilots' door. More than one Aerostar piltot has lost an arm while waving good-bye.
 

Vigilant1

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I believe certified twin aircraft have to climb on one engine at 100 ft a minute. Twin pilots know some out there had to have been certified on a freezing day to get it to climb.
Apache

I was surprised when I learned that the FAA sets >no< minimum SE climb gradient for certified twins weighing 6000lbs or less. It’s “legal” if they can’t even maintain altitude on one engine. For those more than 6000 lbs and type certified (not built) after Feb 1991, the minimum climb gradient with an engine out is 1.5% at 5000' MSL. For a plane flying at 75 knots, that’s about 112 fpm. I’m not saying that is safe, but it is legal.

More info on the FAA regs on this here. Pg 12-3.
 
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aeromomentum

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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.
 

wsimpso1

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Sorry, no data. I do have the commentary of a world class aerodynamicist (I prefer not to be a name dropper) on the topic.

Two opposing things happen when you move the prop from one end of the fuselage to the other:

Tractor prop airplanes have the fuselage washed in higher velocity air than pushers, resulting in more fuselage drag for tractors than for pushers;

Tractor prop airplanes have much cleaner airflow into the prop than with pushers. Angle of attack seen by the prop blades in tractor props as they rotate changes little and stays close to ideal angle. In pushers, the prop is operating in the wake of the fuselage, with cooling inflow and outflow, excrescence from the fuselage, interference drag vortices, resulting in widely varying prop blade angle of attack, flow oscillations, etc..

So, does the lousy air cost you more than decreased fuselage drag? Does the clean air gain you more than the increased fuselage drag costs you? I suspect that it depends upon the airplane...

Then there is the expression that we have been familiar with - "Pushers may not fly faster than sound, but they sure sound faster than they fly". The noted high noise level is evidence of the widely varying flow field that the prop blades encounter...

Burt Rutan's last three personal airplanes all have the prop(s) on the front. The Catbird and Boomerang are very models of modern high efficiency high speed prop driven personal airplanes.

Now as to landing gear disruption - Have you looked closely at deployed landing gear on most retracts? They are drag nightmares. Exposed tires, cylindrical landing gear legs and shock struts, links for the retraction mechanism, over center links, springs, gear doors (some of them perpendicular to the airflow) and their links, and then those sharp edged openings. Drag city! Most of that stuff has drag coefficients around unity, and then there is the interference drag of these parts close to each other. Landing gear down drag has to be as big or bigger than the fuselage. I strongly suspect that the biggest reason most twins on one engine will sink with gear down is simply the big increment of drag the gear adds. Throw some more prop flow disturbance in for a pusher, just loses you a bit more power when you can least afford it.

Billski
 

Heliano

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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!
 

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