Aerodynamic Efficiency: Tractor Propeller verses Pusher Propeller

[Dave_Jackson]

When considering a compounded helicopter, which consists of; a rotor, a propeller and no wing, I believe that the Tractor configuration


http://1000aircraftphotos.com/Contributions/Braas/4338.jpg
SNCASO Farfadet


is more efficient than the Pusher configuration



http://www.sikorsky.com/Images/SAC_Sikorsky_Aircraft_Corporation/US-en/X2_Demonstrator_Aircraft.jpg
Sikorsky X2


This is the argument. (http://www.unicopter.com/1538.html)
http://www.unicopter.com/StickPoke.gif

Opposition requested.


Dave

[NickLappos]

Dave,
The rule on how much efficiency you lose when you interfere with the prop or rotor flow is to ask the simple question, "Am I touching high speed or lower speed air?" The Inflow to the disk is the place to screw with the air, because the losses are performed on lower velocity air, which causes less power lost (the drag force times the velocity is the power).
This presumes that you can make the same "damage" to the air as a pusher and as a puller. Generally, all airplanes would be pushers if the engine package could be properly fitted, but engine cooling and CG packaging generally favor the nose engine design as the lightest weight solution, so the slightly greater losses as a tractor prop are actually the best balanced design for the aircraft. A helo with a transmission right in the middle of Hollywood and Vine makes the trade different, I would bet.

[Dave_Jackson]

Nick,

The argument is a speculative one, at least from my perspective it is speculative.

However, this article concludes "Our usual first reaction when someone approached us with a new pusher application was to try to talk them out of it." DJ Aerotech (http://www.djaerotech.com/dj_askjd/dj_questions/push-pull.html)

Dave

[slowrotor]

I built a pusher prop motorglider with hand made props (twin engine).
One prop took a rock hit on the first flight. That incident forever cast my view about pushers. Also, pushers work in a turbulent airstream, thats hard on the prop.

[Graviman]

A helo with a transmission right in the middle of Hollywood and Vine makes the trade different, I would bet.

With such a huge emphasis on power-to-weight, a helicopter would have to be designed around the powertrain. I remember Prouty commenting somewhere that the aerodynamicist did all he could (CFD engineer in today speak) without affecting the packaging.

Dave, if you believe it is such a problem why not put the pusher prop above & behind the rotor to ingest clean air? Alternately, just accept that there is one speed where performance is poorer, and only use prop above this speed. This is the one case where there may be justification for electric motors & generators, but be warned they cost significantly more than a driveshaft...

Mart

[Dave_Jackson]

The Inflow to the disk is the place to screw with the air, because the losses are performed on lower velocity air, which causes less power lost (the drag force times the velocity is the power).One prop took a rock hit on the first flight. That incident forever cast my view about pushers. Also, pushers work in a turbulent airstream, thats hard on the prop.Nick and slowrotor, the above statements are probably very valid. To add to the above, the gyro community believes that the tractor configuration is more efficient than the pusher, for some reason(s).

Due to all these associated pros and cons, it seemed that the initial consideration should only involve the two actuator disks, and their relationship. That is why the web page mentions "The fuselage and aesthetics are not a consideration."


Dave, if you believe it is such a problem why not put the pusher prop above & behind the rotor to ingest clean air? Mart, what you say may be valid. It may have an efficiency similar to placing the prop below & in front of the rotor. In both instances the disks are in parallel and the same air is not passing through both disks.
The disadvantage of the high prop is that the prop's thrust vector will be way above the craft's drag vector.

Alternately, just accept that there is one speed where performance is poorer, and only use prop above this speed. Naughty, naughty. Got to improve the performance at all speeds. := :)


Dave

[Flingwing207]

I believe the gyro gang likes the tractor more for longitudinal stability purposes than because of efficiency.

Food for thought: how many tractor-style submarines or boats out there? Why is that?

[Graviman]

Got to improve the performance at all speeds.

Why? The rotor will be far more efficient at low speeds than the prop, and the prop only really benefits at high speed by keeping hub profile drag down.

The smart solution would be for the prop control system to come in as airspeed increases. Pusher props (protected from landing gear) then allow rotor downwash to have rearward bias while maintaining horizontal rotor disk.

BTW, auto guys didn't seem that clear cut on tractor to me. :confused:

Mart

[Dave_Jackson]

Flingwing,

A couple of guesses;
~ The rear propeller came about before the global warming. Perhaps early IceBreakers were destroying too many bow-mounted propellers.
~ Submariners weren't too happy about timing torpedo launches with the rotation of the blades.

How about if we compromise and put the prop in the middle. :O
http://home.att.net/~karen.crisafulli/images/ILN1858sepia.gif

Seriously;
~ Perhaps a tractor prop on a ship would spend a lot of its time thrashing air and waves.
~ Again perhaps, the slower speed of the submarine and the density of the water results in a clean flow entering the pusher propeller. Just a guess.


Mart,

I did not mean to imply that the prop should be operational during hover and transition. One of the advantages of a pusher prop is that it can be a folding one, thereby minimizing drag and contact with foreign objects at slow speeds.


Dave

[Graviman]

Dave, a ship needs a narrow bow to minimise wave drag. Powertrain considerations (including buoyancy) would compromise this.

Have a look at a laminar flow aerofoil in a wind tunnel sometime. It is incredible how much turbulence is generated. This and wake contraction says that if engine is already behind pilot, it would be nut's to try to take drive forwards.

On further consideration, i am not convince that rotor wake entering prop is a bad thing. As long as air is directed downwards and rearwards you have lift and thrust, regardless of it's source. If rotor lift power went up due to stream velocity, this would just be an indication of higher thrust. It is just a case of minimising drag from hubs presenting awkward profile etc.

Mart

[Dave_Jackson]

Flingwing & Mart,

Sorry for not replying to your comments that the gyro community considers the tractor configuration to have better longitudinal stability. This may be very true.

I was thinking about a Rotary Wing thread from 4 or 5 years ago where, as I recall, the consensus was that the tractor configuration provided higher speeds than the pusher. However, it was also mentioned in the thread that the few existing tractor gyros had fuselages that were more aerodynamic than the pusher configurations had.

Perhaps the gyrocopter should not even be considered in this discussion. This is because the air is flowing through the rotor in the opposite direction to that of the helicopter.
__________________________

Mart I agree with your wave comment. In fact bulbous bows are now used below the waterline. However, an analogy between air and water environments may not be valid due to the large differences between the viscosity of the mediums, and the difference between their velocities.

I cannot agree with your second argument. Doubling an induced velocity involves more than doubling the required power. Passing air through two disks will be increasing the velocity of that air. Reference the previous comparison between the [parallel] side-by-side rotors and the [serial] close-centered coaxial rotors.


Dave

[Greaney]

In forward flight, a pusher propeller located below
the main rotor would experience some airflow
parallel to the propeller's disk. This would increase
propeller efficiency, as forward movement increases
the efficiency of the main rotor. It will not be as
dramatic an effect as translational lift, but you
need to consider it.

[Graviman]

However, an analogy between air and water environments may not be valid due to the large differences between the viscosity of the mediums, and the difference between their velocities.

Dave, for the past 100 or so years aerodynamicists with far more experience than i have believed that as long as you get the Reynolds number ( http://en.wikipedia.org/wiki/Reynolds_number) of the flow the same, you get the same flow. This comes from studying Navier-Stokes and the boundary layer mechanism (ie well established). This only breaks down at higher mach numbers, since water is practically incompressible.

Doubling an induced velocity involves more than doubling the required power. Passing air through two disks will be increasing the velocity of that air.

Yes, but if that velocity is directed rearwards it just means the rotor power is now becoming thrust. The ideal is just the you produce one laminar streamtube (which i imagine never happens).

Take Greaney's advice, then make the props teetering for insensitivity to flow direction.

Mart

[Dave_Jackson]

Greaney,

Thanks. The idea of the pusher propeller obtaining an advantage from the increased mass flow, due to the rotor generated translational lift is an interesting one. Do you think that tractor propeller might negate this advantage by decreasing the effectiveness of the rotor due to a decreasing of the mass differential between the top and the bottom of the rotor?


Mart,

My knowledge of fluid dynamics is '0'. However, I dumbly believe that the laminar/turbulent flow on the rotorcraft cannot be compared with the laminar/turbulent flow on the submarine. This is because their Reynolds numbers are so very different.

Are you suggesting that the pusher's propeller be replaced by a helicopter's tail rotor'? :eek:
Sorry, but loosey-goosey rotors just won't do. http://www.unicopter.com/No_Tail_Rotor.gif


Dave
__________________________

For a UAV, how will this Interleaving ABC do?
http://www.unicopter.com/1512_C.gif

[Graviman]

Dave,

Flow velocity is normally adjusted to get the same Reynolds number. Submarines at their normal speed will have different flows, but a model helicopter in a water tunnel running at the speed to give the same NR will have the same flow. It is a method to reduce scale, but don't forget it is only the laminar to turbulent transition region which changes. The general laminar (or Bernoulli) flow is comparable.

For UAV design looks fine, but you would need to compare on cost and payload to get a better idea. The downside will always be the need for fatigue proof shafts to keep everything in synch.

Mart