Why are so many powered flying wings pushers?

I'm thinking of adding power to a Zagi-EPP flying wing. I notice that many powered flying wings are pushers. Is this just for looks, or is there an engineering reason?

From Don Stackhouse :

There are generally three reasons for this, one good one, one bad one, and one REALLY bad one.

First the good one:
All propellers act a little vertical and horizontal surfaces. When the air coming into them is not perfectly parallel to the propshaft, they generate a side force, just like a tail surface would. Just like such a tail surface, if it's located on the front of the aircraft (ahead of the c/g) it tends to be destabilizing, while if it's behind the c/g it tends to increase pitch and yaw stability. On most aircraft this effect is minor, but flying wings generally need all the yaw stability they can get. Note that the effect is related to the location of the prop, not whether it's a pusher or a tractor. For example, if you built a DC-9 or A-10 Warthog with tractor props on the fronts of the nacelles, you would still get a stabilizing force from them because they would be behind the c/g.

On the old Northrop XB-35 there were contra-rotating pusher props but no vertical surfaces. When they created the YB-49 by replacing the props with jet engines, they had to add some little vertical fins to replace the stability contributions of the props.

Now the bad reason:

They look "cool".

In general, when you make an aircraft design decision based purely on cosmetic reasons, you often give up some flight performance in the process. A conventional looking aircraft that flies really well will look "cooler" than a radical one that flies like a brick! This naturally brings up..

The REALLY bad one:

There is a sorely misguided notion that pusher props are more efficient. In actual practice this is almost NEVER the case!

The idea is that by putting the fuselage ahead of the propeller, the fuselage will have less drag because it doesn't have to fly through the high speed wake of the prop. What proponents of this idea conveniently overlook is that now the propeller has to fly through the turbulent wake of the fuselage. The fuselage represents only a portion of the total drag of the aircraft (usually a small portion), while the propeller generates ALL of the thrust. In return for a miniscule savings in total drag, you usually pay a HUGE penalty in propeller efficiency!

There are some cases where a pusher prop can significantly reduce fuselage drag, such as the Cessna Skymaster (the "Pushme-Pullyou"). The rear section of its fuselage tapers too quickly, which results in separated flow over that region. The acceleration of the air coming into the front of the rear prop helps keep the flow attached (half the airspeed increase from a prop is in front of the disk,and the other half is behind). Note that in this case the basic problem is a lousy fuselage shape, and that the prop is being used as a "band-aid" fix for it. The prop does pay an efficiency penalty in this case, but that is slightly overshadowed by the benefit to the fuselage airflow.

I used to be an engineer for a propeller company, one that had a lot of experience with pusher installations. Usually our first reaction when a customer came in with a new pusher installation was to try to talk them out of it! I know of only one case where a pusher aircraft had a decent aerodynamic environment for the prop to operate in, and that was because the designers spent over 2006 hours in Boeing's wind tunnel getting it right. Even then there were a host of other problems to deal with, such as F.O.D. thrown up by the wheels or objects left on the airframe ahead of the props (care to guess what a wrench does to a propeller blade?), ice chunks coming off the fuselage, higher noise from the effect of turbulent air going through the prop disk (despite being a turboprop aircraft, the Lear Fan sounded like a P-51 because of the engine exhaust streams being chopped up by the props), and corrosion and heat effects from the engine exhaust on the blades. Many of these don't apply to a Zagi with a pusher prop, but you do have to figure out how you're going to launch it with a spinning prop on the back!

I'm consulting on a design right now that might end up as a flying wing, and where total aircraft efficiency is very important. Yup, you guessed it, we're installing the prop on the FRONT.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/