I was giving some more thought to the twin electric Gentle Lady and I come up with this question for the experts. Motors pushing air over or under the wing? Motors pulling air over or under the wing?
Thanks, Ace

This is a fine thought.

My own reaction is that there would be a lot to be gained by using pods that are mostly on the bottom of the wing with pusher motors. The big advantage is that it would leave the more critical leading edge region and upper surface clear of stuff that could introduce drag and turbulence to disturb the wing and act like permanent spoilers. Also the pods could be short and the props would fold into the slip stream behind the wing and without worrying about fouling anything.

The pods would be shaped and sit much like the outboard flap linkage fairing to the right of the outboard engine nacelle in this picture.



But you know the best and easiest way is still the nose mounted motor. But done right and with internal wiring wing mounted pods like this could be made removable and you still have a pure glider..... Hmmmm...... You may be onto something here.

I see a Bird of Time with two Speed 400 or 480 motor pods on the wing and a belly mounted blended fairing that houses the motor batteries. Less than five minutes to remove some nylon screws and put a couple of small wire access covers back into place and you have a pure glider once again.

Most of the reasons that I know of for using motors/props on the front of the wing are a matter of getting dynamic pressure (q) over the wing. Take a conventional people hauler. The motors always are running. Then instead of having the wing see just the airspeed q and the direct lift from that part of the wing that is blown by the props see the airspeed plus propwash (really big) and the net result is a trementous q increase. The photo that Bruce attached is a perfect example of this.

You end up with a configuration that in times of crises can give extra lift, control, whatever by just increasing the engine rpm or prop pitch adjustments. It becomes more critical with respect to keeping the motors running but then again that is the name of the game when hauling people.

With a glider this is less important and the pluses given by Bruce outweigh any minuses especially if you want to keep a glider capability. The wings will be cleaner regardless with a pusher configuration and there is the inwash to the props that makes the wings more effective in that area. Since the inwash is being sucked (not the right word) from everywhere forward of the prop plane it is not as strong as the propwash effect mentioned above.

I have a photo of a full scale B17 with 5 engines - -one on the nose - yet another approach--honest- I do -
actually it was a P&W test bed setup .

I believe that's a post war test bed used for testing one of the first large turboprop engines. If we are talking about the same thing they actually flew the '17 on just the turboprop with the other 4 feathered and it STILL had to be throttled back a touch to avoid overspeeding the '17 in level flight.

I'm not sure what that big turboprop engine went into for real.

Proace,

I am assuming the the plane is to be flown as a glider, and the motors are just to get up in the air. If this is the case, then reducing drag is probably the most important goal. There is no benefit to using the propwash to enhance lift for a glider. As BMatthews says, getting the motors and props behind the wing is probably best. If possible, I would strive for an even smaller profile than an engine nacelle. If the weight of the motor allows, I'd like to see a short 'sting' jutting straight back from the trailing edge, with the motor in a very smooth pod on the end. I don't know how well a folding prop would feather in this configuration. There might be a tendency for at least one blade to droop down into the airflow, which would not be good. This could be a showstopper for this kind of configuration.

banktoturn

Graupner (I think) and Astro Flight in the past have had pusher motor/props on glider wing configuration airplanes.

I had one of the Graupner motors and the props were a low speed design and were spring loaded to fold and trail behind the motor pod/wing without power.

The pod was essentially the same diameter as the motor and was faired into the wing. It extended a few inches aft of the trailing edge. It worked well.

Ben - back when I lived in Cincinnati -in 75- I did a pusher pattern design (two of em actually -with twin booms
dismal failures both .
the problem was that they looked awful in rolling stuff- the twin booms killed the smooth look.
The winning part of the setup was a great , no propwash setup.
the interesting part was when I examined the prop blast - using a simple tuft test.
The test a piece of string - dangled from my fingers --in front and behind the prop at full throttle .
The apparant airflow was smooth going into the prop--BUT-- the prop blast appeared to be much like wter going down a drain -- the air behind the prop rapidly moved inwardto the hub.
I had a nice big spinner on them - and the fuselage pod on each was essentially a egg in frontal view and about a 3-1 egg in planform.
the prop blast going inward was a surprise.
Would it surprise you ?
(please keep it simple - as others have noted - I am not one of the annointed )

Now that IS interesting Dick. Most of the time we have a big fat ugly fuselage in the way to prevent seeing the "pure" prop wash effect.

I see two possible causes, or at least factors, for your observation.

First is that your pod shape created, in essence, a hole of lower pressure behind the spinner and the wash was moving inwards to fill the void. Sort of the opposite of the fuselage wedging the flow outwards or at least keeping it parallel.

The second is that the energy in the prop wash is responsible for the flow you saw. The center core being weaker than the outer flow and hence the outer flow is drawn inwards.

.......

OK, there is probably a third one here. Remember that prop blades are just small wings. And those little wings are running at full bore life coefficients to help "lift" our models forward. And what happens at the tips of highly loaded wings? Yep, we get tip vortex flows that roll up around the tip and form a cone of swirling air that is rolling inwards towards the upper surface.

I think I'm more inclined to see this last one as the major cause of your inward flow. Although I like my second "weak core" theory as well. What's really needed is some pressure and velocity measurements across the area behind the prop at various distances to get a better picture of what's happening.

My own gut reaction is that a pusher of this sort should be more efficient than a tractor prop. But the relative lack of pusher configurations in full sized aviation is a strong indicator that I'm wrong. Well, that and Ben's and B2T's observation about the prop wash assisting the lift.

I figgered the same - - filling the void made sense - but the other thing that bugged me is that it looked like nature - busily reducing unbalances -- moved the load away from the tips - (more work out there -)
So -If I were to try to prove any of this - I guess I would mount a prop on a long rod - in an open field, then spin the rod from a remote location and see where the the incoming low pressure load came from and where the outgoing - compressed air went -then I would make streamlined shapes (and not so streamlined) in front and then behind the prop to see what it did to the airlow ---and more importantly how it reduced or increased the load on the driving force.
(Country boy engineering)

Yep, that would show the flow pretty well. I don't think it would take that much of an extension either. I have an electric motor gearbox by Leisure with about a 3 inch extension. An inboard model boat drive shaft would work excellent also.

I just have to make sure I keep the wool tufts short enough that they can't wind around the shaft....

I'm not in any rush ( got quite enough on my plate just now) but this would be a fine bit of research for later this spring. I've got a test power supply that runs off a car battery so it would be easy to set this up. Just need twe wires across the airflow with some yarn fibers on it.

And with the lower speed of a gear drive electric one could even attach a thread on the tip of the prop and using a strobe light timed to the revs you'd see a standing prop with a moving thread (actually muliple images of the thread in different positions) so you could see how much of a tip vortex there is. Hmmmm.....