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Old May 05, 2014, 07:44 AM
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Joined Apr 2014
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Motor Position Above or Below Wing

Hello.
I am trying to figure this out, but could need some Help.
I want to build a Twin Engine Plane.
Now, should the Motors and Props be in a Gondola below the Wing, like in most Planes or above like on the Boing YC-14. I was thinking that, if I put the Motor above the Wing I would be getting some sort of Channelwing effect, which results in a more STOL Possibility. That is because the Speed of Air would be higher above the Wing.

Am I right with these Thoughts? Would it make Sense?

Thanks for any Help...
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Old May 05, 2014, 12:24 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,364 Posts
Lots of full size planes have done it both ways. So it would appear that if there is any advantage that it's a minor one. Also the motor nacelles would need to be located well below the wing to prevent the air off the prop going at least partly over the wing's upper side.

I'd suggest that most of the decision should be based on how best to tie the motor nacelle into the wing spars and other structure. This is also pretty consistent with full size prop driven planes.

If you're thinking of being down low on pylons such as used by the big jet airliners keep in mind that they do that for a few reasons. One is that the air and thrust flow is well away from the wing structures. So there's no need for complex internal structures such as you'd find on aircraft like the old Dehavilland Comet or the current Hawker Siddley Nimrod patrol craft. It also aids with servicing the engines since the engines are out in the open and simple covers can be removed or hinged up more easily compared to near wing or inside wing options. And finally the jet engines on pylon system serves well in allowing the engines to break free in a crash landing and dissipate some energy by breaking away.

So bottom line? I'd suggest the KISS principle. Build your wing and from the spars and leading edge support a couple of mount points for a motor nacelle that runs more or less in line with the lower surface. Then add on a top cover which is removable. A carved block fairing that runs up over the leading edge and back along the upper surface for the first 15 to 25% of the leading edge will flare the nacelle design neatly into the wing. This will locate your motor thrust line more or less in line with the wing's leading edge. Make the nacelle long enough that you don't get a siren like beat sound from the prop wash interfering with the leading edge. To avoid this I'd suggest that the prop be located about 1/3 to 1/2 the prop diameter in front of the leading edge.
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Old May 05, 2014, 01:10 PM
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Kirkland, WA
Joined Sep 2007
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The Champion 402 Lancer was originally designed with the thrust line of the engines on the chord of the wing. The FAA decided that they didn't like the idea of not seeing the wingtips, since it was a high wing twin. So they demanded the thrust line to be raised so that the engines were above the wing. The end result was that the performance was not nearly the same and the single engine ceiling ended up below the traffic pattern of some airports!

I'm sure that the drag of the nacelles on top of the airfoil had a lot to do with it. As far as prop blast over the top of the wing, Boeing had a great idea with a -14. Too bad the USAF didn't buy it.

So any way you want to do it, like BMatthews said, keep it simple and remember that the thrust line is also dependent on the whole plane, as in the elevators and angle of incidence. In a model, if it looks about right, it probably will do fine. TLAR school of engineering.
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