Delta mid motor cut-out shape?

[hardlock]

Anyone know the proper shape of the prop cutout for a mid mounted motor in a delta design? It's to be located about the 1/4-1/3 root chord location.

Should the root chord cross section be just like two wings with the front of the cutout tapered downward and the back of it rounded? (a)

Or should it be more like the shape a slotted wing with the front of the slot tapered upward to its trailing edge? (b)

Only thing I'm sure of is it shouldn't be cut square. :rolleyes:

Any other tips on doing this sort of thing would be very helpful also such as best thrust line location etc.

[Ollie]

At 1/3 to 1/4 of the root chord, the slot width is likely to take up most of the width of the wing at that location. Why not just leave off the little triangle in front of the slot and go with a trapizoidal planform? Maybe that little triangle, interfering with the inflow to the prop, does more harm than good. Then you can use a thin symmetrical airfoil for the wing without any worries. You can compensate for the loss in area by increasing the linear dimensions about seven to ten percent.

Alternatively, you could eliminate the slot by putting the prop in front of the wing with an extension shaft.

The above arrangements will make it a lot easier to start the engine if you are using internal combustion power. If you are using electric power the motor can be located in the nose and the CG placed by moving the battery well aft.

If you don't like either of those ideas, you could go with crossection C. C would be symmetrical airfoils fore and aft of the slot. I like this because it results in the same flow conditions for inverted maneuvers as for upright maneuvers.

[hardlock]

Thanks Ollie. Here's the plane so far. The spar is presently below the booms which causes some camber in the front section relative to the back. This should cause the nose to stall first I'd think which could be an advantage like a canard maybe.

However since it's a flat surface in general, the up-pitching nose may not be best for aerobatics and if constructed the opposite way then I may get a pitch down effect at low A of A that could be worse?

The graphic was more for me to get a handle on the effects of "real" airfoils in this configuration.

[hardlock]

This might make my problem clearer:

[Ollie]

I don't think it is going to matter much either way because the diameter of the rods (tubes?) is so small compared to their spacing that the angular differences will be small too. A little up or down thrust on the motor can compensate for most of the difference when under power.

Looks very intresting. Please let us know ho it flies.

[Pook]

Hmm now this is just what occures to me.. i have no idea if im right :)

in the second picture the bottom wing layout - it looks like some of the air going over the top of the wing would be accelerated by the prop and then go under the rear part of the wing.. which to mee would seem to be rather bad..

Piers

[hardlock]

Ollie, - I hope your're right. The tubes are only .125 dia. but by repositioning them over and under the booms I can shape the wing for camber, reflex, dihedral, etc. plus the entry angle to the slot as shown. It's all subtle but from glide tests of the plain wing across the room changes have noticable effects. Add the power and speed of a BL and effects might be greater.

Motor thrust angle is another mystery to me. I know that if level and above the wing it will want to pitch down but that's about it.

I see planes like the A-10 with upthrust on the motors even though they are on top and in the rear. My experience says add downthrust (back of motor up?) to kill the high position nose over moment. I've never put a prop right on the CG before so as to upthrust or downthrust I'd only be guessing. :)

Pook - Don't you mean second picture, top wing layout? With the arrow being the direction of flight (not thrust) then the top pic logically suggests a pitch up moment and the lower a pitch down.

If pitch up then more air looks like it would go under the wing as you say. I need a wing tunnel to really know what happening.

If its nose down, I can always reflex the elevons as necessary to overcome the down pitching which I'd think would also add stability at the expense of some drag. (am I right on that?)

Going the other way with nose up would be interesting as depending on the CG location, I may have to even lower the elevons to raise the rear against the pitching up of the nose. More working wing area so more lift but less stable?

I hear that for full on aerobatics I probably won't want much if any stability anyway but I'm just a hacker designer so any advise I can get here keeps me learning. :)

[Pook]

Ahh sorry i was taking tha arrow as the direction of thrust..

Piers

[BMatthews]

Hardlock, I hope I'm wrong but cutting that big a hole in the middle of the wing just seems like a very bad idea drag and lift wise. It's fine with something like Laddie Musolasko's (SP?) high powered deltas where the slots are a much smaller portion of the wing and the grossly oversized engines make up for the loss of lift but in this case I suspect you'll really need to fly on the power rather than the wing's lift.

I think large slots like this fall into much the same category as large gaps between the wings and fuselage as seen on some pioneer full sized aircraft and the GWS Lite Sticks. It's been shown on many occasoins that a large root gap acts like two extra wing tips and ruins the otherwise strong center section lift thanks to the air leakage spoiling a large segment to either side of the gap.

As I said I'd be happy to be proven wrong for your sake but if it was my design I'd put the motor and prop on the front and just cover that slot over before it's too late. Or at least make provisions to allow for it as a future mod.

As for up and downthrust I wouldn't bother. You're right in thinking that any thrust offset will have very little effect with the prop at the balance point. Just set the balance to where the model is close to neutral in pitch stability and you won't have any problem.

I also wouldn't worry about the airfoil offset. The slipstream on the wing will be compensated for by the prop torque so they will mostly cancel each other out. I think you'd be risking a bad roll being built in more than anything else if you offset it.

Good luck.

[hardlock]

BMatthews - Thanks for the input!

I've never figured out what they are thinking with planes that have that gap between the wing halves. I forgot my gap cover on an UL once and barely got off the ground! Made me a believer that's for sure.

As to the model, I originally designed it as a tandem wing but just couldn't let all that potential wing area go to waste. If looked at this way, the prop gap doesn't seem so bad. :)

I'm a pusher fan and crash a lot so didn't want to use a tractor layout. It would be too much like an IFO then. This plane completely collapses for transport and as is nose overs or nose ins won't hurt it.

I've had problems with other pushers being tail heavy once switching to LiPolys so a mid motor just seemed a natural.

Here's a pic and QT clips of it's predecessor that flies pretty good even on a little LPS motor:

http://www.acesim.com/temp/c-cutlass-lps.mov

http://www.acesim.com/temp/c-cutless-ips-s1.mov

[BMatthews]

That previous one flies quite nicely.

OK. I'll grant you that with all that area and with it probably weighing in as 4/5 of nothing it'll be fine.

Post up how it flies when it's done. I'll keep a look out in Parkflyers for it...

[hardlock]

She flew but not quite as expected. Here's the link I started in the Parkflyer section:

http://www.rcgroups.com/forums/showthread.php?s=&threadid=145186

[hardlock]

As to specific flight test results of the two layouts in the second diagram above, the lower configuaration does cause pitch down at high speeds and needed much more reflex in the elevons for stable flight. One dive couldn't be pulled out of in fact from either flexing of the control surfaces or servo stalling, don't know which.

Once switched to the upper variation of the second drawing however, the control surfaces can remain about flat, yet there is still some dive stability even with a rearward CG. Some of this may be due to the small amount of washout that this setup also adds to the wing.

Very pitch stable at very high A of A also.