Elevon mixing on T-tail

[phacious]

Elevons on a T-tail, bad idea?

What do you call a plane with a puller (tractor) motor mounted on a T-tail? A powered T-tail perhaps? Anywho, I'm building a 30" span belly sliding FPV plane with the motor mounted to the top front of the T-tail (prop will be about 3/4 mark of the overall length). I did this to get most of the control surfaces back in the prop wash. Now I'm wondering if splitting the the elevator into elevons would have any benefit if I have to power out of a bad situation or if it would just cause complicate the way the plane rolls. The main wing will have its own set of ailerons for best roll authority during the normal flight, but they are not in the prop wash. If I loss authority on the main ailerons while in a steep bank I'd like to be able to stomp on the gas and snap my wings level. Any thoughts on this?

Here is the airframe so far. It was designed around a 2200mah 3s and 7x3.8 SF + 2200kv 320w.

 

[Pieliker96]

I think you should be fine using standard ailerons. It looks like about 2/3 of the disk area of the prop will pass over the elevator, which is a very small surface compared to the ailerons. Also, the relative size and position of the elevator mean that it acts as a much smaller momentum arm than the main wing. The (I assume) undercambered outer portions of the main wing should allow them to maintain authority at lower speeds than the middle of the wing, permitting aileron control when entering a stall. So unless you're planning to do hardcore 3D with this thing or need the excessive roll authority, the ailerons should work just fine.

Looks like a nice FPV ship, like the removable noses. I would expect this to be pretty nose heavy though, so make sure to put the battery somewhere under the wing .

 

[PsyBorg]

I would think having that much air pushing over the control surfaces that close would tend to force the foam to bend back straight thus not producing as much change as you would think or wanted. I would find a way to secure the plane and watch what happens as more air goes over the surfaces to see how they react at various throttle settings. Usually ailerons on the wings running the full length like that should do some crazy rolls so worrying over not having enough to get out of a bad situation should not be an issue.

 

[phacious]

Thanks for the input guys, I'm going to skip the elevons and keep it simple. However, I have a new question. Considering the diagram, what do you think the ideal Horizontal Stabilizer angle should be? I designed the main wing with +5 degrees AOA relative to the plane sitting flat on its belly. The hope was to improve lift for takeoff, using the belly instead of having landing gear. The horizontal stabilizer is inheriting the 5 degree simply because I cut everything square and I didn't think it was necessary to change that. Now that I look at it, I'm not sure if the main wing AOA would do much or anything to counteract the offset thrust position. Seems like the horizontal stab would be the ideal device to coarse adjust the negative pitch created by the thrust position, question is how much angle is needed if any? My first guess is somewhere between the 5 - 0 degrees depicted, or it could be more in the negative than that. What do you guys think?

 

[Hai-Lee]

I love your concept and applaud your skills. Your question as to geometry is best sorted on test though I suspect it will be very pitch unstable with varying throttle.

Do not take my post as gospel but again I would have made thrust and flying surfaces all zero incidence to start with and mixed some flaps into the ailerons to assist with lift off from the ground. Be careful banging the throttle open when flying low and slow until you are sure that the prop wash being very strong and so close to the control surfaces does not cause control surface distortion and possible extreme "P" factor effects, even possibly control reversal or servo stall.

 

[phacious]

Zero incidence it is. Thanks for the info, never heard of p factor before now. Hoping to get in a maiden sunday.

 

[Hai-Lee]

Good Luck. I will await your post as to how it went.