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    Foamies (Scratchbuilt)
        Question self stabilizing.

#1 jimmy boo Jan 21, 2013 08:38 AM

self stabilizing.
So, I am thinking about making my own scratch build, but wanted to make it only three channels (rudder, elevator), the problem is, I know this wont work unless it is self stab lazing. The plane would be a 30inch twin engine cargo-ish style plane.

I also have another question on motors. Since its twin engine, 30 inch by 4 inch wingspan, we are talking about 120inch2 of wing. the wing will carry the standard low speed airfoil shape. I am using staples foam board to give the plane some stronger wings to deal with the motor. Sadly, I am pretty terrible at deciding on electronics and am pretty unsure of what motor I would need. I think I will use a standard 2200mah battery.

#2 davidmc36 Jan 21, 2013 09:08 AM

Normally all you need to do is put a nice but of dihedral in the wings to make it fly with rudder only. The Stratos flies with rudder only. Could be a basis for design.

#3 jimmy boo Jan 21, 2013 09:32 AM

yea the problem is, that is tough to do with foam paper. Would vertical stabilizers on the end of the front wings work.

also engine ideas?

#4 davidmc36 Jan 21, 2013 09:39 AM

Your planned size is pretty close to the Stratos so anything in that 180-280 size should work for motors.


Why the vertical stabs on the wings?

#5 davidmc36 Jan 21, 2013 09:40 AM


Originally Posted by jimmy boo (Post 23880404)
....that is tough to do with foam paper....

No need to copy the curved shape of the Stratos. Just make your wings straight with dihedral.

#6 jimmy boo Jan 21, 2013 09:53 AM

^never tried that, what angle are you thinking.

like how high up should the straight wings be?

#7 davidmc36 Jan 21, 2013 10:38 AM

A Piper Cub that I have that was designed to be able to fly with rudder only, is 58" wing and has each wing tip about two inches high.

That span is about twice what you are thinking of so if you were around an inch or so the angle would be pretty close to that.

The Cub wing looks to be somewhere around 7 deg total, so 3.5 deg each wing. Hard to measure the wing with the protractor I have just now, my good one is at work. But if you take half the 58" wing and the two inch rise of the wing tip and do some trig.....

#8 jimmy boo Jan 21, 2013 10:42 AM

ok, That seems good, but if i were to leave my wing in one piece, would vehicle stabilizers on the wings do the same thing, just wondering.

#9 djacob7 Jan 21, 2013 10:55 AM

About how much dihedral, I would say 1" at the tip would be OK.

#10 davidmc36 Jan 21, 2013 10:57 AM


Originally Posted by jimmy boo (Post 23881114)
ok, That seems good, but if i were to leave my wing in one piece, would vehicle stabilizers on the wings do the same thing, just wondering.

What do you mean by vertical stabilizers on the wing?

#11 djacob7 Jan 21, 2013 11:00 AM

Vertical wing tips are just for making the wing slightly more efficient. The tips reduce the trailing vortices and save energy.
You can also making the wing to be ployhedral with the center part having zero dihedral and the outer parts having a dihedral.
To make a 1" dihedral cut the wing in half then glue it back together using 1" spacers at both wing tips. But remember that the wing joint in the middle has to be very strong.
What are you using for spars?

#12 jimmy boo Jan 21, 2013 11:07 AM

^by spars do you mean support bars?

anyways, I don't plan on using support bars, the trick will be having one 30 inch by 4 wing, then having the same cutout with a inch by 30 cutout. placing it at the front and covering with packing tape, thus producing a basic airfoil. the 30 by 1 inch piece will act as a support.

I am not sure about the who dihedral thing, Ill look into it.

#13 KenSt Jan 21, 2013 11:11 AM

Dihedral or polyhedral, as above, is what makes the airplane turn with rudder control. So no, vertical "stabilizers" on the wing won't work.

#14 djacob7 Jan 21, 2013 11:16 AM

Yes, a spar is a support bar.
If your wing is curved into a foil shape, then you may not need a spar because the curvature will prevent the wing from bending. But it still would be pretty weak. Many people use BBQ sticks under the wing connected to the fuse to strengthen the wing.

#15 davidmc36 Jan 21, 2013 11:21 AM

Oh, OK. You were taking about winglets: http://www.flightglobal.com/blogs/fl...48289-241.html

No, they will not make the plane bank and turn like dihedral will with rudder only control.

In a simplified nutshell kinda way: When you initiate a turn with rudder the wing "moving forward" will have an increase in lift and the one "moving backwards" will have a decrease in lift. Thus the plane banks in the same direction you are turning the rudder and turns for you. If you have no dihedral this effect is little or none and the plane will not turn very well.

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