View Single Post
Old Oct 09, 2012, 11:24 AM
kcaldwel is offline
Find More Posts by kcaldwel
The answer is 37
kcaldwel's Avatar
Joined Jan 2007
3,852 Posts
I see you clipped the "much" from my efficiency comment. For most power RC models efficiency really isn't much of a consideration. They generally have a great excess of power, are not asked to lift heavy loads or fly for maximum endurance. Efficiency is not much of a consideration, unlike a sailplane.

More from Dr. Drela:

"Deflected ailerons deform the load distribution away from the ideal near-elliptical shape, and hence increase induced drag. Partially slaving the flaps to the ailerons can alleviate this load distribution deformation, and thus mitigate the ailerons' CDi penalty. The question is what's the optimum amount of ail-> flap mixing.
The lowest-drag aileron system is wing-warping as used by the Wright Brothers -- the wing is linearly twisted from tip to tip. When such a twisted wing reaches its steady roll rate, the load distribution returns to its optimum level-flight shape, and the drag penalty is zero.
With a finite number of hinged control surfaces such a linear twist cannot be achieved. But it can be approximated as close as possible if each surface's deflection is made proportional to its distance from the aircraft's centerline, measured at the surface midpoint.
If the four control surfaces have equal span, we then have:
Surface mid-span loc. deflection
L.Aile. -3/4 -100%
R.Aile. -3/4 -100%
L.Flap. -1/4 -33%
R.Flap. -1/4 -33%
So for this wing the flap motion should be 33% of the aileron motion. Using AVL I've verified that this mixing ratio produces very nearly the smallest induced drag penalty. If the flap span differs from the aileron span, the table above can be adjusted accordingly.
Longer flaps will have larger travel and vice versa.
BTW, this "distance-proportional deflection rule" strongly argues against stopping the ailerons short of the tip. The resulting unhinged tip portion should in fact have the largest deflection.
The "distance-proportional deflection rule" can be fudged if there is a tip stall problem in a sustained turn, where some opposite aileron must is held. By increasing flap travel over its "optimum" amount, the flaps can carry a greater share of the roll power, which reduces the required downward deflection of the inside aileron, and thus delays tip stall. So if your TD glider has insufficient tip stall margin, I suggest increasing the flap mixing and you should see some improvement.
The extreme case would be 100% flap mixing, which mimics full-span flaperons. Flaperons give excellent tip stall resistance, as is obvious to anyone who flies a DLG with a good 2-servo wing. A 4-servo TD wing with decent planform should not need to go to this extreme."

kcaldwel is offline Find More Posts by kcaldwel
Reply With Quote