Wing designe for tow plane

[Manfred Eiberger]

Greetings from Canada and Oakville to be specific. I belong to the CMAS(Canadian Modell Aerotow Society) and am wishing it was summer and flying weather.
We are in the process of designing/building a new towplane which was designed by one of our members. Its wingspan is 102 inches (with foam core) and has a profile at root of 20 inches NACA 2415 with tip profile of 12 inches and a NACA 4415.Plans show a 1.5 degree washout. Leadingedge offset is 2 inches.
Question: do we realy need a washout to prevent tipstall or has the tipprofile sufficient lift compaired to the root profile to prevent this from happening. My belief is that washout may not realy give us more security with our basic designed wingprofile.Would appriciate your thaughts. Thanks Manfred E.

[space_case]

Is this a straight taper from 20 to 12 inches? This does not sound like the best design for a tug. This is the quintessential '0.6 taper ratio' that would in theory give the lift distribution closest to eliptical for a single taper. However, I doubt that is what you really want.

I believe a constant chord wing would serve you better in this application. Concentrate on making it strong and true, and add a couple of degrees washout.

You are concerned with which profile provides more 'lift'. What you really need to be worried about when adding aerodynammic washout, is whether or not the tip of the wing stalls before the inboard portion. So you really need to plot CL_alpha curves for say the average Reynolds number in those regions.

The NACA numbers tell you that the outer profile has twice as much camber as the inner profile. Whether or not this will benefit the tip stall condition, considering the different Re#s, I could not say without running the numbers.

You are refering to plans. Are these plans yours? If not, what is the plane you are building?

If somebody went to the trouble to design it, they probably put in the washout for a reason. I would go with it. A little washout never hurts anyways.

[Ollie]

The zero lift angle of attack of the NACA 2412 airfoil is -2 degrees and the zero lift angle of attack of the NACA 4415 airfoil is -4 degrees. The wing has 2 degrees angle of attack of aerodynamic washout plus 1.5 degrees geometric washout equals total 3.5 degrees washout.

See:
http://aero.stanford.edu/WingCalc.html

The wing design is good for lift distribution and good for spar structure with airfoils tapered thickness.

[Manfred Eiberger]

Thank you for the very quick response re "wing designe". The planned a/c is a new designe by member of the club and the fuselage a type of "ugly stick", very basic for fast and strong building. We have cut foam cores with the 1.5 degree washout and some without this washout. We will probably try both and see what results we obtain. NACA 2415 (root) to NACA 4415 (tip) taper over the length of the wingpanel. The leading edge is set back 2 ". Would the extra washout increase drag, therefore decreasing the effect of washout? Or am I much off the wall here. You can tell I am realy newand inexperienced at designs. Manfred E.