Why the Perfromance Degradation?

[Don Cooper]

I had a plane I liked very much (the 'Sorta Tiny' with the flat bottomed wing mentioned in the earlier "Tiny Roll' thread). Unfortunately, I got overconfident with my flying skills and flew it at almost 90 degrees into a concrete road. The body was demolished and the gear box (GWS S1) broken. I got a new S1 and built a new body - same sideview but widened because I had trouble stuffing the electronics in the old body. The wing, tail surfaces and landing gear were not hurt and were used as is. The new plane is a dud. It is noticeably slower than the previous version, loops are not as tight and the altitude loss during roll is worse. My first thought was that the new motor was weak. So I put the old one (unharmed in the crash) in the new gearbox, but got no improvement. My next thought was that I had increased the drag too much by widening the body. Assuming that thrust was equal for both cases and manipulating the standard drag equation, I got V2/V1 = square root(A1/A2) where V is velocity and A is cross sectional area. With the change in area I had made between the bodies, this equation gives a max speed change on the order of 1/2 mph. I doubt that my eyes are calibrated to pick up such a small change. Does the equation over simplify the effect of the frontal area change? Any suggestions as to other things I can look at? Incidentally, to the accuracy of my scale, the weight is the same with the new body.

[Dave Robelen]

Hi Don,
Almost all equations "oversimplify" the situation, until you can apply them with experience. You did not mention the weight. What seems like a small weight gain can really cut into the performance. The wider fuselage not only has additional frontal drag, but it trashes more of the flow around the wing center, and reduces the slipstream velocity. Also, there is a possibility that you are using slightly different incidence settings, or a slightly differen CG. So many things can add together, each small, to make the difference between a "dog" and whoopee!
God luck with the troubleshooting, Dave

[Don Cooper]

Dave,

The performance loss appears to be largely, if not completely, due to a drag increase. I had already played around with decalage, C.G. and thrust angles without much luck (incidentally, the weight was the same for both versions). Yesterday I clipped 1.5 inches off each wing tip (span is now 27 inches) and tried it this morning. Performance was significantly better, even with only preliminary airframe tuning. I have to admit I am surprised about how much difference what appear to be relatively minor aerodynamic changes can make with these small power-limited planes.

Can you recommend any books on model airplane aerodnamics? Complexity shouldn't be a problem since I worked as a theoretical aerodynamicist for several years (albeit decades ago). The field down the street is small enough that it rapidly becomes marginal with the 'bigger motor/bigger battery' approach, so I prefer to stay with the IPS motors and try to improve the aerodynamics.

[Dave Robelen]

Hi Don,
I can well agree, small changes can make a significant difference. There is very little in book form that deals with the theoretical approach to model aerodynamics relating to small R/C model. I have seen several books on free flight models, mostly rubber powered. You can get a lot of good stuff from the "Ask Joe and Don" section of the DJ Aerotech website. Actually, quite a bit of basic theory comes from full scale, without changes. Issues like stability, CG, tail volume, etc. are all the same. If you have a decent library within reach, you might check there. For in depth reading, Aerodynamics and Astronautics by Barnes McCormick covers these areas extremely well. Barnes is a professor at Penn State, and from working with him, he knows his stuff. The book really is pretty clear, and covers the areas of interest to us. The main place where full scale literature falls short is in the performance predictions of airfoils and propellers due to the huge difference in Reynold number.
Happy tuning, Dave

[Dave Robelen]

Hi Don,
There is a very handy website for predicting wing performance of models. It is called the "Wind tunnel data analyzer". If my luck holds, the following will take you to it. http://soaring.cnde.iastate.edu/calcs/frames.shtml
Happy hunting, Dave

[Don Cooper]

Thanks Dave. That web site looks interesting.