Wingloading & Cruise Speed

[Don Cooper]

A few weeks ago I built an Interstate Cadet from Earl Stahl plans, scaled up to give a wing area of 150 sq.in. The weight, including battery, is about 5.7 oz., which gives a wingloading of about 5.5 OSF. I used a WASP airfoil from Profili.

Last week I resurrected my GWS Cub for at least the 10th time. In its latest incarnation, it came out with a wingloading, including battery, of about 6.7 OSF.

THis morning I flew them one after the other for the first time. I was surprised to see that the Cub, despite its higher wing loading, would cruise markedly slower than my Cadet. I don't understand why. I tried moving the C.G. on tha Cadet back until it got too twitchy for me without significant effect. I started playing with decalage on the Cadet, but ran out of batteries before I could any significant data.

Can anybody give me any clues as to why the Cub will cruise so much slower than the Cadet?

[Dave Robelen]

Hi Don,
There are some pretty marked differences in those two airfoils. The Cub has a fairly thin undercambered section, and this normally means a higher maximum lift coeeficient. Meanwhile, the Wasp section with it's more shallow camber, and stremlined form would likely have a lower maximum lift, and meanwhile a lot lower drag at the cruise conditions. You did not mention, but how far apart is the minimum speed of the two models?
Cheers, Dave

[Don Cooper]

Thanks for the help Dave. After making the post I compared the characteristics of some thin, undercambered airfoils with those of the WASP and had come to the same conclusion. I guess I had fallen prey to the over-simplification that lower wingloading means lower cruise speed.

RE: the speed difference, I have not done any rigorous testing, but I did fly it at the same time that a friend was flying his Cub and the Cadet appeared to be on the order of 25% - 30% faster - it caught up with the Cub pretty fast.