Thorp Air Command - T18.net

I was once told that the LDS airfoil also may shift the center of lift slightly, possibly reducing drag from the airframe due to a more optimum cruise attitude. I'm sure someone will correct me if that's B.S.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Thanks, all, this is very helpful. I do think that the stock T-18 with the LDS airfoil is the direction I am going.

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Matthew Long, Editor
cluttonfred.info
A site for builders, owners and fans of Eric Clutton's FRED

« Voici ce que j'ai fait...vous pouvez en faire autant! »
"This is what I have done...you can do the same!"
--Henri Mignet (1893-1965)

Software such as Xfoil is not 100% reliable. That said, it indicates the LDS will need to cruise a fraction of a degree more nose down than the original airfoil. I have no idea if that is in the "optimum" direction. It also indicates the aerodynamic center is in the same place, but the moment coefficient is a bit greater; in other words, the center of pressure is a bit aft, not much. This won't affect the cruise attitude of the airframe, only the trim position.

The LDS's notable feature is it's improvement in max lift coefficient without significantly altering any other aspect of the original airfoil. Kudos, Lou Sunderland. How useful this is in practice, I can't say. David Read just said both are good if well constructed. If I build a T-18 (just studying the plans for now), I plan to use the LDS.

Karl

The biggest difference between the two airfoils is the stall characteristics, the original airfoil has a very sharp stall break, the LDS airfoil has a mild stall break. That said you can tame the stall break on the original airfoil by using stall strips on the leading edge. This does two things the inner portion of the wing stalls/loses lift before the outer portion and the turbulent air striking the stabilator gives you a stick shaker warning before the whole wing stalls. There is a article or two in the old newletters on installing stall strips.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

I didn't find very much discernible difference in the stall break between mine & the other T-18 that I mentioned in my previous post, which has the original airfoil. My total left seat time in a T-18 was 2 hours, in that T-18, before I test flew mine, & I did a lot of stalls. It stalled nicely, & I wouldn't characterize it as particularly sharp. One thing that is true is that there can be a lot of difference in the leading edge radius of the wing & hence the stall characteristics on these airplanes. I've seen some sharp leading edges on a few Thorps, and they reportedly had very sharp stall characteristics. I'm sure the stall is a little different, but I believe that the difference is not all that great if the wings are properly & well built. I agree wholeheartedly with Bruce about the stall strips giving a nice stall warning & changing the characteristics of the stall a little. That has just been my experience.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE