Wednesday, May 28, 2008

Relation of cruise Cl and wing loading

It is interesting to look the parameters of different airfoils. One notable thing is that the glide ratio of the given airfoil is in relation to the Cl at cruise condition, and the wing loading has a direct effect to the cruise Cl of the airfoil, the higher the wing loading (the smaller the wing area in relation to weight), the higher is the cruise Cl. With some aifoils, this relation is stronger than with others, since the low drag laminar bucket is at some specific Cl, and it is not always in a favorable position for use in a light aircraft, it may be quite often designed for airliners which have very high wing loadings (and very high stall speed as a result as well, unreasonably high for a personal aircraft, which makes surviving a crash unlikely (which would be unacceptable for a personal single engine aircraft)).

I calculated some rounds of weights, wing areas, wing loadings (I have been calculating with wing loadings between 18 lbs/sqft to 25 lbs/sqft (e.g. Lancair Legacy has 23 lbs/sqft)) and subsequently the Cl-cruise and the L/D at the given Cl. Some airfoils are particularly poor at low Cl whereas at least, if Javafoil is at all to be trusted (the methods it uses aren't very accurate), the already mentioned NLF414F is a rare exception. It has excellent lift/drag relation exactly where it should be in a light aircraft with low wing loading. It would be easier to say for sure, if I could see some wind tunnel data for the NLF414F but so far I haven't found enough information. Also it would be interesting to compare to the Wortmann FX63-137. According to Carmichael [1] it does have good L/D charasteristics, but would be great to be able to determine, how good exactly at Cl 0.1, 0.15 and 0.2 (as this is the usual range in light aircraft). The low drag potential is wasted if it can be only utilized at Cl higher than e.g. 0.4, which is not practical or even quite possible in a lightweight personal high performance aircraft and it is also interesting, that many aircraft that are using airfoils which have very low drag potential, may be operating the airfoil outside the best cruise Cl area, and the result is not that good, not that different, or in many cases worse, than if the airfoil was a low drag traditional one, like NACA 66-212. I haven't found so far the wind tunnel data for the Wortmann either, seems like it is not at least available in the Internet, at least not for free.


Jukka Mäkelä said...

Hello Karoliina

Very interesting blog

Doodhwala said...

Nice , really interesting