Archive for the ‘ fx63-137 ’ Category

Why Diamond uses Wortmann FX63-137?

I have been thinking over and over again why Diamond has chosen the Wortmann high lift airfoil FX63-137 on its aircraft. However, I am suspecting what might be the reason (not confirmed though since anybody on Diamond booth e.g. in Oshkosh is usually never able to answer to my questions). Here is my theory about it:
– The FX63-137 has high L/D at fairly high alpha and thus Cl (as the airfoil is such that the Cl rises rapidly as a function of alpha). This is maybe not the best configuration for cruise where a low drag bucket at low Cl is desirable. On an airfoil which has best L/D at low Cl, the climb has more D component (because high lift devices cause drag) and while getting more L with high lift devices. It might be close to the optimal climb optimisation on the chosen aspect ratio on those planes and compromise is drawn to cruise and it is not seen as a bad thing because competition is not faster but usually slower, it does not take so much to win e.g. a C172 in efficiency and speed after all. So it might be that with a lower drag cruise airfoil e.g. DA42NG with the very heavy diesel engines might have somewhat poorer climb rate on single engine situation or it might not climb alltogether if the airfoil was not optimised to provide low drag on high Cl.
– Comparison between the DA40 and Cirrus SR20 kind of potentially shows this: the Diamond shows significantly better climb rates with a quite similar AR and quite similar wing loading (SR20 takes some toll on that, but not that much in comparison if a light loaded SR20 and heavy loaded DA40 is compared), despite of the fact that the SR20 has more sophisticated flaps and the SR20 has 20 hp more engine power available.
– This can be also evidenced on best climb rate speed: with similar wing loading, the best climb rate speed is much higher on the SR20 than it is on the DA40, which partly indicates that the sweet point in the L/D occurs at lower alpha on SR20 than on DA40. SR20 also requires quite accurate angle of attack and thus speed to climb optimally whereas the DA40 is not that critical which would also indicate that the low drag bucket of the FX63-137 is broader than on the (according to UIUC data site) Roncz airfoil on the SR20.

So this is just my home-brewn theory style thinking, is based on collected information and my experience with flying the Diamond DA40, DA42 and Cirrus SR20 and SR22. I might be wrong as always, but here is some food of thought if you have been thinking why there is this airfoil with high L/D at high Cl and the airfoil also has fairly high pitching moment which some find undesirable because of for example trim drag.

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