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.
3 comments:
Interesting. I know that training students/pilots in Diamond Da-20s, when they were new in 2002, They had real trouble getting them to come down on simulated engine out procedures. If they had flown Cessna's before they would end up high and with too much energy.
I prefer a Cirrus, but I like the Diamond flight qualities as well. For some reason, and maybe your critique of the airfoil would explain, they do not inspire confidence in weather. Consequently, you do not hear Diamonds on the radio when there is weather about. My seat of the pants guess is that the airplane just feels so much lighter than it is, both in weight and wing loading. Nice article
Yes, the DA40 feels a bit lighter in both weight and wing loading than the SR20 despite these are quite close to each other.
The DA40 is not so bumpy ride as DA20, it is quite much like SR20 with light load.
Anyhow, the best thing in the DA40 is the stick handling feel and stability, these are not as good in the Cirrus. Other than that quite equal except that Cirrus may provide less bumpy ride when close to gross weight and Diamond wins it in climbs in any condition any time.
The bumpiest experience with the DA40 in our case has been so far on approach to Vagar, Faroe Islands. That was a ride that didn't feel anymore that much fun. But on the other hand, it was only scary, it might have been totally insane in a C172 or worse in a LSA or ultralight. All other landings were without any scary feelings and I would go to equal weather in the DA40 than with the SR20. Even the severe cross wind in Bergen wasn't a problem.
Yeah and I also demonstrated the DA40 on a very bumpy day to another fellow pilot and haven't heard of him since then, maybe it was too much. The wind was 50 kts on 1000 ft and the turbulence was insane (luckily unlike in Vagar, the ground level wind was okay). It was safe with the Diamond though since it has good aileron and elevator control. But I am sure the Cirrus would have been jumping in this weather too and with lesser capable planes it would have been possibly suicidal to take off.
Didn't we nail that earlier indeed? :)
Probably those craft are designed largely for training so cruise is not important, climb is.
As proposed before, how far could you go cruise speed / consumption wise with just new KS118 or something in that vein wings on a Diamond?
Here's how Diamonds are made:
http://www.youtube.com/watch?v=4LhN_T5tGK8
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