Sunday, May 6, 2007

DA40 - Aerodynamics

This post is the first of a series about the DA40, partly about the plastic technology, but more generally.

What is noticeable with the DA40, but also other plastic planes (Cirrus, Columbia, ...) is that they are a brand new generation of planes, designed from scratch, taking full advantage of plastic. This has impact on how wings are designed, but this is also a product of modern computer aided design tool.

One my preferred characteristics about the DA40 is its stall characterstics, which make a stall kind of a non-event. Let me be clear here. I don't say that you can't stall a DA40, if someone design once a non-stallable plane, I would be really interested ;-)

What makes the DA40 stall good and safe is that stalling it does not induce any drop, be it wing or nose. The plane just falls (about 1300 fpm / 40 kts). Compared to nose drop common to PA28 or C172, it is even more uneventful. My basic training took place on a plane that lost at least 700ft in wing / nose drop at any stall.

A part from the pelasant way of stalling, feeling safe and smooth to the pilot, the factor that make it s safe plane is that the risk of spin is really low, as no drop occurs. And given the ENOURMOUS wingspan of DA40, the ailerons maintain efficiency at very low speed.

On the back side, ENORMOUS wingspan leads to a very low wing loading (weight per square meter / feet of lift generating surface) makes it really reactive to turbulence. Any wind speed change, or thermal lift will have strong effect, so flying precisely requires a quick reaction time.

The second consequence of low wing loading is that it glides really well, but precise landing is not easy. Depending on weight at landing, speed can be adjusted from 58 to 70 kts on final, and beleive me, adjusting is important. Approach with 5 extra knots, and you will have a long flare, which can be critical on runways shorter than 800 meters (2400 ft).

Another strange thing is the stall warning, which from my point of view sounds too early. If the wind is slightly turbulent on take-off, it is quite common to have a "stall-warning check" immediately after take-off. I don't feel it good to have a stall warning horning arround 65 when the plane can fly at 45. May be next versions could include a flap dependent stall warning, however as the stall warning is not electrical but pneumatic, it's not so easy.