What is important for getting desired performance out of an airframe

I have been looking quite a while how to get the aerodynamic design optimal and how to save there some drag, or a lot of drag, but a good design has also other parts taken into consideration. One of them which should not be underestimated is the structural and thus weight.

If we look for example EM-11 Orka, what is the problem with it when it is actually slower than aerodynamically less efficient and lower power Tecnam P2006T. It is pretty obvious what is the problem: it is not the aerodynamics of the plane (which is good) but the weight. The gross weight of Orka is very high, even higher than on DA42 that some people consider to be a lead-angel (lyijyenkeli). This has implications obviously to the empty weight too. That is very high as well. The empty weight-gross weight ratio is not actually bad in Orka, it is actually better than average. However, because of the gross weight being so high, the empty weight has to follow too. With the high weight, aerodynamic efficiency goes out of the door.

So it is very important that aircraft has minimum possible empty weight and as high as possible empty weight to gross weight ratio.

From the lighter end of the scale, Dynaero MCR01 is a good example. It is very lightweight, a lot lighter than its competitors. And it really shows positively in the performance. The wings in the ULC-model don’t even incorporate a NLF-airfoil and the fuselage is all-turbulent behind the propeller. Still it is damn fast compared to all competition in its class with the same engine and propeller. The Dynaero’s empty weight-cross weight ratio is not actually much better than on Orka, but because Orka is so much heavier and it is designed to carry so much more, the end result is very heavy (and it requires higher power engines than the Orka prototype originally had).

So this leads to a conclusion:
Previously mentioned gross weight of 818 kg for the twin concept is not unfounded. It represents ratio of 0.55 which is worse than on Orka or Dynaero MCR01. The goal has to be drawn somewhere. If the empty weight has to be more, e.g. 500 kg, that means 900 kg MTOW with ratio 0.55, and already a bit worse performance (speed (because the plane has to fly at higher Cl to maintain level flight on cruise and it is no good especially if the airfoil was designed to give its lowest drag at low Cl value) and climb performance).

Someone might be wondering why I don’t talk about aerobatics much at all – Aerobatic planes require higher empty weight – gross weight ratios more than 0.55, and because of that I am not even thinking about a aerobatic plane which is intended for cross country flying. Efficient cross country machine has to be separate from aerobatic plane unfortunately because of restrictions what is achievable with even the best materials out there. Strength in airplane is not a place where a compromise can be made, it must be strong enough for the intended use or it is a deathtrap, and this leads to that the empty weight – gross weight ratio may not go much lower than 0.53 very easily on a small aircraft, especially without compromising something else like aerodynamics.

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