On the importance of the correct C.G. location in flying wings
I would like to dedicate this lecture to the memory of Robert Kronfeld. He was one of the most successful and famous sailplane pilots in the late 20s and 30s. He was killed nearly exactly So years ago while testing a flying wing. The reason for this accident was most probably a wrong C.G. location of his tailless glider.
Before I come to the history of Kronfeld's fatal flight, I would like to treat this subject of the C.G. location a little more general. For simplicity, in the following only swept back flying wings are regarded. Hence, e.g. flying planks are not considered.
Now, this is a very unwelcome situation:
First: This additional lift may lead to separation of the flow at the tip and, hence, to wingtip-stall with a subsequent roll-over which may result in a spin.
Second: If this flow separation happens simultaneously at both wingtips, then a "rearup-stall" may result, which is especially dangerous near the ground.
Third: If this flow separation happens simultaneously at both wingtips, then a "rearup-stall" may result, which is especially dangerous near the ground.
Forth: Both elevons down means that the wing has negative twist. Such wings with negative washout, however, have a tendency for spiral instability. This may not be as dangerous, but it is unpleasant during instrument flight.
Fifth: This negative twist unfortunately amplifies the unfavorable adverse yaw, which is a nuisance for any flying wing. Thus control around the vertical axis is weakened.
Opposite to popular belief tail-heaviness gives no advantage for the performance. If the lift distribution of the wing is chosen optimal for the correct C.G. position, then tail-heaviness gives more induced drag, i.e. a loss.
The conclusion from this is now obvious:
First: This negative lift prevents the separation of the flow at the tip and stops, hence, any wingtip-stall. Therefore, no roll-over should be observed and a spin-proof aircraft can be expected !
Second: Since no flow separation at the wingtips occurs, also no "rear-up" stall should be observed.
Third: Both elevons up means that the wing has positive twist. Such wings with positive washout, however, have a tendency for spiral stability while circling. This is especially important and pleasant for sailplanes, especially during instrument flight.
Let me tell you, as an example, my own experience while flying the Horten H IIIf (prone pilot position). In 1941 I entered a cumulus cloud with this ship, even that my turn-indicator was not working. At that time I had not nor have I now - an instrument flight rating. But I had trust in the words of Heinz Scheidhauer who had told me: "In the cloud keep the stick completely back and well centered . . . and wait, until the ground appears again". It worked, after a rise of 1000 meters I came out on top of the cloud. The Horten did the constant circling herself never flying faster than 70 km/h. Completely safe! With which other sailplane of that time would that have been possible? With none!
Fourth: By this positive twist the adverse yaw is dampened or even completely annihilated. This strengthens the control around the vertical axis.
Fifth: There is, however, a price to be paid for all these very favorable flight characteristics: If the lift distribution of the wing is chosen optimal for the correct C.G. position, then nose-heaviness gives more induced drag, i.e. gives a loss. If the nose-heaviness is very large, then this loss may also be very substantial.
Now you may ask: "Why would anybody in his right mind move the c g. of a flying wing too much backward ?". I have seen this done very often, it happened twice during the last two years with a disastrous result in one case. The "normal" reason for doing so is the observation, that the elevons are in "up"-position during "normal" flight. The obvious reaction is then: "Oh. the airplane is nose-heavy, otherwise the elevons would be in neutral position". Then, the C.G is moved hack, then wingtip stall and/or lateral instability occurs, then the plane goes into a spin or an other unpleasant flight situation and very often crashes.
If moving the C.G. back is wrong, what should then he done in such a case? Why, the solution should be obvious to any aeronautical engineer:
When the flying wing in normal flight has the elevons up, then the fixed twist of the wing, the build-in wash out, is too small and should be increased !
But, whenever I suggested this remedy, then invariably came the reply: "Well, but this would deteriorate the performances of the airplane". This is true; but flying with the elevons up deteriorates the performance even more !
But, you know, nobody ever did believe me !!! And so, flying wings again and again have been flying with C.G. aft . . . and crashed.