scott@opus.UUCP (Scott Wiesner) (01/28/85)
To prevent anyone from freaking about about this article, let
me say up front that while everything described applies to any
aircraft, the incident occurred with a radio controlled MODEL.
I flew a new plane this weekend. I had bought it at an auction
for what seemed like a good price. As models go, it was a little
overweight for it's size, and grossly overpowered. (The big engine
was the reason for it's being overweight).
The plane had a tendency to snap roll at high speed. Whenever I
pulled a little extra elevator, (such as in a loop, etc) the plane
would start snapping. I decided to take it up to a reasonable high
altitude to see how it handled at low speed. As could be expected,
the plane snapped soon after it started slowing down. The plane started
heading straight down, but was NOT in a spin. Just flying vertically
down, with the engine at an idle. Whenever I tried to give a little
up elevator, it would start to pull out, then do another snap. I
probably went through this snap/recovery sequence 4 times before it
went in.
Now my question. Can anyone out there give me a good explaination
of the aerodynamics that were going on here? It would seem that
when at full throttle, quickly changing the angle of atack would
cause the wing to quickly stall out, resulting in the engine torque
taking the plane for a ride.
--
Scott Wiesner
{allegra, ucbvax, cornell}!nbires!scottscw@cepu.UUCP (Stephen C. Woods) (01/30/85)
In article <1053@opus.UUCP> scott@opus.UUCP (Scott Wiesner) writes: >To prevent anyone from freaking about about this article, let >me say up front that while everything described applies to any >aircraft, the incident occurred with a radio controlled MODEL. > >I flew a new plane this weekend. I had bought it at an auction >for what seemed like a good price. As models go, it was a little >overweight for it's size, and grossly overpowered. (The big engine >was the reason for it's being overweight). > >The plane had a tendency to snap roll at high speed. Whenever I >pulled a little extra [...] started slowing down. The plane started >heading straight down, but was NOT in a spin. Just flying vertically >down, with the engine at an idle. Whenever I tried to give a little >up elevator, it would start to pull out, then do another snap. I >probably went through this snap/recovery sequence 4 times before it >went in. > >Now my question. Can anyone out there give me a good explaination >of the aerodynamics that were going on here? It would seem that >when at full throttle, quickly changing the angle of atack would >cause the wing to quickly stall out, resulting in the engine torque >taking the plane for a ride. You didn't discribe the wing section, but it sounds to me like there was a litle rigging problem. I'd suspect that you had at least 2 of the following problems: (1) CG too far aft. This sounds unlikley given the big engine but not impossable given that the model was overweight. (2) A poorly chosen airfoil, airfoils with sharp leading edges have undesireable stall charisterics (like WHAMO ), there are also airfoils that have VERY poor lift moments (as you increase the angle of attack the center of effort moves forward) in this case if your CG was near the aft limit, all of a sudden it's behind the center of effort and you get an increasing angle of attack, which moves the center of effort forward (divergence so to say). (3) The wing had a lot of wash-in (the wing tip has a higher angle of attack that the rest of the wing), this is very bad as the wingtip stalls before the wing root (Vola snap roll). (4) Elevator had too much throw (very easy to overcontrol). (5) Elevator push rod was too flexable (assuming that it pushed for up elevator). This produces a very wierd effect, as the aircraft slows down the elevator increases in deflection. Usually this is not a very pronounced effect, but I had a glider once that was very difficuly to fly because of this, when I replaced the push rod with a stiffer one it flew a whole lot better. (6) An asymetrical wing (did it always snap the same direction?) different airfoils on right and left wing will produce wierd effects. -- Stephen C. Woods (VA Wadsworth Med Ctr./UCLA Dept. of Neurology) uucp: { {ihnp4, uiucdcs}!bradley, hao, trwrb}!cepu!scw ARPA: cepu!scw@ucla-cs location: N 34 3' 9.1" W 118 27' 4.3"
scott@opus.UUCP (Scott Wiesner) (02/05/85)
A few possible problems have been given regarding my model's set-up. > (1) CG too far aft. Not really. With the tank full, I suspect the CG was quite a bit forward from the "dry" CG. With the tank empty, the plane was "just a touch" nose heavy. > (2) A poorly chosen airfoil, airfoils with sharp leading edges have > undesireable stall charisterics (like WHAMO ), there are also > airfoils that have VERY poor lift moments. Though I can't speak to the lift moment question, the leading edge was incredibly blunt. > (3) The wing had a lot of wash-in. I don't believe there was ANY wash-in or wash-out. The plane was built quite straight. No trim was required for level flight at full throttle. > (4) Elevator had too much throw (very easy to overcontrol). The elevator had plenty of throw, but anything other than VERY large manuevers caused a snap. > (5) Elevator push rod was too flexable (assuming that it pushed for up > elevator). All pushrods were quite stiff. Even if the it were too flexible, I don't think this would be a cause for the snap rolls. > (6) An asymetrical wing (did it always snap the same direction?) different > airfoils on right and left wing will produce wierd effects. I'm not sure about this one. It did always snap in the same direction, but I'd think this problem (asymetrical wing) would have cause a lot of trim problems, of which there were none. The theory I'm most likely to believe is that the plane was too nose heavy, requiring a fair amount of elevator deflection to cause any pitch change. The "extra" amount that was causing the snaps was causing the tail to stall out, which resulted in the snap. -- Scott Wiesner {allegra, ucbvax, cornell}!nbires!scott