dwbebens
Posts: 196
Joined: 4/25/2005
From: Dickson,
TN, USA
Status: offline
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I’ve been very puzzled about the many comments I’ve seen on racing forums and heard from people about the phenomenon of “bad air” causing crashes in Q-40 pylon racing. It seems that one plane following another around a pylon gets into the turbulence from the leading plane and then immediately crashes.
First off, I don’t fly Q-40, but I’ve never heard of this problem much in other racing events. In any racing events I’ve raced in, at worst, the following plane will “bobble” a little, but will not deviate so much as to result in a crash. Since I’m the kind of person that needs to know WHY this problem of “bad air” crashes happens in Q-40 and not other racing events, I started thinking and calculating. There must be a reason. Here’s what I’ve come up with:
Speed V = 180 mph = 265 ft/sec
Turn radius r = 75 ft
Weight of plane W = 4 LB
Wing area A = 400 sq. in = 2.78 sq. ft
Span S = 56 in
The centripetal acceleration resulting during a turn, a = V^2/r = 265^2/75 = 933 ft/sec^2 = 29.2 g’s
Therefore the wings must provide a lift of L = 4 lbs. x 29.2 g’s = 116.8 lbs. during a 75 ft radius turn.
The lift equation is: L = q x Cl x A
Where: q = dynamic pressure = .5 x d x V^2 = .5 x .00238 x 265^2 = 83.3
and Cl = the Coefficient of lift
and d = air density in slugs/ cu ft = .00238 at sea level
Rearranging the lift equation to solve for Cl: Cl = L/(q x A) = 116.8/(83.3 x 2.78) = 0.50
Therefore, we are asking the wing to generate a lift coefficient of 0.50 at a 75-ft radius turn.
Take note that at higher altitudes (Phoenix for example) q is a smaller number because the air density is less. This means that the coefficient of lift required will be even greater than at sea level!
I’m guessing that the root chord is about 8”+ and the tip chord is about 6”+ on a typical Q-40 plane. The root thickness to chord ratio is about 10% and the tip ratio is about 8%. The Reynolds number Re of the tip would be about 860,000 and of the root would be about 1,000,000. Also, the lift distribution out the wing panel is not uniform. The lift profile falls off rapidly going out to the tip. The airfoil sections in the outer portion are operating at a disadvantage as compared to those in the inner portion.
I examined and analyzed a bunch of airfoils in these chord and thickness ranges. I used Martin Hepperle’s “Java-Foil” program. I found that there is a good possibility that the outer portions of the wings could be going into an “accelerated stall” in certain situations An accelerated stall happens when a wing stalls due to additional “g” loads being applied such as during a turn or a steep pull-up maneuver. This kind of stall can occur at full flying speed! According to my calculations, it looks like the outer ends of the wings are normally operating right at their maximum coefficient of lift during a turn. In other words, the wing ends are lifting about as much as they can. So, let’s say that only the lower wing encounters turbulence from a leading plane going through a typical high-g turn. This could “trip” the wing into a partial stall (outer portion probably) resulting in a quick over-banking , and thus the new turning trajectory would quickly and unfortunately intersect the ground (the plane crashes). This would occur very quickly and give the pilot almost no time to react. Once the wing stalls, the only way to un-stall it is to decrease angle of attack. If this happened hundreds of feet up, possibly the pilot could recover, but at near pylon height, no chance. The converse should also be true. If only the outer wing encountered turbulence, then the plane could rapidly go to an under-banked condition part way through the turn resulting in a rapid altitude increasing “zoom-out” type of turn exit.
If this were indeed the case, then leaving this occurrence up to chance would be unacceptable to me, personally. A remedy would be to alter the wing so that it operates with more of a “cushion” for the maximum coefficient of lift; leave some “wiggle room” so to speak. Using a different airfoil at the outer portions of the wing could most easily do this. You could use a higher lift airfoil (greater camber) and/or it could be thicker. Highly tapered wings, of course, are more susceptible to accelerated tip stalls. Or, you could just fly slower or make wider turns! When I get into Q-40, I hope everyone else adopts the “fly slower and make wider turns” strategy.
I hope everyone found this to be informative, if not entertaining. The mathematics STRONGLY suggests that this analysis is valid. Surely this phenomenon is not simply a matter of “dumb thumbs”. The guys flying at this level are just too good for that to be the case.
Doug Bebensee
< Message edited by dwbebens -- 1/17/2007 4:17:26 PM >
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Q-40 "bad air" crashes - 
