Re: Back of the curve ? question
David J. Gall
[WARNING: LONG POST]
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[WARNING: NOT PROOFREAD or EDITED] Peter, Excellent questions you pose. The "backside of the power curve" is most easily described by describing the whole "power curve," first, then isolating the "backside." The "power curve" we're talking about is actually the "powerrequired curve." We all know that the drage of the airplane is made up of two parts, parasite drag and induced drag. Parasite drag increases as the square of the speed. If you draw a graph of the parasite drag as a function of speed, you get a curve that goes up to the right, ever steeper the farther to the right you go. Of course, this assumes that "to the right" is toward increasing airspeed, and that "up" means more drag. Induced drag, on the other hand, decreases with increasing airspeed, so its curve goes down and to the right, ever shallower the farther to the right you go because induced drag is "inversely proportional" to the speed. Adding the parasite drag and the induced drag together gives the total drag, and the curve for the total drag starts high at low airspeeds (high induced drag), comes down to a minimum (just above where the two drags' curves cross), then goes back up as parasite drag starts to predominate in the higher speed range. But drag is not power. In order to get the power required from the drag, we must multiply the drag times the airspeed. This sounds kind of kooky to just say it, but the end result when applied to the drag curve is that the power required curve looks very much the same as the drag curve. The power required is high at low airspeeds, diminishes at moderate airspeeds, then goes back up at higher speeds. The minimum power required point does not quite coincide with the point of minimum drag, though. The minimum power required speed is a little bit slower than the minimum drag speed. But the important idea to remember is that the power required curve is bowlshaped. And that, if you extend the flaps, the whole curve shifts up (higher power required at all speeds) and to the left (lower minimum power required speed). The power available curve sets the limits on what our airplanes can do in level flight. If you fly at too slow an airspeed (too far to the left on the power required curve), the engine can't make enough power to sustain level flight. When that happens, the airplane must descend in order to maintain airspeed. (Fortunately for your "expert" pilot, the Q2/200 is capable of climbing in this condition. Even at reduced power setting, your Q2/200 can apparently climb at speeds close to stall speed.) At the other end of the speed range, the only way to go faster than flatout full power in level flight is, again, to go downhill. There's a difference in the airplane's behavior, though, between the slowspeed case and the highspeed case. In the high speed levelflight condition, a reduction of power results in the airplane slowing down. Slower flight means less power required, so the airplane can continue in level flight. However, at low speeds, below the minimum powerrequired speed  in other words, to the left of the bottom point of the bowl  if you reduce power and simultaneously attempt to maintain level flight, you find it impossible to set a steady airspeed. If you try to maintain altitude, the airplane continues to slow down. Alternatively, if you hold a steady speed, the airplane descends. The airplane demands ever more power as it slows down. So to go slower you find that you end up using more power than you had been using before you slowed down. You are now flying "on the backside of the power curve." To go slower demands the counterintuitive use of MORE power, not less. Enter the Quickie/Q2/Q200 and Dragonfly. Here's a unique situation! Here's where the Q's and D'flys are different from ANY other canard or conventional airplanes out there. Earlier, I mentioned that the power required curve for a conventional airplane was different with the flaps extended. But on Q's and D'flys the FLAP is also the elevator. In order to fly slower requires that the elevator be deflected to a more "nose up" setting, but that is the same as extending the flaps. So, the power required curve of the Q's and D'flys is UNIQUE in that it is a blended hybrid of the two power required curves described above. At low airspeeds, the power required curve is that of the flapsextended case. As the airspeed increases, the flap (elevator) is retracted and the power required curve becomes that of the flapretracted case. Where is the minimum powerrequired speed? Compared to a conventional airplane, the minimum power required speed is much farther to the RIGHT. That's right, the gradual extension of the elevator (flap) as the airplane slows moves the power required curve points up and to the left, so the minimum powerrequired point is way out to the right compared to conventional airplanes since the elevator (flap) is partially extended right up to design cruise speed. In fact, design cruise speed is probably right at the minimum power required speed. This explains a lot about the performance of these planes. Ever notice how fast the best climb speeds are? That's because best climb speed is very close to minimum power required speed. If minimum power required speed is close to design cruise speed, then expect best climb speed to be nearby. Ever wonder why these planes are so good on gas mileage? That's also your answer: minimum power required speed (design cruise speed) is close to our everyday cruise speeds. Notice that I'm referring to the "design" cruise speed, not the 65% or 75% power cruise speed. Chances are that the designers never put this much analysis into the aerodynamic design of these planes, instead concentrating on stretching whatever magic it was that Burt Rutan put into the Quickie into a two seat airplane. Back to the power required curve: the power required curve for these planes is stretched out to the right compared to a conventional airplane's power required curve. That means that the minimum power required point is faster, and that the size of the "backside" of the power required curve is much larger in terms of the range of airspeeds it covers. Whereas a conventional airplane that stalls at 60 might have a "backside" up to 85 or so, ours might extend to 100 or even more. Granted, the effects might not be so noticeable since the curve of our "bowl" is so much shallower. And the really bad stuff at the extreme left end of the curve never really happens to us since the elevator (flap) only goes to 22 degrees instead of the 40 or more on conventional airplanes. So the "backside" effects are probably much less severe (or even unnoticeable) on Q's than on Cessna's. Notice that none of this discussion even mentioned "stall." To finish up, let me point out that the "frontside" of the power curve for these planes is also different from conventional airplanes. Here, the continued deflection of the elevator more trailingedge up as higher speeds are attained, and the use of reflexors for high speed trim, may give an advantage over conventional airplanes. In other words, the power required curve may be flattened somewhat allowing "more speed on less power" than other designs. Let's hope that's true, but note that it would require a reduction of parasite drag more than anything else. One form of parasite drag is socalled "trim drag." Could it be that the trim drag of a Q is significantly less than that of a conventional airplane? That is another excellent question for another day.... JMHO, David J. Gall
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