| Chris, Yes. I over-simplified. The descent rate.... As we descend, power off, gear and flaps down, prop in high pitch, at the Best L/D AOA mark, we see the glide path angle. If it is steep, we reduce the AOA by nosing down a little, which increases the V a little, increasing the reserve lift. If it is shallow we can get closer to the Final AOA mark, nosing up and slowing just a little. Now we have the optimum lift coefficient available for flare. If there's wind, we lower the AOA to compensate for the airspeed loss as we get close to the ground. We use our depth perception to decide just when to start the flare at the right distance from the ground. Flare increases the AOA, using the Cl we've reserved below the lift curve peak... but by then I'm too busy to look at the AOA. Pilotage -- I need to refresh my reworkable depth perception to be able to start the flare at just the right distance above the ground. By practicing. It's a habit. It's fun. How's that sound?
One way I use to understand using an AOA is to look at some lift vs angle curves for various airfoils, as in Abbott & Doenhoff, or on the internet. for one's Lancair's airfoil. (the NLF-0215?). The lift increases to a maximum and hen reduces about 10-15% before the stall where it drops to about half. As you said, increasing the lift alsoincreases the drag, using energy, slowing the plane. The actual degrees aren't important because of fuselage part-span, part-span flaps, and tip losses. I just find the stall break and mark the AOA, and then find the best rate of climb AOA and mark that... which should be close to best glide. (I haven't checked power off, flaps, gear glide yet in the Lancair. Just have about 50 hours on it, Still working on LOP and a com antenna problem.)
Terrence On Jan 27, 2010, at 5:52 PM, Chris Zavatson wrote: Terrence, AOA will compensate for weight very nicely. It doesn't know, however, what descent rate is being flown and how much energy is needed to arrest that sink. The energy needed goes up as a squared function of the vertical speed. In the 360, I span descent rates from 500 to 2,000 fpm on final (ILS to short approaches). The energy thus varies by a factor ~16. It is the higher end of the scale where one can get into trouble. The sink rate introduces a time element that the AOA indicator can't factor in. Here is a thought experiment to illustrate the point. In the flare you bump your AOA from your mark to just under stall. This will generate a new, higher quantity of lift that will immediately begin to decay as the new added drag slows the plane. Now, the higher your descent rate was, the longer that new, higher, but tapering lift must act on the plane to stop the sink (F=ma). The question becomes: Do you have enough time before you run out of speed? If you do, then you stop the sink and all is good. If not, you smack the runway at a high AOA but still descending. Chris Zavatson N91CZ 360std
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