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Terrence,
You said:
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.
Which wing configuration? This is a question I always have with our
very special wing. The standard airfoil is without the flaps being in
reflex. The hi-speed cruise is with the flap in reflex (less lift, less
drag, nose up pitch). I don't consider the flaps to be in an extended
state until they pass through 0 degrees (out of reflex). The drag, lift
and pitch conditions are definitely different in these two configurations.
Also, it seems that the lift characteristics don't change much after
extension beyond about 7 degrees down after zero (total movement = 14
degrees down from reflex) but the drag increases and the pitch moment is also
greatly affected.
I like steeper speedier approaches because of big reduction in
energy during the high-drag round out, thus a shorter float and roll-out.
You are absolutely right about choosing the correct AGL so the descent is
arrested and it is different under different conditions. For me, by the
time I have reached the pattern, the flaps are in takeoff position and that give
me a very stable 120 KIAS without the nose being too high. The gear drop
occurs at a point in the pattern (at an altitude higher than those 172's)
where I have to start the descent and the runway is assured even though the
speed now drops 15 knots.
Scott Krueger
a LOBO seminarian
Lancair 320
In a message dated 1/28/2010 11:48:08 A.M. Central Standard Time,
troneill@charter.net writes:
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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