"Kiwi Jet Jock" wrote in message
...
Additionally, some might be interested in doing some experiments dumping
flap during a simulated engine failure on approach to work out at what
height the additional height loss (in dumping the flap) is (more than)
compensated by the increased gliding range.
Interesting. "Dumping", you mean retracting, right? The idea being to
reduce the drag & preserve energy to extend the glide distance?
I'd have thought that above 'best glide' speed, you'll always extend your
glide distance - without any associated additional height loss if you
pitch to maintain airspeed - by retracting flaps. After all, at constant
speed, you'll be cleaner and dissipating less energy -- energy which
can only come from the loss of your (2.m.g.h) potential energy as you
descend. (If this wasn't the case, the POH would tell you to add flaps
to configure for best glide).
Below best glide, things aren't as clear. It may be that the induced drag
with flaps out is less than that when clean. Well, for low setting flap
settings, anyway. In the worse case, you should only need to get back to
best glide, which means the additional height loss will be the potential
energy needed to increase your speed.
0.5 * m * (v'-v)**2 = 2*m*g*h
(v'+v)*(v'-v) = 4*g*h
so for v' (best glide) = 70knots = 120 ft/sec, v (approach speed) =
60 knots = 100 ft/sec, height loss works out at about 35 ft.
Now in my 182, I typically maintain 70 knots (which happens to be
my best glide speed) until short-ish final, so I know exactly what I'll
be doing if the engine conks out (assuming I've been obliged to take
the 'grand tour' behind someone, or I'm coming in on the final 1/2 mile
or so of an ILS). I slow to 60 when I'm pretty sure I'll make the
airport property, if not the runway itself, so exactly what the optimum
response to an engine failure might be in the low speed régime doesn't
concern me that much.
Of course, with a headwind, you'd normally trim for slightly more than
best glide speed to increase your range. But that's another topic.
|