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Overweight takeoff / flight



 
 
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Old December 4th 03, 12:53 AM
Michael
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(Koopas Ly) wrote
Whoa! At first, I didn't understand what you meant so I posted
another thread yesterday called "Va: maneuvering speed ad nauseam".
After someone else gave an explantion similar to yours, I re-read your
reply and it's now perfectly clear. Thanks. Why isn't this taught to
pilots when they learn about Va?


First, because the average flight instructor can barely handle the
easy parts, never mind this level of complexity. One of my CFI orals
adressed the issue of Va. It went something like this:

"What is Va?"
"It is the maximum airspeed where full deflection of the flight
controls will not overstress the airframe."
"Specifically, which control?"
"The elevator. Va is the maximum speed at which you will stall before
you overstress the airplane. It's the clean stall speed multiplied by
the square root of the maximum positive gee rating for the category
under which the airplane is certified."

That was as far as the examiner wanted to go. Note that neither of
the answers I gave are technically correct. For example, at Va you
can do a full rudder deflection to one side, starting with zero yaw
rate. If you then deflect the rudder to the other stop, you are not
necessarily protected. Therefore, full rudder deflection is not
necessarily permitted. And, of course, if you are rolling as you pull
gees, all bets are off. Therefor, if you are rolling at Va and pull
the elevator all the way back, you risk pulling a wing off. However,
this level of complexity is really beyond the scope of the CFI ride.
Maybe it shouldn't be, but it is. It's certainly well beyond the
complexity of the private ride.

Second, what would be the point? To allow the pilot to competently
asess the consequences of overgross operation? Just because it's done
all the time doesn't mean the FAA will acknowledge the fact and make
it any easier on the pilots. Denial ain't just a river in Egypt.

You make it sound like you wouldn't be able to climb before reaching
Vy when overloaded.


Overload enough, and that's exactly what happens. Just because you
are not stalled is not a guarantee you will be able to climb. At some
maximum weight and density altitude, you will only maintain altitude
at one speed, and will descend at any other speed. To be honest, that
would probably require over 100% overload on most airplanes at
reasonable density altitudes, but you certainly can get there.

Assuming that you upped your weight by 25%, you
should still be able to accelerate to your new Vy speed of
sqrt(1.25)*74 = 1.12*74 = 83 kts in ground effect.


Not necessarily.

Granted, you'd
take a while longer to get there. Then you ought to be able to climb
right *after* reaching your stall speed of sqrt(1.25)*48 = 54 kts.


I repeat - just because your airspeed is over stall for your increased
weight does not mean that you have excess power available.

Granted your climb rate will be quite low since 1/ you'd be climbing
at an airspeed much slower than best rate of climb speed (which has
also increased due to the higher weight) and 2/ you'd experience
reduced excess power at that airpseed compared to the normally loaded
airplane flying at that airspeed, but you should still be able to
climb.


I repeat - not necessarily. For the purposes of this exercise, assume
the plane has a constant speed prop (we will deal with the
complexities introduced by a fixed pitch prop later). For all
practical purposes, that means the power available over a reasonable
airspeed range is constant. At best rate airspeed (Vy), climb rate
will be a maximum. Going to either side of best rate will reduce the
climb rate. Your boundaries are Vs and Vne.

Most airplanes are not so overpowered that they have sufficient power
to climb at Vne (though I know of at least one exception, a highly
modified King Air used to haul jumpers). Most are also not so
underpowered that they have insufficient power to climb at Vs, but I
know of some exceptions there too. However, if you overload enough,
you WILL eventually reach a point where sufficient power to climb at
Vs (or 1.05 Vs, or 1.1 Vs) will simply not be available.

Is it possible to become airborne in that situation? Yes it is. If
you are rolling on a smooth surface, on good tires, with the elevator
neutral and the wings at zero angle of attack or close to it, the drag
forces on a grossly overloaded airplane will not be much greater than
those on a properly loaded one. Most of the drag at typical rotation
speeds will come from parasitic drag (since the wings are at zero
angle of attack, and thus not producing lift), with some contribution
from the weight on the wheels. Parasitic drag is quite low close to
stall speed, and normally rotation speed is close to stall speed. The
increased weight on the wheels hurts some, but not much. The
increased weight will mean greater inertia, which will slow the
initial acceleration, but given sufficient runway you will easily
reach rotation speed. When you rotate, what really happens is that
you lower the tail, immediately increasing the angle of attack on the
wings. This will provide immediate lift, and you will become airborne
in ground effect. Now what?

You have eliminated the weight on the wheels, and thus reduced drag a
little. However, you have added lots of induced drag, because now the
wings are supporting the weight at low airspeed and thus a large lift
coefficient. Your total drag an inch over the runway is MUCH greater
than it is on the runway. At the airspeed you are flying, you have
insufficient power to maintain altitude and airspeed. Either you will
lower the nose (impacting the runway) or you will let the speed bleed
off (settling back onto the runway). Or you may have just enough
power to maintain altitude and airspeed in ground effect, but nothing
extra. Now you keep flying until you hit an obstacle.

A fixed pitch prop actually makes the situation worse. At lower
airspeeds, the prop can't spin fast enough to allow the engine to
generate full power. Thus power available decreases with decreasing
airspeed, and you're even less likely to have excess power available
to climb or accelerate.

Could you take a look at my Nov. 26 post entitled "Angle of climb at
Vx and glide angle when "overweight": five questions"?


I have, and really don't have much to add there unless you have a
specific question.

Michael
 




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