Keith Goodwin
June 29th 04, 10:56 AM
Hi everyone.
This is an extremely interesting thread, and I have been reading avidly.
As a glider pilot, based in the U.K (not that my location makes the blindest
bit of difference), I am reasonably familiar with the process of using a
cable to get airborne, whether behind another aircraft (the tug), or from a
winch. Though I will be the first to admit that the weights and sizes etc.
are a LOT smaller than everyone is talking about here, the principles are
the same.
My physics is not up to 'proving' on paper any of what I am about to write,
but if you don't believe me, then come for a ride in either of my gliders,
or any glider come to that.
The towing aircraft (tug): -
In order to tow an aircraft successfully, the best position for the tug to
have the hook is as far astern as possible. This allows the cable to be
free of the tail and fin when taking off and in flight. It also means that
you are only slightly upsetting the balance of the tug. However, if the
towed aircraft lifts too high above the tug, or kites (I think that's the
proper term for it) it will lift the tail of the tug. If the tug pilot or
glider pilot don't release immediately it will be fatal for the tug pilot.
If you are flying a 747 at close to vne and are forced into an extreme tail
high attitude... I leave the results to your imaginations.
You could put the hook on the c.g. and lift the cable above the fin for
straight and level, turning and climbing, but that would put an extreme
amount of rotational force onto the 747, pulling up the nose, and forcing
down the tail. Even if the 747 has enough elevator authority to cope with
those forces, it would increase the angle of attack and raise the stalling
speed.
You could of course make a rig to change the position of the cable
attachment during flight, i.e. Have the cable hooked at cg, but laid down
the length of the fuselage and hooked at another point at the rear of the
tug, obviously routing the cable to be clear of all control surfaces etc.
During takeoff and climbing the rear hook would hold the cable, but when you
wanted to 'kite', the rear hook would release and the cable would then be at
cg of the tug. This would still make for an interesting ride in the 747
though!
The towed aircraft: -
The problems are very similar for this vehicle, as for the tug. As far
forwards as possible is the best location for the hook for takeoff, straight
and level, turning, climbing etc. The cg hook can be used for aerotowing,
but the stick has to be kept forwards, otherwise the vehicle WILL kite.
So the solution for the towed vehicle is basically the same. Primary hook is
cg, secondary hook is as far forwards as practical. Releasing the secondary
hook when the kiting stage is required.
Something else occurs to me, the towed vehicle is going to have to have
extremely efficient wings to make use of the 747s power at the altitudes
being discussed here, I suspect that even the "Eta" a 30.84m span glider
would struggle at those air densities, and it has a tiny weight by
comparison, and no 20km tether to support either!
I hope this information helps, and look forward to reading all the posts
that follow.
Keith.
This is an extremely interesting thread, and I have been reading avidly.
As a glider pilot, based in the U.K (not that my location makes the blindest
bit of difference), I am reasonably familiar with the process of using a
cable to get airborne, whether behind another aircraft (the tug), or from a
winch. Though I will be the first to admit that the weights and sizes etc.
are a LOT smaller than everyone is talking about here, the principles are
the same.
My physics is not up to 'proving' on paper any of what I am about to write,
but if you don't believe me, then come for a ride in either of my gliders,
or any glider come to that.
The towing aircraft (tug): -
In order to tow an aircraft successfully, the best position for the tug to
have the hook is as far astern as possible. This allows the cable to be
free of the tail and fin when taking off and in flight. It also means that
you are only slightly upsetting the balance of the tug. However, if the
towed aircraft lifts too high above the tug, or kites (I think that's the
proper term for it) it will lift the tail of the tug. If the tug pilot or
glider pilot don't release immediately it will be fatal for the tug pilot.
If you are flying a 747 at close to vne and are forced into an extreme tail
high attitude... I leave the results to your imaginations.
You could put the hook on the c.g. and lift the cable above the fin for
straight and level, turning and climbing, but that would put an extreme
amount of rotational force onto the 747, pulling up the nose, and forcing
down the tail. Even if the 747 has enough elevator authority to cope with
those forces, it would increase the angle of attack and raise the stalling
speed.
You could of course make a rig to change the position of the cable
attachment during flight, i.e. Have the cable hooked at cg, but laid down
the length of the fuselage and hooked at another point at the rear of the
tug, obviously routing the cable to be clear of all control surfaces etc.
During takeoff and climbing the rear hook would hold the cable, but when you
wanted to 'kite', the rear hook would release and the cable would then be at
cg of the tug. This would still make for an interesting ride in the 747
though!
The towed aircraft: -
The problems are very similar for this vehicle, as for the tug. As far
forwards as possible is the best location for the hook for takeoff, straight
and level, turning, climbing etc. The cg hook can be used for aerotowing,
but the stick has to be kept forwards, otherwise the vehicle WILL kite.
So the solution for the towed vehicle is basically the same. Primary hook is
cg, secondary hook is as far forwards as practical. Releasing the secondary
hook when the kiting stage is required.
Something else occurs to me, the towed vehicle is going to have to have
extremely efficient wings to make use of the 747s power at the altitudes
being discussed here, I suspect that even the "Eta" a 30.84m span glider
would struggle at those air densities, and it has a tiny weight by
comparison, and no 20km tether to support either!
I hope this information helps, and look forward to reading all the posts
that follow.
Keith.