In article , Andreas Maurer
writes
On 6 Jan 2004 08:21:29 GMT, Ray Payne
wrote:

ASW 27B is only certificated for aerotow operation when the forward tow
release is used!

This is correct.
And an ASW-27 where no nose hook is installed is certified for aerotow
operation on the CG hook.

A point I made in an earlier contribution to this thread seems to be
being missed but I think is important. That is, where a glider has two
hooks, the rear hook can be placed close to the true C of G position.
With only one hook, the position will normally be somewhat forward of
the C of G position and will be a compromise rather than a true C of G
hook.

In other postings people talk generally about "CG hooks" without making
the above distinction, which could be critical to handling on the
launch. "Belly hook" might be a better term, and many will not be true C
of G positions unless a nose-hook is also fitted.

--------------------

In more detail:

Gliders with only one tow hook:

The hook will be under the belly rather than in the nose, so that it can
be used for both winch/auto tow AND air tow.

Test and certification. The hook will be tested for both wire and air
tow launches before the initial C of A is granted for the design.
Testing will be at a number of glider C of G positions, the most
critical being the aft CG case because it is the least longitudinally
stable.

The hook is unlikely to be aft enough to be strictly a "C of G"
position, so that natural longitudinal stability while on air tow is
maintained. This is rightly a certification requirement.

During a winch or auto tow launch, a positive pull force will be needed
to keep the right climb angle. With some gliders this pull force can be
quite high, but at least is safe in the sense that if the cable breaks,
the nose falls quickly as soon as back-pressure on the stick is relaxed.


Gliders with two tow hooks:

Front hook, for air tows. Straightforward. Better longitudinal
stability while on tow, more directional stability before takeoff. Less
tendency for a "tug upset". Better all round for air tow.

Winching on a nose-hook. One of my early clubs (RAF Swinderby, near
Lincoln, UK) had an old Cadet Mark 1 glider which only had a nose hook.
It also had no instruments! We used to winch launch it and were lucky
to get 600 feet. The back-pressure on the stick to get even this low
height, was considerable. How did we know that it was 600 ft? The CFI
borrowed an altimeter from the RAF Instrument Section and we carried it
in a pocket, consulting it when off the wire. Those were the days !

Rear hook.
Because there is a front hook, the rear hook can be placed close to the
true C of G position. That is, where the vector of the pull of the wire
during the main part of a winch or auto-tow passes close to the glider C
of G. This minimises the hard pull force otherwise necessary to climb on
a wire launch, and maximises the launch height.

Flight Manual and Testing. Such a hook may not be included in the
flight manual conditions for air tow and may not have been tested on air
tow during initial certification testing.

Back pressure during launch. During a winch launch there should be a
pull force, but not a large one.

At aft CG, little if any pull force may be needed on the winch and such
a condition can even be neutrally stable longitudinally (relax stick
pressure during the launch and the pitch angle hardly changes). Under
these conditions it is easy to climb too steeply and break the wire,
particularly with a heavy glider. Answer, look sideways at the angle
that a wing is cutting the horizon to judge pitch angle. At aft CG it
is easy to enter a spin, so careful handling is needed on cable break
recovery.

--
Ian Strachan
Lasham, UK