CG Hook Location Computation

Udo wrote:
wrote:

When the aircraft manf has not done the research or made any
recommendations - how is the location for a CG hook computed?

One example - the SGS 2-33 installations I have seen (not factory) seem
to be far ahead of the actual CG.

TNX

Michael


I do hope that will help a little.
The location is dependent on the gliders total C of P location, but
primarily that of the wing airfoil. Also the height of the fuselage
determents the
relative position. Once the optimum location is calculated, a line is
drawn
from that point usually at or near the wing / airfoil to the bottom of
the fuselage
relative to the position the glider takes, when in full launch. A line
is then drawn
from the calculated point and the intersection at the bottom of the
fuselage.
A glider that has a tall cockpit, the hook appears much further
forward.
Udo


Anyway the words "GC hook" are a misnomer, since I know no glider where
this hook is juste below the CG, all have it more or less ahead of the
CG. "Winch hook" or "ground launch hook" would be more appropriate.
I think the place is determined so that the glider will be stable
at nearly 45 degrees climb with the cable pulling nearly horizontally
and elevator neutral. If it was juste below the CG it would generate
a pitch up moment in these conditions.

Robert Ehrlich wrote:
..../...
Anyway the words "GC hook" are a misnomer, since I know no glider where
this hook is juste below the CG, all have it more or less ahead of the
CG. "Winch hook" or "ground launch hook" would be more appropriate.


I've not measured any full size gliders, but the following procedure
gives the hook position for free flight towline model gliders. I use it
to set up F1A class models:

- locate the CG on the wing's datum line.
- project a line downward from this point and sloping forward at 18
degrees from the vertical.
- the line should pass through the towline attachment point on
the hook.

A tow hook fitted at this point gives a good stable, controllable launch
for a model with its trim fixed to glide at its minimum sinking speed.
Note that this procedure will place the tow hook further forward on
models with a high wing mount than on those with a minimal wing pylon.


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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