CG Hook Location Computation

[[email protected]]

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

[raulb]

Michael,
It sounds like you are describing what is called a "compensation hook."
That is, it it located somewhere between a nose hook and a CG hook.
It is better for ground launching than a nose hook is but not as good
as a CG hook. Likewise, it is better for aerotowing than a CG hook but
not as good as a nose hook.

I am really surprised that no one answered you sooner.

Raul

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

[raulb]

I forgot to add, the CG is usually (but not necessarily always) figured
to be the leading edge of the wing and all calculations are made from
the CG. It is where everything in front of the CG and everything
behind the CG balance. See the Weight and Balance calculations for the
2-33 (or any glider), which should be in the glider's flight manual.

Raul

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

[Jack]

raulb wrote:
> I forgot to add, the CG is usually (but not necessarily always) figured
> to be the leading edge of the wing and all calculations are made from
> the CG. It is where everything in front of the CG and everything
> behind the CG balance. See the Weight and Balance calculations for the
> 2-33 (or any glider), which should be in the glider's flight manual.


My handy SGS 2-33 manual shows the reference point for CG computations
to be Station "0", just slightly aft of the nose of the aircraft. The
"Datum", or leading edge of the wing at the root is at Sta. 66.34".

The CG range for all 2-33's when flown is Station 78.20" to Sta. 86.10",
or 7.9" wide, from 11.86" aft of the Datum (wing root L.E.) to 19.76"
aft of the Datum.

The empty weight CG for a representative 2-33 is around 96.0", with the
main gear axle center at 88.75", which is why 2-33's sit tail low on the
ground when empty.

The text of the 2-33 manual mentions both a "forward" and "CG release",
but the only release I've ever seen on a 2-33 is just ahead of the skid,
and I suppose that is what Schweizer calls a "CG" release on the 2-33.
The skid would interfere with a release nearer the forward edge of the
CG range.

Anybody here know where the "forward release" on a 2-33 would be?


Jack

[chipsoars]

Jack wrote:
> raulb wrote:
> > I forgot to add, the CG is usually (but not necessarily always) figured
> > to be the leading edge of the wing and all calculations are made from
> > the CG. It is where everything in front of the CG and everything
> > behind the CG balance. See the Weight and Balance calculations for the
> > 2-33 (or any glider), which should be in the glider's flight manual.
>
>
> ground when empty.
>
> The text of the 2-33 manual mentions both a "forward" and "CG release",
> but the only release I've ever seen on a 2-33 is just ahead of the skid,
> and I suppose that is what Schweizer calls a "CG" release on the 2-33.
> The skid would interfere with a release nearer the forward edge of the
> CG range.
>
> Anybody here know where the "forward release" on a 2-33 would be?
>
>
> Jack
> My handy SGS 2-33 manual shows the reference point for CG computations
> to be Station "0", just slightly aft of the nose of the aircraft. The
> "Datum", or leading edge of the wing at the root is at Sta. 66.34".
>
> The CG range for all 2-33's when flown is Station 78.20" to Sta. 86.10",
> or 7.9" wide, from 11.86" aft of the Datum (wing root L.E.) to 19.76"
> aft of the Datum.
>
> The empty weight CG for a representative 2-33 is around 96.0", with the
> main gear axle center at 88.75", which is why 2-33's sit tail low on the

We had a 2-33 at PGC with what I understood was a factory kit. As I
recollect, it was installed on the right side of the ship, alongside of
the skid about the skid midpoint. The ship was N33966 - I don't
remember who bought it, but you can look it up and perhaps get more
information.

Chip F.

[Udo]

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

[raulb]

Jack wrote:
>
> The text of the 2-33 manual mentions both a "forward" and "CG release",
> but the only release I've ever seen on a 2-33 is just ahead of the skid,
> and I suppose that is what Schweizer calls a "CG" release on the 2-33.
> The skid would interfere with a release nearer the forward edge of the
> CG range.
>
> Anybody here know where the "forward release" on a 2-33 would be?
>

Excuse me? Somehow I do not think you understand the term "CG."

No matter what Schweizer calls it, the "nose hook" is not located at
the Center of Gravity (CG) and thus cannot possibly be the "CG hook."

The nose hook is in front of the skid. The compensation hook is
mounted somewhere mid-skid and offset to the side slightly so as to
avoid the skid. The CG hook is mounted on (or very close to) the
Center of Gravity possibly also offset so as to avoid the skid. I have
a friend with a 2-33 that has all 3 hooks.

[Jack]

raulb wrote:

> The compensation hook is
> mounted somewhere mid-skid and offset to the side slightly so as to
> avoid the skid. The CG hook is mounted on (or very close to) the
> Center of Gravity possibly also offset so as to avoid the skid. I have
> a friend with a 2-33 that has all 3 hooks.

Curiouser and curiouser.

Three hooks now, is it?

Can you get your friend to post photos?


Jack

[raulb]

Jack wrote:
> raulb wrote:
>
> > The compensation hook is
>
> Curiouser and curiouser.
>
> Three hooks now, is it?
>
> Can you get your friend to post photos?

Do you own homework. See the diagram in The American Soaring Handbook,
volume 3, Ground Launching, page 10.

They call the "compensation hook" the "compromise hook." Symantics.

[Jack]

raulb wrote:

> Do you own homework. See the diagram in The American Soaring Handbook,
> volume 3, Ground Launching, page 10.

Thanks for your help, raulb.


Jack

[Robert Ehrlich]

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.

[Martin Gregorie[_1_]]

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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gregorie. | Essex, UK
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