Tow hook position relates to CG or to CL? - RC Groups
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Jun 03, 2004, 10:58 AM
Gustavo Exel
gustabmo's Avatar

Tow hook position relates to CG or to CL?


Hello all,

It's my understanding that the tow hook position has to be adjusted according to the CL (center of lift) and has nothing to do with the CG (center of gravity = center of mass).

In practice we adjust the CG a little ahead of the CL and as the CG is easier to find, we use it as a guide to place the tow hook.

I'd love to hear your comments on that.

Tks,
Gustavo
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Jun 03, 2004, 11:45 AM
What's wrong with heavy?
dephela's Avatar
We use the CG as an indicator of where to put the hook. It's not the CG that counts.

First you have to go fly the plane and set the CG through flight testing. After getting the CG as aft as you can handle it and as close to the Neutral Point[of longitudinal stability], then it's OK to place the towhook position. The attempt should be made to get the CG as aft as possible and not necessarily "where you like it" for this part of testing. You'll know when you get really close to the NP, your heart willl be in your mouth and you'll be trying to figure how to get the plane on the ground, except that you don't want to be flying it anywhere near the ground the way it's flying now!

At the extreme loading during the launch mode of flight, the CG doesn't matter much.

I've seen several planes launched with ballast broken loose and moved to someplace down in the tailboom where it makes the plane totally unflyable once launched, but while they were on the line, the pilot didn't have a clue as the plane still "flew" well. As observers off to the side, we could SEE the dark block of material in the boom!

Dennis
Jun 03, 2004, 04:34 PM
Gustavo Exel
gustabmo's Avatar
Thanks a lot for the reply... that pretty much confirms what I was thinking!

Gustavo
Jun 03, 2004, 04:39 PM
RCGroups Author
SoarNeck's Avatar
Dennis has pretty much summed it up. I'll only add an easy way to remember these concepts. The c/g in flight is the "lever point" or fulcrum about which the elevator acts. The model "balances" on that point in the air. On launch, that point is the towhook, since it's basically held against the line there. Any changes in pitch by the elevator will act through the towhook "balance point".

Actually...that might not be the "easy way" to remember it...but it works for me!
Jun 03, 2004, 05:00 PM
Gustavo Exel
gustabmo's Avatar
Quote:
Originally Posted by SoarNeck
Dennis has pretty much summed it up
He sure did... I had some related questions ready to post here as soon as I got the first replies but his message answered all of them. Thread closed!

Your observation was very pertinent too. I already had the concept clear in my mind but your analogy brought a different and interesting point of view.

Tks,
Gustavo
Last edited by gustabmo; Jun 03, 2004 at 05:04 PM.
Jun 03, 2004, 11:02 PM
What's wrong with heavy?
dephela's Avatar
SoarNeck, nice addition, thanks!