Where to locate the hook?
|143 forum posts|
Hi, I am putting together a simple glider as a basic step up from playing with chuck gliders for my youngest son. I am planning to tow this one up on the end of a line pulled along the field by his older brother.
My question is where do I locate the tow hook in relation to the CofG ? Should it be ahead, inline or aft? I am planning to just use a simple wire hook with a key ring and ribbon on the end of the line.
The model is my own design so no plans to guide me here. Any ideas/suggestions appreciated.
4482 forum posts
Just ahead of the cg is usually a good compromise between safe & ideal. On the cg is best for the quickest climb & highest achievable launch, further forward becomes safer & more stable but won't get up as high. Many older F/F designs had several hooks to use depending on wind strength.
|Martin Roberts||19/11/2013 11:12:52|
|88 forum posts|
the COG as you know is usually located, dependant on the aircraft design air-foil, angle of attack (incidence) etc..., at about 30% of the wing chord.
The tow hook provides a new centre of rotation (fulcrum), so if you move the tow hook behind the COG you lose all control with a resultant loss of aircraft - life- other OOOOPs. This is because the plane wants to go nose down but because the tow line is in the way the plane wants roll or yaw for the heavier than normal nose to get below the line, if you are lucky the plane starts to spin and the hook releases before you crash - a flat spin to ground is better than being dragged wing in to ground - you can imagine what happens next.
The place I find best is about 1/2 way between the leading edge and COG so that the wing rotates naturally to a positive angle of attack causing the plane to climb without elevator. Too far forward - too much rotation and collapse the wings, not forward enough and you need to use up elevator and start introducing more drag which slows the acceleration and therefore the climb potential.
If you make the tow hook adjustable, you can start with it further forward (say 1/3 from the wing tip to COG) and move it back incrementally until you get the right balance.
At least when the hook is forward the plane will also want to track straight ahead and that would tend to eliminate roll etc... as long as the plane is balanced along the nose to tail axis - this can be done by placing a pin in the nose and another in the fin (near the top) before the rudder is attached, and settling the two pins on stands so the airframe is not touching anything, then balance the plane until the wings are level.
Happy flying - Marty
|143 forum posts|
Thanks for this. Between the GoG and the leading edge sounds as though it's about right. Thanks Martin for the detailed breakdown, all makes a lot of sense.
Hoping to have it ready for the weekend so if all goes well and the weather is kind I will report back once it's flying.
|Tom Satinet||20/11/2013 14:51:34|
519 forum posts
Just to say - you can definitely launch a plane with the tow hook behind the flying CG. When you apply a very heavy load to the point where the tow hook is, that essentially becomes your new CG. Imagine dangling a 20kg block off the tow hook and then measuring the CG. The weight of the glider would not make much difference - the CG would basically be where the tow hook is. You can trim the model to fly on the launch with the elevator.
That's not to say having tow hook on or behind the CG is the best place to start or even the best place to get a good launch. Every model is different!
I would go with the a few mm in front of the CG
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