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Old 03-15-2013, 09:34 AM   #1
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Default Bungee attachment to model

I have various models that have hooks underneath for ring attachment ... catapult Bungee launcher.

Most durable of course is the piano wire bent and sticking down under the fuselage. But I want to try and avoid the extra of a hook even in ply sticking down.

Has anyone created a 'Pin' attachment that sits into a hole in model ? My worry with that is the pin angling and sticking - refusing to exit when bungee slacks of ...

I'm not looking for a Glider release where I need to switch or release by radio.

Please can people photo / link to / show / describe their attachments ? Interested to see if any other way to do it.

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Old 03-15-2013, 03:01 PM   #2
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I've used a tiny gusset made of plywood and epoxied in place at the base of the pin. I expect you could accomplish the same goal with fillet of Bondo or epoxy plus filler.

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Old 03-15-2013, 07:14 PM   #3
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There is no release that is more reliable than the hook on the belly.

Get a model hung on the high start and you will wish you still used the simple hook.

Its not really about it being durable as much as it being reliable for disconnecting at the right time.
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Old 03-15-2013, 10:29 PM   #4
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Interesting thought Nigel. Something that can withstand the rigorous stress of launch but will also just simply fall away after leaving no obvious sign that it was there.

What a nice problem.

I have a couple of ideas and will start working on them and if I come up with something that shows any promise I will certainly let you know.

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Old 03-17-2013, 07:01 AM   #5
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I've wondered about a recessed hook ... basically a hollow in the underside and the hook across it ...

Get a model hung on the high start and you will wish you still used the simple hook.

Its not really about it being durable as much as it being reliable for disconnecting at the right time
Main point of my creating this thread ... I've been involved with Hi-Starts for many years as I used to do thermal gliding as well. The hook underneath has been the standard way ... but maybe there is another ?

I've used a tiny gusset made of plywood and epoxied in place at the base of the pin. I expect you could accomplish the same goal with fillet of Bondo or epoxy plus filler
You still have the hook proud of the body. Good idea though.

Ok - let me throw another factor in ... My car has nice leather seats and carpeted cargo area ... the bent pin hook is not something I want catching my seats or carpet !! I've also caught my hand a nasty scrape on the hooks. You can't put a cap on the end as then the bungee can get 'hooked' ...

So far - I think the only way to not have a hook sticking out underneath is the recessed hook ... leaving aside the glider release gear.

Interesting problem.

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Old 03-17-2013, 08:39 AM   #6
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It seems to me that recessing the hook would end up increasing the weight of the structure as well as sacrificing valuable space inside the fuse near the CG - which is where you want to try to have as much of the weight of servos, radio gear, etc as possible. Its important to try to keep the mass of the sailplane concentrated near the CG as possible. Sailplane fuses do not usually have copious amounts of excess space inside either.

You wouldnt gain anything in the way of drag reduction because of the resulting open hole around the hook. In fact, I am pretty sure it would have a lot more drag than a simple hook.

If you're worried about snagging or tears due to the hook, maybe it shouldnt be so sharp Aside from rounding off the point, you could always just get a largish cork or rubber stopper and stick on the "point" if you insist on having a sharp hook.

That said, I saw a model years ago with what sounds like the solution to your problem.

The model had a tube recessed into the fuse that was sized to be larger in diameter than a pin that would be attached to the hi-start end.

This tube was inserted just ahead of the CG and angled up into the fuse but was tilted toward the front (Im guessing) about 45 degrees. I never saw or dont remember any of the interior details. This was back in the 70's....

On the end of the hi-start is a pin somewhat smaller than the tube. Insert the pin into the tube and tension the hi-start and it will stay in - hopefully until you reach the top of your launch.

I dont think I ever saw him launch it, so no idea how well it worked. I remember the glider was a very very beat up 2 meter something. Maybe an old (new then) 2x4 or similar with a lot of miles on it. Couldnt say if the wear and tear was due to failed launches or just bad piloting in general.

Obviously the idea never caught on though

I think I need a signature.
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Old 03-17-2013, 09:14 AM   #7
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I'm not flying gliders now - if I was - I'd use the Tx controlled release.

I'm looking at EDF's and pushers ... where hook point is more forward than gliders ... ideal I've found is max 40% back from nose.

The tube and pin idea is actually workable and has been used by people I know - BUT you have to be extremely careful about maintaining the pin etc. clean and straight. If it is not pristine - it locks at an angle in the tube ... seen it happen to a nice thermal glider years ago.

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Old 03-17-2013, 09:29 AM   #8
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I am wondering if using a solid carbon rod instead of a metal pin would be more reliable. It wouldnt bend and if the end was rounded it shouldnt gall up the inside of the tube. It would also have less friction than metal on metal.

Might be worth trying.

I think I need a signature.
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Old 03-18-2013, 08:18 AM   #9
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When you think about it - the tube the pin goes into can be excessively larger than the pin anyway - all it does is provide a hard surface for the pin to act on.

Taking this a step further ... it does not have to be a pin ....

If a hollow is made in underside of fuselage ... lined with hard material or epoxy ... with a forward angled rake to the front section ..
Bungee line has a V ply or similar 'hook' to it ...

Engages in hollow ...

OK - lets think a bit laterally ... we always assume the ring should be on bungee line and hook on model ... why not swap over ? Have a ring embedded across the fuselage, with sufficient proud to have hook on line able to 'hook on' ..... you get same ability to fall away - but hook is now on the line. You cannot have ring longitudinally, it needs to be transverse.
With this method - you can still use the line on a conventional hook equipped model - just connect hook to hook ...

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Old 03-19-2013, 10:52 AM   #10
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Nigel, that all sounds reasonable except for putting the hook on the tow line. That wont work.

The reason is that, as far as that hook is concerned, the "pull" direction is always exactly in line with the tow line and will always be at exactly the same spot on the hook - where it touches the cross bar. The hook will never get into a position where it pulls off the cross bar because it will never go over-center no matter what the model does.

Try it with a string and hook set up and you will see that as long as there is any tension on the tow line, the hook will not pull off the cross bar.

I think I need a signature.
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Old 03-19-2013, 01:04 PM   #11
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Nigel, what about something like a spring latch set up. Push once and the pin head sits flush, push second time and the pin head/shaft pops out slightly.

Pin shaft could have a slight notch on the back side for your ring to hang onto.

For transport, push the pin in and it's out of the way. No snagging or poking holes in things. For use, push the pin and out pops your "hook" so to say for launching.

Heck it could even act as a quasi shock absorber, as it's spring loaded the pin could compress slightly on contact with mother earth.


Hmm, now you got me thinking. Trip to the hardware store in the future.

Would work for display too. No butt ugly hook hanging off the nose of the plane. No hook to catch on the flying wires of one of the other planes on the rack. (don't ask)

When I die, I want to go like my Grandfather did, in his sleep...... Not screaming like the passengers in his plane.
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Old 03-19-2013, 01:38 PM   #12
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Keep ideas coming ... I have 2 models waiting for final decision ... Twister and F16 ....

I've made new ramp in 3/4 plastic pipe and wider to accommodate these bigger jets but still suitable for my small stuff.

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Old 03-19-2013, 09:57 PM   #13
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If you're bungee launching jets, thats a different story from sailplanes. With the jets you will want the tow point much further forward of the cg than with a sailplane.

Having the tow hook back causes the model to stay close to 90 degrees to the tow line so that the model achieves maximum altitude - but at the cost of air speed.

Your jets wont want to kite up like a sailplane. In fact, unless you have a very effective rudder it would likely be suicide to try it like a sailplane.

The jets will need the tow point much further forward so that they get a lot more forward speed and a lot less altitude gain.

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Old 04-14-2013, 06:35 PM   #14
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Originally Posted by Larry3215 View Post
If you're bungee launching jets, thats a different story from sailplanes. With the jets you will want the tow point much further forward of the cg than with a sailplane.

Having the tow hook back causes the model to stay close to 90 degrees to the tow line so that the model achieves maximum altitude - but at the cost of air speed.

Your jets wont want to kite up like a sailplane. In fact, unless you have a very effective rudder it would likely be suicide to try it like a sailplane.

The jets will need the tow point much further forward so that they get a lot more forward speed and a lot less altitude gain.
Please go back to a previous post of mine where I make exactly that point. Reason is - Glider has no thrust or power of it's own and you are looking for maximum climb on the bungee ... so hook is back about 60% or more. On a powered model - you only want air-speed to get flight.

Back to hooks .......... when looking at an old Jetex plan for a model ... I noticed a catapult hook illustrated ... instead of being a single wire bent to shape - it was a double wire. ie Length of wire bent in middle to form the hook part ... the ends then buried in the model fuselage. This meant the working end was rounded off and safe. Why didn't I think of that ... KISS !

All my hooks will now be replaced with the doubled wire type .........

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Old 04-15-2013, 03:16 PM   #15
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Originally Posted by solentlife View Post
....Ok - let me throw another factor in ... My car has nice leather seats and carpeted cargo area ... the bent pin hook is not something I want catching my seats or carpet !! I've also caught my hand a nasty scrape on the hooks. You can't put a cap on the end as then the bungee can get 'hooked' ...
A solution to the snagging issue might be to add a humped wire "guard" each side of the hook. The height of the guard would be => the height if the hook. Wire skids in effect, with the hook within their protected area. Such an arrangement would still be proud of the fuselage and not as sightly as a recessed hook. I've used a version of this on heavy machinery to guard against accidental arming of a toggle switch. Still, if esthetics are an issue, this would be a no-go.

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Old 04-15-2013, 06:03 PM   #16
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Originally Posted by Arkitexas View Post
A solution to the snagging issue might be to add a humped wire "guard" each side of the hook. The height of the guard would be => the height if the hook. Wire skids in effect, with the hook within their protected area. Such an arrangement would still be proud of the fuselage and not as sightly as a recessed hook. I've used a version of this on heavy machinery to guard against accidental arming of a toggle switch. Still, if esthetics are an issue, this would be a no-go.

Rick
Ships have similar on switches that are not intended for other than emergency use ... good idea ...

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