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Oct 01, 2016, 09:23 PM
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Build Log

Bungee launch setup


I thought I will list my favorite method of bungee launching aircraft.
There are a few ways people bungee launch models:

1) Use a ramp
2) From the ground
3) From a dolly

It seemed that very few people use a Bungee Dolly method. Ramps lead the way by far, followed by just straight off the ground.

Ramps work ok, but you do need to 'make one' and cart those pieces around - assemble - disassemble....
Because the plane leaves the ramp some distance off the ground it leaves more room for 'heading down', whether with Wings Level, or Wings Angled into a cart-wheel!
The plane also has to 'drag' along the ramp rails - the wings usually - so they can wear, or things could catch too. eg control horns etc.

From the ground is the easiest - you don't need to take much stuff at all. Just the main bungee part.....
But dragging a plane across the ground is not going to be great for its underside - though you do have to LAND on the underside too. But at least only landing means HALF the wear and tear! lol.

After I thought about it a lot I decided that a Bungee DOLLY was the only way to do this in an acceptable and very safe manner!!

1) The plane does not get dragged on the ground - as per ground bungee.

2) The lift-off is exactly like a take-off from landing gear. Far more gradual (angles etc) and even looks like a plane taking off from a runway! Not a plane 'thrown into the air' and 'must fly' - as per from a ramp.

3) There is no 'dragging' - the plane lifts off UPWARDS off the dolly at a shallow Angle of Attack (as per a normal runway take-off), which means a much more 'docile' and 'slow' move into whatever problem state it might head into due to offset trims etc.

So that was that... the Dolly method wins hands down!

....
Last edited by PeterVRC; Dec 03, 2017 at 05:17 PM.
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Oct 01, 2016, 09:35 PM
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Overview of bungee setup


So how do we do this... and the best way.... ????

The basics are as per any Bungee system.
1) You need some pegs to pull the bungee from, and for the foot release end.
2) You need some Bungee - or even better is Shock Cord.
3) Some strong cord.
4) You need a Foot Release mechanism
5) You need some Caribiners and Welded Rings
6) The plane needs a bungee hook.

The pegs are pretty straightforward..... Tent Pegs.... you can get quite large/long ones, and you want galvanised ones so they will never rust. I got some very large 40cm ones.

For the "Bungee" I have both, Bungee and Shock Cord - the Shock Cord is better. Though both work fine really.
For a general purpose 'able to launch anything' system, I use TWO strands of 10mm that are actually made from a 30 metres long piece. Knotted to form a loop half way and thus gives you the two 15m lengths. You can use both in parallel if needed, or just one of them for lighter aircraft.

You want about 5metres or so of strong cord, just to keep the 'stretch' part of the bungee from near the plane.

I use cheap caribiners for all the connections. Typically $2 or so each.
You also need two welded rings - stainless steel, or nickel plated - one for the plane hook, and one for the foot release.

Foot release. There are many ways to make this, but the easiest is using wood. And the 'lever' system, as you will see in the pics.

All these above parts form the 'basic' bungee system as used by all three methods. For ground launching that is all you need.

....
Oct 01, 2016, 09:51 PM
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Bungee or Shock Cord


The aim of the 'bungee' is to give you a source of energy that you can deploy via it pulling the plane forwards to a good airspeed.
The common item used for this is bungee cord. But there is also Shock Cord.

'Bungee' for models use is generally a form of surgical TUBING. It is just a tube of rubber.

Shock Cord can be one solid rubber 'rod' (not a tube), or one that is made up of many strands, which the strands type is better.
Shock cord is rubber covered in a material sheath so it is hardier than bungee. But the sheath will prevent it stretching more than two times in length - which is not a problem.

Both the above can be stretched out by you, and thus 'load' it with energy.
Picture two main aspects of this....
1) A very short high power 'stretch' - much like a bow and arrow use, or a slingshot
2) A long 'power spread out over distance' format.

Either would work, but you don't really want a plane shot off like a missile! Though that would still work....
So I think the best it too have about 15metres of 'bungee' - or at least 10 metres!
20 metres seems overkill to the long side... 10 metres is a bit short.

You also use multiple strands....
But anyway, I will outline stuff to use and how much etc. later.

Whatever you USE for the 'bungee' has to be able to be stretched to the energy level - tension - that you need for the aircraft to be launched! If you have 3 metres of bungee, at its full stretch capability you would probably be lucky to get 5 Kg of tension.
You are best to have approximately SEVEN times the weight of the aircraft, in tension.
Having more is no real problem, but it will just launch it faster - heading up the scale more towards a missile.

When you use a DOLLY that will eat up some energy due to drag across grass, so you will need a bit more tension (energy). Even just EIGHT times will do, but to have a bit more safety leeway, nine or ten times is better.

8mm Shock Cord is ok, but its diameter means it stretch limit is 'sooner' - though it is a good size to work with fairly easily.
10mm Shock Cord gives a better stretch result, but that "just 2mm more diameter" makes quite a big difference in ability to use it - knot it for ends etc.
I have some of both and they sort of end up close to equal. In 8mm you need more length and more strands. eg 3 in parallel, of 15metres each.
In 10mm you can use 1 length for many aircraft, and with 2 lengths you could launch 'anything'!

In 8mm you can just tie knots at the ends onto rings or caribiners. With 10mm you can't really tie knots, so you have to use alloy crimps! That is quite a nuisance - to get them(!) - so maybe I would have to say 8mm is a lot easier to set up.

I don't like bungee at all... though I have sets of that too. I just prefer the Shock Cord 'toughness'.

So look for 50metres of 8mm or 30 metres of 10mm.....
Oct 01, 2016, 09:56 PM
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Cord/Rope - Caribiners and Rings


You will need a few pieces of cord for various points of the setup.
A 5m to 10m piece to go from the 'bungee' to the 'plane harness'.
A few metres for the plane harness pieces.
2m or so to go from the plane to the foot release.

Something about 5mm to 6mm region, of strong cord (Google GRUNT brand to see some examples).

You can buy cheap Caribiners at hardware stores. Aluminium ones are fine, otherwise galvanised or nickel plated steel - or stainless steel.
Probably 4 caribiners.. or get a few spares... and at least two rings of about 50mm diameter.
Oct 01, 2016, 10:02 PM
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Foot Release


There are many ways you could make a foot release!
This one I made is pretty simple and 'safe'!

The release pin is a Bolt that comes up through from the bottom. So note that the 'input' to 'output' of this unit is extremely solid.
The input is the bungee ring to the pin - down along the wood base - into a tent peg that goes through a hole in the foot release rear end.

You might see that there is no weak point - as you need this thing to remain solid for sure, up to almost 30 Kg of tension through it.

The lever part of it is very robust, but does not even have to be. That release function is not a 'strength critical' thing at all. But it is easy to make it robust anyway, so I did....

Pushing down on the lever with your foot, just lift the bungee ring off the foot release pin. Simple....

Oh, the Pin is TILTED a bit rearwards. Just to make sure the ring wants to slip DOWNWARDS further onto the pin, not up and off it by itself!!

...
Oct 01, 2016, 10:18 PM
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Dolly


The dolly is another item that can be made up in many ways. The main criteria is to have large wheels - for lower drag. And a 'cradle' with a SLOPE so that the plane sits with some Angle of Attack when at rest on the dolly.

Dimension are quite lenient, but you want to try to make it 'universal' to fit any aircraft you have.

Mine is 40cm long - that can do 90mm jets fine (and anything smaller). 20cm wide - just enough to be stable and also makes the gap between the rails enough to sit any fuselage down inside of.
It is all wood (bad for wet weather!) and has foam rubber 'pipe insulation' cut up into half circles (U's) and glued onto the rails so that the aircraft never get marked or damaged,

This 'plane resting on soft rails' is the key factor that makes this a zero aircraft damage system! And the plane LIFTS UPWARDS from the rails... it doesn't ever slide along them.

You might note a wire rod that it has.....
The rod extends out the rear of the Dolly and a cord goes from that end loop to the foot release.
The wire rod also runs right to the front 'block' of the Dolly, which also has the front end cord(s) to the bungee on it.
You might see that this means the bungee energy passes right through along the wire rod - never 'through' the Dolly. The Dolly is just an attached item... a passenger... of the wire rod.

If the bungee front end as connected to the front of the Dolly, and then the cord to the foot release was connected to the rear end of the Dolly, then the Dolly would have to made strong so that IT could then pass through the energy. Over time it might break, fail etc. Thus a wire rod removes the need for making some 'extra strong Dolly'.

I used two cords from the front end to the bungee - I am not sure exactly why. You could use ONE central one! (easier!)

I had those wheels that are on it now lying around (spare parts), and it does work fine, but having even larger wheels would be of more use too.

Note the cord harness at the front - there is one lead that is the 'bungee to plane' link.
When the bungee is stretched out and set up, you only need to run the path through the Dolly (Wire Rod)and out to the foot release - with no plane on top yet.
You then put the plane onto the dolly and set the plane cord/ring onto the plane hook and slide the plane rearwards a bit to make that taught.
This means that the cord to the plane needs to be the correct length to the tow hook - but there is some leeway as to where the plane sits on the rails. eg about 10cm leeway.

When you release the bungee (foot release) the bungee thus pulls the Dolly AND the plane equally. As the plane gains more lift it will rise up off the rails and the ring will fall off its hook - and away it will fly!!

Oh... I used the long wire rod out the rear end of the Dolly so that the CORD is well after the plane.
You could use a one piece wood base plate for the Dolly and then tie the front and rear cords to that plate - to have an assured solid link right through. But some people have had the trailing cord (the piece from the Dolly to the Foot Release) FLICK back over the plane, and even CATCH onto something!
RARE .... but POSSIBLE!!!
Thus my solution to make that much harder was to have the wire Rod "TAIL" running well after the rear of the plane!
I might be overkill... LOL... but better safe(r) than sorry! It is just an improvement to the system!

....
Last edited by PeterVRC; Dec 03, 2017 at 05:15 PM.


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