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Catapult Build for Ducted Fan Models

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Roy Wellington10/11/2019 16:43:25
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32 forum posts

Does anybody have a plann for building a CATAPULT for EDF models. I want to launch my 50" EDF Canberra.

Alternatively does anybody know where one might purchase one. I am aware that they are advertised in the US for 1400 Dollars - far too expensive foe me!!

Cliff 195910/11/2019 17:25:59
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283 forum posts
60 photos

Hi Roy, check out YouTube, there are quite a few videos and designs there, I was thinking about building one myself. From what I saw 8mm surgical tubing in the way to go but where to get it is the question.

Doc Marten10/11/2019 19:25:46
386 forum posts
4 photos

I'm pretty sure Hobbyking do one.

Steve Balaam10/11/2019 19:47:57
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110 forum posts

Have a look here **LINK**

kc10/11/2019 20:08:46
6079 forum posts
169 photos

Nigel Hawes article in RCME March 2003 gave a plan. The magazine is available here

leccyflyer10/11/2019 21:36:31
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1290 forum posts
302 photos

Catapults are rather cumbersome and not very successful in my experience, having imported one from the USA some years ago, of the type used for aircombat launching funfighter sized models. My Bearcat made a shuddering ascent of the steep, 6' long ramp on the carrying cradle and simply fell off the end - the catapult didn;t throw it a full fuselage length.

What are described in the links above are bungee launchers, with a launch pedal - surgical rubber bungee and a dog stake. Some incorporate a very shallow ramp, raised a few inches from the floor, and these do work well, if your site permits their use.

Ron Gray10/11/2019 22:31:49
1550 forum posts
367 photos

I built a bungee launcher for my Multiplex Funjet. It’s made from some uPVC waste pipe and has a launch foot pedal. Bungee itself was bought off Hobby King, 8mm dia tubing. Works a treat and saves me worrying about the finger slicing 6s motor and severe torque roll from a hand launch.

Roy Wellington11/11/2019 14:04:54
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32 forum posts

Thanks to everybody who have sent replies to my query about catapult and bungee launchers. Most helpful.

Gordon Whitehead 111/11/2019 14:12:21
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334 forum posts
153 photos

Here’s my bungee launcher based on a design by Bob Partington which we dubbed the "Bob Sleigh". The pics show the components of the bungee cord and ramp. These days I prefer the pin in tube method over the ring and hook, having broken a bungee hook previously.

001 - bungee components.jpg

002 - stake cord.jpg

003 - bungee cord knot.jpg

004 - ring & pin.jpg

Pic below shows pin-in-tube launch connector in Westwings Hunter.

005 - pin & tube in ww hunter.jpg

The foot release uses a yacht cam cleat. The sideways foot movement does take a bit of getting used to, but there’s no ring. There’s a risk that elasticity in the stretched foot release cord can flip the ring forwards and wrap the cord round the tailplane during launch. The plate is secured by two tent pegs, and the third tent peg acts as a safety catch, only being removed just before launch. The cleat is secured to the plate by two 5mm screws into T-nuts.

008 - foot release and safety pin.jpg

006 - cam cleat foot release safety pin in place.jpg

Next pic shows cam on point of releasing cord as trigger is moved sideways with my foot.

007 - cam cleat releases cord.jpg

Gordon Whitehead 111/11/2019 14:20:27
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334 forum posts
153 photos

The ramp is a 1/4in thick exterior plywood board with folding legs and which is sized to fit on the floor of the car boot with the rear seats folded forwards. The legs fold so that the ramp takes up minimal space when laid in the back of the car. The point of using a board is that planes on which the tail is lower than the wing such as the Lightning, TSR-2 etc risk catching the tail on the framework of a pipe ramp. This eventuality cannot occur with a board.

009 - 4ft x 2ft ply ramp.jpg

010 - ramp pvc pipe legs.jpg

011 - folding leg mounting.jpg

012 - leg brace 1.jpg

013 leg brace 2.jpg

014 leg folded flat.jpg

When set up for launch, note how the rear legs elevate the back of the ramp so that the release cord follows the same slope as the ramp.

The ramp slopes at no more than 10 degrees.

I measure bungee tension with a spring balance, and use 4x model weight, ie a 3lb model has 12lb bungee tension.

015 low winger ready for launch.jpg

The Hunter pics show wing tip supports from pipe lagging and the Lightning needs taller supports which peg into the edges of the board seen in later pics.

016 - mid winger ready for launch.jpg

017 - twang.jpg

The Lightning needs the slipper seen in the pics. Without the slipper the belly tank pivots the nose down when the plane reaches the end of the ramp. The slipper is made from foam plastic with a solarfilmed liteply base. It only flies about 3ft from the end of the ramp during launch.

If I’d fitted the ventral strakes to the Hawk, that would have needed the slipper too.

018 - high winger on slipper.jpg

019 - packaging foam rest on slipper.jpg

Gordon Whitehead 111/11/2019 14:39:35
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334 forum posts
153 photos

The next pic shows solarfilm base of slipper.

020 - solarfilm base on slipper.jpg

Next pics shows recesses in packaging foam to accommodate bungee ring and fuselage hook.

021 - ring and hook recess on slipper.jpg

022 - ring and hook recess on slipper.jpg

I used this ramp for many years. We called this one "Gordon's Grasshopper". If the grass is long, the stake cord passes over an A-frame. The frame is just two laths of wood linked by one screw as a pivot, and the cord passes over the vee at the apex.

023 - a-frame to raise bungee over long grass.jpg

Launch sequence follows. Note wing supports for Lightning, and also the way the nose dips at end of ramp as model pivots on tank - cured by later use of slipper.

024 ready.jpg

Model seen on ground is Ron Laden's scratch-built Eurofighter Typhoon.  Ron shot the launch and flying pics during one of our many enjoyable EDF flying sessions using this ramp set-up.

025 nose dips as hook too far forward.jpg

026 safely away.jpg

028 up and away.jpg

029.jpg

030.jpg

031.jpg

032.jpg

033.jpg

Edited By Gordon Whitehead 1 on 11/11/2019 14:43:36

eflightray11/11/2019 14:46:30
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590 forum posts
128 photos

I used a bungee launch originally, then realised it wasn't actually necessary.

Perhaps it was something to do with the models wing loading wink

.
Gordon Whitehead 111/11/2019 14:57:48
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334 forum posts
153 photos

Another variation on the Bobsleigh ramp was that of his club-mate Terry, who used a pasting board that not only conveniently folded for storage, but also had a carrying handle. Owing to Terry's seniority we called this one "Gramp's ramp" !

Gordon

Gordon Whitehead 111/11/2019 15:02:16
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334 forum posts
153 photos

That has to be the most relaxing-to-fly supersonic fighter ever, Ray yes

Gordon Whitehead 111/11/2019 17:20:13
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334 forum posts
153 photos

Not so my Frightnin' seen here surprise

Gordon

Doc Marten11/11/2019 22:43:55
386 forum posts
4 photos

Hobbyking

Stephen Jones11/11/2019 23:22:57
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2737 forum posts
1597 photos

Hi,

I made one for my Fantrainer. Link to Video

Steve.

Gordon Whitehead 112/11/2019 09:29:46
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334 forum posts
153 photos

Hi Steve, your ramp is very impressive in its apparent simplicity. Do the rails pivot up with as the foot release is pressed? What is the bungee material, and how long is it and how much do you stretch it?

A safety note for Roy's benefit which I intended to mention before is that  a very secure bungee stake is essential. It's not been unknown for the stake to come out of the ground when the bungee is stretched and come flying back to hit the operator.

The Hobbyking kit doesn't appear to include any means of linking the bungee to the foot pedal and model, and there's no instruction booklet in the pics.  Plus I can't see how the pipe joiners won't carve chunks from the wing undersides as the model exits the end of the ramp.  A length of pipe insulation on the rails would prevent this risk to damaging the model. 

Another note about the bungee stake. It's worthwhile marking it with some sort of flag. We'd carry the bungee etc to the flying site in a supermarket bag, and fix that to the stake to blow in the wind. This highlights where the stake is in the ground so that at the end of a flight you don't crash the model into it on landing.

Gordon

Edited By Gordon Whitehead 1 on 12/11/2019 09:30:11

Edited By Gordon Whitehead 1 on 12/11/2019 09:36:56

Stephen Jones12/11/2019 18:13:19
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2737 forum posts
1597 photos
Posted by Gordon Whitehead 1 on 12/11/2019 09:29:46:

Hi Steve, your ramp is very impressive in its apparent simplicity. Do the rails pivot up with as the foot release is pressed? What is the bungee material, and how long is it and how much do you stretch it?

Hi Gordon, My setup is the same as yours only My ramp is Different

I think This video will explain things better . Link it also shows my self retracting tow hook.

Not sure if i did a plan of it But if is of any use to Anyone i could draw some up.

Steve.

Gordon Whitehead 112/11/2019 20:56:46
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334 forum posts
153 photos

Hi Steve, thanks for your video which explains the foot release very well. It's a very clever design and looks to me to be more foolproof against accidental launches than the pin-through-a hole style of the H-K foot pedal.

I love your Sipa Minijet, which I believe you featured in a recent build thread. It brings back a memory of my Dad taking me to an air display at Yeadon Airport in the mid 1950's. The display included a handicap air race which had everything from piston bipes to a couple of jets, one of which was the Minijet, the other being the Miles Sparrowjet. I can't remember which was the faster jet, but a vision of the little egg-shaped Sipa flashing along at near to nought feet is still imprinted on my mind. One of the bipes was a Blackburn B-2 and there must have been a Tiggie or two as well. Leo Valentin, the "Bird Man" was there but his launch plane had engine trouble so he never got off the ground.

Thanks again,

Gordon

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