Catapult Build for Ducted Fan Models

[Roy Wellington]

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 1959]

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.

[Former Member]

[This posting has been removed]

[Steve Balaam]

Have a look here **LINK**

[kc]

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

[leccyflyer]

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 Gray]

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 Wellington]

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

[Gordon Whitehead 1]

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.

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

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.

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

[Gordon Whitehead 1]

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.

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.

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.

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.

[Gordon Whitehead 1]

The next pic shows solarfilm base of slipper.

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

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.

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.

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.

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

[eflightray]

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

Perhaps it was something to do with the models wing loading

.

[Gordon Whitehead 1]

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 1]

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

[Gordon Whitehead 1]

Not so my Frightnin' seen here

Gordon

[Former Member]

[This posting has been removed]

[Stephen Jones]

Hi,

I made one for my Fantrainer. Link to Video

Steve.

[Gordon Whitehead 1]

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 Jones]

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 1]

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

[Gordon Whitehead 1]

Re the H-K bungee setup, there's a video here which shows how to set it up and avoid some possible pitfalls. The ends of the ramp are designed to not snag the wing undersides, so I stand corrected there. Whether the distance between the ramp rails is compatible with the distance between the nacelles on a 52in span Canberra is something to consider, and could be cured by changing the lengths of the cross-tubes. The safety pin mod is worthy of note, as are the comments regarding tailplane clearance, and ensuring the bungee pin is well pushed down. Personally I'd use a screw picket/dog stake for that job.

Hope this helps

Gordon

[Stephen Jones]

Hi Gordon,

Thank`s for the nice comments.

When the load is on the peg it locks that pedal up and needs a good press down to release it.

Just watched the HK video Funny thing that cord arrangement is setup the same way as mine.

Also worth noting is that wind direction can change pushing the model on take off to one side if the wind is not head on.

So my solution was to stake out two ends of the bungee and the Centre to have the cord attached and pulled back to the model that way you are guaranteed the model will launch in the desired direction.

Steve.

[Gordon Whitehead 1]

Hi Steve

If I understand correctly, you stake the bungee elastic down at both ends at right angles to the line of flight, and then fasten the cord linking it to the plane at the centre of the elastic. This then has the effect of preventing the bungee elastic being deviated by the wind. During launch, which doesn't take long of course, the model on the end of the linking cord will be pushed a bit sideways by a side wind, but far less than if the cord was in line with the whole length of the stretched elastic.

We fly on a narrow field bounded by trees, often with cross-winds, and if that idea works well it's worth bearing in mind for future reference. The only possible downside seems to be that there are two bungee stakes to try to miss on landing

BTW, I think I also saw another thread about the Miles Student in your avatar. I can't remember if it was ducted fan or pusher, or which forum it was in, but I do enjoy reading design and build threads. After partaking in this thread I'm beginning to feel like having another go at a li'l EDF, having had much joy from the little ones several years ago. Must finish my current bipe project first, though.

Gordon