Dolly for launching EDFs

[MattyB]

On 18/05/2023 at 15:49, Paul De Tourtoulon said:

I can't see the point of your RC 'trigger' Ron, my Asw 19 floats off of my dolly.

A large scale glider on a big wheeled dolly towed by a powerful tug is rather different from a small EDF with low thrust on a small wheeled dolly on a less than bowling green patch. There is a reason the vast majority of successful EDFers use a catapult.

[Dickw]

2 hours ago, leccyflyer said:

The repaired Gnat and Jet Provost had another try yesterday, with exactly the same results -all from good strong hand launches. The Gnat just goes down 10 feet in front of the launch, it simply doesn't have enough power. Will have to make some measurements and a rethink. It has flown previously, but IIRC did fall out of the sky during that flight. 

........................................................

 

I am sorry to hear you are having problems with the Gnat - mine gets away every time from a decent launch (but I am on a 4s setup).

One thing I did was set up a launch mode with elevators up about 1mm and both ailerons down 1mm just to help in those first few seconds after launch while it is slow and I am getting my hand back on the sticks (right hand launch and mode 2 flyer). Once in the air it is great to fly.

Have you read through the Provost/Gnat thread on this forum to see if there are any suggestions in that?

 

Dick

 

[leccyflyer]

Thanks Dick  - yes - I'd read through the thread and downloaded the PDF of the build articles. For yesterday's launch I'd put in a few clicks of up-trim for the Gnat, to no avail. I need to weigh the model and measure what the fan is giving in terms of thrust. I have some 4s1p 1800mah packs which I could try if I fitted the 4s fan instead of the 3s fan unit. I know that it has flown, because I took pictures of it in flight a couple of years ago-

 

including a blurry one of it subsequently falling out of the sky 🙂

.

[Dickw]

Yes, good idea to check weight and thrust.

The only thing I can suggest is to make sure the cheat hole is at least as big as shown on the plan (mine is a few mm bigger all round) and there are no obstructions in the airflow in front of the fan.

Good luck 👍

 

Dick

[Simon Chaddock]

Now if you build really light.........

https://www.youtube.com/watch?v=yuRAhIgO50I

A short edited video of an early test flght.

A big bigger than the Nijhous version so no need for a cheat hole 😉 with a 50 mm EDF but at 12 oz (340g) with a 1500 mAh 3s it is quite a bit lighter. You can hear on the video it can fly on much reduced throttle.  

 

Edited May 21, 2023 by Simon Chaddock

[leccyflyer]

Cheat hole is as on the plan. Think I've got the answer to the lack of oomph though. Model weighs 664g (23.5 oz) and the static motor thrust, simply measured on a zeroed kitchen scale at WOT was a measly 202g, before the rolled acetate thrust tube departed towards the ceiling.

[john davidson 1]

The one in the vid certainly sounded much stronger than your one!

[leccyflyer]

It did John. Once I've figured out how to replace the thrust tube without needing to undertake major surgery, I'll recheck the ESC programming and retry the fan. If that doesn't improve it might mean doing what others have done and fitting a 4S unit instead.

[Chris Walby]

Two options spring to mind from lots of shall we say bitter experiences. 

Dollies, don't bother unless you have good power to weight ratio and the model has a benign stall (worst thing you can do is let it off the dolly if its doesn't have enough go to climb (a bit) and gain air speed to gain control. If you are prepared to retrieve a dolly then the bungee is a better option in my book. Pretty much guaranteed rapid acceleration in the right direction until the control surfaces become effective. The Vulcan I look after has very poor thrust to weight ratio, pusher props and is very heavy (IIRC 6kg) so a dolly, bungee with 25Kg and 20m long, pulled out close to 40m gets the man hold cover, sorry Vulcan into the sky.

 

Now there is an alterative and no doubt some will say you don't need it or its cheating or something else, but it works for me and others that embrace technology. 

I have an SU47 that has a good few issues (power to weight, floppy wings and some plain weird flight characteristics), which means we hand launch a 2.5 kg model and it  sits somewhere with enough AOA to miss the ground and accelerate to controllable air speed or not. Now I might achieve this just about 50% of the time without damaging the model, but if I use an RX with gyro and Safe (or equivalent) for a 5 degree climb out then the success is much much higher. Once the model is away and up to speed, switch safe off and try and fly the unruly beast!

 

PS - the critical factor is power to weight and every time I repair a model...it gets heavier!