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Old Apr 05, 2012, 12:25 PM
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Thrust tube questions for the Guru's

I have a question about thrust tubes. Im relatively new to EDF's but not to flying in general. When I got my first EDF I really struggled as it was grossly under powered. I then I refered back my nitro powered RC days. I live and fly at or near 5000ft in elevation. So with the nitro powered RC planes we would always run the largest recommended engine, or as large as we could fit in the engine mount and or under the cowl.

So initially I replaced the EDF unit and motor with a larger KV powered motor. Went and did a test flight and it was a little better. Then I threw a larger amp ESC at it. That helped a little more. But I still wasnt where I wanted to be. Then I got to looking at the exhaust portion of my ducting. It had all the little foam castings and little imperfections in it that can, and likely do create drag for lack of better words. So I thought what if I made my own ducting? And I did. And it worked pretty good. Then got to thinking a little more about it. What if I tapered the tube down on the exhaust end this should increase the exiting thrust. Thats when I started doing some internet surfing and joined a couple of forums. Because surely there is somebody out there that has done this before and has way more knowledge than I do about this. Ive found numerous threads on thrust tubes, How do design and figure out how long it should be and how big around etc.

Which leads me to my question (s). If found what works well in my EDF aircraft. But Ive noticed that it has a side effect. Not necessarily a bad one but it is a side effect. After the EDF,motor,ESC swap and figuring out the correct dimensions for my thrust tube. I noticed that my take off rolls where much slower and longer than before. Even when I only just fabricated a smooth duct it didnt do this. However I did noticed that once the aircraft was airborne it accelerated rather quickly and provided much better speed.

So my question is. Because the thrust is coming out of a more narrow opening is it similar to a propeller driven aircraft that once it gets moving or airborne and starts to accelerate the propeller (fan) unloads providing more thrust?

Ive only noticed this mostly on take off. On aborted landing approaches where a panic slap of the throttle to WOT to climb out it seems only just a little bit less responsive than with out the tube. Otherwise Im very happy with the designs that have been shared.
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Old Apr 05, 2012, 12:50 PM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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What jet are you flying? size fan, etc?

Thrust tubes are a must for good performance. The sweet spot between max thrust and out out speed is 85% FSA for an exhaust tube.

A good material for thrust tubes is mylar, I use Dura-lar.

And a cone program is useful to get the tube dimensions down to the nitty-gritty.


Good luck.




Julian
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Old Apr 05, 2012, 12:51 PM
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nøtterøy, NORWAY
Joined Mar 2006
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You have been reading up on this, and you have also first hand experience through your tests. I am definately no expert, more of a trial and error guy but here are my thoughts..

In short, narrow outlet gives you slower acceleration (low static thrust), but higher efflux (velocity of the air comming out if the tube´s end) theoretically achieving higher top speed. If you have a draggy airframe, this(high efflux) is not your best option.

If you opt for a wider outlet, static thrust will rise and give your model better acceleration.

The choice is yours
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Old Apr 05, 2012, 02:03 PM
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Quote:
Originally Posted by Boogie_ View Post
What jet are you flying? size fan, etc?
Julian,
Well, the jet that I refered to in initially in this post was the Exceed RC 70mm Mig 15. Nothing super special but it flew (yes passtence FLEW) nicely. When it arrived it had a 28xx (something) KV motor factory fan, and a 45A esc, with a 4s 2200mha 25c battery. Before I even flew it and I forgot to mention this in the original post I was informed by another electric flyer that I needed a better battery. So the first flight in stock form was with a 4s 3000mha 40C battery. And it flew but thats about all it would do.

Then with the fan and motor swap I went with the Dynam EDF combo that came with a 3000kv motor and a 60A ESC. Thats when I started messing with the thrust tube. And truthfully I was quite happy with how it flew.

Now Ive got a Blitz RC 70mm Mig 15. Its much larger and heavier. And Ive got at Sapac EDF, with a Cyclone Power 3300KV motor, and a Dynam 80A ESC. But Im waiting on another fan as I injested servo wires that came loose during a static pull. But it flys nicely considering its weight and I need to make a different thrust tube for it as I think my exhaust opening was a little smaller than it should have been. It was right about 2 inches in diameter. So Im thinking that I will make it just a little larger right about 2 1/8 or a 2 1/4 inches.

The other EDF that I have is a Venom F 86. The fan and motor have been removed on this one also and sitting on my work bench right now is Change Sun 10 blade fan with a Cyclone Power 3300KV motor and a Dynam 100A ESC. But the nice thing about the Venom is that it already comes with a thrust tube installed from the factory. So I will see how well that pans out as far as the thrust tube goes.

As for my thrust tube material. I use a very rigid stencil paper. Its almost like a flexable card board.

Any other info you care bestow any other info it will be greatfully recieved.
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Old Apr 05, 2012, 02:06 PM
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Quote:
Originally Posted by kimhey View Post
You have been reading up on this, and you have also first hand experience through your tests. I am definately no expert, more of a trial and error guy but here are my thoughts..

In short, narrow outlet gives you slower acceleration (low static thrust), but higher efflux (velocity of the air comming out if the tube´s end) theoretically achieving higher top speed. If you have a draggy airframe, this(high efflux) is not your best option.

If you opt for a wider outlet, static thrust will rise and give your model better acceleration.

The choice is yours
I guess I will just have to keep experimenting with lengths and opening and exit diameters till I find a good happy medium. And yes I am flying two rather draggie airframes. 43 inch wing, 70mm, 3.5 lb Mig 15, and an F86 with same wing, fan and weight. But less the external wing tanks. But I need as much efflux as possible as I live and fly at 5000ft in elevation so the air is quite thin and these things arent light so they need everything I can throw at them for power.
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Last edited by SilverState RC; Apr 05, 2012 at 02:38 PM.
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