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Thrust tube question for the Guru's

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Thrust tube question for the Guru's

Old 04-05-2012, 08:31 AM
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SilverState RC
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Default Thrust tube question 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.


Old 04-05-2012, 10:36 AM
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skyhawknut
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Default RE: Thrust tube question for the Guru's

There are a number of things at work here.

First - when you narrowed the exit you increased the efflux (ehaust speed of the air) which is really everything. Narrowing the opening will increase top speed (at the expense of static thrust) and opening it up will increase static thrust (at the expense of top speed) You may have to experiment to fid the optimum opening to get what you want.

Also - just like a prop - different fans have different pitch. Higher pitch will result in higher top speed but longer takeoffs.

Old 04-05-2012, 10:51 AM
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Default RE: Thrust tube question for the Guru's


ORIGINAL: skyhawknut

There are a number of things at work here.

First - when you narrowed the exit you increased the efflux (ehaust speed of the air) which is really everything. Narrowing the opening will increase top speed (at the expense of static thrust) and opening it up will increase static thrust (at the expense of top speed) You may have to experiment to fid the optimum opening to get what you want.

Also - just like a prop - different fans have different pitch. Higher pitch will result in higher top speed but longer takeoffs.

I appreciate your input. That is sorta what I figured. I have plenty of runway to build enough airspeed to take off. So the slow excelleration on the take off roll out isnt really that big of an issue. But the airframes Im flying are rather heavy and have a fair amount of drag to them. Im flying a Mig 70mm Mig 15 and 70mm F 86. So coupled with the airframe drag and the 5000ft elevation that I fly from I need nearly as much available top speed as I can get. But I do agree that If I experiment enough I will find the happy medium of static thrust and top speed thrust. But in short youve more or less answered my question. As the aircraft excellerates the prop (edf unit) unloads and ends up at top operating speeds (rpm)

Old 04-05-2012, 04:47 PM
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Default RE: Thrust tube question for the Guru's

Actually - that's a common misconception. The fans do not unload as airspeed increases. There have been many discussions about this and some have done instrumented flight tests and the fans do not unload as they go faster.
Old 04-06-2012, 02:03 AM
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stoneenforcer
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Default RE: Thrust tube question for the Guru's

your lost thrust cause your exhaust is to narrow

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