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.