thrust tube question
I've been thinking of converting an old glider to edf power. How does the length and taper of the thrust tube effect the thrust. Also, does running a fan without a thrust tube (outside the fuse) effect the amount of thrust it will produce.
I take it that you would be chasing push, not speed.
I don't have the maths to confirm it, but I have read in posts from some of the more experienced guys on here that somewhere between 2 to 3 times the fan diameter is the best length for a thrust tube. I'm sure someone will correct me if I've remembered that wrong :D
An exit diameter equal to the Fan Swept Area (cross sectional area inside the shroud MINUS the cross sectional area of the spinner/hub/motor) should give you the optimum thrust for a given fan/motor/battery setup. This would mean a slightly tapered thrust tube fiited to the fan shroud.
If you further reduce the exit diameter from this by increasing the taper, you effectively reduce the 'footprint' of the column of air that is pushing against the surrounding atmosphere, so you will reduce the thrust.
However, reducing the exit diameter also has the effect of increasing the veocity of the column of air exiting the fan, so while thrust will reduce, potential top speed will increase.
Running a fan without a tube will mean that you will not get the full amount of potential thrust, but that may not matter seeing as how you're only trying to get the glider to altitude. It's just not as efficient as it could be.
When I tried this and mounted a fan above the wing, I found that I needed to angle the fan so that the EDF unit was angled slightly down when viewed from the side (similar to how the Bixler motor is set up) as the EDF was causing the glider to nose over under power when set at the same incidence as the wing.
Good luck with your experiment.
very helpful, thanks a lot. Any other comments are welcome!
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