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Jun 11, 2020, 03:01 AM
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Optimisation of propeller flows


I'm creating this thread to discuss about using toys to study empirical aerodynamics

So, I have a lot of spare parts of toy drones since 2013 and decided to make a plane out of them and a cheap (less than 3 euros) EPP foam plane.

I'm trying to make it fly decently by optimising wing flow . I had to remove a lot a foam for weight reduction.

I put the board so that the plane is stabilized as well (works quite good !). Rc sticks are reversed but it is just a training for my aging brain . Right now, the plane is at 55 gr and can fly straight for 50 meters if almost no wind : it still goes down a bit.

Now, I have put the motors above the wing so that most of the flow goes above the wing (few air under) and make the flow go down with the "flaps" and CL should be far better because more flowing air above the wing than under. I'm reading a lot of studies online about Custer or motor/wing interaction and distributed power etc. .

What I did should improve flight performance for the same motors and battery (i don't want to buy anything), but I'm now wondering if I could not gain more lift (CL) by putting the motors on the wingtips (?) now .

Indeed, this should remove completely the tip vortex and the vortex induced by the propeller should then be in the right direction (the blade going up ABOVE the wing while the descending blade is not "pressuring" the wing since it is "outside").

Do you have any opinion about that process ?
Last edited by newtoon; Jun 11, 2020 at 03:11 AM.
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Jun 11, 2020, 07:59 AM
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While putting the motors on the tips (with props rotating counter to wing tip vortex) may reduce wing tip vortex, you will incur other penalties by doing so:
1. Your weight is spread at the extremities as opposed to being concentrated near the center - this means greater roll inertia and more bending moment on the wings (so may result in a lot of wing flex).
2. With the motors on the tips you lose a lot of that forceful air going over the wings (lose lift) and tail (lose tail authority).

A simpler solution to the tip vortices is what the majority of airliner and lots of models and general aviation planes have: Winglets at the tips. There's good reason this approach has been retrofitted to dozens of designs (both full scale and model) that didn't have them before
Jun 12, 2020, 04:24 AM
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You could probably cut the mounting lugs off the corners of the board to reduce drag.

I would also bury the motor wires inside the wing instead of running them along the leading edge the way they are in the photo.
Jun 12, 2020, 05:06 AM
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We were told by a young Navy pilot , that propellers blowing air over the top of the wing was most efficient …
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Jun 27, 2020, 08:27 AM
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Quote:
Originally Posted by nuteman
While putting the motors on the tips (with props rotating counter to wing tip vortex) may reduce wing tip vortex, you will incur other penalties by doing so:
1. Your weight is spread at the extremities as opposed to being concentrated near the center - this means greater roll inertia and more bending moment on the wings (so may result in a lot of wing flex).
2. With the motors on the tips you lose a lot of that forceful air going over the wings (lose lift) and tail (lose tail authority).

A simpler solution to the tip vortices is what the majority of airliner and lots of models and general aviation planes have: Winglets at the tips. There's good reason this approach has been retrofitted to dozens of designs (both full scale and model) that didn't have them before
Hi ! Thanks for all the comments.

So, I went down to 54 gr by removing stuff and stuff and it still struggles (it flies, sure, but going slowly down, so 50 meters max in straight line). I could change the battery (150 mah instead of 240) but I don't have anymore.

I wonder what I can do to improve this. Is the board limiting the current ? (it s the board for very small motors, not for that kind of motors, so perhaps it is that). I find it strange that it cannot cope the 54 gr anyway, with two motors. Still thinking

Regarding winglets, they have a drawback : they remove the drag of vortex but add drag anyway.
Jun 28, 2020, 02:45 AM
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Quote:
Originally Posted by newtoon
Regarding winglets, they have a drawback : they remove the drag of vortex but add drag anyway.
Of course, but if you do it right, the net drag reduction is what counts.
Namely: Did you reduce vortex drag so much that it offsets the cost of the added drag of the winglet such that there is a net drag reduction.
If this wan't achievable you can bet the airlines (and some light aircraft manufacturers) wouldn't have spent all that money adding winglets to old designs


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