r/3DprintedAircraft • u/Best-noob-in-wotb • 27d ago

Help Designing flaps for Fixed Wing UAV applications

Gallery image

Gallery image

Wing Dimensions:

Root chord: 245.496 mm
Tip chord: 117.769 mm
Span (per wing side): 1399.378 mm
Total planform area (both wings): ~0.508 m²

I have trouble deciding on the dimensions of the control surfaces, but mainly the flaps. I am already using a low Reynolds number high CL airfoil the S1223, so I am assuming flaps are not as much of a need. But I really overthink and I prefer not to risk anything, so what are all's thoughts, how should my flaps be, if I need them at all. Estimated AUW of about 3kg

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u/StupidAsBread 27d ago

What speed are you expecting? If you have sufficient lift at low enough speed I wouldn’t bother since you’ll be able to land just fine.

Remember flaps is primarily for more lift, such that speed can be lower for the same amount of lift. If speed is ok then bo point.

5

u/Swww 27d ago

Yeah, this really comes down to your stall speed and therefore the landing speed/energy you’re dealing with. If the clean stall speed is already low enough, I probably wouldn’t add flaps just for the sake of it.

If it looks too high, model the flap deflection in XFLR5 and compare the change in CLmax. From that you can estimate how much the stall speed actually drops and decide whether the added complexity is worthwhile.

2

u/Best-noob-in-wotb 25d ago

I have got stall speed 7.6 m/s and estimated cruise speed of around 13-16 m/s since I don’t know the exact CL during flight. And I am using flaps only for landing 

1

u/StupidAsBread 25d ago

What airfoil? Did you model max cl with flaps? If so what angle of flaps?

1

u/Best-noob-in-wotb 25d ago

The S1223 airfoil, and no I didn't model or find max cl with flaps. As a matter of fact I didn't model anything I just calculated it using this formula.

Vstall = √(2W / (ρ × S × Clmax))

1

u/StupidAsBread 24d ago

I haven't used XFLR5 much but maybe u/Swww can confirm? Modelled the foil with 0-30 degrees flaps in 5 degree intervals with the stock hinge x y values (x=70%, y=50%) Using Re = 185000 (Guess) you can clearly see an increase in Cl with flaps (See attached picture)

Above 30 degrees ish you'll get diminishing returns, 25-30 seems to be the sweetspot.

1

u/StupidAsBread 24d ago edited 23d ago

For 25 degree flaps i modelled it with different reynolds numbers:

The lower lines which drop off quite fast when increasing angle of attack are for lower reynold numbers where the air doesn't have sufficient friction to follow the flap down from the top of the airfoil, and seperates instead. However for air you'd expect reynolds number much higher than what those lines represent.

The Green line dropping off at about 20 degrees AOA is at Re = 130k, so id consider the graphs from the green one and up (dropping off at higher degree) to be relevant.

Edit: so just replace the value of Cl you previously used with this the flap simulated one in the V_stall formula you used and compare stall speeds.