Questions about basic designs

[Kaos116]

Hello all,
I have a few questions that have got me wondering. I want to scratch build a small slow flying plane. I would like it to be powered by a Speed 280. Controls being ailerons, elevator and speed control.

Question 1. Wing loading. The questions are, what loading is best for a slow flier. Can I just increase the cord to keep the span down? Does the elevator factor into wing loading?

Question 2. Wing Dihedral. What is it's purpose? I think it has something to do with the plane coming back to level flight on it's own

Question 3. Propellers and Gearing. I have never changed the prop on my Simple 400, so I am not sure what effect it has. If I understand it correctly. Increasing length, but decreasing pitch creates more thrust but loses speed? Is there a formular for figuring prop specs, rpm(gearing) and wingloading to give basic handeling characteristics?

Thanks ,

Todd

[Cody WIlson]

1- Keep it as light as possible with alot of camber if you want it to go slow. Keeping a thicker chord is pretty typical on slow flyers.

2- Yep, pretty much. Also- if you have a rudder/ elevator setup the rudder cocks the ouside wing so the wind gets under it more and makes a nice turn.

3- Right again on the width and pitch. I don't know too much about gearing except that the higher the ratio (ie 6.6-1) the larger/ steeper prop you can turn and get more thrust. But I can tell you to get as close to 1-1 thrust to weight as you can. You can get away with half, but it gets boring *yawn*... stretch...

The lower the gear ratio- closer to direct drive- the faster the model will go, and will need a longer, skinnier wing. With the high ratio's and larger props you get more thrust, and typically go with a shorter, fatter wing to create lots of lift. Then you get into airfoils- symmetrical, semi-, flat bottom, undercambered etc...

The more thrust you have the more aerobatic the model becomes. Less thrust, less fun.

FWIW-

Cody

[vintage1]

If its truly a slow flyer you prpbably want to go for rudder rather than ailerons. They are not so effective as a rudder at very slow airspeeds with dihedral.

Prop and gerabox selectin is a bit of a fine art.

If you gear too much you end up with not enough pitch speed - the mdel my hover, but not actually fly...

[feihu]

Question 1. Wing loading. The questions are, what loading is best for a slow flier. Can I just increase the cord to keep the span down? Does the elevator factor into wing loading?
Question 2. Wing Dihedral. What is it's purpose? I think it has something to do with the plane coming back to level flight on it's own
Question 3. Propellers and Gearing. I have never changed the prop on my Simple 400, so I am not sure what effect it has. If I understand it correctly. Increasing length, but decreasing pitch creates more thrust but loses speed? Is there a formular for figuring prop specs, rpm(gearing) and wingloading to give basic handeling characteristics?


Ans.1 As Cody said, keep it light. Depending on structure (balsa or foam), the wing loading for a 280 geared may typically be less than 10 oz/sq ft but not much higher than perhaps 16 oz/sq ft. Elevator does not factor into wing loading. Wing loading is equal to All Up Weight divided by the wing area (expresses in sq feet here)
Ans.2 When rudder is applied, dihedral allows the airplane to sideslip which causes the airplane to bank, and thus turn. Without dihedral, you would not be able to bank the airplane to turn without ailerons.
Ans.3 Same as what Cody said.

To give you a better perspective, here are a couple of photos and specs of one of my park flyers with a geared 280 motor.
Wingspan 30"
chord 6"
Wing area = 30x6=180sq in divided by 144 = 1.25 sq ft.
Dihedral 4 degrees per panel
280 motor with 4:1 gearing and 9x6 APC prop provides ~ 7 oz thrust
HiTec555 recvr, C-10 esc, 2 HS-55 servos, 8cell 230mah batt.
All Up Weight is 11 oz
Wing loading is 8.8 oz/sq ft
Thrust to weight ratio is ~ .63
Airplane flies well!

feihu

[Kaos116]

Thanks for all the information. I think I have a grip on it now :-D I'm going to build something up and see what happens. I'll post a pic BEFORE I fly it to see what you think.