Liftoff Mechanics

[skatj]

I'm trying to figure out the amount of wing area I need to lift off for a 2 lb airplane.

Using the formula L = 1/2 d v^2 s Cl

I rewrote the equation as 2L/ d v^2 Cl = s

I plugged in 2 as the lift needed, 0.002377 as the density of the air, 25 as velocity squared, and 1 as the coefficient of lift (I read somewhere that this was an acceptable number for most model aircraft).

Anyway turns out I needed 67 square feet of wing area to take off. So umm...

I'm retracing my steps to see where I went wrong when some questions came up.

1) Is the amount of lift needed to take have equal to the weight of the aircraft or the wing loading of the aircraft? Because the wing loading would be significantly lower.

2) Does the lift need to be equal to the downward force, or does it actually have to be somewhat greater than it?

3) How do I take into account the angle of the elevator when calculating lift?

4) Is 5 ft/s a good ballpark estimation for a 2 lb plane with a brushed motor on the ground that's taking off?

5) Should I be using 1 as the lift coefficient?


On a sidenote, what is a light wing loading for the slowest, most stable flight possible? Maybe I'll just try to get a wing area estimation from that.

[kcaldwel]

You did it right, except 5 ft/sec is only 3.4 miles per hour. That is really slow, which is why you end up with such a big wing area required. 25 to 30 ft/sec would be closer to reality I think, which would give required wing areas closer to 2 sq. ft.

The lift has to be a bit larger than the weight of the aircraft at take off, to accelerate the aircraft upward. You don't use wing loading here.

Don't worry too much about the tail at this point. It will likely have a small down load, but it isn't too significant for this rough calculation.

Cl = 1 is a reasonable estimate for this size and speed of model.

The lower the wing loading, the lower the stall speed. This doesn't necessarily translate to a more stable model, in fact a lightly loaded model will get bounced around more in turbulence, and will have to fly in lighter winds.

Kevin

[vintage1]

Yup. You don't see many class landings from moths in a hurricane. :)

My 'light wind' planes stall arounmd 13-15mph, 15-18 is a modest wind plane and over 20mph is a pain to hand launch but will cope with pretty heavy conditions. Usually wipes the undercart off in a heavy landing tho. :D

[skatj]

25-30 ft/sec on the ground seems REALLY fast to me, but tbh I never actually flew an r/c plane before.

Keep in mind the motor is just a low-speed brushed motor.

[BMatthews]

When you've never flown before yeah, 25 to 30 feet per second WILL seem very fast. But it soon becomes quite normal.

Even our slower RC gliders land at around 10 to 12 mph which is around 15 feet per second and even most beginners find this speed to be quite easy to adjust to after a flight or two.

The motor you're using doesn't enter into the factors other than if it has the thrust to accelerate the model to the speed needed to fly and the energy output to let it climb. If you're hellbent on using this motor perhaps you should let us in on what it is and that way we could suggest what sort of size of model would be suitable for it.

[skatj]

I actually haven't decided on a motor yet, just giving a rough idea. I'm actually planning on building a UAV, so it cant be brushless since the motor controller I'm using doesn't work with it, so basically I'm just limited to brushed motors. Any suggestions?

(I'm using one of these motor controllers) http://www.parallax.com/Store/Accessories/MotorServoControllers/tabid/160/List/0/CategoryID/35/Level/a/Default.aspx?SortField=ProductName,ProductName

[BMatthews]

Instead of using those set up your onboard controller to output a PWM signal that replicates the pulses coming from a reciever. Then you can feed that into a brushless motor controller and use the better motors. There isn't the wide variety of motors available in the brushed field these days.

PS: not to mention that the high current ones are quite big and heavy compared to the model airplane stuff.