Stall Speed, Prop Pitch Speed,..what's the sweet spot?

[Rob Schaffer]

Looking for some answers to some aerodynamic questions. If I know the models information such as weight, wing loading, type of model (sport, scale, slowflyer), type of wing (symmetrical, flat-bottomed, etc) how do I determine the approximate stall speed of the aircraft.

Once the stall speed is known, is there a general rule to determine the best pitch speed for the aircraft, so I can then determine what prop to use?

I'm looking at optimizing my airframe for the static thrust, watts to the motor, and the expected pitchspeed of the prop. Any ideas?

[Ollie]

Read Model Aircraft Aerodynamics by Martin Simons. Then improve your questions. I like one question at a time. "Sweet Spot" is not a good question. Please, be specific rather than general. Narrow down your purpose.

[yoyoML]

Looking for some answers to some aerodynamic questions. If I know the models information such as weight, wing loading, type of model (sport, scale, slowflyer), type of wing (symmetrical, flat-bottomed, etc) how do I determine the approximate stall speed of the aircraft.

Once the stall speed is known, is there a general rule to determine the best pitch speed for the aircraft, so I can then determine what prop to use?

I'm looking at optimizing my airframe for the static thrust, watts to the motor, and the expected pitchspeed of the prop. Any ideas?

Weight, wingloading and airfoil let you estimate stall speed pretty well:

Weight/Area = Wingloading = 0.5(rho) * v^2 * CL(critical alpha)

Where rho is air density. And you'll need to find CL(critical/stall alpha), which is just max CL, of your specific airfoil. Then v is your stall speed for level flight.

You may next estimate your "cruise speed", which may mean different things, but is usually compared with the stall speed.

Stall speed is the speed that allows you to pull 1G, as in level flight. Twice the stall speed allows you to pull 4G, and three times the stall speed allows 9G. For anything aerobatic you'll want to pull some 5G anytime during cruise, so mark your cruise speed sqrt(5) times stall speed.

Then you'll need to estimate the required pitch speed at cruise. A reasonably efficient prop should give you 80%~90% efficiency [citation needed, lol], which means cruise speed is 80%~90% pitch speed, at cruise throttle and corresponding cruise rpm.

Translating that to motor power will need some understanding of the motor's torque curve. But an electric motor has approximately a straight line as its torque curve (starting from (0rpm, max torque) to (unloaded rpm, no torque)). This torque curve corresponds to a parabolic power curve, which has a maximum at (0.5 * unloaded rpm, 0.25 * max torque * unloaded rpm). In the above, unloaded rpm is input voltage times the motor's effective KV value.

Effective KV value is KV divided by gear ratio. 4000 rpm/V attached to 5:1 ratio gets you 800 rpm/V of effective KV.

If your cruise thorttle is 60%; the battery is 11.1V; the effective KV is 800, then the unloaded rpm is 60% * 11.1 * 800 = 5328 rpm. For most power you'll need a propellor that manages half of that, 2664 rpm. For best efficiency you want the rpm a little higher, but I'm lazy to calculate that now.

You shouldn't expect to understand all these, though. They took me quite a while to figure out, and there's still a lot more I don't know. Please don't hesitate to ask.