Tell me more about the 2.5 times stall rule

[Sail 'n Soar]

Can anyone relate how the pitch speed > 2.5 times stall speed was derived?

I've been doing next model virtual designing on MotoCalc for an electrified version of the Ken Willard Gasser. The Gasser has a 9% foil with ~ a 1% camber. For many of the power train combos I'm considering MotoCalc predicts a static pitch speed of less than 2.5 times the stall speed. On the basis of this, the MotoCalc opinion is it "may result in reduced performance at typical flying speeds and a low maximum speed."

Actually, MotoCalc is doing a reasonable job of predicting max CL and the associated stall speed. I can decrease the predicted stall speed sufficiently by either increasing the foil thickness or increasing the camber that I meet the 2.5 times stall speed criteria. However, both of these actions will decrease the maximum speed and go against what I was trying to achieve. The Gasser was my first RC model and I want to change the airframe as little as possible - weight savings only.

What is the real performance envelope impact of achieving or not the 2.5 times stall? What is the real significance?

[vintage1]

I think its really a rule of thumb. You want to be delivering sensible thrust at the most efficient airspeed - which is generally about 1.5 times stall speed. For things like gliders, thats all you really need - to deliver thrust at climb speed.

For things like warbirds, you want to go faster. So 2.5 times is better, but actually you make want to produce thrust at 4 or even 5 times stall speed. That means a prop with a pitch speed that is up to 6-7 times times stall speed, and that prop is likely stalled itself at anything up to 2 times stall anyway.

No wonder full size planes have variable pitch...

I have flown combinations with far less than 2.5 times stall, and they climb like angels, but don't speed up at all in the dive or on the level. Nice for trainers.

Other things that are more corase pitched take time to get up to serious flying speed.

Its a choice.


I am not familiar with the gasser. If its a trainer go with less than 2:1 pitch speed.

If not, accept a bit less climb rate and pitch up the prop.

[steve lewin]

As v1 says it's just a rule of thumb based on average models. How useful it is depends very much on the type of model. I don't know what a Ken Willard Gasser is either but for example many vintage models are based on FF models and often get nowhere near 2.5 times stall. In fact they fly permanently at or near the stall and you can't speed them up whatever you do. If that's how they're supposed to fly it's no problem.

Steve

[Sail 'n Soar]

Gasser was set up with a kick down elevator to allow high speed straight & level flight. You could also get if flying inverted on single channel. Had the trim set up wrong on my first flight and it did a continuous serious of verticle Immelmans. Went into a large loop from my hand launch. Hard rudder at the top and it would roll into another half loop. Too much for a raw beginner, but slow and easy glide once the engine quit.

[Sail 'n Soar]

Trying jpeg this time.