How to estimate stall speed?

[S55]

In another thread that I could not find again it was said the propeller pitch speed should be at least twice the stall speed. Is this a good rule? Then how can I get even a rough idea about my stall speed? Can I design to achieve a specific stall speed?

Thank you,
S55

[Ollie]

Yes, you can design to a specific stall speed. Start with a trial configuration and gross weight. Select an airfoil. From its polar diagram of coefficient of lift versus angle of attack, you can determine the airfoil's maximum lift coefficient before stall. Then go to the referenced web site and put the wing's configuration into the online program which will calculate the lift distribution. Increase the angle of attack until the maximum of the wing's lift distribution equals the airfoil's maximum lift coefficient. Then read off the wing's lift coefficient which the program has calculated. Put that into the formula below and calculate the stall speed. If the stall speed is too high you can reduce the wing loading by increasing wing area or reducing gross weight. Or choose an airfoil with a higher maximum lift coefficient or change the planform for a more favorable lift distribution that results in a higher lift coefficient of the wing before some part of it stalls or all of the above. The process can be iterated until the objective stalling speed is met.


The constant (29) in the following formula is for a standard atmosphere at sea level. In English units the stall speed in feet per second is equal to 29 times the square root of ( wing loading in pounds per square foot divided by the maximum lift coefficient of the wing). The maximum lift coefficient of the wing can be estimated from the maximum lift coeffient of the airfoil, angle of attack just before stall and the lift distribution of the wing where some portion of the wing stalls first. See:
http://aero.stanford.edu/WingCalc.html

[S55]

Polar diagram of the airfoil…(?!?!?)
Nice Java applet, the problem is I have no clue what CL, Cm, Cdi, and e mean. Same about AR, Sweep, Taper, Twist.
I guess this is what I deserve if I do not like the cut and try method.

Ollie, if you have time and enough drive for some guiding, my airfoil is the good old Clark Y.

Thank you,
S55

[Dick Huang]

Originally posted by S55
In another thread that I could not find again it was said the propeller pitch speed should be at least twice the stall speed. Is this a good rule? Then how can I get even a rough idea about my stall speed? Can I design to achieve a specific stall speed?

Thank you,
S55

S55,
Use Clmax=1.0 in the formula below for the Clark Y.
Stall Velocity(ft/sec)=7.25*(W/S)^0.5*1/(Clmax)^0.5 this is at sea level and (W/S) is the wing loading in oz/ft^2.
(ft/sec)/1.467=mph.

Dick Huang:)

[Ollie]

For the polar diagram of the Clark Y see:
http://www.nasg.com/afdb/show-polar-e.phtml?id=7
The Cl is the coefficient of lift. Alpha is the angle of attack. You can see that the Clark Y has a maximum lift coefficient (Cl) of about 1.19 from the right hand graph. Cm is the moment coefficient but you don't need it for this exercise. Cdi is the induced drag but you don't need it for this exercise. The Oswald efficiency factor (e) is a measure of how closely the lift distribution matches an elliptical lift distribution and you don't need it for this exercise. AR is the aspect ratio and it affects the lift distribution some. Sweep is the angle that the 25% chord line is swept back and it affects the lift distribution. Taper is the ratio of the tip chord to the root chord and it affects the lift distribution. Twist is the angle at which the tip is washed out with respect to the root and it affects the lift diatribution.

BTW, Dick's short cut is a nice simple way of estimating the stalling speed. It is very accurate for a well designed wing but not so accurate if the wing has too much taper, sweepback or twist resulting in premature stall. You should use Dick's method if you have coinfidence that the wing is well designed to avoid premature stall. If you are curious about how well your wing is designed you can check it with the lift distribution applet.

[S55]

Many thanks, folks!

Dick’s formula will work perfectly. Accuracy is not an issue, all I need is to estimate the needed pitch speed.
Here in Oregon, it will soon start to rain a lot. If I can also convince Santa to bring me some spare time this winter, I will try Ollie’s approach.

S55

[Sail 'n Soar]

Stall Velocity(ft/sec)=7.25*(W/S)^0.5*1/(Clmax)^1.5 this is at sea level and (W/S) is the wing loading in oz/ft^2

There is a typo in Dick's formula. It should be Clmax^.5, not Clmax^1.5. That is,

Stall Velocity(ft/sec)=7.25*(W/S)^0.5*1/(Clmax)^.5 at sea level and with (W/S) in oz/ft^2.

The typo doesn't make any difference for CL = 1, but if you intend to use the formula for airfoils other than those like the Clark Y, it will make a difference. For instance, for a thin symmetrical airfoil, Clmax ~ .9. Then using that airfoil in a constant cord wing with an aspect ratio of 6, for the overall wing the average Clmax is a little less than .8. It's this average Clmax that determines the stall speed and should be used in Dick's corrected equation. For the AR = 6, Clmax = .9 example given, the net effect of the typo will be a calculated stall speed 20% higher than actual! In terms of prop speed that is a big difference!

Gerry

[Sail 'n Soar]

Oops, a slight error in my figures. The difference in calculated stall speed for 1.0 vs .8 would be 20% higher. But for .9 vs .8, the example given, it would be closer to 10% higher. It doesn't sound like much, but from my own experience that amounts to about the prop speed difference between using an 11 x 8.5 rather than an 11 x 7 (prop speeds calculated by MotoCalc for my AstroFlight 020).

[JRuggiero]

Years ago, Keith Shaw published a formula derived from experiments conducted on quite a few models.

3.7 times Square Root of Wingloading in oz./sq. foot

It may not hold up well for park or indoor flyers.

Jim R

[S55]

Thanks, Gerry.
S55

[Sail 'n Soar]

Originally posted by JRuggiero
Years ago, Keith Shaw published a formula derived from experiments conducted on quite a few models.

3.7 times Square Root of Wingloading in oz./sq. foot

It may not hold up well for park or indoor flyers.

Jim R

I've seen that formula referenced previously in various threads. Unfortunately, that translates to a CLmax = 3.8+. It can't happen on anything of model scale and would be a real challenge to achieve even with various high lift defices at full scale. My guess is there was a factor of 2 squared dropped somewhere in the referenced published formula.

[Ollie]

Who knows? The velocity might be expressed in furlongs per fortnight or even leagues per day. ;)

[Sail 'n Soar]

Originally posted by Ollie
Who knows? The velocity might be expressed in furlongs per fortnight or even leagues per day. ;)

Ollie,

You got me there. I obviously jumped to quickly to assuming feet per second.

Gerry

[JRuggiero]

Ooops! I forgot to mention that Shaw's formula expressed speed in miles per hour, IIRC.

Jim R

[Dick Huang]

Sail 'n Soar
Thanks for finding the mistake in the stall speed fomula;I have corrected it. Stall speed in mph can be had by dividing 7.25 by 1.467 to get 4.94 if you want stall speed in mph. I believe the Clmax will be much higher than we can obtain if 3.7 is used instead of 4.94.
Dick Huang

For a wing loading of 10 oz/ft^2, The Shaw formula would give a stall speed of 3.7*3.16=11.7 mph. using the correct formula the
Clmax would have to equal 1.78 to get a stall speed of 11.7 mph.
Dick HUang:D