Landing speed / stall speed

[capncrunch]

Trying to ballpark a landing speed, can't find the equation im looking for in my old texts. Do I use CL = 1 (or .7 or something) or do I figure the CL from the airfoil data and go from there?

-barrett

[Sparky Paul]

Even if you find the Cl, what are you going to do with it?
We fly airplanes by looking at them, and judging whether the airspeed is sufficient for the flight condition. What that airspeed is, or the alpha, or the Cl.. we don't know.

[Jim_Marconnet]

Even if you find the Cl, what are you going to do with it?
We fly airplanes by looking at them, and judging whether the airspeed is sufficient for the flight condition. What that airspeed is, or the alpha, or the Cl.. we don't know.Perhaps he has a radar gun.

Also, if it stalls, I know I went too slow or turned too sharply. :)

[groundfx]

A rule of thumb formula is:

stall speed in miles per hour = 4 * square root(wing loading in ounces per square foot)

Note: I've also seen 3.7 used by some authors instead of 4.

Google is your friend. Here's what I found after a quick search:
http://www.auf.asn.au/groundschool/umodule2.html#stall
http://adamone.rchomepage.com/design.htm

Here's the search term I used... you'll find more useful links:
http://www.google.ca/search?hl=en&q=estimated+stall+speed+square+root+wing+loading&btnG=Google+Search&meta=

[Jurgen Heilig]

Have a look here:

http://www.wattflyer.com/forums/showthread.php?t=1702

:) Jürgen

[capncrunch]

Thank you JH and groundfx for giving an ACTUAL answer to my simple and direct question. it looks like it's accepted to solve for v, at L=W and Cl = 1.0.

-barrett

[Sparky Paul]

What's magic about a Cl of 1.0?
Many profiles will go to 1.6 before stalling.

[capncrunch]

*goes back and reads the thread jurgen linked to more closely*

so the idea is to estimate CLmax and work from there. that thread involved a discussion as to whether CLmax =1.6 or =1.0 was more appropriate for the airfoil and wing plan.

also, groundfx, thats the difference between using 4 and 3.7 for that ballpark, it reflects a difference in CLmax for different wings. (3.7 is the higher lift wing)

-barrett

[Sparky Paul]

The only times I've had to know Cl and airspeed was when designing a lifter plane that had to take off within 200 feet.
Knowing the thrust from the motor, and the proposed weight of the plane, the acceleration can be computed. At 200 feet, the velocity achieved is flight speed.
Then, this speed, the weight, and wing areas are considered to find the Cl the plane will be at, at 200 feet.
Appropriate profiles are investigated to find a profile with this Cl at a reasonable alpha.
Once that is done, the plane can be built and flown, and airspeed forgotten about.
It's all motor performance then.
Landings are automatic (mandatory) after taking off. It's up to the pilot to fly the plane so it lands properly.

[portablevcb]

I can see where you might want to know stall speed ahead of time. My own 'rule of thumb' is based on wingloading (as above). Up to about 6oz/sqft the plane will land at fast walking speed. At 10-13oz/sqft it will land at running speed. At 20oz/sqft it will land at a speed I really don't like very much :)

For a detailed determination you can find Clmax from the performance charts. Then use that Cl, wing area, and weight to determine stall speed. There aren't many airfoils that haven't been tried before so you can find one that comes close to yours and get the Clmax from it. Try Profili to get what you want. Will it be 1.0 or 1.7? Depends on your airfoil.

charlie

[vintage1]

Even so CL is not a huge influencer of stall speed. The wing loading dominates. You use that to get near, and then prop accordingly so the model is at least capable of a lot more than stall..some high slow passes will then determine where it falls over the edge..land faster than that.

[groundfx]

portablevcb,
You left out one important point... that is, that CLmax for an airfoil is not the same as CLmax for a wing. Wings have tip losses (especially low aspect ratio wings.) Also, the wing planform can have a huge effect on the stall characteristics. E.g. delta wings can form a vortex on the leading edge that adds lift at high AOA.

vintage1,
I agree with everything you said. This is why proping for maximum thrust (and no pitch speed) is rarely a good idea (you'll be flying around on the virge of a stall) but not everybody gets it.

I'd also add that with a aerobatic airplane (especially one with a high wing loading), I've learned (the hard way) that it's also a good idea to fly up high and test out the stall characteristics both inverted, knife-edge and side-slip. It's good to know the edges of your airplane's flight characteristics so the don't take you by surprise down low. Also, every time you change the center of gravity, it's a good idea to redo these tests.

[portablevcb]

I agree. Also, many wings these days have different airfoils from root to tip and they also include washout. But, from what I gathered from the first post this design won't have as much affect from either of those so should be OK with the airfoil design properties.

With any new airplane you should follow a test regimen to determine stall performance at all conditions, low throttle (landing), high throttle (takeoff) level and in various degrees of turn. Test for stall entry and recovery. Test for spin entry and recovery. Most of these should be attempted after getting a good initial trim and performed first at altitude.

charlie