What coefficient of lift is required?

[Ralph Walton]

As a new user of profilli, I'm fascinated by all the features and particularly the polar graphs that can be produced. I've started concocting my own foils, and I'm working on one based on a combination of RG15 at the front and Dr Dre's ag at the back, to produce a slope foil.

I'm getting some pretty interesting polars, but I'm trying to relate them to real world performance, and I'm struggling. What I can't figure is, what Cl
( coefficient of lift) is required to maintain flight, or how do you figure, for a given wing loading and speed and g's the Cl required?

What do the Cl numbers mean? For example if you have a Cl of 1.0, that is 1.0 what? Degrees?

[hul]

it's a dimensionless number, see its definition here: http://www.mh-aerotools.de/airfoils/velocitydistributions.htm#defCoefficients

And this is how you calculate CL for a given plane and speed:
CL=(mass x g)/ (1/2 x air_density x speed^2 x wing_area)
mass= mass of plane in kg
g= gravitational constant (9.8m/s^2)
air_density= 1.2kg/m^3 (approx)
speed in m/s
wing_area in square meters

example:
m=2kg, speed=20m/s, wing area=0.3m^2 -> CL=0.27

Hans

[Jurgen Heilig]

...
What do the Cl numbers mean? For example if you have a Cl of 1.0, that is 1.0 what? Degrees?

http://en.wikipedia.org/wiki/Coefficient_of_lift

:) Jürgen

[Ralph Walton]

I'm costantly amazed at the depth of knowledge we have here on RC groups. As time goes by I realise how little I really know.

Thanks Guys.

By the way Hull, I see your avatar is a mosquito. My Dad flew one in WW2 and he used to tell me all sorts of interesting stories about shooting down V1's etc

[BMatthews]

Something that I don't see mentioned above and that you may not realize is that the wing's lift coefficient changes as the airspeed changes. At low speeds it needs to operate at higher Cl's to provide the lift needed to maintain flight. At higher speeds the Cl reduces. Obviously the Cl is tied quite closely to the angle of attack.

For a real eye opener it's fun to play with a little Java applet called Foilsim. It only works for one airfoil but you can alter the thickness and camber and set up the wing size and speed and angle of attack and see the lift coefficient or lift as an output. To get "real" numbers from it you need to set up the basic rectangular wing to more or less match your intended model and then set up the speed and angle of attack so that the lift value matches what you think the model will weigh. Then switch to the Cl output and see what the value is.

I was shocked when I did this a few years back to find out that at moderately high speeds the Cl can be under .01 without much trouble at all. And if you slow the speed and increase the angle of attack until you're at a point where the Cl is starting up the flatter upper slope where the drag builds fast you can quickly figure out roughly how slow the stalling speed will be. Because the Cl is independent of the airfoil you can use Foilsim in conjuction with the polars from Profili (actually Xfoil which is the module used by Profili to generate the polars) to more or less find out how slow you can fly without generating high drag numbers. It's a great little learning tool that shows you more when you're new to this than some dry numbers. It also shows how the Cl alters during flying.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

[ghoti]

Ralph and Hull,
The avatar also resembles a Bristol Beaufighter, a Brit nightfighter from WWII. Which is it? Bill

[MarkusN]

Ralph and Hull,
The avatar also resembles a Bristol Beaufighter, a Brit nightfighter from WWII. Which is it? Bill
Nope. Look at the tall fin, the slender fuse and engine pods. Definitely a Mossie.
</offtopic>

[hul]

it's this Mosquito: http://www.rcgroups.com/forums/showthread.php?t=290594. Passed away with reversed ailerons....

Hans

[Ralph Walton]

Ive just been playing with foilsim. Its a lot of fun.

I've found the formula that Hull gave in post 2 to be very usefull. I can now better relate my modified airfoils to real world performance.

The Cl change with speed aspect is covered in the formula, as I discovered when I ran one of my existing slopers thru it. At 10 m/ sec it had a Cl of .6 when the speed is increased to 20 m/ sec it went down to .15

Now I can start running some numbers on my new foil (foils), and also need to learn more about how air density changes will affect the foils performance within the normal range of altitude, temperature, humidity etc that I fly in.