How to Determine Proper Angle of Attack? (to set wing incidence)

[green66]

Hi all,

How can the angle of attack for an actual wing be calculated, knowing the airfoil, desired Cl, Reynolds number, etc?

I've tried the methods by Martin Hepperle Hepperle method to determine AoA (http://www.mh-aerotools.de/airfoils/hdi_aoawing.htm) and Martin Simons, obtaining similar AoAs, but both values seem unrealistically high at 7.3 and 7.0 degrees, and that's after subtracting out the zero lift angle (see screenshot). BTW, the airfoil is an Eppler 214 on a glider wing.

I'm seeking a rational way to get a ballpark figure, not an eyeball or trial/error approach which could otherwise be done w/an adjustable incidence pin.

TIA for any advice.

[gunsguns]

Hi green 66,

I do believe that these calculations from Hepperle and Simons are not for calculating an angle of attack that can lead to a usable angle of incidence but they just tell us that a wing with a certain span is so much less effective than a wing with infinitive span.

[Sparky Paul]

AOA is dependent on airspeed (Reynolds number) as well. I see no airspeed input in those charts.

[green66]

AOA is dependent on airspeed (Reynolds number) as well. I see no airspeed input in those charts. Re is implicitly included in the inputted Cl and AOAs which are from Profili results based on explicit Re.

[Ollie]

You can calculate the wing angle of attack. What is the source of force thru the incidence pin? It comes from the tail that controls the wing angle of attack. Pitch control comes from the tail. The problem is to calculate the decalage angle. You must consider tail moment arm, stab area, stab in the wing down wash in stab angle of attack, stab AR, stab Cl, etc.

[Sparky Paul]

Here's a typical lift curve.. the slope of the line is the same for a wide range of Res.. And is about the same for any airfoil. The intercept might vary, but for all intents the Cl-alpha relationship is constant for any airfoil.
What you need to do is find the Cl your plane will be flying at, and get the alpha from that.. Then you can finger out the longitudinal dihedral.
Here's some stuff I worked up for another problem..
The 1G AOA varies quite a bit for the speed range shown.
(I took some liberties with the curve to simplify things.)
The slope and intercept relate alpha to Cl.. usually it's done the other way around, finding the Cl for the alpha, but for this example we compute the Cl, then find the alpha.
Without this information you really can't much out of the stuff you got from Martin's site.

[Ollie]

Read about Dr.Drela and Phil Barnes:
http://www.rcgroups.com/forums/showthread.php?t=432795&highlight=+angle+of+incidence++
http://www.rcgroups.com/forums/showthread.php?t=438965

[BMatthews]

Go find Foilsim on the net. Google pops it up immediatley. Use theonline version or download the applet.

Set the wing span and chord and what you expect for a low end speed. Then alter the angle of attack until the wing generates the amount of lift that equals the model weight. That's your AoA for that speed. Now set the speed to what you expect as a best cruise speed and play with the angle until the lift again matches the model weight. That's your penetration AoA.

How you set the wing onto the fuselage will then be a compromise based on which end of the spectrum from above you want to enhance for minmal drag. Most folks set it to the lower angle to enhance the high speed option but if it's a floater then you'll want to set it closer to the lower end angle.

And 7'ish degrees AoA if you're floating along at just barely over a stall speed sounds about right. The model will be very draggy at that point however. Mind you at lower Reynold numbers the stall occurs at lower angles so a better minimum airspeed option would be to max the AoA at about 5 to 6 degrees and see what speed you need to keep the model airborne at that angle.

[HELModels]

What is wrong with foilsim? It seems to consider the whole wing when determining total lift.
The application version allows many manipulations of the airfoil shape, so no need to enter foil coordinates to get in the ballpark.