Alpha angle and flaps

[marfish]

Hi guys,
I'm looking at a foil on Profili that gives me maximum L/D (CL/CD) at +2* alpha and 8* of flaps. How do I know when the model achieves 2* alpha?
I already know that when flaps are deployed the model will either pitch up or down to varying degrees of pitch or even none at all. I feel that as long as the tail group is matched to the wing, it is the size of the flaps that dictate the pitch properties. Ie... the ratio between the wing, flap size and tail authority should dictate more of the pitch response than the length of the tail moment. So, a tail group properly matched to the overall wing and model size would still probably see more positive pitch with large, wide flaps than with small, narrow ones.
So, how does one measure when the model is at any degree alpha? :confused:
Thanks in advance for any insight offered.
Marlan

[Ollie]

The alpha is relative to the aircraft's direction of flight. Use an arrow that keeps it's flight direction with changes in flight direction. Pivot the the arrow on it's self CG. Make the arrow's neutral point far behind the pivot, self CG, and huge feathers. Then you can see (measure) the alpha relative to the arrow direction in flight.

[Ollie]

There is more. The air is invisible. The air velocity ( speed and direction) is invisible. The alpha angle is relative to air velocity. So we need to make the air velocity visible so that we can measure alpha relative to air velocity. Be careful of wing air up flow and down flow. Put your arrow pivot way ahead of the nose of the fuselage.

[Sparky Paul]

You have to fly the model at that flap condition, and observe how it performs at different airspeeds.
Do this often enough, and you'll get a feel for when the plane is at the best l/d point.

[marfish]

Thanks guys, I knew you'd come through.
That arrow idea would work great except the carbon arrows I have are all longer than the entire glider.
It looks like short of building a contraption that will support an arrow out front of the plane, I'll just have to eyeball it while practicing pass after pass on the slope like Sparky said, watching for pitch change as the flaps are deflected, and working toward the most effecient turn radius and speed. I guess it would be prudent to dial in the correct amount of elevator trim to keep the plane on a "no-lift" heading as in a vertical dive while testing for best L/D. Then I could get a pretty close estimate of where 2* is so my snap-flap practice will be more productive in a shorter time period.
Lotsa lift,
Marlan

[BMatthews]

It'll only do that at one speed and one speed only. At other speeds the angle will be more or less than your ideal 2 degrees. The speed where this occurs will depend on your wing loading and the corresponding lift coefficient.

Google for Foilsim and set it up to more or less match your airfoil shape and angle of attack. To get it to sort of match your airfoil shape you'll have to know roughly what the effective camber % is for your 8* deflected flap angle. Yes I know it's a very rough approximation but it'll give you some sort of idea on what the flying speed will need to be. Also remember that if the first part of the airfoil is set to 2* and flaps are deflected from that then the REAL angle of attack is more like 4 to 6* when you consider it as an undercambered airfoil. It all depends on what you consider that 2 degrees angle to be governed by.

[marfish]

Agreed. I forgot that dropping the flaps automatically changes the AoA for the wing by moving the TE down. Does Profilli take that into consideration, or does it measure via the mean neutral line of the model?
I have Foilsim too, and have not had good luck getting much applicable info out of it. Maybe I'm not tweeking it in the right place. Lol... So what units are the camber and thickness measured in? It makes a great deal of difference and is not listed.
Marlan

[BMatthews]

Foilsim uses % of chord for both thickness and camber. The little drawing may not look much like the numbers but I'm trusting that the equations inside know the right stuff... :D

It's not a complete answer since it isn't using the actual lift and drag curves for YOUR airfoil but it is good enough for finding the flying speed of a given size and weight. If I need to up the angle of attack past 7 degrees I assume that I've stalled and need to fly faster. It doesn't do a good job at modeling low reynolds number stalling which occurs far sooner at "our" sizes and speeds than full sized.

What I like it for is finding lift coefficients and flying speeds. And to some extent it acts as an interesting indicator of zero lift angles as the airfoil numbers alter.

If Profili allows for flap deflections or you have coordinates for an airfoil with flaps deployed than I'd guess that it would use the proper baseline and alter the camber value. You should see that in the numbers it shows for thickness and camber.

But it's also hard to imagine a high L/D on anything that uses 8* of flap angle. The discontinuity at the hinge line would usually result in separation bubbles at that magnitude of angle.