HN-1033 Airfoil question

[flusterfly]

I'm designing a little light lift sloper/DLG thermal catcher. I'm planning on using the HN-1033 airfoil. Looking at the Cl to alpha and the Cd to alpha chart I noticed a little bubble of low drag high lift when the wing is at 3 to 4 degrees positive AoA when the Re are at 75,000 to 250,000. This high lift low drag state would happen when my plane is going apporx. 15 to 45 mph. My question is - Should I put the wing at 3 degrees positive AoA and the stab. at zero (decalage is quess)? This would create a higher drag senario below 15 mph air speed. Would that be a problem from your experience? Would putting the outboard tip at 2 degree be favorable since the tip will be flying at lower Re #'s? Is my thinking way off base?

Adam

[Ollie]

Let's say you design a flying stab. What is the decalage between wing and stab?

In design, the air speed is controlled by changing decalage! It is controlled by 15 to 45 mph depending the decalage range of angles. If the stability is zero, the decalage angle range is zero for any speed by 15 to 45 mph. If the stability is positive the decalage angle range is more than zero. The stability is increasing with the increasing decalage range of angle. With shift in CG, the stability shifts so the angle of decalage range shifts.

My answer to you is build it and then adjust it to your flying style. There is on optimum answer to design process until you compromise many conflictiing objectives.

From Dr. Drela:
"The problem with talking about "optimum" Aspect Ratio is that it's almost impossible to define in a realistic design problem. You can optimize AR for min sink rate or max L/D in straight and level flight, but RC gliders do not spend their life in straight and level flight at the optimum airspeed. There's also launch, zoom, circling slowly, running back, landing, etc, etc. The end result strongly depends in how these factors are weighted against each other."

[Ollie]

There is more.

The wing design is about airfoil, aspect ratio, taper, sweep, twist, strength, stiffness, angle of attack, etc. The wing lift distribition along the span is to get minimized induced drag. See:
http://aero.stanford.edu/WingCalc.html
Or
http://www.amadistrictii.org/cjrcc/wing2/wing.html

[flusterfly]

Thanks, Ollie for your responses! The links are helpful also. Is a full flying stab helpful here....a little more efficient then just tweaking the elevator or shimming?

Adam

[Ollie]

"Is a full flying stab helpful here....a little more efficient then just tweaking the elevator or shimming?"

Either way is fine if it is well done. A poorly hinged stab with gaps is bad. A poorly flying stab with gaps to the fin are bad. Etc, etc.

I have studied designs for over 40 years. By far, the best I have studdied are by Dr. Mark Drela. Study them! Learn from them!

[flusterfly]

So, Ollie, do you think the CG will need to change as the wing AoA is tweaked?

Adam

[Ollie]

Don't confuse AoA with decalage.

"I'm planning on using the HN-1033 airfoil. Looking at the Cl to alpha and the Cd to alpha chart I noticed a little bubble of low drag high lift when the wing is at 3 to 4 degrees positive AoA when the Re are at 75,000 to 250,000." That data is for two dimensional flow. In a real wing the flow is three dimensional, especially near the tips (vortex). The AoA change along the span is dependant on AR, twist, sweepback, taper, etc.

The purpose of CG is for stability. You should only change CG if you need a change in stability. The purpose for changing neutral elevator air speed is for a change in decalage or elevator angle.

There are two purposes for the tail. The size of the tail and its moment arm shifts the nuetral point (NP) of the whole plane behind the CG for positive stability. The distance between the CG and NP measures the stability. The other purpose of the tail is to control the plane pitch attitude with air speed.

[CloudyIFR]

I have a program in MS Excel that will show you the NP and CG and changes in tail sizes and moments would affect. These formulas are from Mark Drela and Martin Simons book on Model Aircraft Aerodynamics. I just plugged the numbers in.

Curtis

It's free and here:
http://h1.ripway.com/cloudyifr/files.htm

Email suterc@msn.com for any questions.

[Ollie]

Curtis,
It sounds very good! Thanks.