Gliding object - Wind tunnel procedure?

[badoumba]

Hi experts!

I am trying to qualify the flight performances of a non-powered aircraft in CFD. I have no idea about how it is done in a real wind tunnel, but to calculate the Cl and Cd coefficients of the aircraft, I had to provide an incidence angle (which I assume to be 0° without any thrust), and I had to provide an initial value for the speed - as in the wind tunnel (which I choose arbitrary then, close to what I think it should be from experience) + the weight and surface that I know.

My intent is to determine
-the maximum glide ratio (based on the drag polar curve I guess

https://en.wikipedia.org/wiki/File:DargPolarAL.png)

-the coresponding forward speed (based on the glider polar curve I guess

https://en.wikipedia.org/wiki/File:GliderPolar.png)

I am however confused by several aspects:
- Without any thrust, does a gliding object has an incidence of 0°? So how can I deduce the best glide ratio from the drag polar curve which requires different incidence angles to be built ?
- How do we connect the glider polar curve to the drag polar so I can determine the corresponding forward speed?

So to sum up, my question would be : with the possibility to get the Cl and Cd coeffs of my entire gliding object for a given initial angle and speed, what would be the procedure to calculate the maximum glide ratio and corresponding forward speed.

Thanks in advance for any clarification.

 

[rb1957]

@berkshire, I was remembering that as I wrote it, and it makes sense (I think) increasing max L/D speed and increasing sink rate. and of course gliders don't have to pay the price (or at least the full price) to carry the extra weight ... not when they get towed to release altitude.



another day in paradise, or is paradise one day closer ?

 

[badoumba]

Thanks for sharing the doc.

As L/D varies both with AoA and speed, I am using at the moment a dichotomic approach to first determine the AoA with a guessed speed, and then refine it by playing around with different speeds. The final values give me the L/D, so the gliding angle. The speed is experimental and given by the CFD simulation. I have to confront this value to the theoretical one but as I said CFD doesn't ask for any weight value.

 

[rb1957]

ok, AoA for L/D max is slightly affected by Re, but there is an L/D max at a specific Re (close 1E5 by the look of it) and so a specific speed for L/D max.

another day in paradise, or is paradise one day closer ?