# How does a glider in steady descent glide with negative AoA? [closed]

EDIT: I now understand relative wind is parallel to the travel of the glider. Ignorance was the basis of my earlier confusion.

I looked at some toy and commercial gliders and all of their wings were mounted with a 0 or a positive angle of incidence. This means that when a glider is gliding down, say at a glide angle of 5 degrees, wouldn't the angle of attack be -5 degrees? And wouldn't the negative AoA make the glider fall from the sky rapidly because of all of the down force? Or does the relative wind change?

I am making a small glider using the airfoil A18. I am confused because the glider doesn't seem to have any data below an AoA of 2. I am further confused because an angle of attack of 2 degrees has the highest Cl/Cd ratio which I thought usually happened at an AoA of 6-7. Furthermore, the Reynolds number is the highest which means that the wing creates turbulence which again I thought happened at a high AoA.

My glider will have a computer with an AHRS system that can control the elevator with a PID system so it can aim at one specific pitch, and I believe that pitch would be the glide angle. But to find the glide angle, I would need to know the lift and drag force which comes from the lift and drag coefficients. I don't have a wind tunnel so I can't calculate the lift coefficient at a different AoA. Plus, I'm launching at a high altitude (80,000 feet) so the lift force will change throughout the flight. Please tell me if I'm missing anything!

I also desperately need help on reading the graphs on AirfoilTools.com. Which graph is most important? Which polar should I read?

Why do these two different polars have different Reynolds number, with the same angle of attack? Is the Reynolds number something you input instead?

Am I overthinking this? Should there just be one angle, based on guess-timating, that my flight computer pitches down at, like -10 degrees? Or how would I gaurentee a stable AoA thoughtout my whole flight (probably 2 hours)?

Again, I am trying to make a sub-249 gram glider that is dropped, not thrown, from a high altitude. I welcome any tips.

Please excuse my ignorance and sorry for so many questions!

• Roughly what chord and what airspeed? That gives the Reynolds number. Commented Mar 28 at 20:33
• If manueverability isn't crucial, why not just make it very statically stable like the "free flight" (uncontrolled) gliders common in 1930-1960, instead of fussing with PID? Commented Mar 28 at 20:35
• Regarding Reynolds number, this might help Commented Mar 28 at 22:15
• Way too many questions packed into one. You are aware that "free-flight" model airplanes, including gliders, have successfully flown for close to a century now, with no guidance of any kind, right? If not, google it. Commented Mar 28 at 23:02
• If you're really talking about weather balloons and AHRS, then you need a much better foundation in aerodynamics & aircraft design than we're going to be able to give you on a Q&A message board. It's okay to misunderstand what AoA is -- we've all been there -- but to leap from that point on the learning curve to creating new designs is pretty ambitious. May be best to slow down, study some basic aerodynamics, build & fly some gliders that are known to fly well, and build experience. Then you'll be in a better place to attempt the sort of project you're talking about here.
– Ralph J
Commented Mar 29 at 4:55

In a glide, the relative wind is coming from below, giving a greater angle of attack than in level flight.

In a 5 degree glide with 0 angle of incidence and a nose-level attitude, the angle of attack is positive 5 degrees, not negative 5 degrees.

You don't want to pitch to the glide angle- that would give an angle of attack of 0. You want to pitch a few degrees above the glide angle.

Keep in mind that the lift and drag ratio for the airfoil is not the same as for the aircraft. A typical fuselage produces a lot of drag and very little lift. The best ratio for an airfoil tends to be at a smaller angle of attack than it is for a whole aircraft because the drag of the wings is a relatively small portion of total drag. So pitching up produces a lot more total lift and not that much total drag.

• Okay, so how do I make sure that the glider sustains the 2 degree angle of attack that the airfoil says it needs if the angle of attack can change? Does it do it by itself? Commented Mar 28 at 21:43
• @CharlesNicholson It depends on what instrumentation you have. With GPS and vsi you could compute your glide angle and from there determine the required pitch angle. With an angle of attack sensor you could just use that. Or you could just rig your glider so that it's naturally trimmed around the desired angle of attack. Etc. Commented Mar 28 at 21:49
• @CharlesNicholson Though also remember that the best angle of attack for the airfoil is probably not the best glide for the whole aircraft. Commented Mar 28 at 21:51
• How would I rig my glider so it is naturally trimmed? Commented Mar 28 at 21:56
• @CharlesNicholson That's a big question for a comment! It depends on the whole design of your glider. But a typical design has stability in angle of attack, and will tend to trend to one specific angle of attack. You can effect what angle of attack this is in many ways, such as changing the size or angle of incidence of the horizontal stabilizer. Commented Mar 28 at 22:36