# How to maximize L/D ratio by design in a high altitude glider? [closed]

EDIT: Some initial confusion with TAS and IAS confused me, I now understand this better, thanks for everyone who helped.

I'm designing a high-altitude glider, and I'm wondering how the angle of attack changes in a flight. I know a glider will go faster higher in the atmosphere. It's almost certain I'm lacking something crucial about aerodynamics.

Here are my questions:

• Will a glider automatically adjust to the most optimum angle of attack for the given speed?
• If the AoA changes in flight then the L/D ratio and hence the glide ratio must change, too, right?
• The AoA must change in a high altitude flight, and how does one adjust for that?
• How should one mount the wing to the fuselage if the AoA is changing in flight?

I'm horrible at communicating my questions because I don't really know why this isn't working for me, so I apologize in advance.

• I've only flown power driven planes: when you say "adjust" for the change of angle of attack, do you mean something like trimming? The pilot usually makes changes in the pitch to get to the AoA needed to achieve whatever they want at whatever speed/altitude they're at. Apr 3 at 1:23
• Why are we voting to close a new user's Q, without a single word of explanation or comment about why it was insufficiently precise? Tsk, tsk, we shouldn't behave this was as a community. P.S. I don't disagree that the question could be tighter, but it would be kind to provide the OP some guidance before a smack-down. Apr 3 at 13:36
• The Perlan 2 is an excellent reference. This glider has a crew of 2 and can reach an altitude of over 76,000 feet with a maximum glide ratio of 43. Apr 3 at 13:54
• @KennSebesta The system-generated explanation is pretty clear: Needs more focus This question currently includes multiple questions in one. It should focus on one problem only.
– Ralph J
Apr 3 at 15:12
• @RalphJ yes, and no. The "close" is quite abstract, and doesn't hint at what is too out of focus. Putting ourselves in the shoes of a brand new contributor, who doesn't have context gleaned from being here for years, what should the OP do to fix things? There are a literal infinite number of ways to modify the question, many of which will suffice, and many more of which won't. Without good feedback it verges on a "find me a rock" problem (jonathanbecher.com/2020/08/30/the-bring-me-a-rock-phenomenon). Apr 4 at 16:27

Will a glider automatically adjust to the most optimum angle of attack for the given speed?

Yes. Actually, it will adjust to the only possible angle of attack at a given speed. Within limits: If speed is too low, no angle of attack which creates enough lift will exist and the airplane stalls.

If the angle of attack is too high, more lift will be created exceeding weight and the airplane will begin to climb. Without adjusting of the pitch angle this will immediately lower the angle of attack until lift equals weight.

If the AoA changes in flight then the L/D ratio and hence the glide ratio must change, too, right?

Right.

The AoA must change in a high altitude flight, and how does one adjust for that?

Not necessarily. Normally, the adjustment for lower air density is done by increasing speed such that dynamic pressure will stay constant. At the same dynamic pressure, the same angle of attack will cause the same lift, so it can stay constant. What changes is the possible speed range.

How should one mount the wing to the fuselage if the AoA is changing in flight?

Pick the angle of attack for the wing which will be needed at the design point and then adjust the fuselage angle such that it will add the minimum amount of drag. Keep in mind that the angle at which the wing is now attached to the fuselage (called incidence) will also determine fuselage attitude during landing!

With a high aspect ratio wing you will see only limited changes in angle of attack, so the small variation of the fuselage angle will not dramatically change flow conditions around it. Pay more attention to the local pressure gradients to avoid early flow separation in the wing-fuselage intersection.

Let's keep it subsonic to help avoid significant changes in Lift/Drag ratio.

Take heart, this 1322 lb aircraft flew to 96,000 feet under power with an indicated airspeed of around 20 knots. Using the True Air Speed calculator, you should have no problem with 50 knots IAS from 80,000 feet down to sea level.

Once you know the weight of the glider, design the wing area to lift that weight based on the lift equation:

Lift=density×area×coefficient of lift×True airspeed$$^2$$

As Peter suggested, build it with enough wing area to glide at 50 knots indicated airspeed without too high an angle of attack.

If you are really clever, make adjustments with wing size and weight until your best glide airspeed is around 50 knots.

You can make your glider staticly stable so it holds 50 knots in a glide.

Design iterations can be tested at the nearest hill. If you are lifting your glider to altitude with a balloon, this is all very doable. Include a tracking device (and maybe a camera too) in your glider and good luck.

• Disclaimer: please make sure you check with authorities before flying to 80,000 feet. Apr 3 at 2:57