7 ms·

Glide ratio and weight are not related. Weight shifts the glide Polar to higher speeds and theoretically improves it slightly due to Reynolds effects.

by roeles 11mo ago

rkomorn 11mo ago
"Weight doesn't affect glide ratio" is one of these things I learned why taking flying lessons that still feels counterintuitive every time I read it. (Don't worry, reader, I never intended to get a pilot's license.)
- ahartmetz 11mo ago
  I don't think it's completely true. Higher weight increases the speed at which the glide ratio is optimal, and drag (parasitic drag in particular, unrelated to generating lift) increases with the square of speed. Basically, flying faster wastes more energy. That effect is going to dominate at some point, probably about 120 km/h or so with a typical glider. At 200 km/h, the glide ratio is garbage (but it's fun). I have flown gliders. I'm not sure if simple descriptions of the phenomenon that glide ratio is independent of weight are missing an asterisk or if I'm just wrong... A decent glider has a ratio of 1:40, an A320 1:17. Is the A320 a "bad plane" or is it optimized for higher speed with the corresponding worse glide ratio? (It also has engines that produce a lot of drag when gliding)
  - rkomorn 11mo ago
    I don't know. On one hand there's your take. On another hand, there are CFIs, the FAA, books, etc. I've only found one search result that agrees with you, so far, and at least a dozen that disagree, but the one that agrees with you has no math in it, and the ones that disagree mostly seem to depend on the same source info, so that doesn't feel conclusive in either direction. The Wikipedia page on lift-to-drag ratio also believes weight does not matter to the ratio. As a side note, your 200km/h example also sounds like it's just not the correct angle of attack or airspeed for the aircraft, so I'm not sure if that example applies?
  - rkomorn 11mo ago
    As a separate reply, I'll add that I think finding where/if this breaks is pretty academic. Eg: you wouldn't build a glider out of heavy material that gives you huge speeds but also huge sink rates. So I think the entire glide ratio conversation mostly fits in the "your plane is fully loaded" vs "your plane is empty" scenario, and the point is that your best glide ratio will be constant, but you'll be gliding at higher speeds if you have more weight.
  - roeles 11mo ago
    Gliders utilize Laminar profiles, while airliners use turbulent profiles. The Laminar profiles perform better, but only when uncontaminated (no bugs or rain). Contaminated turbulent profiles perform better than contaminated Laminar profiles. Since regulations state that you should carry fuel for the worst case scenario, it does not yet make sense to design airliners with Laminar profiles. Naturally, manufacturers are looking for ways around this.
- hermitcrab 11mo ago
  So, 2 gliders with identical geometry, one standard and one made of lead will have the same glide angle? That sounds unlikely.
  - rkomorn 11mo ago
    There is an asterisk that you have to be at the right glide velocity, but yes: they'll have the same glide angle. The leaden one will just go significantly faster. And yes, it does sound unlikely. That's why I made my previous comment. Here are a couple of many posts on the topic: - https://gronskiy.com/posts/glide-ratio-lift-to-drag-and-weight/ https://gronskiy.com/posts/glide-ratio-lift-to-drag-and-weig... - https://skybrary.aero/articles/glide-performance https://skybrary.aero/articles/glide-performance
    - hermitcrab 11mo ago
      Thanks for the links. Weight may cancel out of the equations, but (being a bit pedantic) I suspect 'glide angle in independent of weight' only holds up to a point. Taking things to extremes, if the glider is heavy enough that you are going to have to go supersonic then I suspect a lot of the assumptions become invalid. NB/ spherical cows are unable to glide in a vacuum.
      - wildzzz 11mo ago
        Make a paper airplane and drop it. It likely won't go much further than your feet. Throw it gently and it will go some distance. Throw it harder and it will go further. Glide ratio is the horizontal distance over vertical distance. The vertical distance is the product of (lift - mass)*t^2 where lift is a function of the shape of the wings and the airspeed. So given a higher mass and the same lift, the time to hit the ground will be less when the glider is dropped at 1000ft. Increase the airspeed and you'll have more lift to negate the higher mass. The increased airspeed also means your horizontal distance will be covered faster. The lead glider will travel the same path as the normal one but will be going a lot faster. The reason why gliders are built as light as possible is reduce the work required to lift them, the speed at which to release them, and the interia required to turn them. You also have the benefit of being able to land them at a lower airspeed without injury.

Glide ratio and weight are not related. Weight shifts the glide Polar to higher speeds and theoretically improves it slightly due to Reynolds effects.

by roeles 11mo ago

Glide ratio and weight are not related. Weight shifts the glide Polar to higher speeds and theoretically improves it slightly due to Reynolds effects.

rkomorn 11mo ago

"Weight doesn't affect glide ratio" is one of these things I learned why taking flying lessons that still feels counterintuitive every time I read it. (Don't worry, reader, I never intended to get a pilot's license.)

ahartmetz 11mo ago
I don't think it's completely true. Higher weight increases the speed at which the glide ratio is optimal, and drag (parasitic drag in particular, unrelated to generating lift) increases with the square of speed. Basically, flying faster wastes more energy. That effect is going to dominate at some point, probably about 120 km/h or so with a typical glider. At 200 km/h, the glide ratio is garbage (but it's fun). I have flown gliders. I'm not sure if simple descriptions of the phenomenon that glide ratio is independent of weight are missing an asterisk or if I'm just wrong... A decent glider has a ratio of 1:40, an A320 1:17. Is the A320 a "bad plane" or is it optimized for higher speed with the corresponding worse glide ratio? (It also has engines that produce a lot of drag when gliding)
- rkomorn 11mo ago
  I don't know. On one hand there's your take. On another hand, there are CFIs, the FAA, books, etc. I've only found one search result that agrees with you, so far, and at least a dozen that disagree, but the one that agrees with you has no math in it, and the ones that disagree mostly seem to depend on the same source info, so that doesn't feel conclusive in either direction. The Wikipedia page on lift-to-drag ratio also believes weight does not matter to the ratio. As a side note, your 200km/h example also sounds like it's just not the correct angle of attack or airspeed for the aircraft, so I'm not sure if that example applies?
- rkomorn 11mo ago
  As a separate reply, I'll add that I think finding where/if this breaks is pretty academic. Eg: you wouldn't build a glider out of heavy material that gives you huge speeds but also huge sink rates. So I think the entire glide ratio conversation mostly fits in the "your plane is fully loaded" vs "your plane is empty" scenario, and the point is that your best glide ratio will be constant, but you'll be gliding at higher speeds if you have more weight.
- roeles 11mo ago
  Gliders utilize Laminar profiles, while airliners use turbulent profiles. The Laminar profiles perform better, but only when uncontaminated (no bugs or rain). Contaminated turbulent profiles perform better than contaminated Laminar profiles. Since regulations state that you should carry fuel for the worst case scenario, it does not yet make sense to design airliners with Laminar profiles. Naturally, manufacturers are looking for ways around this.
hermitcrab 11mo ago
So, 2 gliders with identical geometry, one standard and one made of lead will have the same glide angle? That sounds unlikely.
- rkomorn 11mo ago
  There is an asterisk that you have to be at the right glide velocity, but yes: they'll have the same glide angle. The leaden one will just go significantly faster. And yes, it does sound unlikely. That's why I made my previous comment. Here are a couple of many posts on the topic: - https://gronskiy.com/posts/glide-ratio-lift-to-drag-and-weight/ https://gronskiy.com/posts/glide-ratio-lift-to-drag-and-weig... - https://skybrary.aero/articles/glide-performance https://skybrary.aero/articles/glide-performance
  - hermitcrab 11mo ago
    Thanks for the links. Weight may cancel out of the equations, but (being a bit pedantic) I suspect 'glide angle in independent of weight' only holds up to a point. Taking things to extremes, if the glider is heavy enough that you are going to have to go supersonic then I suspect a lot of the assumptions become invalid. NB/ spherical cows are unable to glide in a vacuum.
    - wildzzz 11mo ago
      Make a paper airplane and drop it. It likely won't go much further than your feet. Throw it gently and it will go some distance. Throw it harder and it will go further. Glide ratio is the horizontal distance over vertical distance. The vertical distance is the product of (lift - mass)*t^2 where lift is a function of the shape of the wings and the airspeed. So given a higher mass and the same lift, the time to hit the ground will be less when the glider is dropped at 1000ft. Increase the airspeed and you'll have more lift to negate the higher mass. The increased airspeed also means your horizontal distance will be covered faster. The lead glider will travel the same path as the normal one but will be going a lot faster. The reason why gliders are built as light as possible is reduce the work required to lift them, the speed at which to release them, and the interia required to turn them. You also have the benefit of being able to land them at a lower airspeed without injury.