Comment by H8crilA
2 years ago
Case in point would be modern gliders (sailplanes). One simple parameter that describes their aerodynamic performance is the maximum achievable Lift/Drag ratio, and that dimension-less ratio has climbed from ~30 in the 1960s to as high as 75 today. That means modern gliders can, using the same altitude/energy, go over 2 times further horizontally. The L/D is not the ultimate decider of performance but it is quite representative of the aerodynamic performance improvements.
BTW, all lift based flying objects have an L/D ratio (which depends mainly on the airspeed), this includes birds, fighter jets, commercial airliners; and the discrepancies can be pretty interesting. For example if one looks at the L/D of the Concorde vs a subsonic jet it becomes clear why it was so damn expensive to operate. Or why the U-2 looks like a glider :). I cannot find any aerodynamic performance data on any famous long endurance (>24h) unmanned drone, but I bet it's rather high as well.
> Concorde
Another good example is the space shuttle. It does actually glide back down. But it glides like a brick at first (1:1 during its initial braking into the atmosphere), and then like a less dense brick (2:1 while it's still supersonic), and then like a brick with shitty wings (a whopping 4:1 or whatever on final approach). Which is about what the Concorde is during landing, 4:1, yea.
Pretty crazy stuff
(Obviously the space shuttle was a tradeoff for, you know, getting it into orbit via rocket)
Your numbers are right but your analogies are misleading. I get “glides like a brick” is hyperbole, but you’ve added enough detail I can see people taking it seriously.
A brick’s L/D is much worse than 1:1. I’m seeing people say 1:10 online, but I can’t find a source and I think that’s incredibly high. A real brick is going to tumble and essentially not make any lift.
A less dense brick will have the same L/D. L/D is about the shape, not the mass.
Space Shuttle pilots themselves referred to it as “The Flying Brick,” I think that is mostly what they were referencing. It was a term of endearment :)
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I’m trying to get my head around the L/D being independent of mass. Does lift scale with airspeed at the same rate as drag? Or is L/D only considering lift-induced drag (whatever the term is) and not total drag including parasitic drag?
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>> tumble and essentially not make any lift
Tumbling itself can produce lift. The difference in drag between one side and the other can result in net pressure differences for a moving object. This is the basis of many baseball pitches. Spin a brick fast enough and it might just be able to climb if thrown horizontally.
If static airfoils are complicated, try looking into airfoils that rotate or otherwise move in relation to airflows. A Russian engineer once said that all problems in aerospace are placed on the tip of every helicopter blade.
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I guess you get maximum L/D out of a brick by giving it some serious backspin (which is a stable configuration so it might be able to maintain it on the way down) to set up the lifting circulation around it. But would this count?
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I saw the space shuttle land once. From my perspective, it seemed to drop like a rock (fast!) and then as it got closer to the ground, it started "flying". I'd never seen anything like it.
The flare-out is really difficult to get right, from what I’ve read. As soon as you start leveling off, air speed is going to drop really fast, and you have very little time to get that bugger on the ground before you get in a stall. They used to practice using a modified jet, flying from high altitude to a landing with thrust reversers engaged all the way!
For whatever reason the way I think about this phenomenon is like autorotation in a rotary craft.
I personally give the space shuttle a little slack on the aerodynamics department when I remember that it enters the atmosphere at nearly Mach 25, and is big enough to produce two separate sonic booms. It was probably a very wasteful design (burdened by the military requirements), but it's like nothing else we've ever built!
The case with gliders and U2 and the max L/D is due to the wing aspect ratio (look up the formula for drag polar). Modern aircraft have much higher L/D because they have long skinny wings and these wings are possible because we moved from aluminum to carbon reinforced composites.