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mogorzalek (Aerospace)
3 Oct 02 11:41
Does anyone know what the L/D of the SR 71 is at Mach 3?
matt00000 (Aeronautics)
24 Dec 02 1:40
I would say the answer to that is no!
have a look in Janes All the world aircraft for cl and cd... it might even have l/d
ferdo (Mechanical)
24 Dec 02 20:23
Hi,
Maybe you can find something interesting here:
http://www.sr-71.org/blackbird/manual/1/index.htm Regards
Fernando
matt00000 (Aeronautics)
30 Dec 02 20:49
that is an awesome site! well found...
Nothing on performance though...
yshij (Aerospace)
10 Jan 03 13:53
字串6
mogorzalek,
I don't think your question is properly defined. L/D is associated with angle of attack. Generally speaking, L/D increases with angle of attack up to stalling angle of attack. The maximum L/D will be reached around of the beginning of stall. However, the conclusion is based on the condition that flight speed is less than sound.
Super-sonic flight is dramatically different from sub-sonic. According to 2-D linear classic aerodynamic theory, except for additional drag, the thickness and curvature don't generate lift in supersonic flowfields. This is totally different from sub-sonic. This is why all supersonic aircraft are thin and sharp, especially for the wing. Only angle of attack can create lift. One thin flat plate is a perfect supersonic shape.
For supersonic flight, actual flight angle of attack is very small. The aerodynamic loads (both drag and lift) sharply increase due to shock wave and because the load is proposional to (speed)^2. Very little angle of attack can provide enough lift to overcome the gravity. In supersonic flight, lift is not a demand. To overcome drag is every thing, very very important. 字串7
For supersonic flight, L/D is not important. The reasons are (1) design theory is totally different from subsonic. The major thing is to reduce drag D with proper pay-volume. (2) L/D will increase with angle of attack. Supersonic aircraft will never reach the stall/flow separation angle, value of which is significantly higher than subsonic (about 12 degrees for subsonic). Before reach that angle, the aircraft would have already been structurely destoried by too much number of gravity.
Therefore, when people talk about maximum L/D, it usually means the flight is under the condition of subsonic flight or that aircraft wing is under subsonic leading-edge.
mogorzalek (Aerospace)
13 Jan 03 14:05
About the SR 71 L/D. I mean the L?D at cruise at Mach 3. The cruise L/D is a measure of aerodynamic efficiency, and can determine the cruise range of an aircraft.
mogorzalek
radomir (Aeronautics)
13 Jan 03 15:29
字串7
I suppose that mentioned Mach 3 cruise speed is at about 50,000 feet.
matt00000 (Aeronautics)
13 Jan 03 18:06
yshij,
it is clear that all aircraft produce lift regardless of their speed and that in doing so create drag. So I dont see why you say it is not properly defined.
In the transonic region shock drag jumps very high but as you go through mach 2 this drops to a level slightly higher than for subsonic flight (depending on the characteristics of the a/c). So it would be intersting to know what it is...
A plot of L/D at increasing AoA is how one typically looks at L/D. True but even so I think mogorzalek was more curious, than concerned with actual L/D curve at M3+
The SR71 is a delta config, I would expect that it can produce lift way beyond 12degrees AoA.
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