Blackbird
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Joined: Wed Oct 06, 1999 10:48 am

Maximum Theoretical And Practical L/D Ratios

Tue Nov 04, 2008 1:57 am

I'm wondering what the maximum theoretical L/D ratio and the maximum practical L/D ratio achievable by an airfoil or airplane/airfoil combination at the following Mach numbers?

SUBSONIC
Mach 0.70
Mach 0.80
Mach 0.90

SUPERSONIC
Mach 2.0
Mach 2.5
Mach 2.7
Mach 3.0
Mach 3.2
Mach 4.0

HYPERSONIC
Mach 5.0
Mach 6.0
Mach 7.0
Mach 8.0
Mach 9.0
Mach 10.0
Mach 12.0

Also does the Breguet-Range equation actually equate to a plane having an actually higher L/D ratio as a result of mach number increased?


Blackbird
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Maximum Theoretical And Practical L/D Ratios

Tue Nov 04, 2008 5:42 am



Quoting Blackbird (Thread starter):
I'm wondering what the maximum theoretical L/D ratio and the maximum practical L/D ratio achievable by an airfoil or airplane/airfoil combination at the following Mach numbers?

Max theoretical would just be the L/D of the 2D airfoil at the speed of interest. But you can never achieve that in reality.

Max practical is a very strong function of aspect ratio (for subsonic). Off the top of my head, I'm not sure what happens at supersonic but the aspect ratio affect must taper off because the wingtips can only communicate with a small portion of the flow.

Quoting Blackbird (Thread starter):
Also does the Breguet-Range equation actually equate to a plane having an actually higher L/D ratio as a result of mach number increased?

Sort of. The B-R equation just looks at what the L/D actually is...it doesn't really care how you got it. However, the integration behind the equation assumes that L/D (and propulsive efficiency and speed) are constant over the flight, and that's probably not a legitimate assumption for some high Mach flights.

Tom.
 
Blackbird
Topic Author
Posts: 3384
Joined: Wed Oct 06, 1999 10:48 am

RE: Maximum Theoretical And Practical L/D Ratios

Thu Nov 06, 2008 5:23 am

Tom,

Quote:
Max theoretical would just be the L/D of the 2D airfoil at the speed of interest. But you can never achieve that in reality.

Okay, can you give me estimates of the maximum L/D ratios you can achieve at Mach 3 in actuality?

Quote:
Max practical is a very strong function of aspect ratio (for subsonic). Off the top of my head, I'm not sure what happens at supersonic but the aspect ratio affect must taper off because the wingtips can only communicate with a small portion of the flow.

Hmmm, could you look up what does happen under supersonic conditions?


Blackbird
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Maximum Theoretical And Practical L/D Ratios

Thu Nov 06, 2008 6:54 am



Quoting Blackbird (Reply 2):
Okay, can you give me estimates of the maximum L/D ratios you can achieve at Mach 3 in actuality?

No idea...you should be able to get a back-of-the-envelope for a basic supersonic diamond airfoil without too much trouble, but what you really want is some old NACA datasets. I'm just not sure if they went up to Mach 3.

Quoting Blackbird (Reply 2):

Hmmm, could you look up what does happen under supersonic conditions?

I'm not sure I've ever seen that exact subject in a text, although I'm sure it exists somewhere. Logically, the 3D performance has got to head towards the 2D L/D curve as Mach number goes up because more and more of the wing can't "see" the wingtip. This is why you see a reverse cutback on the wingtips of some older supersonic aircraft.

Tom.
 
Blackbird
Topic Author
Posts: 3384
Joined: Wed Oct 06, 1999 10:48 am

RE: Maximum Theoretical And Practical L/D Ratios

Thu Nov 06, 2008 10:54 pm

Okay, let's try this again...

Does anybody here have any figures as to the maximum L/D ratio a double-delta or a cranked-arrow wing can achieve under actual-world conditions at Mach 3.0 (including trim-drag)?


Blackbird

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