lehpron
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? About Center Of Lift

Mon Jan 07, 2002 6:13 pm

Is it safe to assume that if the center of lift moves back on a wing as the speed passes Mach 1, that it changes position gradually and not suddenly?

Furthermore, if the speed continues to rise, how far back will the center of lift go?
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vc10
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RE:? About Center Of Lift

Mon Jan 07, 2002 11:13 pm

On Concorde the centre of lift moves rewards gradually,but no linearly, as the aircraft accelerates. By the time the aircraft reaches Mach 2 the centre of lift has moved rearwards by approx 6 feet, which is why fuel is pumped to the rear tanks so as to try and move the Cof G rearwards by an equal amount and so keep the aircraft trimmed.
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seagull
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RE:? About Center Of Lift

Tue Jan 08, 2002 3:03 am

However, the answer is that it is NOT safe to assume that, it depends on the design of the aircraft.
 
lehpron
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RE:? About Center Of Lift

Tue Jan 08, 2002 11:38 am

Okay, it's not good practice to assume, I guess.

What if the speed continues to rise, say beyond Mach 3/4/5..etc., how far back will the center of lift go?

Is there a limit, I think that's what I'm asking.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
ThirtyEcho
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RE: RE:? About Center Of Lift

Tue Jan 08, 2002 12:11 pm

Well, the only way you'll go Mach 5 is outside the earth's atmosphere, for all practical purposes, so the wing won't be lifting at all. You'll be following a ballistic trajectory and dependent on an RCS (reaction control system) to maintain directional stability. Even better, you will have figured out a way to get Congress to fund one last X-15 flight.
 
lehpron
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RE:? About Center Of Lift

Wed Jan 09, 2002 10:00 am

ThirtyEcho, don't you think Mach 5 (about a mile per second) is too slow to fly outside the earth's atomosphere, especially since the Space Shuttle must be going at Mach 25 AND 450,000-feet to maintain an orbit?

Realistically, A plane moving at the upper Mach's should fly in the altitude range of 150000 feet where the air is 1000 times thinner than surface airdensity, hence the speed needs to be increased to maintain a flight.

Like Concorde, it's center of lift might move back farther, but by how much? Consider this pix of the Hypersoar,



then imagine a tailcone at the end so it would represent a future commercial trasport. Now suddenly the plane is backheavy since the presumed center of lift is approximately in the middle and the wieght may be at the 2/3rds mark. So my guess is a temporary wing-fin should lift the back end at low speeds and as it goes faster the change in the center of lift should rebalence it.

Does that make sence, or have I still lost you guys?
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
ThirtyEcho
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RE: RE:? About Center Of Lift

Wed Jan 09, 2002 10:32 am

Uh, Lephron, I said "for all practical purposes." This is true and at 150,000 feet (actually lower) there is no such thing as aerodynamic lift. That is why the high altitude versions of the X-15 needed RCS systems. I think that you are thinking of escape velocity to escape the Earth's gravity.
 
lehpron
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RE:? About Center Of Lift

Thu Jan 10, 2002 5:37 pm

ThirtyEcho:

Okay, explain this to me:

When NASA did supercomputer windtunnel tests of the X-30 @ escape velocity while in the atomospheric range of 250,000ft, the model showed a fiction air temperature of 3000 centigrade. If there is no atomosphere to support aerodynamic lift, then there should also be no friction due to the atomosphere.

The idea that there is air-supportable lift at 80,000 where the SR-71 has gone and not at higher altitudes doesn't make sence.

Granted this was done in a cybertunnel, the X-43 will experience a similar situtation as it flies @ Mach 5 at 110,000 feet later this year. Have you seen this thing? It's built around it's engine, an airbreather, a scramjet. As the air gets compressed by the forward section of the vehicle, the reaction is that it pops up -- as forward lift. That thing would go completely out of control if it had only forward lift.





Pertaining to my original question, the center of lift must move back far enough so the plane is balanced; or is that wrong?
The meaning of life is curiosity; we were put on this planet to explore opportunities.

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