Physics-flight help


by marinepyre
Tags: physicsflight
marinepyre
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#1
Sep7-13, 04:14 AM
P: 6
I need help with understanding bernoulli's principle, how an aerofoil develops lift i have horrible knowledge and understanding on how it works
And how the aerofoil produces lift in regards to bernoulli's principle.
Diagrams would help alot
Thanks
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rcgldr
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#2
Sep7-13, 10:10 AM
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The simple explanation is that using a wing as a frame of reference, the wing diverts the relative air flow downwards. This corresponds to the wing exerting a downwards force onto the air, and the air exerting an upwards force on the wing. The exertion of a downwards force on the air corresponds to a lower pressure above the wing and/or a higher pressure below the wing. Air accelerates from higher pressure areas towards lower pressure areas, and as the air accelerates, Bernoulli equations notes a relationship between the decreasing pressure and increasing speed, assuming no work is done, and ignoring issues related to turbulence and friction with the wing. A wing performs some work on the air (using the air as a frame of reference, a wing adds energy to the air), and the work related portion of lift violates Bernoulli.

Previous threads:

http://www.physicsforums.com/showthread.php?t=707155

http://www.physicsforums.com/showthread.php?t=699059

http://www.physicsforums.com/showthread.php?t=618500
marinepyre
marinepyre is offline
#3
Sep7-13, 08:10 PM
P: 6
How does the air going downwards make air go upwards i don't understand
Im sorry

marinepyre
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#4
Sep7-13, 08:44 PM
P: 6

Physics-flight help


I also have another problem in regards to finding the maximum lift to drag ratio
I have been given a graph of airspeed over drag graph where airspeed is x axis and drag is y axisClick image for larger version

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I have to find out maximum lift to drag ratio at what airspeed and when it occurs
Please help ASAP
rcgldr
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#5
Sep8-13, 05:19 AM
HW Helper
P: 6,930
Quote Quote by marinepyre View Post
How does the air going downwards make air go upwards i don't understand.
The air doesn't have a net upwards flow. The net acceleration is downwards and the net velocity aft of the trailing edge of a wing is downwards. There's a Newton third law pair of forces, the wing exerts a downwards force onto the air, and the air reacts with an upwards force on the wing and also reacts by accelerating downwards. Assuming this is level flight, gravity keeps the aircraft flying level instead of accelerating upwards.

Quote Quote by marinepyre View Post
I also have another problem in regards to finding the maximum lift to drag ratio ... I have been given a graph of airspeed over drag graph where airspeed is x axis and drag is y axis.
There's not sufficient information. The maximium lift to drag ratio occurs where the lines cross, but there's no scale on the drag axis, and there's no information about the weight of the aircraft to compare the drag force against, even if the drag axis had a scale.
marinepyre
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#6
Sep8-13, 07:50 PM
P: 6
Thanks


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