Solve Physics: Understand Bernoulli's Principle & How an Aerofoil Develops Lift

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Discussion Overview

The discussion revolves around understanding Bernoulli's principle and its application to how an aerofoil develops lift. Participants explore the mechanics of lift generation, the relationship between air pressure and velocity, and also touch upon related problems such as finding the maximum lift-to-drag ratio using a provided graph.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant expresses a lack of understanding of Bernoulli's principle and how it relates to lift generation by an aerofoil, requesting diagrams for clarification.
  • Another participant explains that a wing diverts air downwards, creating a downward force on the air and an upward force on the wing, linking this to Bernoulli's principle while noting that the work done by the wing on the air complicates the explanation.
  • Several participants question how downward airflow can result in upward lift, with one emphasizing that the net acceleration of air is downwards and discussing the implications of Newton's third law in this context.
  • A participant raises a separate issue regarding the maximum lift-to-drag ratio, indicating that the provided graph lacks sufficient information to determine the maximum ratio accurately.

Areas of Agreement / Disagreement

Participants express confusion regarding the mechanics of lift generation, particularly the relationship between downward airflow and upward lift. There is no consensus on the explanation of these phenomena, and the discussion remains unresolved regarding the maximum lift-to-drag ratio due to insufficient data.

Contextual Notes

Participants highlight limitations in the provided information, particularly regarding the graph for lift-to-drag ratio analysis, which lacks scale and context for accurate interpretation.

marinepyre
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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 a lot
Thanks
 
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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:

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

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

https://www.physicsforums.com/showthread.php?t=618500
 
Last edited:
How does the air going downwards make air go upwards i don't understand
Im sorry
 
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
ImageUploadedByPhysics Forums1378604607.787760.jpg

I have to find out maximum lift to drag ratio at what airspeed and when it occurs
Please help ASAP
 
marinepyre said:
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.

marinepyre said:
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.
 
Thanks
 

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