GS Q: Bernoulli's

[andafoo]

A plane flies from low to high altitude at constant velocity. Which of the following is true?

1. At the lower altitude the plane will experience greater lift. A
   At the higher altitude the plane will experience greater lift B
   The lift the plane experiences is not dependent on altitude. C
   At low altitudes Bernoulli's effect is not applicable. D

Once again our friend Bernoulli has the answer. This time the velocity is constant but the height 'h' is changing. By looking at Bernoulli's equation it is clear that if the height 'h' increases then pressure 'P' must decrease - and vice versa (answer choice A.) - in order for the equation to remain constant.


The passage says:
"Bernoulli's equation, P + 1/2ρv2 + ρgh = constant, is often modified when discussing an airplane's wing. The "ρgh" component is usually left out since the difference in distance from the top of the wing to the ground compared to the bottom of the wing to the ground is usually negligible."


What I was thinking was that the pgh term in Bernoulli's only accounted for the difference in height between the top and bottom of the wing. The reason is because the 'P' that we ultimately care for is actually the deltaP between the top and bottom of the wing, for that is what causes lift.

I'm not sure what the the answer explanation is talking about... even if the absolute P changes, how does that change how much lift a wing has? That would mean just because an airplane is flying at a lower altitude, it should have more lift (assuming it is flying at the same velocity as an airplane flying at a higher altitude).

Huh?

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[Phlame217]

Air is denser at ground level than at 30,000 ft.

Denser air means that at lower altitudes, it takes more thrust to move the plane at the same velocity than if it were at 30k ft for one.

and two, it means that less molecules running across the wings creates less lift. Bernoullis simply gives us a way to mathematically quantize this.

 

[andafoo]

Air is denser at ground level than at 30,000 ft.

Denser air means that at lower altitudes, it takes more thrust to move the plane at the same velocity than if it were at 30k ft for one.

and two, it means that less molecules running across the wings creates less lift. Bernoullis simply gives us a way to mathematically quantize this.

Sounds like a great answer... but on an exam, seems like this questions would be inconclusive...

There are two factors, the density of the air, which you say gives less lift at lower altitudes, while the altitude itself (as mentioned by the explanation) gives more lift?