- #31
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Consider the following, then, please.
Take a sheet of A4 paper and grip the shorter edge between your fingers. The other end of the paper flops downwards.
Now, draw the paper through the air, and the lower edge rises up!
You only need the pressure from the air underneath it being accelerated down to create lift.
Of course, with an aerofoil design you get both that effect AND the flow of air over the top that stays in laminar flow over the top, due to Coanda (resistance to flow separation from the surface), thus follows the profile of the wing (downwards), making for an efficient wing.
Of course, in the stall this is where the flow separates and this exposes the integral nature of the pressure on the top to the bottom, if the pressure on the top becomes larger than under it (stalled air, high pressure) then of course there can never be lift.
But you only 'need' the extra pressure from underneath. No 'Bernoulli' low pressure on top is necessary, though it is an inevitability and modern wing designs take advantage of that, just not 'essential'. I would say low pressure on the top of the wing is effect not cause, but it's not something that can be teased apart, the two happen together.
To prove that applies to the paper example, here are two alternative variations.
First, glue the top edge of the paper to an A4 sheet of card and then grip them both so the card is held horizontal and the paper flops down. Now draw the arrangement through the air. What happens? The paper STILL lifts, yet air cannot flow over the top because there is a piece of cardboard stopping it.
Second, put lots of holes through the paper with a pencil. Now, the pressure on the bottom can neutralise any low pressure on top. What happens? Still the paper lifts, not so much but it lifts.
In fact you can do it with a string bag and still get the same effect.
Or even more extreme, a rope hanging from a helicopter. The rope still lifts as it is pulled through the air.
Take a sheet of A4 paper and grip the shorter edge between your fingers. The other end of the paper flops downwards.
Now, draw the paper through the air, and the lower edge rises up!
You only need the pressure from the air underneath it being accelerated down to create lift.
Of course, with an aerofoil design you get both that effect AND the flow of air over the top that stays in laminar flow over the top, due to Coanda (resistance to flow separation from the surface), thus follows the profile of the wing (downwards), making for an efficient wing.
Of course, in the stall this is where the flow separates and this exposes the integral nature of the pressure on the top to the bottom, if the pressure on the top becomes larger than under it (stalled air, high pressure) then of course there can never be lift.
But you only 'need' the extra pressure from underneath. No 'Bernoulli' low pressure on top is necessary, though it is an inevitability and modern wing designs take advantage of that, just not 'essential'. I would say low pressure on the top of the wing is effect not cause, but it's not something that can be teased apart, the two happen together.
To prove that applies to the paper example, here are two alternative variations.
First, glue the top edge of the paper to an A4 sheet of card and then grip them both so the card is held horizontal and the paper flops down. Now draw the arrangement through the air. What happens? The paper STILL lifts, yet air cannot flow over the top because there is a piece of cardboard stopping it.
Second, put lots of holes through the paper with a pencil. Now, the pressure on the bottom can neutralise any low pressure on top. What happens? Still the paper lifts, not so much but it lifts.
In fact you can do it with a string bag and still get the same effect.
Or even more extreme, a rope hanging from a helicopter. The rope still lifts as it is pulled through the air.