Lift on aircraft wing

Author
Discussion

silverfoxcc

Original Poster:

7,433 posts

132 months

Tuesday
quotequote all
Putting this in very basic terms ,,and that there may well be plus and minus in the answers with regarding various wing shapes the question is

As i understand it the shape of a wing makes the air pressure passing over it less than the pressure under the wing. so if there is a figure of say 14 psi under the wing, what would be it above?


lufbramatt

5,025 posts

121 months

Tuesday
quotequote all
Depends on wing loading (mass of aircraft divided by wing area, usually given in pounds per square foot)

Divide by 144 to convert to psi

for example wing loading of a Spitfire is 28 pounds per square foot.

28/144 = 0.19psi

so 13.81psi above the wing

Boeing 737 wing loading 104 pounds per square foot

104/144 = 0.72psi

so 13.28psi above the wing




2xChevrons

2,440 posts

67 months

Tuesday
quotequote all
silverfoxcc said:
Putting this in very basic terms ,,and that there may well be plus and minus in the answers with regarding various wing shapes the question is

As i understand it the shape of a wing makes the air pressure passing over it less than the pressure under the wing. so if there is a figure of say 14 psi under the wing, what would be it above?
It's usually less than one psi of pressure difference between the upper and lower wing surface - often barely any difference at all on light aircraft with relatively big wings. Divide the wing loading (weight carried divided by wing area) in pounds/square foot by 144 and you get the theoretical pressure difference.

So for a Boeing 787 it's about 0.85psi (14psi under the wing, 13.15psi above). For an F-35A fighter it's 0.74psi, and for a Cessna 172 it's a hair under 0.1psi. For a modern sailplane it's 0.06psi.

But through the magic of physics, a small pressure over a big area can do a lot of work.

If you crunch the numbers - a fully laden Boeing 787 is generating 496,699 pounds-force via the pressure differential on its wing surfaces. But weighs over 502,000lbs. The Cessna is also just short of being able to lift its gross weight via wing pressure force alone. The difference comes from the lift generated by the wing's angle of attack; the wing is angled down from its leading edge and deflects air downwards.

These numbers are in the weird world of 'accurate but theoretical' - for any given specific flight condition the exact balance between lift generated by the Bernoulli principle and lift generated by angle of attack will be different, and of course the application of things like flaps, slats, droops etc. will change it further.

RizzoTheRat

23,764 posts

179 months

Tuesday
quotequote all
lufbramatt said:
<good stuff>
From the name and content, are you a Loughborough Aero Eng grad?

lufbramatt

5,025 posts

121 months

Tuesday
quotequote all
RizzoTheRat said:
lufbramatt said:
<good stuff>
From the name and content, are you a Loughborough Trans Tech grad?
Loughborough Industrial Design smile

My job involves lots of poring over old drawings of aircraft and although I don't design ones that actually fly it helps to understand a bit about how they work.

I did originally want to go into aero engineering but struggled with A-level maths so took a slightly different course.

Simpo Two

81,665 posts

252 months

Tuesday
quotequote all
I always assumed that 'wing loading' was how much force you could apply in a turn or pull-up before they snapped off!

48k

11,505 posts

135 months

Tuesday
quotequote all
"Bernoulli's principle is not how wings generate lift" can of worms being opened in 3... 2....

hehe

TGCOTF-dewey

3,753 posts

42 months

Tuesday
quotequote all
lufbramatt said:
Loughborough Industrial Design smile

My job involves lots of poring over old drawings of aircraft and although I don't design ones that actually fly it helps to understand a bit about how they work.

I did originally want to go into aero engineering but struggled with A-level maths so took a slightly different course.
My first degree was industrial design... Ah the joy of Magic Markers, pastels, and lighter fluid.

Mave

8,129 posts

202 months

Tuesday
quotequote all
Simpo Two said:
I always assumed that 'wing loading' was how much force you could apply in a turn or pull-up before they snapped off!
It's normally specified in 1g flight - which is why fighters have lower wing loading than bombers smile

Mave

8,129 posts

202 months

Tuesday
quotequote all
48k said:
"Bernoulli's principle is not how wings generate lift" can of worms being opened in 3... 2....

hehe
hehe

Yertis

17,251 posts

253 months

Tuesday
quotequote all
lufbramatt said:
Loughborough Industrial Design smile

My job involves lots of poring over old drawings of aircraft and although I don't design ones that actually fly it helps to understand a bit about how they work.

I did originally want to go into aero engineering but struggled with A-level maths so took a slightly different course.
Have you appeared in that telly programme yet?

LimaDelta

5,684 posts

205 months

Tuesday
quotequote all
Mave said:
48k said:
"Bernoulli's principle is not how wings generate lift" can of worms being opened in 3... 2....

hehe
hehe
...So, how does a flat winged balsa glider fly. Upside down...?

hehe

lufbramatt

5,025 posts

121 months

Tuesday
quotequote all
Yertis said:
lufbramatt said:
Loughborough Industrial Design smile

My job involves lots of poring over old drawings of aircraft and although I don't design ones that actually fly it helps to understand a bit about how they work.

I did originally want to go into aero engineering but struggled with A-level maths so took a slightly different course.
Have you appeared in that telly programme yet?
I did my best to avoid that.

Simpo Two

81,665 posts

252 months

Tuesday
quotequote all
LimaDelta said:
Mave said:
48k said:
"Bernoulli's principle is not how wings generate lift" can of worms being opened in 3... 2....

hehe
hehe
...So, how does a flat winged balsa glider fly. Upside down...?

hehe
Angle of attack would be my first-principle guess (as it was when I was 11 and flying such things). Oh and balsa is very light so it goes up nuts

Mave

8,129 posts

202 months

Tuesday
quotequote all
LimaDelta said:
Mave said:
48k said:
"Bernoulli's principle is not how wings generate lift" can of worms being opened in 3... 2....

hehe
hehe
...So, how does a flat winged balsa glider fly. Upside down...?

hehe
Its almost as if you need to put a vertical acceleration into a mass of air to create a lift force.... wink

Edited by Mave on Tuesday 21st March 18:32

GliderRider

1,592 posts

68 months

Tuesday
quotequote all
2xChevrons said:
If you crunch the numbers - a fully laden Boeing 787 is generating 496,699 pounds-force via the pressure differential on its wing surfaces. But weighs over 502,000lbs. The Cessna is also just short of being able to lift its gross weight via wing pressure force alone. The difference comes from the lift generated by the wing's angle of attack; the wing is angled down from its leading edge and deflects air downwards.
Depending upon its angle of attack, a small amount of lift will be generated by the fuselage. Being pretty inefficient as a wing, fuselage lift would be minimal in the cruise as it would create unwanted drag.

oakdale

1,562 posts

189 months

Tuesday
quotequote all
2xChevrons said:
It's usually less than one psi of pressure difference between the upper and lower wing surface - often barely any difference at all on light aircraft with relatively big wings. Divide the wing loading (weight carried divided by wing area) in pounds/square foot by 144 and you get the theoretical pressure difference.

So for a Boeing 787 it's about 0.85psi (14psi under the wing, 13.15psi above). For an F-35A fighter it's 0.74psi, and for a Cessna 172 it's a hair under 0.1psi. For a modern sailplane it's 0.06psi.

But through the magic of physics, a small pressure over a big area can do a lot of work.

If you crunch the numbers - a fully laden Boeing 787 is generating 496,699 pounds-force via the pressure differential on its wing surfaces. But weighs over 502,000lbs. The Cessna is also just short of being able to lift its gross weight via wing pressure force alone. The difference comes from the lift generated by the wing's angle of attack; the wing is angled down from its leading edge and deflects air downwards.

These numbers are in the weird world of 'accurate but theoretical' - for any given specific flight condition the exact balance between lift generated by the Bernoulli principle and lift generated by angle of attack will be different, and of course the application of things like flaps, slats, droops etc. will change it further.
The figures you quote don't make any sense to me without you stating the speed and angle of attack of the aircraft, do you mean at level cruising speed?

The lift of the wings may be limited to those figures by trim to maintain level flight but the wing will be capable of providing much more lift, if that were not the case the aircraft would never get off the ground.

Wing lift rises in proportion to the square of the airspeed so the wing will have a much higher lift capacity than the mass of the aircraft at speeds higher than take off.

williamp

18,660 posts

260 months

Tuesday
quotequote all
Very interesting thread... and no treadmill in sight!!

Mave

8,129 posts

202 months

Tuesday
quotequote all
oakdale said:
2xChevrons said:
It's usually less than one psi of pressure difference between the upper and lower wing surface - often barely any difference at all on light aircraft with relatively big wings. Divide the wing loading (weight carried divided by wing area) in pounds/square foot by 144 and you get the theoretical pressure difference.

So for a Boeing 787 it's about 0.85psi (14psi under the wing, 13.15psi above). For an F-35A fighter it's 0.74psi, and for a Cessna 172 it's a hair under 0.1psi. For a modern sailplane it's 0.06psi.

But through the magic of physics, a small pressure over a big area can do a lot of work.

If you crunch the numbers - a fully laden Boeing 787 is generating 496,699 pounds-force via the pressure differential on its wing surfaces. But weighs over 502,000lbs. The Cessna is also just short of being able to lift its gross weight via wing pressure force alone. The difference comes from the lift generated by the wing's angle of attack; the wing is angled down from its leading edge and deflects air downwards.

These numbers are in the weird world of 'accurate but theoretical' - for any given specific flight condition the exact balance between lift generated by the Bernoulli principle and lift generated by angle of attack will be different, and of course the application of things like flaps, slats, droops etc. will change it further.
The figures you quote don't make any sense to me without you stating the speed and angle of attack of the aircraft, do you mean at level cruising speed?

The lift of the wings may be limited to those figures by trim to maintain level flight but the wing will be capable of providing much more lift, if that were not the case the aircraft would never get off the ground.

Wing lift rises in proportion to the square of the airspeed so the wing will have a much higher lift capacity than the mass of the aircraft at speeds higher than take off.
To be honest, I think that referring to lift generated by Bernoulli, lift generated by pressure and lift generated by angle of attack is just a confusion.

all the lift generated comes via pressure on the wing (neglecting fuselage effects). If you increase angle of attack, then you increase the pressure differential (at a constant speed and altitude)

Angle of attack is just a reference, there's nothing special about zero degrees that says you get lift via a different phenomena to when you've got extra angrier of attack. It's just a way of setting how much lift you've got when the aircraft is on the runway.

So you've got just under 1g of lift on the take off run, then because you're slow you need a reasonable angle of attack to generate sufficient lift, and take off. Ignoring flaps, as you accelerate (so lift increases) but also climb (so lift decreases) you trim up and down the CL / alpha curve to keep 1g lift (ie 0.85ish psi in the example)

GliderRider

1,592 posts

68 months

Wednesday
quotequote all
Mave said:
all the lift generated comes via pressure on the wing (neglecting fuselage effects). If you increase angle of attack, then you increase the pressure differential (at a constant speed and altitude)
At anything greater than a horizontal thrust line, some lift is coming from the thrust generated by the engine. In a sustained completely vertical climb, if we exclude anything from kinetic energy, then all the lift is from the aeroplane's propeller or jet thrust.