What two types of forces contribute to the lift on a wing?

  • #1
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What two types of forces contribute to the lift on a wing?
 

Answers and Replies

  • #2
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For the scholar solution:

1: Air Pressure differential due to Bernouilli's law
2: forces due to the Newtons laws. accelleration of air and reaction force, The downwash of air creates a reaction uplift in the wing.

A good link: http://www.theken.org/forum/html/article.php?sid=5 [Broken]
 
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  • #3
Matt
If you are using the word "lift" as the actual action of flight (lift and flight have been used interchangedly on the sites I have researched)...

The four main forces of flight are:
1)Lift (opposes gravtiy)
2)Drag
3)Thrust (opposes drag)
4)Weight (aka Gravity)

http://sln.fi.edu/flights/own2/forces.html [Broken]

Within the category of lift, the following exist:

-Another name for lift, besides flight, is airfoil lift or Bernoulli's lift: (http://www.allstar.fiu.edu/aero/flight41.htm)

Main Forces acting on Lift:

1) Newton's Third Law of Gravity (for dynamic lift)
2) "Angles of Attack" = these are the angles at which air pressure is exerted against the airfoil (it can be positive, neutral, or negative pressure)

Source: http://www.allstar.fiu.edu/aero/flight41.htm
 
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  • #4
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1) Bouyancy
2) Thrust

Thats the best I can think of, without repeating other solutions. I'll be interested to see how the ans is worded.
 
  • #5
Matt
Air Pressure Is the Backbone of Lift

"According to Bernoulli's Principle, slower air has higher pressure than faster air. That means that the air pressure pushing up on the bottom of the wing is greater than the pressure pushing down, so the wing goes up."

This statement encompasses the forces of air pressure and the idea of Newton's 3rd Law (the second force)- Every action has an opposite and equal reaction:

"...the air pressure pushing up on the bottom of the wing is greater than the pressure pushing down, so the wing goes up"


Moreover, just the fact that wind is hitting the bottom of the tilted wing (airfoil) gives it lift - think of a kite. The upwardly tilted piece of fabric on a kite catches the wind to direct it upward.

To create lift to begin with, however, one major component is neeed: thrust. The thrust, aka forward motion, is created by the plane's engine. Without forward motion wind and pressure would not pass over the plane's wings.
 
  • #6
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In a bird's wing vacuum also plays a part. The bird slaps its wings together pulls them apart leaving a vacuum to begin its flight.

Vacuum and pressure.
 
  • #7
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In this question lift does not equal flight. Most of you have one answer correct, figure out the other :smile:
 
  • #8
Matt
Arrrgg mateys, back to the drawing board....
 
  • #9
Matt
Ok, let's look at this question in a different light...

Air pressure is obviously one of the correct answers. Air pressure corresponds to Newton's Laws of motion (esp. the 3rd Law - Every action has an opposite and equal reaction) The force exerted on the lower part of the wing produces lift.

But what about perhaps that second critical force...momentum:

"The lift of a wing is equal to the change in momentum of the air it is diverting down. Momentum is the product of mass and velocity (mv)."

The idea of momentum being the 2nd force is interesting because it also coincides with Newton's Seond Law.

"The lift of a wing is proportional to the amount of air diverted down times the vertical velocity of that air (Newton's second law normally states force equals mass times acceleration)." The diversion of the mass and velocity is also linked to the Coanda Effect.

http://137.229.52.100/~physics/p113/nowakowski/Physics_Webpage.html

Therefore, my revised answer is Air Pressure (the diff between the upper and lower half of wing) and Momentum
 
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  • #10
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Fluid motion and pressure.
 
  • #11
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Maybe all you guys are not thinking enough of balloons and Zeppelins. Archimedes said that any body (even a wing) in a medium feels not such a strong downward force, as it would in vacuum. The reason is gravity (of the medium, compared to the body). So my first guess is:
1) Gravity
Next comes all the stuff about Bernoulli, downwash, and so on (i.o.w., dynamical stuff), which is only there because air molecules are inert. So my second guess is:
2) Inertia.
 
  • #12
enigma
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Hi all!

This is my question, so I'm going to give a few hints to point you all in the right direction. It comes straight from one of my textbooks. Not many people (or websites I've seen) outside of aerospace specific fields think about the second answer.

1) Newtons 2nd, d/dt(mv) = F is not a force in itself. It is a way to calculate the change in momentum, but not a force.

2) Air pressure is one of the types of forces, so Andre gets at least 1/2 a point for the first mention of it.

3) Gravity probably should have been one of them, since it does act in the lift direction, but since it doesn't ever contribute to positive lift, I didn't include it.

4) I have seen something close to the second answer given. It is a very small contributor to lift compared to air pressure under normal flight situations, but in certain situations, it can get very large.
 
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  • #13
drag
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I hope the answer is indeed connected to forces directly.
Well, my stupid guess (as it seems everything else was tried)
is pressure and density differences.

(BTW, if I win the whole contest and you still decide to
award me the 15$, despite my objection that I wrote in
the feedback thread, I wan'na donate it to the WWF and help
save endangered animal species. Of course, I won't win, so -
there go the animals I guess... )
 
  • #14
Matt
Thanks for the hint enigma, but besides the pressure mentioned (a form of dynamic lift), the only other contributing factor I can come up with is...

Turbulence - I got this from "Physics" - 5th Ed by Giancoli (the only reason I think this one would work is because you said sometimes it has more of an effect than othertimes)
 
  • #15
enigma
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Nope

Turbulence characteristics are affected by it as well, though.
 
  • #16
Matt
Alright, now I'm just getting downright desperate : )

Ok, so the equation for lift is this:

L = C(subscript L) * S x .5pv^2

Therefore, 5 MAJOR Factors:

1)The Cross-section Shape of the airfoil (not a force)
2)The angle of attack (not a force)
3)The plan form size of the wing (not a force)
4)The Density of the air (mentioned earlier)
5)The Velocity of flight through the liquid air (briefly commented on)

Final Answer? (Oh God, lets hope so)

"Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift."
 
  • #17
drag
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Oops... I wrote my answer and then deleted it after seeing
Matt's answer was first but then I saw he's talking about velocity
between the wing and the environment while my answer is the
velocity difference below and above the wing (Bernoulli's stuff...).
Anyway, isn't it accounted for as pressure too eventually in
this case ?
 
  • #18
enigma
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You're going to kick yourself when you find out what it is!

I can guarantee you know it. I can even guarantee you've done problems dealing with it (although probably not in airplane type problems... first cut through aerodynamics and it is typically discounted).

The formula for lift you gave has the unitless coefficient CL. The actual forces are all tied up within that coefficient (using the Buckingham Pi Theorem).

EDIT: Yes drag, Bernoulli does deal with pressures only.

I'll give a last hint when I get home tonight, if noone has gotten it yet.
 
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  • #19
drag
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making a fool out of myself yet again :
hmm... shock waves ?
 
  • #20
Matt
Well Enigma I have never taken physics before (I have only done the summer work leading up to my AP class next year), so unless it was one of the times you helped me, then I prob. haven't seen it.
 
  • #21
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a wild stab in the dark here, is it air pressure and the velocity of the wing as its travelling through the air? cos if its not moving then its not gonna create any lift is it, unless the air around the wing is moving very fast.

simple answer from a simple physicist.
 
  • #22
Matt
Hmmm....

I wonder if perhaps to actually get lift you need to take into consideration how much displacement is occuring, much like that of a boat. If the plane does not displace enough weight then it wouldn't have a chance of getting off the ground right? Planes are built like birds - hollow where necessary and oftentimes made out of fiberglass and alumnium.
 
  • #23
drag
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I'll be starting my aerospace engeneering first semester
in a few month, so I got no clue either. (Besides, I'm much
more intrested and knowledgable in the space part.)
 
  • #24
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4
Originally posted by Greg Bernhardt
What two types of forces contribute to the lift on a wing?


since apparantly no one has it yet, i will make my guess. to maintain flight you must calculate device weight vrs. area (or weight distribution). if the ration between these two values is below a certain amount, it cannon stay up.
 
  • #25
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I don't have idea but in this page:
http://meme.essortment.com/howdoairplane_rlmi.htm [Broken]
put that the 2 forces are pressure and deflection
 
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  • #26
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If this is a trick question on fundamental forces, then are we saying EM and gravity?

Random guess...
 
  • #27
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i'm sure by now someone has got it. we just need to wait for greg to catch up.
 
  • #28
Matt
Yeah, Greg or Enigma - I think Enigma came up with the question.
 
  • #29
enigma
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I guess I can answer it now.

You're going to kick yourself.

.

.

.

.

.


Air friction is the other contributor.

Like I mentioned above, Newton's laws are the way to tally up the Lift on the wing (airfoil, airplane, etc.) by measuring deflection of the air (or other more complicated methods), but they don't provide the fundamental forces. The information of the underlying cause is swept under the rug.

Similarly, the formula for lift L = CL*q[oo]*S doesn't provide the fundamental forces either. All the force information is bundled up into the experimentally obtained value for CL for a specifically shaped airfoil.

The only types of forces which cause interaction between two human sized objects except for electrical/magnetic or gravitational interactions are friction and pressure. Even point loads are simply approximations to a large pressure distributed over a small area. This holds true also for airplanes, only the friction is very small under normal circumstances.

Drawing from "Fundamentals of Aerodynamics, third ed." by John D. Anderson, Jr. (Chapter 1.5 Aerodynamic Forces and Moments)

Definitions:

L = lift = component of the resultant force (R) perpendicular to the freestream velocity (V[oo])
D = drag = comp. of R parallel to V[oo]

N = normal force = comp. of R perpendicular to the chord
A = axial force = comp. of R parallel to the chord

' - value per unit span
[the] = difference in angle between chord and the surface of the airfoil (clockwise positive)
p = pressure
[tau] = shear due to friction
[alpha] = angle of attack
LE = leading edge
TE = trailing edge

pressure always acts perpendicular to the surface, and shear due to friction always acts opposite the direction of motion of the wing (so, to the back of the wing)

Going over the top surface of the wing, looking at a differentially small portion of the surface, the normal and axial forces per unit span are:

dNu' = -pu*dsu*cos[the] - [tau]u*dsu*sin[the]

dAu' = -pu*dsu*sin[the] + [tau]u*dsu*cos[the]

Similarly, on the lower surface:

dNl' = pl*dsl*cos[the] - [tau]l*dsl*sin[the]

dAl' = pl*dsl*sin[the] + [tau]l*dsl*cos[the]

Putting it all together and integrating,

N' = - [inte] {LE,TE}: (pu*cos[the] + [tau]u*sin[the])*dsu + [inte] {LE,TE}: (pl*cos[the] - [tau]l*sin[the])*dsl
A' = [inte] {LE,TE}: (-pu*sin[the] + [tau]u*cos[the])*dsu + [inte] {LE,TE}: (pl*sin[the] + [tau]l*cos[the])*dsl

From there, lift and drag can be obtained by taking the angle of attack into account:

L = N*cos[alpha]-A*sin[alpha]
D = N*sin[alpha]-A*cos[alpha]

Like I said above, friction's contribution to lift can be safely disregarded in many situations, since [alpha] & [the] are both typically small, and |[tau]| is usually << |p|, but there are situations where neglecting the friction can cause values to be off by 10% or more. An example of this would be supersonic or hypersonic flows about non-symmetric airfoils (or airfoils at non-zero AoA) where the friction is a significant fraction of the pressure.

EDIT: Fixing sub/sub
EDIT2: Fixing a - sign
 
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  • #30
Matt
Owww! Just head-butted the monitor - too hard to kick myself.
 
  • #31
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To end this question I will award andre 1/2 point for getting one of the forces correct.
 

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