Lift - what mechanism?

[Jetstream Rider]

I've been having a discussion on Wikipedia recently about an edit I made to a page concerning lift. We have come to an agreement, but I wanted to ask a couple of questions to clear something up in my mind.

I have an Aeronautical Engineering Masters from London and came across a bit on Wikipedia that talked about the equal transit time fallacy for air travelling above and below and wing surface. It was quite rightly saying this isn't true, but the said "lift is actually produced because air is deflected downward". I changed that bit and it resulted in a tit for tat reversion of what each other had written. Our discussion hinged over what caused lift and Newton's laws.

My edit said that lift is caused by the pressure field around the wing, caused by pressure changes in the air both above and below the wing caused in turn by the fact that air is "turned". I backed it up with references to Anderson's Fundamentals of Aerodynamics and some other excellent books such as Kermode and Aerodynamics for Naval Aviators.

The chap in question kept saying that lift could be calculated by working out the downwash, and that lift was equal to the downward momentum transferred to the air.

I'm quite happy with the fact that Bernoulli's equation is initially derived from Newton's Laws and that Newton's laws can be applied to a control volume to derive the Euler equations and the Navier-Stokes equations. These are of course approximations to actual flow. However, I don't think its as simple as downwash = lift.

Looking at lift on a spinning cylinder, the upwash and downwash are identical, so surely there cannot be any net downwash? Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without downwash.

So have I missed something? Is lift = downwash = integrated pressure around the whole body?

I'm aware that there are some books around, like Stick and Rudder, that say lift is entirely down to downwash and nothing to do with pressure distribution. I reckon this is wrong - but have I missed something?

Looking forward to a discussion.

[cessnarepairman]

I am sure I read somewhere many moons ago that a proportion of the lift from an aerofoil was generated by the downthrust it gives to the air as a result of A of A, about 20% rings a bell. For instance if you hold a perfectly flat board horizontally into an airflow & gradualy tilt it towards the vertical it will give give an upward lift. Obviously it is a very crude wing & drag will soon take over.

Taking another example. if you hold the back of a spoon into a flow of water from a tap it will move further into the water flow(coander effect). No water flows on the underside of the spoon so no lift is as a result of the example above.

A wing will produce lift even with a negative A of A ( about -4degrees in the case of a typical light aircraft wing) so surely this must be entirely generated by the wing camber & not downwash.

This may be an over simplification, but that is my understanding.

[Capt Pit Bull]

This is all a bag of worms, depending on how accurate you want your explanation to be.

The current ATPL (JAA) explanation, aka the 'air deflected downwards / equal and opposite reaction makes lift' explanation is (a) a step in the right direction (versus the 'equal transit time' fallacy) and (b) at least doesn't violate Newton 3. However, it does violate momentum conservation.

Just say to the bloke something along the lines of: "In your explanation, consider level flight. The aircraft hasn't changed its vertical velocity component, but if the air has been deflected downwards, so momentum has not been conserved"

I've spent a lot of time thinking about this over the last couple of years. Over my career I've worked as a ground school instructor teaching PofF, a school teacher of Physics in addition to my flying. What I do not have though is a degree level of understanding regarding fluid dynamics.

In general, for explaining anything mechanical, my yardstick for something to be a good model is: Newton 1,2,3 not violated. Momentum conserved. Energy conserved.

Also, everything else being equal, scalar based explanations are simpler than vector based explantions, e.g. Energy rather than forces (doesn't help much with explaining lift, but helps a lot with explaining induced drag).

As far as I can see , it is impossible (by my yardstick) to give a good model for lift production if you are only looking at a 2d aerofoil. However, once you go to 3d the world is your oyster.

Anyway, I'm short of time to post further at this point.

pb

[Shawn Coyle]

I like to think of it as displacing a mass of air, like a boat displaces a mass of water.

[SpamCanDriver]

Recently in a discussion with a ex Royal Navy test pilot he said that it was explained to him in the way that if the differential pressure theory was correct, how the hell could a jumbo get off the ground? i.e. with a takeoff weight of I guess here at 220t the pressure differential required would suck all the panels off the top of the wings etc!

[Knight Paladin]

I reckon it's magic.

[Jetstream Rider]

It is definitely magic.

Sucking the wing panels off is not a problem. The actual force per unit area is not all that much as the wing surface is large. In Mechanics of Flight by Kermode, it says that a typical "suck" required is less than a baby sucking a nipple. Now I've never had my nipples sucked by a baby, but I'm sure it wouldn't take the panels off a jumbo.

According to the Boeing website

Jumbo wing area = 524.9 m^2
Max weight for ER version 412,775Kg = 4,045,195N

Therefore average pressure = 7700 N/m^2 = 0.77N/cm^2

So thats like a load of 1 apple per postage stamp. Hardly going to break the wing. (I realise that calculation is a bit simplistic).

[603DX]

Jetstream Rider: Not disagreeing with the point made in your post #7, but you have a factor of 10 error in the calculated average pressure.

Try one apple per postage stamp, which illustrates your point even better.

[Jetstream Rider]

Now corrected - thanks very much - early morning this morning. Thought it was bigger than I had imagined....

[barit1]

If you can wade though 160 posts, here is a prior thread

[Jetstream Rider]

Thanks - I'll have a look. I'd love to hear from some of the TPs and FTEs on this forum though, unless of course they have posted on the previous thread.

[Brian Abraham]

I'm just your average (maybe below) bog aviator and the way I have always conceptualised "lift" is as follows. I think of a wing as a propellor blade. The blade produces lift (thrust) to pull/push the aircraft forward. How does the lift manifest itself? By throwing a quanity of air to the back. No air being thrown to the back, no lift (thrust). This view of lift may stem from my helicopter background. No downwash from the rotor blades, no lift.

I'm aware that there are some books around, like Stick and Rudder, that say lift is entirely down to downwash and nothing to do with pressure distribution.

As I infer above I think of lift as being the result of a mass of air being thrown at the ground, and the means by which that is brought about is by the pressure distribution about the wing. So the two go hand in hand, can't have downwash without the pressure distribution. Of course with an airfoil at its zero lift angle of attack you still have pressure distribution but no downwash.

Also, one can turn the trailing edge of a wing up and still produce lift, albeit not very efficiently, but without downwash

My understanding is that reflex in the trailing edge is introduced to control the pitching moment of the airfoil. You still need downwash to get lift (lift, as in to keep a body flying).

[bArt2]

In my opinion lift is produced the way I have allways learned it was, and the way it is still explained in the books I have.

So lift is produced because the air moving on the top of the wing moves faster than on the bottom, causing a drop in pressure, causing an upward lift.

How else can you explain that a cambered wing at at angle of attack of 0 degrees can produce lift.

I do not believe in all these new fancy theories like lift being produced by donwash or by turning the flow or by little eddys on top of the wing.

Bart

[Jetstream Rider]

There is a big difference between the mathematical modelling of the flow and the actual flow. The wing can be modelled as a sheet of little vortices - but we know in reality a wing doesn't work like this.

How can pressure distribution and downwash be the same thing?

If it were just about "throwing air downward", then why don't curved plates that generate downwash but a poor pressure distribution generate so little lift?

Have a look at FoilSim II 1.5a beta

By being a bit creative with your entires to the boxes, you can generate lift with more upwash than downwash. OK, its a sim and not a real windtunnel, but I doubt it is so inaccurate that it is not representative.

For instance, try angle -20 deg, camber 24.5, thick/cord 16.725. You will have a lift of 797 llbs, so its not very efficient as a lift producer, but the air is visibly going up more than it is going down, so there is no net downwash.

Similarly, set the angle 9.56 deg, camber -8.1, thick/chord 12.775. The flow from the back of the wing is slightly upward (see the jink in the pixels on the line) and it is generating a small amount of lift (again, albeit inefficiently).

[603DX]

If downwash does not provide a significant contribution to total lift in straight and level flight, how do the deniers of this explain the well-established phenomenon of ground effect? This "floating" tendency which extends the landing run unless pilot input counters it, can only be caused by the downwards-moving mass of air below the wings coming into contact with terra firma.

This effect is visible when landing on dry bush strips, or in ultra-low flying by the reckless aviator, when a continuous cloud of dust is raised by the aerodynamic "squeezing" or "cushioning" between wing and ground.

To my straightforward (simple?) engineer's mind, it seems blindingly obvious that total lift is a variable combination of pressure-distribution and downwash effects, based on distant memories of fluid mechanics lectures and lab work. This "controversy" about the components of lift all seems a bit artificial to me, due to the apparent tendency to polarise opinions in an "all or nothing" approach. In other words, I think both sides are right, to some degree!

[Jetstream Rider]

Lift usually means downwash.

However, I think pressure distribution is more important. We were taught in ATPL exams that a third of the lift comes from downwash and 2 thirds from pressure distribution. While this is necessarily a gross simplification, there may be a grain of truth in it.

Certainly the lower surface accounts for more lift than upper surface only theories would have you believe.

I think what I am seeing with no downwash lift is purely the pressure distribution, and with flat plates turning the flow is almost purely downwash created lift. A real wing being a mixture of the two. It still means "lift is produced by deflecting the air downward" is no the right way to describe lift.

[bArt2]

If downwash does not provide a significant contribution to total lift in straight and level flight, how do the deniers of this explain the well-established phenomenon of ground effect? This "floating" tendency which extends the landing run unless pilot input counters it, can only be caused by the downwards-moving mass of air below the wings coming into contact with terra firma.

Because when you come into ground effect the wingtip vortices are being reduced because there is no more air below to let them propagate, therefore drag reduces, hence the floating of the aircraft.

If lift would be caused by downward deflection of the air, why would there be wingtip vortices

[Jetstream Rider]

From Wikipedia (and referenced on that page):

"The phenomenon of 'wing in ground effect' is caused by the ground 'interrupting' the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the performance of the aircraft while it is experiencing the ground effect."

Ground effect reduces drag, and hence increases L/D, giving a gain in performance. Its not a "lift increaser", its a "drag decreaser".

Be careful about "downwash" here. There are two definitions, one the downward motion of air behind the wing, the other is due to induced drag of the wingtip vortices which reduces local angle of attack.

[603DX]

I think we are in fairly close agreement, with one reservation, regarding flat plates and the "turning" effect.

Up to a moderate angle of attack, at a moderate airspeed, airflow over the upper surface soon separates and becomes turbulent, i.e. the flat "wing" section stalls, experiencing a drastic loss of lift at the upper surface. However, at high angle of attack, at a low airspeed, vortex lift can develop on the upper surface. This can restore the uplift proportion of total lift such that downwash no longer makes up most of the lift.

If this effect did not occur, neither Concorde nor the Vulcan could have been the practical successes that they were. These aircraft, and indeed delta planforms with slender wing sections generally, would need to have much higher landing speeds and/or highly complex wing slat and flap systems, and land at much lower angles of attack. These factors could well have rendered them impractical, I understand.

[Brian Abraham]

Dug out my old study notes which explains it thus. The only forces that can act on a body moving through a fluid are those produced by friction (shearing stress in the fluid which is a function of viscosity) or those produced by pressure. Except when minimum drag is considered, the pressure forces are by far the most important, and completely responsible for the production of lift. The ambient or static pressure existing around a body moving through the air, cannot produce a resultant force, so dynamic pressure is left as the fundamental source of aerodynamic forces.

The maximum force that can be produced by dynamic pressure would seem to be,

Force = Dynamic Pressure x Area

Most airfoils are capable of producing a total reaction considerably greater than that suggested by the above formula.

Consider a mass of air travelling at constant velocity that is turned through an angle of 90° by a vane whilst maintaining its constant velocity. A vector diagram of the velocity’s will show that that the resultant (acceleration or change in velocity) will be 1.414 x Velocity (being an equilateral triangle with included angles of 90° and 45°). Newton’s second law suggests then that the force acting in the direction of the resultant (on the vane) will be,

Force = Mass x Change in Velocity

Mass/Unit Time = Rho x Area x Velocity

Therefore Force = Rho x Area x Velocity x 1.414 Velocity

= 2.828 x ½ Rho x Velocity^2 x Area

= 2.828 x Dynamic Pressure x Area

Although an airfoil is unlike the vane in that it is immersed in the airstream, it still produces lift by changing the momentum of the air, and producing a greater force than predicted by the simple “pressure x area” relationship. This is equivalent to saying that the airfoil produces an aerodynamic mechanical advantage. Although the forces may be magnified by mechanical advantage, the total energy remains constant. The dynamic pressure produces the resultant force by altering the value of the local static pressure, with the total pressure remaining constant. Until the airfoil produces a change in momentum to the airflow, the dynamic pressure will not affect the static pressure, or will affect it equally, and no resultant force will exist. Once the angle of attack is set so that the momentum of the airflow is changed, the resulting dynamic pressure will produce an out of balance of the static pressures, and a resultant force will act on the airfoil. The ability of an airfoil to change the momentum of the airstream is a function of airfoil camber and angle of attack.

A helicopter with rotor blades that are free to flap give as good an illustration as you could possibly get of the downwash/lift relationship. With blades at minimum pitch there is no noticeable lift and little downwash and the tip path plane of the blades will be at a given position. As you progressively add pitch you increase both lift and downwash and you will see the tip path plane increase in height (that is, the rotor blades increase their coning angle as the lift, and downwash, progressively increase).

To sum up, the wing throws air at the ground. No downwash, no lift.