“Ailerons on a wing are designed to give extra lift on that wing to make the plane roll”
Taken from a book written by a professional pilot with thousands of hours. Any one care to comment?
Ailerons are designed to change the angle of attack of the outer part of the wing by variation of the chord line in a diferential manner.
If air is a fluid why would it transfer a force linearly for any distance? Above ground effect height the down-diverted air will spill in all directions.
Uptimist wrote:
I.e. purely hypothetically the aircraft were flying in an infinitely large volume of air (and ignoring inconvenient side effects like what causes gravity) then there would be some net downward momentum imparted to the air?
NUP.
If the earth was to disappear and was replaced with a piece of string to pull the plane “down” instead of gravity then everything would stay the same i.e. there would be no NET acceleration of air. (the momentum of the air moving down would match the air moving up).
There is only an initial net acceleration downwards as the pressure differential is built up around the wing as we roll down the runway 
and momentarily when in accelerated flight, i.e. rolling wings, raising and the lowering the nose, turbulence etc.
If we got rid of the string the plane would accelerate upwards until a new equilibrium was established. Similar to what happens with a rotor aerofoil on the front our aircraft established in cruise.
Ok fair enough, I see that momentum must be conserved, so, when the aircraft is flying level, somewhere in the system that downward-pushed air has to get back up again or be balanced by upward pushed air but….(and I’m taking the hit here for being the non specialist here trying to make intuitive sense of it!)…isn’t the point that in the general area of the aeroplane the air does get pushed down which is what causes an upward force on the wings? Yes, the maths of how that happens for a given wing shape is pretty complicated, but wouldn’t it be easier for the average pilot to understand how a wing works by saying the shape of the wing being pulled through the air pushes the air down rather than invoking Bernoulli etc.?
Isn’t the point that in the general area of the aeroplane the air does get pushed down which is what causes an upward force on the wings?
More accurately, there is increased pressure underneath wing and less above. At any instant in time, as the wing is pushing forward through the air the area underneath is mostly being pushed forward and down. (Increase in pressure). The area above is mostly up and backward. Slightly below the leading edge the air still be pushed forward and up. This is how the stall warning vane works. This is circulation. At any given point about the wing the direction is different. But it’s not so easy to visualize because the wing moves at 100kts . So one moment if being pushed forward and up and instant later is pushed down and back. In level flight this is all balanced out so there is no net change in momentum. All this moving about causes drag.
but wouldn’t it be easier for the average pilot to understand how a wing works by saying the shape of the wing being pulled through the air pushes the air down rather than invoking Bernoulli etc.?
Perhaps, but trying to use pushing or pulling while it looks seductively simple it starts to get very complicated one you get into detail, where as using pressure changes which is also a way of integrating movement of air is actually easier and a lot easier to measure. How would you explain stalling or stall warning vane, ground effect, vortex generators, why does the front 25percent of the wing produce half the lift. Etc etc
Uptimist wrote:
and I’m taking the hit here for being the non specialist here trying to make intuitive sense of it!
Using only Newton, you simply will not get an intuitive sense of it. Not an intuitive sense that is anywhere near reality. Fluid is all about pressure, pressure fields and velocities. Then add viscosity, turbulence, shear layers, circulation, vorticity. Later add compressibility, Mach numbers and temperature. Then combustion, evaporation, condensation enthalpy, entropy and so on, and you are into turbine land. Thermodynamics and fluid dynamics are closely related subjects, interwoven.
But lift of an airfoil is basically only pressure fields and velocities and most important of all, the magic ingredient – Circulation This is a peculiar thing about airfoils (lifting bodies) in particular. Lift is probably one of the simpler things in all of thermo-fluid dynamics (as a concept), but impossible to understand without a grasp of circulation. Circulation is isolated straight forward. But exactly how the circulation around an airfoil occurs in the first place, is not well understood (see that video above. Hertz’s principle vs viscous effects – vorticity. Chicken and egg, which comes first?)
Maoraigh wrote:
If air is a fluid why would it transfer a force linearly for any distance?
Pressure. A pressure field will propagate with the speed of sound. At low Mach numbers, it’s better to think of that propagation as instantaneous, since all other velocities are small compared with the Mach number. The wing itself only “feels” pressure (+ some skin friction). That is the only mechanical thing keeping a wing in the air.
This is AIUI from something I once worked on and needed “the idiots guide” so I could try and explain it to others. In order to get lift you need the movement of a mass of air over a surface.
Ignore the surface for the moment as its pretty irrelevant until you need to achieve a goal.
The movement of the mass of air hitting and circulating around that surface depends on the speed in which the mass of air is hitting that surface.
Put your hand out of a car window whilst the vehicle is stationary and you feel nothing.
Get the car up to speed say 70mph and stick your hand out and it is a totally different thing.
Palm forward and your hand will be ripped backwards, palm down and your hand will lift palm up and your hand will be forced down.
The only thing stopping your hand disappearing behind, up or down is the weight of your body and the energy you can produce in the opposite sense of the direction the air mass is forcing the hand to travel.
This knowledge has allowed us to design surfaces which will react in different ways to make the air or other fluids to react in a way which we can use.
Eg a design.of oar allows rowers to use the palm forward effect.
The palm down allows us to create lift for the wing of an aircraft.
Palm up allows us to design a racing car which will stick to the surface like glue.
Are you pushing the air or water in a particular direction in any of these cases?
I would answer, no. We are deflecting the air/water or we are pushing against it.
If we drop a feather from a height, despite the effect of gravity it usually does not sink straight to the ground, it will meander. Unlike a brick which will go pretty directly. So weight and the movement or circulation within a mass of air play a significant role. Are the brick or the feather pushing air downwards? Possibly, but only in the sense that they are both compressing the air below them, to some extent, as they fall but it is not enough to push against the surface of the earth and create lift. Pushing or compressing the air against itself might well happen.
However that air with few or no containing forces can only compress against itself for so long before it is forced out sideways.
So you can create more lift by the design of a wing, by weight reduction and by speed of the mass of air hitting the wing. All of which as LeSving wrote is to do with circulation around a surface and very little on pushing the air downwards.
I once tried to make a video about a computer sytem being used by a designer to get the perfect shape and surface for a well known formula 1 team car.
The car went on to win the formula 1 championship and constructors championship that year. Whist he was designing the car he was telling me what the software wouldn’t do. By the end of it my head was spinning. Some of the bits I remember, I have recalled above.
gallois wrote:
Put your hand out of a car window whilst the vehicle is stationary and you feel nothing.
Get the car up to speed say 70mph and stick your hand out and it is a totally different thing.
Palm forward and your hand will be ripped backwards, palm down and your hand will lift palm up and your hand will be forced down.
This is as good example as any as why pressure is easier once you delve into the details. A flat plate stalls at around 10 degrees. So when you stick you hand out the wind initially it will be deflected mainly up or down as you twist it further less “lift” is created and more drag is created etc, so the pushing force will be mainly backward. Why isn’t the most lift created when your hand is 45 degrees to the airflow? etc etc
If you take that same flat plate, and bend it just at the front so that it now more aligned with the airflow (say with a hinge), you will actually increase the lift force, this sort of stuff get hard to explain pretty quickly without using pressure…
this sort of stuff get hard to explain pretty quickly without using pressure
It’s just hard to explain, period IMO
It’s a bit like flying itself. Can you get an intuitive understanding only reading books? I don’t think so. You have to actually fly to intuitively understand what flying is. With aerodynamics and wings, I also think it’s very hard to get the intuitive understanding without the background, and preferably having worked with it doing simulations and tests.
Everyone can take a note pad, curve into a rough foil shape, wave it through the air at a nice angle of attack and feel the air being pushed down. From there it is easy to think. Aha, air is being pushed down, it’s all about “Newton”, problem solved. That’s fine with me, even though it for sure is not “all about Newton”. It’s all about upwash, downwash, acceleration, pressure and circulation. Without circulation, none of upwash etc would have happened, the lift would be zero. It really is all about circulation.
This is a bit unfortunate because although this has been known for more than a 100 years, the very creation of this circulation is still enigmatic. However, to get the intuition that circulation is in fact needed, even though exactly how it is created eludes you, you have to dive into it. After a while, like 2+2=4, lift = circulation. All experiments, all theories shows you that this is how it is.
But to explain the incipient creation of this magic circulation? To this date I have not seen a good explanation. Maybe this Hertz’s principle is it? Or maybe it’s due to viscosity? Both sounds equally adequate IMO, but this Hertz’s principle is perhaps more elegant. Time will show.
I have been wondering an aeromysteric question already for some time – hopefully someone can either confirm my thoughts or prove them false.
We all know that the wing lift comes from the different air speed above and under the wing. When the wind is blowing around in the wind tunnel, the speed above is increasing and below it either slows down, remains the same or increases less.
But what about is real life? The wind is not blowing but the wing itself is moving, let’s say to left. Does this mean that the wing moving to left actually blows the air above to the opposite direction, to right? And below either carries the air with it towards left, does not affect at all or blows also to right with less speed.
In my opinion the wing moving to left must give the still air some speed towards right, which sounds ridiculous. But how else could it be that already with the airplane speed 0,75 mach the air speed above may achieve the speed of sound? So the plane runs at 0,75 and the air above the wing MUST run at 0,25 to right even though before the plane came there, it was standing still. Magic – isn’t it?
Opinions, please!
