airfoil question

[padren]

I am trying to figure out something that kinda puzzles me about how an airfoil works.

 

From what I know, air moves over the top faster than under the wing, creating a lower pressure region of air over the wing, creating a vacuum style lift effect.

 

That all makes sense, but what I don't understand is, is how you can get so much lift from that simple process. The difference in air particles above vs below cannot equal the mass of a boeing 747, so how does it generate that much lift?

 

Am I just looking at it wrong? Should I be comparing the pounds per square inch of air pressure that push up below the wing, to the far more neglible pounds per square inch of very low atmospheric pressure on the top wing surface?

 

If that's the case I probably answered my own question...but I'll ask another one if I had gotten the first one right:

 

For the vacuum effect to occur, there must be a compressed region of air as well yes? Does this occur on an arc above the region of low pressure?

[JesuBungle]

I think the main thing you're forgetting is the speed involved. To take off, a jumbo jet has to be going in the area of 150mph. At that speed the lift is quite incredible. But once the plane drops below that speed, or stall speed, it can no longer remain in the air.

[padren]

I think the main thing you're forgetting is the speed involved. To take off, a jumbo jet has to be going in the area of 150mph. At that speed the lift is quite incredible. But once the plane drops below that speed, or stall speed, it can no longer remain in the air.

 

Speed is only a method to create the vacuum effect over the top of the wing though, it doesn't actually increase the lift surface area at any time. The faster you go, the lower the air pressure is over the airfoil, and the larger the total volume of the low pressure area, which helps increase lift but it can never quite reach "perfect vacuum" over the wing with perfect normal pressure under it quanta of lift, so at best at any speed you'll get a lesser total percent of that perfect lift per square inch, which itself isn't related to speed. Speed+airfoil is just the way to create the low pressure effect.

 

That is my understanding at least - do correct me if I am wrong.

[JesuBungle]

I know what you're asking, and it's kind of difficult to explain. But lift does increase with speed. Now it won't reach a "perfect vacuum", but there are other things involved, such as the elevators. But I don't quite understand what you're saying. Correct me if I'm wrong, but you make it sound like speed has nothing to do with it. So you could basically get a 747 going at 25mph and take off. No, you have to get the pressure over the wing low enough, so the weight of the aircraft can be mostly overcome, and you can pull the nose up.

[JesuBungle]

I forgot to add this, but there are also planes that have no airfoil at all, or a symmetrical wing shape. In that case, speed is the only thing keeping it in the air. I guess the only way i can really explain it is to compare it to waterskiing. Over 18mph, the water basically turns to a solid(not literally of course) but at that speed, a skiier is able to ski on top of the water. Similarly, with air, when a plane gets up to a certain speed, dependent on weight and wing area, the airplane is able to stay up.

[Bignose]

Bernoulli's efffect (this difference in pressures) is often not sufficient to produce lift to lift the entire plane... most wings also have significant flow turning. The fancy fluid mechanics word for this flow turning is circulation. Look at wikipedia's entry on lift, for instance.

 

Both the complete pressure calculation and the circulation calculation should be the same, but when the flow is redirected downwards, the pressure calculations become much more difficult, which is why the circulation calculations are usually easier. The difficult thing is that the pressure acts normal to the wing, so that integral becomes very difficult for fancy shaped wings. Whereas the circulation can be measured on any closed loop drawn around the wing, and typically a large circle is picked for ease.

[JesuBungle]

Thank you bignose. As he said, there is a lot more than just a vacuum at work. There's also flow turning and downwash. Downwash is a big factor in lift. When the air coming off the top of the wing meets back up with the air that went under the bottom, it is forced down which causes thrust and adds to the lift.

[padren]

I know what you're asking, and it's kind of difficult to explain. But lift does increase with speed. Now it won't reach a "perfect vacuum", but there are other things involved, such as the elevators. But I don't quite understand what you're saying. Correct me if I'm wrong, but you make it sound like speed has nothing to do with it. So you could basically get a 747 going at 25mph and take off. No, you have to get the pressure over the wing low enough, so the weight of the aircraft can be mostly overcome, and you can pull the nose up.

 

I am saying speed itself isn't the determining factor in lift. If you had the same low pressure area over the top of the wing via "magic" you could achieve lift off without any forward velocity. Speed is a method by which, in conjuction with an airfoil, you create a low pressure region over the top of the wing.

 

Regarding elevators, and angle of inclination, you are redirecting some of your forward thrust to move you upwards as well. Since the airfoils can overcome gravity well, even when the top facing surface area is reduced by the mild angle of inclination, the engines can push the plane up at a decent enough speed.

 

If I am misunderstanding any of the principles let me know.

 

Another factor is inclined fins, which when you apply a straight forward force to the craft, they impact a certian amount of air particles, deflecting them down. The result is the craft will meet more total resistance, but also deflect upwards somewhat, effectively converting some of the forward motion into upward motion.

 

I forgot to add this, but there are also planes that have no airfoil at all, or a symmetrical wing shape. In that case, speed is the only thing keeping it in the air. I guess the only way i can really explain it is to compare it to waterskiing. Over 18mph, the water basically turns to a solid(not literally of course) but at that speed, a skiier is able to ski on top of the water. Similarly, with air, when a plane gets up to a certain speed, dependent on weight and wing area, the airplane is able to stay up.

 

The issue with waterskiing isn't the water, but the fact you are always moving over a fresh patch of water, at a speed faster than the viscosity of the water allows for it to get out of the way under your weight.

 

This also is made possible I believe, due to the weight and fluidity differences between the air and the water. Waterskiis would not keep a heavy mass from sinking, other than via the effect of the "inclined fin" factor that I stated above.

 

 

When you mention a symmetrical wing shape, is that based on an incline to convert some forward motion into upward motion? I am curious what principle, other than the asymmetrical airfoil lift factor, could utilize speed to increase lift, without relying on pure vertical jet/rocket thrust etc.

[JesuBungle]

The "inclined fin" factor that you mentioned is basically what makes flight possible. An airfoil, being the wing (symmetrical or not), is also used to deflect air causing downwash. This is the force that allows a plane to overcome gravity. The vacuum over the airfoil gets it started, and the wing is pitched to an angle to deflect air downwards, causing a force which pushes the plane up. I know my examples aren't the best lol, but here's another. An airplane wing is the exact same thing as a helicopter blade. The airfoil of a helicopter blade is always spinning at the same speed. But when the collective, or pitch, of the blade is increased, this creates downwash which pushes the helicopter up. An airplane has to move to get air over it's airfoil, then the pilot can pull the stick back and increase the pitch of the wing, creating downwash and pushing the plane into the air.

[padren]

Okay, I had previously thought the vacuum lift factor was the largest element in lift.

 

One more question then: Since we are measuring force effectively, and we are using an engine to create forward force, some of which is deflected to create upward force through a mechanical system (ie, the inclined fin thing) - how is this more mechanically effecient than just taking the forward generating force system (engine) and pointing down to create direct upward force? Or at an incline angle to create both forward and upward force?

 

Is this as simple as the jet or prop is more effecient when air is passing over it at faster speeds, allowing for forward motion to create a larger total force?

I would have trouble believing that, since a prop plane going as fast as it can horizontally, can pull up sharp and start a steep climb at a high speed, but will quickly loose speed since it isn't overcoming 9.8m/s^2 anymore off pure engine power. Yet, that engine and prop is the only force creating the motion required to allow the lift force of the wings to overcome that same 9.8m/s^2 with ease.

How can the mechanical advantage of the foil system achieve a vertical acceration rate higher than 9.8m/s^2 when pushed by prop thrust, yet that same thrust is unable to accelerate the plane up at 9.8m/s^2 when pointed straight up?

[JesuBungle]

You have the right idea. Most of an airplane's energy is in momentum. That's why runways have to be few thousand feet long. An airplane has to slowly build up speed and take off. There are a few planes out there such as the Harrier Jump Jet that can take off vertically, but the majority of planes need a runway. So in short, a normal airplane's engine either pitched slightly up to lift it or pitched down to push it up would not be create enough force or be practical.

[padren]

I understand how velocity can be required to create an effect, such as the conditions for lift via an airfoil, related to factors such as the size of the foil and the viscosity of the fluid etc, and thus the need for a higher airspeed on takeoff and such, but what I don't understand is that since gravity is a force (9.8m/s^2), that if speed (say 300 m/s^1) that if the momentum of the craft cannot be impart any energy into the system without loosing speed, and to maintain speed or increase it, the force of the engine must be able to cause acceleration better than 9.8m/s^2.

 

I guess it boils down to the need to overcome the 9.8m/s^2 downward acceleration of gravity, and the solution is to "move up" with thrust greater than that acceleration.

Airfoils and circulation etc seem to cause sufficient vertical thrust by means that cannot be acheived with the engine (which is designed just to produce thrust) alone.

Since forward velocity can only come from the thrust of the engine, it can catalyze but not add to the upward thrust without reducing forward thrust.

 

All the forces (thrust, lift, gravity) are m/s^2 and velocity is m/s^1, which is why I am confused how speed can be more than a means to achieve the mechanical advantage of the aerodynamic features. All those aerodynamic features do though is manipulate how the engine's thrust is used, even though the thrust itself doesn't seem sufficient hit 9.8m/s^2.

 

This is what I am curious about.

[JesuBungle]

Well I'm going to have to admit, they didn't teach me all the math in flight school lol. But it's just a combination of a lot of forces that make it possible. Thrust, lift, gravity, drag, and a few I don't know about. My main concern is getting the plane up and landing safely. So I'm gonna leave it there and hopefully someone more mathematically inclined can help you out. It's also an easy google, plenty of sites about it.

[Bignose]

Lift L = C * rho * 0.5 * v^2 * A

 

rho=density of fluid,

v = velocity

A = surface area of the lifting surface

C = lift coefficient

 

Now, sure, you can direct the engine in any direction, look at a rocket, or the VTOL Harrier jets. But, there are other options, like lift, and if you have a big surface area, A, which the fixed wings do, the choice is to put the engine's enegry into increasing v. That way you get a big combination of v^2 * A, rather than just using the engine to power the entire craft.

 

Given the same amount fo fuel, a Harrier will fly a lot farther using horizontal thrust and the wings for lift, than lifting itself by turning the jet wash. The pilots pretty much only use the adjustable wash for short take offs and landing and occasionally for super-fancy dogfighting. Oh, and airshows.

[Rocket Man]

this web site may be of interest;

http://www.amasci.com/wing/airfoil.html

it describes how the bernoulli effect introduces unneccesary assumptions about the flow over the wing, the air doesnt often line up infront and after, the downwash behind the wing is what is keeping it up, yes there is a low pressure reigion on the top, but newtons third law acting on the downwash is what produces the majority of the lift.

[Rocket Man]

an explanation for the harrier fuel intake lies in e = 1/2 mv^2

and p = mv.

the momentum applies force, so if you have a greater mass to push against, you require less energy to produce the same momentum, while a smaller mass requires more velocity, the velocity is the squared term so the energy requirement goes up.

With a vtol, you only have the exhaust to play with, so it needs to go very fast using lots of energy.

The wing causes a greater mass to move at a lower speed so using less energy, the engine is therefore pushing against less resistance than the 9.8ms^-2 so the engine doesnt need to overcome 9.8ms^-2

 

you might also want to look up the gossamer albatros, pedal powered air craft.

[gcol]

 

1. Am I just looking at it wrong? Should I be comparing the pounds per square inch of air pressure that push up below the wing' date=' to the far more neglible pounds per square inch of very low atmospheric pressure on the top wing surface?

 

2. For the vacuum effect to occur, there must be a compressed region of air as well yes? Does this occur on an arc above the region of low pressure?[/quote']

 

For what it is worth, here is my inexpert opinion:

 

You have not mentioned speed of airflow specifically. It is the most important factor. Whatever the airfoil section, no flow = no lift. Given enough power and speed even a brick can "fly".

 

1. Yes, lift is the difference in pressure between high (lower surface) and low (upper surface).

 

2. Air moving over the upper surface, following a "steeper" curve than the lower surface, has further to travel between leading and trailing edge, so is "stretched" and "thinned out", causing lower pressure.

 

Now comes the number-crunchingly most difficult bit. Laminar flow, which probably comes under fluid mechanics ( air being regarded in this application as a low density fluid). This has to do with making the air "stick" to the upper surface and flow smoothly without breaking away, which it will try to do being at lower pressure. Uneven flow = turbulence, = lower lift and increased drag = aerodynamic inneficiency and excessive engine power.

 

Corrections invited from aerospace techies.

[Rocket Man]

i'm by no means an expert, but as for a "total vacuum" above the wing, this can occur at the speed of sound where the brownian motion cannot cause the particles to hit the top of the wing.

 

the high pressure reigion under the wing is caused by the mass of the aircraft pressing down on a constanly replenished mass of air and the low pressure reigion above is caused by the same thing just inverse.

the low pressure above the wing does account for a lot of the lift but uses the suction to produce a downward flow just as the lower surface deflects air downward.

this downward flow is an acceleration of particles so it is a downward force, thus, newtons laws say that the particles are applying the same magnitude of force the the wing surface so the plane is held against gravity

 

a stall occurs when the majority of low pressure moves beyond the wing and so the force is lost and the plane loses lift.

[gcol]

a stall occurs when the majority of low pressure moves beyond the wing and so the force is lost and the plane loses lift.

 

When designing model airplanes, I always thought of it as the centre of lift moving too far in advance of the centre of gravity (longitudinal balance point of the airframe) which is always within wing chord, beyond the ability of the empenage to control the resultant nose-up pitching. Drag increases, airspeed drops, no lift, stall, nose drops, dive, airspeed increases again until lift is regained or the ground intervenes.

[Rocket Man]

When designing model airplanes, I always thought of it as the centre of lift moving too far in advance of the centre of gravity (longitudinal balance point of the airframe) which is always within wing chord, beyond the ability of the empenage to control the resultant nose-up pitching. Drag increases, airspeed drops, no lift, stall, nose drops, dive, airspeed increases again until lift is regained or the ground intervenes.

 

i dont know the model air craft terminology but when the low pressure reigion moves behind the wing, there will be a LOT of drag. the result will be a loss of airspeed, of course but when the low pressure (normally over the leading edge) migrates too far back (ie when the wing tilts too far up), there is a loss of the vacuum lift as well.

[gcol]

As far as I know, model aircraft aerodynamic terminology is the same for full size aircraft in general.

 

Yes, the regions of high and low pressure will shift across the wing according to airspeed and angle of attack (incidence), yet will always resolve to a point. It is the range of movement of this point of resolution I am considering.

 

Trying to keep this point within reasonable bounds is largely why there are such an extraordinary number of different wing sections catering for such diverse needs as low speed high lift, through to ulta high speed and low lift. It is terribly complicated, which is why whatever theory and maths says, the final test is always practical in a wind tunnel.