# Lift forces acting upon an aircraft

## Main Question or Discussion Point

I've been trying to conceptualise accurately the lift forces upon an aircraft in physics terms, for a simulator flight manual I've been working on.

What I've got from some rudimentary flight training is that low pressure above the wing rather than high pressure below it causes lift, obviously because the straight line path of air going over the wing is disrupted by the aerofoil plane.

When you go faster you increase your lift. To fly higher you need higher (true) airspeeds.

So this is what I've assumed. The low pressure area above the wing is a form of buoyancy. This is what lifts the aircraft, the wings of course forming the threshold of this buoyancy and being lifted along with it.

When you go faster, it is as if the air pressure has increased due to your higher airspeed. So, since the mass of the aircraft hasn't (consequentially) increased, the same amount of buoyancy has more effective lift.

What is the industry standard math for this?

Secondly, is this effect related to moment of inertia? I figured that due to the curvature of the Earth and its atmosphere and gravity an aircraft is really striking at a body of atmosphere with angular momentum.

I recognise I may be well off on most counts.

## Answers and Replies

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LURCH
We have had long debates in the past in these forums regarding exactly how wings generate lift. Somewhat agree with you that the differential pressure between the bottom and top of the wing surface (caused by the shape of the wing) is what generates most of the lift. Others will say that this is a long-held "urban myth" and that nearly all lift in an airplane is generated by the wings deflecting air downward.

Whichever argument you except, this...

When you go faster, it is as if the air pressure has increased due to your higher airspeed. So, since the mass of the aircraft hasn't (consequentially) increased, the same amount of buoyancy has more effective lift.
... is only "kind of" true. The crucial reason why airspeed needs to be higher and higher altitudes is because the air is thinner. There is a certain amount of air that the wing requires in order to generate enough lift to maintain level flight. It higher altitudes, where there is less air per cubic meter, the wing must pass through more cubic meters to get that amount of air.

Or, if you're talking about the act of climbing in itself, then airspeed does not necessarily need to increase. One need only in the nose of the aircraft upward and maintain a constant speed. However, maintaining this constant speed will require more throttle, for the same reason going uphill in a car requires more throttle; extra energy must be expended to do work against gravity.

Yes, I've just had a taste of differing perceptions of a research thesis once again at a forum. The Luftwaffe almost won the Battle of Britain by attrition, and yet the Luftwaffe never had any chance of winning the Battle of Britain it was all over by the mid-thirties. Well opinions will differ, unfortunately turning the argument into a popularity contest.

I've a habit of opening similar cans of worms. Historical documentation suggests (stipulates according to some) the BMW 801D-2 (14-cyl. radial) engine boost system was a fuel injection charge coolant, but neither computer models by reputable software nor practical experience in racing supports the claimed 18% output gain and some other form of charge coolant or otherwise oxidant/fuel booster must have been used for such a dramatic effect as the listed technical specifications. But some dimwit in 1940 at BMW decided to note on an installation diagram that the boost system was an "emergency power fuel system" probably arising from the engineers stipulation that aviation fuel could be used in the charge coolant tank to extend range...and that was the final word as far as the armchair fanclub was concerned.

One gets used to it at forums, and visits them less.

TVP45 I thankyou very much for the link. It was full of valuable and readily accessible information.
:)

russ_watters
Mentor
On thing: "buoyancy" is a word that doesn't fit here at all. Buoyancy is a force caused by the displacement of a fluid. But the amount of air actually displaced by the low pressure region* is miniscule - so miniscule, in fact, that below about 220 mph, the compressibility/variable density of air is completely ignored in aerodynamic calculations.

*even that is cumbersome to say. Air isn't really displaced, it is simply at a lower pressure and therefore less dense.

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Ouch! I suspect that is Russ's way of pointing out that a wing is "bouyant" in air the way a wing shaped brick is "bouyant" in water.

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<shrug> Why dont you just think of lift as the closed loop integral of the pressure forces around the airfoil? </shrug>

We have had long debates in the past in these forums regarding exactly how wings generate lift. Somewhat agree with you that the differential pressure between the bottom and top of the wing surface (caused by the shape of the wing) is what generates most of the lift. Others will say that this is a long-held "urban myth" and that nearly all lift in an airplane is generated by the wings deflecting air downward.
I hope they realize that 'deflecting air downward' is result of increased pressure on the bottom wing surface, else how can the air be 'deflected downward', by magic?

Changing the direction of flow requires a reaction force on the buttom wing surface.

rcgldr
Homework Helper
Changing the direction of flow requires a reaction force on the buttom wing surface.
The air can be sucked downward from above a wing just as easily as pushed downwards from below a wing.

A typical wing generates lower pressure above the wing, and higher pressure below the wing (even flat bottom wings are flown with a signficant angle of attack). Air accelerates towards low pressure areas, and away from high pressure areas, in all directions, except that air can't flow upwards (or backwards) through the solid wing, so there a net downwards (and forwards) acceleration of air.

Note that the low pressure area above the wing draws some of the air in front of the wing over the wing, "stealing" some of the air that would otherwise flow below the wing, lowering the seperation point (where the air divides into seperate flow over and under a wing). This is speed sensitive, because the rate of information about pressure differentials is the speed of sound. As airspeed increases, there's less time for a volume of air to "react" to the pressure differentials before the wing reaches that volume of air, and at Mach 1 or faster, there's no time for the air to "react", and the wing (and a small boundary layer) litterally collides in to the air.

The amount of lift produced by a wing is related to air density times coefficient of lift (related to effective angle of attack and the airfoil) times the air speed^2. A flat board wing will fly just fine, but it's lift to drag ratio is less than that of cambered airfoils. Most effecient sub-sonic airfoils are tear drop shapes, just stretched and curved, but there are some unusual airfoils, such as this pre-shuttle prototype lifting body:

m2-f2 (glider version).jpg

http://www1.dfrc.nasa.gov/Gallery/Photo/M2-F3/Medium/EC71-2774.jpg [Broken]

High altitude flight

The power requirement for flight is aerodynamic drag times actual air speed, not "indicated" air speed. This means it takes more power to fly at high altitudes. In addition, normally aspirated engines are intaking air at a lower density, which reduces their power.

Two extreme examples of high altitude aircraft are:

SR71 - which flew at speeds over Mach 3.2 (2242mph fastest unclassified speed), highest altitude, 85,069 feet.
http://en.wikipedia.org/wiki/SR71

NASA's Centurion, which flew at 17 to 21mph.
Centurion.htm

NASA's later model, Helios, broke up due to turbulence during a test flight, but set an altitude record for non-rocket powered aircraft of 96,863 feet.

Helios

http://en.wikipedia.org/wiki/NASA_Pathfinder

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While buoyancy by its definition is not lifting the wing the analogy might well be kept. Lift is provided by differential "static" pressure in both cases. I suggest that dynamic pressure is not a real pressure but a numerical value for changes in "static" pressure due to dynamic motion.