Lift Generation

[SgtPappy]

Here I am with yet another question. Kind of makes me look like I do no research of myself but hey. I can only find so much.

Various sites and books state a bunch of different theories of lift. How does an airfoil create lift? Some theorists (like my gr12 physics teacher) claim that the air at the top of the wing must catch up to the air traveling at the bottom of the wing once those air molecules reach the trailing edge of the wing. Since the top part of the wing is curved, the air molecules must travel faster and the pressure difference creates lift. Honestly, there's nothing to support the assumption that air molecules somehow stay together after being split by an airfoil. NASA, too disproves of this theory. It would essentially mean that no matter what the AoA is, the airflow over the top is always faster than the flow at the bottom. Clearly impossible since planes can fly upside down and negative elevator creates negative lift. 


Theory:http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong1.html

Another theory (the most promising one) is that the chord line of an airfoil is angled upwards (leading edge facing up, trailing edge of the wing pointed down). As air travels over the top of the leading edge, the obstruction of the upward-pointed leading edge causes air molecules to be directed upwards. As these air molecules are pushed up, they are pushed down by the air molecules above them in result of Newton's 3rd law (not fully a Venturi principle since the air is pushing on itself and the wing is causing this). This compression speeds up the air over the top of the airfoil. The air at the bottom of the wing is less obstructed and is travels a near straight line. The upward motion of the air above the wing, however, pulls on the air at the bottom of the wing (since air molecules, like water molecules tick to each other) 'loosening' the air and actually spreading it out over the bottom of the wing, slowing it down, causing a high pressure effect. The resultant pressure difference causes the lift. NASA states that the compression of the air cannot be accredited for the velocity field, but the theory seems completely logical. The NASA version of the incorrect Venturi theory, does, however only take airflow on top of the wing into account, when we know that the bottom must be designed to give a slower airflow.


Theory:http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong3.html

So how is lift produced?

[TimRas]

The clearest (correct) explanation I have seen is from late Richard Shevell, a former aeronautics professor at Stanford:

"A simple feeling for lift can be obtained by realizing that the wing is a device for pushing air downward.  The wing gives the air a downward momentum.  The air 'pushes' back on the wing, producing a lift.  The resultant downward velocity of the air is the downwash.
There is nothing wrong with this thinking except that one cannot get any useful information from it.  Questions such as "how much lift for a given angle of attack?", how much drag to produce the lift?", " how much change in the local air velocity on the wing ?", cannot be answered except through the use of potential theory combined with the theory of circulation.

Potential theory is beyond the scope of this discussion.  It allows the calculation of flow around bodies of various shapes, including airfoils.  Forgetting viscosity of the air for a moment, i.e. considering a 'perfect' fluid, the theory is excellent for symmetrical shapes but failed for airfoils at some angle of attack because it showed zero lift and zero drag.  For over 150 years this fact was used to show that the theory was useless because it was known that there is lift and drag in experiments.  Then in 1902 a German scientist named Kutta realized that the theory showed the flow on the bottom surface of the wing curled around the trailing edge and moving forward for perhaps 5% to 10% of the chord before turning aft in the direction of the freestream.  But observation showed that the fluid flowed smoothly to the trailing edge of the airfoil (except of course when the airfoil is stalled at too high an angle of attack, a viscous phenomenon).
Kutta assumed that some correction must be applied to the theory so that the flow continued to the trailing edge.This is the well known "Kutta condition".

The correction was to assume a "circulation", a vortex flow, superimposed on the airfoil. When a circulation is applied of an amount necessary to make the flow go smoothly to the trailing edge on both top and bottom surfaces, (meet the Kutta condition) the theory showed a lift very close to experiment and calculated the velocities along the surfaces very well.  A wing can be thought of as a vortex extending from one wing tip to the other. At the wing tips the vortices trail aft as many of you have actually seen on a high humidity day. Having established the local velocities which are generally higher on the top surface, the Bernoulli equation can then be used to determine the local pressures and the lift.

The lift produced by the wing 'bound' vortex (so called because the center of the vortex is bound to the wing) is similar to the forces produced on spinning golf, baseball, or tennis balls to make their trajectories curve.

For further information on lift and circulation refer to "Fundamentals of Flight", R.S.Shevell, Prentice-Hall 1989 , "Aerodynamics of the Airplane",C.B. Millikan,McGraw-Hill, circa 1943, or similar textbooks.

Some people including some authors, think that circulation is a mathematical theory not a physical thing.  However, it is truly a physical process and the theory explains not only lift but drag due to lift also known as induced drag, ground effect, why birds fly in an approximate 'V' formation, etc.  If you want to see circulation develop on your dinner table, place a soup spoon in soup with fat particles or chopped parsley.  Set the spoon at a low angle of attack and move it through the soup, and watch the vortices develop. World's cheapest windtunnel!"

[Gianlupo]

Kutta's theory is probably the best to explain lift. there is a good explanation in this online book:

John Denker's See how it flies. The author is a pilot and a physicist, it's not an easy reading (at least for me ) but I think you'll find Chapter 3 and 4 very interesting.

Another good site for flight theory and piloting, is the Australian Ultralight Federation's, where there are a lot of info about flying:

AUF's Flight Theory guide. The "Airfoil and wings" section is what you need.

[CAP1]

best thing to do.......head out to your local airport, and find a CFI. they can explain it to ya very clearly.

try here too......http://www.allstar.fiu.edu/aero/airflylvl3.htm

[Yossarian]

Here's the best website I've ever seen on the topic:

http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-summary

I've linked to the summary, but be sure to scroll up!

EDIT: just realised Gianlupo had posted a link to the overall website.

[Angus]

Don't hook up on the airfoil alone, for lift requires area, thrust and angle of attack.
Think of it as holding a metal sheet in a storm, - if you apply angle of attack you may be airborne....

[CAP1]

Don't hook up on the airfoil alone, for lift requires area, thrust and angle of attack.
Think of it as holding a metal sheet in a storm, - if you apply angle of attack you may be airborne....

easier yet........stick your hand out the car window, palm down and flat. now tilt the leading edge up slightly, then down slightly.

[Gianlupo]

easier yet........stick your hand out the car window, palm down and flat. now tilt the leading edge up slightly, then down slightly.

That's how I did it the first time I was brooding over lift!

[gripen]

A cool demonstration of an airframe featuring complete flat wings are current RC indoor aerobatic planes. The reason for this is that the flat wing starts to stall (ie airflow separates) at very low AoA - just few degrees - so there is no sudden separation or stall at higher AoA as with normal wing profiles (typically somewhere around 15-20 deg). In practice this means that RC indoor planes can be steadily flown through high AoA maneuvers because the separation has allready happened unnoticed at lower AoA and this also demonstrates well how the AoA is the key factor for the lift generation despite there is no profile and wing is actually stalled during maneuvering.

[Wmaker]

A cool demonstration of an airframe featuring complete flat wings are current RC indoor aerobatic planes. The reason for this is that the flat wing starts to stall (ie airflow separates) at very low AoA - just few degrees - so there is no sudden separation or stall at higher AoA as with normal wing profiles (typically somewhere around 15-20 deg). In practice this means that RC indoor planes can be steadily flown through high AoA maneuvers because the separation has allready happened unnoticed at lower AoA and this also demonstrates well how the AoA is the key factor for the lift generation despite there is no profile and wing is actually stalled during maneuvering.

Yep, and the fact they can be flown like they are flown is the power to weight-ratio which is "out of this world" compared to the WWII fighters, for example. Good example is the "slowing to hover - move" when a flat plate rc-plane translates from normal controlled flight into a hover. "It just can do it" because of the rediculous thrust to weight ratio.

[CAP1]

A cool demonstration of an airframe featuring complete flat wings are current RC indoor aerobatic planes. The reason for this is that the flat wing starts to stall (ie airflow separates) at very low AoA - just few degrees - so there is no sudden separation or stall at higher AoA as with normal wing profiles (typically somewhere around 15-20 deg). In practice this means that RC indoor planes can be steadily flown through high AoA maneuvers because the separation has allready happened unnoticed at lower AoA and this also demonstrates well how the AoA is the key factor for the lift generation despite there is no profile and wing is actually stalled during maneuvering.

well...that and even the smallest ones are severly overpowered.

[Blooz]

Funding.

That's what makes your birds go up.

No bucks, no Buck Rogers.

[Serenity]

Funding.

That's what makes your birds go up.

No bucks, no Buck Rogers.

lol. Love it!

[RTHolmes]

No bucks, no Buck Rogers.

 

[gripen]

Yep, and the fact they can be flown like they are flown is the power to weight-ratio which is "out of this world" compared to the WWII fighters, for example. Good example is the "slowing to hover - move" when a flat plate rc-plane translates from normal controlled flight into a hover. "It just can do it" because of the rediculous thrust to weight ratio.

Some WWII planes could do sort of stalled maneuvers, like the one in your signature; at full power it could hang on propeller, mushing forward 130-140km/h at around 60deg angle (see Kokko's report). Probably some other planes like the P-38 could do similar things. Anyway, the control is probably very limited at such condition.

Harrier style maneuvers does not require ridiculous thrust to weight ratio, just good control at slow speed. I remember when the first IFOs came around, these had thrust to weight ratio some what below 1 with the power setups available that time. However, these could easily do 60-70 deg harriers with good control despite being unable to hover due to limited power (or too heavy batteries, that was before the high current lipos).