Google really isn't cooperating with my search for answers.

I've come across an 80/20 rule of thumb for airfoils, which states that the low pressure distribution above the wing contributes about 80% of lift and that the high pressure distribution contributes about 20% of lift. Lots of things seem to follow this 80/20 rule.

Further, wing structure toward the wing root generally contributes more to lift than wing structure toward the wing tip, and depending on angle of attack, the center of pressure generally trends toward the leading edge.

1) Is there a similar rule for the low pressure distribution across the surface area of the wing? For example, does 20% of the surface area of the low pressure distribution contribute 80% of the lift?

At least one source asserts that the fuselage of a commercial jet can contribute 10-20% of total lift. Nature tends to be very efficient in design.

2) How much lift does a bird's body (minus wings) contribute to the total lift?

3) How strong must a bird's wing (or an airplane's fixed shoulder joint) be for flight (takeoff, landing, etc.), based on weight, wingspan, lift, airspeed, etc?

Originally Posted by skyfridge

I've come across an 80/20 rule of thumb for airfoils, which states that the low pressure distribution above the wing contributes about 80% of lift and that the high pressure distribution contributes about 20% of lift. Lots of things seem to follow this 80/20 rule.

Does the 80/20 rule hold up? It depends how willing you are to squint at the results. The attachment below shows the lift contributions of the upper and lower surfaces of a NACA 0012 as a function of angle of attack. At higher angles of attack, the ratio tends toward more like 75/25. At lower angles of attack, the 80/20 rule of thumb is wildly inaccurate.

Originally Posted by skyfridge

Further, wing structure toward the wing root generally contributes more to lift than wing structure toward the wing tip.

While true in a sense, you have to be careful with this assertion. It's easy to look at a plot of the span loading of a wing and conclude that the structure near the tips isn't contributing much to the lift. However, if you were to remove the structure near the tips, the lift generated by the structure near the root would drop dramatically. In other words, while it may not contribute much to the lift directly, the structure near the tips makes possible much of the lift contributed by the structure near the root. Looked at this way, the lift indirectly contributed by the tips is significant.

Originally Posted by skyfridge

depending on angle of attack, the center of pressure generally trends toward the leading edge.

Trends toward the leading edge as you increase angle of attack? This is not generally true. A wing that has significantly reflexed sections or significant sweep will see the center of pressure move toward the trailing edge as you increase angle of attack.

Originally Posted by skyfridge

1) Is there a similar rule for the low pressure distribution across the surface area of the wing? For example, does 20% of the surface area of the low pressure distribution contribute 80% of the lift?

This is even less generally true than the 80/20 rule applied to wing sections.

Originally Posted by skyfridge

At least one source asserts that the fuselage of a commercial jet can contribute 10-20% of total lift. Nature tends to be very efficient in design.

If the span loading across the fuselage doesn't result in a roughly elliptical lift distribution, then the aircraft will suffer an induced drag penalty. So fuselages do tend to contribute a significant amount of the total lift.

Originally Posted by skyfridge

2) How much lift does a bird's body (minus wings) contribute to the total lift?

An amount roughly equal to the total lift being generated by the entire bird minus the amount being generated by its wings.

Originally Posted by skyfridge

3) How strong must a bird's wing (or an airplane's fixed shoulder joint) be for flight (takeoff, landing, etc.), based on weight, wingspan, lift, airspeed, etc?

For an elliptical spanwise lift distribution, the spanwise location of the center of pressure is at about a third of the way from the root to the tip. This means that the wing root must be able to support a bending moment equal the weight of the bird or airplane times the wing span times the maximum load factor divided by 12. How strong does the structure need to be to be to support such a moment? That depends on the details of the structure (such as the depth of the spar).

ShoeDLG:

Wow, I was all over the map and didn't word that very well at all. Thank you for muddling through that with your very informative reply, especially your answer to 3).

On 2) I was hoping for a numerical answer, though that undoubtedly depends on many factors. Let's say there's a raven with a 48" wingspan and weighing 3.5lbs, gliding along at a given slope at an optimal speed. Who knows what his effective angle of attack is? Make up the numbers if you must. What percentage of total lift does the body minus the wings contribute? With all of the control that the raven has over the shape of his body's surface, does that contribution approach 35%? More? Or is the contribution of the airframe minus the wings of that commercial jet comparable to that of the raven, because we've optimized the heck out of it?

You're trying to nail stuff down that can't be nailed down.

I also seriously doubt that the fuselage lift thing is accurate other than perhaps during high angle of attack flight modes such as during climbing and during descent. A fuselage is a very poor shape for a wing. Any lift it develops is going to be attached to a higher than optimum degree of drag. So during the most efficient cruise speeds the fuselage is going to be angled such that it produces the least drag and no lift. At least nothing significant.

The pressure stuff you're looking for is called the "pressure distribution" and is commonly shown by a pressure distribution diagram. And yes the low pressure on top of the wing and higher pressure below is not even across the chord.

If you download and play around with Foilsim you can easily alter the specs for a basic wing and airfoil and see all this occurring very easily. It's a superb training aid for trying to get a handle on how changes in flight affect lift.

The bit you've read about the lift near the root being greater is related to the spanwise lift distribution. This curve changes depending on wing taper and any washout or washin twist in the wing.

I would suggest that you don't try to nail hard numbers down to any of this stuff because it all changes radically as the angle of attack changes. Only airliners or other long distance planes that cruise at one altitude and speed for long stretches at a time will see the numbers for anything remain constant for that portion of the flight. Pretty well any sort of model flying tends to be constantly moving around or at most flies in a straight line for only a few seconds.

The lift distribution of a soaring bird is likely pretty close to elliptical (this distribution gives the least drag for a given span and lift). By unloading the tips a bit compared to an elliptical distribution, it's possible for a bird to reduce the bending moment that must be carried by its wings. This suggests that any deviation from an elliptical distribution would be toward (slightly) less loaded tips. Having the tips unloaded would mean having the root sections carry a greater fraction of the lift.

To get a reasonable estimate of the lift directly generated by a bird's body, I would draw an ellipse over a front view of the bird. The area over the bird's body divided by the total area of the ellipse should give you a pretty good estimate of the fraction of lift carried by the body...

Originally Posted by BMatthews

A fuselage is a very poor shape for a wing. Any lift it develops is going to be attached to a higher than optimum degree of drag. So during the most efficient cruise speeds the fuselage is going to be angled such that it produces the least drag and no lift. At least nothing significant.

Not true. If the fuselage generated no lift, the the wing roots would behave just like wing tips... meaning that you would have not only strong tip vorticies, but equally strong root vorticies. In other words you would effectively have two wings of less than half the airplane's span flying side by side. The induced drag penalty for this arrangement compared to a single lifting wing is enormous. So, while it may be somewhat costly to generate lift with the fuselage, the cost of not generating lift with the fuselage is much higher.

Is it true that the greater lift contribution of the upper wing surface is only true for airfoils with non-zero thickness?
On a zero thickness cambered airfoil, the streamlines directly adjacent to the upper surface have the same curvature as those directly adjavent to the lower surface, so pressure distribution is the same (50:50). Now an increase in airfoil thickness leads to a local reduction of curvature on the lower side or even locally opposite curvature which lowers pressure in that region compared to the zero thickness case. This reduces the high pressure contribution. Right?
Here I neglected boundary layer effects which effectively increase airfoil thickness even on a zero-thickness foil, but I'd think this effect is neglible compared to typical geometric airfoil thickness.

I still do not understand why the center of pressure moves as angle of attack increases
- toward the leading edge for cambered airfoils
- stays nearly constant for symmetric airfoils
- toward the trailing edge for significantly reflexed airfoils
Can anyone explain the physical reason for this?

BMatthews:

Thanks. I had been looking at 2D pressure distribution plots all day. A difficulty that I've had with that is many of the images don't label all of the pertinent information, such as angle of attack, which led me to strange generalizations. ShoeDLG's earlier reply turned me on to the 3D plots, and now I'm seeing it more clearly.

ShoeDLG:

Right on! You nailed it for me. Thank you.

Makes me wonder if we are talking about African or European swallows.

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Incredible--
Take away the sections which contribute less then re "analyze"
This stuff is nonsensical
Which side of a wing contributes more ?
Gee - there is no one sided wings
why bother with trying to assign numbers to sections when everything is interdependent ?
just bored?

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The British Swallow --




Ray.

The results of not knowing the differences of swallows:

The Bridge of Death - Monty Python and the Holy Grail (3 min 17 sec)

That is right in keeping with the "pressure distribution answers "