The physics of reduced incidence towards the wingtips

SixPapaCharlie

SixPapaCharlie

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Got to thinking...

Most of our planes have reduced angle of incidence toward the tips so the wing stalls from the root first then gradually out toward the wing tips.

If I am cruising along straight and level at a high rate of speed, is the wing some how splitting that difference? I know it is only like 3 degrees but it seems if I was in a cruise attitude per the root AOI then there would be a tremendous downward force on the wing tips.

If cruising based on attitude of the wingtip AOI, then the root should want to climb.

Note: this question is coming from someone that believes Newton and not Bernoulli is the reason we can fly.
 
0 degrees or -3 degrees the airfoil is still positive for lift
 

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You're describing washout. That's always a hot topic for Supercub builders. It's well known that zero washout allows the plane to fly slower but also make it harder for an average pilot to control the plane at the stall. A typical Cub wing has 1 3/8" of twist between root and tip. And even that's controversial because it's supposed to be measured from aircraft centerline.
 
The horizontal line is AOA. The vertical line is coefficient of lift. At 0 degrees AOA the coefficient o lift is +0.5. At -3 degrees the coefficient of lift is +0.25.
Lift coefficient for this airfoil does not go negative until -5 degrees.
 
The lift coefficient is how much lift you are getting at a given speed and air density per unit of wing area. If it be bigger than zero, you are still getting lift.

The angle of attack (alpha) is measured using a line from the trailing edge to the most forward part of the leading edge (in the middle of the leading edge curve). For a symmetric airfoil (top = bottom) lift = 0 at 0 angle of attack. When you put some camber in the wing (curve it to flatten the bottom and make the top stick up more) you still get lift at small negative angles of attack due to the overall curvature.
 
Stewartb said:
You're describing washout. That's always a hot topic for Supercub builders. It's well known that zero washout allows the plane to fly slower but also make it harder for an average pilot to control the plane at the stall. A typical Cub wing has 1 3/8" of twist between root and tip. And even that's controversial because it's supposed to be measured from aircraft centerline.
Click to expand...


Geez. It seems 0 degree would allow it to fly faster.
Having the angle Δ means presenting more surface to the oncoming wind.
This should increase drag and reduce speed. :dunno:
 
SixPapaCharlie said:
Geez. It seems 0 degree would allow it to fly faster.
Having the angle Δ means presenting more surface to the oncoming wind.
This should increase drag and reduce speed. :dunno:
Click to expand...

Slightly faster plane that blanks out the controls first when getting slow or a slightly slower plane that lets you control it in slow flight and in the pattern.

Your choice.

BTW- you cannot separate Bernoulli and Newton.
 
Stewartb said:
You're describing washout. That's always a hot topic for Supercub builders. It's well known that zero washout allows the plane to fly slower but also make it harder for an average pilot to control the plane at the stall. A typical Cub wing has 1 3/8" of twist between root and tip. And even that's controversial because it's supposed to be measured from aircraft centerline.
Click to expand...
Washout also helps control the spanwise lift distribution - the ideal distribution is elliptical but it is expensive to build an elliptical wing. One solution is to put in some twist (washout) that reduces the lift out at the ends to reduce the induced drag (and improves cruise speed for a given horsepower).
 
SixPapaCharlie said:
Geez. It seems 0 degree would allow it to fly faster.
Having the angle Δ means presenting more surface to the oncoming wind.
This should increase drag and reduce speed. :dunno:
Click to expand...

Check out the twisted wing Mooney. Didn't work as well in reality as THEROY. Which supports your assertion. They added stall strips to get the root to stall first when they dropped the twisted wing..
 
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SixPapaCharlie said:
Geez. It seems 0 degree would allow it to fly faster.
Having the angle Δ means presenting more surface to the oncoming wind.
This should increase drag and reduce speed. :dunno:
Click to expand...

Newer Maules have negative degrees of flaps for cruise flights. My understanding is these reduce lift and as we all know drag is a part of lift.

Frontal area is not all there is to drag, as it matters more about how the air moves around the surface. For example, landing gear on Cessna's have a significantly higher amount of drag attributed to them than the fuselage despite their smaller frontal surface area.
 
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Wing washout is a different topic that overall angle of incidence. Airplane designers had to address many parameters and in doing so incorporated some compromises. Like washout.
 
SixPapaCharlie said:
Geez. It seems 0 degree would allow it to fly faster.
Having the angle Δ means presenting more surface to the oncoming wind.
This should increase drag and reduce speed. :dunno:
Click to expand...

Only for a symmetrical wing.

Most of us have wings that curve at the top (the exception is some aerobatic airplanes designed for inverted flight).

For a related exercise, what happens to angle of attack if you maintain pitch attitude as you deploy flaps?
 
SixPapaCharlie said:
Geez. It seems 0 degree would allow it to fly faster... :dunno:
Click to expand...

Stewartb was talking about Supercubs. The word "faster" has no meaning in that realm.

Wing twist or washout, the offset position of the vertical stab, fixed trim tabs, dihedral, motor mount skewing - these are all tweaks in design to eliminate undesired characteristics or to incorporate characteristics that provide safer or more stable flight. They are compromises from the "pure" design but the end result is better overall. You could, for instance, do everything to optimize the speed of the aircraft but then you couldn't let go of it for a second to pick up a map.

It's said that Lindbergh had the NYP purposely designed as unstable so he couldn't fall asleep.
 
Sleepingsquirrel said:
Check out the twisted wing Mooney. Didn't work as well in reality as THEROY. Which supports your assertion. They added stall strips to get the root to stall first when they dropped the twisted wing..
Click to expand...

Aren't the strips there to provide the warning buffet?
 
The COL doesn't go "negative' the slope of the line goes negative. The COL is still quite positive past the critical angle. In fact, it's pretty symmetrical on both sides of the peak. The reason they stop plotting is when you get way past the critical angle you typically have so much drag that you've got other issues.
 
Back to the original question - I am willing to bet that what it means is that the lift per square foot of wing decreases from root to tip. I suspect that in normal level cruise flight, all parts of the wing contribute to lift, just not the exact same about of contribution.
 
pilot1956 said:
Back to the original question - I am willing to bet that what it means is that the lift per square foot of wing decreases from root to tip. I suspect that in normal level cruise flight, all parts of the wing contribute to lift, just not the exact same about of contribution.
Click to expand...
ii-yup
 
Silvaire said:
Stewartb was talking about Supercubs. The word "faster" has no meaning in that realm.
Click to expand...

The reason I used Supercubs in my example is that they're probably the most discussed examples for washout and that includes builders and the FAA. In fact the FAA has done flight testing and written rigging instructions to clarify washout for Cubs. As I see it there are two reasons. Washout changed between PA-11 and PA-18s but lots of airplane guys, like Atlee Dodge, used the original PA-11 washout to build and modify airplanes. The reason the FAA took interest is because Supercub low-level stall/spin accidents are frequent and they were trying to identify the reason(s). Wing washout was one they addressed. It continues to be a contentious topic today within the Cub crowd.
 
flyingron said:
The COL doesn't go "negative' the slope of the line goes negative. The COL is still quite positive past the critical angle. In fact, it's pretty symmetrical on both sides of the peak. The reason they stop plotting is when you get way past the critical angle you typically have so much drag that you've got other issues.
Click to expand...

"A number of airfoils, including 14 commonly used airfoils and 10 NACA airfoils, were tested through the negative angle-of-attack range in the NACA variable-density wind tunnel at a Reynolds Number of approximately 3,000,000. The tests were made to supply data to serve as a basis for the structural design of airplanes in the inverted flight condition. "

There can be negative COL we're usually not interested in it.
 
Isn't the simple answer that the wingtips still have a positive angle of attack, just not as much as the root? In cruise the overall AOA is maybe 4-8 deg so 3deg of twist still has positive AOA. No need to whip out the calculus books. I don't think . . . . .
 
SixPapaCharlie said:
Got to thinking...

Most of our planes have reduced angle of incidence toward the tips so the wing stalls from the root first then gradually out toward the wing tips.

If I am cruising along straight and level at a high rate of speed, is the wing some how splitting that difference? I know it is only like 3 degrees but it seems if I was in a cruise attitude per the root AOI then there would be a tremendous downward force on the wing tips.

If cruising based on attitude of the wingtip AOI, then the root should want to climb.

Note: this question is coming from someone that believes Newton and not Bernoulli is the reason we can fly.
Click to expand...

It all wants to climb, it just wants to climb less on the tips, and that also helps the roll stability and ride in turbulent and gusty conditions.
 
warthog1984 said:
Slightly faster plane that blanks out the controls first when getting slow or a slightly slower plane that lets you control it in slow flight and in the pattern.

Your choice.

BTW- you cannot separate Bernoulli and Newton.
Click to expand...

Wonder how many aviators bought it before this was figured out...
 
Blatham489 said:
Isn't the simple answer that the wingtips still have a positive angle of attack, just not as much as the root? In cruise the overall AOA is maybe 4-8 deg so 3deg of twist still has positive AOA. No need to whip out the calculus books. I don't think . . . . .
Click to expand...

In level attitude a Cub AOI is something like 1.7. That for a famously slow airplane. A PA-12 is 1* less, which equates to approx 7/8" at the wing root.

Anyone familiar with getting the plane "on step" so it cruises faster?
 
Stewartb said:
Anyone familiar with getting the plane "on step" so it cruises faster?
Click to expand...

My familiarity is limited to OWT-spreading pilots who think of airplanes moving through the air like boats planing on the surface of water.
 
Gee, if only a couple of brothers could invent a system of wing warping that would allow the wing to develop symetric lift across its length and then when we need to turn change the lift capability at the wingtips.

You know, then some motorcycle mechanic will come along and design little winglets at the rear of the wing that change the lift capability and allow for turning. To keep the little wings viable in slow flight he will twist the wing and create less lift at the tips when in straight and level flight.

Don't get me started about what he might do to the tail wheel.
 
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With respect to AOI/AOA/cruise speed, think about why a Cessna is faster with an aft CG. It's contrary to common sense. Or maybe there's more to it that just AOI?
 
Stewartb said:
With respect to AOI/AOA/cruise speed, think about why a Cessna is faster with an aft CG. It's contrary to common sense. Or maybe there's more to it that just AOI?
Click to expand...

Aft CG = less trim drag applied via the horizontal stab. Makes perfect sense.
 
Mtns2Skies said:
Newer Maules have negative degrees of flaps for cruise flights. My understanding is these reduce lift and as we all know drag is a part of lift.

Frontal area is not all there is to drag, as it matters more about how the air moves around the surface. For example, landing gear on Cessna's have a significantly higher amount of drag attributed to them than the fuselage despite their smaller frontal surface area.
Click to expand...

That is called "reflex"... I tried that trick on my 801.. Lost 3 mph...:sad::sad::sad:
 
cgrab said:
Gee, if only a couple of brothers could invent a system of wing warping that would allow the wing to develop symetric lift across its length and then when we need to turn change the lift capability at the wingtips...
Click to expand...

A good example of necessary compromises in developing design. The aileron, invented by Alexander Graham Bell for Curtiss, was clearly better than wing warping but the Wrights, in patent battles with Curtiss, clearly couldn't adopt them and infer that someone had "one-upped" them. So the later model C and D military flyers had large forward finlets, which the Wrights called "blinkers" to offset the severe adverse yaw inherent with the design. Not to say the Curtiss machines had no adverse yaw but the Wrights design was clearly up against a formidable shortcoming.
 
N801BH said:
That is called "reflex"... I tried that trick on my 801.. Lost 3 mph...:sad::sad::sad:
Click to expand...

We found that the reflex setting would be of advantage in some situations but for the most part, meh, didn't do much.
 
IIRC, the hershy wing Cherokees have little or no washout. Still the wingroot stalls first, and in my old Cherokee 140 the ailerons were still effective in a stall. If loaded lightly I could try to power-on stall mine and all it would do is shake like an unbalanced washing machine and still climb at 200FPM with the yoke all the way back and the airspeed indicating 40 mph and the nose pointed ridiculously high as long as you kept the ball in the center with the rudder. If you let it get uncoordinated that way, it'd drop a wing and roll off into an incipient spin which was instantly recoverable.
 
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