Ever feel like you don't know enough?

[PilotAlan]

Then, two hours later, I remembered the phrase "relative wind" and realized that it's my understanding (albeit potentially incorrect), that I could use a fan to cause the air to move over the wing, and it would still be the same - lift from a pressure differential due to the two air flows meeting at the same time at the back end of the wing, and therefore, the top flow moving faster than the bottom flow.

And, it's gone.

Howdy,
The key is that the air is moving as a mass. The air on top can't slow down. If it did, it would have to separate from the air behind it,right? If it slows down, then there have to be a break, and that would create a vacuum.
That can't happen, so the air must keep moving as a mass.

You picture the air moving from left to right, and looking at the theoretical "one particle of air". But it isn't starting from a vacuum on the left side of the diagram and moving to right, it's part of the much larger body of air, and it must stay in its place relative to that volume.

It took me a long time to get it. I tend to ask "why" rather than just accept it. So when you are wondering why something doesn't do something, ask what would happen if it did. Why doesn't the air slow down? Because it would create a vacuum.

 

[denverpilot]

In supersonic aircraft, isn't there a term for air that can't make it to the trailing edge fast enough and starts to separate in wild directions, or something like that? Non-laminar flow? Part of the reason we see a shock wave form?

 

[TMetzinger]

Hopefully this won't introduce the Centipede's Dilemma, but I'm not sure there's any magical force that requres the air on top to "meet" the air on the bottom at the trailing edge.

The cause and effect in Bernoulli's theory is one he chose.

One could say that the air on top moves faster (this is observed) as a RESULT of the lower pressure (also observed) on the top of the wing relative to the bottom of the wing, and now you have a Newtonian model.

Put another way...

The OBSERVED FACTS are that there is higher pressure at the bottom of the wing and lower pressure above it. Airspeed above is higher than airspeed below.

Whichever theory you use to describe why that occurs is less important than understanding that it does occur, and that there are theories to explain it.

 

[denverpilot]

I've never understood why aircraft with obviously "traditional" airfoils, longer across the top than the bottom, still manage to successfully fly inverted. I'll admit it.

I've thought about it and figure that they must fly at a significantly higher angle of attack when inverted to overcome this handicap. But there has to be more to it.

Recently I've read some stuff that seems to help. It's stuff that talks about just accelerating a completely flat airfoil and then turning it upward, and that much of what we attribute to Bernoulli is really just equal-and-opposite flat plane getting shoved upward, just like putting your hand out the car window. Accelerate a brick enough and control it's angle of attack, and it'll go up.

What I'm mostly curious about is what percentage in a typical light aircraft airfoil is Bernoulli and what percentage is just accelerating a brick and angling it upward.

I'm sure the answers are buried in Aerodynamics for Naval Aviators somewhere, but I suck at much of the math in there.

Let me toss out there those airfoils that are symmetrical for more cannon fodder on the topic. Seems like those are definitely the accelerate a brick and twist it at an upward angle, variety of wings.

 

[Clark1961]

Nate - It seems that some folks (qualified academics) think all lift results from the downward acceleration of air.

Since I don't know the right answer I just stick with the explanation that money makes aircraft fly. If that position is attacked then I go with lift fairies and thrust demons. As always YMWV.

 

[MAKG1]

Nate - It seems that some folks (qualified academics) think all lift results from the downward acceleration of air.

Really? I've never seen that in the literature. Common statements seem to be just the opposite. Most of the lift comes from "sucking" the wing upwards. Stalls don't make any sense otherwise.

As for the statement (repeated at least three times so far) about airflow meeting at the trailing edge, no one can understand it because it isn't true. There is no vacuum if a parcel doesn't meet its previous neighbor at the trailing edge. It just meets a different parcel. If the flow is turbulent (due to a pending stall, for instance), there will be contact discontinuities there. Not a big deal; this happens all the time in subsonic flow -- it's what a "front" is in weather phenomena.

Maybe a better way to think about it is that following a streamline requires constant total pressure (or the flow won't follow the streamline, but will go elsewhere), and a hard boundary must be a streamline.

 

[Clark1961]

Really? I've never seen that in the literature. Common statements seem to be just the opposite. Most of the lift comes from "sucking" the wing upwards. Stalls don't make any sense otherwise.

Why do you think stalls don't make sense otherwise? When the F generated by forcing air downward no longer equals (or exceeds) the weight of the aircraft then it's going to stop flying...(or the lift fairies are going to stop lifting).

 

[MAKG1]

Because, with increasing angle of attack, you'll force more mass downwards, not less. The stall occurs because the flow over the top of the wing is disrupted, erasing the "suck" (for lack of a better term).

 

[Threefingeredjack]

Sarah, first of all, welcome. You have probably already heard the old saying that a pilot's license is a "license to learn". I am not the most experienced pilot around, but I've done a bit of flying, and I can tell you that in my mind you should never feel that you know enough. The fact that you have this hunger and drive is a great thing; nurture it and it will serve you well. Enjoy this fantastic journey you are beginning.

 

[Clark1961]

Because, with increasing angle of attack, you'll force more mass downwards, not less. The stall occurs because the flow over the top of the wing is disrupted, erasing the "suck" (for lack of a better term).

Why do you think increasing angle of attack will force more mass downward? Velocity is also a factor so as the wing slows it will force less mass downward resulting in a stall.

 

[MAKG1]

Why do you think increasing angle of attack will force more mass downward? Velocity is also a factor so as the wing slows it will force less mass downward resulting in a stall.

As an exercise, explain accelerated stalls with this. You can make an accelerated stall at any airspeed if you pull the yoke hard and fast enough, as long as the structure will take the forces without breaking (so, do this below Va for your weight).

You don't have to fly slow to stall. And the stalling angle of attack is always the same, about 18 deg.

You can explain this with loss of suction -- it goes away as soon as flow separates from the surface (it still sucks, just on other air parcels, rather than your wing); such is determined exclusively by geometry, not by speed or force. You can't explain it with downdrafts. Particularly the constant stall AoA.

 

[Clark1961]

As an exercise, explain accelerated stalls with this. You can make an accelerated stall at any airspeed if you pull the yoke hard and fast enough, as long as the structure will take the forces without breaking (so, do this below Va for your weight).

You don't have to fly slow to stall. And the stalling angle of attack is always the same, about 18 deg.

You can explain this with loss of suction -- it goes away as soon as flow separates from the surface (it still sucks, just on other air parcels, rather than your wing); such is determined exclusively by geometry, not by speed or force. You can't explain it with downdrafts. Particularly the constant stall AoA.

As far as the wing is concerned lift just has to equal the effective load. It doesn't matter what those loads actually are and slow is a relative term.

 

[TMetzinger]

Because, with increasing angle of attack, you'll force more mass downwards, not less. The stall occurs because the flow over the top of the wing is disrupted, erasing the "suck" (for lack of a better term).

Up to a certain point, (the critical angle of attack) that's true. Above that point you're moving so slowly through the air that you cannot generate enough downward force.

Assume a symmetrical airfoil, and describe the effect of increasing AoA. The distance from leading edge to trailing edge is the same on both top and bottom.

 

[MAKG1]

Tim, if all this is due to "downforce," why is there a critical angle of attack at all? It should be a function of forward speed, not angle.

Frankenkota, you're going to have to go deeper than that if you want to understand it. That's a tautology. Suppose I'm cruising along in my 172 at 97 KIAS (Va) straight and level at max gross. I yank up suddenly on the yoke, up to 45 deg pitch angle Vector analysis from "downdrafts" says that 70% of the relative wind is now deflected downward, whereas previously it had been much smaller, just a few percent. The forward speed is unchanged (that's the point of doing it fast). This analysis would suggest a massive increase in lift right away -- it's almost equivalent to an elastic collision. Why does this stall immediately? Try a less extreme accelerated stall to experience this. You don't have to slow down.

The answer requires flow separation. You can't explain it by deflections.

In a real wing, there is some pressure below the wing, and a partial vacuum above it. The partial vacuum is much more significant, except during a stall. The pressure doesn't change much during a stall, so the remaining lift while deeply stalled is a measure of how much the pressure side really matters. It's not zero, but it's very far from dominant.

Tim, thinking about a symmetrical wing is useful here, as it helps get past not terribly relevant complications from incomplete Bernoulli models.

 

[denverpilot]

Heh. I'm almost sorry I brought it up.

Does a 172 have enough elevator authority to go into an accelerated stall from cruise flight? I'd think it might exceed the G load rating on the airframe and break something. Ouch.

Maybe a better example for anyone crazy enough reading this thread who goes out and tries things that aren't advisable, would be a Pitts?

 

[MAKG1]

Does a 172 have enough elevator authority to go into an accelerated stall from cruise flight? I'd think it might exceed the G load rating on the airframe and break something. Ouch.

Unless I've seriously misunderstood the point of Va, you won't break anything. Va is the point at which the stall speed is equivalent to the maximum rated G-load. Va is pretty close to cruise speed at max gross for a 172N. Now, solo, it's a lot slower.

But it is intended to be more of a thought experiment, attempting to get to specifics instead of the general statements made earlier. 45 deg pitch angles have other problems (like, where to put the required parachute -- and you're well out of utility category at that weight, so there are certification issues as well). It happens to be easy to calculate.

 

[docmirror]

I got pretty frustrated trying to understand manifold pressure and how its regulated by a constand speed prop.

I think you know now that MP is not regulated by the constant speed prop. The constant speed prop regulates the 'speed' or RPM of the engine. When you set the control for the prop, it will remain at that RPM unless the control is changed, or the engine can no longer provide sufficient torque to maintain that RPM.

MP is controlled by the throttle inside the carb(or the air metering plate in a FI) which allows a calibrated amount of air pressure into the intake.

Many of the questions you've asked have to do with aeronautical engineering. It will take time to get these down if you don't have a degree in eng, but you don't need the details to be a safe pilot.

I think the most important thing at this time is you have learned something critical. You know what you don't know, and you realize the limits it places on your progress. It'll all come together, and your learning curve will flatten out after a while. Knowing the chord line, or mean aero chord of a prop isn't very important in setting the RPM, or power.

 

[Clark1961]

Tim, if all this is due to "downforce," why is there a critical angle of attack at all? It should be a function of forward speed, not angle.

Frankenkota, you're going to have to go deeper than that if you want to understand it. That's a tautology. Suppose I'm cruising along in my 172 at 97 KIAS (Va) straight and level at max gross. I yank up suddenly on the yoke, up to 45 deg pitch angle Vector analysis from "downdrafts" says that 70% of the relative wind is now deflected downward, whereas previously it had been much smaller, just a few percent. The forward speed is unchanged (that's the point of doing it fast). This analysis would suggest a massive increase in lift right away -- it's almost equivalent to an elastic collision. Why does this stall immediately? Try a less extreme accelerated stall to experience this. You don't have to slow down.

The answer requires flow separation. You can't explain it by deflections.

Perhaps you can't explain it by deflections, that doesn't mean it can't be explained...in your example the g-load increases (the accelerated part) which causes the stall. The deflected mass isn't enough to equal the load. Don't try to make it any more or less complicated than it is.

 

[denverpilot]

Unless I've seriously misunderstood the point of Va, you won't break anything. Va is the point at which the stall speed is equivalent to the maximum rated G-load. Va is pretty close to cruise speed at max gross for a 172N. Now, solo, it's a lot slower.

But it is intended to be more of a thought experiment, attempting to get to specifics instead of the general statements made earlier. 45 deg pitch angles have other problems (like, where to put the required parachute -- and you're well out of utility category at that weight, so there are certification issues as well). It happens to be easy to calculate.

Understand. And I don't have any V-speeds for 172s memorized anymore. Been too long.

I'm not a huge fan of pushing 30+ year old airframes to their design limits either... Sure, inspected and all that, yadda, yadda... but not spamcans... Stuff might fall off.

 

[MAKG1]

Perhaps you can't explain it by deflections, that doesn't mean it can't be explained...in your example the g-load increases (the accelerated part) which causes the stall. The deflected mass isn't enough to equal the load. Don't try to make it any more or less complicated than it is.

That doesn't work when you get quantitative.

The G-load at Va is no higher than 3.8 G -- the maximum rated load. The "deflected mass" is about 20 times bigger at 45 deg than it is at 2 deg (sin 45 deg / sin 2 deg), initially. Taking your model at face value, this says the stall angle of attack at Va is (much) greater than 45 deg. That's wrong.

A good physical description is as simple as possible, but no simpler.

DenverPilot, I hear you. Va is convenient because we can know the G-loads immediately. But I do agree -- don't do this at home. At least not nearly so extreme. The point is made as long as you're well above the unaccelerated stall speed.

 

[Clark1961]

That doesn't work when you get quantitative.

The G-load at Va is no higher than 3.8 G -- the maximum rated load. The "deflected mass" is about 20 times bigger at 45 deg than it is at 2 deg (sin 45 deg / sin 2 deg), initially. Taking your model at face value, this says the stall angle of attack at Va is (much) greater than 45 deg.

No it doesn't. Deck angle doesn't equal AOA.

 

[kontiki]

I'm pretty sure I don't know enough.

 

[MAKG1]

No it doesn't. Deck angle doesn't equal AOA.

It does when you're cruising straight and level. That's why I set it up that way. For this problem the relative wind is known to be parallel to the ground.

This is also why we consider the initial response, and use a rapid control motion to initiate the stall. Before the aircraft has had a chance to climb, the relative wind is still parallel to the ground.

The key to understanding a complex phenomenon is to separate out the variables as much as possible, by picking reasonable physical parameters that make some of the variables go away.

 

[Clark1961]

It does when you're cruising straight and level.

If you're cruising straight and level then you aren't pulling up or are you imagining an instantaneous pull-up? If so, I'll leave you to your imaginings at this point.

 

[MAKG1]

If you're cruising straight and level then you aren't pulling up or are you imagining an instantaneous pull-up? If so, I'll leave you to your imaginings at this point.

Please reread what you are responding to.

The idea is a quick change in attitude, to minimize change in vertical velocity. Your aircraft has inertia. This is always possible; the attitude will lead the velocity signficantly, so the transient gives you your information.

I hope you don't think that the conventional power-on stall is the only way to stall the aircraft in cruise. It most certainly is not. A rapid change in attitude to beyond the critical angle of attack will stall it.

So, let's review: (1) fly straight and level. (2) yank the yoke back suddenly, to a pitch angle well above the 18 deg critical angle of attack.

At that instant (not a later one), what direction is the relative wind?

 

[docmirror]

Gentlemen, I don't think your contributing to this student's understanding of wing loading or stalls anymore.

 

[Clark1961]

Please reread what you are responding to.

The idea is a quick change in attitude, to minimize change in vertical velocity. Your aircraft has inertia. This is always possible; the attitude will lead the velocity signficantly, so the transient gives you your information.

I hope you don't think that the conventional power-on stall is the only way to stall the aircraft in cruise. It most certainly is not. A rapid change in attitude to beyond the critical angle of attack will stall it.

So, let's review: (1) fly straight and level. (2) yank the yoke back suddenly, to a pitch angle well above the 18 deg critical angle of attack.

At that instant (not a later one), what direction is the relative wind?

I'll leave you to your imaginings at this point.

 

[MAKG1]

I'll leave you to your imaginings at this point.

Suit yourself. But please don't quote that "downdraft" model if you don't spend the effort to understand why it's wrong.

Those of us who model for a living try to get the quantitative responses right. tan(a) is not equal to sqrt(a) even in the small-angle approximation.

 

[Clark1961]

Suit yourself. But please don't quote that "downdraft" model if you don't spend the effort to understand why it's wrong.

Those of us who model for a living try to get the quantitative responses right. tan(a) is not equal to sqrt(a) even in the small-angle approximation.

I don't agree that it is wrong. I've been writing models for more than 20 years now.

 

[denverpilot]

Got it. Thanks guys.