Overbanking v. Adverse Yaw

[meritflyer]

Whats the relationship of these two with regard to aerodynamics?

Here is how I teach it and I am looking for a little more detail so feel free to add any input -

Overbanking - during turns the outside wing travels at a greater resultant velocity v. the inside wing. This results in an increase in lift which tends to create a rolling tendency in the direction of the turn. The pilot must use aileron somewhat opposite the turn to counteract such effects.

Adverse Yaw - during a turn, the aileron which deflects downward (outside wing) creates a higher lift/drag situation and tends to yaw the nose opposite the direction of the turn. This is counteracted by rudder inputs.

[DE727UPS]

You forgot to mention the coriolis effect on the horizontal stabilizer. That's the key...

[meritflyer]

Actually, never heard of it.

[DE727UPS]

That's cause you didn't go to Riddle.

[meritflyer]

Care to share your thoughts on it?

[Doug Taylor]

It was all about the "Coanda" at ERAU/PRC back in the early 1990s!

But for the technical aspects other than saying "trigonometry and physics", i really don't know. It's certainly in the realm of "minutia" but if you really want to know, I'd talk to Aero_Engineer.

[MidlifeFlyer]

merit,

They both sound fine to me for basic explanations. The only thing I do a bit extra with overbanking is talk about stability as well. Overbanking tendency exists in all turns, but you don't need to counteract it in shallow turns. That's because of the airplane's positive lateral stability.

In shallow to medium turns, the stability wins. If you put the airplane into a 15° bank and let go, it will roll out itself. If you put the airplane into a steep turn and let go, overbanking wins. There is a "pivot point" where the two tendencies are equal.

I like demos and this is one of the concepts I teach in the first intro lesson (mostly for the purpose of "advertising" how stable the airplane is). I don't do the steep turn, but I show a shallow turn righting itself an then do a 30° turn (retriming for level flight) and let go so the new student can see the airplane maintain it's bank (although it eventually degrades, I can usually get 2 180s out of it)

[tgrayson]

Originally Posted by MidlifeFlyer

merit,
In shallow to medium turns, the stability wins.

This concept is from the Airplane Flying Handbook and I disagree with it. It's based on the premise that an aircraft's lateral stability tends to roll an aircraft out of a bank. This isn't exactly true, because an aircraft has no way of knowing that it's banked. What it does recognize is sideslip. In an uncoordinated bank, the aircraft will be in a slight sideslip, which will activate the geometric dihedral and tend to roll the airplane until the sideslip goes away.

In theory, if your turn is coordinated, there will be no sideslip and the aircraft will have no tendency to roll wings level.

[tgrayson]

Originally Posted by meritflyer

Adverse Yaw - during a turn, the aileron which deflects downward (outside wing) creates a higher lift/drag situation and tends to yaw the nose opposite the direction of the turn. This is counteracted by rudder inputs.

There is also a yaw due to roll rate. As you roll left, the downward moving wing creates its own relative wind due to its downward motion. Lift is perpendicular to the relative wind that causes it, so the lift vector points forward somewhat and pulls the wing in that direction, creating adverse yaw.

According to some aerodynamics books, this is more significant than aileron drag.

Also, during a steady turn, there is the effect that I mentioned in another thread, due to the yaw rate of an aircraft that will require rudder in the direction of the turn.

This is probably excessive detail for a student pilot. They will do well to remember the aileron stuff. ;-)

[DE727UPS]

"This concept is from the Airplane Flying Handbook and I disagree with it"

You're disagreeing with the FAA blessed resource on how flying machines work. Now THAT'S an interesting topic of discussion...

[tgrayson]

Originally Posted by DE727UPS

You're disagreeing with the FAA blessed resource on how flying machines work. Now THAT'S an interesting topic of discussion...

The FAA is notorious for having a poor understanding of aerodynamics (and physics).

Take a look at the explanation in the predecessor to the AFH, Flight Training Handbook, as to how dihedral works. And until recently, they've included both centripetal and centrifugal force in their diagrams of turns.

One advantage of the AFH compared to Flight Training Handbook is that it leaves out a lot of explanations as to how things work; keeps them from being wrong. ;-)

[Dugie8]

Originally Posted by tgrayson

The FAA is notorious for having a poor understanding of aerodynamics (and physics).

Take a look at the explanation in the predecessor to the AFH, Flight Training Handbook, as to how dihedral works. And until recently, they've included both centripetal and centrifugal force in their diagrams of turns.

One advantage of the AFH compared to Flight Training Handbook is that it leaves out a lot of explanations as to how things work; keeps them from being wrong. ;-)

You had me until centrifugal and centripetal forces. Lay it down for me how those don't "exist" in a turn (if that is what you are saying).

[tgrayson]

Originally Posted by Dugie8

You had me until centrifugal and centripetal forces. Lay it down for me how those don't "exist" in a turn (if that is what you are saying).

One or the other "exists", not both. Otherwise, they cancel. In general, centrifugal force is considered a "fictitious" force.

Centripetal force is the force that accelerates the aircraft toward the center of the circle and is provided by the horizontal component of lift. A proper diagram should only represent this force, because there is no force that opposes it. A turn consists of a constant acceleration towards the center of the turn, so there are no balanced forces in the horizontal plane.

What we call centrifugal force is merely the result of our inertia slamming us into the bottom of our seats as the airplane accelerates in the opposite direction.

Now, if you want to conduct your analysis from inside the airplane, then centrifugal force can be considered real, but that's a frame of reference shift. Aircraft mechanics is usually analyized by an outside observer, so centrifugal force should not be considered.

[MidlifeFlyer]

Originally Posted by tgrayson

This concept is from the Airplane Flying Handbook and I disagree with it..

Sorry, I never read it there. For me this concept is from doing it. And I try to keep those turns coordinated.

[Dugie8]

Originally Posted by tgrayson

One or the other "exists", not both. Otherwise, they cancel. In general, centrifugal force is considered a "fictitious" force.

Centripetal force is the force that accelerates the aircraft toward the center of the circle and is provided by the horizontal component of lift. A proper diagram should only represent this force, because there is no force that opposes it. A turn consists of a constant acceleration towards the center of the turn, so there are no balanced forces in the horizontal plane.

What we call centrifugal force is merely the result of our inertia slamming us into the bottom of our seats as the airplane accelerates in the opposite direction.

Now, if you want to conduct your analysis from inside the airplane, then centrifugal force can be considered real, but that's a frame of reference shift. Aircraft mechanics is usually analyized by an outside observer, so centrifugal force should not be considered.

That is what I thought you were getting at. I agree and I disagree, but it is really more a point of semantics than physics.

[meritflyer]

fish, where are you??

[tgrayson]

Originally Posted by Dugie8

That is what I thought you were getting at. I agree and I disagree, but it is really more a point of semantics than physics.

Not really...include centrifugal force in a statics/dynamics or physics course in college and you'll get a nasty red mark on your exam.

Including centrifugal force in a diagram of a turn indicates a fundamental misunderstanding of the physics involved. Curved flight requires a net force. Centripetal and centrifugal forces are equal and opposite, resulting in no net force; no curved flight would be possible if they both were real forces in the same frame of reference.

[MidlifeFlyer]

Originally Posted by tgrayson

Not really...include centrifugal force in a statics/dynamics or physics course in college and you'll get a nasty red mark on your exam.

When I have an engineer or physicist as a student, I'll worry about it. As it is, I explain to my students that the FAA description is simplistic at best and if it doesn't fit their needs in terms of a practical understanding of aerodynamics they can apply to the airplane, I'm happy to point them in the right direction and they can deal with all the Rhos and Thetas they want.

[tgrayson]

Originally Posted by MidlifeFlyer

When I have an engineer or physicist as a student, I'll worry about it. As it is, I explain to my students that the FAA description is simplistic at best and if it doesn't fit their needs in terms of a practical understanding of aerodynamics they can apply to the airplane, I'm happy to point them in the right direction and they can deal with all the Rhos and Thetas they want.

Nothing at all wrong with that. Most students will not care to know about the rho's and theta's. Even the ones that do won't likely achieve the knowledge they want while pursuing a PPL.

However, some pilots at some point want to have a fuller technical understanding of what they and the airplane are doing. These needs cannot normally be met by even the above-average flight instructor. That's when they post in forums such as this. I'm glad they ask.

[MidlifeFlyer]

Originally Posted by tgrayson

That's when they post in forums such as this. I'm glad they ask.

And I'm glad there's someone around who understands it.

[meritflyer]

Originally Posted by tgrayson

However, some pilots at some point want to have a fuller technical understanding of what they and the airplane are doing. These needs cannot normally be met by even the above-average flight instructor. That's when they post in forums such as this. I'm glad they ask.

Can you give me a reference for your thoughts on this subject?

[tgrayson]

Originally Posted by meritflyer

Can you give me a reference for your thoughts on this subject?

Absolutely. I'm pleased you asked. Which thoughts in particular would you like references for?

[meritflyer]

Any and/or all if possible.

[tgrayson]

Originally Posted by meritflyer

Any and/or all if possible.

Hmmm....we'll start with this. Let me know if there's something else you'd like references to.

From Sears and Zemansky’s “University Physics”, 10th edition, by Young and Freedman, p. 140.

Caution:>In doing problems involving uniform circular motion, you may be tempted to include an extra outward force of magnitude m(v^2/R) to “keep the body out there” or to “keep it in equilibrium”; this outward force is usually called “centrifugal force” (“centrifugal” means “fleeing from the center”). Resist this temptation, because this approach is simply wrong! First, the body does not “stay out there”; it is in constant motion around its circular path. Its velocity is constantly in direction so it accelerates and is NOT in equilibrium. Second, if there were an additional outward (“centrifugal”) force to balance the inward force, there would then be no net inward force to cause the circular motion, and the body would move in a straight line, not a circle. Third, the quantity m(v^2/R) is not a force; it corresponds to the “ma” side of Sum(F)=ma and does not appear in Sum(F). It’s certainly true that a passenger riding in a car going around a circular path on a level road tends to slide to the outside of the turn, as though responding to a “centrifugal force.” But such a passenger is in an accelerating, non-inertial frame of reference in which Newton’s first and second laws don’t apply. As we discussed in Section 4-3, what really happens is that the passenger tends to keep moving in a straight line, and the outer side of the car “runs into” the passenger as the car turns (Fig. 4-8c). In an inertial frame of reference there is no such thing as a “centrifugal force” acting on a body. We promise not to mention this term again, and we strongly advise you to avoid using it as well.


http://www.amazon.com/Sears-Zemansky...e=UTF8&s=books

For something more accessible, here’s the Physics FAQ:

http://math.ucr.edu/home/baez/physic...al/centri.html

[MidlifeFlyer]

Originally Posted by tgrayson

As we discussed in Section 4-3, what really happens is that the passenger tends to keep moving in a straight line, and the outer side of the car “runs into” the passenger as the car turns

In other words, merit, what we =call= "centrifugal force" is actually a shorthand description of a body's tendency to keep moving in the same direction while being pulled to the center.

As I recall (it's been a long time) the standard elementary school "experiment" to show centrifugal force is swinging a ball around on a string. The apparent movement of the ball to the outside of the circle is actually the result of the ball's straight line momentum and the centripetal force exerted by the string. If the string broke, the ball would travel in a straight line at a tangent to the circle, not outward. If centrifugal force really existed, the ball would travel outward.

There's a pretty good graphic at http://www.phy.ntnu.edu.tw/oldjava/c...cular3D_e.html. Note the string (centripetal force) and the straight ahead (not curved) velocity vector of the ball.

That's why "centrifugal force" is not a "force" that acts on something. Ball or airplane, it's just a convenient shorthand to describe the net result of the centripetal force and velocity vectors. Maybe "centrifugal phenomenon" would appease those who object to the technically inaccurate use of the word "force?"

tgrayson, how'd I do?