Magnus effect

Walboy

Pre-takeoff checklist
Joined
Feb 11, 2014
Messages
402
Display Name

Display name:
Walboy
.
 
Last edited:
I'm studying material for the CFI test(s) and have a question regarding the Magnus effect on pages 3-5 through 3-7 in the PHAK (FAA-H-8083-25A).

The concept is introduced to illustrate how viscosity and friction affects the flow of a fluid around a rotating cylinder. I get it. The surface friction and viscosity of the fluid cause a lower pressure due higher velocity over the top of the rotating cylinder as the molecules are pulled in the direction of rotation. At one point in the discussion the rotating cylinder becomes a proxy for an airfoil without any explanation.

Just exactly how is a rotating cylinder analogous to an airfoil on a fixed non-rotating wing aircraft?

I have a pretty decent understanding of Bernoulli's principle, so no need to go there.


An airfoil has circulation analogous to the Magnus effect.

Now, when you say you have a decent understanding of Bernoulli's principle, what do you mean? You understand the actual equation, or you associate Bernoulli with some sort of shape and/or distance, or a wing looks like half a venturi, or...
 
I'm studying material for the CFI test(s) and have a question regarding the Magnus effect on pages 3-5 through 3-7 in the PHAK (FAA-H-8083-25A).

The concept is introduced to illustrate how viscosity and friction affects the flow of a fluid around a rotating cylinder. I get it. The surface friction and viscosity of the fluid cause a lower pressure due higher velocity over the top of the rotating cylinder as the molecules are pulled in the direction of rotation. At one point in the discussion the rotating cylinder becomes a proxy for an airfoil without any explanation.

Just exactly how is a rotating cylinder analogous to an airfoil on a fixed non-rotating wing aircraft?

I have a pretty decent understanding of Bernoulli's principle, so no need to go there.

Because with movement it creates a wake contained by a boundary layer that shapes itself into an airfoil, causing a rearward deflection (angle of attack) in the direction of the spin. Same principle used in throwing curves and sliders with a baseball.
 
Would you please elaborate? Why is it analogous when the circulation is caused by the rotation?

My understanding up until now had been that since the path of the flow of air over the curved upper surface of a wing is longer than the lower surface, a relatively lower area of pressure develops on the upper side due to the increased velocity.

The rotating cylinder analogy was used to illustrate how molecules cling to surfaces and each other due to friction and viscosity. I'm just having a hard time understanding the leap from a rotating cylinder to a non rotating wing. It seems to me that phenomenon of friction and viscosity exist on both sides of a wing.

Maybe I'm overthinking this and should just move on.

The Magnus effect is only relevant if the rotating cylinder is moving with respect to the wind. Say you had pie crust roller, held it by the pins in front of you horizontally, gave the roller a downward whirl and then ran forward, you would effectively create a lifting airfoil.
 
Would you please elaborate? Why is it analogous when the circulation is caused by the rotation?

My understanding up until now had been that since the path of the flow of air over the curved upper surface of a wing is longer than the lower surface, a relatively lower area of pressure develops on the upper side due to the increased velocity.

The rotating cylinder analogy was used to illustrate how molecules cling to surfaces and each other due to friction and viscosity. I'm just having a hard time understanding the leap from a rotating cylinder to a non rotating wing. It seems to me that phenomenon of friction and viscosity exist on both sides of a wing.

Maybe I'm overthinking this and should just move on.

Over thinking? Perhaps. The FAA doesn't expect you to actually know anything. But it would be nice to be able to tell the truth and not resort to fairy tales, eh?

Longer vs. shorter flow length stuff falls into the total nonsense fairy tale category. If it were true, how would changing the angle of attack have a significant effect on lift? The geometry doesn't change. How do you fly upside down? etc. etc. etc.

You can explain lift a few ways - you can invoke circulation as here which should answer your question about why the FAA brings up the Magnus effect, or you can attempt to avoid the whole circulation thing as much as possible as in here.
 
Wow! What a fantastic website! Thanks for helping.

No kidding…I'm truly impressed with that treatise.

The best part is, unlike some publications (including the PHAK!), it doesn't have very many errors. IMO, it's the best out there short of an aerodynamics textbook.
 
Magnus effect is the only reason to brake wheels from rotating after takeoff.

When they rotate they are producing lift DOWN.
 
I gotta admit I'd forgotten what "Magnus effect" referred to. I cannot recall even once in my 41 years as a CFI having reason to discuss that effect while giving flight or ground training. That ought to tell you how important it is to actually being a flight instructor. Has anyone here ever heard that question on a CFI practical test?
 
Magnus effect is the only reason to brake wheels from rotating after takeoff.

When they rotate they are producing lift DOWN.

No. They are rotating in the correct direction for positive lift.

However, it is turbulent flow. You may have noticed that some airframes shudder right after takeoff due to that and stiffness that is just right to excite that mode. Particularly if the airplane doesn't have wheel pants. The Magnus effect is not zero with wheel pants, but the flow becomes much less turbulent.

And it is definitely not the only reason. Cessna retracts can rub the wheels on the frame holes if they are still spinning during retraction, due just to centrifugal distortion of the tire.
 
Last edited:
I just hate the rumbling sound and vibration. Never thought of it in terms of lift produced, and I doubt it's a significant amount, anyway.
 
No. They are rotating in the correct direction for positive lift.

Not in the lift diagrams I've ever seen.

0199210896.Magnus-effect.1.jpg
 
I just hate the rumbling sound and vibration. Never thought of it in terms of lift produced, and I doubt it's a significant amount, anyway.


In addition to very hard tire compounds and groved runways (and HEAVY LOAD), vibration sounds like flat spots and out of balance tires. Don't forget t brake disks are essentially loose inside the wheels. You can hear them constantly falling when towed. Loose gear doors are another source.
 
Last edited:
And it is definitely not the only reason. Cessna retracts can rub the wheels on the frame holes if they are still spinning during retraction, due just to centrifugal distortion of the tire.

Wrong again!

Any airplane that needs to have its wheel stopped in retraction have "snubbers." Otherwise, it is a maintenance problem.
 
Wrong again!

Any airplane that needs to have its wheel stopped in retraction have "snubbers." Otherwise, it is a maintenance problem.

Well, the Cessna 172RG POH disagrees with you. They aren't always correct, but I still stop the wheels prior to retraction on that airframe, like the checklist says.
 
The easiest visual display of the Magnus Effect would be to take a straw, lay it near the edge of the table, pinch the back end off your finger, and watch it climb and then fall to the ground. That slight climb is a visual indication of the lift being created by the airflow over the straw.

When you pinch it to flick it off, you're rotating it backwards while propelling it forwards, if that makes sense.
 
No. They are rotating in the correct direction for positive lift.

However, it is turbulent flow. You may have noticed that some airframes shudder right after takeoff due to that and stiffness that is just right to excite that mode. Particularly if the airplane doesn't have wheel pants. The Magnus effect is not zero with wheel pants, but the flow becomes much less turbulent.

And it is definitely not the only reason. Cessna retracts can rub the wheels on the frame holes if they are still spinning during retraction, due just to centrifugal distortion of the tire.

Rotating backwards would be the correct direction.

This one time, I was taking off 500 pounds over gross and I couldn't get out of ground effect. The end of the runway was coming near so I stomped on the brakes to reduce the negative lift, and I started climbing out safely.

Thanks Magnus!
 
The thing about Magnus and Magneto, they both have the same root. Mag....

hope that helps :)
 
I would guess that if you went into any FSDO and asked a group of Ops Inspectors about Magnus Effect, 90+ percent would not know what you were talking about. Glad I got my CFI in 1968.

Bob Gardner
 
I would guess that if you went into any FSDO and asked a group of Ops Inspectors about Magnus Effect, 90+ percent would not know what you were talking about. Glad I got my CFI in 1968.
I was not asked about that term during either my initial ride in 1973, or my reinstatement ride in 1977, or my second reinstatement ride in 1990, so you probably would have been OK long after 1968.
 
I was not asked about that term during either my initial ride in 1973, or my reinstatement ride in 1977, or my second reinstatement ride in 1990, so you probably would have been OK long after 1968.

I never would have expected to hear the term outside of fluid dynamics class.
 
Dizzy Dean once said, in reponse to a claim that a curve ball was an optical illusion, "Stand behind a tree 60 feet away, and I'll whomp you with an optical illusion!"
 
Back
Top