What's the deal with the Magnus Effect?

[pilot#476398]

I was doing some reviewing recently and I came across the Magnus Effect.

 

It talks about airflow about a rotating cylinder, but since the blades aren't spinning around the feathering axis, what are we supposed to do with this explanation of lift?

 

 

 

[Tom22]

Use Magnus Effect to explain why a baseball pitcher can throw a curve ball or why golf balls have dimples .

[Tom22]

You can use Magnus Effect to explain German engineer Anton Flettner's boat: he replaced the sail on his boat with a rotating circular cylinder with its axis vertical to the deck.

[terminal_velo]

Yea, I've never understood the relevance either, and no pilot I've asked has been able to explain it.

 

Apparently someone's tried to use it to fly though:

http://en.wikipedia.org/wiki/Magnus_effect#In_flying_machines

[Mikemv]

In a IHST, USHST work shop in August at Flight Safety, Tim Tucker and I were talking about mistakes and other stuff in the new handbooks. His take was "Magnus Effect", WTF?

[crashed_05]

Its in the Rotorcraft Flying Handbook, but it has been left out of the newer Helicopter Flying Handbook. I guess they figured we didn't need to know about it.

I remember I did have to teach it on my CFI checkride, though.

[troll.detector]

I think they were trying to relate faster moving air over a curved surface = lower pressure and therefore = lift. Awkward and unnecessary step in learning about lift on an airfoil in my opinion and glad it is gone from Helicopter flying handbook. Cool ship though!

[pilot#476398]

Its in the Rotorcraft Flying Handbook, but it has been left out of the newer Helicopter Flying Handbook. I guess they figured we didn't need to know about it.

I remember I did have to teach it on my CFI checkride, though.

 

Yeah, serves me right for still using the old one for reviewing! I just checked the new one (I have it, but I don't look at it much because its on my computer which is a bitch to use for reading!) and its funny that even though its not there, there is a picture of the Magnus Effect on the title page!

 

I guess the new kids won't know what the hell we're even talking about here?

[Goldy]

I guess the new kids won't know what the hell we're even talking about here?

Who's Magnus?

[Rupert]

Magnus effect uses rotor wash to create low pressure on one side of the tail boom and thus reduce amount of tail rotor thrust required.

[pilot#476398]

Magnus effect uses rotor wash to create low pressure on one side of the tail boom and thus reduce amount of tail rotor thrust required.

 

What book is that from?

[Eric Hunt]

For those people stuck in the Bernouilli frame of mind, the Magnus effect is hard to understand. But it is the real reason why an airfoil behaves the way it does - you put a rotating vortex into a moving free stream, and the free stream gets deflected around the vortex. Deflected air = (in Newtonian references) acceleration = force = lift.

 

An airfoil has a vortex rotating around it - look at the streamlines and see that when there is no angle of incidence for a symmetrical airfoil, there is one streamline attached to the leading edge, and one to the trailing edge. The rest divide equally top and bottom, there is no net force generated.

 

But add an angle of attack, and things change - the line which was attached to the leading edge moves under the nose and attaches there, and the trailing line also moves forward and under to attach itself at the back. So now the streamlines are spread out - less on the top, more underneath. Airflow just above the leading edge line will approach the airfoil from slightly below, then actually move forward and up a little to get around the nose, before it gallops down the back, and does a tricky thing by again moving forward to near the attachment point, before sliding away behind the airfoil and moving downwards.

 

The overall effect is a net deflection of air downwards, giving a net force upwards to the airfoil.

 

The vortex is a closed system, like a big rectangle based on the wing - as soon as you move, you leave a Shed Vortex in the air, and you drag the rest with you - planes are continually shedding their wingtip voices, and so are helicopter blades. The "Bound Vortex" stays on the airfoil, and is finally shed when the airflow stops. It can only exist as a closed system.

 

Bernouilli hadn't discovered Vorticity at the time. And our academy didn't introduce it to us until the post-graduate year of aerodynamic studies - it is a bit above what your average Joe needs to know to fly. But it works.

 

The Notar uses it on their tail boom, the strakes down the boom of newer Hueys and Eurocopter machines also use it to get some free help from the wasted downwash.

 

You can do it yourself with a balloon - try to bat it in some direction - it wobbles around and does very little. Now bat it with a rapid backwards rotation - it will move forward and climb until the spin or the forward movement stops, then wobble to the ground.

Edited October 19, 2013 by Eric Hunt

[troll.detector]

Magnus effect uses rotor wash to create low pressure on one side of the tail boom and thus reduce amount of tail rotor thrust required.

isn't that Coanda effect not magnus? I thought magnus required a cylinder to rotate and the tail boom does not rotate.

[rotornut67]

isn't that Coanda effect not magnus? I thought magnus required a cylinder to rotate and the tail boom does not rotate.

 

You are quite right sir...

[iChris]

I was doing some reviewing recently and I came across the Magnus Effect.

 

It talks about airflow about a rotating cylinder, but since the blades aren't spinning around the feathering axis, what are we supposed to do with this explanation of lift?

 

The story is laid out in the first paragraph of Eric Hunt’s post above. You have to start with the true origin of lift. All that is necessary to create lift is to turn a flow of air. The airfoil of a wing turns a flow, and so does a rotating cylinder, the spinning ball also turns a flow and generates an aerodynamic lift force.

 

It’s based on Newton’s observation that for every action there is an equal and opposite reaction. For the hovering helicopter, the action is the production of an upward rotor thrust and the opposite reaction is found in the downward velocity imparted to the air in the rotor wake.

 

Newton’s relationship says that a force is equal to acceleration times mass (ƒ= a x m). Therefore, all that is necessary to create lift is to accelerate (a) a given mass of air (m) in the opposite direction.

 

In physics the term acceleration means a change in velocity. Velocity is given as a vector quantity having both direction as well as magnitude, in this case, the speed and direction of the airflow.

 

Magnus’ observation holds that a spinning ball or cylinder, like an airfoil, can also turn or deflect (accelerate) airflow and produce an aerodynamic force. The Coanda observation holds that a fluid flow has the tendency to be attracted to a nearby surface and follow its curvature. Here again we can design a curved object and place it in an airflow to turn or deflect (accelerate) the airflow and produce an aerodynamic force.

 

LINK:

 

So the common factor in all is shown in the figures below. The spinning ball, airfoil, and the tail of the NOTAR all turn or deflect (accelerate) airflow to produce the desired aerodynamic force. In addition, changes in flow velocity and pressures across the surface of the airfoil or object are byproducts accompanying the airflows deflection.

 

A word on the Bernouilli’s theorem:

 

“The reader is cautioned against explanations of the origin of lift based on Bernouilli’s theorem. Bernouilli made it quite clear that his theorem relates to conservation of energy in flowing air such that the sum of the static and dynamic pressures remains constant. Bernouilli’s theorem only applies if no energy is put into the air. However, a wing or a rotor blade is an actuator that is exchanging energy with the air. This is clear from the presence of induced drag. When energy is put into the airflow, Bernouilli’s theorem simply doesn’t apply and the explanations based on it are flawed” and should be disregarded.

 

The Art of the Helicopter, John Watkinson, pg. 65

 

Also see prior post:

Helicopter Lift Theory - Equal Transit, Skipping Stone & 1/2 a Venturi - all wrong...

 

 

 

 

 

http://youtu.be/23f1jvGUWJs

 

 

 

 

Edited October 21, 2013 by iChris