Gravity and the loop

[Hal Jordan]

This is one of the places I trained as a flight test engineer:

www. etps. qinetiq. com/Pages/default.aspx

Hey! I've worked with some of the good folks at qinetiq! Absolutely brilliant bunch of guys! :worthy

[gordonjudd]

The Gatti-Bono and Perkins analysis are using time tested theories derived from the derivations and practical experiments of Robins, Magnus, Euler, Navier and Stokes. They make perfect sense to me.

Vince,
I am glad we now agree that Dr. Gatti-Bono has provided the correct way to compute the lift for different loop shapes. Does that mean you have changed your mind about your claim that:

Vertical drag only exists when there is a rotational component of velocity. That is the Magnus effect.

An inclined piece of fly line is not spinning so there is no Magnus effect. There will be a vertical component for both the tangential and form drag as Dr. Gatti-Bono derives in her paper, and just like like putting your hand out of car window those drag forces will provide a vertical component of lift.

Whichever way you cook it, your flat plate drag axis is in the wrong place.

Can you give a free-body diagram to show what that might mean? I have no idea of why the choice of axis would impact if there was a vertical drag induced force on an inclined piece of fly line or when the stream from a fire hose is directed horizontally at an inclined steel plate.

Your last 2 quotes appear to be mutually exclusive, do you believe in drag or rotation?

As noted earlier the forces that are producing the dolphin nose loop have nothing to do with aerodynamic drag. Thus those two statements are not mutually exclusive since we have an apples and oranges situation on what causes the DN loop shape, and what produces lift in an inclined piece of fly line.

As noted before I think Alejandro has explained that drag forces are not the cause of the DN loop shape. His experiments where he produced the DN loop shape in different ways with a length of bead chain showed me that I was wrong about attributing it to drag.

As noted before there is rotation of the line mass as it goes around the loop, and that will impact the tangential drag on the line as it goes around the loop. As Dr. Gattio-Bono has shown that tangential drag will have a small lift component. Rotation is different than spinning, however, so even though the line is rotating around the loop there is no Magnus affect to consider as you speculated.

As Dr. Gatti-Bono has computed the form drag on an inclined piece of fly line produces most of the lift force in the loop. That calculation just assumes the inclined line section stays at a fixed angle as it propagates horizontally. Therefore there is no rotation component for the form drag that is providing the majority of the lift in a climbing loop shape.

If you think the lift can only come from the Magnus effect, then how would you use it to compute the lift on an inclined piece of fly line?
and
I did not say that.

That is a relief, but if that is the case what did you mean by your observation that?

Vertical drag only exists when there is a rotational component of velocity. That is the Magnus effect.

"Only exists" is a pretty strong characterization, especially when it is flat out wrong.

Gordy

[VGB]

Hal

I don't think our paths have crossed yet but its a small world. I no longer work for them but do routinely work with them.


Gordy

I have just returned from some flight test activity abroad to find your post. I do not have the time for playing silly games to see who can wee furthest up the wall but lets look at your previous mutterings:

I am glad we now agree that Dr. Gatti-Bono has provided the correct way to compute the lift for different loop shapes.

I don't understand how you could think that. Anyone who has put their hand out of a car window knows there is lift from form drag when you angle your hand up up or down.

Drag is what causes lift, but I was referring to factors that impact whether or not you see the DN shape in a propagating loop.

If you understood the discipline, you would be aware that aerodynamic drag is the restraining force that acts on any moving body in the direction of the freestream flow. This sets the axes so that lift is the perpendicular axis to the flowstream. The airflow around the object results in an aerodynamic force vector that is resolved in terms of lift and drag axes.

It appears as usual that when you have something wrong you attack and twist the message and judging from your response, the penny is beginning to drop. As well as getting the lift/drag axis wrong in your chart here:

http://www.sexyloops.co.uk/cgi-bin....y243521

You are also getting the point at which the aerodynamic force acts upon the object wrong. On a flat plate it is through the aerodynamic centre at about the 25% chord point:

http://wright.nasa.gov/airplane/ac.html

I have no idea of why the choice of axis would impact if there was a vertical drag induced force on an inclined piece of fly line or when the stream from a fire hose is directed horizontally at an inclined steel plate.

I have given you this before:

http://www.grc.nasa.gov/WWW/k-12/airplane/kiteincl.html

Take particular note of this extract:

The aerodynamic force on a kite is a vector quantity having both a magnitude and a direction. The aerodynamic force is resolved into the lift which acts perpendicular to the wind direction and the drag which acts along the wind direction.

Perhaps you did not understand the firehose explanation but be careful how you use it:

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

At the moment you are not in a position to even making pronouncements about more advanced concepts such as the Magnus effect. Additionally, if you had a better understanding you may have noticed that the Gatti Bono paper is based upon a static not a dynamic loop. As I mentioned in my original post, you need to be able to model the dynamic loop using specialist Computational Fluid Dynamics to understand what is going on in the boundary layer and where the aerodynamic force is acting. Even then you would need wind tunnel data for validation because the base data is unvalidated. The Gatti-Bono paper is fine for what it is trying to achieve but is incomplete because the level of complexity of the problem rises rapidly once you add rotational forces.

As noted before I think Alejandro has explained that drag forces are not the cause of the DN loop shape. His experiments where he produced the DN loop shape in different ways with a length of bead chain showed me that I was wrong about attributing it to drag.

You need to re-read what Alejandro said:

Similar phenomena to DN can appear for diverse causes, as small changes in the way of the impulse, or for aerodynamic drag in some circumstances. But the experiment shows that the typical DN manifests itself more clearly the smaller is the loop, so the force that creates it appears to be related to its diameter.

His discussion on high rotation, high speed casts is excellent but I was discussing low rotation, low speed casts with James. You have not offered any comment on this subject. Although you did offer the following.

Rotation is different than spinning, however, so even though the line is rotating around the loop there is no Magnus affect to consider as you speculated.

You have not listened to when I gave you the simple swimming explanation from here:

http://ffden-2.phys.uaf.edu/211.fal....ft.html

this is the piece you needed to understand when I talked about bound vortices and Reynolds numbers:

Another way a swimmer produces lift is explained by the Magnus effect. The Magnus effect is how a fluid flows around a rotating cylinder, the forearm for example. The fluid flowing past the forearm appears as if it is rotating around the arm, this is called a bound vortex. Moving the arm with the vortex through moving water decreases the pressure above the arm and increases the pressure below the arm, Bernoulli's principle. In this way the arm produces lift in a similar way as the hand.

Have you ever heard of propellors, knock yourself out:

http://ntrs.nasa.gov/archive....976.pdf

I finally got to the end of the drag induced lift thread and see why so many fine minds no longer visit this section.
In case you have not noticed, I have also lost the will to live to discuss this with you and will not re-visit the board.

Vince