Thrust factors

[Smokin' Joe]

I tried to recollect coherent thoughts while going to the grocery (I taught grad classes in hydrodynamics a few times). Here's a summary in a nutshell. You start with D'Alembert's Paradox, that in the flow of an 'ideal fluid' (viscosity=0) past an object there is no net force exerted on the object (the sum of all local forces over the entire body adds exactly to zero). There's no drag, no lift, no nothing. The object is not only not dragged downstream, it isn't even wet (first layer of fluid slides past without sticking). Next comes the fact that drag (the main fluid force, the one that's always present) is due to the (real, viscous) fluid having been decelerated in layers flowing near the object (think of a fixed object, flow runs past it, the first layer of fluid sticks to the object, this is how the object gets wet and experiences a net force). That fluid forms the wake, and mathematically is represented by vorticity. Vorticity and circulation are related but there's no circulation here. A plate at an angle to such a flow experiences a small side force but mostly drag, the lift/drag ratio is low. Now, circulation (related to vorticity) is approximately conserved if the viscosity is not too large (as in air, water). When a plane runs down the runway, the wake has the form of a vortex sheet (a velocity discontinuity, with speed higher on top side of sheet). But the sheet is unstable and eventually rolls up into a vortex/eddy (with finite circulation about it). Since circulation is nearly conserved, a vortex/eddy of opposite sign has formed about the wing. When the trailing vortex splits off then the wing lifts because an eddy in a flow experiences a side force. Prandtl explained this all in a short paper ca. 1916.

It's the circulation about the wing/foil that produces the high lift/drag ratio necessary for flight (or for a prop or sail). Otherwise, you have a small lift/drag ratio, as with a flat plate in a flow with no circulation about it.

Theoretical physics can't be avoided if you want to understand props.... .

Yes, "stall" comes in at the extreme of excessive angle and low speed, I was referring to the other end of the curve.


All wings and props meet a stall limit somewhere, even yours.

[Smokin' Joe]

You can think of the sail as a cambered sheet with wind moving fast over the convex side, and with 'dead air' (a big trapped eddy at some Reynolds nr.) inside the concave side. So, taking the boundary layer into account, the net profile looks more like an airfoil of finite thickness, thickest near the middle of the sail. Edgar Rose would surely enjoy this discussion, given that physics has entered .... .

Agreed on what Gerr's book is about and I was only offering it to provide some basic info for prop theory and agreed that surfacing props are difficult to understand and predict performance. But I have found my rather simple relations have proved to be very useful in setting up my hydro.

As far as the lift/drag theory of wings, props and sails by nonintuitive I would add that for sails there is another theory out there. Since a sail is a thin cambered sheet when properly set in the wind there is no relative difference in cambered shape of the front and the back face like there is with a wing and prop blade. Therefore the usual wing theory of lift by pressure difference from the lower pressure on the greater curved side than the flatter does not totally apply to a sail. A more reasonable explaination results from the sail simply changes the direction of the air from the leading edge to trailing edge that passes around it and that deflection constitutes a force from Newton's F=ma. The angle that the air enters the sail is different than when it leaves at the trailing edge thus resulting in sail force. Therefore for sails the greater the deflection the greater the force (within limits of course). No doubt that shedding eddies has some contribution as well as other items. If interested here is a fairly good explaination of the theory:

Prelude
http://www.sailtheory.com/wrongtheory.html

Theory
http://www.sailtheory.com/sail.html

[kenboyd]

Yes, "stall" comes in at the extreme of excessive angle and low speed, I was referring to the other end of the curve.


All wings and props meet a stall limit somewhere, even yours.

I would like to add that angle of attack, is the main factor in producing stall. A stall can occur at any speed, if the angle of attack is great enough to cause the fluid medium to separate from the foil.

[Smokin' Joe]

Correct. A good picture book of flow patterns is S. Goldstein's "Modern Developents in Fluid Dynamics", vol. 1, boundary layer separation is shown there. There's a real picture book of flow patterns but I can't recall either the author or title.

I would like to add that angle of attack, is the main factor in producing stall. A stall can occur at any speed, if the angle of attack is great enough to cause the fluid medium to separate from the foil.

[Smokin' Joe]

When testing race boats, motors, props, I kept written records of everything. Otherwise I couldn't remember from one time to the next what I'd done and what was the result. I made some good props, and some good boat bottom modifications. I would not have found it fun to buzz around on the lake without improving performance. Unfortunately, propeller theory is nearly non-existent, nearly everything we know comes from trial and error. I would still like to understand how a surfacing prop works (I mean REALLY works).

After you finish working all the numbers you may have to test, if for no other reason but to find out you wasted a lot of time you could have spent on the water having fun.

[nonslip tip]

Robert Kress has done some good papers on surface or super cavitating propellers. Also to add to all that has been said,look up patent #6352408, and yes it does work. Airplne tips , props all work the same. Alway so props with these tips have been run through the Navy's David Taylor model basin and a folding prop for sailboats has been truogh the test tank in Berlin.

[Mark75H]

Wouldn't the added weight on the tip limit this kind of prop to relatively low rpm use?

[Smokin' Joe]

I'm always bothered by the term 'supercavitating' and the assumption that a cleaver with a thick trailing edge is necessary. In Mod VP preparing for Havasu in 1981 we found that choppers and other round eared props with normal trailing edge were better. I doubt that a cavity of any size can form over the low pressure side of an efficient (i.e., one that does the job) suction side of a blade. A cavity would represent an effectively very thick blade and more drag. Maybe there are just some air bubbles, underwater photos that 'stop' the blades would be necessary. One thing is fairly obvious: a normal submerged prop has a triple helical wake. A surfacing 3-blade prop with one blade entering as the other leaves has less than 1/3 of that wake, so less drag.
Clear that a prop with 4 or more blades will create more drag, ceteris paribus.

I haven't understood the bent upward tip on a wing, haven't done my homework. Can imagine that the purpose is to make the trailing vortex sheet (wake) smaller, drag reduction. Not clear why that would apply to a helical vortex sheet ... .

Wouldn't the added weight on the tip limit this kind of prop to relatively low rpm use?

[Smokin' Joe]

Can you give me the references, or email them in pdf form? Am reading
Gerr's nice book and haven't read prop papers since ca. 1980. Do you have the paper by Railton & Kamen on surfacing props? I'm landlocked in an alpine village til Aug. Where's a photo of the bent-tip boat prop?

Thanks,
jmccauley@uh.edu

Robert Kress has done some good papers on surface or super cavitating propellers. Also to add to all that has been said,look up patent #6352408, and yes it does work. Airplne tips , props all work the same. Alway so props with these tips have been run through the Navy's David Taylor model basin and a folding prop for sailboats has been truogh the test tank in Berlin.

[Mark75H]

Any idea why racers are having success with 4, 5 and 6 blade props?