Thrust factors

[Smokin' Joe]

Sorry too, I had to try (with all books and papers in Houston) to remember some of what I knew 30 years ago. The purpose of a bent-upward tip on an airfoil is to try to constrain the flow over the foil to 2D (2 dimensions). Here's what's going on.

Circluation about a foil (wing, blade, sail) is like a vortex/eddy about the foil (Ludwig Prandtl, ca. 1916). A vortex cannot end in the fluid (conservation of circulation), it can only end at a boundary or interface (or close on itself like a smoke ring). One end of the vortex ends on the body of the plane (hub of the prop). so the vortex extends way beyond the wing/blade and must end someplace. With the plane, the other end of the tip vortex ends on the ground where it started. A tip vortex coming off a prop blade ends on the surface of the water, where it started. Vortices are unstable against viscosity, so they diffuse away. You can best see them by playing with your paddle while sitting in the boat. The tip vortex is one 3D effect that only adds to drag, but cannot be eliminated (due to circulation conservation in the fluid). Here's a related 3D effect.

An infinitely long wing would have strictly 2D flow over it. A real wing has a velocity component transverse to the wing (radial to the prop blade) and only stretches the vortex over the wing/blade, adding drag but no lift/thrust.
Adding the bent-upward tip forces the radial component of fluid velocity (let's use prop language) to be zero along the bent-upward tip (the fluid velocity vanishes at any solid boundary). This constrains the flow over the suction side of the blade to be 'more like 2D flow', reducing the drag. Of course, the bent-upward tip adds drag in the form of extending the vortex sheet off the trailing edge, just as would adding prop diameter. So a bent tip should work on a prop.

Yesterday (during a break in a jazz festival near the Inn River in NE Austria) I saw a 5-6m dia. variable pitch ship prop, a rusty relic on display. The prop had 'bent-upward leading edges' (welded on) along a chord short of the max. radius. This is the same idea as the airfoil tip (try to make the flow over the suction side more 2D), so the notion's been around for ships for some time.

Sorry it took so long to get back,but had to finish a bunch of props so people can bang up them up this weekend and i can do it again next week.Anyway to answer some questions,no it does'nt add weight by having tips because you have to reduce dia. when you tip because if the increased loading of the prop,other wise if you tip at dia. you will overload the engine because of the increase in efficiency of the prop.How tipped props work is by tipping you eliminate or greatly decrease tip vortices,for as water comes off the tip it creates vortices which draw water off of the suction side and the pressure face,so if you can stop this you create more suction on the suction side which in turn creates more pressure on the pressure side which creates more efficiency. The benefits are more thrust straight back,improved handling,decreased fuel,more overall boat lift, not transom or bow but overall more lift all of which leads to go speed. So in a nutshell you decrease your slip.I do have Krees's papers and the Railton&Kamen papers i will dig them out(Doug Railton used to spend a great deal of talking to my dad Bob Kilian and myself at Pitchometer Propeller years before any papers were written)trkilian@yahoo.com 510-357-3480

[nonslip tip]

My Grandfather and my father started doing tipped props in the late 40's and early 50's.Their are patents on various tipped props going back to the 30's, so the idea is not new,making them work has always been the trick,especially at speed beyond displacement and semi-displacement speeds. I still have some of the patterns left from when they were making them.

[Smokin' Joe]

Tim,

Posted is interesting for the community, I can then download, thanks!

Joe

I do have pictures of all types of props we have tipped,I will get them scanned and post next week,also if you want i can mail you copies.Tim R. Kilian