Hi there again Eric,
I dunno about building a few winglet prototypes.
Like my past idea for making lead acid batteries, I was throwing the
idea out so that others can work on it, not me!
I am a talker rather than doer, but eventually get things done if I live
long enough, and the wife can back off on loading me up with useless
jobs like mowing the lawn.
Seems like any gains with the winglets might be minimal, if at all.
Not to throw away the idea, but there are probably things that can be
done that reap greater effect. There might be a good reason why winglets
are not used on airplane props, and sadly, it might be as simple as
nobody thinking of it, or if thought of, being rejected out of hand by a
supervisor with no vision or risk tolerance. After all, from what you
say, winglets were known about for years before they became common.
Your Kort Nozzle comment caught my eye.
My understanding, based on reading it somewhere, was that propulsion via
a ducted waterflow was far less efficient that a prop. This was relative
to jet-ski's, I believe, so it might very well be different due to the
pump system.
Now, I can get well enthused about the ducted prop idea. I noticed long
ago that jet engines have static turbine blades that prevent the column
of gases from rotating. Now, apply this to a prop in water, and it is
obvious that part of the rotating energy, due to prop pitch, is spent on
turning the water, rather than thrusting it straight backwards.
Ideas lead to ideas, I guess.
I have also had the passing thought that seeing as we are trying to
create thrust, and by nature are pressurising the water behind the prop,
that the water, being incompressible, might well be spraying out in all
directions, thus wasting power. The prop slippage energy is going
somewhere. So, it is possibly the case that the flow behind a prop
creates a funnel shape of moving water, with the prop at the narrow end.
So, if that splayed out water was 'crushed' back in line with the prop,
it would add to the flow already there, I think. One wonders ('One'
being in this case 'I') if the blades of a prop could be curved inwards
at the tips, to direct the flow inwards towards the propshaft
centreline, but behind the shaft. There would still be water moving
outwards though.
So, has anyone tried ducting a prop using thin sheet stainless for low
wetted drag, and together with the duct, having static blades (that
support the duct tube?) to lessen the column of water from rotating?
Maybe Kort has! I should have looked that up before posting.
This has probably never been looked at by ICE designers, as it is easier
and cheaper to just increase the engine power.
I have just taken a quick look at the Kort idea, and the drag of the
duct might well be a factor, due to the airfoil shape of the duct
itself. As most electric applications are probably for sailboats, the
penalty of drag increase is greater, unless it can be lifted clear when
not in use.
There is one thing that I can and will test though, unless someone beats
me to it, and that is a straight, thin parallel metal cylinder duct to
contain the 'funnel' of water. The drag would be virtually zero, and it
somehow strikes me that a gain in thrust would be almost certain to
occur.
I haven't checked, but I suspect, guessing, that if you look at the
aftermarket outboard ducts, you will find that they extend in front of
the prop as well as behind it. This would cancel out the 'good' effect,
by restricting the inflow.
I would try having small static vanes in front, and the duct with static
vanes behind. Another test would be to have the rear static vanes very
slightly curved, as, if the water column is rotating, straight vanes may
sap away energy by trying to straighten the flow out too much. The trick
would be to have them curved less than the natural curve of the water.
It might be too small to consider, especially as the flow increases.
Also, in the final design, for least drag, the duct would have to be in
line with the boat motion, rather than the (angled) prop shaft.
John
3b. Re: Propeller Idea
Posted by: "Eric" ewdysar@yahoo.com ewdysar
Date: Mon Aug 22, 2011 9:38 am ((PDT))
Hi John,
Those "little vertical fins" are called winglets. Here's a quick
history on the concept.
The initial concept dates back to 1897, when English engineer Frederick
W. Lanchester patented wing end-plates as a method for controlling
wingtip vortices. In 1905, the Wright brothers used a concept they
called "blinkers" on the canard of their Flyer III and later, on their
Wright Model A aircraft. In 1910 they installed "side curtains" and
modified blinkers on their production Wright Model B aircraft, to
improve its stability. In the United States Scottish born engineer
William E. Somerville patented the first functional winglets in 1910.
Somerville installed the devices on his early biplane and monoplane
designs.
Dr. Sighard Hoerner was a pioneer in the field, having written a
technical paper published in 1952 that called for drooped wingtips whose
pointed rear tips focused the resulting wingtip vortex away from the
upper wing surface. Drooped wingtips are often called "Hoerner tips" in
his honor. Gliders and light aircraft have made use of Hoerner tips for
many years.
Hoerner's concept was further developed by Richard T. Whitcomb, an
engineer at NASA's Langley Research Center, in response to the sharp
increase in the cost of fuel after the 1973 oil crisis. Whitcomb's
designs were flight-tested in 1979–80 by a joint NASA/Air Force team,
using a KC-135 Stratotanker based at the Dryden Flight Research Center.
Even before NASA did flight testing on winglets, Burt Rutan incorporated
them in his innovative Rutan VariEze homebuilt aircraft design, which
made its first flight with winglets on May 21, 1975. The VariEze
pioneered glass-reinforced plastic composite construction in homebuilt
aircraft, which simplified fabrication of the winglets. He reduced the
resulting drag penalty by assigning double duty to the winglets; they
also serve as vertical stabilizers and rudders in his canard, pusher
configuration aircraft....
I wanted to mention Rutan's contributions. During the 1970's, my father
and I spent a reasonable amount of time in Burt's shop in Mojave as he
was developing many of his "revolutionary" ideas. So, as a teenager, I
was privy to numerous engineering discussions while he vetted various
concepts with aeronautical engineers and rocket scientists like my
father and others. So I've known why and how winglets work for 35
years, and I was pretty jazzed when Boeing added them to their airliners
almost ten years later.
Here's a brief description of what winglets actually do.
The upward angle (or cant) of the winglet, its inward or outward angle
(or toe), as well as its size and shape are critical for correct
performance and are unique in each application. The wingtip vortex,
which rotates around from below the wing, strikes the cambered surface
of the winglet, generating a force that angles inward and slightly
forward, analogous to a sailboat sailing close hauled. The winglet
converts some of the otherwise-wasted energy in the wingtip vortex to an
apparent thrust.
So, from what I know about props, they don't generate a significant tip
vortex. Notice that we don't see winglets on aircraft propellers, wind
turbines or similar devices that have had billions of dollars of
research thrown at them to make them more efficient.
What we do know is that props can be more efficient if they are put
inside a duct (like ducted fans). With very tight tolerances, the duct
kind of acts like a stationary endplate for the propeller blades. So
one would believe that ducting a boat propeller might increase the
efficiency of the drive. That brings us to Kort nozzles.
The Kort nozzle is a shrouded, ducted propeller assembly for marine
propulsion. The hydrodynamic design of the shroud, which is shaped like
a foil, offers advantages for certain conditions over bare propellers.
Kort nozzles or ducted propellers can be significantly more efficient
than unducted propellers at low speeds, producing greater thrust in a
smaller package. Tugboats are the most common application for Kort
nozzles as highly loaded propellers on slow moving vessels benefit the
most. The additional shrouding adds drag, however, and Kort nozzles lose
their advantage over propellers at about ten knots (18.5 km/h)
There are number of after-market ducts that can be added to the drive
legs of outboard motors, but they haven't really delivered on the
theoretical improvments on boats like ours.
Now that you have a little more background on your general concept, you
can start making a few prototypes and collecting some data on whether or
not the concept actually helps in the scale that we're dealing with.
Even though I don't think that this will produce a siginficant
breakthrough, I'm personally interested to see how well your prototypes
will work.
Keep us posted.
Eric
Marina del Rey, CA
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