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Old Jan 04, 2011, 12:13 AM
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Just thought I'd share this list of videos, all of which utilize a kf variant.

http://cmreel.com/?page_id=82

- Jeff
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Old Jan 04, 2011, 07:46 AM
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here is some food for thought...It may be crazy but has anyone fully KF'd a prop..after all a prop is an airfoil, and it should be easier to measure the effects of the step..ie revs Amps thrust...
I know a small tip step was tried with some positive results...
Ive ebeen thinking to try layers of sticky tape to build a KFm4 step..
daft or what?
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Old Jan 04, 2011, 08:42 AM
electron fly'r
United States, AZ, Green Valley
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Not daft just do it. What are the results?
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Old Jan 04, 2011, 09:18 AM
fix-it-up chappie
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Quote:
Originally Posted by davereap View Post
here is some food for thought...It may be crazy but has anyone fully KF'd a prop..after all a prop is an airfoil, and it should be easier to measure the effects of the step..ie revs Amps thrust...
I know a small tip step was tried with some positive results...
Ive ebeen thinking to try layers of sticky tape to build a KFm4 step..
daft or what?
I would think you need two matched props to start with. Adding tape means you are adding weight, so my first question would be how will you balance the KF prop so it weighs the same as the untreated prop? Perhaps both a bit of tape, and a bit of material removed? Maybe instead of tape one could use thin layers of wood, glued down, and sanded into shape. That will give you a smoother leading edge.

Just changing one prop will be fun, but not necessarily objective. That's not to stop you, only to point out that any data which is not objective and repeatable, will not past muster with the nay-sayers. Even though I am a believer in KF airfoils, I do agree that exceptional claims require exceptional proof.

The best bet would be to cut two props on a CNC machine. That way they could both start with known airfoils. But then you need to model them first in the proper software, and find someone with a CNC machine to "print" them out. I actually know someone with a CNC machine, but I don't know the first thing about 3D software.
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Old Jan 04, 2011, 11:43 AM
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yes I was thinking you will need to add tape on a second prop so as to have two with at least identical weight. I was thinking of a KFm4 or KFm2 then the few layers to make the step could be smoothed with a wrap around top layer that goes round the leading edge.. the prop for comparison will need complete covering to make up the mass..Usually a heavier prop will take more power to spin so we will see what happens
I need to get a scientific approach going here, but it has to be easy done as well..
wooden props and laminations are another way to go .... CNC well I dont have that sort of experience...
Will a KFm2 produce more thrust as it produces more lift?

lots of thoughts and no answers so far...feel free to play
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Old Jan 04, 2011, 01:00 PM
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Thoughts on modifying & testing props

Dave & friends,

I've always been intrigued by the reports of KF'ed prop tips increasing thrust. I'll be very interested in any results.

You may want to investigate further on the subject of super low Reynolds numbers aerodynamics; in my experience, the shaping of the leading edge of a prop is super-critical to it's performance... making it fatter & blunter will reduce it's performance, so adding tape layers- especially to the front half of the prop blade front surface- may change the game entirely... it changes the L.E. radius, the camber line, & likely will increase the drag.

(As an example, I've bought modest-priced folding props from BPHobbies which have fatter, more poorly shaped leading edges than the Fruedenthaller Aeronaut folding blades which I prefer to use... the amount of thrust which they produce for a certain wattage of input power is far less... (just goes to show that sometimes a low priced 'deal' isn't much of a deal at all...) So I use the mid-parts, but replace the blades on those folding props.

The forward 25% of the prop blade's shaping is hyper-critical in relation to the amount of thrust which a prop can generate with minimum drag.... very similar to wings in some ways, but since props operate at such low Reynolds numbers, it's even far touchier.

To do it with tape, you might first compare the performance of a 100% tape layer covered prop with an identical 'virgin' unmodified prop to get an idea of the affects on thrust / drag / power consumption.

I'd advocate removing the the step material, rather than adding anything to the prop. If you look at the best of the APC propeller blades as a benchmark to start from, then figure out how to make a jig so that you can use a Dremel router to cut in a KF type step at 50% of chord for the tip ~15% of each blade, and leave the front 50% of the blade's shape undisturbed, you should have a very good experiment. (If later you want to add a secondary step at 70% or 75% of chord, you certainly could... but leaving the forward 50% of the blade un-touched would be my recommendation as one starting guideline.)

From there, suspending a motor with prop from a mount that hangs from the ceiling with a digital fish scale, and plugging in a watt meter into the motor wiring will give a readout of ounces of thrust for watts of power consumed. If you see an increase in thrust (ounces) at the same or less power consumption (watts) when starting each run with the identical peaked battery pack, then you know that you're on the right track with your KF type stepped discontinuity carving.

Forget all of the cheap props that can't match the APC in thrust produced for input power consumed... if the alternative un-modified prop can meet the APC benchmark in a head-to head test run- (same diameter & pitch, thrust produced for power consumed), let everyone know about it!

[But improving the performance of a bad prop really wouldn't prove as much as if you can come up with results showing improvement of the performance of the best props out there... and from the testing I used to do, the APC prop aerodynamics engineering is the benchmark I'd start from.]

In my earlier experimentation with electric flight back in the mid and late 1980s, I used this type of test setup to test a lot of different props to find which one would best fly a given motor / battery / aircraft setup most efficiently. [The NiCad batteries we were working with at that time really motivated us to get the best use of what power capacity we could carry... ]

If someone has the time and inclination to get involved in this type of experimentation, it's really interesting how much can be learned from a relatively inexpensive test setup. Getting the KF steps carved away cleanly and consistently is the challenging part of this process, so coming up with a serviceable prop blade holding fixture & router guide or CNC setup seems like the first major challenge to end up with repeatable consistent results.

After completing the static tests, the Eagle Tree data logger's capabilities should tell the rest of the story as the prop unloads in flight.

VIKING
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Old Jan 05, 2011, 05:30 AM
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all good points there...thanks
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Old Jan 09, 2011, 02:26 PM
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Ive not had a go at the props yet.

But here is a Super UFO 26" span KFm4... flying the flag... 14oz without the battery... so 18oz with a 1300 3 cell and 4 sqft area..4.5oz sqft loading..the motor gives 26oz thrust

These circular designs are great....highly recommended..this a much modified design based on the UFO by GPW
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Old Jan 10, 2011, 08:42 AM
gpw
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There's No Free Lunch !!! Seems if we increase the efficiency of the prop ,viz. more lift, then we up the amp draw and the drag due to lift ... or is that faulty logic ... logic does allow us to be Wrong , in a logical manner...
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Old Jan 10, 2011, 10:24 AM
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More thoughts on props, & a couple of examples

Friends

It may be possible to improve a prop blade's shape to increase the thrust without increasing the watts of input power consumed... and it may be possible to modify a prop blade's shape to reduce it's drag while still producing the same (or higher?) thrust with the same amount of input power (or less?)

The earlier prop blade modifications were done on a wooden prop, since it's easy to cut, shape, and sand the rock maple. But it also should be kept in mind that those relatively thick square blade tips really did have room for improvement in the first place.

Two examples from my experiences:

[1]: Many years ago, I had a flying buddy who put floats on a sport aircraft (.60 glow power) and we went to a float-fly for his first flights. He had a classic black Master Airscrew prop on it, probably a 12x6, which he had used for sport flying before installing the floats. When he tried to take off from the water, it couldn't do it successfully. We then swapped his prop out, replacing it with an APC prop of the same diameter and pitch. Result? His plane now had much more thrust, & could get up 'on step' and lift off the water nicely. Same prop diameter & pitch, different blade shape, same power available = noticable difference in generated thrust.

[2]: In setting up my 1/6 scale J3 Cub which was scratch-built with an Astreo-Flight 25SF motor in the nose, I tested props. The classic square tipped prop blade style did poorly in the tests of ounces of thrust for watts of input power. Curiously, however, the Master Airscrew brown plastic "Oldtimer" style prop was one of the best ones I tested at that time (in about 1985-1986?) The different shape of the blades made a very significant difference - more thrust, less drag, = more efficiency.

(I just looked - 25 years later, I still have the same prop on that 1/6 scale Cub!)

VIKING
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Old Jan 11, 2011, 06:50 AM
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here are the lines for the UFO..
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Old Jan 11, 2011, 02:48 PM
just Some Useless Geek
Chicagoland
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Bruce, is it not true that a prop is a wing turned sideways? If so, then what's good for the goose, right? Would it not follow that the effects we are seeing with KF discontinuities in wings would have similar effects in props? How can we measure these effects without raw speed/power/thrust data gained from those measurements? Is there some other science at work here that we should be aware of?
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Old Jan 11, 2011, 03:55 PM
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Geek,

Yes, A prop is a device which deflects & pushes air (back / down, as needed in a given application) thereby producing what we refer to as thrust (simply another form of 'lift'.) Even an angled flat plate when rotated produces lift / thrust... but there are far more efficient shapes for doing this.

The test & measurement methodology which I outlined in post 1761 above is the best way I know of to do tests of props on the typical foamie builder's budget. Starting with a high performance benchmark (a well-shaped prop), & working to produce improvements on that was the heart of my proposal.

(There are still a lot of poorer-performing props out there, and of course they can be improved in several ways without a lot of effort... but those are not a realistic starting point, in my perspective.)

VIKING
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Old Jan 11, 2011, 05:28 PM
Arizona Rim Country
Joined Nov 2001
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KF prop consideration

Quote:
Originally Posted by A Useless Geek View Post
Bruce, is it not true that a prop is a wing turned sideways? If so, then what's good for the goose, right? Would it not follow that the effects we are seeing with KF discontinuities in wings would have similar effects in props? How can we measure these effects without raw speed/power/thrust data gained from those measurements? Is there some other science at work here that we should be aware of?

I thought I read that KF is best suited to relatively slow flight applications. Slow flight, high lift, high drag. Now I know that there have been some speedy and nicely streamlined planes that are a marvel to be sure, but in comparison to the speed and drag of a prop . . .

The whole KF mystery continues to unfold
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Old Jan 11, 2011, 05:34 PM
Onward through the fog.
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Bohol Philippines
Joined Aug 2008
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First day it's not rainining in 2.5 Weeks.

And we have to go to Tagbilaran on business. The RC Gods are not happy with me for some reason.

I flew the cobbled together Taube during a lull in the rain. Just managed one flight here on my beach but it's fast due to the weight. It flies in a manageable way but won't slow down much for the landing.
I have the Firefly, and another Delta of my own design to maiden when time and weather permit and I"m working on a design that is based on the AP-12 with some liberties taken to accommodate my building methods and the very forward CG. (See Pic.) I need to maiden the two deltas which have a different untried leading edge to see if it will work on the Samba.

Dave,
You just can't stay away from those round things can you?


Geek,
Some early props were rotating paddles but Orville and Wilbur gave us the twisted airfoil shape we use today. Or at least they used and popularized it.

I lost 3 BW motors last month so I'm due to put in an order from HK pretty soon. Need to get the car situation sorted out first. Maybe I'll research rewinding and give that a try.

Happy Flying all,
Steve.
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