** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 69 - RC Groups
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 Dec 25, 2012, 08:57 AM Jack The wind tunnel sensors discussion is still going on over on this thread: https://www.rcgroups.com/forums/show....php?t=1788242 A recent post included a link to a search result from the NASA Technical Reports server: https://www.rcgroups.com/forums/show...07&postcount=8 I was not aware of that server or of the potential depth and diversity of the knowledge to be found on that server. I'm sure that all the scientists here would want to know where they can go to find 10 pages listing a hundred or so *.pdf files with details about wind tunnel designs... Jack
Dec 25, 2012, 09:42 AM
Quote:
 Originally Posted by g828 Can you expand on why the air has to move faster on the top? The reason that it has to catch up to the air underneath does not make much sense to me.
The air that goes over the low pressure side (the top for airplanes but the bottom for race cars) does not just keep up with the air that goes over the high pressure side, it goes much faster and leaves the trailing edge first. This is because the air on the low pressure side is accelerated AND the air on the high pressure side is slowed down. If you add up all the velocity vectors around a wing and subtract the free-stream you get the bound vortex going FORWARD on the high pressure side and backward on the low pressure side with a little vector at the leading edge pointing up and one at the trailing edge pointing down.
 how wings work? Smoke streamlines around an airfoil (1 min 54 sec)
Last edited by nmasters; Dec 25, 2012 at 07:59 PM.
 Dec 25, 2012, 11:53 AM Registered User nmasters, Do you know why a bound vortex forms? I have tried researching into that but never found a clear answer. I have also been taught about the bound vortex at the leading edge of the airfoil but never understood why it occurs. In a book I read, it discussed molecules of air impacting the leading edge of the airfoil and then being redirected and so on. That explanation really confused me. Have you heard that explanation before? Thanks
Dec 25, 2012, 09:41 PM
Quote:
 Originally Posted by g828 Do you know why a bound vortex forms?
It's just a velocity distribution. There isn't actually a vortex around a lifting wing. However if the airfoil suddenly disappeared the air would start spinning and the vortex would drift downstream. Likewise, when the plane dives, the wing will shed a vortex from the trailing edge because the lift has reduced and the strength of the bound vortex is proportional to the lift. That shed vortex is the excess energy of the bound vortex being dumped overboard. Dr Drela describes this as well as any book I've seen. Arvel Gentry also describes a simple experiment that you can do in a bathtub to show how the flow will roll up if the wing disappears.

Quote:
 I have also been taught about the bound vortex at the leading edge of the airfoil but never understood why it occurs.
It's not at the leading edge. It's more like a wave traveling along with the wing. Just like a wave in water the molecules do not travel along with the wave rather they move up and down, and back and forth. One small slug of fluid at the center of this wave moves in a circle. As you move farther away from the center the movement becomes elliptical with the major axis of the ellipse being vertical. The farther away from the center you measure this motion the more stretched out the ellipses get . I think the center of the wave is inside the wing, not at the leading edge. But what do I know, I'm just a liberal arts school dropout?

Quote:
 In a book I read, it discussed molecules of air impacting the leading edge of the airfoil and then being redirected and so on
That's called the stagnation point. It's actually a bit below the leading edge when the wing is producing lift. In the video above you can see the streamlines curving upward in front of the airfoil. One streamline will hit the wing just between the points where they divide to go over and under it. The point where this streamline hits the airfoil is the maximum pressure point and, if the wing never changed angle of attack, there would be a few molecules that would stay there. However, since the stagnation point moves up and down on the leading edge when you change AoA, this little slug of air only stays put for a short time so the name can be confusing

--Norm
Last edited by nmasters; Dec 25, 2012 at 10:04 PM.
Dec 25, 2012, 09:58 PM
Quote:
 Originally Posted by xlcrlee 2. An easy way to describe what happens in an airflow is to picture or draw a curved section of a meandering river. If you've looked down from an airplane you can see that the river tends to move away from the shore on the inside of the curve [depositing sand & debris there], and "eat" into the shore on the outside of the curve. If the water was replaced by a lot of moving steel ball-bearings, you'd see something similar. Now draw a curved line through the center of the curving river, cut the paper where you've drawn this curving river [or as a thought process] and REVERSE the outside with the inside parts of the curved river. THAT is what happens with an airfoil in a subsonic airflow. Lee
I see. The water on the outside of the bend accelerates and the water on the inside of the bend slows down. The flow does not bend because its speed changed! The speed changes because the flow is bent. The faster water has more kinetic energy and can carry more material so it cuts into the bank. The water on the inside of the bend has less kinetic energy and therefore drops material that it has picked up upstream.

--Norm
Dec 26, 2012, 05:18 AM
Registered User
Quote:
 Originally Posted by nmasters I see. The water on the outside of the bend accelerates and the water on the inside of the bend slows down. The flow does not bend because its speed changed! The speed changes because the flow is bent. The faster water has more kinetic energy and can carry more material so it cuts into the bank. The water on the inside of the bend has less kinetic energy and therefore drops material that it has picked up upstream. --Norm
Hi Norm:

I haven't rigorously thought out this analogy, it's really a heuristic "explanation".

In general I picture the fluid flow to be like a stream of molasses which might be freely falling downward & outward, like water from a garden hose, and which is then curved more downward by gravity. As noted in the pulsed-smoke video segment in the video you posted above, if one pictures a "wave-front", it's clear how the fluid flow bends, and how your statement about how the flow on the top of a lifting airflow leaves the T.E. before that on the underside, describes the fact > it's a geometric part of how the fluid bends and changes direction .... similar to a marching band turning a corner, or a centipede turning.

Lee
Dec 26, 2012, 09:53 AM
Quote:
 Originally Posted by xlcrlee similar to a marching band turning a corner, or a centipede turning.
Or a rubber duck race
Dec 26, 2012, 10:46 AM
Registered User
Quote:
 Originally Posted by nmasters or a rubber duck race:d

### Images

 Dec 29, 2012, 12:31 AM Registered User Just started to design a drone using Kfm2 wing. Anyone knows how to calculate the lift force? http://subsonichobby.blogspot.com/20...cdrone-v1.html
Dec 29, 2012, 02:01 PM
Registered User
Quote:
 Originally Posted by SubSonichobby Just started to design a drone using Kfm2 wing. Anyone knows how to calculate the lift force? http://subsonichobby.blogspot.com/20...cdrone-v1.html
Here is a 60" KFm3 drone built by RCFoamFighters and it is launched in very strong winds. But the wind was not a problem. This video may give you some ideas.

 Flight of theBoomBox part 1 (10 min 1 sec)
 Jan 05, 2013, 12:41 PM OpenTX University Staff Subsonic, lift force depends on the airfoil shape you plan to use. You can make just about any airfoil you want into a KF2. What you don't want is to use the flat plate design. It has very little lift and huge amounts of drag. You can easily approximate a reasonably good airfoil by folding the wing at the leading edge and using a raised step. Look in this thread and in tyhe KF building thread for examples of airfoil designs. You might also consider higher aspect ratio wings for your drone. Short fat wings are not good for roll stability. Longer thinner wings would serve you better.
Jan 25, 2013, 07:55 AM
Suspended Account
https://www.rcgroups.com/forums/show....php?t=1174874
OEM kit using KF air foil.

### Images

Last edited by zeroback; Jan 30, 2013 at 11:05 AM.
Jan 31, 2013, 06:35 PM
Build straight - Fly twisty

# KF and Ca hybrid

This is not *advanced* science but it is, I think, a mildly interesting experiment.
The aim was to test the flying properties of a KFm2 to that of a conventional airfoil (Ca) under identical conditions.
A small chuck glider was made from \$tree foam and balsa. The port wing was made with the KF profile and the starboard wing was sanded to resemble a Ca. The same leading edge profiles were used on both sides.
Upon completion the first advantage of the KF became evident in that it was lighter than the Ca.
Obviously for this test this test lateral balance was important so a small amount of ballast was added to the port wing-tip.
Hand launches showed that the little glider flew very well. After several flights it was felt that the plane slightly favoured turning towards the KF side but it sometimes chose to turn the other way. In practice it was very much the result one would expect from a plane with identical wings.
Stalling behaviour was tested by launching the plane at an angle and speed that would result in loss of airspeed. In this test both wings seemed to perform identically. The resultant stall was straight, the wings remained level.
This test did not show that the KF was more stall-resistant than the Ca.