Espritmodel.com Telemetry Radio
Reply
Thread Tools
This thread is privately moderated by maguro, who may elect to delete unwanted replies.
Old Dec 25, 2012, 07:57 AM
Jack
USA, ME, Ellsworth
Joined May 2008
16,680 Posts
The wind tunnel sensors discussion is still going on over on this thread:

http://www.rcgroups.com/forums/showthread.php?t=1788242

A recent post included a link to a search result from the NASA Technical Reports server:

http://www.rcgroups.com/forums/showp...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
jackerbes is offline Find More Posts by jackerbes
RCG Plus Member
Sign up now
to remove ads between posts
Old Dec 25, 2012, 08:42 AM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
2,153 Posts
Quote:
Originally Posted by g828 View Post
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)
nmasters is online now Find More Posts by nmasters
Last edited by nmasters; Dec 25, 2012 at 06:59 PM.
Reply With Quote
Old Dec 25, 2012, 10:53 AM
Registered User
Joined Jul 2012
43 Posts
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
g828 is offline Find More Posts by g828
Reply With Quote
Old Dec 25, 2012, 08:41 PM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
2,153 Posts
Quote:
Originally Posted by g828 View Post
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
nmasters is online now Find More Posts by nmasters
Last edited by nmasters; Dec 25, 2012 at 09:04 PM.
Reply With Quote
Old Dec 25, 2012, 08:58 PM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
2,153 Posts
Quote:
Originally Posted by xlcrlee View Post

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
nmasters is online now Find More Posts by nmasters
Reply With Quote
Old Dec 26, 2012, 04:18 AM
Registered User
Zurich
Joined Apr 2006
3,535 Posts
Quote:
Originally Posted by nmasters View Post
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
xlcrlee is offline Find More Posts by xlcrlee
Reply With Quote
Old Dec 26, 2012, 08:53 AM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
2,153 Posts
Quote:
Originally Posted by xlcrlee View Post
similar to a marching band turning a corner, or a centipede turning.
Or a rubber duck race
nmasters is online now Find More Posts by nmasters
Reply With Quote
Old Dec 26, 2012, 09:46 AM
Registered User
Zurich
Joined Apr 2006
3,535 Posts
Quote:
Originally Posted by nmasters View Post
or a rubber duck race:d
xlcrlee is offline Find More Posts by xlcrlee
Reply With Quote
Old Dec 28, 2012, 11:31 PM
Registered User
Joined Mar 2011
63 Posts
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
SubSonichobby is offline Find More Posts by SubSonichobby
Reply With Quote
Old Dec 29, 2012, 01:01 PM
Registered User
Dickeroo's Avatar
Joined Dec 2006
1,180 Posts
Quote:
Originally Posted by SubSonichobby View Post
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)
Dickeroo is online now Find More Posts by Dickeroo
Reply With Quote
Old Jan 05, 2013, 11:41 AM
Registered User
maguro's Avatar
United States, DE, Bear
Joined Apr 2007
1,547 Posts
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.
maguro is online now Find More Posts by maguro
Reply With Quote
Old Jan 25, 2013, 06:55 AM
Suspended Account
United States, FL, Pompano Beach
Joined Oct 2011
1,425 Posts
http://www.rcgroups.com/forums/showthread.php?t=1174874
OEM kit using KF air foil.
zeroback is offline Find More Posts by zeroback
Last edited by zeroback; Jan 30, 2013 at 10:05 AM.
Reply With Quote  (Disabled)
Old Jan 31, 2013, 05:35 PM
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,158 Posts
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.
Whiskers is online now Find More Posts by Whiskers
Reply With Quote
Old Jan 31, 2013, 10:23 PM
KlonWarz
Joined Dec 2012
454 Posts
Sounds like a reasonable test to me, W...
IIRC, that's how they got started with the KFm... tossing gliders.

So, a conclusion might lead someone to believe the KFm performs for less weight.

and I've already found a problem with that... if you want to bury servos in the wing, or, run a wet wing for fuel, which they did on some racers.
rc
rusty case is offline Find More Posts by rusty case
Reply With Quote
Old Feb 01, 2013, 05:48 AM
Jack
USA, ME, Ellsworth
Joined May 2008
16,680 Posts
The test is interesting. But I think a better test would be to build two planes, one with each wing. Ballast them out to the same weight and then you'll see a better comparison.

Chuck gliders or DLG's are not really a flying venue for KFm wings though, especially so if the intent is to compete with planes with non-KFm wings.

The inferiority of KFm against a S-series foil on DLG's has already been demonstrated in one comparison build. The KFm wing could only attain about 65% of the duration that the other plane did.

Unpowered flight with KFm wings is fun and entertaining. But it is not likely to be competitive across the board with the other foils used for the same flying.

Jack
jackerbes is offline Find More Posts by jackerbes
RCG Plus Member
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion ** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** jackerbes Foamies (Scratchbuilt) 7152 Jul 17, 2014 04:25 AM
Cool Here is my KFm-5 DLG GLider (Kline-Fogleman) dougmontgomery Foamies (Scratchbuilt) 151 Apr 21, 2014 09:08 AM
Discussion ** Kline-Fogleman Airfoiled Flying Wing ** Tony65x55 Foamies (Scratchbuilt) 3945 Apr 08, 2014 10:40 AM
Video Kline Fogleman Airfoil on a flying wing Tony65x55 Electric Plane Talk 3 Jan 30, 2009 07:37 PM
Idea Per Dick Kline, Kline-Fogleman test dougmontgomery Hand Launch 49 Apr 13, 2007 02:13 AM