HobbyKing.com New Products Flash Sale
Reply
Thread Tools
This thread is privately moderated by maguro, who may elect to delete unwanted replies.
Old Feb 16, 2012, 06:41 PM
RC beginner
New York
Joined Oct 2008
6,046 Posts
Quote:
Originally Posted by jackerbes View Post
I don't think I remember anyone ever claiming less drag as a KF benefit. But If you are flying a flat plate wing I think there is an increase in lift.
heres just one of a dozen or so on a quick search:

Quote:
Originally Posted by Lavaelous View Post
The KFm Airfoil helps you achieve speed by reducing drag
more commonly described as improved "glide ratio" or better "lift to drag" which implies less drag. as time goes by these kind of claims are showing up less often as the fervor wears off and level heads prevail.

i have personally witnessed contests against flat plate and, more importantly, single surface undercamber (king of lift). and assure you kf does not win. i still have these wings and it shouldnt be too hard to replicate when the weather improves. meanwhile ill post pictures dick requested in a few days. if i cant get them off the ide drive ill pull out those wings and snap new photos myself.
dave1993 is offline Find More Posts by dave1993
Reply With Quote
Sign up now
to remove ads between posts
Old Feb 16, 2012, 06:55 PM
Registered User
Joined Feb 2012
70 Posts
Off topic and seemingly random question: How many small UAVs, Ravens, have gone down due to stalls?

Wondering because it seems the major department for the KF airfoil is its stall characteristic. If the number is high, perhaps the KF airfoil would help there.

Also, how hard would it be to add one little strip to the KF airfoil to test the Pick up truck idea? I think it would be worth considering and might improve the L/D ratio while still preserving the stall characteristics. Just in case anyone asks, the reason I can not do it is because I do not really know how to fly yet. I am still learning.
teflyer is offline Find More Posts by teflyer
Reply With Quote
Old Feb 16, 2012, 07:24 PM
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,215 Posts
There is no reason that a UAV should ever have trouble due to stalling. The flight program will keep the thing in the correct flight envelope and everything will be sweet.
The UAV will know its airspeed, not guess at it like we line-of-sight pilots have to do.
The guys most interested in good L/D ratios are the competition sailplane fanatics. Commonly they will have one section at the wing root blending into another at the tip. A wing based on their design principals would, I think, be the best for a UAV seeking good performance.
The pickup truck thing is simply a "Flat plate verses a Streamline shape" thing.
I read about a small record-breaking car (driver in one pod, motorcycle engine in the other) that had the same profile drag as a 6 inch diameter disk.
A pickup with the gate down is aerodynamically more like a flat plate than it is with the gate up. Its as simple (and complicated) as that.
Whiskers is online now Find More Posts by Whiskers
Reply With Quote
Old Feb 16, 2012, 07:34 PM
Registered User
Joined Feb 2012
70 Posts
Ok thanks! How do you know when an airfoil has stalled?
teflyer is offline Find More Posts by teflyer
Reply With Quote
Old Feb 16, 2012, 08:44 PM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
3,991 Posts
Lift goes down (or in some cases levels off), drag goes up when you increase angle of attack beyond that point.
Don Stackhouse is offline Find More Posts by Don Stackhouse
Last edited by Don Stackhouse; Feb 16, 2012 at 09:03 PM.
Reply With Quote
Old Feb 16, 2012, 08:59 PM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
3,991 Posts
Quote:
Originally Posted by Whiskers View Post
Way back in 1901 the Wright Brothers did splendid work with their 6' long by 16" square wind tunnel. ....
Yes and no. Their wind tunnel did not properly account for the effects of Reynolds number. As a result, they concluded that very thin airfoils with moderate camber were necessary (which is true at the Re's existing in their tunnel). At full-scale Reynolds numbers this is not the case.

However, because they convinced themselves that thin airfoils (which are too thin for a cantilevered spar) were required for their manned aircraft, they condemned themselves and all the others who copied them to go with wings that required external bracing. This set the trend that stubbornly persisted until the 30's, although others (such as Fokker and Junkers) did experiment.with thicker, cantilevered wings before then.

What finally ended the reign of externally-braced flying surfaces was the advent of all-weather operations by the airlines. It's easy enough to fit de-icer boots on a wing or a tail, but utterly impractical to fit them on all those wires and struts.
Don Stackhouse is offline Find More Posts by Don Stackhouse
Reply With Quote
Old Feb 16, 2012, 09:17 PM
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,215 Posts
Usually when the plane stops flying and starts falling.
Let's make a plane do a simple stall. We are flying straight and level at cruising speed. We reduce power and feed in up elevator to maintain level flight which the plane can do because the wing now has a greater angle of attack (AoT). If we keep reducing power and increasing AoT we eventually arrive at a situation where the airflow over the top of the wing becomes a mass of turbulence and the lifting power is no longer enough to support the weight of the plane. The plane typically drops its nose and plunges towards the ground. Recovery is by applying down elevator and power to attain airspeed and resume flight.
Of course there variations on the theme. Sometimes one wingtip is the first to stall leading to a spiral dive or perhaps a spin.
But not all stalls are bad. The classic 3 point landing happens when the plane stalls about a poofteenth of an inch above the runway.
A stall turn involves a stall. No surprise there.
A snap roll requires one wing only to stall.
Then there's high-speed stalls, but that's another subject.
Whiskers is online now Find More Posts by Whiskers
Reply With Quote
Old Feb 16, 2012, 09:38 PM
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,215 Posts
Quote:However, because they convinced themselves that thin airfoils (which are too thin for a cantilevered spar) were required for their manned aircraft, they condemned themselves and all the others who copied them to go with wings that required external bracing. This set the trend that stubbornly persisted until the 30's, although others (such as Fokker and Junkers) did experiment.with thicker, cantilevered wings before then.

What finally ended the reign of externally-braced flying surfaces was the advent of all-weather operations by the airlines. It's easy enough to fit de-icer boots on a wing or a tail, but utterly impractical to fit them on all those wires and struts.


And they were right about thin wings. All fast modern aircraft have thin wings. It is modern material and design that makes them structurally possible.
Those old thick monoplane wings were no better than the thin biplane setup.
Of course the Wrights didn't understand about Reynolds numbers. In those days the sum of human knowledge of aerodynamics was close to zero.
However, with the data from their tunnel they were able to design a propeller. There was no try and see. There was no make a lot and see what works. They used their own data, did their own calculations, and it worked.
I call that splendid work.
Whiskers is online now Find More Posts by Whiskers
Reply With Quote
Old Feb 16, 2012, 09:56 PM
Registered User
Joined Feb 2012
70 Posts
In a stall, would the pressure on the top of the wing equal that of the bottom of the wing? sorry for being kind of off topic
teflyer is offline Find More Posts by teflyer
Reply With Quote
Old Feb 16, 2012, 10:15 PM
Registered User
Dickeroo's Avatar
Joined Dec 2006
1,184 Posts
Closing the tailgate on the KFm2.

Maybe we can close the tailgate before the vortex gets out?
Dickeroo is online now Find More Posts by Dickeroo
Reply With Quote
Old Feb 16, 2012, 10:23 PM
Registered User
Joined Feb 2012
70 Posts
How does the Kamm Effect work? it would seem that the KF airfoil is rather similar. maybe...
teflyer is offline Find More Posts by teflyer
Reply With Quote
Old Feb 17, 2012, 08:10 AM
RC beginner
New York
Joined Oct 2008
6,046 Posts
Quote:
Originally Posted by teflyer View Post
In a stall, would the pressure on the top of the wing equal that of the bottom of the wing? sorry for being kind of off topic
this thread does NOT need to go off in that direction. there is another one going on right now in the model science area for "bernouli heads" who bear a striking resemblence to kf extremeists in terms of logic and discussion methods. classic centuries old conflict between Faith and Reason:

http://www.rcgroups.com/forums/showthread.php?t=1539175
dave1993 is offline Find More Posts by dave1993
Last edited by dave1993; Feb 17, 2012 at 08:16 AM. Reason: link
Reply With Quote
Old Feb 17, 2012, 08:23 AM
Registered User
Joined Mar 2008
627 Posts
Quote:
Originally Posted by springer View Post
I'm still not sure i would need a 3'x4' tunnel section to test 6 or 12 inch span sections of a bunch of different foamie wings.

Dave: I noted the point, and alsoI saw in one of the sites that the open ended tunnels were susceptible to uneven flow/turbulence, but I would think that it should be possible to see it (smoke) and try different straighteners to eliminate it. After all, this whole exercise is pretty much just another "tinkering" like building a hot wire cutter, or a wing baking jig. (bit more complicated, of course, but just another thing an old retired guy can play with out in the barn, at least until the pallets of foam show up....)
Unfortunately this is a somewhat complex topic that can't be easily addressed in a short post on an internet forum.

Because subsonic air is "connected," turbulence anywhere in the tunnel can propagate throughout the tunnel (both upstream and downstream). There are a variety of sources, eliminating or mitigating turbulence through the addition of "flow smoothening devices" (straws, tubes, etc.) can introduce even more turbulence and thus the design of such devices is not a simple task. If you have eough time and instrumentation, you can probably find an effective design using a "brute force" empirical method.

Because of the boundary layer effect, a boundary layer will form in the tunnel and the test section must be designed to account for this. This is called "displacement thickness" and the test section must be made larger in order that the center of the test section is out of the boundary layer and possesses the correct properties for the test.

This is all fairly well documented stuff, suggest you get a textbook to learn more.
Cap_n_Dave is offline Find More Posts by Cap_n_Dave
Reply With Quote
Old Feb 17, 2012, 08:24 AM
RC beginner
New York
Joined Oct 2008
6,046 Posts
Quote:
Originally Posted by Dickeroo View Post
Maybe we can close the tailgate before the vortex gets out?
lol! what was it my mom occasionally said to me on my way out of the house? something about a horse and a barn door?
dave1993 is offline Find More Posts by dave1993
Reply With Quote
Old Feb 17, 2012, 09:19 AM
Registered User
Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
3,991 Posts
Quote:
Originally Posted by Whiskers View Post
...And they were right about thin wings. All fast modern aircraft have thin wings. It is modern material and design that makes them structurally possible.
Those old thick monoplane wings were no better than the thin biplane setup....
No. Thin wings are required for supersonic flight, the F104 being one fairly extreme example (3% thick, no camber to speak of, leading edge radius 1/64", so sharp that they had to put covers on them on the ground so mechanics wouldn't get injured if they walked into one).

However, for subsonic flight, much greater thickness is better. Full-scale high performance sailplanes frequently have airfoils 15% to 18% thick, including outboard on the wing where the structural challenges of high aspect ratios are no longer a problem.

There are some outstanding 18% NACA 65 series laminar flow sections with unusually high section L/D's and good stall characteristics, if you are working with full-scale Re's. For full-scale general aviation aircraft, typical thicknesses run 12% to 18%.

Those thicknesses at model airplane Re's are a disaster. For model sailplanes, going thicker than about 8% to 9% can actually reduce max lift
Don Stackhouse is offline Find More Posts by Don Stackhouse
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion ** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** jackerbes Foamies (Scratchbuilt) 7171 Aug 25, 2014 09:42 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