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Old Dec 13, 2012, 09:28 PM
just gotta mess with it!
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Originally Posted by Whiskers View Post
It can be an attractive thought until you start thinking about the disturbed air-flow around the vehicle, the fact that airspeed and ground speed may differ and bumps and undulations will play havoc with lift readings... The list goes on.
Ye of little faith! Choose a straight, quiet, flat road for starters, have an anemometer to check the airspeed, rather than relying 100% on the speedo. I've been thinking about doing model tests for temporary structures this way.... just need to find the sensors.
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Old Dec 13, 2012, 10:04 PM
Build straight - Fly twisty
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I'm not of little faith.
I'm of no faith whatsoever!
But I'd be very happy to be proved wrong...
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Old Dec 14, 2012, 03:39 AM
just Some Useless Geek
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Doggs, you'll find that many people have offered the same suggestion and have seen the results: inconsistent, useless data gathered under conditions that are never the same twice. The "natural wind tunnel" appears to be out of our grasp.
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Old Dec 14, 2012, 08:08 AM
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The Clark Y airfoil is not a good comparison to almost any KF airfoil I've seen. If you wan a true comparison, build a KF airfoil and the same airfoil with a continuous curve to the trailing edge. See my test airfoil shapes earlier in the thread. What we are trying to get data on is what effect the steps have on a models wing. Comparing two different airfoils will not tell us anything. I think a MH32 airfoil is relatively easy to duplicate in FFF or similar. If you are going to cut the wing with a hot wire, then any airfoil will do. Just make one with steps and one without.


I tried flying an instrumented and tufted test plane. Maintaining a constant airspeed within 5mph long enough to gather reasonable data was almost impossible. It sounds easy to do, but it is not. Even a flight stabilization system doesn't help much. A decent autopilot system might do the trick, but they are out of my price range.
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Old Dec 14, 2012, 08:54 AM
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Does visual evidence prove anything?

I have viewed a number of FPV videos and have noticed that in most cases the camera is jumping frequently. Rarely does it hold steady.

However, when I look at videos produced by Trond of Norway and FPCRC of Latvia, I see an extremely stable platform at all altitudes and with a wide range of wind conditions. Personally, I believe that the KF airfoils are at least equal to true airfoils as far as a FPV platform is concerned. For me, the proof is in the videos. If I am off base on this, please correct me.

Some examples...

FPVRC Intermezzo...KFm7
Green is the color of Latvia (HD) FPV (4 min 42 sec)


KFm4
FPV49 v3 New Prop & Motor (7 min 13 sec)
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Old Dec 14, 2012, 03:20 PM
Build straight - Fly twisty
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I agree that KFm airfoils are well suited for use in FPV, but the steady flying in the above videos can be largely attributed to the perfectly calm air conditions.
Note in the 'green' one that the trees are perfectly still, and the dust behind the car just hangs in the air and does not get blown away.
And look at the mirror smooth water in the 'new prop' vid. Perfect conditions.
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Old Dec 14, 2012, 04:10 PM
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Quote:
Originally Posted by Whiskers View Post
I agree that KFm airfoils are well suited for use in FPV, but the steady flying in the above videos can be largely attributed to the perfectly calm air conditions.
Note in the 'green' one that the trees are perfectly still, and the dust behind the car just hangs in the air and does not get blown away.
And look at the mirror smooth water in the 'new prop' vid. Perfect conditions.
Well, Whiskers, then how about this KFm2 flying wing that is flying in extremely windy conditions. The wind is so strong it pushes the flying wing backwards. You can see a field down below, then again when the field is much further away. Considering the conditions, the KFm2 wing still remains fairly stable. As noted in the video, Trond wouldn't even attempt to fly any of his other planes in this environment.

Also, in the Latvia video, he takes the plane up into the clouds. I have to believe that the wind is a bit stronger up there than it is close to the ground.

My only point is that the KF airfoil seems to deal with strong winds quite well and there are a number of videos that illustrate this.

SHORT: FPV49 v2 vs the Wind (4 min 37 sec)
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Old Dec 14, 2012, 05:01 PM
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i suspect the stall resistance at certain angles of attack may be the biggest advantage. its always been one problem with wind specially with quick changes in direction.
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Old Dec 14, 2012, 05:06 PM
Build straight - Fly twisty
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An interesting thing about the plane in the above video in that the wing-sections are so asymmetrical. Clean on the starboard side and far from clean on the port..
We certainly get away with things at 'our size' that would be a disaster at full-size.
It does indeed do a good job in the turbulence, however I'm sure piloting skills play a fair part in it as well.
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Old Dec 16, 2012, 05:26 PM
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Step right up!

Here's a video that mimics the feathers of a bird's wing with nice results.

Dynamic morphing Airfoil - gliding flight test (2 min 42 sec)
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Old Dec 19, 2012, 12:46 PM
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This is an interesting video. I wonder how the feather tiles would respond over a traditional airfoil. In nature, I wonder how much air flows through the cross section of the wing?
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Old Dec 24, 2012, 08:07 PM
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praxis

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Originally Posted by jackerbes View Post
On August 20, 2006, in the ** Kline-Fogleman Airfoiled Flying Wing ** thread, tony65x55 posted the following explanation and figure as to how he thinks the Kline-Fogelman airfoil works. It is as good an explanation as we have ever gotten.

Tony65x55 wrote:

I've been thinking long and hard about the KFm and why it flies the way it does and I may have some answers. More likely, new questions.

I think the step is important as it creates the lift but not in terms of the size of the step but more as the angle between the step and the LE. That is to say, the height of the step determines the angle of the airfoil's underside.

I think the underside angled portion of the airfoil does the lifting. On the KFm Wing the lower wing root angle is +4 degrees. Air striking the LE parts and goes up and down. The air flowing over the upper surface is deflected 0 degrees but the airstream hitting the lower surface is deflected downward under higher pressure at +4 degrees, creating the upper/lower pressure differential and pushing the wing up (viola..lift!).

As AoA increases, the airfoil maintains its upper/lower angle of attack differential, always deflecting the air downward 4 degrees more than the angle of attack of the upper surface.

On a conventional airfoil the air must follow the sloping rear section of the airfoil, which at 0 degrees AoA is already at a 8-10 degree slope away from the airstream. This slope away from the airstream make it much easier for the airflow to detach itself from the upper surface and when it does...stall.

The KFm airfoil would have to reach an AoA 8-10 degrees greater to place its upper rear section at the same AoA as a conventional airfoil. At these extreme angles of attack drag becomes a much greater force and the airfoil can no longer maintain it's altitude without a great deal of power and so begins to sink. However, it has not yet stalled and so it maintains it's extreme AoA and mushes down, without the nose breaking through the stall. Simply reducing the extreme AoA restores a lower drag and the airfoil simply resumes normal flying.

[

That's it, I finished my silly theory. I'm probably full of it but after two weeks of pondering this, that's what I came up with. Any (polite) comments are welcome.

Tony
A long time ago I was making very low Rn flow studies, using smoke or thread tufts in my wind tunnel.

I saw what tony65x55's KFm dwgs illustrate, which is that at very low Rn's the airflow just keeps on flowing and there is a negligible amount of circulation beyond any obstacle, the negation of the Coandă effect! [ http://en.wikipedia.org/wiki/Circula...luid_dynamics) ].

It is the reason T.E.'s are best shaped like ] , with sharp edges, whereas this shape ) engenders draggy burbles [airflow left to right].

One finally learns in grad school that the high-pressure/ low-pressure concepts of lift usually taught in American HS & undergrad colleges is confusing cause with effect*. Lift is the result of changing the momentum of the airflow and turning it more downward [called the Delta Rho Vector, pointing with a scalar value in the direction of the momentum-change]. In the process the air on top has to move faster to catch up as best it can with the air it was with just before it encountered the wing. And the pressure is lower.

But in the end it's a simple action-reaction process, like a rocket motor. AND, if you measure the airflow pressure on the bottom of a vertically rising rocket you will find that it has more pressure at the exhaust end [bottom] than at the nose [top]. And it also LIFTS off ....

Anyway, at very low Rn's the air acts enough like the shockwaves of supersonic effects, so that similar shapes will produce this momentum change.

The KFm shapes are like some of the hypersonic aircraft designs that have been proposed and variously tested.

Nothing magic or mysterious: the airflow just doesn't see or notice that it's left the wing behind, and proceeds on its merry way, following the direction in which it was simply deflected!

An effective design concept when used appropriately [as in most of the foamies in this thread].


Lee



*One time at Boeing some admins decided to make and distribute a large poster to schools, pictorially illustrating the air-pressure nonsense these non-aerodynamicists had learned in their schools. The real aerodynamicists flipped out, but it was too late. And so the silly myth continues, blythely ignoring Mr. Delta Rho. [insert Charlie Brown sigh here].


.... and, oh yeah. ρ /rho is the symbol used to represent momentum, mv
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Old Dec 24, 2012, 09:14 PM
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Originally Posted by xlcrlee View Post
One finally learns in grad school that the high-pressure/ low-pressure concepts of lift usually taught in American HS & undergrad colleges is confusing cause with effect*. Lift is the result of changing the momentum of the airflow and turning it more downward [called the Delta Rho Vector, pointing with a scalar value in the direction of the momentum-change]. In the process the air on top has to move faster to catch up as best it can with the air it was with just before it encountered the wing. And the pressure is lower.
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. I mean, suppose you have airflow and separate that airflow into two different sections. Let's imagine the first section has a significantly longer path than the other for the air to travel through. As a result, the air from the second section should come out first while the air comes out of the first section later.
It seems rather doubtful that the air on the top of the airfoil somehow knows where the air on the bottom is and moves faster to try to be on the same x-coordinate.

Please explain how the air moves faster at the top.

Edit:

This is an old argument, but I feel that anyone reading should have both sides of the argument.

So, I respectfully disagree that the lift on an airfoil comes only from the transfer of momentum. At least some of the lift comes from the difference in pressure of the airfoil, hence the existence of pressure graphs of airfoils. Also usually the center of lift is estimated around 25% of the wing chord, thus suggesting that there is lift from the front of the airfoil as well. Therefore, some of the lift comes from the difference in pressure.

Please correct me if I misinterpreted your statement.
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Old Dec 25, 2012, 05:23 AM
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Sure:

1, Physics does NOT try to "explain" things, merely describe observations [pls see my response in the above/other KFm thread], As such, nothing "makes sense" or is "explainable": think "cooking" > when you add this ingredient, at this temperature, this happens. Period. One can like it or dislike it, but what happens, happens.

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.

If you either "don't like" or "don't understand" this ..., fine. No worries, no problem, as that does not change the observed realities of the two actually-similar situations.

And as to "why" the water- & airflows re-join, it's what we mean when we say that these are viscous fluids which try to "stick together" [like molasses]. Just like gravitational and magnetic attraction, chemical or electrical attraction, like surface tension, is something we observe happening and we still don't exactly know "why" ....


Happy/Merry,

Lee
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Old Dec 25, 2012, 06:04 AM
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afterthought ;-)

.... in the river AND airfoil situations, the significant thing is that the river HAS in fact curved and that the airflow, after passing around the airfoil, has changed direction [been re-vectored].

This takes energy, slowing the river and the airplane unless extra energy is added [from gravity, in a glider, thermals in a sailplane, or from a motor, for ex.].

We see the sand deposit on the inside of the curving river, moving the shoreline [this is a similar to pressure or force] and we see the shoreline being eaten away, moved outward, on the outside of the curve. Of course we could use earth-moving equipment and force the river to turn. But the curving will nonetheless continue to curve even more because of the natural meandering process.

When the river turns and the airfoil changes the direction of the airflow, we observe these pressure, flow-velocity and moving-forces. We COULD blow another airflow down to interact with the horizontal airflow, and THAT would also cause the horizontal airflow to change direction and move more downward [momentum-vector addition].

But that is not what we do when we fly an airfoil -- of ANY shape -- through the air.

"Down by the Riverside" and the reaction force is what we call Lift.

But one is allowed to call it "Elmer" if one wishes ....
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