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Old Aug 06, 2011, 05:14 PM
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Roger... outstanding work. It looks like the steps help prevent separation at various AOA. And, thank your wife as well for her contribution to this effort.

Dick
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Old Aug 07, 2011, 01:17 PM
just Some Useless Geek
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Ahh...Jet Steve, I think you're right. I talked about moving the wing fore and aft on the boom, but forgot to mention that I also lengthened the boom. Sorry about that. I had talked about that aspect of experimentation on another thread, but not this one. Oopth.

In any case, I agree with you that this plane seems to present some unique quirks that don't show up in other KF planes, even my own designs. But the fact that I can link the weird performance directly to the step height of the KF indicates that the step is either causing or exacerbating some aspect of the plane's behavior. This is what I wanted to point out in my previous posts; that the KF may cause weirdness to show itself, and we should be aware of that whilst gathering data.

-- Marty, hanging head in Sloppy Science Shame --
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Old Aug 07, 2011, 01:30 PM
just Some Useless Geek
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Roger, next time set up a script for what you are going to do with the aircraft before taking it up. Do some more invert, with slow and fast invert passes, stalling, rolls, loops, etc. See if you can figure out how to get the tufts moving around. It may take you several flights to get a good script figured out, with transitions from one maneuver to another causing the tufts to flail. Take a laptop with you to see the video immediately on landing. That's what I do.

Once you've got a good test plan then shoot the flight and do a voice-over or text annotation to describe what's happening with the plane while we see what the yarn is doing. This will be some really good science on the behavior of the air over/under that wing in flight.
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Old Aug 07, 2011, 02:58 PM
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Marty,

The step height could certainly be compounding the problem.. By increasing the thickness of the airfoil and leaving the bottom flat you are adding camber. More camber gives more nose down pitching moment. This isn't a 'KF step thing' it works the same way on all airfoils.

On a very short coupled and small tailed plane like this then you will probably get best results from some sort of symmetrical airfoil (stepped or otherwise). If it had ailerons reflexing them might also help.

Steve
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Old Aug 07, 2011, 03:18 PM
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Roger,

The in flight video is interesting. also very useful to have the on-board and external videos running together so we can see what the model is doing... great work!

To be honest the behavior of the wing looks in most respects pretty much like you would expect of an ordinary airfoil. The stall appears entirely 'normal', certainly no one can really argue after watching the video that KF airfoils don't stall.
I don't see any sign of any inflow behind the step as was discussed previously. I also don't see any evidence of a stable vortex behind the step. If there was a vortex sitting there as some have speculated, then the airflow immediately behind the step would be forward, you would therefore expect the tufts right behind the step to all line up pointing forward and that's just not the case. The tufts right behind the step seem to flop around in all sorts of directions, probably indicating a quite unstable flow regime at that point. The tuft's near the TE show that flow re-attaches, which is good. The second row of tufts from the TE seem to show some slight instability which probably indicates some degree of turbulence at that point as the airflow re-attaches after passing the step. The tuft's on the top edge of the step are interesting too.. they seem to show that the under low AoA conditions the airflow curves down quite sharply after passing the step, so any turbulent 'pocket' that sits behind the step is probably quite small in these conditions. When the lane turns tight, pulling more 'g' and increasing AoA, the tufts on the step point almost straight out indicating that the pocket behind the step grows in size.

Anyone see anything different?

A similar video of the conventional airfoil would be good for comparison. Guess that's on the list of 'things to do'?

Steve
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Old Aug 07, 2011, 04:28 PM
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[QUOTE=JetPlaneFlyer;18980311]Roger,



To be honest the behavior of the wing looks in most respects pretty much like you would expect of an ordinary airfoil. The stall appears entirely 'normal', certainly no one can really argue after watching the video that KF airfoils don't stall.

Anyone see anything different?



Steve... my question to you is: Would you equate the stall of a normal airfoil and a KF airfoil to be one in the same type of stall?

From my perspective, the KF airfoil is more of a "porpoise" type of stall where the airfoil loses some height but recovers rather quickly. Therefore, I would not consider it having the same characteristics as a stall from a conventional airfoil which tends to lose a lot more altitude. Please correct me if I am wrong on this.

– Dick
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Old Aug 07, 2011, 05:42 PM
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Dick,

I've not flown a KF wing plane so I cant comment specifically on the KF... However I've flown a lot of different planes and they all had different stall behavior. Some were real nasty, they would drop a wing and spin in with little or no warning (those planes usually didn't last long). On some the stall was a non event, just a sort of high alpha 'mush'.. Most planes being somewhere in-between these two extremes. As an example my current 3D aerobatic planes with conventional symmetrical airfoilds can be flown in high alpha harrier way past the point of stall under full control without any loss of altitude, in fact they can and do climb.

There are a lot of things that influence the way a plane stalls, airfoil being one but wing planform, washout, CG location and wing loading are just as important if not more so.

So what I'd say is that's it's impossible to say that all planes with conventional airfoils stall one way and all planes with KF airfoils stall another.. there are many variables.
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Old Aug 07, 2011, 05:56 PM
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Dick,

The 20' KF sailplane video posted earlier in this thread is a case in point where a KF wing plane stalls and looses considerable height.. The ground stopped it losing more but even if it had been two or three times higher I don't think it would have recovered: http://www.rcgroups.com/forums/showp...&postcount=378

It looks to me that too far back CG was the main problem with that glider, compounded by being a bit underpowered... but it does show that not all KF planes stall the same way, and neither do conventional airfoiled planes.
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Old Aug 07, 2011, 08:09 PM
Jack
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"..I've not flown a KF wing plane so I cant comment specifically on the KF..."

With your interest in flying, you really ought to bang some kind of KF wing together and see what we are always talking about first hand. What are you flying now? Maybe we can help you convert one of them?

I have built about 15 KF wings in the different variants and different kinds of planes and every one of them had what I can only characterize as a "very gentle stall". They were not all the same because the planes flew differently but it has to be said that a stall that ranges from gentle to almost imperceptible (depending on the plane and situation) is a characteristic of the wing.

And that include wings with and without dihedral, with and without ailerons, etc.

Jack
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Old Aug 07, 2011, 08:47 PM
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that's pretty much what I've observed. I'd never say they are stall proof, but of the planes I've made/flew with both KF and other types of airfoils, the KF always stalls softer/later/gentler. And the KFM stall is more often than not the porpoising or bobbing small stalls with quick recovery, instead of a long drop to pick up flying speed. I suppose one could attribute that to different style wing/planes, but it is pretty consistent across all the planes I've done as KFM's.
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Old Aug 07, 2011, 09:27 PM
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A smoking book and a trip down memory lane.

I thought people might find this interesting... in 1973, Floyd Fogleman and I made a trip out to South Bend, Indiana to the University of Notre Dame. There we met with Dr. John Nicholaides, who was heading up their aerospace division. We brought with us a cord section of the KFm1 airfoil for testing purposes in their smoke tunnel. Dr. Nicholaides was kind enough to give me a copy of a book called See The Wind Blow by F.N.M. Brown who had done extensive work on all different types of objects in a smoke tunnel that he had designed. He was one of the pioneers of smoke tunnel testing. The third picture shows an airfoil at 12, 14, 16 and 17.5. The separation shows up very nicely in these pictures as the AOA increases. I had misplaced this book and just came across it again. It feels like a life-time ago.
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Old Aug 07, 2011, 11:36 PM
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Quote:
Originally Posted by JetPlaneFlyer View Post
Dick,

The 20' KF sailplane video posted earlier in this thread is a case in point where a KF wing plane stalls and looses considerable height.. The ground stopped it losing more but even if it had been two or three times higher I don't think it would have recovered: http://www.rcgroups.com/forums/showp...&postcount=378

It looks to me that too far back CG was the main problem with that glider, compounded by being a bit underpowered... but it does show that not all KF planes stall the same way, and neither do conventional airfoiled planes.
Lets put it this way. When you have a plane that balances at 85% from the LE of the wing. And runs of 23 watts per pound, you will get a nasty stall, whatever plane it is (excerpt for 3D planes as that is not the plane we are basing our conversation on). Also, consider this. I had close to 20 degrees of down elevator just to keep it in the air. When it stalled, it was my fault. I pulled ta far back on the stick. This aggravated the already stall prone plane.

Now take this into consideration. Watch the 2nd video of this plane. Do you see a stall? I don't. The difference on the plane is this. I lost over 1lb of off the tail. (rebuilt it). I also took one foot off of the fuselage.
When I remaidened the BFF, I didn't have a trim problem. I could have used more power, but I have to take into account that I gained about 3,000 feet of elevation.
... It balanced at about 30% from the LE.....

I don't believe that you should be able to say that these KF airfoils stall this way, and don't stall that way. when you have never flown one.


Do your research, then tell me that this particular plane stalled because of the airfoil, not the circumstances it was under.
~Cole
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Old Aug 08, 2011, 02:08 AM
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Quote:
Originally Posted by anti-gravity View Post
Do your research, then tell me that this particular plane stalled because of the airfoil, not the circumstances it was under.
~Cole
Cole,
I think you are being rather unfair and accusing me of saying things i simply did not type or imply.. I DID NOT say or imply that the plane stalled because it has a KF airfoil or that the KF airfoil was in any way to blame for the severity of the stall.. In fact I clearly stated that my opinion was that the stall was due to too rear a CG location exacerbated by lack of power, and from what you have just said it looks like i was right.

What i actually said was that airfoil shape was just one factor that influenced stall behavior. What the video does prove is if you have some of these other factors 'wrong' then any airfoil can stall badly. In this case it would not have mattered what airfoil was fitted to the model, the stall would have been the same because the CG was miles out.
What it also does prove beyond doubt is that KF airfoil can stall. This is contrary to the "KF airfoil remains unstallable" claim made in post #6 of this very thread and elsewhere.


Perhaps KF airfoil planes do, all other factors being equal, stall in a gentle way, i did not say they didn't and would not say that because I've not flown one... All I said was that you cant say that ALL KF planes will stall in a certain way and ALL conventional airfoiled planes stall in another; there are many factors that influence the way a plane stalls, CG being one, as you found out.


I think there is a certain amount of paranoia creeping in among some.. I'm not trying to 'do down' KF airfoils with every post i make. In this case i was simply responding to a post Dick made where he asked for my thoughts on stall behavior.
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Old Aug 08, 2011, 02:29 AM
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Quote:
Originally Posted by jackerbes View Post
With your interest in flying, you really ought to bang some kind of KF wing together and see what we are always talking about first hand. What are you flying now? Maybe we can help you convert one of them?

.....Jack
Jack,

I've got really lazy as far as building is concerned. I've gone from someone who drawn their own plans and built from absolute scratch to a flyer of ARF's. This maybe says something about the time i have available these days?

Anyway my current active RC fleet is:
  • Precision Aerobatics Ultimate AMR (3D/aerobatic biplane, 40" span)
  • Extreme flight Yak 54e (3D/Aerobatic, 48" span)
  • Vladimir's Models Elf (1m span discus launch glider)

I've got various other models in storage including a 1.5m discus launch glider that could be in the air with a couple of hours work re-installing the RC gear.

Of all the models i guess the Elf would be easiest to convert to a KF wing, though it would have to be strong to stand the stress of a discus launch.
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Old Aug 08, 2011, 02:37 AM
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PS.. why not include 3d planes in the discussion? The Ultimate AMR and Yak 54 3D models I'm currently flying are just normal RC model airplanes that are perhaps a bit lighter and a bit more powerful than 'average' sport models. If i reduce the control throws to low rates they fly just like a good sport model. Even on low throws and not using 3D power to hover, flown just like a conventional sport model, these model stall in a quite gentle way.. probably as gentle as any KF airfoil plane (especially the Ultimate).

Steve
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