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Old Jul 29, 2011, 09:49 PM
treefinder
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yup, that's why I only did one every inch or so. I also noticed that the longer ones tended to get tangled in each other and not align with the flow. You may want to try it this way and if they tangle, cut them to about half an inch. I really like the idea of being able to see the flow along the steps, tho.
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Old Jul 29, 2011, 10:28 PM
In the 20' glider range
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Boise, Idaho
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In a few weeks or so I am going to fly my 20 footer, and wondered, If you guys would be interested in me doing this yarn thing.
I have a small camera that I can put on the wingtip....
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Old Jul 29, 2011, 10:39 PM
Build straight - Fly twisty
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Interested? Interested? INTERESTED?
Drooling...
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Old Jul 29, 2011, 11:08 PM
In the 20' glider range
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What kind of length of string should I be looking at, distance apart, what material?
Tell me everything you want from it. I will try and do it.
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Old Jul 30, 2011, 06:39 AM
treefinder
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Well yeah! On your size there's room to do lots more, and not worry about the test setup affecting/skewing the results. I would suggest 1 to 1.5" long tufts of the softest yarn you can find. On a part of the wing try them spaced 1.5" apart on the tape like maguro is doing, then on another portion, use individual taped tufts, same length, same spacing. Do rows at LE, TE, and step top at minimum. Shooting from wingtip will allow seeing how the flow goes toward wing center.
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Old Jul 30, 2011, 08:45 AM
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The yarn will cause a bit of extra drag, but not that bad. It certainly should not have a major effect on flight characteristics. My first flights will be with the regular streamlined top deck, so I can get an idea of the effect of the yarn, and check out the telemetry. Then I'll swap in the camera deck and take some photos.

If the yarn is too long, some scissors will solve the problem. Too much yarn can be solved just by tugging out every other piece. It's a lot easier to shorten and remove, than lengthen and add.

Roger
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Old Jul 30, 2011, 09:36 AM
just Some Useless Geek
Chicagoland
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Hey, um, JetPlaneFlyer -- are you paying attention to the level of detail here? No wind tunnel, but we're still collecting useful data. Eh?
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Old Jul 30, 2011, 10:07 AM
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One more thing.

Quote:
Originally Posted by A Useless Geek View Post
Hey, um, JetPlaneFlyer -- are you paying attention to the level of detail here? No wind tunnel, but we're still collecting useful data. Eh?

I might add that when Springer did his tests using yarn to see the airflow over the upper surface of a KFm2, he discovered that the yarn was drawn toward the root of the wing. I seriously doubt that a computer would have been able to uncover this movement of airfoil towards the fuselage. Real world testing is still a step ahead in my opinion.
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Old Jul 30, 2011, 11:54 AM
Jack
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Another way of testing air flow is to use a clear oil (mineral oil maybe?) and put soot (carbon black from an artists supply?) in it. Then paint (or apply with an eye dropped?) thin lines of that, parallel to the direction of flight, on the wing surface. When you land you'll see some flow patterns that are like "averaged out" over the duration of the flight.

Jack
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Old Jul 30, 2011, 04:52 PM
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Jack, that will only show flow in the boundary layer.
Some weird stuff can happen there. At CSIRO here in Oz (a long time ago) they did a lot of work on delta wings, using a method similar to yours.
They found the air in the boundary layer that started at mid-span turned in a big arc and traveled in the direction of flight near the root.
The pilot freaked a bit when he saw the flow going 'the wrong way.'
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Old Jul 30, 2011, 05:53 PM
treefinder
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And it would work best on monocote wings, where one can wipe it off, vs leave it forever on a foam wing! Actually, I noticed the reversal of le tufts on one of the vids. They laid right along the le instead of trailing back aft. I think it was on landing approach.
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Old Jul 30, 2011, 06:45 PM
Jack
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It should wipe off OK from the colored tape if it is worth doing it or meaningful. I don't use monocote.

And I didn't know about the boundary layer thing, I just saw a film of them doing that somewhere over the years.

Jack
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Old Aug 03, 2011, 04:44 PM
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I got a test flight in today. The front camera refused to go into video mode, but the rear camera was the one that I needed to work, and it did. My wife got the test on video so we have a view from the ground to match with the view from the air. The FDR collected the data as well, so as a first test, I think it worked out pretty well. I had the telemetry receiver on, but my wife couldn't look at it and video at the same time, so video won out. You can hear the sounds of the variometer in the background making noise as the plans climbed or descended.

I did high and low speed passes, a stall some high alpha, a spin, a loop, roll, and a bit of inverted flight. The tufts worked out OK. I will remove every other tuft at the top and bottom of the step. I'll stagger the top and bottom rows, so the tufts are offset. Next time I'll remember to straighten all the tufts before takeoff.

I reviewed the video a couple times, but not in great detail. What I noticed was that the tufts above the step showed the flow going downward and toward the trailing edge. The tufts at the base showed that near the base of the step the air is relatively dead, further out it flows toward the trailing edge. This is the case for most aspects of flight except stalled conditions and high alpha. This leads me to believe that there is no vortex acting here. From what I see it looks like the step creates a low pressure that draws the air flow down to the lower surface of the wing. The flow stays attached, except in a stall, or when hanging on the prop. In the latter case the flow reverses from the trailing edge up to the step. The flow at the leading edge detaches, but remains in a front to rear direction. It is as if a large low pressure area develops above the wing drawing the air from the front and rear towards it. Obviously this needs further study. It all does of course, but it is a very interesting start.

I'm going to try to make a picture-in-a-picture video, so the ground view can be compared to the view from the wing at the same instant. Then we can go through the video one frame at a time. I'll post the FDR data then.

I don't believe the FDR data will be of much use until it is compared to the data collected from other runs. I guess I will have to put together a script so I can (attempt to) perform the same maneuvers in the same sequence. The problem is trying to read the script and fly at the same time.

Roger
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Old Aug 04, 2011, 08:10 AM
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I woke up last night thinking about the flight testing. It dawned on me that we have three distinct types of flight testing that need to be done: handling, data acquisition, and flow visualization.

Handling is a subjective evaluation at best, but, given the time I've spent flying this airplane, I think I can be reasonably objective as to the change in handling qualities each wing version exhibits. I plan to try an determine if a wing version is more or less stable. That is: does it act like an aerobatic wing, or like a trainer. How well does it recover from stalls and spins, what is the roll rate like, does it respond well to aileron input, and any other difference I might notice. Anyone have an idea of something I should be on the lookout for?

Data acquisition is what the FDR is for. I realized last night, that data acquisition should be done separately from the flow visualization. This allows me to use the streamlined fuse top, and it allows my wife to read out the telemetry data to me, so I can make repeatable passes. I have two almost new batteries and a couple old ones. I plan to use the new batteries and record top speed with a couple passes immediately after takeoff. This will give the most repeatable results. After the high speed passes come the constant speed (within a few MPH) passes. The plane with the camera mount stalled at about 22 MPH, so I think 28-30 is a reasonable speed to try for. Averaging the data for multiple passes is the best we can hope for here. The last data point I think we need is stall speed. Can anyone come up with another data point that needs to be gathered?

For the flow visualization, I think we need straight and level high and low speed passes, stalls, spins?, inverted, and high alpha done in stages (as best as I can). We also need to test roll rate here when the cameras can record as accurately as possible. I plan to do rolls at full throw on low rates. That will give us identical roll inputs at a lower roll rate. We are not interested in absolute speed here, so much as the difference in roll rates between the wing types.

I'm tired from lack of sleep. If I missed anything important, please let me know.

Roger
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Old Aug 04, 2011, 09:30 AM
just Some Useless Geek
Chicagoland
Joined Oct 2008
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Roger, be on the lookout for pitch stability problems with KFm2 wings. It's the only problem I've run into with any form of the KF. My Yardarm gets completely stupid when the KF step is anywhere near the recommended height, and even the Smarty Pants shows this effect in a minor way.

The symptom is the plane will want to suddenly pitch down in level flight at higher speeds. The same plane will fly just fine at very low speeds, since the nose is pitched up higher and the wing's aspect to the apparent wind is different. I experimented quite a bit with various wings, step heights, wingtips, wing offsets, and wing aspects on the Yardarm, but didn't find anything that worked as well as the original wing with the single layer FFF step. YMMV.
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