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  #11  
Old 05-01-2014, 09:51 AM
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Mike S Mike S is offline
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Location: Dayton Airpark, NV A34
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Quote:
Originally Posted by cjhukill View Post
I fabricated several different versions of spoilers that I taped to the inboard side of the wheelpant, and test flew them, with camera plane. When I finalized the optimum size, shape and placement of the spoiler, I built the permanent one out of micro and repainted to match. Problem solved.
Any chance of getting some photos of this??
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Rv-10, N210LM.

Flying as of 12/4/2010

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  #12  
Old 05-01-2014, 10:01 AM
John Courte John Courte is offline
 
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Location: Los Angeles, CA
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Thanks for the advice, and an update:

Gear-leg fairings: I've checked, they're in trail. The intersection fairings are possibly the worst fiberglass work I've ever done, and that's saying something. Hoping the RVBits ones are still available. Hitting the brakes on takeoff is something I've started doing, and it does help, but not to the degree I thought it would.

I think my own lack of piloting experience may have contributed to the issue, because I now realize that the buffeting I was experiencing was from passing through a small layer of turbulence, which happens plenty often near the beach at that time of day, I was just hyper-vigilant and ready to attribute anything other than smooth flight to my changes to the aircraft. Subsequent flights have been different, although I still get a little pull to the left in level flight, but that's another story involving the future testing of a rudder trim tab.


The oscillation is now cured. After searching VAF with different terms, I came across a few others who have had this issue, and there were several solutions that seemed like they'd work pretty well.

One issue was that the autopilot pushrod was on the outermost hole of the servo arm. I had been having trouble with autopilot stability and overcorrection, even at settings recommended by MGL and others (These are Trio servos with a MGL Odyssey G2 EFIS). So I figured the easiest course was to reduce the amount of throw and enabling more precise correction by moving the pushrod to the innermost hole on the bracket. Since I had the wing cover off, I took the opportunity to install the bellcrank bracket, and with the AP on and set to test-mode center, I adjusted the pushrod so that neutral was neutral. I also found that the other aileron was about 1/4" out of line, so I adjusted that one as well.

This did work, sort of, but I could still start the oscillation with a tap on the stick. The AP struggled to keep it damped down, and wasn't always successful: a track intercept was a herky jerky mess, and even in straight/level flight, the slightest turbulence would set it off again.

I remember having a horse like this in military school - just when you think you have the thing bested, it does its own thing, resulting in unpleasant and unexpected motion.

The other remedies involved the squeezing of the trailing edge, similar to what to do for a heavy wing. I don't have the post in front of me right now, but whoever suggested checking to see that the control surfaces were flat or concave as they rolled off to the trailing edge, thank you.

I checked the ailerons (QB, factory built) and didn't notice anything until I put a straight edge on them and there was definitely some bulge. Not much, barely enough to get daylight under the straight edge, but given the sensitivity of flight characteristics to the TE-squeezing process, I figured that would be enough to cause a problem.

Since I didn't have a heavy wing, it was necessary to squeeze both edges, just barely, not enough to notice with the naked eye: I measured the curvature afterwards with the straight edge, verified I was flat or concave, then just for kicks, checked the trailing edges of the elevators and trim tab. The elevators were OK, but the trim tab was a mess, so I took care of that, just because.

Problem solved! Tapping the stick results in a single bounce to the other direction, then settling back down to the trimmed position. The autopilot can now intercept a heading with no weirdness, although it's rather insistent and urgent about it: I need to adjust the track intercept force setting on the EFIS, which will come when I do the operating system update.
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  #13  
Old 05-01-2014, 02:41 PM
Jim F Jim F is offline
 
Join Date: Dec 2008
Location: Canby OR
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Default Wheel pants...

John
Sounds like you got the problem solved, but just for grins let me check.
You did align the wheel pants and gearleg fairings with the plane jacked up so there was no weight on the wheels right???

Just checking...
Jim Frisbie
RV-9A 350 hrs
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  #14  
Old 05-01-2014, 05:20 PM
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rv7boy rv7boy is offline
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An even more elegant method than aligning the fairings and pants with the plane is to align them with the AIRFLOW.

how? There are several methods but the one I always liked was used by Dave Anders on his screaming RV-4. He made test flights with a few dots of melted crayons along the stagnation line on the fairings. When the streams of flowing crayon came together on the trailing side of the fairings, he knew he had the fairings in the right position.

I'm not sure if he found any difference between cruise and top speed positions. He told me at Oshkosh you cant just set the fairings and wheel pants straight with the fuselage if you want to eek out every knot from your RV. You have to allow for the corkscrew effect from the prop. I confess I haven't tried it yet but everything Dave Anders did to his RV-4 seemed to work.
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  #15  
Old 05-01-2014, 06:03 PM
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Quote:
Originally Posted by crabandy View Post

When I added the gearleg/transition fairings I knew things were not right shortly after takeoff. I needed way more (about 3 times normal) once airborne.
I'm curious - 3 times more what? Rudder? Enquiring minds want to know.
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  #16  
Old 05-02-2014, 08:46 AM
John Courte John Courte is offline
 
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Aligning with the airflow is a really good idea, but I think I should probably do that after I've sorted out the rudder trim. The pull to the left was happening before the wheel pants and fairings went on, so I'm pretty sure I need to fix that before I do much to the undercarriage.

However, I'm curious about this crayon method. Was this an in-flight test? How did he keep the crayon melted enough to indicate?

Jim F, the spats/fairings were aligned with the aircraft hanging by the engine mount from the tines of a surplus forklift (my hangar mate uses it for everything he can). There was no weight on the wheels at all.

Quote:
Originally Posted by rv7boy View Post
An even more elegant method than aligning the fairings and pants with the plane is to align them with the AIRFLOW.

how? There are several methods but the one I always liked was used by Dave Anders on his screaming RV-4. He made test flights with a few dots of melted crayons along the stagnation line on the fairings. When the streams of flowing crayon came together on the trailing side of the fairings, he knew he had the fairings in the right position.

I'm not sure if he found any difference between cruise and top speed positions. He told me at Oshkosh you cant just set the fairings and wheel pants straight with the fuselage if you want to eek out every knot from your RV. You have to allow for the corkscrew effect from the prop. I confess I haven't tried it yet but everything Dave Anders did to his RV-4 seemed to work.
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  #17  
Old 05-05-2014, 02:25 PM
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rv7boy rv7boy is offline
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Default Dave Anders's method of aligning landing gear fairings

Quote:
Originally Posted by John Courte View Post
Aligning with the airflow is a really good idea, but I think I should probably do that after I've sorted out the rudder trim. The pull to the left was happening before the wheel pants and fairings went on, so I'm pretty sure I need to fix that before I do much to the undercarriage.

However, I'm curious about this crayon method. Was this an in-flight test? How did he keep the crayon melted enough to indicate?
<SNIP>
You know, looking over the list of things Dave Anders did to his RV-4 to get it up to 260 mph, I probably spoke out of turn. Because the fairing alignment item is but one little item on a whole list of things he did to maximize speed.

I looked for a site that had photos or a diagram, but as I recall what I read, he mixed some coloring crayon, (color doesn't matter) with a thinner that was safe on his paint, just enough to make the consistency enough to run (think of honey's viscosity). He then placed three dots of gooey crayon on the leading edge of each fairing. Then he'd go fly, real fast, and land. By looking at how the gooey crayon ran back to the trailing edge, he could get an idea of how to adjust his fairings.

The goal was to get each gooey crayon dot to split into two streams which each trailed back and met at the trailing edge. It's not a perfect method, but a compromise design method, (we used to talk about the "empirical method" on SSME's). And since he was obviously concerned with top speed, he did this at top speed. Cruising speed may give you a different configuration.

I think he had to make three or flights to get both sides adjusted. And I think he may have tried a similar method on his wheel fairings, but I'm not sure.

All in all, it probably doesn't make too much difference for us average pilots, but since I brought it up, I felt I owed you an explanation as I remember it. I still think it's an "elegant" method of aligning the fairings.
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?Certainly, travel is more than the seeing of sights;
it is a change that goes on, deep and permanent, in the ideas of living." Miriam Beard
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  #18  
Old 05-06-2014, 10:27 AM
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LettersFromFlyoverCountry LettersFromFlyoverCountry is offline
 
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Is paint thinner safe for paint?
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  #19  
Old 05-06-2014, 10:59 AM
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Bob, I didn't say "paint thinner." I said "a thinner." The word "thinner" here is a generic term. I remember seeing the article but have been unsuccessful in finding it. Notice I added the modifying phrase "thinner that was safe on his paint" in my description. Whatever he used (it may have been rubbing alcohol), it was both a thinner for the crayon and safe to use on his paint. [Perhaps the word "solvent" should have been used instead of "thinner" to describe a liquid that would dissolve a coloring crayon.]


If I had had any thought that Doug was going to plaster this on his front page today, I would have researched a little harder to see if I could find that article.
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N79599/ADS-B In and Out...and I like it!

?Certainly, travel is more than the seeing of sights;
it is a change that goes on, deep and permanent, in the ideas of living." Miriam Beard

Last edited by rv7boy : 05-07-2014 at 03:11 PM.
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