What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Divergent Oscillation - Educate me

John Courte

Well Known Member
I had a real test-pilot moment yesterday.

After putting my wheel pants on, I took the plane up to see what those extra knots would be like. I noticed a bit of buffeting after takeoff, during climbout but it wasn't bad. I couldn't really tell if it was buffeting or light turbulence, and it went away after a bit. It repeated itself on subsequent takeoffs.

Once I got up to 5k or so, I leveled off with the throttle opened up and my RPMs around 2300. Trimmed up for level flight and let go of the stick to see if it would fly hands off.

That's when it got interesting. Slight rocking motion, building up slowly from almost nothing to what could have been a real lunch-tosser. Before it got too crazy, I grabbed the stick and it settled down. This behavior is 100% repeatable.

The RV7 is supposedly a neutrally stable aircraft, and this (before the wheel pants and gear leg fairings) proved out in earlier flights. There are any number of things it could be, but the obvious culprit is misaligned gear leg fairnings.

What about the ailerons being out of rig? If they're both pointed slightly downward, would that counteract what little roll stability the RV7 has?
 
Seeing as the only change you made to the airframe before a change in flight characteristics was the pants/fairings that puts them at the top of the list as to the cause of the issue.

Wheel pants only, or gear leg fairings, and top and bottom fairings on the gear legs also???

Sounds like you may have an alignment issue with one or both of them. Or, something is not firmly attached??

The fairings can act like control surfaces and cause a lot of strange behavior if they are not accurately installed, and properly fixed in place.
 
I installed pants, leg fairings, and upper/lower intersection fairings.

Is it OK to fly without the whole set? That would make testing a bit easier.
 
You might also hang a camera under the ship so you can see what they do. I did the same thing on some wheel pants I designed - ran it through all the speeds, landings, etc.
 
I had an issue with my gear leg fairings as well.

I flew it first with just the wheelpants and all was well.

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. The airplane also yawed to the left with a power reduction, it felt as if it was being steered from in front of the wings.

I used the string-line method for alignment, obvious fail. I borrowed a friends upper intersection fairings (mine were homemade, his were commercially made) and when I fit them to my plane it showed the trailing edge of the right gear leg was about 1/4 inch too high.

I re-aligned the right fairing and then test flew with the gear leg fairings and wheelpants but no transition fairings. It flew great. I was worried about flying without the transition fairings but mine were stuck in place with just the hose clamp.
 
Here's a question: How tight should the roll trim springs be? Could the spring tension be at the magic point of causing an oscillation in certain autopilot modes or hands off flight?
 
This may boarder on a answer that is not an answer but since the post heading was "educate me," I thought I would look for a good reference. As a result I found something that was out on the web that I thought was a pretty cool, for a nerd (me).
Look at http://ocw.mit.edu/courses/mechanical-engineering/2-003j-dynamics-and-control-i-spring-2007/

Look at lesson 20 & 21 and you can download the course content from MIT.
The system works as a mass, spring, and a damper.
I can try to boil it down to easier to understand, but do not have right now.
 
The roll trim springs need only be slightly in tension when the lever is in neutral. Just enough to keep them from sagging. The active spring will tension as the lever is moved to envoke some trim. I made the mistake of tensioning them too much on my 7A during the build and quickly realized they do not need much tension to function properly. You need to use a few wraps of safety wire to lengthen to get a proper fit. I am assuming you have manual aileron trim.

Roberta:)
 
The gear leg fairings seem pretty well addressed here. As far as the buffeting on take off, it could very well be your wheels spinning and slightly out of balance. Try hitting your brakes shortly after lift off.
 
Wheel pants "Flying"

I had a similar event with my RV8. I had a friend with an RV4 whom we did a lot of formation flying together. He flew just below me with an observer in the backseat filming my undercarriage. What the viewing of this video showed was my left landing gear was oscillating inboard, outboard ever so slightly. So my suspect was gear leg and/or wheel pant misalignment. I jacked up the airplane and checked alignment, and it was PERFECT. I flew again with the RV4 photo ship, and the same phenomenon was still there. During debrief we decided that what was happening was the swirling prop-wash was striking the inboard side of the left wheelpant which is an AIRFOIL shape! The increased airflow was causing a low pressure area on the inboard side of the pant, which caused the left gear leg to "fly" inboard, ever so slightly, then spring back out. This process varied with intensity, as speed and power were changed. 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.
 
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??
 
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.
 
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
 
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.:)
 
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.
 
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.

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.:)
 
Dave Anders's method of aligning landing gear fairings

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. :)
 
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. :eek:
 
Last edited:
Back
Top