Divergent Oscillation - Educate me

[John Courte]

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?

[Mike S]

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.

[John Courte]

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.

[Toobuilder]

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.

[crabandy]

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.

[John Courte]

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?

[BenNabors]

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/mechanica...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.

[robertahegy]

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

[Joker]

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.

[cjhukill]

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.