stability question

[hevansrv7a]

Before I get to the question, a fact that may or may not be relevant. I tweaked the angle of my HS so that it is trim-neutral at a slightly higher speed - around 110 KIAS. The purpose was to reduce trim drag at higher speeds to see if that would let the airplane go faster. It did not. But, it's also my best climb speed and it makes the pilot workload effortless at a busy time in the flight so I left it that way. I can undo the change by removing the shims. Easy.

Now for the issue:

As some of you may already know, I experiment a lot with what are probably new techniques for understanding the drag curve in a powered aircraft without either using the Norris-Bauer zero thrust device or going to extremes like towing the airplane sans prop. The results to date of that work can be found on my web site in the Oshkosh presentations of 2010 and 2011.

In the course of that experimentation I've tried to set the trim for climb and add or subtract a known amount of power via either rpm, fuel flow or indicated percent power (GRT). The airplane does not cooperate. It oscillates. Translation - the nose gently moves up and down enough to ruin the test (for climb or sink rates). I can hand-fly it, but I'm looking for how the airplane behaves without being forced or corrected, so that's not a solution.

Note, though, that the airplane is stable in level flight.

Has anyone else observed this in their RV (either the 7 series or any other)? Any ideas besides the obvious one of trying it without the shims?

Thanks.

[F1Boss]

We found a similar instability problem with SOME of the F1s. Turns out the elev at neutral has some slight stalling happening on either the upper or lower sfc, which allows the elev to deflect the other direction, which then causes the stall to move to the other sfc, which causes the sfc to move the opposite direction....an oscillation develops.

Our was happening at 200KIAS +, so the effect that the pilot felt was a bit more dramatic. The 'fix' in our case was to strengthen the H Stab, which fixed the problem; changing the incidence had very little effect. One of our customers made up folded TE elevs, following Vans RV4 design. This particular ship has not yet flown.

I doubt your particular problem is H Stab flexing at 110KIAS. My guess is that the H Stab sfc is not true to the design, or maybe the fwd area of the elevs, and this is causing a flow separation at exactly neutral elev.

Fix:
Some A/C built in the 30s/40s, and maybe later, used a thicker section at the elevs & rudders (about +10%) to eliminate the tendency of the sfcs to 'hunt' due to stab/fin section shape or sfc irregularities, which is what I suspect is happening to your ship. The Pitts type ailerons follow this design theory, and function quite well, but the design is nothing new. Unfortunately, this fix is expensive in your case.

Put 'er back to design specs, where the elevs are slightly loaded in climb, and carry on!

Mark

[Dogcowdaddy]

Out of curiosity, what is the period of the oscillations you encounter? Also is the longitudinal control moving around during the oscillations or remaining fixed.

Cheers

Jim

[ferret]

Its normal. It called a phugoid frequency.

[Mike S]

Jim, welcome to VAF

With a login name like "Dogcowdaddy", there must be a story behind it.

We need to hear that.

[Bob Axsom]

Originally Posted by hevansrv7a

Before I get to the question, a fact that may or may not be relevant. I tweaked the angle of my HS so that it is trim-neutral at a slightly higher speed - around 110 KIAS. The purpose was to reduce trim drag at higher speeds to see if that would let the airplane go faster. It did not.

...

Thank you very much!

Bob Axsom

[AlexPeterson]

Hmmm, more info please. My 6A has, in certain situations, two stable elevator positions. Meaning, I can move the stick slightly forward, and it will stay. Then a slight pull aft and I can find another aerodynamically stable (the elevator, that is) position. They are very close to one another, and the resulting pitch change is almost not noticeable. Very subtle, and not anything other than a curiosity. Probably something with geometry at the trailing edge of the elevator. Mine does not oscillate though.

[hevansrv7a]

The oscillation is several seconds up, then several more down. I never timed it. It's not unlike the way a C-150 behaves when you are trimmed out and then, on purpose, bump the pitch control and let it recover. Except it doesn't stop. It's not rapid or scary, but it makes some aspect of testing bad to impossible.

For example, if I am testing sink rate by reducing power by 1 gph, it will go to zero sink as it slows then go through the neutral point and keep on reversing. When I conceived of this test I wanted the sink rate to be in proportion to the power reduction and I keep the wings on autopilot during the experiment, leaving stick and trim alone. If I do a test with added power, I get pretty much the same behavior.

It flys OK level; it only behaves like this when there is a change and it is so gentle that I would not have noticed it without having tried this experiment. My normal climb out is rock solid at 110 kias, but I usually have my hand on the stick during that, so the issue would go undetected.

I don't know if it did it before I shimmed the HS. I guess I will find out by removing the shims.

Except for the shims which came later, the airplane was built from a QB kit without modification. Exception: SJ cowl, plenum and all three wheel fairings. FWIW, the prop is a Catto 3-blade.

I have heard of phugoids, but my C-150 did not, as far as I know, do this. Maybe it is normal for an RV, but that's what I'm trying to find out.

Thanks for all the input so far..

[simpkinsona]

My 9a does this and so does my dads Cherokee 180. I don't know what causes it or how to prevent it (other than using stick pressure).

-Andy

[scsmith]

There are a couple of different things going on in the various follow-up posts here. All are interesting and worth comment.

First, for the Original Question: If the period of oscillation is long (many seconds) it is most likely a Phugoid mode. It is hard to change stick-free phugoid damping. I do not think your tail incidence change will have an effect, and doubtful that it caused it.
Bottom line: you will need to hand-fly the test points, or get an autopilot to do it.
I think cleaner airplanes tend to have less Phugoid damping. My sailplanes have always had unstable phugoid modes.

What may be affected by your h-tail incidence change is the stick-force-gradient, the change in stick pressure as you change speed above or below the trim speed. This will be most noticeable at aft c.g. and slow speed - you might find a stick-force reversal where you need forward pressure to keep the airplane from slowing down more below the trim point.
And I'm glad you have validated my assertion that there is very little drag associated with the elevators not trimming perfectly in trail. In the case with no balance horns sticking up, it would be extremely little drag from the slight elevator deflection at trim. If your balance horns are nicely rounded, then there's just not much drag there.

The vague stick feel, or two stable stick positions very close together, are caused by the rounded trailing edge fold. Pinch them together a tiny bit. But be careful - if you make them too sharp, you will notice more stick force gradient.