[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]

neutral trim instability

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]

Welcome to VAF!!!!

Jim, welcome to VAF

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

We need to hear that.

 

[Bob Axsom]

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]

follow ups

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]

A couple of things

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.

 

[Dogcowdaddy]

Definitely sounds like the Phugoid. It's how the airplane returns to equilibrium once it is disturbed from a trim condition. It happens in any stable airplane. The period of the oscillation is typically .25*trim airspeed (in knots), and it is very lightly damped (the damping ratio is .707/(Lift/Drag ratio) which means that it will take several complete oscillations to finally damp out. When you have the airplane in trim and then make a power change without touching anything your have altered the energy balance and the Phugoid gets going to sort out the balance. If you want to check it out the easiest way to do it is to trim the airplane and then slow it down say 10 knots from the trim airspeed and stabilize it there (without retrimming). Then slowly release the control back to trim and watch the airspeed. It should speed up past the trim airspeed and then slow down below the trim airspeed with the excursions from the trim airspeed getting smaller and smaller until (if you wait around long enough) it will settle back at the trim airspeed. If you have an angle of attack gauge it is interesting to note that the angle of attack is constant during this whole thing, even though the pitch attitude is changing markedly.

As far as the aircraft with two apparent airspeeds for the same trim, I suspect what you are seeing is the effects of friction in your control system. In a stable airplane, each tail position corresponds to a single angle of attack, and thus airspeed (at a given Gross Weight, power setting...) when you move the stick from trim and then let it come back, friction keeps it from returning to exactly to the original trim position, resulting in a new trim angle of attack, and airspeed. This effect produces what is technically referred to as a trim speed band.

Thanks for the welcome. The dogcowdaddy thing would take a few beers to explain. If you are going to be at Oskosh I could meet you at the beer tent. First round on me.

FWIW

Jim

 

[49clipper]

Phugoids

Interesting. I have never been able to trim out my -6 during a climb or descent. I can only get a phugoid going and I never can wait long enough to see if it would trim out stable. I can get it close to holding altitude in level flight but it takes a while. Climb or descent is another story.

My other problem is my trim tab at cruise is up 29 degrees from in trail position. (looks like a spoiler) Almost all the nose down trim I have. The manual says anything over 10 degrees up and I should shim the HS. But which way? I thought Up, but when I tried that it only made the plane fly slightly nose down attitude in cruise. Hevans6A, which way did you go and how much shim?

 

[hevansrv7a]

More

..snip..Hevans6A, which way did you go and how much shim?

As DiNiro said to the mirror, are you talking to me? (joke). But to answer the question: The purpose of the HS is to push up or down to make the wing's angle of attack correct for what the pilot is trying to do. If you are trying for very fast, level flight, it has to push up because that lowers the nose which would otherwise be trying to go up simply because higher speed means more wing lift and you only want enough lift to equal weight. So if you are trying to have the HS optimal for a given speed and that is, in this case, higher speed, you want to get a higher, positive angle of attack. That's why I shimmed the front spar by either 1/8 or .063 (don't recall which) and it found its neutral point at a slightly higher IAS. If you are going to try this, it would make sense to start small. I was a little worried about making the airplane uncontrollable, but that turned out not to be an problem.

There is no issue with inappropriate slop nor friction in the pitch control mechanism. My autopilot is only an altitude holder, so that's no help. I do have angle of attack (LRI) but by the time I get to 110 KIAS, it's past the right side max deflection. Much of my "testing" is at that speed or higher. BTW, months before either GRT or Dynon added calculated AOA, I pointed out in a post on this forum that it could be done. I don't know if I caused anything, but I'm an AOA fan.

I am very grateful and impressed with the quality of the responses to the original post. RV'ers are a wonderful bunch and I'm happy to count myself among you. I hope to meet some of you, again, at OSH, espcially the beer tent as I'll be staying at that little camp ground near it.
--------------------------------------------------------------------------------
If you are wondering why I even found this, let alone care, here's the story. For background, see my presentations at Airventure 2010 and 2011, but especially 2010. It's about methods of finding your airplane's true drag curve (for the 1% who care). This is about only one of the methods presented.

In a nutshell then: If you are cruising in perfect trim and level and know your power setting (not necessarily your actual power) or, as a substitute, your fuel flow at a known mixture, then you are almost ready to find your THP at that speed. Here's an example, using the CAFE RV6A: 6000' density altitude, 150 mph TAS, best power mixture. Your GPH should be 6.45. If you reduce your fuel flow 10% and tweak for best power again, maintaining 150 mph, you will observe a sink rate. That sink rate should be 122 feet per minute because we know that the THP is 61.0. If we did not know the THP (and for our own airplanes, we don't) then we can compute it from the sink rate. To get this right, we need a steady-state sink rate and airspeed at the reduced power setting. That's where I have the problem.

If you want to experiment with this on your computer, download my triangle spreadsheet from my website. I would be very pleased to have some results from the field!

An additional benefit is that if you are willing to make a reasonable assumption about SFC (.50 for best power for example, leading to a BHP estimate) then you can arrive at a good estimate of prop efficiency, too.

The major areas where this approach is not perfect include mixture (SFC) and prop efficiency. That's why I recommend 10% as being a change that's large enough to measure and small enough to minimize SFC and prop efficiency changes.