Interesting pitch oscillation

[MCarlton]

Flew the funfly again today, trimmed out nicely, no real issues at all, but I did notice on the first flight that at times, a small pitch oscillation occurred in flight, which I don't think was me (it could have been I suppose), but it looked more like I had flown through turbulent air, but there was no roll or yaw oscillation.

CG too far aft? But the model didn't feel like that, it was pretty solid

Interference? But then surely ALL channels would be affected?

Dodgy Servo? Checked out on the ground ok, no slop, second flight with no oscillation

Too much movement around neutral? maybe I was twitching the stick, perhaps a touch of expo would help?

Any other thoughts on what could produce pitch oscillation?

[HerkS]

It could be from having the CG too far aft, but usually it's a control setup that doesn't consistently come back to the same neutral position. Friction in the linkage, slop in the servo or slop in the linkage can all cause this.

[vintage1]

Turbulence. Or flutter. If you meet a nice rolling flow head on, there isn't much roll or yaw..autopilot apart, what airliner get tossed SIDEWAYS in bumpy air. It just feels like riding bumps..oddly enough..:D

I had a speed 400 model once with a massive Permax turbo450 in it..steady as a rock in calm air, but a high speed pass would show it jumping up and down several inches in wind..and also wagging its tail as it did so..I never puzzled that out till someone else was talking about gyroscopic precession, and I realised that the massive spinning armature was kicking the tail sideways as the nose went up and down.....

Anyway, its my experience that ground level turbulence in reasonably open spaces is like bales of hay rolling down a slope, Little tubes of air with up on one side, and down on the other. If you fly into wind, its like meeting a wave head on. No roll at all. Crosswind is another matter.



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[Work in Progress]

Not flutter. You can always hear that unmistakable buzz, and usually see the parts blurring if the model is close by.

What frequency was the oscillation? Did it tend to damp out or diverge? What kind of pitch angles was it producing? Was there. in fact, any wind or thermal activity?

[MCarlton]

I think it must have been turbulence, there was nothing to indicate flutter, and it wasn't a fast enough oscillation.

If you fly into wind, its like meeting a wave head on

I think you might have it there, I flew again this morning (early early) and it didn't happen, so I am assuming that it was turbulence, the area I fly is a pretty large microlight airfield, which is fairly open and flat for a few hundred metres in each direction.

It had been a warm day, so I assume there may have been some convection turbulence as well as the light wind.

What frequency was the oscillation? Did it tend to damp out or diverge? What kind of pitch angles was it producing? Was there. in fact, any wind or thermal activity?

I would say each oscillation went from 10 degrees positive to 10 degrees negative, about twice in a second. It didn't diverge certainly, and seemed to damp out, which makes me think the CG is probably about right.

Wind was about 5mph, but possibly quite thermic as it had been a warm day, lots on sunshine onto a flat grass field.

[vintage1]

MCarlton checks aerodynamics in highly unstable conditions..

[MCarlton]

Well, you know, I like to do things right....

Oddly though, its not something I've ever had happen when slope soaring, but maybe thats something to do with a more slippery aircraft moving faster?

Thats a rather brilliant picture there V1, I can almost imagine the chap thinking "oh lawks, now that doesn't look quite right"

[vintage1]

Actually I nicked it off the Financial times..interesting article on where military spending is being spent..many more UAV's, many less Raptors etc..

It struck me it was a Pushy Cat more or less :D

[MCarlton]

With the proliferation of UAV's, I wonder if us RC types are vulnerable for national service? :d

Its not far off a PC is it, I wonder if it shares some of the same issues....

[peterangus]

Any other thoughts on what could produce pitch oscillation?

Decades ago, I flew a FF scale model which had a very steady pitch oscillation on the glide.

Perhaps it was shedding vortices from the bluff nose of its fat fuselage.

http://www.peterangus.pwp.blueyonder.co.uk/1949.htm

[MCarlton]

Interesting peter, you could be right, which makes me think perhaps I had something of a similar ilk going on, as the model was banked at about 60 degrees at the time, with the nose high, so I guess its possible that there was some blanking and/or strange vortices being generated. I don't know, not being an expert :)

[Cap_n_Dave]

Decades ago, I flew a FF scale model which had a very steady pitch oscillation on the glide.

Perhaps it was shedding vortices from the bluff nose of its fat fuselage.

http://www.peterangus.pwp.blueyonder.co.uk/1949.htm


Phugoid?

[Al M]

A pitch oscillation in a glider is ususlly a sign of a cg that is too far forward.