Elevator Slop?

For some reason, they say that a tiny bit of "play" in the elevator linkage seems to help a "hunting" problem, which I now have. The model is a profile flapped stunter with OS40. There is no elevator play because I'm using a ball clevis back there. Bellcrank is no longer available. The flap horn is available and using a wire with 90 deg bend. So flap "play" is possible to install.

Would some flap "slop" be as good as elevator "slop"?

Floyd,

I don't think so. Try putting the Ball link on the flap horn and use a pushrod with some adjustment. Then put a little down into the elevator adjustment. I'm not sure if that will help your situation but it might be worth a try?

SNIP>>>>Would some flap "slop" be as good as elevator "slop"?Floyd

Never have FLAP Slop. Slight SLOP on the Elevator is the way to go.
Hunting is one problem that is hard to remove from the plane.
Down Thrust on Engine, Nose Weight, CLEAN LINES, Move Lead outs back are all things you can try.

Scott

>Hunting is one problem that is hard to remove from the
>plane.
>Down Thrust on Engine, Nose Weight, CLEAN LINES, Move Lead
>outs back are all things you can try.

How about 0 incedance? Thrust,wing,stab

Floyd,

I agree, droop the elevator a bit (couple of degrees) may help with the hunting problem. Worked for me.

Floyd,

You've got an alignment problem. Find misalignment and fix it.

Hope to see you and perfectly aligned model at NW Skyraiders Roundup, Labor Day weekend.

Not sure what to try first! The scientific method is to try only one thing at a time. But that takes a lot of time.

\Dan: I will be at the NW Skyraiders in Sept. You will beat me, of course, But I'm used to it. I'll bring my best plane, but your best plane is better than my best plane (?)

Confessing here: The problem model is a Fancher DOCTOR. It flew pretty well without flaps, but I wanted a better turn. I didn't fly it enough without flaps to see if it had a "hunting" problem.

Without NASA-style equipment, it is hard to tell if everything is aligned. I was hoping that some quick-fix would work. It is too late to change alignment problems except for a turn or two on the control rods.

Floyd,

Might also try sealing the hinge lines. This can also help with a hunting problem. Air leakage between the flaps and wing or stab and elevator can do some weird stuff.

LAST EDITED ON Aug-17-04 AT 08:19 PM (CDT)
 
In my experience, sealing the hinge lines makes the flaps/elevators MORE sensitive, due to the control surfaces being more efficient. With that said, however, the hunting problem on my current ship was totally caused by the elevator/flap linkage being out of adjustment, plus needing the CG moved forward. Absolutely no hunting now. I still have a couple of other small issues, but not hunting. It varied from some hunting to really bad, depending on where the linkage and CG were set. I am using ball links, so there is no slop at all. Hinge lines on both flaps and elevator are sealed. I do have a slight amount of positive incidence in the stab, and at first, I thought I had overdone it. Admittedly, this does not equate to a non-flapped setup.

>Not sure what to try first! The scientific method is to try
>only one thing at a time. But that takes a lot of time.
>
>\Dan: I will be at the NW Skyraiders in Sept. You will beat
>me, of course, But I'm used to it. I'll bring my best plane,
>but your best plane is better than my best plane (?)
>
>Confessing here: The problem model is a Fancher DOCTOR. It
>flew pretty well without flaps, but I wanted a better turn.
>I didn't fly it enough without flaps to see if it had a
>"hunting" problem.
>
>Without NASA-style equipment, it is hard to tell if
>everything is aligned. I was hoping that some quick-fix
>would work. It is too late to change alignment problems
>except for a turn or two on the control rods.
>
>Floyd

You might want to make sure both flaps and stab are 0 at the same tme

Hi

Another thing to consider is dynamically ballancing the control surfaces. In unbalanced elevators, any upward acceleration of
the tail causes some down trim due to enertia. That adds to
the upward acceleration of the tail. It's a bit like feedback.
If you extend a counter weight ahead of the hinge line, so the
control surface ballances on or in front of the hinge line,
this flutter effect is eliminated.

I suspect flutter can contribute to hunting though it's probably
not the primary cause.

A tad of down thrust has been known to work wonders!

BIGS

" What constitutes the dignity of a craft is that it creates a fellowship, that it binds men together and fashions for them a common language. For there is but one problem - the problem of human relations."

Didn't Phil Ganderson mention increasing elevator throw to stop hunting on one of his planes?

Floyd,

Elevator slop used to be vey popular for models with a tendency to 'hunt' in level flight. That was in the day of small % tails...

With modern large horizontal tail area layouts, we don't need slack in the linkages for level stability, tracking, or recoveries.

The baddest thing about built-in slop is that it increases itself with airtime on the model. The excess clearance needed, somewhere, hammers to its limits, and grows larger.

Dave Fitz, Brett and a few notable others have found a few degrees down elevator at neutral handle and flap seems to be a better solution to 'same feel, both ways' than slop ever dreamed of being. Some of those threads may be archived here in SSW...

Nobody mentioned the obvious cures - narrower handle spacing and farther forward cg.

Hunting is a function of an underdamped feedback loop. That is, the model goes up, the stationary handle automatically applies down, and the reverse as the model now overshoots to below the handle. This can be overdamped, neutral, or underdamped. With the cg back and the spacing wide, this is underdamped and dynamically unstable (I can probably dig up my thesis from MIT and show you, but trust me).

Usually, you compensate for a slightly underdamped model with your own mental feedback to keep the oscillations slight. The less stability, the faster the model will turn, so we do tend to keep them on the razors edge.

Better dampening (longer tail moment, more tail area), less feedback (narrower handle, or more flap to elevator ratio), and forward cg will all help cure the problem.

If you want to see how well damped your plane is, just fly level, and lock your handle to your chest with both hands. If it tracks true, it is at least neutral. In all probability, it will start to oscillate, and you will have to unlock your hand to save it.

Now, many planes are stable near neutral, but if you hit a large gust and get the oscillation going, then it becomes unstable.

What slop in the elevator horn does, is eliminates the feedback loop for small oscillations. The elevator gets no handle feedback until the model has moved some distance from neutral, and it's inherant stability is enough to control it. (CL models are normally stable until the control feedback is added)

I hope this clarifies the possibilities for a cure.

Thanks to all for the suggestions. And there are many! I think I mentioned that I have flown this Fancher DOCTOR a lot without flaps. It didn't show any pitch instability without flaps. Your comments have caused me to pursue this problem until it is cured! First, the easiest fix: I have modified the ball link at the elev. horn to provide a little bit of play in the linkage. Will fly it again Wed. evening and report back.

Engine down thrust, stab incidence alignment are now difficult to implement without tearing the plane apart.

My apologies to Ted for screwing up his design! But this is only an experiment!

Right, Larry...

I have seen your thesis, years back. Nice work.

The 'compensations' we (often) can succeed with are practically in the muscle memory range -- like the miniscule balance shifts we make on a bicycle, or even walking! (Wanna fall on your face? Try walking by thinking out your next motion BEFORE you commit to it <g>)

Where the heck did you see that? Did I send it to you?

>Would some flap "slop" be as good as elevator "slop"?
>
>Floyd

Ron B.
F4Fguy

Floyd:

NO!