Plane that "hunts"

[Tom Taylor]

I have a question about why a plane hunts.  I have a new to me plane that when I try to fly it level it keeps starting to go up or down.  When it starts to go up I ease the handle down then the plane goes down, and so on.  What causes it to not fly level?  Thanks for any help with this.

[Dick Pacini]

Well, from what I remember from years ago, some of Jack Sheeks' advice:  A tiny bit of play between the elevators and flaps can alleviate a slight pushrod length error that may cause a plane to hunt.  The second thing is a leading edge that is too sharp.  I would check alignment and tweak horns if required.  Maybe increase nose weight a tad.

[Bryan Higgins]

Stiff Controls.  They need to be free and smooth.  My two cents....

[Tom Taylor]

Thank you for the replies.  The plane is one that was given to my as the previous owner no longer flies.  It has a 4" bellcrank.  The wings are foam that I believe were similar to the Genesis configuration.  I have been flying on 63' lines with a Super Tigre 60 with a 13/5 prop. Line spacing is about 1 1/2 to 2 inches. 

[Randy Powell]

Might check the alignment of the elevators to the flaps. If handle is at neutral and the plane has a tendency to drift up or down, on cause is the elevator is slightly down or up. Also, air leakage between the fixed surfaces and the control surfaces can cause a plane to hunt. It's why we seal hingelines.

[Howard Rush]

This reminds me of my grandfather telling me of cures for warts.

As Bryan said, control system friction would be the first thing to look for. If the controls are really free, next seal the hinge lines (without introducing control friction).  Those two are prerequisites to any trimming, and friction will cause the airplane to hunt for sure.  The next step is stabilizer or elevator configuration.  A sharp stabilizer leading edge usually helps prevent hunting.  If the stabilizer is mounted above the wing centerline, it should probably have some positive incidence (stab LE up) relative to the wing.  Dave Fitzgerald (but what does he know?) concluded this after a bunch of experimenting a few years ago.  This seemed mysterious to me until I saw some CFD data from Igor Burger that suggested that the laminar-turbulent transition point on the stab can move back and forth with slight lift changes in level flight, causing the tail lift to change.    The incidence and sharp stabilizer LE probably make the transition stay put at the LE.  There are other stab-elevator arrangements that prevent hunting: the Impact, for example, seems to violate these rules, but I think it controls the transition by different means.  If the airplane has flaps, you can fiddle with the relative position of flaps and elevators by changing pushrod length. 

Moving the CG forward will cure hunting, kinda like lung cancer cures smoking.  If the airplane is hunting with the CG in the design range for the airplane, there's some nonlinearity that's causing the hunting.  You could mask it by moving the CG forward at the expense of having a crummy-flying stunt plane.  I think you'd be happier finding and fixing the nonlinearity.  I'm assuming this is a stunt plane, though.  If you only fly level, moving the CG forward will do the trick, although it may introduce another form of hunting in the wind.

However, do as I say, not as I do. Lately I'm having trouble flying my own plane level.

[Will Hinton]

I had this problem on my Crosswind # 1 and Crist Rigotti told me to add .010 shims under the forward motor mount screws to give it a hair of downthrust.  It cured the hunt without moving the cg, which was a real plus as the ship tracked beautifully in the manuevors so I didn't want to change the cg if possible.  I have since had the same problem with a different ship and cured it the same way.
It's worth a try as it's a very simple solution if it works in this airplane.
Will

[John Miller]

Ya beat me to it Will. Down thrust, just a little, like about 1 degree will do wonders.

[Alan Hahn]

And I was going to suggest buying that plane a bird dog!

As a WAG, a little downthrust probably provides a similar correction as a positive stab incidence--or adjusting the pushrod from the elevator to the flap to give a little down. But a shim is a lot easier.

I have also had a crummy handle adjustment do something similar. I think my hand wanted to be one way, but my head thought drilling the plane into the ground wasn't a good idea. So the head and hand were fighting. In that case a small handle adjustment fixed it. It wasn't way off, but just enough to cause the problem for me.

[Randy Powell]

Engine down thrust and elevator "down" (or positive incidence in the stab) are the two most common ways to fix this after hinge lines have been sealed. But, I still have a classic Simons Shoestring hanging on the wall that hunts around like a drunken sailor and nothing has worked to fix the problem. Something's out of whack, but I have yet to find it.  Which is why it's hanging on the wall.

[Bob Reeves]

I've became a complete believer in engine down thrust and stab positive incidence. Best flying ship I have has Legacy numbers with 1 deg down, 2 deg right and 1.7 deg positive stab. The 1.7 was a goof as I intended it to be a little less than 1 (little miss calculation) but it has worked out great.

[John Sunderland]

Me too. And if you also believe in aft CG, this allows you push it a little further without fear of hunting.

[Bob Reeves]

Me too. And if you also believe in aft CG, this allows you push it a little further without fear of hunting.

And miss the ground at the bottom of the verticals with the wind blowing 25+ MPH.

[Steve Helmick]

"I have been flying on 63' lines with a Super Tigre 60 with a 13/5 prop. Line spacing is about 1 1/2 to 2 inches."

If this means handle spacing...that's a likely cause...non-responsive controls. Possibly with other complications, of course. Or maybe it means leadout spacing? Less than optimal, but if they're in the right place, and they're not adjustable, then oh, well. If the LO's are in the wrong place...the plane might hunt because of it. Check for stiff controls while really tugging on the lines, not just doodling with them. Report back!    Steve   

[Larry Fulwider]

This reminds me of my grandfather telling me of cures for warts.
 . . .
 . . .
Moving the CG forward will cure hunting, kinda like lung cancer cures smoking.  If the airplane is hunting with the CG in the design range for the airplane, there's some nonlinearity that's causing the hunting. . . .
. . .

Don't forget the obvious though! -- If the CG is a tad too far back, a (tail-heavy) plane will hunt, even if all the incidences are spot on, and the controls are as smooth as a pickle seeder.

Never fly any plane without having the CG in a "safe" and measured location.

    Larry Fulwider

[Kim Mortimore]

I've became a complete believer in engine down thrust and stab positive incidence. Best flying ship I have has Legacy numbers with 1 deg down, 2 deg right and 1.7 deg positive stab. The 1.7 was a goof as I intended it to be a little less than 1 (little miss calculation) but it has worked out great.

Bob,
May I ask what method you use to measure 1.7 degrees positive stab?

Thanks,
Kim Mortimore

[Bob Reeves]

Digital angle indicator.. A good incidence meter is the new Hobbico digital. Everyone should own two...

[Howard Rush]

Don't forget the obvious though! -- If the CG is a tad too far back, a (tail-heavy) plane will hunt, even if all the incidences are spot on, and the controls are as smooth as a pickle seeder.

Never fly any plane without having the CG in a "safe" and measured location.

    Larry Fulwider

If the CG is too far back, an airplane will be outright unstable.  As far as I can tell, stunt fliers use "hunting" to describe a limit cycle, which is something else. As the CG goes back, limit cycles caused by stuff like control friction or stab transition point wandering will get worse.  If an airplane is hunting when the CG is in the recommended position (assuming the recommender knows what he's doing), the CG isn't too far back: something's wrong. 

I went flying today and was fixing to try some downthrust after seeing all the enthusiasm for it, but I moved my leadouts and now there's no evidence of hunting.  I was reminded of another cause of friction when dew started to appear, namely sticky lines.  Wipe the goo off your lines with solvent every few flights.

[Ted Fancher]

If the CG is too far back, an airplane will be outright unstable.  As far as I can tell, stunt fliers use "hunting" to describe a limit cycle, which is something else. As the CG goes back, limit cycles caused by stuff like control friction or stab transition point wandering will get worse.  If an airplane is hunting when the CG is in the recommended position (assuming the recommender knows what he's doing), the CG isn't too far back: something's wrong. 

I went flying today and was fixing to try some downthrust after seeing all the enthusiasm for it, but I moved my leadouts and now there's no evidence of hunting.  I was reminded of another cause of friction when dew started to appear, namely sticky lines.  Wipe the goo off your lines with solvent every few flights.

Hi Howard,

I was going to bring up the subject of leadout position (as well as that comment about 1.5" line spacing--although I suspect Steve was right and our friend is thinking of the spacing at the wingtip (which is about four or five times too much, as well).

Excessive leadout sweep can be a cause of hunting all by itself.  When the CG lines up with a too far aft leadout position the the outward yaw combined with the constant slip to the left due to flying on a tether makes for a substantial amount of vertical P-factor which tries to pitch the nose up forcing down control to be applied to stop the climb.  Throw in a significant wind and the forces will be constantly changing forcing the pilot to have to constatnly compensate.  It's the same phenomenon that makes downthrust and/or positive stab incidence valuable in producing a stunt ship that is equally responsive both in inside and outside maneuvers. In a well trimmed stunt ship the necessary thrust/incidence adjustment is modest because the ship will be trimmed to be tangent to the circle. A ship with excessive leadout sweep or (heaven forbid) rudder offset will need greater amounts of offsetting adjustments.  The trick is to trim it right in the first place.

P.S. The original Trivial Pursuit has flown with a couple of thin shims under the front engine mounts for almost twenty years! Still flies pretty good.

Another great argument against yawing a controlliner away from the pilot in search of line tension.

Ted

[Larry Cunningham]

Actually, a model can also hunt when the CG is too far forward. My brother and I meticulously moved the CG back and forth on a stock Twister to determine its working limits. The model would hunt as it approached either limit. We also found that too much tip weight would make our model hunt.

If your models hunts, there is a pole on the right side of the S (complex) plane, and you're approaching it. Avoid that area, or move the pole outside your operating area. Seriously, this is where instability exists, where a minor oscillation can grow in amplitude.

Remember the good old days when some cars would hit a bump and start to "shimmy"? It's caused by positive feedback, where an error signal acts to worsen an error. Resonance is a phenomena closely associated with this. Nowadays we don't see it much but such things as gross misalignment, worn tires and suspension systems can still induce it.

The way to deal with a resonance in a mechanical system is to relocate it to an area where you do not operate. This typically entails changes of mass and/or mechanical stiffness or damping. Change the resonant frequency! If there are no poles in the right half of the S-plane, no oscillation can be sustained.

Slop in the control system can be effective by creating a small "dead zone" around neutral, where an oscillation is prone to start. Misalignments can introduce some unwanted aerodynamic forces, of course, and some deliberate misalignments can also stabilize a system.

As a resonant point is approached, a very small disturbance can kick off oscillation. And some very minute change can often fix the problem.

Good luck.

L.

PS - I nearly forgot to mention alignment and position of your model's gear. A skewed wheel pant, for example, can induce a shimmy!

"If you want an oscillator, try to design an amplifier.
If you want an amplifier, try to design an oscillator." -Rules of Op Amp Design, page 7..

[Randy Powell]

Larry,

Great, I'll box up my Shoestring and send it to you. Maybe you can figure it out.   

[John Sunderland]

My fathers last build was Robbie Hunts Geo/ Saturn.....shoot name escapes me now. Point is this fine design was originally wrapped around a fairly heavy 40 sidewinder. The old man decides his Aerotiger 36 will haul the mail fine for his needs. Problem is, this engine is an oz or two lighter than the average 40 back in the mid 1990s. I was honored with first flights and trimming. It was a hunting dawg and super sensitive to input at the handle. Pops built practically no adjustment features into the bird. So, he cut in and added some down trim, a near equivalent to positive incidence in the stab. We narrowed handle spacing a tad and double checked the LO position. Try it again... hunting dawg! On further inspection of Daddy'Os new ride I find a sloppy thin pin adjustable clevice in a larger than optimal hole in the elevator horn. In other words more slop than was ever necessary even when a little was desirable in years past. Back home the cutting commences and we sleeve the hole. Everything is better except she still hunts a little and is still sensitive. Put an OZ in the nose, thin shims under the front mounting bolts. BINGO! Sealed the hinge lines, fine adjust the handle settings and its pretty good now.

[Howard Rush]

"If your models hunts, there is a pole on the right side of the S (complex) plane, and you're approaching it. Avoid that area, or move the pole outside your operating area. Seriously, this is where instability exists, where a minor oscillation can grow in amplitude."

I guess that's a useful way of explaining hunting to the average modeler, but doesn't it just describe having the CG too far back, rather than having a limit cycle with the CG in the normal range?

[John Witt]

We're trying to have airplanes that have two mutually exclusive properties, i.e., marginal pitch stability, so that it will turn well, then a high degree of pitch stability when it is flying level.

My admittedly inexperienced observation is that we get the planes to turn well and rely on a high degree of pitch damping to get the straight line stability we also want. This results in large stabilizers for dampng, and  big flaps and elevators to overpower the large damping coefficients. When the plane is level and the thrust line, wing and stab all have zero incidence, it seems to me that little damping is being developed by the tail, so it becomes possible for small perturbations to cause Howard/Larry's limit cycle, or periodic oscillation, to develop.

As was pointed out, it doesn't take much to set it off or even provoke resonance. The pilot is also providing another feedback loop that is probably running at the same frequency as the oscllation, but with a time delay superimposed.

John

[Larry Cunningham]

Sorry, I should not be flippant about this subject. I'm not very good at communicating a good model airplane example here, not very good at communicating in general. Apologies.

I'll simply say that your model airplane is a more complex mechanical system than you might suspect; a number of things can contribute individually or in concert to destabilize it.

L.

"Only learn to seize good fortune, for good fortune is always here." -Goethe

[Norm Faith Jr.]

My latest plane had the same problem...At the Memphis Classic, it was suggested to me to set the flaps to neutral and add about 1-2 degrees of down to the elevator. The hunting went away...Now! if only I can figure out how to stop it from getting light on the lines during the outside loops, especially the  outside squares. 
Norm