Good morning, Yorkshire--from California.
Yes, we call that hunting.
Sometimes the fixes are simple--but in my experience, it can be the most difficult thing to trace down and fix. I have one plane that has defied attempts so far--and probably needs to have a knife taken to it....
A few more details about your plane and setup might help.
I have attached the content of a prior thread where I attempted to list many of the causes that I have seen. It should be a good starting point for your consideration.
Divot McSlow
Causes of Hunting
1. The CG is too far back. If too far back, the tail downforce is less for the static flight condition. You may then get into the threshold issue, likely caused by any turbulence. See number 5, below.
2. Sticky and draggy controls. Everyone's opinions differ on how "free" is really free. On a stunter with big surfaces, if you simply move the control surface to neutral (no lines connected) and let go, they ought to droop of their own weight. If the hunting gets worse once the engine quits it is either tail heavy or the controls are too sticky. I had to fix this on a foam wing job where the pushrod was rubbing on the inside. The bellcrank bushing was not a close fit, so on the ground the controls seemed free. In the air I suspected they were not. To prove this, we tried moving the controls with flight tension on the lines. It was instantly clear what the problem was. On the other hand, I have seen sport stunters that had horribly stiff controls with a lot of spring rate in them that did not hunt. Generally, they were heavy for their size, probably had good line tension and were mostly flying round maneuvers well and would never fly great corners, so a little wobble at the top was likely no matter what.
3. Notchy control action. For some reason, probably tweaking controls after assembly to fix some other presumed fault, the controls get a slight "catch" near neutral. Same bad effect as number 1, above. I have a plane that I built that is like this now, and while it did not cause the hunting it was born with, it can't be helping, either. I probably won't fly it again until I cut into it to find out what was damaged in the attempt to reverse the controls at the flap horn.
4. Control surface gaps. I'm guessing this could occur, but have not experienced a miraculous resolution of hunting by taping hinge lines--yet. The plane I would like to fix has the wrong geometry to tape. Hopefully, I will remember on future planes to shape the flaps and elevators to allow taping.
5. Poor alignment (decalage). This is the ever-popular topic of adding just a hair of positive AOA to the stabilizer to avoid hunting. The range of discussion on this one has been inspiring and the champions vocal! I think it also pre-supposes that the layout of the plane is standard; ie. the wing is below the thrust line, the tail is well above. Otherwise this one makes no sense. I think it is really an attempt to load up the tail in level, upright flight so that it never goes thru a lower downloading threshold. I have no idea what download threshold would be, except to guess that it has to be larger than any transient from turbulence. What is less clear to me is why this would still work well when inverted. I’m not saying it doesn’t, I’m just saying that the tail is more likely to be exposed to more downwash. Maybe that is why it works inverted, too. This problem can be very hard to measure. I am less than convinced that it can be reliably identified with the “incidence meters” on the market. I further wonder if adding reflex via the flaps essentially isn’t proof of this effect. (Note that the thrustline would need to drooped to avoid introducing a new issue.) So the fix for evening up inside vs. outside by drooping the flaps might actually make any tendency to hunt more noticeable?
6. Engine thrust line. Seems like too much up or down would cause the tail to be loaded differently and cause hunting trouble upright (upthrust) or inverted (downthrust). This one is usually pretty easy to alter for a flight check even on an inverted engine installation.
7. Controls are too tight and don’t have enough “slop” in them. I have issues with this classic “cause.” If the controls are not actually sticky, then the only merit to this as a cause of hunting would seem to be that your hand is not steady enough. On a fast racing plane, I can see the plane changing line as you run and bump in the middle. That’s not hunting. But at slow walking speed on a stunt plane seeing the plane either wobbling around not synched up to your steps or worse, making a slow rise and fall, say twice in each lap, has seemingly got nothing to due with inaccurate hand motion, which adding slop to the controls would seemingly address.
8. Control surface are too effective and/or too efficient. The supposition is that the elevators ought to be thinner than the stabilizer to “bury” them in the boundary layer and reduce effectiveness right around neutral. So even if your hand is not completely steady, and your controls are “too tight,” the aero geometry desensitizes this. Some variations of this thinking include not tapering the TE of the elevators too thin, because then they are too clean and too efficient and even very minor motion shows up as significant change in tail downforce and hunting. This argument then also encompasses whether the TE of the elevator should be squared off or fully radiused. I think some guys also favored rubbing chicken guts and feathers on the TE…but I may have not heard that correctly since that guy was yelling really, really loud to get his point across….
9. Twisty flaps and elevators. Not sure how this works, but if you’ve tried fixing everything else and it didn’t work, this would be a tech-sounding scapegoat, right?
10. Transient separated airflow caused by poor shapes. This is a cause of hunting that I solved on a particularly recalcitrant Goodyear racing plane made from the SIG Buster kit. It is a happily resolved case, and a good one to think about if your are having issues. The plane has been a real workhorse now for years, but it hunted badly for a long time. I tried all the usual fixes like shifting the CG forward (and then aft), bigger HS/elevators, tighter pushrod guide, engine thrust line, and so on. The controls were always quite free. None of this had a positive effect. So I sawed off the ¼” thick wing and replaced it with a 3/8” thick semi-symmetric wing. This actually helped some. Note that there are actually three different things happening, or potentially happening, at the same time with this rework. The airfoil is different; the wing/tail decalage may be different (I tried to keep the zero lift line parallel to everything); and ….? This improvement was not simply a fluke. My racing partner liked what he saw and he scabbed another ¼” plank on the bottom of the wing of his Buster and reshaped the airfoil. While his Buster never hunted quite as bad as mine, his improved also. But I still was not happy. What made a major improvement was putting a much larger radius on the front edges of the cheek cowl. This was a clean experiment, since no other changes were made at the same time. This is one reason I am not a fan of bluff features such as the canopy on the Nobler. Another feature that makes me leery is a large cooling inlet with an exhaust ramp that has a large angle. This creates a large turbulence zone and may blanket any surface that is in its wake. What you want is something closer to parallel flow ejection. Go look at cowl exhausts for light planes to get some ideas. A particular feature on a particular plane may not affect things at all—but when you go fly it is trial by error. In a kit you presume your risk is low because any problems should have been solved by then. But the design may not have much margin relative to stability (freedom from hunting) and the mods you make might not be aerodynamic improvements.
Other comments on the discussion so far:
There is some pretty good anecdotal data from some pretty experienced guys that putting slop into the elevator controls tamed hunting. What is hard to confirm is that the controls were equally free before the slop was added. If the controls were slop-free but had significant friction initially, then they might be attributing the improvement to “slop” and not “sufficiently friction free.” So the experiment is not really conclusive or complete.
Relative to the discussion on control friction and ball links and rod ends: If the plane has plenty of margin on controls performance, then a bit of friction shouldn’t be noticed. A good, straight design with no other likely causes may be immune. Now compare size. If you used the same exact ball link setup on a Ringmaster at 28 oz. running slow lap times, the line tension needed to overcome friction is a much larger percentage of available tension than on a 60 oz. ship running ProStunt everything and using the exact same controls. The use of smaller bellcranks on the smaller plane makes this even worse. So if (put your favorite ProStunt name here) uses XYZ controls, they must be the best, and better than you need for your 28 oz. Ringmaster—well, maybe not. What did the same ProStunt guy use on his Ringmaster?
Something getting lost here is the stick/slip action that is likely occurring as one root cause. It takes a certain control input force to overcome the stiction. (Also known as breakaway friction, which is always higher than running friction.) When the contacting surfaces break free the friction goes down. It is highly non-linear. So the input from the pilot goes past what he intended, and he immediately has to try to compensate in the other direction. No pilot can do this. (Think of our setup as “servo between the ears” which is pretty accurate. But because of this, the bandwidth of the system is really low!) As Tim alluded, even creating software to simulate this plant response is problematic because it has so many dependencies like temperature, vibration level, whether the rubbing surfaces are “contaminated” by engine exhaust or brand new clean, and so on. So the solution is mechanical, not in software, as was the root cause—you have to reduce the stiction. I have had pretty good luck with using a kerosene/turbine oil mix on ball links and rod ends. You can always try it to see if it helps. I would not agree that “…a little bit of friction…is [adding] damping and…assists in stopping hunting.” If there is no control slop, then all friction can be doing is turning the pilot into a bang-bang controller with the range large enough to visibly see the result of the plane going up and down. This overshoot characteristic is similar to the heater control in your house—well, at least in my old house. If you don’t want to alternate between being hot and being cold, then you have to reduce the deadband—and you can’t unless you can measure smaller temperature differences and you can make smaller adjustments. Just like our pilot with servo between the ears. The only way he can make smaller control inputs is to have less friction in the controls and thereby reduce overshoot.
Note that the sticky controls are on a differ leg of the root cause tree than the aerodynamic disturbances than might be root cause like my Buster example. I spent a lot of time “improving” the controls and it had no effect, because the controls weren’t the cause.
Relative to Ken’s replacement of controls and his experience that hunting was reduced (eliminated?) even though the ball links (rod ends?) may not have allowed a “free dropping” low friction condition: note that you like changed multiple relevant things at the same time, whether you intended to do this or not. With a new pushrod or horns or bellcrank the rigging has been changed. Just as a minimum example, if it was a 4-40 ball link, your increment to get things to line up is one full turn. That is .025” or an angle of about 1.4 degrees. This sounds small and is difficult to measure without fixtures. But for reference, some of the ProStunt guys argued a lot about using 0.5 degrees positive on the stab during build. Not 1 degree or 1-1/2 degrees. So even if you did not intend to change the rigging, there is a limit on your ability to measure, and the prior and new setups won’t be the same. It may be very close if both measurements were highly precise. Then the question is how much margin did the plane have to this adjustment? If a lot of margin, then it suggests this is not an additional cause. So we reject it out of hand, or discount it. But it is real and in some cases of hunting, maybe it is the cause.
Topic: Level flying problem. (Read 2048 times)