Trim problem

[Russell Shaffer]

I am fighting with the second airplane to have a rolling tendency.  The airplane does not have flaps.  It will do inside loops and outside loops without trouble.  The wing is level upright and inverted and as far as I can tell, the stabilizer and wing are parallel. But, when going from an inside to an outside (horizontal eight) the plane rolls into the circle. It also rolls in after the intersection on the square eight.  It does ok on vertical eights and overheads.  I think the wing is straight and the plane flies without obvious yawing in or out.  Help.   

[Jim Hoffman]

Hi Russell,

Your plane appears to have a problem when you transition from up to down control.  Look hard from the front of the plane and verify that the wing and stab are indeed parallel.  I put a small coin on both stab tips and use that as a sighting aid.  ANY lack of parallelism can be problematic.

Are you and Bill coming to GSSC?

Jim Hoffman

[Russell Shaffer]

Exactly, Jim.  When I come out of the inside on the horizontal eight and give down, the plane banks in and gets loose.  No flap alignment problems since the plane doesn't have any.  Bill hasn't flown in at least a year, to busy.   Thanks.

[Dave_Trible]

These things are hard to guess without seeing them in person but I can think two possible issues.  If there is rudder offset I'd try to eliminate it.  Also engine torque can get you right at this spot where the nose is pointed up and the flying surfaces are less useful for damping a roll or countering the torque.  You can try a small trim tab under the inboard wing out near the tip and see what happens.

Dave

[Brett Buck]

Exactly, Jim.  When I come out of the inside on the horizontal eight and give down, the plane banks in and gets loose.  No flap alignment problems since the plane doesn't have any.  Bill hasn't flown in at least a year, to busy.   Thanks.

      So you see the *bottom* of the wing when you turn outside? Add tip weight.

       Brett

[Al Rabe]

Airplanes (all with normally rotating propellers) Have a tendency to yaw outward on outside maneuvers and inward on outside maneuvers. The cause is most likely gyroscopic precession which almost nobody takes the time to try to understand.  The yaw is seemingly accented on the transition from insides to outsides  because the precession isn't just a yaw from a single pitch change from level flight but a yaw to a yaw in maneuver.  In the maneuver transition the airplane is  experiencing the sudden loss of gyroscopic outward and replacing it with a gyroscopic inward yaw which is effectively doubling what we see with a sudden change from an inside maneuver to an outside maneuver.  Changing suddenly from an outward yaw to an inward yaw  seems to be virtually doubling the instantaneous effects of the transition.

Moving leadouts and playing with tipweight, engine alignment and rudder offset  aren't going to solve the problem.  A larger vertical fin and rudder should help some by increasing directional stability to minimize yaw.  Using a lighter propeller would also reduce unwanted gyroscopic precession.

Personally, I solved my problems with gyroscopic precession a half century ago by making my rudders movable and linking them to the elevators so that down elevators would move the rudder to the right to balance an inward yaw.  Also, moving the rudder to the left on up elevator would reduce outward yaw.  Outward yaw typically isn't generally a problem, so I use less left rudder than right  and trim them individually to obtain just the desired line tension for both upright and inverted maneuvers.

Gyroscopic Precession is a natural force of nature. It can't be eliminated but it can be adequately compensated.

When Russell's  airplane began with an inside loop it was experiencing a bit of outward yaw out from Gyroscopic precession.  It might be said that it was also experiencing centrifugal force which helped it to stay out on the end of the lines. When the controls were reversed in the transition, the outward yaw suddenly became a destabilizing inward yaw  with a roll and loss of line tension.  Where was the line tensing centrifugal force then?

Al

[Russell Shaffer]

Brett, I see the top of the outboard wing. The airplane banks toward me.  Jim, I found a slight upward warp in the outboard wing which should cause it to lift. Fixed that.  There is no rudder offset that I can see.  The down line is in front and was very close to the CG.  I moved it back some. About Motorman's comment, wouldn't too much tip weight cause the outboard wing to dip in all fast maneuvers?  Test fly time. 

[Brett Buck]

Airplanes (all with normally rotating propellers) Have a tendency to yaw outward on outside maneuvers and inward on outside maneuvers.  the cause is most likely gyroscopic precession which almost nobody takes the time to try to understand.  The yaw is seemingly accented on the transition from insides to outsides  because the precession isn't just out and inward but in the transition it is necessarily swinging from outside to inside  which is double what we see when we maneuver in just one change of direction. 

    It's far simpler than that, in fact, it's apparently not going to the direction you would expect (or I assumed from the description).

   Precession is generally a trivial problem swamped by far larger and far more basic trim issues, particularly since we left the "bad old days" of giant props.

     Brett

[Brett Buck]

Brett, I see the top of the outboard wing. The airplane banks toward me.  Jim, I found a slight upward warp in the outboard wing which should cause it to lift. Fixed that.  There is no rudder offset that I can see.  The down line is in front and was very close to the CG.  I moved it back some. About Motorman's comment, wouldn't too much tip weight cause the outboard wing to dip in all fast maneuvers?  Test fly time.

      OK, so where exactly does it show the top of the outboard wing?  If it is right after the transition, it is not banked towards you, but away from you, because the lift is in the negative-G direction. To first approximation, if accurate, you have too much tip weight or your leadouts are too far apart.

   Too much tipweight = see the bottom of the wing on insides and the top of the wing on outsides
   too little tipweight = see the top of the wing on insides and the bottom of the wing on outsides
 
    Having a hitch or change in the roll angle right at the intersection is the classic and most sensitive way to detect tipweight issues.

     Brett

[Russell Shaffer]

Ok Brett, I think I am starting to understand what you are telling me. I wasn't grasping the negative G part. The plane gets loose right at the transition - as it goes over the top of the outside part of the eight.  I do think I have seen a downward bobble of the outboard wing on hard pull outs.  So, tip weight may be the major problem? 

[Brett Buck]

Ok Brett, I think I am starting to understand what you are telling me.  The plane gets loose right at the transition - as it goes over the top of the outside part of the eight.

   Which side of the wing do you see when this happens, top or bottom?

     Brett

[Russell Shaffer]

I'm seeing the top.  The airplane stops turning and sort of levels out and I have to bail rather than attempt to keep turning down. The last airplane that did this to me I tried to fix by adding tip weight.  Didn't work. Guess I should have asked sooner for advice.

[Tim Wescott]

Just as a check on terms: you guys are in agreement that "top" always means "canopy side", even when the canopy is pointing toward the center of the earth -- right?  Ditto "bottom" and "wheel side"?

[Russell Shaffer]

Good point, Tim.  Yes, I am calling the "top" the canopy side regardless of orientation.

[Tim Wescott]

I'm seeing the top.  The airplane stops turning and sort of levels out and I have to bail rather than attempt to keep turning down. The last airplane that did this to me I tried to fix by adding tip weight.  Didn't work. Guess I should have asked sooner for advice.

Sounds like too much tip weight, but something else, too -- it rolls because of the tip weight, but in the "right" direction (i.e., it should be giving you more pull on the lines).  But then it goes light on the lines.  I'm thinking it's got excessive yaw, which is getting excessive-er on the sudden roll.  For this I would check the leadout position (or spacing, because Brett is always right -- I always run my leadouts close together, because I'm not smart enough to know what to do when they're far apart, so I can't speak to them being too far apart).

[Brett Buck]

Sounds like too much tip weight, but something else, too -- it rolls because of the tip weight, but in the "right" direction (i.e., it should be giving you more pull on the lines).  But then it goes light on the lines.  I'm thinking it's got excessive yaw, which is getting excessive-er on the sudden roll.  For this I would check the leadout position (or spacing, because Brett is always right -- I always run my leadouts close together, because I'm not smart enough to know what to do when they're far apart, so I can't speak to them being too far apart).

     There's definitely something else going on, probably some yaw motion (almost certainly NOT from precession) that is nosing the airplane in during the transition. If you are seeing the top of the wing due to roll, that should be improving the line tension, not reducing it.

   I have seen a spate of cases lately where the airplane had excessive tipweight to compensate for erroneous yaw trim, usually, inboard rudder or leadouts too far forward, but I am not sure what is happening in this case. It could be a misalignment somewhere, too.

    Brett

[Tim Wescott]

If you haven't left yet, toss it in the truck and bring it to the Follies.  There will be plenty of people there to look at it and make suggestions -- some of them may even be useful suggestions.

[john e. holliday]

Yep, it always helps to see the problem in person.

[Brett Buck]

I'm seeing the top.  The airplane stops turning and sort of levels out and I have to bail rather than attempt to keep turning down. The last airplane that did this to me I tried to fix by adding tip weight.  Didn't work. Guess I should have asked sooner for advice.

      OK, after a bit more time to think about it, first, I presume that you have sealed the hinge lines. If not, seal the hinge lines and start over with the trim process.

     I would suggest starting from first principles - set the CG, set the leadouts, check for any misalignments (tail to wing, primarily, both in skew and in tilt, a;so wing to fuselage) and correct them, remeasure the fin/rudder to make sure the fin is *dead straight ahead*, cut loose and straighten out if not. Make sure there is enough engine offset to tell that it is not inboard, and a few degrees won't hurt anything.

     Depending on the tail area and design of the airplane, you want the CG to be set right or *slightly* ahead of what might be ideal. Ted's rule of thumb is pretty good for flapped airplanes, set the CG at the percentage of the mean chord based on the ratio of the tail area to the wing area. If the tail is 20% of the wing area, then, the CG can be about 20% of the mean chord - or forward. For non-flapped airplanes, set it at 15% of the mean chord to start, it is much less critical than on a flapped airplane.

    Set the center of the leadouts about 3/4"-1 1/4" behind the CG. This may or may not be ideal, but it will at least be safe to fly with. Add enough tipweight to be sure that you have enough. It varies depending on your wing asymmetry. With normal asymmetry (1/2-3/4"-1") then start with enough to counterbalance 1 1/4 ounces temporarily attached to the inboard wing tip. That is, put 1 1/4 ounce on the inboard wingtip and either balance it or temporarily tape it in place. Then add enough tipweight to the tipweight box to just balance it when held by the fin and engine shaft. Then remove that from the inboard. For symmetrical wings, same thing, except use 2.5-3 ounces. This will almost certainly be too much, however, you can cover a lot of other problems with too much, and you can't run 60 mph if you guess wrong and wind up with too little.

   Set the rudder with just a hint (1/32-1/16" at the TE) of right rudder. This will also probably be wrong in the long run, but makes sure you are making any errors in the safe direction.

     Then go fly it. Upright, check if the wing is level. The outboard wing will probably be hanging down, which is good at this point. Check the control response, and do an "s" maneuver (1/4 turn inside followed by a quick transition to 1/4 turn outside to get back to level - just like you round 8 intersection) to check what happens when you go to from positive to negative G. Again, it should be pulling pretty hard and it will likely roll rapidly from right to left at the intersection due to excess tip weight, but not lose it or get loose at any point. If safe, then fly it inverted level, and note or have your buddies note what happens to the roll angle. What I would expect is the outboard wing will still be down. What is of interest is whether it is down the same amount upright and inverted. If it is the same, the wing is probably straight, if not, it has a warp that you should correct or tweak before you try again.

    Check that the wheels line up such that you can see maybe 1/2  wheel radius to an entire radius of the outboard wheel sticking out behind the inboard wheel. If they are lined up, or the outboard juts ahead of the inboard, move the leadouts back a hair, if you see more than half the wheel behind, move the leadouts forward.

     Once you get it the same upright and inverted, then, start adjusting the tipweight to try to get the wings level. Go very easy. Work up to it, and check what happens in various load conditions. If it looks level flying level, rolls away a little (right on insides left on outsides) in round loops, then rolls more in hard corners, you have a bit too much. If it should ever start rolling in (left on insides and right on outsides) put more in.

   Try all that and it should be at least safe to continue and pull decently well.

     Brett

[Russell Shaffer]

Thank you Brett.  This particular airplane is a TEOSAWKI with a built up wing (no foam LE).  The other plane that had the same problem was a much smaller old time combat plane.  Neither have flaps and both wings were symmetrical. I corrected a small wing warp and moved the lead outs back some.  The wheels were pretty much even in flight.  I'll do your tip weight thing before the next flight. 

[Tim Wescott]

This particular airplane is a TEOSAWKI with a built up wing (no foam LE).

Hey Russel:

When I first built my Cessna 37 stunter, I scaled the fin down by the same factor as I was scaling down the cockpit and cowling.  I ended up with no end of weirdness on turns -- in my case the thing would oscillate on the end of the lines, coming in, then flying out and banging on the lines, then coming in, etc.  I tried everything except for making the fin bigger -- because hey, control line planes don't need fin area, right?

Once I caved to social pressure and increased the fin area by about 30%, the plane flew just fine.  There was still a hint of weirdness at first, but I got that smoothed out with trimming.  That's why my cartoon-scale Mooney has a big fin.

So I looked at your TEOSAWKI, and boy, does it have a little fin -- kind of a compromise between a combat fin (zero area) and a stunt fin.  You may want to stick a fin extension on there, at least temporarily.  Bathtub caulk works well; depending on what your current fin is covered with it can be as temporary or as permanent as you want (it's held the fin on the Cessna for years, and through about a dozen crashes).  It may be ugly -- but ugly and flyable will get you more points than a pretty wall decoration.


(Russel's TEOSAWKI, from Flying Lines)

[Brett Buck]

Hey Russel:

When I first built my Cessna 37 stunter, I scaled the fin down by the same factor as I was scaling down the cockpit and cowling.  I ended up with no end of weirdness on turns -- in my case the thing would oscillate on the end of the lines, coming in, then flying out and banging on the lines, then coming in, etc.  I tried everything except for making the fin bigger -- because hey, control line planes don't need fin area, right?

Once I caved to social pressure and increased the fin area by about 30%, the plane flew just fine.  There was still a hint of weirdness at first, but I got that smoothed out with trimming.  That's why my cartoon-scale Mooney has a big fin.

So I looked at your TEOSAWKI, and boy, does it have a little fin -- kind of a compromise between a combat fin (zero area) and a stunt fin.  You may want to stick a fin extension on there, at least temporarily.  Bathtub caulk works well; depending on what your current fin is covered with it can be as temporary or as permanent as you want (it's held the fin on the Cessna for years, and through about a dozen crashes).  It may be ugly -- but ugly and flyable will get you more points than a pretty wall decoration.


(Russel's TEOSAWKI, from Flying Lines)

     The larger fin or  aft fuselage area  will tend to cover up other trim issues but it should be trimmable with it as is. It won't hurt anything to make it bigger.

    Brett