Trimming for zero roll in maneuvres

[Paul Van Dort]

Hi all,
French top flier Serge Delabarde was acting as a judge in the latest international stunt contest in Pepinster Belgium.
The super friendly man was not only judging F2B, he was also giving advise. How cool is that.
He told me : "Tu te bat avec ton avion". or "You are "fighting" with your model". It was very windy during the competition and wind creates more of a "fight" than calm conditions. So I tought I understood his comment.
Weeks later his comment still was is in my head. So i started watching the behavior of the model better. He was right, although level and inverted flight don't show any rolling tendencies (Wing perfectly parallel to the ground), in maneuvres the bottom of the wing became visible much more than the top.
It had a rolling issue when the flaps were deflected .
This has to do with a warped wing or warped flaps or a combination of the two ...My model certainly has a slightly warped flap...

The issue was discussed by Paul walker in his "Flying lines" trimming series. http://flyinglines.org/pw.trimflow3.html
In the article Paul is pessimistic , stating that a model with a warp issue will always give issues here or there..  Trimming out 1 issue will cause another issue....

Now my model "No Panic 3" has flaps shorther than the wing  length. The remaining 5 inch is taken up by adjustable trimtabs on both sides.
This is giving me more trimming possiblities than just tweaking the flaps. So I am now trimming for 1. Flat level/ inverted flight and 2. no rolling in maneuvres. For point 1, the trimtabs are handy and for point 2 I tweak the flaps. Experimenting with this during the afternoon really was an eye opener.
I managed to get rid of the rolling in harder corners by tweaking the flaps. This however resulted in a lousy level/ inverted. The top of the wing became visible then. This was corrected with the trimtabs and now I don't have to fight my plane much less than I used to. Thank you Serge Delabarde for your feedback.

[Dave_Trible]

From what you describe your issue isn't a warp.  It's a matter of flap asymmetry.  One flap is overpowering the other one as they are deployed in maneuvers preventing the model from turning flat or co-planer with the lines.  You can tell which is more powerful by watching what the airplane is doing during control deflection which trait will vanish when the controls are neutralized.  A quick way to judge it is to do horizontal eights tight and high,  having the intersection at the 40-45 degree mark.  Watch what happens at the intersection right when you change the control input from 'up' to 'down' etc.  You will see the airplane roll one way, then reverse and roll the other way.  You will be able to then see which flap is over-achieving.  Doing the test up high removes most of the tip weight related slinging that could fool you here.  Once you see which flap is too large you can either shave it down a little or add a little area to the opposite side by gluing on a wood strip to the trailing edge.  Obviously some experimenting will be required to find the right amount.  Then you can do whatever cosmetics you wish to the flaps.  This is standard practice with me on new airplanes.  Exact duplicate airplanes should work out pretty close to these dimensions on the next airplane though never guaranteed.  You can sometimes offset this issue with slight tip weight variations and learn to live with minor asymmetry issues but the airplane isn't in it's best trim that way.

Dave

[Douglas Bykoff]

Wasn't it the French who made their models with very small flaps, very wide and high wing roots and small and thin tips? Models totally out of the standards then in force?

[Brett Buck]

From what you describe your issue isn't a warp.  It's a matter of flap asymmetry.  One flap is overpowering the other one as they are deployed in maneuvers preventing the model from turning flat or co-planer with the lines.  You can tell which is more powerful by watching what the airplane is doing during control deflection which trait will vanish when the controls are neutralized.  A quick way to judge it is to do horizontal eights tight and high,  having the intersection at the 40-45 degree mark.  Watch what happens at the intersection right when you change the control input from 'up' to 'down' etc.  You will see the airplane roll one way, then reverse and roll the other way.  You will be able to then see which flap is over-achieving.  Doing the test up high removes most of the tip weight related slinging that could fool you here.  Once you see which flap is too large you can either shave it down a little or add a little area to the opposite side by gluing on a wood strip to the trailing edge.  Obviously some experimenting will be required to find the right amount.  Then you can do whatever cosmetics you wish to the flaps.  This is standard practice with me on new airplanes.  Exact duplicate airplanes should work out pretty close to these dimensions on the next airplane though never guaranteed.  You can sometimes offset this issue with slight tip weight variations and learn to live with minor asymmetry issues but the airplane isn't in it's best trim that way.

Dave

  Maybe, but it's likely to be simpler than that, until he adjusts the tip weight enough to really discover the problem, we don't know if it needs a tab, the wing is warped, he has differential hinge line bleed, etc.

  There could be a host of other issues, particularly, he doesn't say if he has sealed his hinge lines, which can easily cause all sorts of issues, and hasn't said he checked the stab tilt and skew. Also, depending on how the flaps are arranged, if they are not full span, tweaking the flaps causes other issues because the flaps and "fillers" don't necessarily line up the same.

       There are almost no details given here, aside from Serge's very apt observation.

     Brett

[Paul Van Dort]

Gentlemen, thanks for the feedback. I think that tipweigth always has its effect in maneuvers. adding or removing weigth helps big time in controlling symmetrical roll. Asymmetrical roll requires flap tweaking / trim tab adjustment in my opinion.

Some more details:
Flap hinglines are sealed. Stab is perfectly aligned.
Flaps are equal length, outboard wing 1/2" shorter
Outside flap is 3/8 wider towards the tip.
flaps are not full length. about 5 inch fixed trim tabs at the tips.
The roll issue was not symmetrical.
Flaps and trimtabs don't line up in neutral. They are tweaked/ trimmed to avoid most malicious roll
laptime 5.2 sec
Rabe Rudder
lines: 20,5 meter staystrate 015'

[Dave_Trible]

There are sure a lot of possible variables here.  Might be possible the vertical CG is favoring one way or the other and yet another variable popped up-Rabe Rudder!  Throw a yaw issue into the pot and ..........Just for test purposes I think I'd disable that temporarily and work through everything else-then work that in to see if it adds or detracts anything. 

Dave

[Paul Van Dort]

Wasn't it the French who made their models with very small flaps, very wide and high wing roots and small and thin tips? Models totally out of the standards then in force?

That was his friend Gilbert Beringer, now world famous as manufacturer of brakes for motorcycles, cars , planes.... I think Serge also had a model featuring the Beringer design elements

[Paul Van Dort]

There are sure a lot of possible variables here.  Might be possible the vertical CG is favoring one way or the other and yet another variable popped up-Rabe Rudder!  Throw a yaw issue into the pot and ..........Just for test purposes I think I'd disable that temporarily and work through everything else-then work that in to see if it adds or detracts anything. 

Dave

Many variables make our hobby so exciting. I fully understand your remarks about flap differential, but the thing is that it only rolls 1way. It likes to show the bottom of the wing. The top is never shown. During level flight however the model flies perfectly flat. No rolling whatsoever. Adjusting for symmetric roll In maneuvers resulted in outside wing higher, so I was able to make a compromise with the trim tabs for level fight. More testing this afternoon. Next Sunday contest in Genk. I need to be ready. The small Rabe rudder is absolutely required to counter the B/E 13-5 prop effect. Thanks for the feedback!

[Dave_Trible]

Let us know what you find out.  If the airplane is electric I'm wondering if your battery is too high in the fuselage causing it to want to bank one way in maneuvers.  If it really flies pretty level upright and inverted it's hard to see what else could cause rolling just one way when control input is applied as long as the flaps are joined with one solid horn and not two horns with slightly different inputs.  Good Luck!

Dave

[Brett Buck]

There are sure a lot of possible variables here.  Might be possible the vertical CG is favoring one way or the other and yet another variable popped up-Rabe Rudder!  Throw a yaw issue into the pot and

   Or you might say "yet another yaw issue".

   100% agreed, disconnect it, put the rudder straight ahead and fixed, then trim it, then and only then hook it back up with just enough movement to get rid of whatever residual yaw motion you can't get rid of otherwise.

    Brett

[Paul Van Dort]

Thanks for taking further interest in my trim issues.
The engine is a ST51.
I have been doing some more testing today. I went to extreme flap tweaking. Although during level flight the top of the wing was clearly visible (= roll to the left) , still the outer wingtip dropped excessively in hard inside corners (rolling to te right).  The outside corners remained pretty stable, but the outside loopings began to suffer reduced linetension as expected.
As the outside turns are very decent and line tension remains good, I am not inclined to disconnect the Rabe Rudder. It does what I expect from it. It took a long time to define the correct throw for good outside corners. The issue is definitely in the inside corners. (very little rudder throw to the outside) The roll results in less usable lift, as a part is sacrified as extra linetension. Less usable lift results in faster stall.
It  is very well possible that yaw, pointing the nose out, reduces lift on the slower wing, letting it drop. In the past I tried adapting the rudder for left rudder for inside corners to cope with this roll, but that was not a good idea w.r.t. line tension...
Now as the increased rolling force from more flap throw on the steerboard wing is not resolving anything, there must be something forcefull to drop the outside wing in sharp corners. I re-tweaked the flaps again to have equal inside versus outside line tension.
Tipweigth is trimmed to avoid hinging in the loops/ and figure eigths. All good, except the hard inside corners. Adapting tipweigth will affect both inside and outside corners, so that is not what I want to do. So still not  happy, although I am fighting the model to a lesser extend than before ...It actually feels rather good....

[Brett Buck]

Thanks for taking further interest in my trim issues.
The engine is a ST51.
I have been doing some more testing today. I went to extreme flap tweaking. Although during level flight the top of the wing was clearly visible (= roll to the left) , still the outer wingtip dropped excessively in hard inside corners (rolling to te right).  The outside corners remained pretty stable, but the outside loopings began to suffer reduced linetension as expected.
As the outside turns are very decent and line tension remains good, I am not inclined to disconnect the Rabe Rudder. It does what I expect from it. It took a long time to define the correct throw for good outside corners. The issue is definitely in the inside corners. (very little rudder throw to the outside) The roll results in less usable lift, as a part is sacrified as extra linetension. Less usable lift results in faster stall.
It  is very well possible that yaw, pointing the nose out, reduces lift on the slower wing, letting it drop.
So the increased rolling force from more flap throw on the steerboard wing is not resolving anything, so there must be something forcefull to drop the outside wing in sharp corners. I re-tweaked the flaps again to have equal inside versus outside line tension.
Tipweigth is trimmed to avoid hinging in the loops/ and figure eigths. All good, except the hard inside corners. Adapting tipweigth will affect both inside and outsie corners, so that is not what I want to do. So still not  happy, although I am fighting the model to a lesser extend than before ...It actually feels rather good....

   I am having trouble following your descriptions, but *you need to disconnect the rudder*, because it can cause exactly the sorts of problems you seem to be describing. You can't treat the axes as uncoupled, and excessive/erroneous yaw motion will definitely/inevitably  cause it to roll, too - in exactly the manner you describe.  For reference, for most models trimmed conventionally, you will need about 1/16-1/8"  of total motion to just compensate for precession. Any more, and it is doing something else. Most of them I see are vastly more than that, and also, move far more outside than inside.

   Do as you wish, of course, but you cannot hope to figure out how much rudder movement is required without first getting all the other trim aspects correct.

      Brett

[Ken Culbertson]

What is your throw and size of the Rabe?  A tiny bit of Rudder is nice to add tension in maneuvers.  I have used Keith Trostle's cam rudder for years and love it BUT anything over an 1/8" is going to do things you don't want AND I disconnect it until the plane is nearly trimmed.  I am curious over the size of your plane.  I have tried 13" props but went back to light 12" because of what it did in corners.   I don't recall what power you are using.  A poorly setup timer or improperly alligned accelerometer sensor can cause this exact problem.  I hate asymmetrical trim issues .

Ken

[Paul Van Dort]

   I am having trouble following your descriptions, but *you need to disconnect the rudder*, because it can cause exactly the sorts of problems you seem to be describing.
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I understand that my desciptions are hard to follow. The fact that English is not my native language does not help. I was trying to explain my findings of today. 
The bottomline is the fact that outside corners work very well (with a lot of rudder throw on a relative small rudder). The inside corners give rolling issues: dropping the outside wingtip. The Rabe rudder is hardly doing anything in inside corners, so that's why I don't see the connection between the Rabe rudder and the cornering issue.

Tomorrow I will be looking at flap stiffness. Perhaps a flap has non equal stiffness up versus down..

You can compare the rudder on my model with the Maxbee rudder of Igor Burger. Small but large throw at full down. I can post pictures tomorrow.

[Dave_Trible]

Paul while you are experimenting try putting a temporary flap tab on the outboard flap close to the outside end.  Try something like 3/4" wide by 3" long, made of balsa or cardboard and taped on with maybe some rubber cement.  A simple test with little effort and not doing any damage to the airplane.  It might not achieve what you are after but could provide more clues into how it's behaving...

Dave

[Brett Buck]

   I am having trouble following your descriptions, but *you need to disconnect the rudder*, because it can cause exactly the sorts of problems you seem to be describing.

I understand that my desciptions are hard to follow. The fact that English is not my native language does not help. I was trying to explain my findings of today. 
The bottomline is the fact that outside corners work very well (with a lot of rudder throw on a relative small rudder). The inside corners give rolling issues: dropping the outside wingtip. The Rabe rudder is hardly doing anything in inside corners, so that's why I don't see the connection between the Rabe rudder and the cornering issue.

Tomorrow I will be looking at flap stiffness. Perhaps a flap has non equal stiffness up versus down..

You can compare the rudder on my model with the Maxbee rudder of Igor Burger. Small but large throw at full down. I can post pictures tomorrow.

  Large throw and asymmetrical?  I can almost guarantee that is at least a contributor to your problem, because precession takes only tiny motion and is a symmetrical effect.The reason that people tend to end up with asymmetry is that they are trying to force the yaw angle to be non-zero, and while precession is symmetrical, the restoring force from the lines is definitely not.

   I have seen this same problem with nearly ever Rabe Rudder setup I have ever seen, with a few notable exception- excess throw, particularly to the right, causing wild and asymmetrical roll moments.

  But of course, as always, just an observation, do as you wish. Alternately, trimming airplanes over the internet is not as good as in-person, get Serge or someone else knowledgable to watch and advise you in person.

    Brett

[Paul Van Dort]

I understand that Rabe rudders can have bad effects, absolutely. But in the inside turns where I have problems, the rudder is hardly moving. It stays a few degrees to the right as if it was fixed. In attachment my model with the small tail and rudder.

[Paul Van Dort]

Paul while you are experimenting try putting a temporary flap tab on the outboard flap close to the outside end.  Try something like 3/4" wide by 3" long, made of balsa or cardboard and taped on with maybe some rubber cement.  A simple test with little effort and not doing any damage to the airplane.  It might not achieve what you are after but could provide more clues into how it's behaving...

Dave

Hi Dave, thanks for the hint of doing small horizontal eigths to adjust for equal line tension, inside versus outside. It worked pretty well. W.r.t the hint of the flaptab, I must say that my outboard flap already is larger than inboard. I also expect the effect of a tab to be symmetrical, while the issue I have is on inside turns only. Especially the 3 corners of the triangle, and the last corner of the hourglass.

[Paul Van Dort]

What is your throw and size of the Rabe?  A tiny bit of Rudder is nice to add tension in maneuvers.  I have used Keith Trostle's cam rudder for years and love it BUT anything over an 1/8" is going to do things you don't want AND I disconnect it until the plane is nearly trimmed.  I am curious over the size of your plane.  I have tried 13" props but went back to light 12" because of what it did in corners.   I don't recall what power you are using.  A poorly setup timer or improperly alligned accelerometer sensor can cause this exact problem.  I hate asymmetrical trim issues .

Ken

Hi Ken, I don't have timers or accelerometers to worry about. Using an ST51 with some mods. The rudder is small and almost symmetrical to the  fuselage (with a dorsal part) so it should not induce too much roll. Throw is similar to the elevator on DOWN. It hardly moves on UP. Now the issue I am having is in the "UP" direction. Aggresive inside turns roll the plane to the outside, showing the bottom of the wing. The resulting increased linetension, counters the effect somehow, but in the last corner of the triangle and hourglass, I lose lift and track, when steering hard. Next session I will install my camera again on the model to better analyse the movements. The camera itself will have effect on the models behavior. That is a worry. We'll see...

[M Spencer]

I think its Frank Mc Millan , who went for a leadout guide with VERTICAL adjustment .
The sliders mounted to other things , which are adjustable up & down .

If it were the case that your rolls induced by the lead out guide HIGHT , that;d be the trick .
From the remarks , it sounds like its worth considering , as otherwise  it'd be ineradicable .

[Paul Van Dort]

I think its Frank Mc Millan , who went for a leadout guide with VERTICAL adjustment .
The sliders mounted to other things , which are adjustable up & down .

If it were the case that your rolls induced by the lead out guide HIGHT , that;d be the trick .
From the remarks , it sounds like its worth considering , as otherwise  it'd be ineradicable .

Perhaps too late to install those adjustable ledouts, but I can try heavier wheels.... Static measurements however show no issues with the vertical CG...

[Ken Culbertson]

Let me add a comment on the Rabe rudder.  It was successful on his planes because he had very tall rudders on all of his planes and a very small deflection had a huge impact on roll because it acted like a vertical aileron more than a rudder.   If yours is like the MaxBee it is more a source of yaw and is not producing much to offset procession other than line tension.  Mine is like yours, closer to the vertical CL of the plane and I have outboard deflection both inside and outside.  I fly electric and the motor is in heavy brake mode in both of the corners you are having trouble with so I do not have the issue to deal with.  Like I said before, I hate asymmetrical trim issues    

Ken

[Dan Berry]

I would look very closely for a warped elevator.

[Brett Buck]

I understand that Rabe rudders can have bad effects, absolutely. But in the inside turns where I have problems, the rudder is hardly moving. It stays a few degrees to the right as if it was fixed. In attachment my model with the small tail and rudder.

   That more-or-less proves my point.  You are doing my experiment for me, the fact that it is wildly out of trim when the rudder does not move (very much) proves beyond a shadow of a doubt that your other trim conditions are wrong - because conventional airplane with fixed rudders do not have any problem with inside corners, assuming they are in trim. So, you need to trim it without the rudder moving, and straight ahead, until it is as good as you can get it,

     In this case , the other trim settings are almost certainly wrong, and you are causing them to seem OK by having a lots of rudder offset on outsides. It may well be balanced for the rudder movement in one condition, and not another. And anticipating the next possibility - no, the answer is not to put in a lot of offset on insides, too.

    The mere fact you have a lot of differential movement proves you are not using it to compensate for precession - because precession is a symmetrical function and if that was all you wanted to deal with, you would need only tiny amounts of offset, to the right on outsides and to the left on insides. This same differential movement will cause your roll issue, it will look just like a warp, because you are flying with the more-or-less tangent to the circle on insides, and yawed nose-out on outsides, causing it to also *roll* differently inside and out just like you had a warp - which is what you are seeing. Yaw *very heavily* couples into roll, yawing around differently inside and outside will also cause it to roll differentially inside and outside.

   I also note that this is *exactly the problem everyone else seems to have using Rabe Rudders*, this is why I tell people to forget about them entirely. Or if they insist on using them, to *disconnect them, put the rudder in a fixed position straight ahead*, then trim the airplane. As above.

   As always, it is up to you, it's just advice, but this case is an absolutely clear (and typical) example of a near universal issue.

    Brett
   

p.s. earlier you said your rudder was small, so it had to move a lot. Even taking foreshortening by the wide-angle lens into account, that rudder is HUGE, probably 2/3 the size of one of the elevators, and there is very little else trying to keep it straight. The chord is more than twice what mine is, and it's easily 3/4 the height. Point being, it should be extremely powerful and even a tiny motion will have a tremendous effect on the equilibrium yaw angle.
   

   
   

[Paul Van Dort]

   That more-or-less proves my point.  You are doing my experiment for me, the fact that it is wildly out of trim when the rudder does not move (very much) proves beyond a shadow of a doubt that your other trim conditions are wrong - because conventional airplane with fixed rudders do not have any problem with inside corners, assuming they are in trim. So, you need to trim it without the rudder moving, and straight ahead, until it is as good as you can get it,

     In this case , the other trim settings are almost certainly wrong, and you are causing them to seem OK by having a lots of rudder offset on outsides. It may well be balanced for the rudder movement in one condition, and not another. And anticipating the next possibility - no, the answer is not to put in a lot of offset on insides, too.

    The mere fact you have a lot of differential movement proves you are not using it to compensate for precession - because precession is a symmetrical function and if that was all you wanted to deal with, you would need only tiny amounts of offset, to the right on outsides and to the left on insides. This same differential movement will cause your roll issue, it will look just like a warp, because you are flying with the more-or-less tangent to the circle on insides, and yawed nose-out on outsides, causing it to also *roll* differently inside and out just like you had a warp - which is what you are seeing. Yaw *very heavily* couples into roll, yawing around differently inside and outside will also cause it to roll differentially inside and outside.

   I also note that this is *exactly the problem everyone else seems to have using Rabe Rudders*, this is why I tell people to forget about them entirely. Or if they insist on using them, to *disconnect them, put the rudder in a fixed position straight ahead*, then trim the airplane. As above.

   As always, it is up to you, it's just advice, but this case is an absolutely clear (and typical) example of a near universal issue.

    Brett
   

p.s. earlier you said your rudder was small, so it had to move a lot. Even taking foreshortening by the wide-angle lens into account, that rudder is HUGE, probably 2/3 the size of one of the elevators, and there is very little else trying to keep it straight. The chord is more than twice what mine is, and it's easily 3/4 the height. Point being, it should be extremely powerful and even a tiny motion will have a tremendous effect on the equilibrium yaw angle.
   

   
 

Thanks Brett. Now I understand the rationale better. That is worth checking out. I am adding a fix point for the rudder and will restart the trimming process with a fixed adjustable rudder. I must admit I am worried about the outside corners, but let's find out. I expect an entirely different model. I will keep you posted.

[Paul Van Dort]

I would look very closely for a warped elevator.

No apparent warp in the elevators. Was also my suspicion...

[Brett Buck]

 

 Key thing to understand about trimming, after the first hour,:

Roll and yaw are very heavily coupled, in very dynamic ways
Any variation in the line tension causes changes in the control response
Roll has no inherent stability aside from the lines
The lines act like 3-dimensional springs and oscillate accordingly, if you put any significant side force on them
The effects of the lines on the airplane are wildly asymmetrical, rolling or yawing "out" to increase the tension will have far more restoring force than rolling or yawing "in"
Tiny amounts of rudder offset result in extremely large equilibrium yaw angles
 

     Brett

[Douglas Bykoff]

I'm not an expert but many years ago I helped a friend who had bought a Magician.

The model had strange behavior. It Lowered the outer wing in tighter maneuvers. Inside pra outside.

We even put weight on the inner wing! After a close look I finaly found the problem.The wire that connected the two sides of the elevators was very thin and flexible.

This caused a very pronounced asymmetry on both sides of the elevator, inner and outer.


We also healed an Full F2B model by replacing the flexible flaps by others heavier and more rigid.



DouglasB

[Paul Van Dort]

Based on Paul Walkers trimming series, I will also have a closer look on the lead out position:

Paul writes:
"There is another issue in the trim that may be contributing to roll in corners. If the leadouts are way out of adjustment, they may induce a yaw which will affect the roll situation. If they are too far forward, they will create a nose in yaw which will tend to lift the tip (roll in) both inside and outside. If they are too far aft, the opposite situation exists. This yawing condition is more difficult to see, as sometimes I have to ask myself which came first, the yaw or the roll (chicken or the egg!). For the purpose of this discussion, it is assumed that the plane has been bench trimmed prior to flight and they are not in an extreme location."

[M Spencer]

Sheeshe !
and it says " NO PANIC " .
 

Ordinarily , You hang it from the leadouts ( a sky hook ) AND OBSERVE .

Fuse should be FLAT to a few degrees Nose Out ( down )

Line up the wing with a door jam. Assuming the door jam is vertical ! .The wing should be too .

No 1 eyeball - from each side - F & R . The TAILPLANES easyer to see , if its ; flat plate .

And theres a funny square thing on the fuse. at the spar , breaking up all the airflow to the Vert. Stab. !
Meesteer Rabe nominates 1/4 Inch rudder out at trailing edge as being adequate . So youd need less .

Maybe try it with the rudder locked straightish .
If IT were askew , then youd be where you are .  Otherwise its something else .
Might pay to check hinge to hinge distance exactly matches too . Seeing You Asked .
----------------------------------------------------------------------------------------------------------------

That CURVED WIRE to it isnt a sound engineering concept .
The O A Dimension would float about . SO Give it the big heave ho , could be wobbling all over the show  .

( ive a very light wire on mine . STRAIGHT . Figuring it only needs force in tension on the widdle Strega wudder .

[Brett Buck]

Based on Paul Walkers trimming series, I will also have a closer look on the lead out position:

Paul writes:
"There is another issue in the trim that may be contributing to roll in corners. If the leadouts are way out of adjustment, they may induce a yaw which will affect the roll situation. If they are too far forward, they will create a nose in yaw which will tend to lift the tip (roll in) both inside and outside. If they are too far aft, the opposite situation exists. This yawing condition is more difficult to see, as sometimes I have to ask myself which came first, the yaw or the roll (chicken or the egg!). For the purpose of this discussion, it is assumed that the plane has been bench trimmed prior to flight and they are not in an extreme location."

  Yes, of course, that is always part of conventional trimming. It's also the adjustment that you pretty much can't do effectively with a movable rudder, which is why you need to disable it, because, otherwise (as Paul notes) it is already hard enough to tell when you have the (fixed) rudder offset and leadouts correct, you can't also have the rudder intentionally moving, because that's too many variables and not enough equations.

   When I say "trim it conventionally" that means all the adjustments, not just those explicitly mentioned in this thread.

         Brett

[Paul Van Dort]

Dear all,
I have been doing some extra tests today. There was not a lot of time to experiment. 4 flights had to be sufficient to be ready for the national contest of Sunday in Genk. So a full basic trimming session was not possible....
Flight 1: Leadouts 1/2 inch forward, 3 gram extra tipweigth: No improvement anywhere. in general less linetension especially in the overheads.
Flight 2: Leadouts back to the original position. Rabe rudder uncoupled and fixed in the neutral setting: small (5 mm) offset to the right. No improvement anywhere. in the square 8, a yaw in/ yaw out sequence in the top corner turning left, reducing linetension. The 2 top corners of the hourglass: Loss of linetension due to yaw in. As I am flying with my arm stretched (Not completely), I could regain linetension easily by pulling in and finish the maneuver.
Flight 3: Rabe rudder re-activated. 3 gram extra tipweigth. No improvement compared to Sunday.
Flight 4: 3 gram tipweigth removed. No improvement. Flight was filmed from the model to the pilot (me!) to assess the yaw/ roll movements.



Quality of the clip is not top due to the rolling shutter syndrome, caused by engine vibration, but it is helpfull
Watching the footage in slow motion shows a lot of instability around the roll and yaw axis. It shows that rolling direction is consistent with the nature of the turn (inside -outside). Loops show little hinging, harder corners show more roll and yaw . Chicken or egg can be deducted from the footage... I still need to analyse more, but I think reducing the tipweigth might make things more stable.
Next sunday I will arrive in time at the contest side, So i can test the effect of reduced tipweigth. As a start I will take out 5 grams from a total of 40 grams. This 40 grams is the force on a scale under the outer wingtip, when the inverted fuselage is supported by the fin and spinner. Fingers crossed...

[Paul Van Dort]

Sheeshe !
and it says " NO PANIC " .
 

Ordinarily , You hang it from the leadouts ( a sky hook ) AND OBSERVE .

Fuse should be FLAT to a few degrees Nose Out ( down )

Line up the wing with a door jam. Assuming the door jam is vertical ! .The wing should be too .

No 1 eyeball - from each side - F & R . The TAILPLANES easyer to see , if its ; flat plate .

And theres a funny square thing on the fuse. at the spar , breaking up all the airflow to the Vert. Stab. !
Meesteer Rabe nominates 1/4 Inch rudder out at trailing edge as being adequate . So youd need less .

Maybe try it with the rudder locked straightish .
If IT were askew , then youd be where you are .  Otherwise its something else .
Might pay to check hinge to hinge distance exactly matches too . Seeing You Asked .
----------------------------------------------------------------------------------------------------------------

That CURVED WIRE to it isnt a sound engineering concept .
The O A Dimension would float about . SO Give it the big heave ho , could be wobbling all over the show  .

( ive a very light wire on mine . STRAIGHT . Figuring it only needs force in tension on the widdle Strega wudder .

Bench trimming has been done. No worries . Hanging the model with the leadouts from the ceiling , measuring the offset and adjusting the leadouts. Checking the vertical C.G. Equal distance of the hinge lines. Parallel tail w.r.t  main wing. Tip weigth measured with scale under tip.
....
I replaced the curved pull rod of the Rabe Rudder by a straigth one 5 months ago. Good remark and good eyesight!!!

[Matt Colan]

Dear all,
I have been doing some extra tests today. There was not a lot of time to experiment. 4 flights had to be sufficient to be ready for the national contest of Sunday in Genk. So a full basic trimming session was not possible....
Flight 1: Leadouts 1/2 inch forward, 3 gram extra tipweigth: No improvement anywhere. in general less linetension especially in the overheads.
Flight 2: Leadouts back to the original position. Rabe rudder uncoupled and fixed in the neutral setting: small (5 mm) offset to the right. No improvement anywhere. in the square 8, a yaw in/ yaw out sequence in the top corner turning left, reducing linetension. The 2 top corners of the hourglass: Loss of linetension due to yaw in. As I am flying with my arm stretched (Not completely), I could regain linetension easily by pulling in and finish the maneuver.
Flight 3: Rabe rudder re-activated. 3 gram extra tipweigth. No improvement compared to Sunday.
Flight 4: 3 gram tipweigth removed. No improvement. Flight was filmed from the model to the pilot (me!) to assess the yaw/ roll movements.



Quality of the clip is not top due to the rolling shutter syndrome, caused by engine vibration, but it is helpfull
Watching the footage in slow motion shows a lot of instability around the roll and yaw axis. It shows that rolling direction is consistent with the nature of the turn (inside -outside). Loops show little hinging, harder corners show more roll and yaw . Chicken or egg can be deducted from the footage... I still need to analyse more, but I think reducing the tipweigth might make things more stable.
Next sunday I will arrive in time at the contest side, So i can test the effect of reduced tipweigth. As a start I will take out 5 grams from a total of 40 grams. This 40 grams is the force on a scale under the outer wingtip, when the inverted fuselage is supported by the fin and spinner. Fingers crossed...

I would try moving your leadouts back next time you try. The loss of line tension on the top of the hourglass and the yaw in at the top of the square 8 is what makes me think it might have a contributing factor.

Do you see the lines whipping back and forth in maneuvers, especially in the middle of square maneuvers? To me, that’s an easy indication of leadouts too far forward and then maybe a tip weight adjustment

[Brett Buck]

   I assume you know that you are going to need a lot more than 4 flights, and those are huge changes to the leadouts.

   

   First - bench trim it.


Put the CG at about 20% of the mean geometric chord
Put the leadouts about 1 1/4" (32 mm) behind the CG
Set the (fixed) rudder offset straight ahead or *very slightly* (1mm or less) to the right
Remove all flap and elevator tweak, double check very carefully that everything is lined up the same, side to side.
Verify that the elevator is at neutral or slightly (0-2mm) *down* at neutral flap
Add 1/2 ounce (14 grams) of tip weight

   This should get you in a safe condition, probably not optimal - nose heavy and probably too much tip weight. Then fly the airplane upright and inverted, have helpers check which way you are rolled in level and inverted flight. You are trying to solve two problems at once here  - tip weight and flap tweak.

If it is rolled in at you both ways by the same amount, you need more tip weight
If it is rolled away from you both ways the same amount, you need less tip weight
If it is rolled in at you upright and away inverted, you need to tweak the inboard flap down
If it is rolled away from you upright and in at you inverted, you need to tweak the inboard flap up

Note that both conditions will likely be present at the same time,  you want to make sure that the airplane is either level, or very slightly rolled away from you both upright and inverted. Don't be too clever taking out tip weight (which per the bench trimming is likely to be excessive since you are adding 1/2 ounce on top of what you already have). At this point, adjust in 1/4 ounce increments, when in doubt, leave the extra 1/4 ounce in.

   Do another flight, do inside and outside round loops. Note which way the airplane rolls, and how much line tension you have in each direction. If it rolls to the left on insides (i..e you see the top of the wing) and right on outsides (you see the bottom of the wing), you need more tip weight. If you have more tension on insides than outsides, then you need a *small* tweak of the inboard flap up, if you have more tension on outsides than insides, you need a *small* tweak of the inboard flap down. This is MUCH more sensitive way to determine the tipweight and tweak than flying around level.

   Once you get those even, do some horizontal round 8s, watch what happens as you switch quickly from inside to ouside and outside to inside at the intersection. Adjust similarly, you should be able to see the rapid roll rotation (if any) right at the intersection. Make the same adjustments as above. As you get very close, start watching the yaw angle, too, and see which way it yaws if at all through the rapid switch in load factor in the intersection.

   At this point you should be pretty solid all around. If the round loops tend to "open up", that is, come out bigger than you intended and you feel yourself having to to put extra control pressure at the bottoms to keep them at 5 feet, move the CG back a percent or two (remove 7 grams nose weight or add 3-4 at the tail). If the round loops want to tighten up and come out too high, move the CG forward a percent or two (add 7 grams of nose weight or remove 3-4 at the tail).

    Now start doing inside and outside square loops.  Watch very carefully which way the yaw angle goes right at corner entrance. This is the part even Paul notes is sometimes difficult. If the first motion the nose makes is away from you, move the leadouts forward 1/8" (3 mm) - not **1/2 an inch**! If the first motion it makes is nose-in, move the leadouts back 3 mm. This adjusts your leadouts to the "minimum reaction" point for your rudder and other offsets. It might not be optimum, but should be safe and flying far better than before. Note that nose-out yaw will likely cause right roll on insides and left roll on outsides, nose-in yaw will tend to roll it toward you.

   Either condition of the leadouts will tend to look like you have the wrong tip weight for your corner. You should able to get this yaw motion down to nearly nothing, then and only then should you fine-adjust the tip weight for the corners. 

    This should get you pretty close starting point, safe enough for complete patterns in most conditions. I would also suggest that you err on the side of "too fast" until you really get it nailed, so when in doubt, faster is always safer.

    Note that this just gets you started and safe to fly, optimizing at different variations on the rudder offset, typically, to try to get the roll angle the same in level flight, rounds, and squares. That is, adjust the rudder a millimeter or so, then repeat most of the above process to optimize around a new condition. This is also where Dave's suggest flap tab/"wart" might come in.

   I realize that you have a contest shortly, I still think these are the right steps,  and we have had cases where we went back to square one bench trim *in between rounds at a one-day contest* even with airplanes with Expert class wins under their belts. Get as far as you can before the contest, do the best you can, then come back and we will work on it some more. This does require *very careful observation* and patience to execute. If you are having problems, find one of the more accomplished fliers at the contest and they will help you. Serge would do, I think, if he is not judging.

    Brett
 

[Steve Helmick]

Perhaps the size of the rudder is deceiving by the camera view angle, but my impression is that the moveable rudder area is about 3 or 4 times bigger than I'd attempt for a Rabe Rudder...not that I have or would use one. What I've observed from those who did use a Rabe Rudder, the less it moved, the better their plane flew, leading me to think it's a complication I don't need.

I'm also not a fan of the aero-balance on the elevators. It seems to me like there would be a lot of disruption of airflow that would not be a good thing as the elevators deflect from neutral, and the airflow at neutral wouldn't be a good thing either.

Having the camera onboard is great if you're looking for fuselage bending, stabilizer twisting, or flutter, but otherwise, it seems less beneficial than a well-located camera on a tripod.

Just my thoughts. When in doubt, always refer to Brett, Howard and Paul's advise and ignore mine!  Steve

[Ken Culbertson]

I have found slow motion video most helpful in diagnosing what a plane is doing.  However, filming it from the upwind side (judges' position) tells you virtually nothing about trim.  You need to film it from a tripod directly downwind and just far enough from the circle to capture the overheads and from the left side pointing at the plane as it comes out of maneuvers.   When you play this back you see everything the plane is doing.  Another useful exercise is to fly it on yellow Spectra lines.  You see everything the lines are doing so much better and that can be quite disturbing, especially in corners.

Ken 

[Brett Buck]

  Another useful exercise is to fly it on yellow Spectra lines.  You see everything the lines are doing so much better and that can be quite disturbing, especially in corners.

   I haven't done enough Spectra flying (and none with my real airplane) to judge the effects completely. But I note that the mass per unit length is drastically lower, so the line whip frequencies are going to be *vastly higher* and the amplitude for a given forcing function much smaller. If it looks bad with Spectra, figure it is much more dramatic with SS stranded lines. Additionally, the same reduced linear density reduces the inertial restraint of the lines, effectively reducing the inertia of the airplane at high frequencies.

     Brett

[Paul Van Dort]

   I haven't done enough Spectra flying (and none with my real airplane) to judge the effects completely. But I note that the mass per unit length is drastically lower, so the line whip frequencies are going to be *vastly higher* and the amplitude for a given forcing function much smaller. If it looks bad with Spectra, figure it is much more dramatic with SS stranded lines. Additionally, the same reduced linear density reduces the inertial restraint of the lines, effectively reducing the inertia of the airplane at high frequencies.

     Brett

Brett , Thank you for the clear and pragmatic trimming advice. I printed it out and will share with my F2B colleagues overhere. Off course my model is able to do an acceptable pattern, but it has some bad habits I want to trim out. Trimming has been going on for a long time, but I am convinced that I often solve an issue with the wrong cure. What I solve for 1 case, appears as an issue in another case. (Reminds me of my Software design/ test career) . Coming weeks I will go again trough the steps , eventually with limited fuel per flight, just  to test changes more quickly. The model is competitive enough for the coming contest. I will not beat Christoph Holterman, Not even close.  I know my limitations :-) . 
By the way, Serge is living in France, about 1000km from my place, so we rarely meet. 
I also have prepared spectra fishing lines, but I am afraid to use them. Another experiment for autumn I reckon.

[Ken Culbertson]

I also have prepared spectra fishing lines, but I am afraid to use them. Another experiment for autumn I reckon.

  I did the same thing.  How can fishing line be better than steel.  I had dreams of the knots slipping.  Finally, I got up the guts and put a set on an old ARF Nobler.  They were great and it didn't crash!   After learning how to tie the knots and properly stretch them IMHO there is nothing better.   They fly and trim different so by all means do it after the contest season, like on the way home from the last one!

Ken

[Steve Helmick]

One thing I noticed but didn't mention previously, was the amount of rudder deflection being specified at "full down" elevator. The problem is that we don't know how much deflection "full down" is, and I promise that you'll never be able to use more than 15 degrees on any hard corner. If you specified rudder deflection at 15 degrees, that'd be more useful information, IMO. Please?

If you think 45 degree elevator deflection is needed, nope, not so. The only benefit is if you break a line or leadout, and you're really really really lucky, the plane will slow enough during the multiple (maybe!) loops, it'll hit the ground with less energy and be more repairable. Some might look at that as a bad thing, though.  Steve

[M Spencer]

Not only to  be entirely annoying .  BUT .

A ' useful ' THING IS to have it ' hang on a line ' in dire circumstances .
Not perhaps the usual trimming proceedure , but a certain handle movement will have the controls at full deflection ,
Even if it is with the handle ' flat ' = square to the lines . 4 inch od leadout  travel , or is that 2 .

SO , anyfing should ' go round ' once , easy , or do a 180 . But , if it staggers and wobbles , Somethings up .
A good nasty little peacemaker ( .15 flite streak ) will go around indefinately. Then your stuck , as past 22 times you might reverse it by slamming on opposite control . So dont try more than 18 .
ANYWAY , in the  ' Combat Days ' it was a prerequisete - for if a big fat slob sat on you and you couldnt see . HOWEVER most stunt ships might go two ' tight loops ' o.k. After youve slowed it down with
some injuditious handle wagging .

Try  perfecting it first on a combat THING . DONT BLAME ME if you plant your stunt ship . But as a ' test ' I will slam on full UP & DOWN ' hang it on a leadout ' as it were . Still Air you WALK BACK .
generally your going to slow for it to have much wake .

And a Big Nasty Unstopable Engine may well stuff up the equation , like electric . You want the prop / engine - semi stalled too . BUt if a plane will  do a few without drama , its NOT FAR out of Useable for aerobatics . 

Full Tit and the wing might fold . So a few wiggles go a long way . Highly subverseive .

[kevin king]

I would think the size and shape of the rudder could also play a part in the equation.  Some have more mass at the bottom than the top, some angled some not, but I'm far from being an expert.

[M Spencer]

Id try another prop or two way differant . It looks a bit like 'P' effect ( more effective pitch on tight insides from vertical yaw ) and a touch of tourque reaction -
as , If it suddenly pulls harder ( there ) at the handle , old ironstien or suchlikes " Every Action has a Equal and Opposite Reaction ' ! might be coming to lunch .

A sudden increase in tension , will do something . Like that maybe . Tho I think your tip wobble might not be as observeable from the judges seat ? ? .

[Paul Van Dort]

Hi all, Thanks for the feedback. Indeed my hard corners are still very conservative wrt elevator deflection. it shows in the video. So also the Rudder is not doing full throw. Further examination of the videos revealed that in the overheads, the model was slightly rolling towards me. Hardly noticable. Both it was apparent in the insides as well as in the outsides. But I prefer it rolling a bit  away from me :-) . So I added 5 gram (about 1/4 ounce) extra tipweigth and bingo. This is curing my issues. Contest went very well. No linetension issues, althougth there was very little  wind and it was shifting continously . I managed 3 decent flights, so very happy . I am convinced the machine can still improve, but the basic setup is working pretty good now.