Line rake and leadout drag

[Dennis Toth]

Guys,
I was flying my Still Stuka last weekend and it was flying very well, smooth and consistent. While flying I happen to look down the lines to see where the angle of the lines would hit the fuse. I had the leadouts set about 1/2" back from the CG point (pretty normal position). What I noticed was the imaginary extension line of the flying wires to the fuse would hit ahead of the CG. Since things were so nice I decided to see what would happen if I moved the leadouts back another 1/8". I was surprised with the result. This ship had always had a very long period hunt but with the more rearward leadouts it claimed down quit a bit. Tension up top was still very good. The ship is some 25 years old and has had several different power system (now electric) and solid leadouts. My guess is that the solid leads are more sensitive to drag if the rake is causing them to bend at the exit of the leadout guide which caused the hunt.

I will continue to trim these back a bit more but my question is as you move the LO's back what would be the signs that you are going to far (beside the obvious loosing line tension) and will the flight lines appear to hit the fuse at the CG when they are in the correct position.

Best,    DennisT

[Tim Wescott]

Read this here article by Paul Walker for the answer on where to put the leadouts.  And pay attention!  I have it printed out in my flight kit, although it's been a long time since I've trimmed a new plane.

My understanding is that if you put the leadouts so that the lines -- as they're dragged by the air -- point exactly at the CG, then you're close to the correct leadout position, but you'll want to do fiddling based on the way the plane flies.  I know that each plane will be different.  I use Line III or my expertly calibrated eyeball to bench-trim the leadouts.  On a stunter this typically ends up with the lines between 1/2" to 3/4" behind the CG -- that's a good starting position that'll leave you with an airplane that won't crash because of line position if you're reasonably nimble, and if the years have caught up with you then just refrain from doing overhead maneuvers until you've investigated the plane's performance at 45 degrees.

Usually when trimming out a new plane I don't get as far as line position until I'm a couple of flights in, and often I start finding out about leadout issues by flying at 45 or 60 degrees, rather than just jumping straight to the overheads.

[Trostle]

I think much of the information that is used now to calculate line rake is based on the work that Pete Soule and Bill Netzeband did years ago.  It was Pete Soule that came up with the notion that the shape of the lines, in-flight, between the handle and the model resembles a catenary curve which is the shape of chain suspended between two points.  In one of his articles on this subject, Bill Netzeband explained that when you look down the lines toward the in-flight model that approximately the first 1/3 portion of the lines from the handle to the model is very close to a straight line and will point to some position behind the model's CG.  On a windy day, this position shifts as the model travels around the circle.  On a really windy day, this position shifts significantly.

It makes for an "interesting" observation.

Keith

[Brett Buck]

Guys,
I was flying my Still Stuka last weekend and it was flying very well, smooth and consistent. While flying I happen to look down the lines to see where the angle of the lines would hit the fuse. I had the leadouts set about 1/2" back from the CG point (pretty normal position). What I noticed was the imaginary extension line of the flying wires to the fuse would hit ahead of the CG. Since things were so nice I decided to see what would happen if I moved the leadouts back another 1/8". I was surprised with the result. This ship had always had a very long period hunt but with the more rearward leadouts it claimed down quit a bit. Tension up top was still very good. The ship is some 25 years old and has had several different power system (now electric) and solid leadouts. My guess is that the solid leads are more sensitive to drag if the rake is causing them to bend at the exit of the leadout guide which caused the hunt.

I will continue to trim these back a bit more but my question is as you move the LO's back what would be the signs that you are going to far (beside the obvious loosing line tension) and will the flight lines appear to hit the fuse at the CG when they are in the correct position.

Best,    DennisT

      That's a common story, leadout position errors tend to cause hunting because it binds up the controls. It's also another good reason not to use solid leadouts, particularly on small airplanes.

      More generally, the leadouts have to be further aft if there is any rudder offset. A common misuse of adjustable leadouts is to use them to try to set the yaw angle of the airplane. You can put in lots of rudder offset, and then put the leadouts forward, and more-or-less keep the fuselage tangent to the circle in steady-state conditions. It will put a bunch of drag on the leadout guide, maybe or maybe not causing hunting, but that's not the worst problem.

      The bigger issue is that any time the line tension changes, the airplane yaw angle will change - like when you try to do a maneuver. It will also cause the airplane to roll in  - because as soon as it yaws in at you, the outboard wingtip is leading the inner, causing the flow over the outboard tip to cause much more lift and the inboard to cause much less lift. Of course, 1/2 a line whip period later, it all reverses itself. The result is wild wobbling all over the place, the line tension changing all over the plane, and since you are trying to do a corner at the same time, the control deflection to change (since the torque available to move the controls is also changing). So you also get wild hopping.

    I have referred to this in the past as "Twister Disease" because that's exactly what happens if you set up a Twister like the plans show. Thats a bit unfair because just about every other kit shows something similar. I think you should trim tangent to the circle for other reasons but even if you trim for a large yaw angle like Igor, you still have to match the leadouts to the angle or you will get something similar. So if you have a lot of rudder offset, you are choosing to trim with a yaw angle (and it take only a TINY offset to get a big angle), and you have to move the leadouts back - instead of trying to use them to drag the airplane back to tangent.   That's why if you look at David's first NATs winning airplane, it has a bunch of Zona Saw cuts in the (fixed) fin where we successively made saw kerfs to straighten out the fixed offset, and why Ted's 95 NATS winning airplane first had the fin cut off and reglued, then cut off completely and replaced to make sure it was dead straight ahead. Any why my airplanes all have adjustable (but fixed) rudders.

    I am a big fan of calculating the right position and adjusting the rudder to compensate, and then seeing if it is really tangent and adjusting from there. However, no matter how you come up with the baseline trim,  it's always useful to search around systematically to look for possible improvement. You can save tremendous time by a few simple computations, but it doesn't completely eliminate experimentation.  As long as you know what you are looking for, it will become obvious very quickly whether you are moving in the right direction or not.

    Brett
   

[Dennis Toth]

For grins, I found the LineIII program and ran it for the Stuka with my current line length and lap time and model weight. It came up with 0.85" ish back of the CG. Then I pulled the model down and measured from the current CG to the center of the leadouts (this has one of Bob Hunts single slider line guides). Current position is just about 0.75" (3/4"). Interesting, I will try to move back 1/16" (0.0625") at a time over a few flights to see where the sweet spot is.

I assume that once I go to far it will start the slow hunt again from the solid leadouts bending the other way?

Best,    DennisT

[Brett Buck]

I assume that once I go to far it will start the slow hunt again from the solid leadouts bending the other way?

   Other bad things are liable to happen before it starts hunting - unless moving it back causes the leadout to drag on something inside the wing.

    Don't overlook the rudder offset (if any) - 1/16 of an inch of rudder offset might move the "idea" leadout location by 1/2-3/4".

      Brett

p.s. as you move the leadouts back, you might end up wanting/needing to remove tip weight as well. At each change, you make the change, then adjust everything else to optimize it at that change, THEN compare it to the way you started. Just moving the leadouts without doing anything else and doing a flight might mislead you because you may have needed another change on top of the one you are adjusting. That's why the old-style "Brownian motion" purely experimental approach takes so long.

[Tim Wescott]

For grins, I found the LineIII program and ran it for the Stuka with my current line length and lap time and model weight. It came up with 0.85" ish back of the CG. Then I pulled the model down and measured from the current CG to the center of the leadouts (this has one of Bob Hunts single slider line guides). Current position is just about 0.75" (3/4"). Interesting, I will try to move back 1/16" (0.0625") at a time over a few flights to see where the sweet spot is.

I assume that once I go to far it will start the slow hunt again from the solid leadouts bending the other way?

Best,    DennisT

If you are within 1/10 inch of what Line III says, then you're close enough to start trimming by the way the airplane behaves.  Line III gives a guideline.  Truth comes from trimming and practice.

[Paul Walker]

 I guess Line III doesn't work for my ELECTRICS.


Currently on my P-47 the lead outs are 3.6" aft of the CG.

[Tim Wescott]

I guess Line III doesn't work for my ELECTRICS.


Currently on my P-47 the lead outs are 3.6" aft of the CG.

We need Line IV, with a "Paul" checkbox.

[Dennis Toth]

Guys,
OK, flew this morning, did first flight with the new setting. It felt very good as it did last week. The Stuka has only about a 15% elev/stab area of the wing area. I had been moving the CG back to get a little tighter loop radius. I wound up around 25% back from the leading edge. This worked and I got the tighter loops (which helped in the vertical eight) but it is a bit further back then most would use on this ship. For the next flight I removed a small amount of tail weight. This moved the CG forward about 1/8" hence the leads back the same amount (this matches LineIII). The ship flew great and had a little more tension at the top of the V eight and still has most of the tighter loop radius. I think this is pretty good. I think it is pretty good now. Thanks for your inputs.
Best,   DennisT

[Will Moore]

I posted a question about line rake earlier .

If you change line length or line diameter, does it change where the leadouts need to be?

For example if you go to .012 solids instead of .018 braided, or if you go 64 feet instead of 66 feet, does it change Significantly where the leadouts should go?

Will Moore

[Tim Wescott]

I posted a question about line rake earlier .

If you change line length or line diameter, does it change where the leadouts need to be?

For example if you go to .012 solids instead of .018 braided, or if you go 64 feet instead of 66 feet, does it change Significantly where the leadouts should go?

Will Moore

Answered in your thread.  I think people are holding off with your answers because it's Yatsenko-related, and in the absence of experience, we're all hesitating to say anything about them.

[Dennis Toth]

I know we are just about done with this thread but Paul's comment about his LO's being 3.6" back from the CG is interesting. Why does this work for him and what are we and LineIII missing here?

Best,      DennisT

[Brett Buck]

I know we are just about done with this thread but Paul's comment about his LO's being 3.6" back from the CG is interesting. Why does this work for him and what are we and LineIII missing here?

  LINEII is not missing anything. What seems to be happening is that Paul is trimming the airplane to have a large static yaw angle. How he (and Igor) get away with that without causing other issues, or why it is so different for electric VS IC, is still TBD.

    Brett

[Igor Burger]

I guess Line III doesn't work for my ELECTRICS.

I wanted to check if it works for my Max Bee, but I did not find rudder offset box in that program, so I cannot tell 

[Tim Wescott]

  LINEII is not missing anything. What seems to be happening is that Paul is trimming the airplane to have a large static yaw angle. How he (and Igor) get away with that without causing other issues, or why it is so different for electric VS IC, is still TBD.

I wonder if it's because they have the Igor timer to juice the thing up in the overheads?  My understanding of why yaw reduces tension overhead is because it causes drag -- if you have a gizmo that gooses the motor when the plane slows down, that would certainly go away.

Just thinking, no proof.

[Igor Burger]

My understanding is that if you put the leadouts so that the lines -- as they're dragged by the air -- point exactly at the CG, then you're close to the correct leadout position

Exactly, I posted long message about it somewhere on Stuka I think back in 2004, this is most important picture from that, someone can remember, it tells that is position where all forces are in balance and invariant to line tension - mean do not change overhead, in wind and turbulence:

[Igor Burger]

I wonder if it's because they have the Igor timer to juice the thing up in the overheads?  My understanding of why yaw reduces tension overhead is because it causes drag -- if you have a gizmo that gooses the motor when the plane slows down, that would certainly go away.

I do not think " yaw reduces tension overhead" is exact, I would say "LO aft of the CG" (in polit view pojection) reduces line tension - imagine that situation, if centrifugal force concentrated in CG front of the LO tends to yaw fuselage out, then if you go overhead, where the centrifugal force is lower because of lower speed and plus lowered by gravity must cause negative yaw, and that makes line tension lower.

[Dennis Toth]

On another thread in the design section Peter Germann gave an explanation that desires consideration. Peter suggests that as the ship goes up top that gravity starts to reduce line tension and the ship also slows down reducing it further. If you reduce the weight of the ship say 50% (example guess) LineIII moves the lines back the same %. On my ship it when from 0.83" to 1.66" back, something to think about. It seems speed up top works.

Best,   DennisT

[Brett Buck]

On another thread in the design section Peter Germann gave an explanation that desires consideration. Peter suggests that as the ship goes up top that gravity starts to reduce line tension and the ship also slows down reducing it further. If you reduce the weight of the ship say 50% (example guess) LineIII moves the lines back the same %. On my ship it when from 0.83" to 1.66" back, something to think about. It seems speed up top works.

   That effect is definitely real, however, it doesn't explain why it want to be so different with electric than IC.

    Brett

[Igor Burger]

   That effect is definitely real, however, it doesn't explain why it want to be so different with electric than IC.

I do not think there is anything aerodynamically different regarding yaw trimming. The only difference could be another 4-2-4 break of yawed engine, that can cause another (wrong) feeling of model retrimed with high yaw angle.

As I wrote, I posted that message back in 2004 when I flew piped IC engine and later I reworked that model for electric motor in 2008, without moving LO at all.

[Igor Burger]

On another thread in the design section Peter Germann gave an explanation that desires consideration. Peter suggests that as the ship goes up top that gravity starts to reduce line tension and the ship also slows down reducing it further. If you reduce the weight of the ship say 50% (example guess) LineIII moves the lines back the same %. On my ship it when from 0.83" to 1.66" back, something to think about. It seems speed up top works.

Clear, the formula is lines drag balanced by centrifugal force, so if centrifugal force changes its value, the line rake must change also. That means:

1/ hard to trim the model for level and also for overhead
2/ changing yaw during flight is the lats I want on my model and it certainly does not make well flying model
3/ models with line at the place given by that program still fly well, contrary to 1/ and 2/ ... means for me, that the hypothesis used in that program is not so well applicable for stunt models 

[Brett Buck]

3/ models with line at the place given by that program still fly well, contrary to 1/ and 2/ ... means for me, that the hypothesis used in that program is not so well applicable for stunt models 

        Or, alternately, 1 and 2 aren't correct. The hypothesis for the program itself is perfectly simple, it computes and integrates the effects of the line drag. Using it as a baseline to set the leadout position is my idea/observation, on the theory that it you are flying tangent to the circle, you don't want the leadouts to be applying any torque to the airplane.

     It's just a theory, but it seems to work out time and again. I can't see why it would be different for electric, but I don't deny there is something going on that I don't get, like the extreme forward CG.

      Brett

[Igor Burger]

        Or, alternately, 1 and 2 aren't correct.

ok, 2/ is subjective, so it is not true/false, but the 1/ is easy to try, simply run the program for base weight and the for the weight minus gravity, it will make different value and that is proof for 1/ :- ))

That program is done for speed models without taking side area and rudder to the equation, that is reason why it does not model whole stunt c/l airplane, and does not give answer why mine and Pauls way works well, if you add fuselage area to the equation, you will have well working math model for "tanget" trimming and also for "yawed" trimming. 

[Igor Burger]

     It's just a theory, but it seems to work out time and again. I can't see why it would be different for electric, but I don't deny there is something going on that I don't get, like the extreme forward CG.

I am sure it is not different (if I ignore the engine run). That program gives some "usefull" number for line rake, but if you count with the side area and use my model, you will see that any "usefull" line rake will work well, also usual 1" also mine 2.5" also Pauls 3.6" and also extreme 30 degrees yaw which we use on indoors.

[Dennis Toth]

Ok, what I am getting out of this discussion is that on a current PA stunt design with fairly large rear fuse area and significant rudder area the line rake could go more rearward then the current programs predict. Seems this large area aft of the CG tend to hold the ship tangent and the line rake goes rearward to reducing most of the yaw bending of the leadouts (this is more noticeable with smaller ships and solid leadouts). Could be that with the IC power the vibration breaks the friction at the leadout guide so the bending friction is not noticed as much as with electric.

This sounds pretty good until you think about what happens to the low level maneuvers down wind in a higher wind conditions. Does the large aft area get pushed out and cause an inward yaw? Does the higher rake angle off set this?

Best,    Dennis

[Keith Miller]

Very interesting thread!
Question: What steps do you folks take to reduce the friction at the leadout guides to accommodate the rake of the leadouts?

My LineIII calcs for a given model has a line projection of about 15" on 52' lines. A quick guesstimate tells me the rake is probably on the order of 3 degrees as the cable enters the leadout.
I'm guessing/betting that there's really no worry with that little bit of drag on the guides, eh?

[Dennis Toth]

Keith,
One thing you can do is make sure the leadouts are cable not solid wires (use a nylon bellcrank). Make sure there is a little room around the leadout wires and the eyelets/tubes. You don't want to have the leadout wires tight in guides which could cause drag on them and which could cause some hunting.

Best,    DennisT

[Brett Buck]

Very interesting thread!
Question: What steps do you folks take to reduce the friction at the leadout guides to accommodate the rake of the leadouts?

My LineIII calcs for a given model has a line projection of about 15" on 52' lines. A quick guesstimate tells me the rake is probably on the order of 3 degrees as the cable enters the leadout.
I'm guessing/betting that there's really no worry with that little bit of drag on the guides, eh?

   The sweep angle at the tip is directly calculated by the program, or at least the tangent of the sweep angle.

    Brett

[Bob Reeves]

Happy to see some are using LineIII, was allot of work putting a user friendly interface on Pete's work and makes me feel good to know it wasn't a completely wasted effort. It has always worked well on my 4 stroke powered stunt ships but I never progressed past winning Advanced at Brodak's. My flying days are all but over due to my Clumsiness getting worse as I get older, I really have to be careful to make sure my feet are actually doing what my brain thinks they are doing.

Anyway, just happy to see you guys making use of LineIII, it will get you in the ballpark and a much better starting position than a WAG...

[Perry Rose]

If I understand this right the bell crank should be as far forward in the wing as possible which should ease a lot of the drag at the leadout guide. Does Line 3 factor in the bellcrank location?

[Keith Miller]

   The sweep angle at the tip is directly calculated by the program, or at least the tangent of the sweep angle.

    Brett

Brett - where do I find the calculated sweep angle? Don't see it in my copy of LineIII. (screenshot)

[Bob Reeves]

It's under the hood, it calculates sweep angle then uses it to calculate lead-out position. Didn't display it as it's not a very useful parameter.

[Dennis Toth]

Perry,
There hasn't been a lot of discussion on bellcrank location but generally speaking I agree that since we are raking the lines back it makes sense to have the BC as far forward as you can to reduce any bend in the leadouts. Most of the discussion have established that the position of the leadouts dictate the position of the ship relative to the circle not the bellcrank.

The main advantage to the forward BC position is to reduce drag on the leadouts against the leadout guides. Any drag on the leadouts could cause the ship to hunt in level flight. This is much more prevalent with solid leadouts. Back in the day most of us use solid leadouts because the cable would saw through the soft aluminum bellcranks we had at the time. With the cable and nylon BC we have now there is not as much of an issue particularly is IC power as the vibration breaks some of the drag friction.

Best,   DennisT

[Brett Buck]

Brett - where do I find the calculated sweep angle? Don't see it in my copy of LineIII. (screenshot)

 
   I use LINEII (which was the one Pete Soule came up with), because I could never get LINEIII to work. Bob's version is a lot slicker.

    In LINEII,  it's at the bottom of the list of things it calculates.  I would show you a screenshot, but I hadn't run it in a few years, and now it also doesn't run under my emulator.

    If you have the leadout position, I am not sure why you would care about the projected line angle, aside from curiosity. It might make evaluation of Igor's observation easier - take a white thread, paint a black spot on it ever 6" or so, trail it behind the airplane, then sight down the line to get the actual projected angle, compare to computed angle, see if it is different. For instance, you would expected it to move further back overhead, and you could check if it was actually -1g different by altering the airplane mass you enter in the program.

     Brett

[Brett Buck]

Ok, what I am getting out of this discussion is that on a current PA stunt design with fairly large rear fuse area and significant rudder area the line rake could go more rearward then the current programs predict.

    Mine is more-or-less the king of the "aft fuse area" (large passive yaw stability) theory. It *cannot* be flown crabbed sideways and you *cannot* attempt to drag it sideways by moving the leadouts aft. Unless you like a lot of bad things to happen in the corner, at least.  The idea behind the theory of using LINEII to calculate the leadout position is based on the concept that you never want the leadouts to apply any force to the airplane in level flight. In a hypothetical perfect situations, you use the leadout position that corresponds to tangent to the circle, and adjust the rudder until there is no load applied to the leadouts. When it is right, you can imagine the leadouts floating in the middle of the guide and not touching.

       That reduces the trim problem to other items that screw up the trim, all of which are generally much smaller effects. Varying precession (from gyroscpic effects) have negligible effect, so you can just ignore them for the most part. That's why it has all that fuselage/fin/rudder area in the first place - to provide passive yaw stability to handle more power.

     If you try to fly it crabbed sideways (i.e. with a static yaw angle) by moving the leadouts, what you will get is a bunch of load on the leadout guide in level flight, and the line tension/leadouts fighting the fuselage and rudder. In exactly steady-state conditions, it will stay at some yaw angle where the fuse/fin/rudder are trying to point it tangent to the circle, and the leadouts are forcing it nose-out, and it reaches some equilibrium.

      As soon as you touch the controls, all these relationships changes, the speed changes ,and the line tension changes, and then it will start yawing (and rolling, since yaw couples into roll) all over the place.

    The airplanes that I have known about or seen that could handle flying at a substantial yaw angle were all very weakly stable in yaw, not strongly stable like mine with lots of fuse/fin/rudder area. Combat planes, for instance, can fly at a big yaw angle and it varies all over the place with little impact to the results.  Igor's airplane is kind of like that, too, with what I would consider a lot of area up in the nose and a "vestigial" fin/rudder (which is also why his Rabe rudder can/has to move so far). On mine, the rudder is the only thing that is touchy to trim - 1/32 of movement or warp has drastically ill effects on the trim.

     Brett

     

    The airplanes that have I have seen flying with a

[Dennis Toth]

From the discussion so far we seem to agree that line rake is one trim item that is not simple or easy to explain its objective (who knew setting the line rake could be this hard?). I always thought the objective of the line rake was to get the ship to be tangent to the circle. We would set the leads with the front lead on the CG (so you wound up about 1/2" back for the center). We checked this doing simple wingovers, having a helper observe the ships attitude as it started the climb over the top. Looking back I can't remember really seeing a ship hanging way out so all was well. Now I have realized that maybe the trick with the line rake is to set it to minimize drag on the leadouts and let other trim components adjust the tangent line.

Best,    DennisT