Leadouts position

I am starting this thread because I think it is aout of topic in "bellcrank position" ... so ...

Serge wrote:

>>>Because of the plane's circular path, there is already inherent out-thrust and "right" rudder<<<

That is one of, but not the only unanswered questions which I noted above.

Brett wrote that centrifugal force is enough and he also wrote that leadouts should be at that "neutral" point where drag of lines is balanced by moment of centrifugal force. So it looks for perfectly tangent fuselage. We all do it that way with good result (including myself) but if you think about it it just can not be true. Look:

1/ Let's assume that we can perfectly balance line drag and centrifugal force in level flight - it is easy calculation and we have even program for that. But what happens if we fly overhead with centrifugal force less gravity. The line tension is smaller and result is excessive line drag. It will turn nose inwards and model will fall to the circle. ... but in reality it does NOT ... we know it ... we even know that if we put lines aft of that neutral point (what will be proper for flight overhead because of smaller pull) we can get even worse result - it is just counterproductive.

2/ We fly on circular path. Circular flow from that flight makes lift on fuselage oriented toward the center of circle. That is because air is rotating around the wing (fuselage in this case) and that makes lift. So if we do not want parasitic effects (many times mentioned by Brett in original thread ) then fuselage needs some offset out of the circle which I wrote already - it is ~1.5 deg. And surprise, from my experience - if I have well trimmed model it always points little bit out of the circle. It is very well visible on hingeline or on landing gears. If I measure it, it is always very close to 1.5 deg. But if we have our sweet yaw different then zero then how can that "drag against the CG" calculation work? - in my eyes, that calculation tells us to put lines 1.5 deg front of "real" neutral point.

3/ We fly on circular path. So there is that mentioned "effective" rudder offset - even if rudder is straight. But not only that. We have also engine thrust misaligned with AC of wing - we know that it is right of the fuselage. (do not think of line drag - that is considered to be balanced by its position to the CG). And there is also pitching moment of fuselage - because every flat (straight) body in air flow wants to continue in straight movement (no I am not speaking about mass inertia, I am speaking about aerodynamics) so fuselage rotating in air at lap time speed makes moment against that rotating.

So I my eyes: the theory about CG matching leadouts is only static vision, but reality is little different. Lines are in level flight pretty ovehanged and they are forcing in leadouts backwards (I am speaking about trimmed model). It means lines are front of that neutral point of balance and thus if we fly overhead, limited pull does not lead to yawing toward the circle. It can make even opposite effect - if lines are front of CG, CG in level yaws inwards at stronger pull. That pretty well explains why front placed lines brings more uniform line tension over head and in level.

However this what I wrote does not change well proven fact that lines adjusted by that calculation are very good starting point ... however I think it is only coincidence and not some great theory. ... at least I do not see any ... if someone has better I would like hear it

Igor,

Thanks! Very nicely put.

Now, not a better answer, but a side effect of your comments? As you say, we can come close with design calculations for steady level flight, then trim to best possible by adjustments we see we need.

Design calculations -- and their fine-tuned final arrangement -- can get us pretty much centered in the middle of the range of loads and conditions the model meets in flight. The deviations from that centered trim are nearly balanced from one side to the other. They are not stacked to favor of inside turning loads, or outside loads.

There should be less difference in response and feel between easy or hard inside and outside turning... That sounds to me to be more consistent, which MUST feel better...

You have my spreadsheet and you can find most of data to this problem. If I add an enumeration of the aerodynamic yaw (fuselage is wing with known camber and aspect ratio – I do not see problem). It should be easy to find point wit ANY response – positive or negative, technically I do not see problem. But you know – the question is not technical, question is what exactly we want. If it is perfectly neutral response – means we have always line tension exactly equal to centrifugal force, then the CG should be perfectly aligned with leadouts guide. If we need positive response – means little yawing out at low line tension, the CG must be little back. … but how much? Too little can be danger in critical situations, too much can lead to hinging or too much drag and it will lead to lower centrifugal force … that is that proper question.

Igor, need some clarification.

"1/ Let's assume that we can perfectly balance line drag and centrifugal force in level flight - it is easy calculation and we have even program for that. But what happens if we fly overhead with centrifugal force less gravity. The line tension is smaller and result is excessive line drag. It will turn nose inwards and model will fall to the circle. ... but in reality it does NOT ... we know it ... we even know that if we put lines aft of that neutral point (what will be proper for flight overhead because of smaller pull) we can get even worse result - it is just counterproductive."

Are you speaking of drag created by the lines moving thru the air or are you speaking of the increase in line angle at the entry point of the wing (line sag due to reduction of tension) which causes inward rotation of the nose?

"So I my eyes: the theory about CG matching leadouts is only static vision, but reality is little different. Lines are in level flight pretty ovehanged and they are forcing in leadouts backwards (I am speaking about trimmed model). It means lines are front of that neutral point of balance and thus if we fly overhead, limited pull does not lead to yawing toward the circle. It can make even opposite effect - if lines are front of CG, CG in level yaws inwards at stronger pull. That pretty well explains why front placed lines brings more uniform line tension over head and in level."

Makes perfect sense to me but I'm not sure I'm comfortable with very light tension in level flight. I guess I'm conditioned to "pull is my friend" and start to backpedal anytime the line tension drops.

Old habits die hard. (and so do Old Wives Tales)

Ok, little different:

We are in level flight. Assume that we have no aerodynamic forces beside one: line drag. The drag is constant at constant speed. The drag yaws model inward. We have centrifugal force concentrated in CG. Lines are attached to wing at leadout position little aft of CG. So centrifugal force make counter moment yawing nose out of the circle. Let’s assume fuselage is actually perfectly tangent. If we fly exactly overhead, the line tension is smaller because of gravity. Therefore line drag is stronger then original centrifugal force in level flight and thus stable position of CG which can balance that drag is more front – and fuselage WILL turn to that position, but that means the nose must point toward the circle little bit, but it will make even smaller line tension ….

>>> Makes perfect sense to me but I'm not sure I'm comfortable with very light tension in level flight.<<<
I do not see problem here … you have always line tension equivalent to centrifugal force. That all over is not any call for change that is attempt to describe formally what we now do intuitively.

Quote...We are in level flight. Assume that we have no aerodynamic forces beside one: line drag. The drag is constant at constant speed. The drag yaws model inward. We have centrifugal force concentrated in CG. Lines are attached to wing at leadout position little aft of CG. So centrifugal force make counter moment yawing nose out of the circle. Let’s assume fuselage is actually perfectly tangent. If we fly exactly overhead, the line tension is smaller because of gravity. Therefore line drag is stronger then original centrifugal force in level flight and thus stable position of CG which can balance that drag is more front – and fuselage WILL turn to that position, but that means the nose must point toward the circle little bit, but it will make even smaller line tension ….

Again, forgive me for being dense but I visualize line drag as the resistance created by it moving through the air. This would be independent of tension. I believe that the lines roughly form a catenary curve (parabolic) from the handle to the leadouts. Tension would affect the "sag". Then I see the sag changing the exit angle from the leadouts which has the effect of yawing the nose inward causing the effect that you are talking about. We maybe saying the same thing.

Also the reduction of G force would cause the CG to swing in due to that little torque arm created by the leadouts being behind the CG again causing the nose to yaw inward.

Am I on the right track?

Now I know what you mean. I think we not need to speak here effect of bellcrank. Those pictures in another thread are jokes of course. So does not matter what is inside the wing, flexible leadouts act as attached to wingtip at leadouts guide without any mechanic moment in that point (as they are FLEXIBLE).

So since there is no moment, does not matter what is the curvature of lines. It is certainly not parabola, because the line drag is not uniform down the lines, but exact name of that shape really does not matter here. I say that the shape of that curve does not affect the force on tip from lines. It is only drag of lines what makes force, which is oriented against the flight direction and is tangent to the circle.

If you agree with that over, all is clear I think.
I am too lazy to make analytical reasons or force diagrams. If you do not agree, try to make counter argument, we will see … may be you are right … but I do not think so. In any case I will certainly answer.

Igor,

I got carried away last night with the previous post -- a bit wordy...

Maybe the other guys in this thread would like to work THIS idea over: the yaw effect of leadout guide placement.

I see it as the result of a *torque* with line pull as the acting force (essentially centrifugal force, plus/minus aerodynamic and propwash effects), and the *radius distance* as how fore or aft of the CG the pull force's line of action passes.

We can estimate quite well the angle at which the lines come to the tip line guides. (Once the lines/leadouts are inside the wing, air drag no longer affects them.) Line force pull continued, at that angle, toward the CG determines any arm distance for the yaw-stabilizing torque effects.

If leadouts 'aim' pull force aft of the CG one length unit(inch, cm, whatever) and pull is 6 force units(lbs, n, whatever), the CG, solidly mounted in the model structure (Oh, I HOPE so!!!) rotates the model in yaw to line up with the pull force that enters the tip guides. The torque causing rotation in this example is 1(length unit) * 6(force units).

If you are into estimating the stability effects of the lines, this makes an easier approach.

We know static CG location.

We know where the tip guides are, and how they'd relate to the CG if the body centerline lies tangent to the flight path.

We can time laps and measure actual flight radius to CG, weigh the model, and calculate the CF.

A first approximation of where the leadout guides SHOULD be, ideally, will have either NO yaw torque from pull and CG alignment/ misalignment, or with the offset we've found to work in past experience.

With about 3+g of CF, we have quite a bit of torque trying to rotate the model to align the CG with the pull force. Other offsets and conditions -- I presume -- apply forces smaller than the 15 lb (~7Kg) level flight pull a heavy modern stunter can generate. These others still belong in the force diagram, though, and require additional trimmng for the very discerning flier.

From that other post:

>>>Then, leadouts supposedly 'float' in their guides with no fore/aft load in low acceleration flight conditions.<<<

It could happen, but it does not mean that lines have no yawing moment. Because the only reason for floating is that bellcrank more front of original point. In that case CG moment to pivot replaces force in leadouts. We know that position of pivot is irrelevant – because CG moment to pivot is always equal to force in leadouts (longitudinal) therefore we can ignore bellcrank as it is and count to picture only drag as force concentrated at leadout point.

>>>We can estimate quite well the angle at which the lines come to the tip line guides.<<<
Yes, that is. The angle depends on two perpendicular forces (as it is flexible) – the line pull and line drag. Is that true? If you have harder pull lines are straight, if you have lower pull, lines are curved. But longitudinal force at tip of wing is still the same and its value is line drag

Dick and Igor,

The curve taken by the lines in flight is an "accelerated catenary" per Peter Soule' in an article in 1971 or thereabouts Aeromodeller Annual. He wrote of estimating drag and line rake in regard to speed CL models. Great article! (Suspension bridge cables follow an UNaccelerated catenary, as they support the road against a steady 1 g. Air drag loads vary along our lines.)

Igor, when we swapped spreadsheets several years back, I was impressed at how much further you had gone with detailed design data on the 'other' aspects of model design. You didn't stop there, but have kept improving and refining... Thank you.

I tried to stay more simple, with the strong forces first and a glance at others to see if they were worth a lot of further work. Of course they are, but many of them have VERY much less affect on our models than the strong forces.

Lift is (at times)the strongest force our models encounter. Centrifugal force, Induced Drag (during high g loads), and thrust (at times) can measure to several pounds/kilos. Not much else can.

The cardboard bellcrank demo has one value. The separation between the leadouts and the bellcrank positions, as seen along 'flight radii,' acts, but internally, WITHIN the structure of the model. The static demo arrangement works pretty well for the dynamics in flight, BUT with the addition of aerodynamic and thrust effects.

Sharply bent (even flexible) leadouts 'tend' to 'saw' on the tip guides, when the bellcrank pivot is not near the CG. (I have had a model or two --not recently-- showing this nature of wear.) I try to design my models, or redesign existing plan and kit originals, subtly, to locate the bellcrank pivot very near the CG, and to put the tip leadout guides where they meet the 'trail angle' of the lines as they approach the wingtip.

Then, leadouts supposedly 'float' in their guides with no fore/aft load in low acceleration flight conditions. (Turning the flight path circle IS a form of acceleration.) (There is NO YAW INDUCING EFFECT at these times. Pull is always aimed AT the CG, along the curved lines.) The pull load in level or straight flight is evenly divided, of course, until airload resistance on moved control surfaces must be met by a shift of the part of pull to the 'acting' line, and off the other one.

On a correctly proportioned and balanced model, the overall drag force should also act through the CG -- and I include line drag in that. Ideally, and in fact, the lines are NOT connected at the leadout guides, but at the bellcrank. Pull AND drag Forces from the lines -- if bellcrank pivot, rake and tip guides are properly arranged -- all pass to the near vicinity of the CG without touching anything else. (IMHO)I also accidentally chose to design leadout guide position, initially, for the low velocity (-- and low pull --)conditions Bill Netzeband advised me were present in maneuvers at the top of the flight hemisphere. Yes, that means slightly more right-yaw than ideal in level flight. Highly practiced, modern fliers will probably find further trimming desirable, but for me it's been a good starting point.

Yes, the straight fuselage centerline acts as a slight nose-out, tail out 'rigging' -- how significant?

On a 70' (21.3 m) radius, lap distance for the CG is about 440' (134 m) If the model is 4'(1.2 m) long, with CG at 30% of fuselage length, the value of the tangent of angle between radii from center to CG and center to prop is 1.2'/70' (.366 m /21.3 m)-- about 1º. A straight fin on this model, using the same method, is less than 2.3º aft of CG on the flight circle. These numbers can be used to find the effective offset of thrust line and fin. Those offsets are similarly small...

Dick, in addition to the trig effect of static weight reducing 'pull', consider also that the turn into the climb is a high-g move. Induced drag also slams the brakes on. As Brett mentions a lot lately, even constant RPM, or run mode, or even airspeed, do not mean we see or feel constant conditions. Wind gives us -- if not the model -- a sense of speed variation. Maneuvering loads reflecting in induced drag changes WILL affect even a SAITO 92 powered model.

- A fly landing on a cantilever bridge WILL cause a deflection... microscopically small, but still there...

We have to learn to use what we can get out of our stunters. Trying to order Mother Nature to obey us is not very productive. REDUCING the nuisance of some things IS possible, but we should keep clear the difference between those and what we cannot 'correct' completely.

Playing with this side of our hobby is a joy to some of us, a bore to others. There's room for both ends of the range, and all flavors between -- so long as we enjoy what we do.

>>>Yes, the straight fuselage centerline acts as a slight nose-out, tail out 'rigging' -- how significant?<<<

Right now I am too lazy to calculate that all, but may be following will tell more:

We are trimming model to be independent on line tension (I yaw). Means we do not want to be sensitive to strong pulls in corners and also to make differences in line tension overhead and in level. So I guess the CG is very close to aligning with leadouts. If that is true, then the only force which can counterbalance the line drag (and that is not insignificant) is the fuselage moment, rudder moment and prop thrust moment. If we know that prop thrust in level flight is relatively small, then also prop yaw is not very large (Howard wrote about it somewhere below). It means that we have that fuselage moment which is enough to keep that 1.5 deg offset also in level flight against line drag.

I think the rudder force will be also relatively small because at angle =1.5deg the rudder is relatively in low lift AoA, but fuselage can go up to moment coefficient Cm=0.02 that can make on larger fuselage which is making relatively large area but also LONG area (we know that Cm is FORCE, not moment and that force needs application to LE – means on arm fuselage length /4 to get the MOMENT) and the resulting moment is thus relatively strong – and almost independent on yaw. So therefore I think the fuselage moment is biggest contributor of those parasitic yaw sources. … one day I must enumerate all of them, but it will mean some research how aspect ratio changes moment … I have no idea right now.

BTW did you detect that smaller models need more rudder offset? Does not it fit my theory?

>
>The cardboard bellcrank demo has one value. The separation
>between the leadouts and the bellcrank positions, as seen
>along 'flight radii,' acts, but internally, WITHIN the
>structure of the model. The static demo arrangement works
>pretty well for the dynamics in flight, BUT with the
>addition of aerodynamic and thrust effects.
>
>Sharply bent (even flexible) leadouts 'tend' to 'saw' on the
>tip guides, when the bellcrank pivot is not near the CG. (I
>have had a model or two --not recently-- showing this nature
>of wear.) I try to design my models, or redesign existing
>plan and kit originals, subtly, to locate the bellcrank
>pivot very near the CG, and to put the tip leadout guides
>where they meet the 'trail angle' of the lines as they
>approach the wingtip.
>

Agree 100%, and I have the sawed through leadout guides to prove it. A little "face-saving" here, admittedly, but I did offer this opinion in the middle of all the bellcrank stuff (#39. in "Bellcrank #2" thread):

It still sounds like we are agreeing, and the rest may be semantics. Moving the bellcrank may cause undo friction and stress at the leadout guides, and the plane will attempt to rotate to relieve that stress. If that rotation is different than the thrust line (assuming some aerodynamic differences), then yaw will occur in flight. Line rake does the same thing, as everyone will agree. The key is that both are altering the centerline of the mechanism with respect to the orientation of the plane."

maybe it wipes a little egg off ... still suffering in Engineer Eat Crow Heck ...

>2/ We fly on circular path. Circular flow from that flight
>makes lift on fuselage oriented toward the center of circle.
>That is because air is rotating around the wing (fuselage in
>this case) and that makes lift. So if we do not want
>parasitic effects (many times mentioned by Brett in original
>thread ) then fuselage needs some offset out of the circle
>which I wrote already - it is ~1.5 deg.

I have been using Brett's trimming methods exclusively without deviation. I have found in all cases so far that the yaw "nuetral" is not zero. The rudder has some deflection to achieve the most "nuetral" yaw setting. Airfoiled rudders have also resulted in satisfactory yaw "nuetral".

Once the leadouts are correct, the best performance settings result in a very slight "yawed out" look. I measure performance with the corner radius, wiggles, and performance at 45 degrees. 45 degree performance, in my mind, is the most easy to see. At perfect yaw "nuetral" the airplane will definately turn the fastest corner with the highest "swivel rate".

>>>I have been using Brett's trimming methods exclusively without deviation. I have found in all cases so far that the yaw "nuetral" is not zero. The rudder has some deflection to achieve the most "nuetral" yaw setting. Airfoiled rudders have also resulted in satisfactory yaw "nuetral".<<<
I even ended up at inward rudder deflection. I got very uniform line tension and clean flight in calm. But in turbulence I got very often problems with complete losing of line tension. Speaking/ thinking about subj., I tend to believe there is some optimal ruder deflection for this problem. But Brett’s way to trim the udder is completely about something else. It is more about roll in tight and soft maneuvers, or if you want about horizontal adjustment of wing and tail. It seems to me that from this (different) point of view there is ANOTHER optimal rudder offset. Now comes the question what to change to put those two optimizations of the same property to one point. Since roll and yaw mixes together (or even converts) very easy I do not see simple way how to trim it by intuition. It could be done by adjusting rudder for one and then adjusting outer flap chord size for another. But I do not think I am able to do it just by seeing wha model does in air. So that the reason to find way how to get it properly before building – by calculation.

>>>Once the leadouts are correct, the best performance settings result in a very slight "yawed out" look.<<<
Yes that is what I think. But it is in contradiction with way how we did it – if we did it that way – by simple calculation of drag and centrifugal force in level flight. It should be perfectly tangent in that case, but it is not. … So we use theory which works well for us, but that theory is simply wrong I my eyes

Igor-

I'm not sure I understand your question.

FWIW, I meant that if one views the circling control-line model from above, the relative motion of the air to the model is circular (a la Frank Zaic). Thus, even if the model is flying with the fuselage exactly tangent to the circular flight path at its c.g., the air hits the nose slightly from the inside and the tail slightly from the outside of the circle, both nose and tail tending to create yaw outward from the circle.

I have to leave town in just a few minutes and will respond further if needed on my return - IF this #*?_&^#@ modem will continue to allow it.

>>>I'm not sure I understand your question.<<<
If there is a question then only if that my thought is OK. But as I think about it, I see it makes sense – so may be another question which is in reply to Lou ... how much.

>>>if one views the circling control-line model from above, the relative motion of the air to the model is circular (a la Frank Zaic). Thus, even if the model is flying with the fuselage exactly tangent to the circular flight path at its c.g., the air hits the nose slightly from the inside and the tail slightly from the outside of the circle, both nose and tail tending to create yaw outward from the circle. <<<

Yes, I agree you can do it that way or that way how I wrote it or even “relook” at that flow as straightened – in that case the fuselage is round. It acts like a cambered airfoil. That will also answer why it makes lift toward the center of circle if it is tangent. – it is nice visualization what is happening there.

I suspect that a lot of side force comes from air deflected by the prop disk in sideslip. I know it's a lot more than thrust * sin (beta), but I don't know how much.

Depends what is beta. If you count with yaw angle only, it could be but if you think of model with equal pannels the engine thrust line offset to AC (which is well right of fuselage) makes stronger yawing. Anyway, I think the strongest yawing makes rudder (beside lines and CG to lines).

Yes, I was thinking of the local beta at the prop disk.

I don't know if this is relevant or not. I built a Humongous with some rudder deflectrion built in. I ended up with the leadouts right at the CG and still getting a little outward yaw. I dithered about moving the leadouts a little furthur forward, but the airplane flew well enough that I never did. I decided not to do anything after flying the model in a high wind. Coming into the wind, the line drag picked up enough that the model was flying tangent to the circle. I thought if I had moved the leadouts forward a little more I would be in trouble in the wind, so I did not.