Overhead line tension?

[Paul Taylor]

Found this thread and it seems to explain what I need to do but want to make sure something was not lost in translation.
Soft overhead eights I need to move the leadouts forward?

Thanks guys
Paul

Posted on SSW By Igor Berger. A rather interesting approach and runs contrary to some people's opinion.


 http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=130453&mesg_id=130453&listing_type=search

Yaw and leadouts on pictures after all"
Fri Dec-03-04 03:55 AM

           I see my texts are not very clear, so I did some pictures. Hope this will show what I mean. I do not know if my ability to make self explaining pictures is better than my English, but I hope all together will be enough to understand what I mean after all.

So first of all usual understanding what is happening here.

I assume:

1/ Model is tangent to the circle
2/ fuselage & engine has no side aerodynamic forces
3/ lines are outside of wing
4/ bellcrank (BC) is at (CG)

Here is Ted’s visualization from "Bellcranks and CGs redux":

We have dragy lines and moving mass point CG at end of them. Centrifugal force of CG makes tension in lines, so they tend to be straight, but drag makes its usual curvature.

So let’s call the centripetal force Fc. The line drag of lines Fd. It is clear, that stable situation is, if the angle makes another force Fy which is in size equivalent to Fd but with opposite orientation. All is on pic1.



Static (flat) theory tells us where to put lines in leadouts to keep tangent position of fuselage.

So we know the shape and we can convert the CG point to real model and fix lines on place, which will match actual position of lines. Nothing happens. Line drag is counterbalanced by centrifugal force of CG to LO position and forces are balanced. Pic2.





Now assume that we have the same model and we fly overhead. The shape of lines curve is different, because the force FC is less gravity FG. The drag is the same. It means lines are more round. Pic3.





Therefore if LO is in wing fixed on the same place, the nose will yaw inwards. That is happening because CG position is not aligned with LO position and therefore any change in that force leads to yawing. Pic4.




Such a model is not flyable, so “flat” theory cannot work for aerobatic model. … at least at those conditions over.

Now another try.

Assume that we have little bit functioning rudder and it makes constant force Fr on tail. Tail is of the same length as wing. So that force is permanently yawing nose out and thus inboard tip forward. Just opposite than the line drag Fd is. Pic5.





It means that it is the force, which counterbalances line drag instead of CG position. If we want reach no friction in LO, we must put BC far forward, but we know that BC position has no effect to yaw and thus we can live it in CG.

Both line drag and also force on rudder are aerodynamic forces and every change in speed has proportional effect to both of them, so they are in balance at every speed. It means that CG can stay aligned with LO not making any yaw.

As the CG is aligned with LO and not making yaw, then also variation in line tension does not make any VARIATION in that nonexistent yaw. Aerodynamic forces are still in balance, thus also if curvature of lines is different, the resulting stable orientation of fuselage still tangent. Pic6.





We can fly overhead or strongly pull handle and model will still keep its angle.

It is not only CG or (exclusively) only rudder what can balance the line drag. They can work together. Assume the rudder is little smaller and its effect is too small for line drag. Its force is not enough, model tends to yaw in, but we can put lines little back and give CG chance to balance the rest. No problem, but it will make lower line tension overhead.

Opposite situation – if rudder is stronger than necessary, it will lead to opposite situation. We will move leadouts FRONT, CG will fall aft of LO thus not allow outboard yaw caused by excessive rudder force (Fr>Fd) and we are still at tangent position. But lack of line tension will point nose OUT … Dick, are you watching? No gismo, no screws, no tricks, just simply proper design/trim. It means LO moved forward will improve line tension overhead – sounds familiar?

I am not calling for any change. We are able trim models and they fly well. I am only explaining what is happening here. So if we use calculation in hope that “flat” theory is proper and works also for our stunt models, then we simply get situation on pic 2. But we fly on circular path and that makes forces permanently yawing out. The CG can in that case fall to in-flight level of LO, or front of or aft of LO. That situation is on Pic 7.





So the rudder, LO position can very effectively place CG on proper place making that proper response not allowing too much yaw, but also keeping good tension overhead. Pic 8 shows detailed configuration. Fvr is variation of line tension and it gives idea what is its effect on yaw.









 

[Dave_Trible]

Paul that can be true if they were too far back to start with.  There are other things working here like misalignment (-), a little engine offset (+), airspeed (MO BETTA), rudder offset (-), airplane weight (+=-), the CG of the airplane and wing asymmetry /tip weight.  Lets forgo this last one for now.
If you are trying to fly the airplane a little too nose heavy it may try to turn in at you overhead as the airplane slows and goes more knife edge.  ( Also an overly large rudder and rear fuselage area may cause more wind vaning, hence moving side area forward is a +.  This plays stronger if you are flying slowly).  Try to fly the airplane not-so nose heavy and reduce handle line spacing to compensate.  Be sure you are flying the airplane at a reasonable clip for it's weight and power.  Anything slower than 5.4 for most airplane setups is likely too slow.  Once you have the CG set you can hang the airplane from the leadoffs on a single hook and measure how much the nose of the fuselage is hanging down from level.  For most airplanes that are otherwise fairly close in trim it should be about one to one and a half degrees.  This should be very close to the final leadout setting for an IC airplane.  They are doing some other things with electric setups that I haven't looked into much so thats for someone else to answer.  If your airplane meets these standards then i'd try at first to creep the leadouts BACK in tiny increments and fly it to sense the results.  Tweak these backward and forward to find the sweet spot.  If its still not holding well enough then look at more power and props with narrower blades and a little more pitch at the tips especially.  Don't be afraid of a degree or two of engine offset.

Dave

[Paul Taylor]

Thanks Dave for the input I appreciate it.
It's my make over Vector 40. It's no longer as tail heavy as it was before. I do have some nose weight and engine off set. It's now balancing on the CG according to the plans. I will check the nose by hanging it and get a real lap time.
This trimming stuff is a dark art for sure. 😊

I will report back. Thanks again.

[Dave_Trible]

Thanks Dave for the input I appreciate it.
It's my make over Vector 40. It's no longer as tail heavy as it was before. I do have some nose weight and engine off set. It's now balancing on the CG according to the plans. I will check the nose by hanging it and get a real lap time.
This trimming stuff is a dark art for sure. 😊

I will report back. Thanks again.

It's one long course you never quite graduate from Paul.  Then you forget half of what you learned and it comes back around and bites your behind sometimes.  Good luck!

[Dane Martin]

Like Dave said, that flying too slowly thing is a funny one. I had no tension in certain things on the twister. Because it was flying too slow, it just came in a little when trying to maneuver. After engine and prop set up, that was corrected. Then I moved the lead out back because my engine was much lighter than the 4 stroke I built it with. Voila, it gets through a pattern repeatably now.

I'm certainly not qualified to give advice, just pointing out the interesting reasons these problems can occur.

[Gary Dowler]

A most interesting and informative post.  Thanks for taking the effort to put it out there Paul.

Gary

[Paul Taylor]

Update. Did the leadout hang. Looks like 2 degrees nose down.




Sent from my iPhone using Tapatalk

[Motorman]

blank

[Dave_Trible]

Update. Did the leadout hang. Looks like 2 degrees nose down.




Sent from my iPhone using Tapatalk

OK.  That may be just a little too much.  Move the lead outs forward maybe 3/32" and try it.  If that helps go another notch. When It definitely turns turkey then back up one spot and that should be about the best you'll get.  Then if need be go in a little on the needle.  If you are using carbon props try putting in a little more pitch and more at the tips.  A little goes a long way.  If using something like an APC 4" pitch try a 5".  You may just have to pull more grunt out of the engine.  Thrust can overcome a lot of issues. 

Dave

[Paul Taylor]

Thanks Dave!

Will make the adjustment and start playing with props.

[Al Ferraro]

What prop, engine? How long are your lines and OD?
Al