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Author Topic: Effects of rudder offset line sweep  (Read 1056 times)

Offline Bob Reeves

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Effects of rudder offset line sweep
« on: October 30, 2006, 01:33:09 PM »
Am posting this here mainly because Brett and Ted seem to visit once in a while and would like their input.

LineIII is about ready to release and need some opinions on a paragraph I am getting ready to add to the help files. Below is a synopsis of information I have gathered from several sources, it makes sense to me but want to make sure I am not saying something not exactly correct.

PS: Linedrag2003.PDF will be included in the zip file of the release.

***************************************************************
LineIII accurately calculates the theoretically correct leadout position based on an extremely complex equation that first calculates the drag and arc of the lines. For an in depth review of how LineIII works see Pete Soules Line Drag article (Linedrag2003.PDF). This calculation should be within 1/4 inch of final flight trim or something else like engine offset and/or rudder offset is effecting the models flight attitude.

Engine/Rudder offset will make the model fly crabbed out instead of tangent to the circle, this situation requires the leadouts be set further forward to compensate. The net result is two forces fighting each other wasting engine power. Also, anytime the lines go a little slack the model tries to resume its crabbed-out flight attitude. This whole situation gets more complicated because any sudden yaw change will also result in a roll. If your pride and joy is waging itís wings this could very well be the reason.

******************************************************************


Offline Bob Reeves

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Re: Effects of rudder offset line sweep
« Reply #1 on: October 31, 2006, 10:36:10 AM »
Wow 53 views and nobody has any opinions.. This has to be a first.. maybe if I added BOM to the title  ;D

Offline Terry Fancher

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Re: Effects of rudder offset line sweep
« Reply #2 on: October 31, 2006, 11:23:44 AM »
Am posting this here mainly because Brett and Ted seem to visit once in a while and would like their input.

LineIII is about ready to release and need some opinions on a paragraph I am getting ready to add to the help files. Below is a synopsis of information I have gathered from several sources, it makes sense to me but want to make sure I am not saying something not exactly correct.

PS: Linedrag2003.PDF will be included in the zip file of the release.

***************************************************************
LineIII accurately calculates the theoretically correct leadout position based on an extremely complex equation that first calculates the drag and arc of the lines. For an in depth review of how LineIII works see Pete Soules Line Drag article (Linedrag2003.PDF). This calculation should be within 1/4 inch of final flight trim or something else like engine offset and/or rudder offset is effecting the models flight attitude.

Engine/Rudder offset will make the model fly crabbed out instead of tangent to the circle, this situation requires the leadouts be set further forward to compensate. The net result is two forces fighting each other wasting engine power. Also, anytime the lines go a little slack the model tries to resume its crabbed-out flight attitude. This whole situation gets more complicated because any sudden yaw change will also result in a roll. If your pride and joy is waging itís wings this could very well be the reason.

******************************************************************

Hi Bob,

Brett is better at this stuff than me, but I do think the second paragraph should probably be eliminated or slightly restated. What you say is more or less true but only begs further questions. 

Brett would probably say that rudder offset, for instance, would actually need the lines further aft in order to optimize the ship for flights "as is".  The use of a forward leadout location to try to force the ship back to tangent would only work under a given set of conditions -- i.e. level flight at a given airspeed.  Any change from that condition would result in greater yaw oscillations as the various physical and aerodynamic fixtures/parameters fight for dominance.

I suspect Brett's position will be that if you do use rudder offset (he would stoutly argue you shouldn't in the first place) that the leadouts should be further aft so that the offset rudder is aerodynamically "neutralized" i.e. The yaw allowed by the leadout position will match that which would result from the offset rudder.  Thus, there would be no tension between the two at that given state.  Minor deviations in speed during maneuvering would therefore have a less dramatic effect on the yaw attitude of the airplane (still wrong from a drag point of view but less yawing back and forth as the physical demands of the the leadout position and the aerodynamic desires of the rudder fight one another).

The ideal location of the leadouts (to the extent that it can be ideal given the change in drag, gravity, etc. on the lines as speed and aircraft attitude change during maneuvering) is such that the fuselage will be as close to tangent to the circle as possible throughout the maneuver cycle.  Excessive yawing can be minimized even with rudder offset but only at the cost of excessive drag due to the yaw of the fuse to the circular flight path.

Further, because that drag is greater there will be more speed changes during maneuvering which will, once again, aggravate the yaw/roll situation.

Does any of that make sense?

Ted

Offline Bob Reeves

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Re: Effects of rudder offset line sweep
« Reply #3 on: October 31, 2006, 11:41:25 AM »
Yes Ted it does make allot of sense and close to what I was trying to say in my own unknowing way and still keep it within one paragraph.

I believe most Intermediate to Advanced fliers have been told so often to look at the wheels and make sure they are lined up that they will move the leadouts forward to get the darn wheels lined up and then wonder why the thing bobbles all over the sky.

Just need to come up with a short understandable way to say it .

Offline Bob Reeves

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Re: Effects of rudder offset line sweep
« Reply #4 on: October 31, 2006, 11:50:56 AM »
Oh, OK. But whatnl is "Line III"???

LineIII is an update to Pete Soules LineII which among other things calculated the leadout position relative to the CG. I found it very accurate but hard to use so enter LineIII..


Offline Dick Fowler

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Re: Effects of rudder offset line sweep
« Reply #5 on: October 31, 2006, 12:52:23 PM »
Wow 53 views and nobody has any opinions.. This has to be a first.. maybe if I added BOM to the title† ;D

I think most of us have an opinion but I think the lack of responses was that you specifically requested information from Brett or Ted.
Dick Fowler AMA 144077
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Offline Jim Thomerson

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Re: Effects of rudder offset line sweep
« Reply #6 on: October 31, 2006, 01:06:19 PM »
Old contrarian here.  I built my Humongous with some built in rudder offset, not as much as shown on Ted Snow's 1/4 scale drawing, but  some to look sort of like it's supposed to.   I ended up with the leadouts exiting in front of the CG.  Not a perfect solution, still a little yaw out, but good enough that it was a favorite airplane to fly.  On really windy days, coming around into the wind, I could see the lines blowing back and the airplane pointing in a little.  Again not a problem, but something to be aware of.

Offline Bob Reeves

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Re: Effects of rudder offset line sweep
« Reply #7 on: October 31, 2006, 02:00:20 PM »
I think most of us have an opinion but I think the lack of responses was that you specifically requested information from Brett or Ted.

Your supose to read what I meant to say instead of the way I said it  **)

Offline Dick Fowler

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Re: Effects of rudder offset line sweep
« Reply #8 on: October 31, 2006, 06:51:43 PM »
Your supose to read what I meant to say instead of the way I said it† **)

I know what you mean... actually I thought it was a rare display of self-control by all... usually we are more than willing to give an opinion whether its asked for or not!† ~> #^ ~>
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Offline Don Hutchinson AMA5402

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Re: Effects of rudder offset line sweep
« Reply #9 on: January 17, 2007, 09:17:38 PM »
Do the simple trig on an decent sized stunter with the fin/rudder dead on the centerline. The model is flying in a ~65 foot radius circle and assuming it flys perfectly tangent to the pilot, you will find out that the airstream strikes the ruddder at quite an angle thus giving you the effect of outrudder. It has been a long time since I did this but I seem to recall it was equivalent to having the entire fin/rudder offset ~3/16"* from LE to TE. There is a similar effect on the engine as well, thus you get some outthrust free in the deal too.

* I think I did it with the Impact as I had a drawing of it.

Don

Offline Jim Pollock

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Re: Effects of rudder offset line sweep
« Reply #10 on: January 18, 2007, 08:57:50 PM »
Gee Don,

I always thought it was a slight effect of inboard rudder since the slipstream is approaching the airplane from slightly inside the flight path in a skidded left turn caused by a tether?  Wouldn't that affect the inside of the rudder in a no wind situation?   ???

Jim Pollock   n~

Offline Dick Fowler

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Re: Effects of rudder offset line sweep
« Reply #11 on: January 21, 2007, 03:15:41 PM »
Jim, maybe this will clarify the situation. The blue arrows are analogous equivalent to the moving airplane hitting stationary air.

Edit _ Was Small I enlarged it a bit.
Dick Fowler AMA 144077
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Offline Jim Pollock

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Re: Effects of rudder offset line sweep
« Reply #12 on: January 21, 2007, 06:55:23 PM »
Dick,

The diagram appears to verify what I said about air vector being slightly from the inside of the circle assuming there is no crab either direction.  Very slightly from the inside.  I have always offset my vertical fin about 3/32 of an inch to the outboard side to compensate for this effect.  It looks like more may actually needed.

Jim Pollock   HB~>  n~

Offline Don Hutchinson AMA5402

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Re: Effects of rudder offset line sweep
« Reply #13 on: January 28, 2007, 09:29:09 PM »
Please read my response again. I stated that the assumption was that the model was flying perfectly tangent to the circle. If the model is biased with the nose out there will be much less "circular airflow" against the right side of the rudder, BUT you will then have a bunch more outthrust form the engine. Do the simple trig and you will find the angles suprisingly high! Actually all control line flyers ought to read two of Frank Zaic's Yearbooks, 1951 and Circular Airflow.
He explains it much better than I have room for here.
Don

Offline Dick Fowler

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Re: Effects of rudder offset line sweep
« Reply #14 on: January 29, 2007, 06:57:18 AM »
Don, I think you and I are in complete agreement.
Dick Fowler AMA 144077
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Offline Jim Pollock

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Re: Effects of rudder offset line sweep
« Reply #15 on: January 31, 2007, 10:50:53 AM »
Don,  Dick

I think that I'm beginning to see what you are seeing.  It kind of gets to be a convoluted mess when applying trig though.  But, yes still air and exactly tangent would cause air to come slightly from the outside in the skidded turn.  Whew.... n~

Jim Pollock   mw~

Offline Bill Netzeband

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Re: Effects of rudder offset line sweep
« Reply #16 on: March 13, 2007, 06:12:00 PM »
Hi Gang, just dropped in to maybe twist the rudder-lines rake discussion back to reality.  Think of the rudder offset to correct stuff sticking out and setting up a yawing moment. Like a muffler on the inboard side of an otherwise perfectly aligned construction job. This can be trimmed out by offsetting the fin/rudder to the outboard side, and verified by observing the plane in situations of reduced velocity. The secret here is both of these forces are proportional to the dynamic pressure (velocity). The lines rake receives all sorts of varying input from the plane during maneuvers, as well as level flight as you circulate through the wind.

Otherwise, the guru's are in the ball park about lines rake. I, at this moment , would like to have back the time I spent dealing with lines rake!

Have fun out there.    WildBill

Offline Jim Pollock

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Re: Effects of rudder offset line sweep
« Reply #17 on: March 13, 2007, 09:19:52 PM »
Bill,

Everyone today owes you a huge debt of gratitude for all that time you did spend on figuring out correct line rake!
My hat goes off to you sir! H^^

Jim Pollock


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