Stuka Stunt Main Forum · 16 of 16 known posts recovered
Just a quick response.
I've always thought Len's suggestion was a touch short of the whole story. Oftentimes, moving the leadouts forward will increase tension. This assumes, however, that they were too far aft to start with. Sometimes true, but certainly not always. Take this to the bank; if the leadouts are already too far forward, moving them forward will make overhead tension worse.
Obviously, the trick is to get them in the right place to start with.
In direct response to your question; if the leadouts are too far aft the airplane will always be flying with the nose pointed out of the circle. As a result, the fuselage is creating a lot of drag flying sideways. If you concentrate only on level flight you can get a ship so it is pulling very firmly on the lines by doing so, although, strangely enough, if they get too far aft even in level flight you will start to lose tension because you will start wasting power trying to fly out of the circle and the airplane will slow down. Airspeed is the primary source of line tension (centrifugal force and all that) so if the drag starts to slow the airplane tension will decrease.
What happens in maneuvering flight is that all that additional drag hangs around while you are adding a bunch more drag for the maneuvers plus fighting the pull of gravity as you get higher in the circle. This drag increase can be too much for the engine power to overcome and the airplane slows and, therefore, line tension decreases.
You can get your leadouts very close to the right position just by recognizing they need to exit the wingtip slightly aft of the point the airplane balances when holding it by the wingtips. this is true because the lines have drag and will "bow" aft of the direction of flight as a result. thus, we place the leadout's exit point about three degrees aft of where the CG (measuring the rake angle from the center of gravity at the acft's centerline ... not absolutely technically correct but so close who cares).
With the leadouts at that location and the wings level in level flight you'll be very close. Variation from that points will depend on engine power, line length and diameter, acft speed and acft weight.
The effect is as follows: If the acft is heavier, faster, on shorter or thinner lines , or has a more powerful engine, the leadouts will move forward because all of these increase centrifugal force. The opposite of the above will result in a correct leadout position a little aft.
If you ever get more than a half inch or so off the three degree preset in either direction on nominal 50 to 60 inch wingspans (that's alot) you are probably looking for the solution to a problem in the wrong place.
final thought. Combat planes appear to fly in the face of my contentions. On deeper consideration, that's not true. These ships fly with lots more offset without negative reaction because they have essentially no side area. thus, the thrust of the engine can be biased out of the circle to allow adequate or more tension anywhere on a circle even in pretty high winds ... a necessity for chasing each others tails. It is undoubtedly true that a combat ship would fly faster if the leadouts were moved forward a bit at at time since the forward thrust vector would be greater ... again, at the cost of tension into the wind.
Hope this helps.
Ted fancher
Disclaimer: I am neither a NATS Champion (yet), professional engine reworker, engine producer, column writer, Hobby Shop owner, *rockeet* scientist, nor otherwise. Just an old Advanced flier. 
Jim
Some thought provoking stuff there but none that isn't understandable.
The fast rat has no significant line tension problems in level flight. If anything it'll pull so hard as to be difficult to fly.
Because it flies so fast the line tension due to centrifugal force will be very high. This will result in a decreasing amount of line bow even with the higher speeds. To get it to fly tangent to the circle will require very little leadout sweep.
In fact, it is not uncommon to purposely run the leadouts as you suggest or even a little forward of the CG to both reduce line tension and fatigue for the pilot and to increase the airspeed. By having the leadouts exit forward of the CG whatever line bow does develop will actually nose the acft slightly into the circle (the CG "will" line up with the leadout exit point!). With an excess of tension already that's not a bad thing.
The second thing that happens is the thrust vector of the engine more closely aligns with the direction of flight and, when optimum, will actually have 100% of that thrust working to move the acft forward and 0% trying to make it fly away from the circle. That's a good thing if you're flying a go fast event.
Ted
Well, I thought that too - that the leadouts on combat planes seem awfully far back. But after it came up the other day, I ran it though the line sweep program with some guessed-at parameters (100 mph, .018 lines, 20 oz) and bingo, 4 degrees+. And given that they can get away with significant outborad yaw, experiment and theory once again seem to be in accord.
By the way,on a combat plane as described above, the lines take more power all by themselves than it takes to fly a stunt plane - .32 HP.
Brett
I think it depends on what vintage Combat plane you're talking about, and what event. Current Fast Combat planes (tapered foam wing, Nelson engine) have leadout guide positions that are fairly forward - behind the CG, but not by much. They pull pretty darned hard, incidentally (though not nearly as hard as a .46 sized stunter - geesh..). Older ones had more sweep. Speed limit planes also have more sweep. Still, not as much as older designs.
First of all, yes, I should have said the location is based on the mid point of the leadouts. I should have also mentioned I think the leadouts should be as close together as practical because, remember, the airplanes yaw attitude is going to depend on that location and when the load is biased to one line or the other the yaw angle will change slightly due to an offset between the two.
Second, the 1/2" to which I referred is the amount of "variation" I think is a practical limit from the "predicted" location which will be roughly three degrees aft of the CG.
On most 50-60 inch wings this'll work out to a predicted location of around an inch to and inch and a half aft of the CG at the wingtip. Somebody with a calculator will certainly tell us exactly how much three degrees is at 25 and 30 inches (halfspan, remember!).
Ted
p.s. There are some other dynamics at work with the go fast planes that can make for some squirreliness. In particular, on take-off, torque must be considered and it is important that the thrust line run inboard of the lateral (spanwise) CG. From a standing start a "moment' will develop between the thrust line and the CG and, if the CG is inboard of the thrust line, that moment will turn the aircraft into the circle on release until such time as speed is great enough to generate the centrifugal force to overcome it.
25 inch span
Each degree = .43" x 3 = 1.29" Center of LO aft of CG
30 inch span
Each degree = .52" x 3 = 1.56" Center of LO aft of CG
Crist Rigotti
"A driver trying to be a pilot."
http://www.clguy.com
If you were laying out a new wing plan, or putting adjustable leadouts in an older design, would you assume this number to be the middle of the adjustment range and then have at least + and - .5 inch?
In addition to what Ted mentioned, I would also add once again that the "LINEII" program by Pete Soule gives the leadout position WRT the CG to within a gnat's <<insert body part>> of the right value, assuming everything is straight and you put the data in properly.
I might ask for the source code and make a super-stripped-down version (console app sort of thing) that might be easier to put the data in for.
Brett
Yes indeed, a very helpful way of looking at it, at least for me. Maybe not another apple clonking Newton on the head, especially for experts, but possibly a fresh angle on it for the rest of us.
Actually, I found the whole thread helpful. But wait . . . isn't this one of THOSE topics:
>We keep answering a good
>number of questions over and over - "how do I break in my
>Fox 35", "WHAT DO I DO WITH MY ADJUSTABLE LEADOUTS", "Why
>can't I fly my ARF Nobler in Open at the NATS "
>...without having to ask the same question over
>again. How does a uniflow tank work? WHERE SHOULD I SET MY
>LEADOUTS? The trim-flow chart. Where do I set my pipe for
>a starting point?
caps added to point out, not yelling. 
Kim Mortimore
Sorry to reply to my own post, but its seems that the link to LINEII on Pete Soule's site is broken. I will email him to see if it's OK to post it somewhere for download.
I can't over-emphasize that this analysis (and the program that implements it) pretty well solves the leadout issue, assuming everything is straight, and you are not intentionally trying to yaw it outboard (in which case you simply add the desired offset to the calculated value). I keep trying "parametric studies" on my airplane and see it on others, and the right answer experimentally is usually within 1/16" of the calculated position, and in those case it isn't, has always indicated some misalignement.
Brett
p.s. via the miracle boon of typing, use this directory link:
http://www.geocities.com/AMA271/Aeronotes/
and then scroll down to where it says lineII.zip and hit the link - then you'll get it. I don't know why the direct link does not work - it looks fine to me.
Any chance to get the source code to this so that those of us with Macs could generate a version that runs on our OS?
Cy
The Cat Whisperer