stunthanger.com
Building Tips and technical articles. => Building techniques => Topic started by: FLOYD CARTER on August 13, 2012, 09:32:25 AM
-
I notice that some people make the front leadout as the "up" line. I've built both ways and I can't find any difference. After all, there are just as many maneuvers which pull hard on the down line, as well as the up line. If there is any aerodynamic advantage, I can't find one.
Floyd
-
Has nothing to do with aerodynamics. It's for yaw control. I believe Al Rabe developed it.
-
What Milton said......& Al Rabe is the "man"
-
Last weekend at "The Scobee", I overheard a conversation between Chris Cox, Alan Resinger and Paul Walker. Chris was complaining about a problem with his "Crossfire", which I think was yawing on one of the top corners of the Hourglass. It's electric powered, flies CCW and uses a "pusher" prop. Next one will have the "up" line aft. Alan's "Crossfire" is also electric powered, but uses a "tractor" prop, because he flies CW, and has the up line forward and no problem with it. Paul had already figured that out, from testing both pusher and tractor props on older models.
I guess the moral of the story is that just because you can reverse polarity and use a pusher prop doesn't mean you should, unless you build the plane with the leadouts appropriately reversed. That said, most of us just aren't at the skill level where we'd notice the difference. :'( Steve
-
Last weekend at "The Scobee", I overheard a conversation between Chris Cox, Alan Resinger and Paul Walker. Chris was complaining about a problem with his "Crossfire", which I think was yawing on one of the top corners of the Hourglass. It's electric powered, flies CCW and uses a "pusher" prop. Next one will have the "up" line aft. Alan's "Crossfire" is also electric powered, but uses a "tractor" prop, because he flies CW, and has the up line forward and no problem with it. Paul had already figured that out, from testing both pusher and tractor props on older models.
I guess the moral of the story is that just because you can reverse polarity and use a pusher prop doesn't mean you should, unless you build the plane with the leadouts appropriately reversed. That said, most of us just aren't at the skill level where we'd notice the difference. :'( Steve
Mine are all conventional (rear line up) and it seems to work as well as anything else.
Backwards props are interesting and the difference in the trim is clearly evident just from watching. Not necessarily better, but different.
Brett
-
I've always understood that having the up line at the front helps counter precession induced yaw (both in and out). The difference in line tension between the up and down lines tends to yaw the model in the opposite direction to precession.
-
Chris and I spent the past two mornings working on the problem with his model yawing. Paul judged and commented about the amount of yaw Chris' plane compared to mine. I've used the reversed bellcrank for years to counter yaw. Chris has never reversed his. Because we are both flying Hunt Crossfires with the same setup apart from props, we started checking adjustments. We were going to reverse his leadouts and try that but then noted that his up line was almost 1" aft of mine. We moved his leadouts closer together moving the aft line (up line) about 3/4" forward. His yawing problem was greatly reduced. Moving both forward another 1/4" improved the problem further. When Paul first started flying electric, I commented about his use of the pusher prop. His answer started "well you know how during the first corner of the outside square the model" he stopped and said "I forgot, you know all about that", his reference about the fact that flying CW, I have always taken advantage of the motor torque to aid in outside manouvers. The same reason that most electric flyers use pusher props to take advantage of the direction the motor spins.
Alan Resinger
-
I have a set of plans for the early 1950's Trixy and the leadout wires cross over each other. What do you think was the reason for that. Was that there way to get the front leadout to be the up line. :!
-
Put the leadouts one over and under the other.
Bob Palmer did it. I did it in my twin boom Ares back in the early 60's.
They could still be adjustable if they were over and under.
Looks nice too.
Took a while to find this photo, I haven't posted it in a while.
I'm slipping. ;D
Charles
-
In some approximate calculations I did decades ago, prop Gyroscopic Precession can (sort of) be estimated. It DO exist, and numbers that fell out of those "studies" suggest it may be noticeable. - In CCW upright flight, an inside turn causes precession torque tending to yaw the nose out, and vice versa. Depends on RPM, diameter and weight of prop and a few other things.
Generally, the effect is small. However, I have two models that served as experimental 'goats.' Profiles - not a big thing to switch the elevator horn to the opposite surface. (Same thing as running UP line forward if you switch the horn to above the elev...) The smaller profile - an M&P Ercoupe - was very twitchy in yaw on hard inputs when UP line was aft; the larger - a Goldberg Shoestring - twitched less w/UP line aft, but the effect was still observable.
Our CLPA cornering is severe enough that I would rather not have to counter precession effects any more than absolutely necessary. Combining various trim conditions with a potentially hard, brief precession kick may worsen tendencies to "hinge." Thus, I've used UP line forward since. if it's only a feel good thing, so...?
-
In some approximate calculations I did decades ago, prop Gyroscopic Precession can (sort of) be estimated.
No "sort of" about it, it's a simple matter to get the real value to any degree of accuracy. What you aren't getting very well is the P-factor that in most cases goes the other way, nor the magnitude of the other disturbances. It has been my observation that in virtually all cases tiny errors in the other trim elements completely swamps the yaw component of precession, P-factor, etc. The pitch component is a lot easier to see and to deal with.
Brett
-
I ran the Over/Under setup for a number of years and moved away from it in the last 2 models.. In trimming it I found the "up" line ended up further forward to the down line anyway so the added weight almost proved counterproductive.
-
Most of the things we do in controline are compromises that work together to benifit the overall performance. While the difference in up/down leadout postion may be small or could be trimmed out using other methods, the position that makes the most sense would be that which works in our favor, particularly in hard corners, i.e. up leadout in front (normal CCW flight and CCW engine rotation).
Here's a quote from Al Rabe a while back on this forum:
"OK. My answer is that it doesn't make much difference, but I have always thought that a front UP line would offer a slight advantage with regard to gyroscopic precession. The front down line may have a slight negative effect. Since there isn't enough difference to worry about, why not go for the slight possible improvement?"
Al
-
Hi.
How notable is the difference when you swap from front/up leadout to front/down? I remember Igor mentioning somewhere, that he allways tries it both ways.
As I'm using a rather big 3-bladed 14,2x5,5" pusher prop in my plane, I feel a slight problem in controlling the outside square maneuvres. OK, the line tension in top of hourglass is great, but I'm sure that I could manage with less. The problems are biggest in outside squares, too much line tension makes it difficult to fly accurately.
There is also noticeable wear in the rear lead-out guide and also the rea lead-out cable shows some signs of abuse. The distance between front and rear cable is about 15mm in tip.
The model is a Yatsenko Shark. Being fully take-apart it's not a big deal to replace the bellcrank with a new one, even though I must move the bellcrank pivot slightly outwards. I'm just wondering if it's worth the trouble. Or should I concentrate in making the engine go slower in outsides? Power is not an issue.
Lauri
-
I would think the spacing between the leadouts at the tip has a lot to do with it. Mine are within 3/4" of each other.
-
Paul Walker finally beat into my head to have the leadouts independently adjustable. That has been nice for trimming out yaw.
-
Mine are too independently adjustable. Should I try moving them closer to each other first? Minimum is about 1/4" L