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General control line discussion => Open Forum => Topic started by: Peter in Fairfax, VA on July 26, 2019, 01:26:24 PM
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If a flapped plane turns tighter in one direction vs. the other, such as tighter inside loops vs. larger outsides, is there any possibility to make the turns more even by adjusting the clevis in or out at the elevator control horn? Assuming the control horn is in the typical stunt plane location, below the elevator, would the clevis be turned in or out to get tighter outsides? Or, does the flap/elevator position bias have no effect, or a different effect? Thoughts?
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Just read Paul Walker's trim articles (http://flyinglines.org/pw.trimflow1.html). Short answer -- yes. Adjusting the handle neutral also affects it, but differently. If you want level flight to be effortless and at equal height upright and inverted, and you want it to turn the same inside and outside, then you need to tweak both the flap-elevator bias ("neutral" is what Paul calls it in his articles) and the handle bias.
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I have had the same problem but handle adjustment or clevis adjustment were not the answers for me. The problem was due to sloppy building and there was unwanted stab incidence.
Engine thrust line was mentioned as a possibility but what about stab incidence? Do you have a way to determine if engine thrust line, wing chord, and stab are parallel?
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I have always taken the meaning of "the same" to mean that the same amount of effort turn the plane the same either way. After wings level I go for Fuselage level which is either CG or incidence. If the elevators and flaps are aligned then I use up/down thrust. Now that it is aerodynamically as level as it is going to get, I use handle bias to get the feel the same. At this point it doesn't matter to me if the plane is actually turning the same, only that it feels and looks like it is.
ken
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FYI, it's a profile, so engine thrust angle up/down is not an option.
The way I read Paul Walker's book, using his terminology, the flap/elevator neutral should first be set by looking at the attitude of the fuse in level flight. If that fails, then try moving the neutral one way and other, to taste.
Handle bias sounds too subtle for this condition. I think pulling more or less line at the handle is not my trim issue.
Moving CG aft might be good for my turns.
thanks for the input,
Peter
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Loosen the bolts, pull the nose of the engine down and tighten the bolts, worth a try?
That usually works.
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If you observe that the model turns tighter to inside, that means the flaps is too down position, when the elevator (precisely described: horizontal stab + elevator combo) is in neutral*.
In usual arrangement you need to lengthen the long pushrod, this way you "pull up" the flaps.
Adjusting the lines' length corrects only your hands "neutral", nothing else.
Istvan
*you cannot correct this way too much amount of uncorrectly mounted horizontal stab: it must be mounted absolutely parallel to the wing...
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On a profile plane the way I check engine thrust is to take the engine glow plug out and remove the fuel tank. Then put an 10" length on 1/8" wire on the engine cooling fins and have to wire go to the wing leading edge. This will tell if the engine has any up or down thrust. However, if you use electric power figure out a different but similar method!
My 2¢
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Thanks for the suggestions regarding setting engine thrust on a profile. I'll check into that.
However, I'm having a little trouble following how the handle bias described in Paul Walker's stunt trimming guide has an effect. In the example scenario from the guide where the line spacing is kept the same, but both line are moved down relative to the center of the handle, is this expected to influence how the bellcrank reacts to handle input from the pilot? For example, if the pilot moves the handle 10 degrees, would the resulting bellcrank movement be different in the two presented scenarios?
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Here's my problem with understanding handle bias, in steps:
1. The bellcrank measures the difference between the two lines. e.g. At the handle, if you pull the up line an inch, and relax the down an inch, that is the same net effect as pulling the up line two inches and leaving the other where it is. At the leadout clips, both measures the same.
2. If you tilt the handle a certain amount, say 10 degrees, it does not matter where along he handle you have chosen as the pivot point, the difference in pull is the same, provided the lines remain spaced apart the same amount.
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To get an accurate read on whether you are getting a good perception of equal insides and outsides you have to set your neutral with the handle grip straight up and down. If you have a slanted pistol grip handle, replace it with a vertical grip handle. Then fly the model and assess the condition. Your hand now will have more equal leverage in up and down motion, and you can more accurately perceive any turn bias.
If bias is detected, lengthening and shortening the pushrod length between flap and elevator horns will balance out the turn reactions.
Good luck.
Chris...
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To get an accurate read on whether you are getting a good perception of equal insides and outsides you have to set your neutral with the handle grip straight up and down. If you have a slanted pistol grip handle, replace it with a vertical grip handle. Then fly the model and assess the condition. Your hand now will have more equal leverage in up and down motion, and you can more accurately perceive any turn bias.
If bias is detected, lengthening and shortening the pushrod length between flap and elevator horns will balance out the turn reactions.
Good luck.
What Chris said!
Chris...
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What Chris said!
Chris...
Ditto!
Ted
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Thanks for the suggestions regarding setting engine thrust on a profile. I'll check into that.
However, I'm having a little trouble following how the handle bias described in Paul Walker's stunt trimming guide has an effect. In the example scenario from the guide where the line spacing is kept the same, but both line are moved down relative to the center of the handle, is this expected to influence how the bellcrank reacts to handle input from the pilot? For example, if the pilot moves the handle 10 degrees, would the resulting bellcrank movement be different in the two presented scenarios?
Yes. Moving both lines down favors more control input being applied to the down line than the up line. For purposes of understanding, consider your hand and the handle combined as a bell crank with your wrist being the pivot point. Moving the down line out away from the pivot means it travels farther for a given movement while the up line travels less since it is moved in closer to the pivot. The result is a little more down elevator will be applied than up, for a given movement of the handle. Your feel or perception on the handle seems that you are applying equal amounts of up or down but the airplane is getting slightly different inputs that balance the turning rates.
A couple decades ago I built and flew an airplane with a 30 degree 'V' tail. It flew pretty well but require a HUGE handle bias-Like both lines dropped 1/2" below center to get equal turns. In the up mode I doubt it ever took more then 10 degrees for the sharpest corner. In the down mode I'd guess it was 35-40 degrees to get equal turns. The only knock on the airplane was that with any but the lightest wind conditions the stabilizer at times thought it was a rudder and would execute some pretty sporty but unplanned aerobatics on it's own.
Dave
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I am going to add one thing I haven't seen mentioned anywhere here. While yes, you can play with controls, or droop the elevator to help equalize a turn rate, or fix hunting, etc. the other 900lb gorilla that can sneak up on you if care about such things is differential turn quality even though the rate is now feeling more equal.
Bear with me here... but if the elevator is drooped a 1/4" or whatever you end up with, you must also have moved the flaps 1/4" the other way with regards to delaying when that flap will add camber to the wing in the other direction. This sensation drives me a bit batty in certain spots... (So now you have a clean pullout with no hint of stall on outsides, but now your insides could potentially mush or stall a little, or visa versa, depending on which way you played with your pushrod)
I "think" we are better off fixing geometric thrust line issues with other means, either thrust line (engine shim) or stab incidence (if a take apart or still in the build phase).
I may be all wet, and I certainly don't have the aerodynamic schooling chop's of others here, so I'll bow to their greater knowledge, but this is at least how I currently understand the situation and putting it into practice has worked very well for me.
EricV
To get an accurate read on whether you are getting a good perception of equal insides and outsides you have to set your neutral with the handle grip straight up and down. If you have a slanted pistol grip handle, replace it with a vertical grip handle. Then fly the model and assess the condition. Your hand now will have more equal leverage in up and down motion, and you can more accurately perceive any turn bias.
If bias is detected, lengthening and shortening the pushrod length between flap and elevator horns will balance out the turn reactions.
Good luck.
Chris...
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Drawing showing net difference.
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Ditto!
Ted
You ought to know, sir. You taught me this very fact long ago.
Chris...
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One time, I tried correcting inside/outside turn size by offsetting the line's attachment points to a TED handle. It didn't seem to do anything good. I'm pretty sure there is a pushrod problem with the CF tube pushrod in that airplane, which turned equally previously. D>K Steve