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  1. #1
    Jetdesign's Avatar
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    Need trim help.

    Hey guys,
    Igot a 69" Yak with which I'm having a tough time getting it trimmed out right. There are a few other people with the same plane out there that don't seem to have any of the same issues (so they say, though I'm not sure how many people actually try to fly precision with it).

    For some reason my plane needed a LOT of lead in the nose to balance at the location specified in the manual - something like 4oz of lead in the nose.

    The plane's habits are that it pulls out of a power-off vertical dive, pulls to canopy in a vertical up line, and pulls to canopy in knife edge flight.

    Inoticed that the plane has 'up elevator' trimmed in my radio for level flight. Removing the up elevator trim helps in all instances mentioned above, except of course for straight and level flight.

    Someone suggested that my plane might be nose heavy, so Iremoved more than half the lead from the nose - the habits got worse, and the plane now needs too much down elevator to fly inverted, where before it just required a touch. Sounds backwards to me.

    The first thing Iwas thnking was trying to add a bunch of up thrust to the engine line, thinking that it would let me remove the up elevator trim in the plane and thus help the above problems. The weight thing threw me for a loop so here Iam hoping to get some proper trimming guidance.

    Ibought this plane last fall with the intentions of trying some IMACwith it. People who fly their own versions of it say it handles precision well and Imet a guy who won some sportsmans pattern meet with one last year, so I"m hoping with a little help Ican get there.

    Thanks in advance.
    Joe Marri
    Enjoying all things aviation.

  2. #2
    Moderator Jake Ruddy's Avatar
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    RE: Need trim help.

    Well the best thing you can do is go through the Peter Goldsmith trimming guide.

    However, I would be curious if you checked the incidence at all?

  3. #3
    Jetdesign's Avatar
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    RE: Need trim help.

    Ok. So the first thing it talks about in the trim charts is CG.

    Do I do the CG trim with the elevator trimmed for level flight, or neutrally aligned with the stabilizer? I'm confused as to how removing nose weight is causing me to need more down elevator to maintain inverted flight.

    Also, when it says 'roll into vertically banked turn (>60 degrees)", does it mean just roll with aileron, or actually to make the turn using some elevator?
    Joe Marri
    Enjoying all things aviation.

  4. #4
    bubbagates's Avatar
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    RE: Need trim help.

    Joe,

    Best way is to trim the plane for level flight, then check for CG. If your fuel tank is over the CG point, most are, then you can do this anytime in your test flight. If the tank is forward the normal CG point then it's up to you but I always trim with a full tank.

    Do the CG as follows. Take off, and then into the wind, pull a 45 degree upline and go to at least 80 percent power, roll over to inverted, make sure the plane is still on the 45 degree line, let go of the sticks. If the plane's nose stays on that line for a long time then the CG is neutral, if the nose raises you are tail heavy, if the nose lowers you are nose heavy. For IMAC we normally like to see the nose lower, Mine lowers to level in about 7 or so seconds.

    Basically the faster the nose moves in whatever direction (nose up or down), the more heavy (tail or nose) you are in that direction. Anytime you trim a plane. it's a must to get the CG the way you want it first. Everything else builds off of it. Another thing is make sure you have the prop on that you intend to use. Changing the prop will change a portion of the trimming process

    Then go and do Peter Goldsmiths trimming article. It can be had on the IMAC website under Info Central and is called How to Reduce your flying workload

    Peter Goldsmith: How to reduce your flying workload!





    Bill James

    IMAC NorthEast Regional Director
    2009-2010
    2013-2014

  5. #5
    Moderator Jake Ruddy's Avatar
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    RE: Need trim help.


    ORIGINAL: gaRCfield

    Ok. So the first thing it talks about in the trim charts is CG.

    Do I do the CG trim with the elevator trimmed for level flight, or neutrally aligned with the stabilizer? I'm confused as to how removing nose weight is causing me to need more down elevator to maintain inverted flight.

    Also, when it says 'roll into vertically banked turn (>60 degrees)", does it mean just roll with aileron, or actually to make the turn using some elevator?

    This is why I asked about the incidence. Your description of how it behaves with CG changes doesnt make sense.

  6. #6
    bubbagates's Avatar
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    RE: Need trim help.

    Jake,

    I'm thinking of some thrust issues myself, though a incidence problem can cause what he is seeing
    Bill James

    IMAC NorthEast Regional Director
    2009-2010
    2013-2014

  7. #7
    bubbagates's Avatar
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    RE: Need trim help.


    ORIGINAL: gaRCfield

    Ok. So the first thing it talks about in the trim charts is CG.

    Do I do the CG trim with the elevator trimmed for level flight, or neutrally aligned with the stabilizer? I'm confused as to how removing nose weight is causing me to need more down elevator to maintain inverted flight.

    Also, when it says 'roll into vertically banked turn (>60 degrees)", does it mean just roll with aileron, or actually to make the turn using some elevator?
    Joe,

    I think that you might be over thinking the trimming article a bit too much. The article assumes that you have done basic TX trimming and are starting at the CG. Do not skip around the article, it's important to follow the steps exactly. IF the one you are using is the one that looks like a table, that's the old one, use the article I linked to.

    To answer to your question, just fly the plane like you normally would

    One thing ,unless I missed it would be to tell us all about the plane we are dealing with here. Make/model/engine/servos/batteries

    Also, are you sure you measured the initial CG location correctly, did you balance it without fuel, a bit more detail would really help, no matter how small it may seem


    Bill James

    IMAC NorthEast Regional Director
    2009-2010
    2013-2014

  8. #8
    Jetdesign's Avatar
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    RE: Need trim help.

    Guys thanks so much for your help. Ihave the trim chart but having these questions answered for this relatively new pilot is a tremendous help.

    I've got the 69" Nitromodels Yak, a 120AX engine and fuel tank just behind the firewall. I've got Hitec 69xx and 79xx servos, Airtronics RDS8000 radio, and a NMP? 2600 2cell lipo and regulator. Here are some pics.

    Ok so today I tried the 45 degree inverted thing. It was terribly windy but I was able to determine that all of the 4oz of lead could come off the nose. Now the nose still dropped when inverted at 45 degrees, but not nearly as bad. I will have to wait for another calmer day to finish finding the proper CG. With the tank ahead of the CG it might end up staying just where it is.

    Unfortunately it was too windy to continue flying so I don't know how much elevator Ineed to maintain inverted flight. What is strange is that with all that lead on the nose, this plane needed the least amount of pressure on the elevator stick for inverted flight out of every plane I've ever flown - it was nice (except for tons of yaw and coupling included).

    The odd thing is that everyone else who has this plane has it balanced at the point suggested in the manual. I needed all that lead to do the same. Now the plane must balance a good inch behind the recommended spot but it is still fliable.
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    Joe Marri
    Enjoying all things aviation.

  9. #9
    bubbagates's Avatar
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    RE: Need trim help.

    Joe,

    One thing to remember is do not worry about the suggested CG position once you set it where you like it when it flies. I could not tell you where my IMAC planes are balanced at any longer. I started out at just a tad behind the recommended position but I can tell you I'm way ahead of that by now.

    Do not assume the CG is correct until you can get a good calm or semi-clam day.

    Also, from this point on and once you have the CG set, the elevator trim you are carrying will also change once you get to changing the engines thrust angles. So for the time being, don't sweat that at this point as well

    It's also perfectly normal for a plane to pull to the canopy on a vertical downline. As the plane builds speed coming down, the wing is making lift and since the lift vector is now horizontal versus vertical when level, the plane will want to pull to the canopy, it's an aerodynamic principle. In IMAC trimming, which is what the trimming articles and charts are teaching you, we mix that out using a transmitter that is capable of mixing (most are these days) Elevator trim does have an effect as well. Engine thrust angles will also change this but to a much lesser degree.

    Again, please use the article I linked to. It's much cleaner and easier to follow and it's still written by Peter Goldsmith. The original one was written many years ago when you built airplanes from kits and had the decent ability to change what he says in that one, like incidence angles, things like that.

    One thing I just noticed in your post is you mention tons of yaw and coupling when inverted and holding elevator, you should make sure your servos are moving EXACTLY the same amount at EXACTLY the same time all the way through their complete travel. IF they are not equal, then all sorts of problems not related to CG can happen and you'll do nothing but chase the settings and never get it trimmed.

    Easy to do, take two spring type close pins, glue long straight balsa sticks to them, clip them to the same spot on each elevator half and have them meeting behind the rudder, now SLOWLY move the elevators using the radio and watch the end of the sticks, then should stay exactly in sync all the way through both up and down. MAke sure your elevator are set to prefect neutral before you start. A good way is to place the plane on a decently level surface, turn the plane and radio on and measure from a point on each elevator (make sure it's the same point on both elevators) to the surface. They should be the same.

    Why we are checking things, do the same thing with the ailerons. You do not need to be on a level surface for this. Make sure any aileron trim you have in the radio is taken out, IE make it neutral, then make sure the ailerons are aligning the the end of the wing at the root. Now move the ailerons one way and measure, now move them the other way and measure, the through would be equal. If not and if your radio supports it, just use the travel adjust to fix them. This only works of you are using two channels for the ailerons instead of a Y connection
    Bill James

    IMAC NorthEast Regional Director
    2009-2010
    2013-2014

  10. #10
    Jetdesign's Avatar
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    RE: Need trim help.

    Ok I'm working on the CG. I had a good calm day out a few days ago (before all the rain!) and found that the plane would drop it's nose from 45 degree inverted flight at a moderate rate though much better than it was. I moved things back for a mild improvement. Unfortunately my tank is not at the CG. With an empty tank the plane did not want to descend very much which leads me to think that most of my issues, at least at this point, are CG related.

    I would love a suggestion on how to deal with this. One solution is the Iron Bay regulator I have coming in the mail, but my luck with Perry pumps has me skeptical, as does the advice from the rep sending it to me who warned that it is not really suitable for an engine as big as a 1.20 two stroke. I'm noticing that the plane doesn't drift to the canopy, it actually turns. In a vertical upline, the plane will invert itself in a relatively short amount of time which I'm thinking is due to the up elevator trim for level flight.

    I borrowed an incidence meter and throw gage and will check my throws and stuff over the next day or two just to know where things are at. Changing the incidence on this plane would require surgery and I'd like to avoid that if possible.


    Joe Marri
    Enjoying all things aviation.

  11. #11

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    RE: Need trim help.

    Hi Joe,

    Been following your posts on the ARF forum too.  I have the same Yak, just began working on it.  But I took a 48" straightedge to the fuselage and checked the wing, stab, and thrust lines.  At least on my plane, it sure looks like everything is parallel to each other, so in other words every thing is set to zero with respect to each other.  For the engine, that means with respect to up/down thrust only, not right thrust.  Since this plane has apparently laser cut parts that lock together to form a rigid frame, it seems like there would not be much variation among the planes coming off the production line.  While ours could be different and you somehow got a lemon, I'm just saying that I don't think incidence is a problem unless in mounting your stab, you created a different situation from mine.  I had to do minor sanding on the hole in the fuse to make the stab sit level with the wings when viewed from the front and back.  Good luck, I think you will get this worked out so that you have a great flying Yak.  Jon
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  12. #12
    Jetdesign's Avatar
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    RE: Need trim help.

    Ok guys, here's an update:

    For some reason the recommended CG is nowhere near close to the proper CG for neutral flying. Removed all the nose weight and added 1/2 oz to the tail -Perfect! I also programmed my servos instead of trying to work out the EPA's on my radio, which it really can't do. Big improvement and the plane is straight as an arrow through rolls.


    Plane flies really nice. Very straight on downlines, slight pitch to canopy on uplines, very, very straight flying airplane with no more than 3% coupling mixes
    Joe Marri
    Enjoying all things aviation.


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