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PZ UM T-28 trim issue

Old 02-23-2011, 11:59 PM
  #1  
NJSwede
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Default PZ UM T-28 trim issue

It's me again!

My PZ UM T-28 needs a CRAZY amount of rudder trim to fly straight. Once it's trimmed, it flies great, but it looks completely insane when you look at the rudder (see below). I've been trying to figure out what's wrong, but I haven't been able to so far. I don't see anything banana shaped or anything. There's no significant crash damage (I think).

I'm posting some pictures. Maybe someone has an idea? Maybe this is normal and I'm just being a noob who doesn't understand that...
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Old 02-24-2011, 12:18 AM
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Wrongway-Feldman
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based on those pictures it looks your vertical control is out of alignment with the fuselage.
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Old 02-24-2011, 12:29 AM
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dumo01
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Originally Posted by NJSwede View Post
It's me again!

My PZ UM T-28 needs a CRAZY amount of rudder trim to fly straight. Once it's trimmed, it flies great, but it looks completely insane when you look at the rudder (see below). I've been trying to figure out what's wrong, but I haven't been able to so far. I don't see anything banana shaped or anything. There's no significant crash damage (I think).

I'm posting some pictures. Maybe someone has an idea? Maybe this is normal and I'm just being a noob who doesn't understand that...


Interesting, usually I would expect to find some sort of crash damage to the rudder or perhaps the thrust angle on the motor has been changed. I think you have something going on though, it should not need that amount of trim.
Good luck with it hope someone else has a better idea than I do

I looked at the alignment issue as did Wrong way and wondered about it but thought it was just the angle you took the photo at, I thought you were looking down left side of the fue instead of straight on from the nose.
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Old 02-24-2011, 12:38 AM
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Wrongway-Feldman
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Originally Posted by dumo01 View Post
I looked at the alignment issue as did Wrong way and wondered about it but thought it was just the angle you took the photo at, I thought you were looking down left side of the fue instead of straight on from the nose.
If you look closely at the horizontal stabilizer you can see how it is aligned with the main wing but over too far in relationship to the vertical stabilizer.
Is this a brand new plane? Did it used to fly trimmed and now is having these problems? has it crashed?
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Old 02-24-2011, 12:44 AM
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Originally Posted by Wrongway-Feldman View Post
based on those pictures it looks your vertical control is out of alignment with the fuselage.
It does? On which picture do you see that? I've been looking for exactly that, but I can't find anything!
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Old 02-24-2011, 12:47 AM
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Originally Posted by Wrongway-Feldman View Post
If you look closely at the horizontal stabilizer you can see how it is aligned with the main wing but over too far in relationship to the vertical stabilizer.
Is this a brand new plane? Did it used to fly trimmed and now is having these problems? has it crashed?
It did a cartwheel in soft snow on the maiden, but the reason for that (apart from too much wind) is that it made a sharp left turn right after launch. So it feels like it was there from the beginning...
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Old 02-24-2011, 12:51 AM
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Wrongway-Feldman
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the third picture seems to show it the best. The right side looks hinky to me. In that picture looking slightly down the left side of the plane you can see a little of the side of the vertical. But in the 4th picture which looks slightly from the right side of the plane the vertical is on line with the viewpoint.
It really looks to me like the tail is slightly of to one side. It doesn't take a lot to mess with the flight characteristics.
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Old 02-24-2011, 12:56 AM
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Hmmm... Now I see it! You may be on to something here! I'm going to see if I can figure out a way to measure it. Thanks!
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Old 02-24-2011, 12:59 AM
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I think its what Wrongway said,lay a staightedge on the vert stab and see where it comes out toward cannopy
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Old 02-24-2011, 01:06 AM
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Other than that, I'm absolutely in love with this plane! It's really surprisingly easy to fly, even for a relative newbie like me. Once she's trimmed she's stable as a rock with a lot of dihedral and self correcting characteristics.
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Old 02-24-2011, 01:09 AM
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Wrongway-Feldman
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I hope that's all it is. should be a relatively simple fix.
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Old 02-24-2011, 03:34 AM
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One of mine is similar, it crabs through the air once trimmed to fly straight....
Still flies okay though....
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Old 02-24-2011, 03:47 AM
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Also if it's a motor angle issue, easy enough to diagnose.

#1 check it manually to see if the prop/motor will wiggle in it's mount.

#2 in the air see what happen if you cut the throttle abruptly. If it immediately veers in the direction the rudder is trimmed, your motor is loose.

"P" factor of the prop wants to pull the nose of the plane up and to the left, that's why the motor is angled down and to the right. If the motor is loose as soon as you apply throttle it will want to move either/or to the left and/or up, which will cause the nose of the plane to follow.

In yours the mount may not be holding side to side, so the motor cranks over to the left as soon as it's throttled up. In turn you have to apply right rudder to compensate.
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Old 02-24-2011, 05:40 PM
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This remains a mystery!

It turns out that what we thought we saw on the photos is probably an optical illusion. I measured and it seems the vertical stab is straight. It's a little difficult to measure, since the foam is really thin and flimsy, but I'd say it's as straight as it gets.

The motor mount seems solid and there's a clear right-facing thrust angle.

Shady, you said you had a similar issue, right? Same thing? Insane rudder trim, some crabbing, but Ok flight other than that? Did you investigate?

I wonder if I should call Horizon Hobby support? Maybe it's a known problem?
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Old 02-24-2011, 09:54 PM
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Swede - call them! That is why they are there, to make customers happy so they return. Even if it is not a defect they may well have seen it and have a solution at hand. Now, pick up the phone and dial .....
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Old 02-25-2011, 11:56 PM
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Glacier Girl
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Swede, could still be a motor angle issue.
Try it in the air. Trim for straight and level flight 1/2 throttle or more.
3 mistakes high and cut the throttle. If it instantly veers off to the side it's a motor angle problem.

Also something else I just thought of. I've seen this on a couple of Trojans.

Make sure the H stab is square to the wing. Both one the same level. Like I said I've seen a couple where the plastic mounts for the H stab were misaligned, one sat slightly higher then the other. If so it will cause the plane to bank towards the high side, and inputting rudder trim will compensate for it.

Also check your lateral balance. One finger on the prop shaft end, and one on the bottom of the rudder and lift the plane up. It should stay level. If one wing drops, your bird is heavy on that side and will want to bank that way. Again rudder trim compensates somewhat for it. Easy fix, add weight to the other wing.
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Old 03-04-2011, 05:16 PM
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NJ,

I'm wondering why you are using the rudder to keep her flying straight? Usually with aileron models, especially low wingers, the ailerons are far and away the most effectve 'turning control'. You would normally trim for straight flight using ailerons, not rudder. You will probably find that with the rudder centred just a touch of aileron is all thats' needed to fly straight.

Assuming you have checked that everything is straight, true, twist free, and laterally balanced then the left turn is most likely due to torque/prop effects. This is easily confirmed by checking if the model only turns left when power is added. If so a little right thrust should fix it.

Steve
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Old 03-04-2011, 05:25 PM
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Well... If I centered both sticks, the plane wanted to fly in a circle, not get into a perpetual roll. So I figured the ailerons are probably ok, but something's up with the rudder. In hindsight, it's probably more likely to be the thrust angle. It's been too windy here to fly micros, so I haven't had a chance to check if the behavior changes if I cut throttle.
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Old 03-04-2011, 06:09 PM
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NJ,

A little aileron trim will stop it 'circling' MUCH more effectively than rudder. On a model such as the T-28 aileron is your primary turn control, the rudder is really there only for take off and aerobatics (not wanting to start the usual; 'bank and yank' argument here). If it were mine then I'd not even think of touching the rudder trim, I'd do it all by ailerons. it's a fact that on many aerobatic type models rudder has no turning effect whatsoever, it just produces yaw.

Centre the rudder and give it a hint of right aileron trim and you will see what i'm talking about


Once you have it flying straight with aileron then do some power on/power off comparisons to see if it is thrust that's causing the turn. I suspect a little right thrust might be the 'final solution'.

Steve

Last edited by JetPlaneFlyer; 03-04-2011 at 08:51 PM.
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Old 03-04-2011, 07:29 PM
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Yep, power on/off is a quick way to tell if it's a motor angle problem.

Power on, trim for straight flight. Power off. If it veers in the direction opposite from what it was doing before you know it's motor angle related.

Power off and it stays the same, you know it's a trim or CG issue that you are dealing with.

Note though on power. You trim for a certain speed. If you increase speed, more then likely the plane will climb. Not nose up, it will climb. Back off the throttle and it will start a shallow dive, not nose over.

If you are seeing items like this, nose up or down, or a hard pull to one side or the other, when you throttle up/down, you have a motor angle problem you are fighting.


And compensating with trim instead of correcting the angle can get real ugly on you.

My first Gee Bee had a really bad up motor angle problem on maiden. Every bit of down elevator trim to get it somewhat under control at speed.

All fine and dandy till landing. Come in hot, and cut the throttle way back. Instant snap into the ground. Backing off throttle, removed the up thrust motor condition but all that down elevator trim was still there. Nailed the wheel spats and then just cartwheeled and rekitted itself.
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Old 03-05-2011, 06:13 PM
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Well... A little aileron trim did the trick. Almost. Now it flies straight, but here's the next problem: When I do loops, it rolls out of them at the top. Automatic Immelman turns. I'm suspecting an uneven hstab. Have to check when I'm off daddy duty.
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Old 03-05-2011, 06:56 PM
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A tilted stab could certainly cause some strange behaviour so if it is twisted fix it... Also double check the lateral balance. If there isn't any right thrust add some to allow the ailerons to be centralised.
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Old 03-06-2011, 01:44 AM
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Also the use of too much elevator will cause such behavior in a loop. The plane is actually stalling at the top of the loop and snapping.

Make your loop a larger diameter and see if it improves. If it does, reduce your elevator throw slightly.
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Old 03-06-2011, 03:05 AM
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Originally Posted by Glacier Girl View Post
Also the use of too much elevator will cause such behavior in a loop. The plane is actually stalling at the top of the loop and snapping.

Make your loop a larger diameter and see if it improves. If it does, reduce your elevator throw slightly.
Maybe that's it... These micro planes are really finicky! I maidened my "big" T-28 (love that plane) today and I can just slam the stick and it does an nice tight loop. The micro one, not so much...
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Old 03-06-2011, 05:37 PM
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Swede - JEALOUS ! I ALWAYS have to work on Sunday, gotta have enough folks in the tower/RADAR room. BUT, got today off to go the the club field to maiden the App 15e as well as re-maiden the J5 and play with the S Cub. Well, storms last night and it is COLD, windy, and pretty wet! what does a guy have to do to get a break?
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