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Unexpected flight characteristics

Old 06-17-2013, 08:40 PM
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Andrewmc
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Default Unexpected flight characteristics

A beautiful sunny, wind free day yesterday and I had the flying field to myself for the afternoon. Was flying a 4 star 40 with a Saito 56 proped with an APC 12x6.

The last time I flew this plane was about 4 weeks ago and I recall being generally happy with how it flew as I had been experimenting with a throttle to rudder mix to help with straight take-offs and felt that this was working. The mix is 0% rudder deflection below half throttle and 10% rudder deflection at full throttle.

Yesterday however the plane pulled left on take-off and once in the air needed right aileron trim (several clicks) to counter a tendency to roll left. However once that was done at full throttle the plane was flying very nicely. However when coming in for landing as the plane slowed down in started drifting right (as if rudder had been input below half throttle no deflection on mix!). I added left rudder to keep the plane straight on the runway but felt I was coming in too fast and blew off the approach and went to WOT. The plane immediately rolled right and almost touched a wingtip. Came round again and landed and checked the rudder deflection - up to half throttle the rudder was neutral and at full throttle a slight deflection to the right - as expected.

Why then would the plane be veering right on approach (under half throttle) and why would it be rolling right with full throttle input. Also trying to understand why I needed aileron trim input from one flying day to the next - this I think is the issue!

I will check c of g and lateral balance later today, in the meantime any ideas?
Old 06-17-2013, 08:51 PM
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JohnBuckner
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Default RE: Unexpected flight characteristics

Andrew that mix is not in common use and I think you found out why. The very first thing I would do is to clear that mix and make sure it is inhibited then fly the airplane agine.

John
Old 06-17-2013, 09:16 PM
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Andrewmc
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Default RE: Unexpected flight characteristics

The reason I added the mix was I found the plane extremely "squirreley" om the ground. However point taken. I will take out the mix - add 90% expo on the rudder , and try again.

I will also check for a warped wing as what concerns me is that "something changed" from the previous flying session to require aileron trim.
Old 06-17-2013, 10:13 PM
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Default RE: Unexpected flight characteristics

Get rid of the mix as that plane should not need one. Make sure it is straight. Take off is high power and slow speed; P factor taking over. High speed trim and takeoff trim will not be the same. You trim for flight and put up with for take off. If you have a TX that can do conditions you could make two trim settings but that is overkill on a 4*.
Old 06-18-2013, 05:04 AM
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da Rock
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Default RE: Unexpected flight characteristics

90% expo?

That's an amazing amount of expo. You've basically blanked out rudder response for the majority of your stick movement. The last bit of movement then slams the rudder over for all it's movement. Huge amounts of expo work for 3D planes but not so good for trainers like the 4Star. You're turning the rudder stick into an on-off switch.
Old 06-18-2013, 06:41 AM
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jester_s1
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Default RE: Unexpected flight characteristics

I think he was joking about the expo. But the others are right. Left thumb skills are the way to get straight takeoffs, not mixes. A 4 star should be pretty easy to get a smooth takeoff with though, so if it's shaking its tail you probably have a gear setup problem or not enough right thrust.
As for your flying trouble, what you describe doesn't make sense with the mix you did. So I'd go looking for something broken or damaged in either the control system or the wing mount.
Old 06-18-2013, 07:45 AM
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Andrewmc
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Default RE: Unexpected flight characteristics

Yes the expo was a joke - sorry

I am currently remeasuring all the wing surfaces, CG, incidence etc. and recording these against design. I hope to post these tomorrow.

Thanks to all for your support so far.
Old 06-18-2013, 08:55 AM
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Default RE: Unexpected flight characteristics

Sorry about this but it sounds like you need to kill the mix and LEARN to taxi . Take your time and learn what your plane needs to taxi straight on TO. Just yesterday I told one of my students that his TO's were looking great, nice & straight , smooth and easy . That's what you need . ENJOY !!! RED
Old 06-18-2013, 10:24 AM
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Default RE: Unexpected flight characteristics

+1
and check your engine mounts.
Old 06-19-2013, 08:20 PM
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Andrewmc
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Default RE: Unexpected flight characteristics

I only seem to have problems with the ground handling on this plane. But practice, practice, practice!

I measured some of the key attributes of the plane last night as follows:

Wing incidence 0° (should be +1/2)
Down thrust 2° (2° is as per plans)
Right thrust 0° (0° is as per plans)

I also check the wings for warping. The left wing is fine the right wing is 0° at the root and +1/2° at the tip. This is however due to the strip aileron and not the wing itself (I think).

This implies that the TE is lower at the wing tip than at the root. This would tend to lift the wing tip though not drop it - have I got that right?

I will add some washers temporarily at the TE under the wing bolts to set the wing incidence closer to the half degree positive and the reset all the neutral points on the flight surfaces.

I should be able to get to fly on Sunday (Good Lady permitting ) .

Anything else I should check before flight testing (other than the tail wheel alignment)?
Old 06-20-2013, 04:59 AM
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Default RE: Unexpected flight characteristics

Andrewmc,
The plane needs 2 degrees of right thrust. It's a low wing so 0 degrees of down thrust. High wings need down thrust. The right thrust will help to taxi the model.

Good luck,
Darrolair
Old 06-20-2013, 05:33 AM
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jester_s1
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Default RE: Unexpected flight characteristics

You still haven't found what caused your flying issue. I'd suspect something has let go in either your control system or maybe your tail. On the ground handling, check your main gear wheel alignment. That plane should be pretty easy to ground handle. Lift the tail and roll the plane. You should feel no rolling resistance and you should be able to push the tail straight toward the nose and have the plane roll straight. It's possible with a taildragger to have crooked main wheels that you've fixed with tail trim. That will cause a snappy yaw when your tail wheel lifts, making your plane boogie all the way down the runway until the wheels are off the ground.
Old 06-20-2013, 09:00 AM
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Default RE: Unexpected flight characteristics

Hi!
That +1/2 degree wash-in is what makes all the trouble!!! Get rid of it , or better make that -1/2-1 degree at the wing tip (wash-out) in both winges and the plane will behave much better.
No mix should be used! Also check that the front wheel (if you have one) is not turning too much as the plane will be very sensitive at take off it if it does.
Old 06-20-2013, 09:23 AM
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Default RE: Unexpected flight characteristics

Set the plane on a flat surface, zero all trims and manually set the rudder straight in line with the vertical stab using a straight edge. Roll the plane on the ground and MANUALLY adjust the tail wheel/linkageso that it rolls straight when pushed . Do not alter the rudder itself or try to fix with sub-trim or digital trim.Adding a few degrees of toe-in to the main wheels helps the model self-correct when taxiing and taking off. You want the initial set-up "squared" properly.

Now you should only need a tiny bit of right rudder when taking off and the rest of the time fly with no odd mixes. You will probably get some torque/P-factor that may or may not need some flight trim.

Some planes require more rudder as well as elevator management to keep the tail-wheel in contact when taking off (short-coupled WWI biplanes are notoious). Some folks blip the throttle to lift the tail and then they are flying low with the wheels touching. Just part of the fun. The Four Star is a tame tail dragger.
Old 06-20-2013, 01:25 PM
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jester_s1
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Default RE: Unexpected flight characteristics

If you're getting anything besides a slight left yaw as you advance the throttle on takeoff, you have a landing gear problem.
Old 06-20-2013, 07:58 PM
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Default RE: Unexpected flight characteristics

I will check the landing gear mains and tail wheel for correctness. Adding toe-in on the mains would be difficult as the LG is fibreglass and would require drilling the hole for the axle rods at an angle. However I will make sure that they are at least parallel to each other and to the fuselage centreline.

The ground handling is one issue but the reason for hard right roll on full throttle is something else. It is possible that a servo cable was stuck on the wing saddle although fairly sure that was not the case. Speculation! I will fly the plane again this weekend and report back one way or the other.

Thank you to all for the ideas and support.
Old 06-23-2013, 08:52 PM
  #17  
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Default RE: Unexpected flight characteristics

Bad weather this weekend so no flying. But I did get the opportunity to scruitinise the plane more closely. The covering on the underside of the right aileron had delaminated over a length of about 20cm. This could pull back in flight causing a roll left requiring the right aileron trim that I had to input. Then a slow speeds the covering "popped" back into position causing the plane to roll right. Does that all sound plausible?

Anyway there was some oil residue on the seam (probably why it delaminated in the first place) so I opted to recover the entire wing, just got the trim to go then all done.

On the wheels the right wheel inside hub was broken. This wasn't affecting the rotation but I did allow for a greater freedom of movement on the axle so I replaced the wheels too.

I will post the final setup prior to flight and a flight report when the weather is better - Might get a chance during the week.
Old 11-12-2013, 07:48 PM
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Wow four and half months to get back to the flying field to test the modifications to the plane . Thats what happens with a new born baby in the house.

Weather was a bit windy on the weekend but I had a window of opportunity from family responsibilities and off I went.

Take-off was still all over the runway but once in the air everything was predictable and clean looking. Landings were fine - no more slow speed rolls.

Therefore safe to conclude that the loose covering on the aileron was the issue and has been now been resolved.

Regarding the ground handling when taxiing at a fast idle the plane would travel straight - so landing gear not the problem

Increasing the throttle sees the plane drifting to the left (as expected?) but attempts to correct withe right rudder even at 50% expo the plane would over correct and the zig zags started.

I have no problems with the ground handling on my extra so I didn't think it is all down to poor technique.

Took the plane home and checked down thrust - 1 degree, right thrust - 1.75 degrees. rudder throws - ah.

The standard throws for rudder are low rate - 3/4" high rate - 1.5". Mine was set up with low rate 1.5" high rate 2".

I reduced the low rate throw to 3/4" and hope that will now enable more picturesque take offs.

Thanks to all for their assistance and suggestions.
Old 11-13-2013, 07:42 AM
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if it's at all possible, reduce the throw to the nose wheel steering, by (either or both) moving the NWS clevis/Zbend to the innermost hole on the servo, and by moving the other end of the NWS pushrod as far away from the nose wheel strut as possible. (typically very small amounts of nose wheel steering are required.. less throw than the rudder itself typically needs)
and congrats on the new baby AND on getting back to the field!
Old 11-13-2013, 08:02 AM
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Hi Jim

This is a taildragger - so no nose wheel.

I like your signature - very good - maybe it would help with my ground handling
Old 11-13-2013, 08:57 AM
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How much maximum rudder throw left to right do you have set ?
Do you have dual rates set for the rudder?
What type of surface are you flying from?
I would set the rudder for maximum throw and set the dual rates/H/L rates on the transmitter so Low is at 40% and high is 100%. When you are on the ground for taxi /ready to take off switch to low rates and this should "tame down" the tail movements until you get more experience with it. You may have to lower the low rate percentage even more but once you get the feel for it you can bump it back up. Once you are in the air you can switch to high and then back to low when coming in . Only drawback when on the ground in taxi in low rates is making tight turns.....you may have to switch high to turn and then switch to low before take-off.
Once the tail lifts up the engine torque will want to pull the plane a bit to the side and you need to correct with a little opposite rudder. You need to learn how to use the rudder basics for taxi, take-off, and landings and then for coordinated turns and eventually knife-edges when you move on to different /larger,possibly more aerobatic planes. This is a great plane to practice on.
Also ,in my opinion, don't use Expo on a plane like this....it doesn't need it. Use dual rates for all surfaces and set the low rates to the minimum recommended throws or around 50% of the high rates(maximum throws). If you feel it's a bit jumpy in the air then program a little expo for the elevator ...like 20% and then dial it down as you become comfortable with the plane and eventually turn it off. The best thing is to learn to fly the plane using the transmitter not having the transmitter try and fly the plane.

Last edited by flyinwalenda; 11-13-2013 at 09:00 AM.
Old 11-13-2013, 09:02 AM
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+1 Expo can be what's causing the problem.. on a taildragger the rudder can be very sensitive during the takeoff rollout...
if the issue is that you're over correcting, as has been mentioned, try using either a low rate setting for takeoff or dial the rudder down.
it takes a little getting used to (and some airplanes are more sensitive than others) but you'll get a feel for exactly how much rudder to apply and when.
(just breathe on it )
Old 11-13-2013, 09:23 AM
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Originally Posted by flyinwalenda
How much maximum rudder throw left to right do you have set ?
Do you have dual rates set for the rudder?
What type of surface are you flying from?
I would set the rudder for maximum throw and set the dual rates/H/L rates on the transmitter so Low is at 40% and high is 100%. When you are on the ground for taxi /ready to take off switch to low rates and this should "tame down" the tail movements until you get more experience with it. You may have to lower the low rate percentage even more but once you get the feel for it you can bump it back up. Once you are in the air you can switch to high and then back to low when coming in . Only drawback when on the ground in taxi in low rates is making tight turns.....you may have to switch high to turn and then switch to low before take-off.
Once the tail lifts up the engine torque will want to pull the plane a bit to the side and you need to correct with a little opposite rudder. You need to learn how to use the rudder basics for taxi, take-off, and landings and then for coordinated turns and eventually knife-edges when you move on to different /larger,possibly more aerobatic planes. This is a great plane to practice on.
Also ,in my opinion, don't use Expo on a plane like this....it doesn't need it. Use dual rates for all surfaces and set the low rates to the minimum recommended throws or around 50% of the high rates(maximum throws). If you feel it's a bit jumpy in the air then program a little expo for the elevator ...like 20% and then dial it down as you become comfortable with the plane and eventually turn it off. The best thing is to learn to fly the plane using the transmitter not having the transmitter try and fly the plane.
On low rates I have 1.5" total throw which is 40% of the total throw on high rates. So spot on your suggestion.

Just need to catch another flying session and give it a whirl. I have been flying this plane for around 6 years the ground handling, while never great has been bad recently. I think I did try and increase the rudder movement to make the plane more aerobatic a while ago so probably that is wher ethe ground handling crept in. It is only now that I am trying to finesse the take-offs that I consider it an issue - full power and jump into the air is effective but not pretty.
Old 11-13-2013, 06:55 PM
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It does take a fine touch with some tail draqgers to not over throw the rudder control. Hey I think we all still ground-loop one every now and then! Perhaps dial the percentage down a bit more and keep practicing . You will get the right touch and figure out how it's going to react so it's not so jumpy.
Old 11-13-2013, 08:34 PM
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2 things to consider based just on what you've said in this thread:

1. Proper takeoffs with a taildragger don't come from seeing the left turn tendency and then correcting for it. Proper takeoffs comes from knowing how much correction is needed to prevent the left yaw from happening in the first place and inputting it before the yaw happens (actually, as it would be happening if you didn't correct).

2. Your gear can still be crooked if the plane is taxiing straight. If the main gear pulls left and the the tailwheel is set to make the plane roll straight, you'll get a straight rollout and then a hard left yaw as soon as the tailwheel loses traction. I had Cub like that that gave me fits. I could taxi all day long perfectly straight down the runway, but as soon as I lifted the tail on a takeoff it would hang a hard right and head for the fence. Once I got the gear straight, truly straight, it became a very predictable airplane on the ground.

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