J230 Trim

[jcruffle]

I have only had my factory-built J230D for about 45 hours. Has anybody experienced a lack of forward trim at cruise (75% power or more), especially with a lot of fuel and one POB, or a load in the back?? I am not sure how adjustable this is considering I need all the rear trim I can get in the landing configuration. The problem is not critical, but a bit annoying at times.

 

I would be interested to hear how other J230's perform.

 

John

 

 

[Guest brentc]

Common problem on the Jabs, particularly the 200/230/400/430. From my experience, you could chop off half if not all of your evelator trim tabs, but if you're worried about mess and certification etc you should be able to adjust the spring on the trim to force it forward. Just remove the ventral fin and change the position of the spring retainer. You can also change the cable-ends so that as much forward trim can be achieved as you can get.

 

As for needing as much forward trim as you can get, this became a further problem with the introduction of the hydraulic lifters which are heavier along with the latest oil cooler which has quite a bit of weight to it. Since then the header tank has been moved to the rear of the aircraft versus under the seat to try and combat the problem. If you're only a 230 and don't have rear pax, suggest a little bit of weight in the rear if landing is a problem.

 

A mate with new J430 has an issue where he can't flare with 2 heavy people in the front but yet is also out of forward trim in flight. It's a bit of a balancing act so to speak.

 

 

[Captain]

Sorry to question your post Brent ............ but if John is running out of fwd trim (John ... please confirm that your post is referring to your trim lever being well to the front of the arc at cruise), then doesn't that mean that it needs bigger trim tabs so that the tabs do more of the work and thereby reduce the amount of fwd trim lever travel that is needed?

 

 

 

This is interesting, as I was thinking of adding tab to mine, which are cut down a little from std.

 

 

 

Regards Geoff

 

 

[Guest brentc]

Feel free to question me dearest Captain, however it's actually the opposite to what you have suggested - the trim tabs face down (or atleast I hope yours do!) which pushes the rear of the elevator up which makes the aircraft want to pitch up, thus removing them will give you more forward trim.

 

When I flew mine I had pressure on the stick pulling it back, so I hack-sawed the tabs back to about half and adjusted the cables a little and the problem mostly went away. Every J-2/4 I've been involved with has had the same problem. Two X J430's here didn't fit them on my advice and I believe they are better off - last thing you want is an untidy spot where they used to be.

 

For comparitive purposes, on mine for example with 1 person I can't do a powered (2800rpm) descent hands-off because I run out of forward trim. I can cruise with 1 or 2 people on board with no input required on the elevator but if I put someone in the back depending on how heavy they are, I may require some slight elevator forward stick input.

 

If you wanted to check what's best for you, just get a small wedge of timber or foam with about the rough angle of a trim tab and tape it onto the top of the elevator at the rear with duct tape.

 

The trim tabs are supposedly only there as a result of the certification of the LSA55's and are to stop the number of fugoids when the controls are released. Probably something to do with the aircraft gracefully crashing if the pilot has a heart attack and the engine isn't running - in theory it will gracefully glide down and land without puorposing. (no idea how to spell that but we'll call it Dolphining so you know what I mean)

 

 

[Guest airsick]

Reading this I can't help but think there is a design fault somewhere here. If everyone is running out of trim in one direction or another that seems to put up a red flag in my book. :ah_oh:

 

 

[jcruffle]

Thanks for the input, guys. Brent, if you change the spring to bias the trim in the forward direction, what effect does this have on back (up) trim??

 

 

[Captain]

Reading this I can't help but think there is a design fault somewhere here. If everyone is running out of trim in one direction or another that seems to put up a red flag in my book. :ah_oh:

I don't agree

 

 

 

All I think we are discussing is the tuning of the trim settings.

 

 

 

As Brent has mentioned there is an adjustment in the trim mechanism and it looks like that is where John needs to look next to make a small change.

 

 

 

Mine has never run out of trim in any direction, but I'd like to make it a bit better, and perhaps a little more fwd trim would be nice so that BC can sit back, hands off his stick, in a 2800 powered descent. But then Brent often has something yummy sitting next to him, so a hands off descent probably has advantages in his case. Me? I'm quite happy to back off the power for descent.

 

 

 

And thanks for the tip, BC. I'll try giving mine a test flight with a wedgie (or 2) in place and see what happens.

 

 

 

(moderated - reason: Tone - Ian)

 

 

 

[Guest brentc]

Agreed, no design fault. If anything to fix the problem Jabiru could order the trim cable to be 10mm shorter which would then put the mounting point in a different location and change the whole set-up slightly. It only takes the smallest change to make a big difference.

 

Mr. Cruffle, you'll need to strike a balance. If you are suggesting that you are concerned about running out of rearward elevator then perhaps changing the trim setting mechanically are not your solution and I wouldn't recommend it.

 

Given that your problem of not having enough flare on landing? is not airflow related I would suggest that a temporary wedge be fitted, say 200mm x 30mm x 2mm tapering to 1mm in a triangle type shape on top of the elevator. I don't believe that this will affect landing your aircraft in the flare because there won't be enough airflow to make enough of a difference to change the flare. Your problem sounds like it's W&B related. Remember that kilo in the tail isn't going to do much considering that I can happily fly mine around with a 120kg passenger in the back.

 

Personally if I were you I would do this if I have your symptoms correct:

 

1. Try a wedge for in flight to see if your tabs need to be removed

 

2. Check and see if this affects your flare (highly unlikely)

 

3. Consider adding a small amount of lead to the ventral fin

 

4. Adjust the trim mechanically forward

 

5. After trim adjust, try to determine if you need to cut the trim tabs off by flight testing

 

Edit - when making trim changed via control rod-ends, make sure you still get the same elevator movement after as before. Jabiru use a template for measuring movement via angles. Make sure you know what the angles are before changing anything so that it's not affected and control problems cause you issues. In theory the trim adjustment should only bias the neutral position of the elevator and not affect it's full movement.

 

 

[jcruffle]

Brent, my concern is not running out of rear elevator, but rear elevator trim. I would simply like to be able to trim the aircraft in all configurations. I have a lot to consider from the suggestions the members have provided. Thanks.

 

 

[Guest ROM]

Brentc, you are very brave suggesting that a piece of wedged shaped foam be taped to the rear edge of the elevator.

 

The Nomad crash near melbourne many years ago was the direct result of changes to the elevator trim tab. To get a better trimming range, small sections were progressively cut off the Nomad's elevator trim with very little effect.

 

The final cut was about a half inch off the trim tab but it was enough to suddenly change the elevator's total cyclic response leading to a loss of elevator control and the subsequent very dramatic crash.

 

The fixed trim tab is a requirement to limit any tendency for the aircraft to continue to nose over into a high speed dive.

 

I haven't checked how the fixed trim tab affects stick fixed and stick free stability but would think that the stick free stability will develop into a phugoid or dolphining as BrentC says rather than just nose over into a high speed dive.

 

The trim tab should be set so that as the vne is reached the aircraft will have a strong tendency to nose up thereby reducing the air speed.

 

Modern gliders have a very shallow concave shaped lower surface on their elevators to achieve the same result and without the need for a draggy trim tab.

 

Very small variations in the angle that the elevator stabiliser is attached to the fuselage will create quite large changes in the trim position and response differences between aircraft of the same make and type. This will account for the diferences in trim settings and the need to possibly modify the fixed trim tab on the elevator.

 

 

[Guest brentc]

Yes I'm brave! Call me braveheart.

 

I'm posting with authority because I've done it several times with complete success. A Jab will happily fly with or without trim tabs on the elevator. When building mine Jabiru told me not to bother even fitting them!

 

I once saw a J series at Lethbridge that had both an up AND a DOWN trim tab on the elevator! I suspect at the time the owner was going through the same testing scenarios that we are here.

 

Rom - if the trim is running out at the forward end, would you not have literally heaps left when it's towards the rear?

 

If it is the case that you are running out of forward trim in flight and rear trim when landing, I'd suggest an aerodynamic modificaiton in that case.

 

If you want, send me a PM with your number and we can discuss.

 

 

[Yenn]

I have heard and read that you should always trim out control forces, but would a prudent pilot try to trim out the full backstick forces when landing. In my opinion that could lead to a dangerous out of trim situation in the event of a go around.

 

There is no real need for trim on most ultralight aircraft as all it does is release you from holding pressure on the stick, being a comfort sort of thing.

 

I don't have trim on my plane and fly level at anything between 60 and 120 kts with no problem.

 

 

[Guest ROM]

No argument BrentC if you have done this before and certainly no argument about not fitting the usual trim tabs.

 

I suspect that the only real reason for those elevator trailing edge tabs on the Jabs is to conform with some requirement somewhere on the lines that I posted above, to prevent or attempt to prevent overspeeding in a dive.

 

It sorta made me sit up real quick when you suggested the wedge on the trailing edge of the elevators as I have heard too many stories and have seen, heard and experienced aircraft flutter in flight, in my case fortunately a very low frequency rudder flutter.

 

In my personal case it was low frequency rudder flutter on a glider that I was doing a test flight in.

 

I was just starting to accelerate in a shallow dive for the high speed test run but I was still well below the VNE of the glider.

 

The nose of the glider in front of me just started to flex around 4 or 5 inches from side to side at about a two seconds cycle and I was severely rocked from side to side.

 

It was one of the weirdest feelings I have ever experienced in some 3000 hours of flying.

 

I have seen and heard aileron flutter on an old glider where the aileron cables had not been correctly adjusted to the correct tension.

 

I have talked to pilots that have experienced flutter, including one who had to bail out as his glider broke up under him so when people start to talk about adding odd bits and pieces to various control surfaces I get rather jumpy at the possible consequences if they get it wrong.

 

Sorry if I jumped but I would not like to see somebody with a lot less experience than you starting to play around with adding odd bits and pieces to the control surfaces and then find out the deadly way that it was not a good idea to do that!

 

Cheers!

 

 

[Guest Bruce Knowles]

The threaded ends of the trim cable are in excess to what is required when screwed all the way in. By cutting a minute amount of the tail end linkage bolt first allows more forward adjustment. I think both ends could be treated the same for the same result. I have done this to tail end linkage bolt with a improvement in forward trim. I have still got room to move to improve it further.