J430 Trim

[Phil Goyne]

I am running out of forward trim with rear load in my J430. Have cut back the elevator trim tabs to 15 mm and adjusted the springs but still need more forward trim. Does anyone know where I should proceed from here ?

 

Phil G.

 

 

[Phil Goyne]

Thanks DB will phone !!

 

 

[Captain]

Phil

 

Do a search for "trim tab" as we had a good discussion on this about 12 - 18 months ago ..... I think in J230 @ YSWG in Kitbuilding, but it may have been separate.

 

Some are cutting them back further than you and mine are down to about 9 mm on a 230.

 

I assumed that "trim" tabs meant that they are there to be trimmed.

 

Hope this helps.

 

Geoff

 

 

[Phil Goyne]

Thanks Geoff- I will make a search.

 

 

[Guest brentc]

G'day Phil, it's been a while, good to hear you are up and aviating.

 

For what it's worth many of us are cutting them off completely or not fitting them at all and in some cases tabs in the opposite direction have been fitted.

 

 

[Guest Andys@coffs]

The whole thing that worries me about this is that Jabiru arent idiots, why are the 230 /430 being configured this way if its so simple to fix, in fixing the trim problem what are we breaking elsewhere that will rise up and bit us on the a#se?

 

Andy

 

 

[Guest brentc]

Not likely Andy - the trim tabs are a hangover from the certification process and are there to lower the number of Phugoids before the aircraft settles in. It was on advice from Jabiru that a number of us either cut off the tabs or didn't fit them at all. Remember that Jabiru don't 'build' the 430's as such so whether or not they are required is up to the owner / builder as these machines are not certified. The problem arises because of the extra weight in the rear in the 400/430's, thus requiring less trim tab length. In the end trim tabs are are normally able to be changed from up to down and vice-versa, so it's really just bad planning on their behalf by using a solid fibreglass trim tab versus an adjustable one - an impractical way to keep costs down and have a simple aircraft. Mine was getting annoying on the weekend with pax in the back as I also was running out of forward trim at MTOW.

 

 

[Guest ROM]

This trimming problem and trim tab was discussed at some length quite some time ago.

 

I can't verify my facts here without a lot of research but I think it is quite possible that a European requirement for their microlight / ultralight aircraft which the Jabiru UL-D fits into and possibly an American / European requirement for the LSA type aircraft requires a trim tab to be fitted to the elevator or an elevator design that will automatically bring the nose up if the aircraft exceeds it's VNE.

 

The theory being that this will automatically slow the aircraft back down to below it's critical airspeeds.

 

This is a design requirement and is certainly the case with European designed gliders.

 

Modern high performance sailplanes have a very shallow concave designed into the lower surfaces of their elevators to do this job of automatically deflecting the elevator upwards when the critical VNE speeds are being approached.

 

Jabiru in their usual manner have arrived at a solution that suited the need for their test aircraft to meet the specifications and have just gone right on doing the same thing on all their models.

 

Some adjustments for individual aircraft are more than likely needed which may entail trimming of the trim tab through removal to even a reverse tab as above.

 

A reverse trim tab is probably starting to play with fire and I would be most wary of doing this.

 

 

[Phil Goyne]

:juggle:Hi Brent- good to hear from you and that you are still enjoying your Jab. I will take in all the suggestions then have a go at fixing the problem. Cheers !!

 

 

[ianboag]

I have a (not built by me) J200. Trim range is from centre (at takeoff) to 95% forward (cruise). That's a J200 with nothing in the back. I have not been able to understand how a J400/430 could possibly work with any sort of weight in the back given that I am essentially at full forward trim. I have inspected the trim spring and find that a bracket was shifted by about 3 cm to increase the tension.

 

I noticed that with the trim at full forward on the ground the elevator is slightly down as one would expect with spring-driven trim.

 

I wondered about hacking off the trim tabs. Looks like I might do that now ...

 

Ian Boag

 

 

[Guest brentc]

I have not been able to understand how a J400/430 could possibly work with any sort of weight in the back given that I am essentially at full forward trim. Ian Boag

It doesn't 'work' as you put it! or atleast in my 400 with weight in the back. That being said, even with some forward input to the stick the whole time, it's not that bad and my arm hasn't fallen off yet. I should get back onto this but I'm afraid I'll make a mess with the hack-saw.

 

 

[Geoff]

Use tin sips or a thin cutoff wheel.

 

 

[ianboag]

Interesting. Like I said, I fly two-up with the lever at 95% forward. Takeoff is centralised. The back half of the trim lever travel is never used. I'm pretty nervous about any weight at all in the back given that I seem to be out of forward trim.

 

Perhaps leaning on the stick all the time is OK - I just haven't done it. Being unable to trim out stick forces would be a bit of a novelty for me.

 

I have figured that the way to deal with the problem is to effectively shorten the trim cable. Doing anything to the cable is way too hard though.

 

One approach would be to cut about 2-3 cm off the back end of the spring-loaded rod. This would mean that the trim lever would be about in the middle for level flight instead of 95% forward. The drawback is that one would have to remove the rod and drill/tap a new hole in the end.

 

An easier way is to take off the fishplate which connects the trim cable to the elevator horn and replace the fishplate with a triangular piece of ally so that the ball joint connects to the elevator horn about 2-3 cm BEHIND the current position. So "trim lever in the middle" with the new configuration will be the same as "trim lever 95% forward" in the current configuration. I will have a heap more forward trim .....

 

Takeoff will be pulled back some, cruise will be more or less in the middle etc etc. If there's extra weight in the back then I should have enough trim to deal with it.

 

Any/all comments gratefully received. I can send a photo to anyone who is interested.

 

IB

 

 

[Captain]

Ian

 

Have you used up all of the adjustment in the split pin holesthat retain one end of the spring on one end of the trim tube in the trim mechanism below the elevator?

 

If so, would you consider drilling a new hole there to put more load on that spring?

 

Regards Geoff

 

 

[ianboag]

The situation I am describing is with the split pin all the way in. I added five washers underneath it to further compress the spring (equivalent to the new holes suggested) but that didn't make any real difference.

 

IB

 

 

[Guest ROM]

Might be worth checking the angle of incidence of the stabilizer against another similar model Jab that has a different trim regime.

 

Jabiru does not seem to very fussy about the elevator stabiliser's angle of incidence to the fuselage.

 

Not that you can do much about any problems there but it might give a starting point for any significant changes to the trim cable anchorage or the trim spring mechanism.

 

 

[jeffcb52]

Ian,

 

I bought a new spring form Jabiru and it is considerably stronger than the original.

 

Regards

 

Jeff

 

 

[ianboag]

Stronger spring. There's a good idea. Now if someone had told me such a thing was available I'd probably have bought one ....

 

The fishplate idea is not particularly radical - I'm just changing (by not a lot) the relationship between the trim lever position and the trim mechanism. I expect that this will mean I now use the back half of the trim lever travel. I'll post a pic when it's done.

 

IB

 

 

[ianboag]

Today I made a bracket which connects the end of the trim rod to the elevator horn about 15mm further back that it was before. Then I went for a flight. The trim lever makes sense now - pulled back for takeoff - in the middle for normal flight - still enough back trim on landing. :)

 

The springs are set on the third hole (of five) both ways. Before I did this the rear spring was on the innermost hole and had five washers under it to scrunch it up some more.

 

Happy to post a photo if anyone is interested.

 

IB

 

 

[Captain]

Happy to post a photo if anyone is interested.IB

Ian. Photo would be much appreciated. Regards Geoff

 

 

[Yenn]

All this problem with running out of elevator prompts me to ask the question, are these planes loaded within the weight and balance specifications?

 

 

[Guest brentc]

Ian, they are not running out of elevator in flight but rather the trim is an issue because of the trim tabs and indeed a J230 with limited cargo in the rear would be well within C of G limits, in fact unless it had somewhere near 120-130 kilos in the back it will be well within tolerance.

 

 

[ianboag]

Here's the pix. All seems to work OK now.

 

Ian Boag

 

 

The mod - I put two holes in the connector because I was not sure how much "shortening" would be needed. This is

 

"level flight" configuration - bellcrank more or less vertical and trim lever in the middle. Springs on the rod both on 3rd hole.

 

 

Level flight setup pre-mod. Note that the clip holding the trim cable has been shifted - before that was done I imagine there

 

would not have been enough forward trim at all. Trim lever and bellcrank both well forward. Spring on the rod was pushed

 

up hard with split pin and extra washers.

 

 

Trim lever as it was when set up for level flight pre-mod. The plane flew OK .... I wonder if this was a build error and the

 

trim lever should have been on the (left in the pic) front bolt rather than the (right in the pic) back one. I think that might

 

have sorted it out the "cable length" issue as well. Note that the pivot bolt is actually BEHIND the trim neutral position

 

on the placard. When vertical it is just behind the M in "TRIM"

 

 

[Captain]

Here's the pix. All seems to work OK now.

Terrific Ian. Thanks for posting them. I don't actually run out of trim on my 230, but the lever is certainly a bit forward. That is a nice simple mod that is easy to do, so I will. Regards Geoff

 

 

[Guest brentc]

I'm glad it worked for you however I'm not confident in making my own bracket which is a replacement for potentially one of the most important parts of the aircraft. There is no redundancy in the elevator attachment so I will leave it be!