Changing Tyre Size On 24 Reg Jabiru 230?

[jackc]

What the eye does not see, or the ear does not hear, the heart does not grieve……over 40 years ago I had a pilot land at Coober Pedy and blew a tyre on an Aztec, he asked where he could get it fixed, along with his crook radio. I said an AME from Pt Augusta or Alice Springs will be the closest and may take a couple of days.  Or, you can look the other way and I will fix it 🙂 

So I sourced an industrial tyre and tube in town and fitted it up, repaired his radio, broken wire and he flew away happy.

You don’t need a piece of paper to fix anything in this World IF you know what you are doing. You DONT need a qualified LAME to fix an aircraft 🙂.  Yeah I can hear the Cupcakes of today bleating about  illegal,  insurance, liability etc etc.  It’s all only a problem IF you don’t know what your doing. Unless  some snowflake snitch dobs you in. IF you can access a LAME by all means use him, if not…..then do your own thing.

 

 

[jackc]

I don’t think it can work for. 24 reg aircraft unless its aircraft manufacturer supplied and certified by them.  But then what would I know?  I ain’t qualified…..just a dummy in many peoples eyes 😞 

[RFguy]

Turbs has an Interesting point on the larger tyres (diameter)  and brake  performance. Yes indeed !

 

That is a real issue. I mean real because Jabs have terrible brakes at the best of times. 

Some have upgraded to smaller diameter master cyl

some have put in steel lines,

some have updated to bigger better brakes ....

 

So, If I were to provide an opinion if an AP, I would take fine on the tyres except... :

 

1) I want to see the braking performance is the same as book  specification.  That would likely require a brake variation/upgrade.

- produce paperwork to show you did your sums and methodology for the brake upgrade.

2) I would want to see stall test performance  and the POH updated to reflect and that test data being neatly filed somewhere.

- produce paperwork to demonstrate compliance with stall requirements (method and test data). that is an easy 1 hour flight).

 

Dont say "modification", say "Variation / upgrade"...

 

 

[skippydiesel]

It seems to me that many/most aircraft in this category have difficulty/cant remain stationary during a full or near full engine RPM run up.

So I would speculate that this poor brake performance would also be true at landing speeds.

I would go further and guess that relying on braking performance for  stooping will get the pilot into a looooot of trouble.

All this  means that RFguy & Turbs concerns are merely a question of poor brakes becoming slightly worse (due to larger tyre circumference /principal of moments) and a good pilot would not be relying on them anyhow.

[BrendAn]

Has anyone mentioned the matco kit made for Jabiru USA. I was going through old posts and found matco are approved for aus factory built jabiru.  Fixes all the braking problems apparently.

[turboplanner]

7 hours ago, skippydiesel said:

It seems to me that many/most aircraft in this category have difficulty/cant remain stationary during a full or near full engine RPM run up.

So I would speculate that this poor brake performance would also be true at landing speeds.

I would go further and guess that relying on braking performance for  stooping will get the pilot into a looooot of trouble.

All this  means that RFguy & Turbs concerns are merely a question of poor brakes becoming slightly worse (due to larger tyre circumference /principal of moments) and a good pilot would not be relying on them anyhow.

This thread talks about bigger tyres more suitable for grass strips, and tundra tyres have had a mention, so you could think ahead to more adventurous landings sometimes without a prior inspection, and regularly encountering short paddocks, so braking ability becomes one of the more important requirements. I certainly never experienced a Jabiru not holding very comfortably during an engine runup, and I found most of the issue with lever length and position. You can't expect to buy a new RA Aircraft with the same level of engineering design found in GA with starting proces above $300,000. For example, A Cessna 152 can touch down on the edge of the white keys at the end of a runway and stop on the white keys. A Cherokee can land on the same spot and stop not far from the end of the keys. In their history both of these aircraft, and both nosewheel have landed plenty of times on rough grassy paddocks, and they certainly relied on braking performance. And yes, a good pilot doesn't do what I said above about stopping distance and looks after the linings.     

 

So perhaps the first question about a 230 should have been, "Is this aircraft suitable for landing in paddocks requiring the unknowns that high flotation tyres may handle.

 

While the 24 Reg aircraft may prohibit a tyre change, going back to brake efficiency, one an aircraft which is legal to modify, if you fit tundra tyres, the equation is based on the static loaded radius of the tyre (axle centreline to concrete) if the Tundra figure is 40% bigger it requires 40% more leverage. You can get back to 100% of original efficiency by increasing the disc and caliper radius by 40%. 

 

Similarly you can increase the effectiveness of your existing brakes by increasing the radius of the disc/caliper.

 

However, in both cases, you have to ensure that when the tyre wall ballons on landing it doesn't hit the caliper, and also that the disc can't hit the ground, so there is some engineering skill involved.  

[skippydiesel]

Turboplanner;

 

I don't think its quite as straightforward as you suggest - calliper clamping pressure, brake pad material, disc design/material,  all play a part in brake efficiency, not just circumference and bigger pads/callipers.

I am old enough to remember the introduction ( to mass produced cars) of brake boosters on drum brakes and then front disc brakes and later still disk brakes all round.

I am not up with current motorcycle brake technology & no recollection of boosted brakes in this application and yet they can easily lock up a large circumferential wheel from a relatively small circumference drum/disk.

[turboplanner]

22 minutes ago, skippydiesel said:

Turboplanner;

 

I don't think its quite as straightforward as you suggest - calliper clamping pressure, brake pad material, disc design/material,  all play a part in brake efficiency, not just circumference and bigger pads/callipers.

I am old enough to remember the introduction ( to mass produced cars) of brake boosters on drum brakes and then front disc brakes and later still disk brakes all round.

I am not up with current motorcycle brake technology & no recollection of boosted brakes in this application and yet they can easily lock up a large circumferential wheel from a relatively small circumference drum/disk.

You’ve strayed a bit wider than I was talking about. You are correct for the initial design, but once that is working and you just change the disk ratio the equation is simple.

[old man emu]

Unusually for me, I think this discussion is drifting too far into the physics of brake function, and wandering away from a discussion on the act of reducing the ground speed of an aircraft from landing speed to taxying speed.

 

Think about how you drive your car. Say you were driving along at the speed limit of 80 kph. That's close to the landing speed of a recreational aeroplane. Being an attentive driver, you see that a couple of hundred metres ahead, the traffic lights have changed to amber. Do you stomp on your brake pedal then and there? Nope. You take your foot off the GO pedal and let the rolling and air resistance start to slow your vehicle. As you approach the traffic lights, which are now showing red , you use your experience and decided to lightly press on the brake pedal to get a better rate of slowing. You monitor the effect of that application so that by the time your vehicle is about 30 metres from the traffic lights its speed is down to about 20 kph and so rolling and resistance forces can deal with slowing the vehicle to walking pace. Then you clean up the stopping process by a little more brake pressure application. At no time did you stomp on the brake pedal and cause the brake system to stop the rotation of the wheels completely so that they skid over the road surface.

 

Now, what's the difference between the braking process you have been doing since you were a pimply-faced teenager and the one you employ now as a responsible PIC?

 

In going to Tundra tyres, I would look at the possibility of the tyres interfering with stall characteristics, because they may increase drag, which may affect pitch behaviour. There could be prop-strike problems if the sidewalls flexed too much on touching down. That flexing could also allow the sidewalls to contact surrounding assemblies, damaging the tyre, and even the assembly.

[facthunter]

 A bigger section tyre with a lower pressure will give more grip on rough surfaces as long as you use rim locks and it will reduce shock loads on the plane's structure. as well. People take spats on and off without encountering any significant trim issues and that would be a greater order of magnitude trim effect. Most planes can be allowed to be ferried gear locked DOWN odr ditch gear up without trim problems.  Nev

[jackc]

This is what I did to my Aeropup, the drum cable brake from what appears to be off a moped was checked on a test flight as part of my condition report, the video I was sent showed a landing roll at Gympie longer than a 747.  I queried the L2 who said if I was you, I would not fly with those brakes.  So, the next pic shows the hydraulic disc brake kit I upgraded it to.  Now this aircraft has not flown with this setup, the reason is the work continues…..now a full rewire.and some added instruments before  taking to the sky.  

Not wanting to put this tyre size on a Jabiru, but just a slight increase?  Maybe a CAR35 engineer could solve the Jabiru problem? 

 

[facthunter]

Rod Stiff is a practical bloke. For some conditions a bigger section tyre is safer. Nev

[jackc]

Well, I have made the enquiry to Jabiru so will see what happens?

[skippydiesel]

10 hours ago, turboplanner said:

You’ve strayed a bit wider than I was talking about. You are correct for the initial design, but once that is working and you just change the disk ratio the equation is simple.

Fair enough Turbs however what I am trying to say is there may be options (cheaper?) to try before throwing the old ("initial design") brakes system out

 

EG

 

Change of brake pad material (if such option available).

Replacing oil/brake fluid contaminated pads. 

Change of master cylinder to one offering potentially greater brake pressures.

Even removing the "glaze" from discs can sometimes restore/improve brake efficiency.

Replacing the brake fluid (on a regular basis) - its amazing how many people cant remember when/if  their brake fluid has ever been changed.

It seems a lot of pilots find brake fluid changes so challenging, they are willing to put up with air/water in the system - both of which degrade performance significantly.

Using the correct fluid is a big plus

 

ETC ETC

[turboplanner]

11 minutes ago, skippydiesel said:

Fair enough Turbs however what I am trying to say is there may be options (cheaper?) to try before throwing the old ("initial design") brakes system out

 

EG

 

Change of brake pad material (if such option available).

Replacing oil/brake fluid contaminated pads. 

Change of master cylinder to one offering potentially greater brake pressures.

Even removing the "glaze" from discs can sometimes restore/improve brake efficiency.

Replacing the brake fluid (on a regular basis) - its amazing how many people cant remember when/if  their brake fluid has ever been changed.

It seems a lot of pilots find brake fluid changes so challenging, they are willing to put up with air/water in the system - both of which degrade performance significantly.

Using the correct fluid is a big plus

 

ETC ETC

Ask Facthunter.

[RFguy]

The best mod appears to be a smaller diameter master cylinder (to get more pressure at the cost of travel ) . I am not sure how much 'stretch' is in the plastic lines. I don't know the material/pressure. There was one Jab I saw a pic of in the USA that had the brake lever split into left and right  ( two master cyls ) - full Matco upgrade. Again, there is a limit to what arms and hands can do compared to feet, and or brake boosters. 

Edited October 3, 2022 by RFguy

[jackc]

Heard back from Jabiru…..

 

If the aircraft is registered under the  ASTM LSA (i.e. 24-rego) then the aircraft has to remain standard (500 x 6 nose and mains). Under ASTM there is no STC ruling, it is a manufactures approval.

 

Jabiru have done no load testing etc to come up data that allows such modifications to a 24 reg J-230, for approval by them.

Therefore I will just have to let the tyres down a bit, for softer landings 🙂 

 

 

 

[RFguy]

Jack just put the other tyres on and do your tests. 

[jackc]

IF I get this aircraft, I will have a good look, the front wheel yoke will not allow a size increase anyway, but Jabiru said also that the load mount points for undercarriage had not been tested for increased stress on those points of the fuselage, either.

Cant see how that could be, even for a small increase but they design aircraft and I dont……and being 24 rego etc etc, we all know the story.

[facthunter]

A larger softer tyre would arguably put less strain on the main undercarriage in rougher conditions. The nosewheel could be left standard and the usual technique applied.(ie Keep weight off the nosewheel). It costs a fortune to certify stuff, but many get upgraded brakes etc (VH and not exp). ) EXP VH is best many say. Most warbirds fly that way. Nev

[skippydiesel]

14 hours ago, RFguy said:

The best mod appears to be a smaller diameter master cylinder (to get more pressure at the cost of travel ) . I am not sure how much 'stretch' is in the plastic lines. I don't know the material/pressure. There was one Jab I saw a pic of in the USA that had the brake lever split into left and right  ( two master cyls ) - full Matco upgrade. Again, there is a limit to what arms and hands can do compared to feet, and or brake boosters. 

Cant say I did an exhaustive market search, however when I did the refurbishment on my Zephyr & got to doing up the brakes,  I asked for short samples from a number of different small bore, "plastic" hose /line suppliers. The one I chose had very little "give" in the walls while maintaining sufficient flex to move with the undercarriage. The combination of high quality brake line & a smaller bore, hand operated, master cylinder radically changed the performance of the stock disk brakes. We went from just about holding for ignition checks, at 3000 RPM (on a very good day) to holding at 4000 RPM every time (wet grass not included)..

[RFguy]

Talking about VH-exp. Does anyone know if conversion from 24- to VH-experimental is actually permitted ? (RAaus of course will not talk about it)

-glen.

 

[jackc]

It’s all the talk at SAAA, there was a seminar on it a Narromine apparently, lots of interest.  Car medicals for GA will probably make many go back to GA, many planes just sitting in hangars will take to the skies once again.  RAA people will upgrade and change over to VH and be able to fly in controlled airspace, too?

[APenNameAndThatA]

8 hours ago, jackc said:

It’s all the talk at SAAA, there was a seminar on it a Narromine apparently, lots of interest.  Car medicals for GA will probably make many go back to GA, many planes just sitting in hangars will take to the skies once again.  RAA people will upgrade and change over to VH and be able to fly in controlled airspace, too?

I'm pretty sure that if you have a RPL then you can fly a 24 reg plane in controlled airspace. 

[BrendAn]

5 minutes ago, APenNameAndThatA said:

I'm pretty sure that if you have a RPL then you can fly a 24 reg plane in controlled airspace. 

correct as long as plane has transponder and radio according to what i read.