Improving Jabiru/J230 brakes ?

[RFguy]

Even nicely bled, my J230 doesnt want to pull up that well. They are dual caliper brakes.

 

Not sure if it is a combination of the partial  plastic brake lines + small master cylinder 

 

or maybe the pads are glazed (see that on motorcycles and bicycles - need to roughen the pads again on some concrete) 

 

There is of course limit to how much SHEAR you can put on the mains- the undercarriage is not braced in any way to resist front to rear forces (heavy braking).

 

Need also to get my idle down a bit. lots of thrust from that prop even at low RPM.

 

Edited September 12, 2021 by RFguy

[facthunter]

High idle speed low slowdown is a common problem with Jabs. Brakes? well they'll never be like a modern car .You will be putting a lot of weight on the nosewheel too which can lead to directional instability. Maybe groove/ drill the discs and use a softer pad material?  Keep your touchdown speed under control.  I think a Jab 230 is not a bad jigger by the way but it's not stol. Nev

[RFguy]

yeah idle is about 1050-1120 right now. way way way too much.

I will try a different pad material. I had a similar problem on the Volvo. changed pad material...

 

Touchdown speed is critical. Over the fence speeds , half fuel, full flap - 48-50 with one up or 53-55 two up.  (benign condix) 

[facthunter]

yes I don't think you can really go much slower unless you use a high power approach  Used to be called a submarine approach You'll trip on a fence wire one day doing those. A 230 driver I knew who used to  go everywhere often cut the motor when on the ground  on limited length strips rather than give the brakes a hard time..  I've often thought that when holding short of the runway when another plane is operating it would be a disaster if the brakes went to the floor and you rolled onto the runway and it might be prudent to shut the engine down in those situations. Nev

[RFguy]

guess it depends how close the fence is to the keys.  At Cowra there is alot of room. 

 

48-50 or 53-55 over the fence (benign condix)  applies in that case to 'over the keys'. 

 

I would be 5 kts higher over the fence. or whatever low obstacle.

 

As engine failure there and you still want to be able to make it in. So I like to approach at power idle  in late final where I am assured of making the strip.

leads to a slightly steeper than powered on approach. 

 

There are  a few places though with a tree  where the fence might imaginarily be.... . what's what I have been practising for. 

 

Certainly not shallow though Nev, IE not dragging it in..... I've done a few now,  - a power idle -  non-shallow , but not too steep descent and then exchange the (excessive )  airspeed  for roundout. 

 

becomes a critical manouver in anything but benign conditions.  I'm realising the money is earned on short strips in non zero wind condix. either know your aircraft very very well or plan to need 200m extra strip.

 

 

 

 

 

[facthunter]

A powered approach gives you the extra control to be more precise with touchdown speed , touchdown point and extra control authority for rudder and elevators.. Use it when the need arises. ie gusty and/ or short strip. Nev

[RFguy]

Indeed. horses for coures- the performance pilot exercises good judgement as to which approach to use and be ready to vary it.....

 

Takes quite a bit of power to drag it in at 50-55 kts. no room for engine failure .

Again  - exercise judgement and caution.  

 

I did quite a few short fielders in training in the heat,  on shortish dirt strips with 50% of Xwind allowance.  high workload, lots of sweating. 

 

[facthunter]

I'm talking of being very active with power not just adding it. In level flight or on a fixed approach path it's all you have to vary airspeed. You have spoilers on a glider. and start off being high.  Nev

[turboplanner]

50 minutes ago, RFguy said:

Even nicely bled, my J230 doesnt want to pull up that well. They are dual caliper brakes.

 

Not sure if it is a combination of the partial  plastic brake lines + small master cylinder 

 

or maybe the pads are glazed (see that on motorcycles and bicycles - need to roughen the pads again on some concrete) 

 

There is of course limit to how much SHEAR you can put on the mains- the undercarriage is not braced in any way to resist front to rear forces (heavy braking).

 

Need also to get my idle down a bit. lots of thrust from that prop even at low RPM.

 

Given that brakes are a safety component it's even more important to know what you can legally modify. Armed with that knowledge:

 

If the J170 is any example you just about need to start braking five miles out from the threshold!

Not a pleasant experience after a GA aircraft.

 

Starting at the activating point, from memory  the brake arm is very short. In a hydraulic braking system there are rules of thumb for the ratio between master cyliner length and activation length, usually the foot sole. I've forgotten the number but it's way more than that tiny Jab lever, and as well, you are trying to control it with your muscles applied diagonally, so even with the Jab Master cylinder, the ratio might be too short to allow the slave cylinders to do their job properly.

 

You could check the specification of the plastic brake lines to see if they might be expanding, but I used plastic in race cars and gave them a good workout under many scary moments.

 

The ratio of Master cylinder size vs slave cyclinders size as you say will be part of the equation. The smaller the master cylinder diameter the more distance you have to move the lever, but the more pressure you will be exerting on the slave cylinders.

 

I don't think it's glazed pads; every Jab1 60/170 I flew was pretty much the same - about half the braking capacity of a Warrior.

 

When you look at Jabiru design evolution, you can see they eliminated any rough landing damage when they could and the mains beam is one of the better suspensions in aviation. If by some chance you managed to get braking efficiency up to the point where it exceeded torsional limits of the suspension, it would be around the clamp joint which is not likely to budge, but you could get some toe-in/toe-out flutter as a warning you were getting close, however, in my opinion you would have to break your finger on the brake level first.

 

If you've exhausted the possibilities it would be worth talking to a Brake Engineer from someone like Bendix, to get all the techical calc and ways to calcuate your way out of the problem.

 

The centre console of the Jab is a squeeze fit for all the cables - a trade off for ease of fat bums.

Some interfere with others; so you likely will have experienced the sight of your trim level coming away from where you set it etc.

I would not drop throttle rpm below what Jabiru sets - below that sometimes you get an embarrasing acceleration lag, spit back or other exciting experiences.

Having said that, with the throttle closed there should be a specification or adjuster for the carb. If the idle is erratic, i.e. one day 600 rpm, another day 750 rpm, then the cosole bulge may be the culrpit, otherwise, if it always drops back to the same figure but that is nore than you want, go up the cable section by section until you find the restriction.

 

 

 

 

 

 

[IBob]

RFguy, if it hasn't been touched on here before: have you checked that your callipers are floating properly, so the pads are pressing fully on both sides of the disks?

The brakes on my aircraft are very basic, the float is provided by dowels that are a very neat fit, with no way of cleaning or lubricating without dismantling. And float can be hard to check too: the brake can look okay  with pads on both sides contacting the disk even if the calliper is no longer floating. So far, my braking is excellent, but if it starts to fade that will be the first place I will be looking.

[RFguy]

Nev  - yeah well summer is a time to be VERY ACTIVE on the power dealing with thermal  sinks and rises. You got to be onto it like a tiger.

Why is it bad luck that sinks seem to occur just before the fence ? 

 

Turbs. great writeup. 

OK, I will gather all the information in the entire system and go talk to a brake engineer.

The Brumby I learned on was super in brakes- but it had differential tow breaks pushed by my legs

IDLE should be 800-900 rpm. currently 1050-1100.

 

Bob. OK on the float. I wll check.

 

Edited September 12, 2021 by RFguy

[facthunter]

There's a magnet in the ground just before the threshold. More seriously a heated runway surface will cause that. You usually choose an into wind landing and the warmer air comes to meet you.. Nev

[Thruster88]

Glen it would be interesting to calculate the hydraulic pressure given x amount of pull on the lever. I suspect there is a lot more pressure generated by two feet than one hand. A smaller diameter master cylinder might help. 

[RFguy]

mmmm. I know another who fitted a smaller master cylinder (VH exp J430) 

Stuart, any other hand brake operated GA around, how long is the lever compared to the short Jab one ?

Edited September 12, 2021 by RFguy

[Thruster88]

Not aware of any GA without toe brakes.

[turboplanner]

28 minutes ago, RFguy said:

mmmm. I know another who fitted a smaller master cylinder (VH exp J430) 

Stuart, any other hand brake operated GA around, how long is the lever compared to the short Jab one ?

It's not just the one measurement (lever), it's the combination ratio.

[facthunter]

That's the most common layout. Individual toe brakes. Austers have a heel operated CABLE set up which is a bit challenging. On a Jabiru I still rely on the rudder a lot for directional control and not allow much  weight onto the nosewheel till you slow up. Maybe real good brakes would create problems They are only small wheels.. Nev

[turboplanner]

Just now, facthunter said:

That's the most common layout. Individual toe brakes. Austers have a heel operated CABLE set up which is a bit challenging. On a Jabiru I still rely on the rudder a lot for directional control and not allow much  weight onto the nosewheel till you slow up. Maybe real good brakes would create problems They are only small wheels.. Nev

Strong nosewheel for RA though.

[facthunter]

Yes  the later ones are OK but I'm more worried about excursions off the side which some experience mainly on grass   I've been advised and  always taught that technique and had no issues even in pretty windy situations. Nev

[kgwilson]

The brakes on Jabirus are not that flash and I hear a lot of people commenting on their very average performance. They look like pushbike discs to me and that hand operated lever is certainly not my cup of tea though it has a simple handbrake lock function. If I ever built a Jab I'd do what I did when I built my aircraft.

 

I installed Matco MHE51B Disc brakes with MC 5 individual master cylinders for differential toe brakes when I built the Sierra. They can be coupled with MC 4 slave cylinders for the co-pilot but I didn't bother. With the Matco  wheels I added a bit of weight but that is more than compensated by performance. They cost about $1100 back then I think. Well worth it though. I can lock the wheels on a sealed runway if I want to and of course with a castoring nose wheel it makes turning tightly easy. I can do a 360 turn in the aircrafts own length. With a 24 rego though you won't be able to do this I guess.

[Old Koreelah]

22 hours ago, facthunter said:

High idle speed low slowdown is a common problem with Jabs…

Good point! An often-overlooked dimension to landing control. I had mobs of problems with idle revs until a LAME showed me how to adjust the idle screw. 

800 rpm is now a reliable idle speed, but some have reported it as low as 350 rpm on Jab engines with EFI.

21 hours ago, turboplanner said:

The ratio of Master cylinder size vs slave cyclinders size as you say will be part of the equation…

I had major problems stopping my early single-disk Ducati until I replaced the standard master cylinder (which I assume had been designed for a dual-disk setup) with a smaller-diameter one off a Honda 500. 

 

Bicycle shops might be a good source of very light-weight hi-tech hydraulic brakes.

[turboplanner]

6 minutes ago, Old Koreelah said:

Good point! An often-overlooked dimension to landing control. I had mobs of problems with idle revs until a LAME showed me how to adjust the idle screw. 

800 rpm is now a reliable idle speed, but some have reported it as low as 350 rpm on Jab engines with EFI.

I had major problems stopping my early single-disk Ducati until I replaced the standard master cylinder (which I assume had been designed for a dual-disk setup) with a smaller-diameter one off a Honda 500. 

 

Bicycle shops might be a good source of very light-weight hi-tech hydraulic brakes.

or Go Kart suppliers

[RFguy]

Well, half a turn CCW on the idle screw  bringing the RPM down to around 800 at warm stationary idle made a big difference  to the brake performance....

It's a little plane with a big engine and prop. Back to idle in the flare though at 800 versus 1050-1100 rpm is dramatic !
 

Edited September 19, 2021 by RFguy

[skippydiesel]

Brakes on our light aircraft are a vexed issue;

 

On the one hand you need sufficient "performance" to do your engine run ups.

 

On the other

 

Your dont want so much that you are in danger of locking the wheels, particularly on a sealed runway as this may overdress the undercarriage, cause a skid/hydroplane and lose of control

 

So its a bit of a balancing act. I found that a change to a smaller diameter (longer stroke)  hand operated master cylinder, allowed for a 4000 rpm (Rotax) stationary, ignition/condition check and to prevent wheel rotation/rumble in the air, first stage, on the park lock (just enough to stop the wheel) meant that if I had forgotten to disengage  the brake (happened a couple of times) on landing this did not cause an issue.

[facthunter]

It CAN  though. Landing there may be little weight on the wheels at times and a locked wheel is easy to do. With brakes like Clevelands it's easy to lock a wheel anytime. This can flat spot or even cause a tire failure. Your prelanding should include a brakes OFF check,

   Anyhow a small plane will never approach the braking capability of a motorcycle or car. It's an undersized wheel on a pretty rubbery and long leg which also has to put up with a fairly rough surface often. Mix that with a significant braking requirement is hard on things.  With our restricted stall speed and landings into  wind you should only be looking at pulling up  from about 80 kms/hr  Some of your most demanding braking work will be done when taxiing in a strong crosswind   I've actually  ruined a brake doing that but I had no option with a long taxying distance. Nev