PKnight

I don't think that the wear on the outside edge of the tyre is caused by scrubbing while taxying as a result of toe-in. The Jab is a bit pigeon toed and when in the air the main undercarriage bends inwards as the weight has been removed. As a result on landing the first part of the tyre to impact on the runway is the outside edge. I think that this is the cause of the wear as it is a very aggressive event and will certainly cause tyre wear particularly on tarmac runways. I find that my tyres are good for about 100 - 150 landings before I think that the wear is getting excessive. If I wanted to economise I could simply rotate the tyres round so that the worn part was now on the outside edge but I have tended to avoid doing that as I felt that it could be a false economy. If my conclusion that the wear is caused by the landing impact then changing the toe in would have no effect and I would recommend staying with the factory recommendations. Peter G-PHYZ J430 1105 hours,

I have had two Jabiru engines, a 2200 and my current 3300 which has 980 hours on it. Both these engines have been significantly tighter when hot; so what is going on. My apologies to all those who know this but I'd like to get the physics bit out of the way first. All metals expand when hot but at different rates. Aluminium alloys expand at approximately twice the rate of iron and steel based alloys. The forces generated by these expansions are enormous. So as the temperature increases a steel shaft in an aluminium bearing (crank shaft situation) should get looser as the aluminium bearing will expand at a greater rate than the steel shaft. Conversely an aluminium shaft (piston) in a steel cylinder will get tighter as the temperature increases and the aluminium expands at a greater rate than the steel that is containing it. The increase in size are very small unless the temperature increase is significant, for aluminium an increase of 100 deg C will give a percentage increase in the component of about 0.0023%. This would be enough to make something that was already tight into very tight but it is still a small change. Sorry about the lecture but I started to look at the behavior of my engine (3300 No A1336) as it got hotter. First bit of information is that the engine rotates freely when cold and has 6 good compressions. If I take the plugs out I can almost whizz the prop round by hand, it is only constrained by the need to put energy into opening the valve springs. When hot, the engine is stiff. First test was to look at the possibility of movement in the crank. I could still detect movement when pulling the prop backward and forward. This shows that there is still some end float and the crank is not squeezed in its bearings as a result of expansion. This is exactly what you would expect of a steel shaft in an aluminium crankcase. I appreciate that you could generate additional compression on the crank as a result of the through bolts squeezing the crankcase halves together but that was not what I was seeing and I think I would expect a much tighter cold engine for this effect to show. I cannot comment on the possibility of fretting between the crankcase halves contributing to the effect as I have not experienced it. So the most likely source of the 'stiff when hot' effect has to be the differential expansion of the aluminium pistons in the steel cylinders but I don't think it is straight forward. I was wondering about how much power was being lost by the need to move these stiff pistons in the cylinders when the engine is running. I don't think it is much as I initially thought as I noticed that immediately after stopping the engine it is not that tight. If you move the prop immediately after shut down (very carefully on a hot engine) there is stiffness, certainly more than with a cold engine but it is not excessive. However if you wait 15 to 30 minutes and try to move the prop I found that the stiffness had increased significantly. I initially thought that this might have been caused by the oil draining away from the cylinders but I don't think that is what is happening. I think the excess stiffness when hot is caused by the cylinder cooling at a more rapid rate than the piston that is inside it. So you not only have the effect of an aluminium piston expanding in a steel cylinder you also have, after a few minutes, a differential temperature as the piston is staying hotter (and so larger) as the cylinder shrinks more as it gets cooler. After about 45 minutes to 60 minutes everything seems to have stabilised and the engine will rotate more easily as presumably the piston and the cylinder temperatures have both reduced and equalised. I assume that having a small element of expansion in a piston over a cylinder is a good thing as it helps to seal the piston against the cylinder wall and so avoid blow-by and keeps the compressions high. Whether the current Jabiru configuration has got the balance correct between ensuring a good seal and an unduly tight engine when hot I simply don't know but from my own experience I am confident that the stiffness is piston rather than crankshaft based and it is increased as a result of the differential cooling between the cylinder and the piston on shut down. I would be interested to know if others experience with their engines mirrors my own. Peter

It's some years since I flew a twin but I recall that all the skill is in flying the aircraft with one engine out of operation. As the props will both rotate clockwise (from the pilot's view) the left engine will be the critical engine in all phases of flight. So the questions that will have to be explored will include issues such as whether or not the aircraft can maintain a positive rate of climb at MTOW with a left engine failure and will there always be sufficient rudder authority to overcome the tendency to turn to the left if the left engine has failed, particularly at low speed with a non feathering prop while climbing. Some serious twins cannot always achieve this in the gap between take-off speed and best climb speed. The feature that really helps the proposed design is that the two engines are comparatively close together and near the aircraft centre line. This means that the amount of asymmetric thrust from a failed engine, even the critical engine, may be modest and easy to overcome with Jabiru's reasonably powerful rudder. In level flight I would have thought that the amount of asymmetry was sufficiently small that it did not require a rudder trim. If your leg starts to get stiff land the thing and work out why the engine stopped! Best of luck with this innovation. Nothing ventured; nothing gained. Peter

In the UK there is an approved thermostatic oil cooler adapter for Jabirus. Works very well and improved oil flow and pressure to the galleries. Made by Steve Rance. Details can be found by Googling SM13017 LAA Oil. That should take you straight to the LAA modification document for the device. With this installed there is no need for any duct tape etc. Peter

This is a copy of a post I made on the Jabiru Owners Forum. Your problem sounds like a classic case of carb ice, may be in the induction manifold. I didn't expect that to happen... -- Peter Knight, 20:09:10 06/04/10 Fri (88.105.183.89) G-PHYZ is a 230hr J430 and 'normally' performs flawlessly. It has an ST Aviation electric carb heater with one element permanently ON and the other switchable. It has a temperature probe thermopasted to the carb body and a second probe in the airbox. It also has the standard Jabiru carb hot air which I use. I though the belts were very well attached to the bracers I was flying from Halfpenny Green to Haverfordwest today in haze at FL60. I had used carb hot air from time to time as the relative humidity was high. When I pull Carb hot air on the airbox temperature reaches plus 50C in about 10 secs and the carb body temperature starts to rise after about 45 secs. I normally put Hot Air off after another 15 secs. The carb body temperature continues to rise for a bit as there is a bit of thermal energy still stored in the air box. Anyway I was dodging a few clouds when I had that feeling that all was not quite right with the engine. Carb ice perhaps? Couldn't be that as carb body temperature was reading 15 degs C. No ice is going to form on the carb if it is that warm surely. (I was once a physicist and thought I recalled somthing about ice and 0 deg C!). Any way I applied Hot Air and left it on while I watched the carb body temperature rise to 18 deg C. Intake air temp was over 50 deg C. I done my duty so switched off Hot Air. About 15 - 30 seconds later engine goes very sick and twitchy just like it is trying to eat ice cubes. Bit of a pnaic as I am over the lumpy bit of Wales with no flat fields in sight so straight to 121.5. They were brilliant and gave me vectors to Swansea while I put all the carb heat on I could find together with the fuel pump and got the engine back to smooth running. Landed safely at Swansea (got a free cup of tea as I was gibbering slightly) although engine was now running perfectly. After discussion with Gary problem seems to be that ice can form in the induction pipes downstream of the carb. I must admit I had not realised this as I had assumed the pipes were kept sufficienty warmed by the hot air from the ram air ducts to remove this possibility. The 'gotcha' is that even with electric carb heat permanently on and the carb body temp reading 15 deg and occasional use of hot air you can still get ice forming that you cannot detect. I admit that the met conditions were extreme for giving Carb icing and my cloud dodging and level changes probably made matters worse but I was very surprised at the induction ice particularly as every other indication was that the actual carb temp was OK. Hope these observations help others as the carb icing seasons starts up. Peter

Just to let you know that my daughter was responsible for this invention! That's six years of UK higher education for you. Just think I could have spent that money on my Jabiru. No sense of priority us Brits, Yours in light relief. Peter Knight