On J230, the heat, and suitability of the Rotax 912ULS

[RFguy]

Have been musing my options for my J230. I love my J230 airframe  there is nothing else like it in RA... but when summer comes  there will be trouble with summer and the engine temps.

I first look at heat removal options for the Jab engine, and then I  explore if the Rotax 912ULS is a suitable replacement powerplant. 
 

Firstly, some background : My calcs say CHTs MUST stay less than 165 C in cruise or at or during TO  / roll/climb (WOT)  . 5 and 6 are always the hot ones , the others are 15 deg C cooler.

At OAT =14, my temps may be, if I have to hold short for a 5 minutes, up around 160-163C  by the time I start backtracking, and 160C in a long climb at 80 kts WOT (EGTs are nice and cool 640-650C ish ) .  In cruise. hottest is 140C.  . If I add 25 deg C to the ambient, cruise is OK just (get up high to fix that) , but climb, even if cruise climb, and TO is going to be way way too hot.

So what to do about it. Bear in mind, I have improved the cooling already with some targeted deflector mods and holes in the back of the cooling plenums.

I've had temperature probes in the plemnums -  and the rear cylinders are simply supplied with hot air from everything in front of them ! They almost need a separate intake, but that's alot arty glasswork to change.

 

1) Apart from LCH, one solution I have that will work is two targeted blowers into the plenum, they'll easily keep the heat down. I have them, These blowers can drive 150 cfm at 3" of water, that are axial mode compressors...They need 80W per blower which means either the Rotec outboard alternator kit, or a LIFEPO battery to provide the juice. The battery is the obvious solution , since a 6kg LIFEPO can store 480 watt hours, and that would be enough supplementary cooling for 3 hours and would be recharged by the system alternator while you were not running the blowers, so no issue there.  ( Blowers for ground ops and during TO/climb) . 

 

2) Water mist injected into the middle of the plenum just ahead of the rear cylinders. Good option. Needs about a half a cup a minute for both sides at max.... Small brass misters could be used. Takes the heat out of the hot air reaching the rear. The fact the air is heated by about 15 to 20C  by the time it gets to the back is the problem.  

 

3) Install a Rotax 912ULS. It fits..... and everything is in the right spot..... (use an adaptor plate between the stock engine  mount and the stock rotax ring mount )  

 

Takeoff / climb  power for the 3300 jab is about 110 HP.     Rotax 912ULS do 96 to 98HP apparently on dynos out of the box.  Let's call it 100HP for the sake of it.

AT 10% down on HP, TO rolls and climb etc will be 10% poorer. Not really so bad.  it's very good as it is IMO.

Bear in mind, the J170D with same wings and MTOW has only an 80 HP engine, and its acceptable generally, except when really hot and high (summer Jindabyne). 

 

If a CS prop was used, that 10% could almost be got back, since the prop efficiency at 80kts climb is about 10% down on the max prop efficiency that occurs about  120 kts CAS - 
 

IE : CS prop would go close to restoring the TO/climb performance. 

 

J230 cruise at 75% power = 90HP , this could be replicated by the 912ULS fine as Rotax will run  90% all day, all night. So we have the same cruise performance

You can easily improve cooling for the Rotax- that's easy- larger oil and water coolers. It'll be fine. Might be into the rich part of the bing needle, no biggie.

 

Now, the PROBLEM comes when the DA is in the toilet.  ISA is fine   , but hot and high are another story. Jindabyne 1005 pressure, 28 C DA = 6015.

 

At 6000' DA the 912 max power  is going to be down to 82% of 100HP or about 82HP.  You'll get the 82HP on takeoff - IF the rotax has the  CS prop for max 5800RPM for takeoff
...Jabiru fixed prop static RPM will be the same, 2850 ish, so the Jabiru TO is going to be down from 110HP  to 90HP ....  so the  CS prop of the rotax is useful here. 

The Jabiru prop efficiency will suffer a bit compared to the CS prop  at the 80 kts TO, IE the CS prop can get you 8 to 10 %, so the performance is likely to be similar. 

- If the Rotax was pitched for 5400 static, there is 3 to 6 HP left on the table by not going to 5800. The Jab has flat torque curve , rotax has a max torque at 5200 and dives beyond that, so the extra power is only very marginal beyond 5250.  so- pitch for 5400 static/ TO roll.  

So the CS prop does a good job to make up for the deficit for TO/climb. 

 

Now, let's look at cruise performance, 7500 ' cruise altitude in summer- DA could be (30 deg ground temp) ~ 9500'

Cruise 115 kCAS (132 TAS at 7500 in ISA)    needs ~90 HP.   At this DA, Jab will be 72%, and max power near the prop highest efficiency point will be  81HP at WOT.  So it wont make IAS at hot and high (it will be airspeed slower) , but it will make the 120kTAS (104KCAS)  as specified by Jabiru- Jabiru are true with that one. 

 

Rotax in this case, max continuous output 90HP WOT at ISA, will be only 65HP at the above example. Now, we are down quite a long way.

This is an indication that the power the jabiru has ' to spare' comes into requirement at extreme hot and high. 

 

While the Jabiru power is linear with RPM (ruler flat torque curve) It doesnt help you to spin the prop much faster because the dive in prop efficiency when the airspeed  gets away from the max efficiency speed hurts.

 

WHat to do ?  remembering this is not about increasing the sea level output, the problem is the reserve at high DAs.

 

1) 114 HP big bore kit. from Zipper , STOLcreek  or the norway bunch . That pretty much fixes the deficit WITHOUT needing a CS prop.  

It's affordable.

 

2) electric supercharger- We only want another 15% to fix the DA problem- we're not using the turbo  to increase sea level engine output. 

The provide pressure increment of 1.15 needs approx 2.8kW  power (after efficiency loss  of compressor and motor)  to generate that boost. ( IE about 2.3 lbs for the 1.3litre 5800 rpm rotax) .

Now, we could use a LIFEPO battery  to cover the supercharger  electrical requirement for TO , but as above, the TO issue is mostly dealt with by a CS prop, the issue is a hot and high cruise.  so that;'s not going to fly.  Only weighs couple of kg.  could still use to deal with hot and high strips, and just accept the cruise performance when hot and up high will be lower.
Heating the air (adiabatic compression)  is a corresponding byproduct, so numbers above will be higher.

 

3) a 914 turbo. The 914 is a very expensive lift in price compared to a 912. there are a few 914s in J230/430 EXP registered aircraft.

 

The big bore is the bext solution, IMO

The CS prop will get it close for TO/climb rolls. for all but extreme  (>7kft) DA conditions. 

 

Edited August 15, 2021 by RFguy

[Blueadventures]

Also gain some real time data from Jab 230 / 912 drivers, I don't know any.  Cheers.

[Thruster88]

Cowl flap like most six cylinder aircraft. https://www.aopa.org/news-and-media/all-news/2017/november/flight-training-magazine/how-it-works-cowl-flaps

Edited August 15, 2021 by Thruster88

[RFguy]

not much improvement for this setup Stuart, the Jabiru head shapes, fin design,  has too many spurious paths and the air wont go down the fins because the high pressure drops  through the fins mean the air will get out any other way it can- the Jabiru heads, unlike the Lycoming heads, have lots of fins on top ! they really needed to have bigger gaps between the pots and have wide finning on the sides of the cylinders, like the lyco, and then there would be the only-path choice for the esacping airflow....... so  without judious applications of extra seals, baffles, blanking plates you name it, it just isnt sufficient  because the system really relies on forcing air through the top fins .  the side fins, one ONE side of the head are the only heat removal path. ... and there are HUGE gaps for air to go around the high pressure drop path.

 

which is why Jabiru plenums have those deflectors on the top, the airflow is deflected into the middle of the top of the  head fin section.... 

 

The heads are built to be economical  - economical materials and method of manufacture. ...That's how they can afford to make a nice  engine for that money. - which is fine, as long as people remember, they dont have the capability to handle the temperatures that a Lycoming can deal with all day. 

 

IE 180C all day in the Lyco is no big deal compared to 180C (on top = 195+ underneath )  in the jabs where you get head recession, valve seat trouble, cooked valves- the lyco doesnt have any of those issues as much because the build is quite robust at those temps for extended periods. 

 

 

 

 

 

Edited August 15, 2021 by RFguy

[corvairkr]

RF maybe you could speak to Ole at Australian aircraft kits. I believe he did the tidy conversion posted by pylon 500 in this thread.

Jason 

 

 

 

 

[RFguy]

Hi Jason

yeah, I know a couple of the 914 conversions, that's a super option but lots of money. They're all experimental cat.

 

The 912ULS into a J230 all fits and the weight is in the right place, the prop is in the right place (no cowling change ! )  and the mount is only a 2D drawing hole adaptor plate. 

So the CG etc , the thrust location etc is all unchanged so the stall, spin recovery will be essentially unchanged. That's the  non negotiable. I   am not sure if the presence of a CS prop would change the spin recovery character significantly. 

With a CS prop, it would meet the book TO roll , and might do a bit better on landing roll. 

 

Jabiru could sell 3x the airframes if they went along with it (they dont wish to be involved so I am  not seeking their blessing) .

J230  is a super airframe . Fast, strong, roomy, quiet, safe.

 

Again, it should be pointed out, that a 80HP engine  in the J170D with the identical wing and MTOW  passes muster for most applications. Of course the J230 performance is a sports car compared to a J170, and also J230s are "more often fully loaded".  A user with a 912ULS, would have to accept lower cruise airspeeds during high DA conditions.

 

Sure the big bore kits fix that (114HP)  without a CS prop, but that's one further step away from a plug-in which I seek to meet.  If it can be a simple bolt on, then we have a goer. Once an additional step is in there, the taste for the change to be applied en masse is reduced  and the path to approval is more difficult

 

 

[Kyle Communications]

Glenn

 

Just look over in the USA. They are flying all sorts of aircraft in the LSA and Experimental also Jabs in far higher DA than your talking about. I think you are stressing way too much over the facts and figures...we all know that what you put on paper is maybe not so in the real world.

 

So many I know over there their takeoff std height altitude is 6000 or 7000 ft so the DA is far worse in their summer and all have no difficulty doing lots of flying. Look at Youtube for Gravity Knight Flying. Todd used to have S6 with a std 912ULS and his home airfield is over 6000 ft and he flys 10 to 12k ft all the time. He has since added stuff to his engine and also now a different aircraft but his previous hundreds of videoes were done with a bog stock 912ULS.

 

Just stick a rotax in it and go fly...I really dont think you will be dissapointed

 

[RFguy]

Hi Mark

 

Agreed. the corner cases dont look too good, but I guess that's what they are - corner cases.

 

and the same aircraft wing and MTOW  with an 80HP is deemed acceptable..... 

 

 

 

[Kyle Communications]

Try with a std Rotax first then if you need more grunt then do the bolt on HP with a big bore kit. Maybe also add a CSU later. I think adding a turbo later is a much bigger job and a lot of testing and fiddling to get it right. I know Todd has had a few issues from time to time with his turbo "add on"  that required a lot more fiddling as he did have some issues and he had to do a lot of tuning to get the wastegate etc right. At least you can do this with a Rotax...cant do it with a Jab engine...I havent seen any bolt on and go HP increases for those engines

 

[RFguy]

Hi Mark. RRR . yeah the turbo is conceptually so simple but there is devil and detail.  Big Bore kit is dead simple. Need extra cooling ? use a bigger cooler, that's the nice thing about the water (and oil) cooling.    As you know,the  GEN3 Jabiru engine does not have any thermal margin for increasing power output. The head shape is the problem for cooling. And it comes back to the  distance between the pots. air needs to be forced thru the top side fins.

 

Apart from the heat issues, the Jabiru engine (late Gen3) is fantastic  if kept below 165C CHT on top  AT ALL TIMES. easier on the 4 cyl. harder on the 6 cyl.  and use an additive if using avgas.

 

 

 

[jetboy]

A couple of considerations... I thought the Rotax props rotate the other way, if so then the engine mount could not be adapted because the whole engine at the rear mounting has to move across and at a different angle - not really a problem just a bit more $.

 

On the + side with any geared engine you get to turn a bigger prop because it can run lower rpm and gain efficiency, this is a performance increase without going CS - which could yield additional increase.

 

The 2 jab in NZ I think are running as microlight class and doing well.

Edited August 16, 2021 by jetboy

[corvairkr]

3 hours ago, RFguy said:

Hi Jason

yeah, I know a couple of the 914 conversions, that's a super option but lots of money. They're all experimental cat.

 

The 912ULS into a J230 all fits and the weight is in the right place, the prop is in the right place (no cowling change ! )  and the mount is only a 2D drawing hole adaptor plate. 

So the CG etc , the thrust location etc is all unchanged so the stall, spin recovery will be essentially unchanged. That's the  non negotiable. I   am not sure if the presence of a CS prop would change the spin recovery character significantly. 

With a CS prop, it would meet the book TO roll , and might do a bit better on landing roll. 

 

Jabiru could sell 3x the airframes if they went along with it (they dont wish to be involved so I am  not seeking their blessing) .

J230  is a super airframe . Fast, strong, roomy, quiet, safe.

 

Again, it should be pointed out, that a 80HP engine  in the J170D with the identical wing and MTOW  passes muster for most applications. Of course the J230 performance is a sports car compared to a J170, and also J230s are "more often fully loaded".  A user with a 912ULS, would have to accept lower cruise airspeeds during high DA conditions.

 

Sure the big bore kits fix that (114HP)  without a CS prop, but that's one further step away from a plug-in which I seek to meet.  If it can be a simple bolt on, then we have a goer. Once an additional step is in there, the taste for the change to be applied en masse is reduced  and the path to approval is more difficult

 

 

The Jab in the link is a 912 conversion. 😊

 

jason 

[RFguy]

Jetboy :
1) no- rotates same way
2) no - no performance increase that way, no free lunch.  geared engine doesnt buy you performance for that reason. . . Gearbox in rotax case enables engine to run at higher RPM hence higher output for given displacement /fuel-air charge  AND use a reasonably efficient  prop configuration. It's not the case that rotax uses a gearbox for better efficiency - If there was a prop that could operate with the rotax without the gearbox, it would be used, but aerodynamically that doesnt exist so they use a gearbox. Doesnt do the job of a CS prop. A fixed prop is specificed  for efficiency  at some single airspeed/RPM combination  and overall compromise to designed service.   CS prop enables this maximum efficiency point to track with airspeed and best engine speed.

 

more exotic fixed pitch prop profiles can better maintain efficiencies away from their most efficient airspeed/RPM. 

 

happy to explain . plenty of prop stuff online.
 

Edited August 16, 2021 by RFguy

[Kenlsa]

As yours is 24 reg you will have to get the ok from RAAus as to E24 reg requirements : 912 ups has been approved for the jab but no other options at this stage.

Don’t plan the hell out of it before you are sure of the paperwork 

Ken

[skippydiesel]

It seems to me that you are ignoring the pioneering (successful) work of others, who have opted for the Rotax 914.

 

Why reinvent with a smaller wheel ??

[Old Koreelah]

Glen I’m probably too older and tired, but the thought of replacing a workable engine with a whole different type make me sleepy. How many enjoyable flights will you miss out while you’re transforming your plane?
You must have lots of spare time on your hands or not enough challenges. You could spend huge amounts to get a marginal improvement in performance.

 

[RFguy]

I'll just need two planes then !

I am looking around for a Tailwheel Thruster or something like that...

 

[planesmaker]

Rf I think you have done too much paper work. A 912s will be no problem and will not be down on performance compared to your Jab engine, you will be running a larger more efficient prop. May even be better performance 😀

[RFguy]

Hi Tom

yes, that is true

- The 912ULS is only a sub-optimal in the corner cases of high DA. 

[facthunter]

A six is far smoother than any four. I've operated j230 at over 42 degrees. Just increase climb speed a  bit if needed.. If you are holding downwind facing for extensive periods you'll get a liquid cooled motor hot too.. Shut down and restart in that situation.. Nev

[Bruce Tuncks]

A king-hit way to give cooling is water injection. This can be into the cylinders or into the ram-air ducts. Yes it is a limited thing just for climbing.

There is also the option of Rotec heads.  I have seen these on Mike Sharples plane, which uses a Jabiru 6 as a top-mounted pusher.

I think there are options to explore before going to an import.

[Bruce Tuncks]

A kg of water absorbs 2,500kJ of heat when turned into steam. This would cool a 60kg lump of Aluminium down about 50 degrees C.

So a spray of water into the ducts would have an interesting effect...  surely this has been tried?

I think that Bleriot was saved by sea-water spray when he got really low over the channel. Surely we could do better than in those days.

[RFguy]

actually,  the misting into the cooling ducts would do the job.  doesnt take much mist, I did the calcs...

 

remember my limit- CHT=165C.  

 

What is pleasing, is that it is only the rear pair 5,6 that need cooling work.  They get 'preheated' air from the fins in front of them ! They almost need their own inlet ducting. 

[Bruce Tuncks]

It wouldn't take much to try it...  a squeeze bottle and a length of plastic tube would be all that was needed. The spray nozzle should be on the very end of the tube I think.

For a permanent job, I can imagine temperature-controlled spray valves doing the "emergency" cooling automatically, well for as long as your water supply was not used up.

BUT beware of causing damage from shock cooling. On a motorbike forum, a guy was told to watch out for this if he squirted water onto his Harley engine.

Apparently several WW2 aircraft used water injection into the intake manifold to cool and prevent detonation. This may be so efficient it overcomes the problem of cooling all cylinders and not just chosen ones.

I reckon there is a lot of fun to be had here to experiment and research.

[RFguy]

A fine brass misting nozzles will do it... I reckon.  inserted about 2/3 the way down the plenums.

 

you want to decide if the water will vaporize  and cool the air hitting the fins, or whether  the droplets will land on the fins and do their magic there.

 

Edited August 19, 2021 by RFguy