Jabiru 3300

[Dorsal]

I hope we can keep this thread civil as it is a good discussion. As Tim pointed out your engine is a big investment so folks get pretty passionate about their decisions. I, for one, am glad there are options, the more the better. Which is best is likely to be one of those debates that will go on forever hopefully with ocasional new choices thrown into the mix. I chose the Rotax because it was the most common and best understood power plant for this plane. I also don't like having a reduction unit but it does provide one solution for the following engineering challenge. To optimise the prop based on desired performance envelope will result in the best length, pitch and RPM. To optimise an engine for best HP/weight and other considerations will also result in a best RPM it just wont be the same as the best RPM for the prop. A direct drive engine will compromise these two in exchange for simplicity (a very real advantage in an airplane engine). Consider the following relationship (this has a bunch of hand waving but I believe basically correct). For constant tip speed and static thrust HP needs to increase linearly with the prop RPM at which it is produced. This is to say that if you have 20% more HP at 20% higher prop RPM (and therefor a 20% shorter prop) it should be about a wash in static thrust.

[kitfox5v]

Dorsal, Thanks for the info. That is why I started the thread. I just want all the info I can get. This is for sure the 2nd biggest investment on our project. Or as may wife said, the reason she upped my life ins. as you said, hope we can keep this civil because some rotax people are closed minded on this subject. Thanks again, Eddie

[avidflyer]

Not wanting to quibble about things, but a few posts back it was mentioned that you can hand prop a Jabiru. The standard Jabiru ignition needs about 300 RPM to make spark. I know I was never able to spin my 2200 that fast by hand. Kind of interesting though, the 582 Rotax also needs about 300 rpm to make spark. Having the gear box will let you hand prop it if you really flip it fast. Good thing to, my battery was nearly dead today, and so I fired it up by hand. Got some nice flying in afterall! Take care, Jim Chuk

PS I've never heard of anyone handproping a 912 even though it has a gearbox. Anyone done it?

[DesertFox4]

PS I've never heard of anyone handproping a 912 even though it has a gearbox. Anyone done it?

Yes. I've done it twice on the 80 hp but that was before I read you couldn't do it. Now of course I wouldn't even try. I was much younger then too.
BTW, I've been living with my Rotax decission for 14 years and 2,500 plus hours of reliable, cost effective , fuel efficient, low maintenance very enjoyable flight. My engine decission worked out great. Hope yours does too.

[Av8r3400]

I wouldn't even want to try hand-proping a 912. They say a minimum of 200 rpm to make the Ducati boxes fire. Besides with the style of charging system these motors use, you need some battery to make it charge. Running without a battery in the loop will ruin the rectifier/regulator.

I don't know how the Jabs work, but you can prop a 2-stroke Rotax. A member of our chapter has a 503 powered Ridge Runner and he always props it. Here it is being done by one of our own (DaveF) on a 582.



http://www.youtube.com/watch?v=W867v6beoo4

[DesertFox4]

The batteries were not totally dead on the two I did it on but not enough charge to turn them through the compression stroke. Don't try it Larry, at our age we're both too slow to get out of the way if/when it starts now.

[foxkit3]

Did a complete tear down and inspection of my 912 ul with 1829 hours on the hobbs. The only component that measured out of spec was the gear box thrust washer. (old style brass washer) lots of lead build up. My only complaint from the engine is alittle rough running at certain r.p.m`s. My model 3 with a 912 and ivo inflight prop will out perform my friends speedster with a jabiru. The Jabiru sounds sweet, very smooth running. More of a simple engine.

[kitfox5v]

Thanks Foxkit3 for getting back to the question. Just trying to get input on the jab pros and cons.