Jabiru powered Fox / Bushbaby...again!

[justin.schoeman]

Sorry Nick, the theory is easy, the maths is horrible . If you want actual numbers, get an aerodynamics text book, and run them yourself .

Anyway, I am not saying that the motor will not work, but as Morph says above, you won't get the same performance. At least take-off and climb. But cruise may be better.

Even those Apollo Fox figures you mention aren't that great. Why do they quote a cruise at 2600RPM, when typical cruise RPM for the Jab motor is 2800RPM? Probably (not, I don't know this for a fact) the airframe drag is so high that adding more power will not really get you any more speed. So the airframe is not well matched to the motor. You are not getting the most out of either in the cruise.

So the motor does work, but performance would be better on a 912.

Another example - the VW motor. Put a VW motor direct drive in a BB, and see if you can get it off the ground. No chance.

Gear it down a bit, and it works quite well.

Now put the same direct drive VW in a KR2, and wow! It flies like a dream. Put the geared motor in the KR2, and you won't be able to climb, because you can't get the speed up to Vy.

The difference is the KR2 only rotates at about (exaggeration, I know) the cruise speed of the BB.

All up weight for both is the same, but the drag of the KR2 is much less.

-justin

[RudiGreyling]

Reminds me of something I posted a while back on the RV12:
http://www.vansairforce.com/community/s ... light=rudi

<SNIP>
Lets look at power:
The 912S is rated at: (2.43 reduction gearbox)
95HP continues @ 5500 engine RPM / 2.43 = +/- 2260 prop rpm
100HP max @ 5800 engine RPM / 2.43 = +/- 2400 prop rpm

the Jab 3300 is rated at: (Direct drive)
105HP @ 2750 engine/prop RPM continues
120HP @ 3300 engine/prop RPM intermittent.

There is only a 10HP difference in continues power, but look at what the Prop RPM is doing at that power setting. The Rotax is below the magical 2500rpm range the whole time, which means it will swing bigger more efficient props without the tips going supersonic.
<SNIP>

Now keep the prop diameter, airframe frontal area behind the prop and cruise speed in consideration and you should have your answer.

Regards
Rudi

[justin.schoeman]

That seems to be an old (or a new) spec. My Jabiru manual specifies max continuous of 3150rpm @117hp...

Skews the comparison a bit , as there is now a 20% power difference!

-justin

[smallfly]

This is indeed very iterresting information as i have often ponderred on

this topic, however, although the jabbi will be slower/less powerfull on

direct drive than the rotax, will it not be more feul efficient, turning at

lower rpm, and therefore also last longer?

Surely the tbo should be longer?

[Nick]

Picked up some gen on a 2200 Jabiru powerd Kitfox in the States.

9,915 km total trip
158.3 kph average for ALL flying
Total fuel used 931 litres
Fuel consumption 9.4 litres per 100 km

I did the convestions from US gallons and miles Source data

I don't know if that's good, bad or indifferent compared to a Rotax 912S.

Do Jab 3300 have an issue with blade tips going supersonic...I don't think so, I think I'd have noticed. Thought that was a Harvard speciality

Any comments on fitting variable pitch props, seems some do this?

[Morph]

Nick,

it appears you already have your mindmade up.

go ahead and install the jabi motor and give us your feedback. We can all learn from this experience

[Nick]

Nick,
it appears you already have your mindmade up

Far from it, I'm just trying to learn. The Rotax is clearly a great engine, but it's expensive, and servicing costs I understand are high. A significant but lesser point is that being high revving units, they are considerably less agreeable on the ear than the Jabs.

The 120 hp Jab engine can be had for less than the price of the 912s, and develops more horses than the 914 turbo. Though, thinking aloud - turbos are less effected by altitude, so I wonder how they would both compare at 10,000 feet.

There are a goodly number who fit the Jab engines to the Kitfox in the US, they must have reasons so to do. Jab engines are winning a great reputation.

Some folks are even putting Jab 2200 engines in the Zenair STOL 701.

I'm also rather interested in the new UL260i - shipment began end 2006, another direct drive unit. This time fuel injected and 95 HP. Though it's probably simply too new for me to bet on.

[Morph]

I hear you, and as I say I also wanted to go the Jabi route because the motors are good. But it's almost as if you are trying to find a compelling reason to use it where there are already compelling reasons not to.

Like I said, it's your choice. I have learnt many years ago that questioning is good, but sooner or later you realise, sometimes at great cost to yourself, that the wisdom of the learned(pronounced "learn-id") outweighs the enthusiasm of youth. In other words, just because you wish it so, it isn't and you will eventually stop fooling yourself and realise the choice was not the right one. I've been there, not listened and eventually had to change my mind.

BTW a little birdie told me that ZULU1 is fitting a turbo to the HKS, pushing power to 95HP at about R20K cheaper than a 80HP 912UL. This will give a fully altitude compensating fuel injected 4-stroke at a really good price.

[Nick]

Morph wrote: But it's almost as if you are trying to find a compelling reason to use it where there are already compelling reasons not to. Shocked

Possibly that's how it comes over, and that may have something to do with my rather robust 'in yer face' style, for which I apologise. But that is not the reality.

I'm just - as an admitted and fully confessed new boy, who really knows squat - just applying basic critical thinking and scepticism, hopefully providing a catalyst for some discussion of general interest 'to the gallery'.

It was more than strongly suggested to me that the Jab engine is not compatible with the Bushbaby, clearly that's less than a fair exposition of the reality. There are folks in the States fitting them to Kitfoxes and my maiden flight was in a Kitfox clone, the Appolo Fox, toting a Jab 2200. Jab engines are listed with Kitfox firewall forward kits (including the 120 hp 6 cylinder 3300) on the Jab USA website - a country almost insanely litigeous, where you better believe, that if it wasn't 'kosher', Jabiru's legal advisors would not have let this 'pass muster'.

Can you blame me for parsing the data, for cutting and dicing?

What would convince me, would be credible performance data from multiple sources citing the performance of the Jabs Vs the Rotax on the Kitfox or a clone's airframe and these all being reasonably congruent. A summation of servicing costs/ issues would also be dandy. These, thus far, I have been unable to locate on the interweb; which I find rather surprising. Because I have been unable to do this, to my mind, the whole issue is open, and I'm going to keep delving in my irascible style.

Now I'm expecting, at least half, possibly a tad more, that I will end up coming to the conclusion that the Rotax 912s is the current optimum - price performance, reliability, risk ...sure bet, power plant for a Kitfox clone. But...I aint there yet! And I see folks with more experience than me (not difficult) shooting down my ramblings as a good thing. It's good for me, and hopefully for all who read this thread, and each can apply their own critical faculties and judge the debate for themselves. To my mind this is the very stuff of open debate, reasoned argument and freethought, the basis of the scientific approach and the best mechanism we have for approaching 'truth'....and now I've done gorn all philosophical...shucks!

Again - sorry for my gobby, in yer face style....I'm a rather difficult Rooinek!

cheers Nick Good

[Morph]

Honestly,

I'd love you to prove us wrong on this one.

BTW I am building a Bushbaby Explorer with a 912 80HP. The only reason, and I seriously mean the only reason, I went this route is because I got the engine for a miserly R50K with only 125 hours on it. I am not in any way umbilically tied to Rotax.

When I am flying it I'll give you the data you need. I suspect that the 2200 based BBs get their cruise speed of 85mph but seriously suffer from climb performance. However the proof is in the pudding.

Your posts are greatly welcome to the forum, don't stop

[Nick]

Thanks Morph, I've already been a sad git, and been through your build thread, a number of times.

Huge respect and admiration, and more than a little envy, for anyone that has the guts and determination...and self confidence, to undertake such an project.

An enterprise such as this, must be food for the soul....with a goodly bit of chewing finger nails down to the elbow!

Your posts are greatly welcome to the forum, don't stop
Go on, admit it, you've got a thing for Norwegian chicks...

[Morph]

Hugely satisfying, immensely frustrating, requires an absolutely understanding wife and must be done at home not far away at some airfield.

Will definitely build more. I love the challenge and having something other than IT to do with my hands and mind.

[Boet]

:D Jaaaa swaer. If you insist: Put the 3300 Jabbi in your BB. You will have a VERY nice performer, comparable to the 912S. The smaller Jabbi only gives so-so performance. A no-go. But, give it a try. We are all eagerly awaiting to learn from your mistakes.

[Mogas]

Not seen mentioned here is the cooling on the Jab.
Lots of forward speed and clever ducting required.
Have seen more than a couple of Jab (airframes) have problems here in the hot Lowveldt, nevermind slower cowled BBs.

[justin.schoeman]

Not seen mentioned here is the cooling on the Jab.
Lots of forward speed and clever ducting required.
Have seen more than a couple of Jab (airframes) have problems here in the hot Lowveldt, nevermind slower cowled BBs.

For tractor applications, the jab motor comes with all the required ducting. Just provide frontal intels that line up with the ducts, and the recommended outlet area, and you are all set.

-justin