Jabiru powered Fox / Bushbaby...again!

[Nick]

The topic of powering a Kitfox clone - such as the Busbaby - with a Jab engine again I'm afraid.

Had my first flip last week. Well 2 in fact, both down in Nailspruit where I was visiting. Both courtesy of Andrew Papas at Micro Aviation. I'd met Andrew at his display at the Hoedspruit airshow the week before.

The first flip was in a Bantam, powered, surprisingly to me, by the 3300 6 cylinder 120 HP Jabiru engine, the same unit that powers the Jabiru J400/ 430 4 seater Antipodean 'plastic fantastics'.

I had my first ever go at the tiller on the Bantam, Andrew rather surprised my by handing over control almost immediately after take off...it was great fun - the Jab engine sounds wonderful. The Bantam didn't require any rudder input in the level flying and figure of 8s I did - I guess that would mean that it's particularly stable around the yaw axis. The Bantam could practically take off in the length of a tennis court!

The second aircraft was the Appolo Fox ZU-EIZ. This is a Kitfox clone, factory built I understand in Hungary (Micro Aviation has the SA agency for both aircraft).

This had a Jab 2200 4 cylinder 85 HP engine. A lovely little aircraft it is too. During my stint at the tiller - my second ever - it was quite noticeable how much rudder work was required to keep the little ball thingy in the middle of the whatnot.

Anyway, thanks to Andrew Papas for being so hospitable.

All this contextual pre-amble brings me onto the thrust of this post...the stories I hear about the Jab engine not being compatible with Kitfox clones, including the Bushbaby, seem rather less compelling now. I understand that ZU-EIZ is fitted with a 64 inch prop.

Given the relative pricing of a Rotax verses a Jab engine, the servicing costs on a Rotax, that the Jab is a direct drive unit, with no gearbox, why do not more folks fit Jabs on Busbabies?

Also, given that the 6 cylinder 120 HP Jab unit only weighs 20 kg more and yields 35 HP more than the 4 cylinder unit - 120 HP vs 85 HP - could one fit one of these into a Kitfox clone, such as a Busbaby for highveldt use giving a slightly faster cruise - say 100 knots? The slippery Jab J430 cruises at 120 knots with the same engine.

[RV4ker (RIP)]

Boet

popcorn, coke and billies lined up.

PS
I have asked same question a couple times and response has often been, Why change what works, engine mounts, cowls, prop diameter etc...

For me personally
Too much power has always been (almost) just enough in my book. My RV4 has 180hp for 450kg airframe, although it will fly happily with the 135hp Lycon engine. (MAUW 1650lbs). So why not BB with 120hp vs 582 for eg. maybe drag is also a issue.

Is there a chance it will pull from firewall as has been touted about the 450hp Steerman for eg?

[Nick]

If they can fit a 120 HP 6 cylinder Jab engine onto a Bantam, and it's fine and dandy, a Bushbaby with one seems a little less than outrageous. Also I notice Jabiru list a Kitfox firewall forward kit for the 6 cylinder engine on their website.

The other thing that Andrew said, is that if he is flying with other Bantams with less powerful engines he taps back so he does not leave them - he mentioned flying to Pretoria - and when he does so, his fuel consumption in comparison to them is considerably more frugal. Now this is of course hearsay, so I'm naturally cautious about such claims - I like to 'triangulate'…part of the reason for this post. That said, I have noticed this phenomenon myself in other contexts - 2 engines doing the same work, the more engine operating at 55%, the less powerful at perhaps 80% and the larger, more powerful engine returns considerably better fuel consumption. I’m no engineer, but I understand that this is because engines operate most efficiently at mid range (I'm like this myself...but let's not digress here).

I’ve also heard, from more than one source, that maintenance costs on Jab engines are considerably less than on the Rotax.

The only issue seems to be the extra 21 kg of mass on the nose of the 6 Vs the 4 cylinder Jab engine, given a 2 seater with a useful load of 300 kg – as per the Appolo fox (not sure about the Bushbaby – allegedly the airframe is a little heavier because its mild steel). Given that 100 litres of Mogas or Avgas weighs about 71 kg and I’m unlikely to fly with 2 folks weighing 200 kg total…this 21 kg would seem to not be a problem.

Anyone feel free to comment on any of this?

[Mogas]

Hey Nick
Real pity we did not make contact, I am also in Nelspruit. We could have flown my 912S Bushbaby and it may have answered some of those questions!

[skybound®]

When I owned a Kitfox, I was part of the Kitfox owner e-mailing list. Seemed quite a few owners had installed Jabiru and Subaru's in their Kitfoxes. I had not heard any real cons of the use of either of these motors.

[KFA]

Hi, to answer your quest. The Jabi engine is a direct drive which means it can only rev up to a max of 2700rpm ish. If you look at the torque curve you will also see that a rotax whith the same amount of hp has got a lot more torque due to higher rpm and a reduction drive. That's why a 85 hp Jabi can only turn a toothpick instead of a 72 inch prop that you will need on a BB or Kitfox. There is a good reason why Rotax is king

[Nick]

Yup the US Jabiru site details FWF kitsfor various incarnations of the Kitfox.

Cost for a new 3300 Jab 120 HP engine R 103,000 (not including VAT), for a 2200 85 HP R 73,000.

I think a 100 HP Rotax 912S comes in at about R 130,000 ?

I'm begining to thing Why would you NOT fit the Jab engine!

[RV4ker (RIP)]

Heard Placo have been appointed as Rotek agents in SA. Maybe a third alternative...

[Nick]

Hi, to answer your quest. The Jabi engine is a direct drive which means it can only rev up to a max of 2700rpm ish. If you look at the torque curve you will also see that a rotax whith the same amount of hp has got a lot more torque due to higher rpm and a reduction drive. That's why a 85 hp Jabi can only turn a toothpick instead of a 72 inch prop that you will need on a BB or Kitfox. :roll: There is a good reason why Rotax is king :wink:

What's special about a Kitfox aerodynamics that makes it need a longer prop?

If you compare the performance of Jabiru aircraft toting Jabiru engines with similar 'slippery' plastic aircaft with Rotax engines - a 2 seater Jab with a Foxbat say, the performance figures would be what you would expect based on horsepower. This makes me very sceptical indeed of this 'short fast propeller is bad, long slow propeller is good' argument, which I keep hearing.

What are the perfromance figures of a 912s 100 HP Bushbaby compared with a Jab 4 and 6 cylinder 85 HP and 120 HP Bushbaby?

Put another way, if there is nothing peculiar about Kitfox/ Bushbaby aerodynamics - are you arguing that if you put a Rotax 912S engine in a Jab J430 it would go better, if you're not arguing that, why not?

[justin.schoeman]

OK, I have posted my comments on this before, but why not again .

Just consider the dynamics of a propeller. There are two aspects to a propeller. Thrust, and pitch speed. Thrust is the physical 'pull' of the propeller. Pitch speed is effectively a measure of how fast the aerie can go, and still produce thrust.

This is grossly oversimplified, but thrust is most closely related to prop RPM and prop diameter. Pitch speed is most closely related to prop RPM and prop pitch.

The power that a prop 'consumes' is (more or less) proportional to the prop RPM and the square of the diameter (with a lesser proportion to the pitch).

Now with these basics out of the way, we can go two ways with a prop. We can deliver a power of 120hp to a prop by:
1) A small diameter prop spinning very fast.
2) A large diameter prop spinning very slow.

In the first instance, you can have a high pitch, and you end up with a propeller that does not produce much static thrust, but does have a lot of pitch speed, so it keeps on producing thrust, even at high speed. This sort of combination gives you longer take-off rolls, and poorer climb out, but a more efficient cruise, at higher speed.

In the second instance, you have a lower pitch, and you end up with a propeller that delivers a lot of thrust, but virtually no pitch speed (think of a chopper spinning the big rotor nice and slowly, but producing enough thrust to hover - as an extreme case). This gives you great take-off, climb, and low speed performance, but efficiency falls off very quickly as speed builds.

It is obvious that the 2nd case is ideal for most microlights - relatively slow planes that operate out of short/poorly prepared fields. While the 1st case is better for faster aeries that operate off long runways.

Now, back to the Jabiru. This is a direct drive motor, that swings a prop at 3300 RPM at full power. This means the Jab motor is squarely in case 1. Small prop spinning fast. (In fact, in most cases, it is heavily overpitched, and does not produce maximum thrust static - it actually produces highest thrust at a higher airspeed.)

A Rotax motor is geared, and you can decide which option you want, but most go for the 2nd case.

Now compare a 912S vs a 3300A. The 3300A has 20hp more, and this can often overcome (at least some) of the low speed thrust issues. But it will be difficult to use this motor effectively on a slower plane. The motor/prop combo will only really come into its own at 70kts+.

The 912S is slightly less powerful, but can be geared to give optimal performance at the desired operating speed of the aircraft.

-justin

[KFA]

Couldn't have said it better myself. The moral of the story is that BB and kitfoxes are mostly high drag aeries and not the best cruisers thus the big prop and not the toothpick

[grosvenor]

with similar 'slippery' plastic aircaft with Rotax engines - a 2 seater Jab with a Foxbat

Nick, bad example, the Foxbat is neither 'plastic' nor 'slippery'. It's a metal aircraft with a very thick wing intended for safe low speed operations. It probably wouldn't exceed Vne with 150hp

Dave

[Mogas]

Well said Justin, propeller theory is a complex subject (which I do not profess to be a master of!).
All I can say is that it is well proven that it is better for slow planes to swing big props at slow revs.

[Morph]

here we go again

I too would have liked to have put a Jabi in my BB. Compared to the Rotax the price is much better I spoke to Monty, the BB manufacturer and he tells me there is one BB with the 2200 engine installed. They have been stuggeling to get comparible performance (compared to a BB with an 80hp 912) out of this plane due to the limited prop size vs frontal area and the limited choices in prop pitching.

[Nick]

Thanks for the overview of prop dynamics Justin.

It was interesting in general terms, but there were no specific numbers. I still find myself un-convinced that - when it comes down to the specifics numbers - the Jab engine is unsuitable for a Kitfox clone, or even a worse proposition than a Rotax.

There was an article about the Apollo Fox, featuring the very one I flew in, in July's African Pilot magazine - viewable on-line (registration required, but it's free).

With the 85 horsepower 2200 4 cylinder Jab engine the figures quoted in the article are...

Cruise at 2600 rpm - 85 knots (157.42 kph) burning 10-12 litres per hour
Stall speed of 35 knots.

They also specifically list the 6 cylinder 3300 Jab, 120 hp engine as an option.