Live prop with 912?

[pmccarthy]

Good practice with the 912 is to burp the engine twice before the first flight of the day. This may take 8-10 prop rotations for the first burp, the another two or three for the second, with the oil filler cap removed.

 

I was originally taught to treat every propellor as potentially live. However, this daily procedure leads to complacency and no doubt if the engine fired while burping, most pilots I have seen would be chewed up. I have a key ignition switch so always check that I have the keys in my pocket.

 

My question is - is there any conceivable way that a Rotax engine could fire with the ignition key switch (or toggle switches for that matter) turned off? I suspect the answer is no, but perhaps there is a possible mode of failure as simple as a frayed wire behind the switch that could turn this into a lethal situation.

 

Do you treat a 912 prop as potentially live when turning it?

 

 

[Old Koreelah]

Adopt the precautionary principle: assume every prop can bite you, that each hand-turned rotation can start the engine. Stand behind the prop holding something substantial, with mags off, brakes on, throttle closed, door open. Make allowances for Murphy Law.

 

 

[Guest Maj Millard]

PCM , there is safety circut built in to the Rotax ign moduals where the engine has to turn around 400 rpms to activate the ign systems. Unlike the impulse spring loaded couplings on GA style magnetos which can kick and fire at will.

 

As far a burping goes I have found it is much more efficient to burp the engine when it is hot right after shutdown. The engine case pressure is up and it'll burp quicker. After a couple of good burps I replace the oil cap and go home. Next time you wan to fly checking the oil is easier with everything cold, oil has settled in tank andyou get a more accurate reading.

 

I have been using the above techniques now for several years with good results..............Maj......

 

 

[Old Koreelah]

What is "burping" the engine?

 

 

[Geoff13]

I have been told that it is only necessary to burp the engine if the Oil appears low.

 

In practice I have only found that to be the case when the plane has been flown in the last 1/2 hour or so or first up in the morning.

 

Also it seems that burping once is more than sufficient to get the oil back into the tank and register acceptable levels.

 

Old Koreelah My understanding of burping is the procedure of removing the cap from the Oil Tank and turning the prop/engine over to feed the oil back into the tank. You can get false readings of oil level if all the oil from a dry sump system is through the engine and not in the tank. It is simply a matter of turning the prop and listening for the oil to gurgle back into the tank, sounding a bit like a burp.

 

 

[Old Koreelah]

...so it's not needed with a Jab engine?

 

 

[Geoff13]

Sorry I have no experience with Jab engines so cannot comment.

 

 

[dazza 38]

...so it's not needed with a Jab engine?

No the Jab is a wet sump engine where as the Rotax 912 is a Dry sump engine.

 

 

[Old Koreelah]

No the Jab is a wet sump engine where as the Rotax 912 is a Dry sump engine.

Yep, I know that. My reason for asking is I suspect oil bubbles accumulate in the narrow Jab dip stick tube. With a hot engine I often get oil readings well above the indicator line.

 

 

[frank marriott]

Yep, I know that. My reason for asking is I suspect oil bubbles accumulate in the narrow Jab dip stick tube. With a hot engine I often get oil readings well above the indicator line.

I find if you clean the dip stick and recheck(whilst hot) you will get a reading lower then actual i.e. After sitting for some time the level will rise as the oil settles out of the engine (exactly the same as you experience with a car engine). I haven't seen the indication you refer to other then prior to cleaning the dip stick and rechecking.

 

 

[Old Koreelah]

Interesting, Frank. Even after wiping the dipstick I sometimes get some oil part way up the stick, and a drop on the minimum line. As if the dipstick passes thru bubbles of oil as it goes down the tube.

 

 

[Dafydd Llewellyn]

Interesting, Frank. Even after wiping the dipstick I sometimes get some oil part way up the stick, and a drop on the minimum line. As if the dipstick passes thru bubbles of oil as it goes down the tube.

I've seen this on the engine in my test cell; however if you let it stand for 1/4 hour, (probably less with a hot engine - but it needs time for the oil to drain back into the sump) the oil drains out of the dipstick tube and you can get an accurate reading.

The 912 is a "dry sump" engine (like the Gipsy) and it uses crankcase pressure to blow the oil back into the oil tank; it does not have a scavenge pump (as did the old Gipsy Major). The oil system is entirely different to that of the Jab (or for that matter, any wet-sump Lycoming or Continental).

 

 

[Downunder]

You've got a test cell !!!!! Get the fudge.....

 

 

[metalman]

 

Treat EVERY prop as live,,,I only burp mine if it appears to have low oil level, if I do burp one it's "check switches, set park brake , check throttle , never wrap fingers around blade"

 

 

[Guest Maj Millard]

Saw this video some time ago....and believe it less now than I did then !...........Maj....

 

 

[rankamateur]

What is not to believe?

 

 

[Dafydd Llewellyn]

You've got a test cell !!!!! Get the fudge.....

And your meaning is?

 

 

[SDQDI]

I'd tend to think Maj would be right as far as the 400 goes (WOT on the jab is just above idle revs on a rotax) but having said all that what would the revs of the prop be to get a rotax to 400rpm?

 

 

[SDQDI]

Found my book and to get the engine to 400 rpm you would have to spin the prop at 165 rpm, as for if it is possible or not I'm not sure, maybe if I have a doughy battery out in the never nevers I'll give it a try

 

 

[Downunder]

And your meaning is?

Could be taken the wrong way but I mean it's pretty awsome and a great thing to have....

Not really something every back yard mechanic has lying around...

 

You said " in my test cell" like everyone would have one....

 

 

[metalman]

What is "burping" the engine?

It's called burping because the oil makes a gurgling sound as it runs back into the reservoir ,sounds like a burp,

 

 

[Oscar]

Could be taken the wrong way but I mean it's pretty awsome and a great thing to have....Not really something every back yard mechanic has lying around...

You said " in my test cell" like everyone would have one....

A supply of fudge would be good to have for the interminable hours of sitting in the control room running the damn engines through the test sequences for certification... 50 hours at least of block tests of start, idle for X minutes, full power for X minutes, cruise power for X minutes, idle for X minutes, rinse and repeat..

 

 

[Dafydd Llewellyn]

Could be taken the wrong way but I mean it's pretty awsome and a great thing to have....Not really something every back yard mechanic has lying around...

You said " in my test cell" like everyone would have one....

Ta - you had me wondering. I suppose it depends upon what one visualises by the term "test cell" - mine is definitely not one of the "gold plated" variety; but it's proving to be not a lot less work than building a small aircraft. It happened because I had a torque stand (i.e. a piece of hardware that mounts an engine in such a way that one can measure its torque) laying around, that we had built for calibrating engine test clubs up to 400 HP; plus a steel-lined cargo container, plus a set of instruments in a light-proof box so they can be recorded using a video camera. And I live on a rural property, with the neighbours sufficiently far away that the noise won't bother them. That provided the opportunity, and Ian Bent provided the need, so . . .

One cannot conduct the certification endurance running on a dynamometer, because it must be done with "a representative propeller"; and previous experience starting with the Jabiru 1600C, showed that it needs fine control of engine temperatures, because the run must be conducted with the engine on its red-line temperatures, so it works out that one has to be able to control each cylinder to within about 2 degrees C. As always, the devil is in the detail, and the details of making sure the instruments are not telling lies is very tedious.

 

The rules for instrument calibration and auditability of the results put some obstruction in the way of simply (!) recording it all digitally directly into a computer; so the video recording method, whilst extremely tedious to analyse, does provide an unarguable basis for auditing. One needs to measure things in terms of certified test weights and things that can be measured accurately, otherwise one ends up in a morass of "how did you calibrate the thing you used to calibrate the thing you used to calibrate the instrument?"; and designing around those issues means one has to sometimes do things in what may appear to be archaic ways.

 

 

[Old Koreelah]

Dafydd you are probably aware of efforts to fit Peugeot diesels to Jodels.

 

About ten years ago many of us got quite excited about the concept. PSA were building over a thousand of these engines every day, for use in several brands of vehicles. One Frenchman must have been deeply involved, and reported that he'd tested ten engines to destruction. The first failed at 12,000 hours (a dropped valve). Applying conservative rules, he predicted a service life of 10,000 hours.

 

Although potentially a world-beater, few conversions made it into the air. The extra weight and complications of a PSRU have made simple engines like the Jabiru quite appealing.

 

 

[Guest Maj Millard]

The Rotax service bulletin mentions 'pulling the prop through' to burp the engine. Not spin it like you are trying to start it. Don't believe I've heard of anyone unintentionally starting a 912 when using the recommended technique. Plus anyone silly enough to move a prop without the switches being in the OFF position is asking for trouble anyway.. ...........Maj......