912 100cv, RPM overspeed on take-off?

by Bruno M. in 912 / 914 Technical Questions 12 years ago

Hello,
Our aicraft has a Rotax 912 - 100cv engine with a variable pitch 3 blades IvoProp propeller.
During a full throttle take off, the Rotax RPM gauge indicates the limit of the yellow arch (6.000 RPM).
The aircraft is also equipped with a digital RPM gauge which always indicates 150/200 RPM more than the original Rotax RPM gauge (https://dl.dropbox.com/u/17099565/IMG_3289.JPG).
If I consider the digital gauge more accurated than the original Rotax, I'm doing an overspeed RPM at every take-off (6.200 RPM). Do I have to take care of this or can I consider the original Rotax gauge accurated enough for its porpouse and consider that 6.000 RPM for about 1 minute at takeoff are not dangerous for the engine?

Thanks in advance

Bruno

Re: 912 100cv, RPM overspeed on take-off?

by Roger Lee » 12 years ago

Roger Lee

126 5.13k 4.54k

Hi Bruno,

First I would make whatever adjustment to the plane or mentally not to be up in that range. I always advise everyone to glance at their gauges when at full throttle whether in cruise or climb to make sure that doesn't happen. Since you aren't sure which gauge is 100% then make sure you limit yourself to 5800 rpm on the highest gauge reading. I would not be going to 6000 rpm.
You need to find out which one is right and use that one and or fix the other if it is fixable.
That said I don't think you damaged anything, but I would not keep do this.
Keep your eyes and ears open and pay attention to the small details on the engine and its gauges to make sure nothing comes of this. Make sure you do a really good inspection when your time comes. If anything seems to be out of the ordinary before then, then address it right then and there.

(6K should not be a one time issue, but I wouldn't make a habit of it)

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Bruno M.

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Re: 912 100cv, RPM overspeed on take-off?

by MARK JACKSON » 11 years ago

MARK JACKSON

12 332 11

Hi Bruno

Ive just seen your post and I think you need to take a good look at your engine. You can do serious damage if you overspeed and there are things you need to check.

The first thing to do is check which tacho is reading right by checking with an optical tacho.

As you say this is not a "one off" and happens on every take off.

You need to read page 77 of http://www.flyrotax.com/portaldata/5/dokus/d05567.pdf

You need to check your crankshaft , and check your pushrods are straight.

I would suggest you don't fly again until you have looked into this, you shouldn't ignore it, or bury your head in the sand and think that it won't cause a problem.

Roger is wrong in suggesting that 6,000rpm, or 6,200rpm is ok as a 1 off, it isn't and it is happening every flight.

Get it checked out.

Mark

Thank you said by: Bruno M.

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Re: 912 100cv, RPM overspeed on take-off?

by Roger Lee » 11 years ago

Roger Lee

126 5.13k 4.54k

Hi Mark,

I'm a big believer in not going over 5800, but don't panic when someone tells me they bumped 6K for a minute. Unless someone can show me different I have never seen or heard of anyone having engine damage because they hit 6K. Maybe Rob at Rotech can chime in and see if he has ever seen any damage from a 6K rev.
If Bruno hit the 6K mark once and only for a few moments then he would be okay. Not as a steady diet. I got my info right from Rotax and from actual run test. If you ask the right person in the organization you may find that the engine has been run higher in test and that the 5500-5800 is a good safe company liability rpm to make it last its life span. You don't think Rotax would publish its absolute max destruction rpm of 5800 and say it's okay to run there for 5 min. You don't think there is a safety margin for liability? If 5800 was the absolute rpm then many people around the world that use an in flight adjustable prop and see 5800 on a regular basis would have more problems. Tachs can be off and people don't always pay strict attention to the rpm and if 200 rpm would do destruction then 5800 would be way too close for engine reliability. On all of the Rotax limits and when they did all their test they had to draw the safety and liability proverbial line in the sand somewhere to make the engine consistently reliable for many years in its life span. I can tell you where it will start to float a valve and 6K isn't even close. If all Bruno did was hit the 6K mark momentarily then he should not have an issue. If he keeps that up on a steady diet then later on he may develop some issues. My first sentence tells him to back off that 6K rpm as a normal rpm setup.

Quote

"First I would make whatever adjustment to the plane or mentally not to be up in that range. I always advise everyone to glance at their gauges when at full throttle whether in cruise or climb to make sure that doesn't happen. Since you aren't sure which gauge is 100% then make sure you limit yourself to 5800 rpm on the highest gauge reading. I would not be going to 6000 rpm.
You need to find out which one is right and use that one and or fix the other if it is fixable.
That said I don't think you damaged anything, but I would not keep do this."

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: 912 100cv, RPM overspeed on take-off?

by MARK JACKSON » 11 years ago

MARK JACKSON

12 332 11

Hi Roger

Please don't take it personally, I was just saying that I think your advice was wrong.

He says that his engine could be revving to 6,200rpm on every take off, not just once and not just for a moment.

Im not sure how many hours it has over revved for, but at least referring him to the Rotax recommendation might have been the better advice rather than just saying it will be ok.

Rotax recommend that if the engine over revs above 5,800rpm, but less than 6,500rpm for more than a minute, that the crank should be checked and the push rods should be checked for straightness.

Are you saying that we should take no notice of this recommendation and do what "johnny at the garage" says, even though he hasn't even seen the engine.

Again, please don't take this personally.
Mark

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Re: 912 100cv, RPM overspeed on take-off?

by Rob Seaton » 11 years ago

Rob Seaton

3 1.32k 240

We have had a few overspeed inspections and have not found any damage. (over-boosted 914s are a different story). That said I think the inspection is worth doing and I would not recommend ignoring the requirement; it is not worth the risk.
Just out of interest (I think it’s interesting); The 9 series are all FAR-33 certified; part of the FAR-33 certification is to run at redline for a total of over 100hrs. It is then torn down and all parameters must be within limits
Unsupercharged engines and engines incorporating a gear-driven single-speed supercharger. For engines not incorporating a supercharger and for engines incorporating a gear-driven single-speed supercharger the applicant must conduct the following runs:
(1) A 30-hour run consisting of alternate periods of 5 minutes at rated takeoff power with takeoff speed, and 5 minutes at maximum best economy cruising power or maximum recommended cruising power.
(2) A 20-hour run consisting of alternate periods of 1 1/2 hours at rated maximum continuous power with maximum continuous speed, and 1/2 hour at 75 percent rated maximum continuous power and 91 percent maximum continuous speed.
(3) A 20-hour run consisting of alternate periods of 1 1/2 hours at rated maximum continuous power with maximum continuous speed, and 1/2 hour at 70 percent rated maximum continuous power and 89 percent maximum continuous speed.
(4) A 20-hour run consisting of alternate periods of 1 1/2 hours at rated maximum continuous power with maximum continuous speed, and 1/2 hour at 65 percent rated maximum continuous power and 87 percent maximum continuous speed.
(5) A 20-hour run consisting of alternate periods of 1 1/2 hours at rated maximum continuous power with maximum continuous speed, and 1/2 hour at 60 percent rated maximum continuous power and 84.5 percent maximum continuous speed.
(6) A 20-hour run consisting of alternate periods of 1 1/2 hours at rated maximum continuous power with maximum continuous speed, and 1/2 hour at 50 percent rated maximum continuous power and 79.5 percent maximum continuous speed.
(7) A 20-hour run consisting of alternate periods of 2 1/2 hours at rated maximum continuous power with maximum continuous speed, and 2 1/2 hours at maximum best economy cruising power or at maximum recommended cruising power.
Rob

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912 100cv, RPM overspeed on take-off?

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