Tachometer problems

Paul,

The fact that this has always been a problem makes me think it’s systemic rather than a part failure. I believe you need to know if Tecnam and/or Garmin has seen this on other Rotax powered aircraft using the GEA 71.  So, you may have to engage your Garmin dealer or Tecnam - as painful as that may be!  

With both engines now exhibiting this behavior this can’t be isolated to your aircraft.  The number of Rotax engines installed in certified aircraft with the G1000 must be very small, so this kind of thing has not had much chance to get worked out.  There are some folks on this site that are quite good with electronics, can you post a simple diagram of how the coils are wired to the GEA 71?  Maybe just markup one of the diagrams in the GEA 71 installation manual.  Good luck with this Paul.  

 All

First, the topic is the tack picking up interference.  The issue is not with the tack signal wire picking up the other trigger wire signals, it is the tach wire picking up the "secondary" high tension, read spark plug wires, field when they fire.  This pulse give a false signal to the tach reading.  It is understandable that when one of the ignitions is off, hence stopping some of the high tension firing from the coils, that the signal is fine.  Check the route of the wire in relationship to the spark plug wires themselves.  

As for prop speed vs crank speed.  The desire for many flight schools for years was to have a common understanding of engine speeds to the pilot.  In any direct drive engine we think of this as prop speed.  If the Rotax can give you prop speed then all these different aircraft in a training fleet will have a common thread for the most part.  

Example here is let's look at the 912ULS, 5800 WOT take off power.  divide by gear ration this is about 2400 prop.  Checking for mag drop, Rotax is 4000 and divide by 2.43 is about 1650.  Rotax idle, recommended for most at 1800, divide by 2.43 is about 740.  For a student pilot these numbers are very similar to each other and it starts to make it easier to switch between direct drive engines and geared ones.  Rotax used to made mechanical tack drives as part of the certified engines back in the 1990s, now it is done by just using an instrument that can calculate the gear ratio electronically.  It is simply a matter of choice, I don't believe it is more than that.

Cheers

Thanks RW,

I understand the rational  however would ask one simple question;

Is the pilot of a IC powered aircraft managing propeller or engine speed, as an indicator of system condition & performance?😈

Hi Sean

The ultimate goal is the control of the propeller.  The dynamics of the air bonding to the blades is why they selected the gear ratios we have.  The design of the power curve is to best give as flat a power curve as possible.  Fixed pitch props are always a compromise to an extent as we give up some peak HP to keep the engine in control for best climb.  Constant speed is expensive but is the best for both HP and it allows for a better climb.  

As a norte the first 912 (80 HP) engines were 2.27 to 1 ratio.  This was best as long as the prop blade diameter was only about 67 inch.  With the development of the 914 a few years later and the 912 100HP after that the desire to power bigger aircraft and to be able to absorb the HP the ratio changed to 2.43 to 1.  This was fine for 68 inch props, in some cases 70 or even 72 as long as the tip speeds were not excessive.  The prop producers will say you can go up to 0.8 MACH (speed of sound) on tip speed but I personally feel that is a bit excessive.  

The advent of the large turbo charged engines, the 915 and 916, saw a total gearbox redesign and a drop in ratio to 2.54 to 1.  This allowed larger mass inertia and longer props again to absorb the extra power.  It is important that the engine can absorb the power, buy pitch or adding blades, and control tip speeds and not cavitate the prop.  

The system condition follows that if you have the max RPM and can pitch to the known blade settings for that engine/airframe combination, you have the system power.  In overhaul the overhaul shop has to have a test prop or test club, assuming they do not have a dyno as at the factory, that will load the engine to achieve peak power at WOT.  If not then it is not developing full power potential.  

If the question is how do you know if your engine is developing full power?  First determine your actual field barometric pressure, generally in inches of HG.  Do this by going to your known field elevation and set that altitude into your altimeter.  Now read the HG setting.  Knowing this, assuming now it is not turbocharged, at WOT on takeoff when you get to your peak RPM in the climb (if you have adjustable prop) look at your HG reading.  It should be as near to your field elevation as possible.  In general terms it will be about 1 to 1.5 lower as this is the MAP at that point.  If you can achieve this you have the max power for that altitude density.  If you have a fixed pitch is is a bit more difficult because you will be at a lower RPM in climb than when you level off at WOT because you have to unload the propeller to the max.  Propeller setting of a fixed pitch should be set for max continuous power, not peak power as with a constant speed.  The fact that that small difference of 300 RPM, 5800 down to 5500 max continuous, will drop you to about 95% of peak power.  

Anyway as you can see this would be how you can determine if your engine is working to the max for the given conditions you are flying in.

Cheers

"The ultimate goal is the control of the propeller. "

What comes first the chicken or the egg?

Seems to me that the prop will always be second to the engine.

A windmilling/unpowered prop isn't much use to a powered aircraft pilot. Just saying.😈