Well the tach needs replacement.. any real reason to get a digital one? A new Mitchell mechanical is about $180 and a digital whiz bang $500-600

Personally, I find digital instruments to be too " busy" for me to read fast and accurately.. A mechanical tach is just a quick glance for me to confirm RPMS. IMHO.

I got frustrated by the inaccuracy of mechanical tachs. My old tach bit the dust, so I ordered an overhauled one. Put it in, and checked it with a stroboscopic tach checker and it was 50 rpm off, and somewhat variable at different rpms. I sent it back and ordered a new one. That one was 75 rpm off. The manufacturer stated that anything within 100 rpm was legal, and to just placard the tach with the error.

Instead I bought a Horizon tach. If you're wondering why your airplane consumes more gas than your neighbor's nearly identical plane.....erroneous tach could be part of the reason. I like to know what my engine is doing.

The Horizon is spendy, but it is dead accurate. It also has other features, like warning lights that come on if a magneto has failed, or if you have a broken P-lead. Also, there's a yellow light to warn that you're in the "5 minute" range and a red light that comes on if you overspeed.

I have my governor set so that the engine will spool up on takeoff and run at between 2690 and 2705 which lights up the yellow light, and the red light flickers. Those are very visible in my peripheral vision, so if I see a yellow steady light and a flickering red light, I'm making power.

As to cruise power, just set what you want and the tach will roam around a little, because the engine is doing so. But, just put the rpm where you want it, and let it vary a bit. No worse than a mechanical tach.

I really like the Horizon, and I've had it in here for 700 hours and a number of years, totally reliable.

MTV

I do agree that the old mechanical tachs are not very accurate. They fail internally and the silly cables fail with great regularity. On my 185, the cable is just a joy to replace, with about .0001 clearance between it an the firewall, so far back you need a special set of loooong needle nose pliers to get to the ring to tighten and loosen it. So on my upgrade I went digital, just to not screw with the cable anymore. I have to admit, I was never up for the pure digital display, too much work for my ever slower mental processing power. Nice that the new ones have digital and analog display to get over that issue. However, check them for accuracy as well, just because it is digital does not mean it is accurate.

mtv wrote:I got frustrated by the inaccuracy of mechanical tachs. My old tach bit the dust, so I ordered an overhauled one. Put it in, and checked it with a stroboscopic tach checker and it was 50 rpm off, and somewhat variable at different rpms. I sent it back and ordered a new one. That one was 75 rpm off. The manufacturer stated that anything within 100 rpm was legal, and to just placard the tach with the error.

Instead I bought a Horizon tach. If you're wondering why your airplane consumes more gas than your neighbor's nearly identical plane.....erroneous tach could be part of the reason. I like to know what my engine is doing.

The Horizon is spendy, but it is dead accurate. It also has other features, like warning lights that come on if a magneto has failed, or if you have a broken P-lead. Also, there's a yellow light to warn that you're in the "5 minute" range and a red light that comes on if you overspeed.

I have my governor set so that the engine will spool up on takeoff and run at between 2690 and 2705 which lights up the yellow light, and the red light flickers. Those are very visible in my peripheral vision, so if I see a yellow steady light and a flickering red light, I'm making power.

As to cruise power, just set what you want and the tach will roam around a little, because the engine is doing so. But, just put the rpm where you want it, and let it vary a bit. No worse than a mechanical tach.

I really like the Horizon, and I've had it in here for 700 hours and a number of years, totally reliable.

MTV

Thanks, I was looking at the horizon. I was also considering a JPI engine analyzer with all the sensors, tach, manifold, etc. I forget which one but one of the new ones with larger screen.

I just got word that the engine mount appeared to have a crack. It was freshly powder coated with the replaced engine 120 hrs or so ago. Plane went through an annual right before I bought it a month ago. Went ahead and had them strip the paint and sure enough a large crack and 4 other ones. So now I will see what this is going to cost before I decide on the horizon or not.

Moving to Phoenix, AZ next week and I guess the plane will be living in Mack, CO for a bit. Monument Aviation will be doing the work. Well referred and great guys.

That reminds me of a buddy that raced cars. He had all round analog gauges, and he would rotate each gauge in the dash so that when they were operating normally, the needle would point straight up. It looked goofy with all of the cockeyed instruments, but he could scan in an INSTANT, and if a needle wasn't pointing straight up, then he would give it a second glance to determine which gauge it was and the nature of the problem.

However, even if you don't rotate your gauges, you can scan the needles quite quickly and verify that they're in their usual location. That's a little faster, because eventually you don't even have to look at the number behind it. Also, the needle can vibrate, and your mind just tunes it out. However, if a digital number keeps changing, i.e., 2411, 2397, 2405, 2399, etc., your mind has to read the digit symbols and convert them to a number, round to your desired precision, then compare it to the target rpm. Then, you will stare at it for a couple of more seconds so that you can average a few readings to get a more useful answer. That was my experience with the digital tachs I've used, but they weren't in a plane, so maybe aviation digital tachs have a better averaging algorithm in them to smooth out the numbers and avoid that problem.

Actually, my biggest gripe with digital instruments is that there is almost always some set of lighting conditions where they are hard to read, either too dim with light reflecting off of them, too bright at night, etc.. The analog gauges are usually far more readable in those same conditions.

I want the accuracy of the digital tach, then pipe it into a digital-to-analog converter, and then hook it up to a needle meter for display!

Try one of these:
http://www.ebay.com/itm/Inductive-Tachometer-RPM-Digital-Sensor-Any-Rotating-Shaft-Tach-Meter-/190673356587?hash=item2c6504872b&item=190673356587&pt=Motors_Car_Truck_Parts_Accessories&vxp=mtr

I found mine for $12 plus shipping somewhere.

I got one a bit over year ago, snaked it into the cabin, and found out my mechanical one is reading around 50 RPM slow. I just attached it to the top #1 plug wire.

I don't mind the discrepancy- I just wanted to explain why my takeoff RPM was lower than rated RPM, and I cruise at 2150 instead of 2200 most of the time.

kevbert wrote:That reminds me of a buddy that raced cars. He had all round analog gauges, and he would rotate each gauge in the dash so that when they were operating normally, the needle would point straight up. It looked goofy with all of the cockeyed instruments, but he could scan in an INSTANT, and if a needle wasn't pointing straight up, then he would give it a second glance to determine which gauge it was and the nature of the problem.

However, even if you don't rotate your gauges, you can scan the needles quite quickly and verify that they're in their usual location. That's a little faster, because eventually you don't even have to look at the number behind it. Also, the needle can vibrate, and your mind just tunes it out. However, if a digital number keeps changing, i.e., 2411, 2397, 2405, 2399, etc., your mind has to read the digit symbols and convert them to a number, round to your desired precision, then compare it to the target rpm. Then, you will stare at it for a couple of more seconds so that you can average a few readings to get a more useful answer. That was my experience with the digital tachs I've used, but they weren't in a plane, so maybe aviation digital tachs have a better averaging algorithm in them to smooth out the numbers and avoid that problem.

Actually, my biggest gripe with digital instruments is that there is almost always some set of lighting conditions where they are hard to read, either too dim with light reflecting off of them, too bright at night, etc.. The analog gauges are usually far more readable in those same conditions.

I want the accuracy of the digital tach, then pipe it into a digital-to-analog converter, and then hook it up to a needle meter for display!

I used to believe that about mentally processing digital inputs too, till I got used to them. It's really not an issue, and this is rapidly becoming another of those closely held aviation old wives tales. Get in an airplane with glass displays and fly it for a little while, and you find (or at least I have) that, even with the analog backup gauges in clear view, I hardly ever look at them, pointers or not.

And, the Horizon digital tach has GREAT viewability in all light conditions.

MTV

Kevbert,
Your buddy wasn't strange, almost all British military aircraft are done that way. When I first flew the Wessex helicopter, I thought their mechanics where all walleyed, until I ran it up and puled into a hover for the first time. All straight up for normal, kind of where the saying comes from.

Don't know how cold of temps you fly in, but around here the cable can bind in the sleeve of the mechanical ones at -20F or so if it's not all preheated well. I think I'll check out MTV's recommendation
-DP

mtv wrote:
Get in an airplane with glass displays and fly it for a little while, and you find (or at least I have) that, even with the analog backup gauges in clear view, I hardly ever look at them, pointers or not.

MTV

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