hilltop170 wrote:I thought my original comment might gin up some discussion on the subject.........again.
I have used cable-drive tachs and both the Horizon and EI electronic digital tachs. The digital are both are accurate, easy to read and work all the time without any problems. It does take some getting used to looking at the digital display after being used to the analog tachs. But once you use one of the digital tachs long enough to get used to it (more than an hour or two), either one is preferrable to the cable drive's, twitching, and questionable accuracy. Getting used to the single digit rpm flickering is a small price to pay for the added safety and other features of the digitals.
If all you want is some sort of idea what rpm is, then the original tach does that. The digitals indicate exact rpm plus have additional features not on the original.
The Horizon has some diagnostic tools that will indicate if a mag or mag switch fails, it will record max rpm and actual mag drop, and will warn of overspeed or caution range rpm (info from the Airplane Flight manual).
The EI will display all of the same items as the Horizon plus elapsed flight time.
The EI is a 2-1/4" instrument and the Horizon is a 3-1/8" so you have an option if you need it. Whether you like the original tach or newer technology digital tachs with additional features that improves the safety of the flight, it's usually personal preference in the end kinda like ice cream, some like chocolate, some like vanilla.
Richard, ...for those of us too lazy to study the advertisements for the various tachs, and as someone who obviously is already familiar ... can you give some fast/simple answers to the following questions?
1- How is a digital tach that diagnoses bad magneto/switches better than an ordinary/regular run-up where the pilot checks his magnetos and switches?
2- How would a digital tach be better in-flight for such diagnosis, when the pilot makes an in-flight check of both mags (perhaps due to noticing an increased EGT, RPM loss, or rough engine?)
3- How is a digital tach better at recording the time or the elapsed time than an ordinary recording tach and clock (which most aircraft already have?)
4- How is a digital tach (relative to an analog tach) better at depicting trend ...or relative change at a glance?
The fast/simple answer is I never said digital tachs are better.
Fast? Simple? George? Really?
Miles
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
KG wrote:
Here's an update.... I just talked to Spruce and ordered a tach. They said they had them on back order from Feb of 2010 until Nov, when they got in a shipment of, I think she said, 60. Those are all gone and they are again on back order.
As I waited, she called Mitchell and asked about shipping dates and she told me that if I ordered with custom markings, which I planned to do anyway, it would be shipped direct from Mitchell and I would get it quicker. They said to expect about two weeks. We'll see.
I'll keep you posted.
K
Update: My tachometer arrived today direct shipped from Mitchell. I ordered it from Spruce on Jan. 14th and it was delivered via UPS on Jan. 27th. So... their expectation of "about two weeks" was pretty darn close.
Strangely enough, the people at UMA advised that they would not guarantee that their tach would work with P Lead magneto type of pick up with Bendix mags.
When I installed the Cont. IO-360 in my C170B, that particular engine came out of a C-337 and was not internally equipped with the necessary gears for a mechanical tach drive. When I questioned Continental, they advised me that in order to activate the tach drive on the rear case I would have to pull the case and install the necessary gears at quite some expense.
This discovery necessitated my looking into a P Lead tach set up. Thats when UMA told me about their tach sometimes not working with Bendix mags. Reason --- unknown. That was why I ended up with the Horizon digital tach, which I never did really like because of the unit of resolution.
I guess just because I'm an old geezer, I think that all instruments should look like black and white faced clocks, with a sharply barbed pointer like an arrow on one end of the needle and an open "O" on the other. I used to love the hydraulic pressure gages on the dizzy three. They were big and round and looked just like the ones on my granddads steam locomotives. I always used to think that Douglas bought them from the Southern Pacific Railroad. Thats undoubtedly where the "Steam Gages" name came from.
My custom marked tach from Mitchell is in the plane and, even though you wouldn't think I would have had enough time to find out about their customer service, I can say they have pretty good customer service.
My mechanic called one day and said that they had installed the tach with no problems, went home for the night, and when they came back the next morning the glass on the front had cracked. Yep, just sitting there it cracked. I called Mitchell and they said that this happens occasionally. They shipped a new glass to my mechanic right away with instructions for him to replace it.
Works fine now.
My only complaint is that the tach that is supposed to be for the 170, as previously discussed in this thread, is geared for 2300 rpm. I flew for exactly 36 minutes and the tach says 8/10 of an hour. I don't like that math. In all fairness, this was the first flight after my engine overhaul and I flew it hard and did not spend much time at reduced power. Maybe over time with longer warm ups, cool downs, and more time spent in reduced power descents it will average out to a more accurate reading.
The calibration-RPM can be altered by ordering the unit for the 172:
Mitchell PN D1-112-5025 (AC RT-11) Clockwise 2566 RPM
Of course...if you rent your plane out to others....that 2300 works in your favor. If you don't rent it out...and you're Part 91...it doesn't much matter, does it? (It would mean a 6% error...or that 56.4 minutes cruising at 2450 would equal 1-hour on the hour-meter, and you'll be performing your oil changes 1.5 hrs early....or that every 100 hrs flying time you could actually log 106 hrs!)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
gahorn wrote: (It would mean a 6% error...or that 56.4 minutes cruising at 2450 would equal 1-hour on the hour-meter, and you'll be performing your oil changes 1.5 hrs early....or that every 100 hrs flying time you could actually log 106 hrs!)
Help me with my math..... if I flew for 36 minutes (.6 of an hour) and my tach reads .8, that is a bit more than a 6% error. It is a 33.3% error.
Looks to me like if this trend continues, when I have 1200 hours on the airplane my tach will read 1600.
Or is converting hours to decimals screwing me up again?
gahorn wrote: (It would mean a 6% error...or that 56.4 minutes cruising at 2450 would equal 1-hour on the hour-meter, and you'll be performing your oil changes 1.5 hrs early....or that every 100 hrs flying time you could actually log 106 hrs!)
Help me with my math..... if I flew for 36 minutes (.6 of an hour) and my tach reads .8, that is a bit more than a 6% error. It is a 33.3% error.
Looks to me like if this trend continues, when I have 1200 hours on the airplane my tach will read 1600.
Or is converting hours to decimals screwing me up again?
Your math may be just fine...your POWER SETTING may need help tho'.
If the thing is calibrated correctly... accurate at say 2300...and you run it at 2450 for one hour.... then it will read slightly more time elapsed than actual. If you cruise around at 2150 for an hour then it'll be back on schedule. heh-heh
By the way.... tenths of an hour are not particularly accurate over a half-hour's flight. Why? Because 1/10th hour is 6 minutes...but the odometer (horometer actually) may have JUST flipped the next digit over into view as you take the reading. You may have begun the time-keeping in the last-minute of the previous 6-minute 1/10th hour....and shut down in the first minute of the next 6-minute 1/10th hour....and therefore indicate .8 hours when in fact you've operated 38 minutes (.61 hours)
If, on the other hand, it is calibrated incorrectly, all bets are off....not only will it be inaccurate, but you won't know at what rpm it is calibrated nor will you know at what power setting you actually operated.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
Kind of related to the original posting: I have a new mechanical tach that I installed. After 3/10 of an hour it stopped working. I partially disassembled it and found it was jammed. It now works fine on the bench, and I'd like to install it again here soon. I heard that when installing a new tach there should be a placard or info sheet advising one to trim the tach cable, because a cable that is a titch too long will jam the tach, and possibly ruin it. Anyone heard that before?
spduffee wrote:Kind of related to the original posting: I have a new mechanical tach that I installed. After 3/10 of an hour it stopped working. I partially disassembled it and found it was jammed. It now works fine on the bench, and I'd like to install it again here soon. I heard that when installing a new tach there should be a placard or info sheet advising one to trim the tach cable, because a cable that is a titch too long will jam the tach, and possibly ruin it. Anyone heard that before?
Yes Sir. Also to short and it will disengage the tach drive which will cause intermittent operation. D
Hmm. So would a seasoned mechanic know how to remedy that? I don't have said info sheet. Can it only be done with the spec sheet from the manufacturer, or is it something that can be done without?
spduffee wrote:Hmm. So would a seasoned mechanic know how to remedy that? I don't have said info sheet. Can it only be done with the spec sheet from the manufacturer, or is it something that can be done without?
Yes. it can be done without a spec sheet. No not all seasoned mechanics know how to do it. A lot have never done it. What happened to the original tach drive cable? The old one is usually measured first then a new cable is fitted for the same size.
Well, I assumed a tachometer was a plug-n-play device. I took out the original one, connected the new one and away I went. I was using the same tach cable as before. Was that not a good thing...?
spduffee wrote:Well, I assumed a tachometer was a plug-n-play device. I took out the original one, connected the new one and away I went. I was using the same tach cable as before. Was that not a good thing...?
Sorry I was not so clear. In most cases it is a plug and play type operation. Except in your case. If you jammed your tach with the cable being to long then the cable was to long or it is coming out of the tach generator.(The cable is screwing itself out of the sheath). This is not a owner/pilot operation. It is an A&P job.
Cessna® is a registered trademark of Textron Aviation, Inc. The International Cessna® 170 Association is an independent owners/operators association dedicated to C170 aircraft and early O-300-powered C172s. We are not affiliated with Cessna® or Textron Aviation, Inc. in any way.
