Digital vs. Analog tach

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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FastEddieB
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Digital vs. Analog tach

Postby FastEddieB » Wed Apr 25, 2018 6:51 am

I figured I’d spin this off from a post Pete Roemer made in a more general Sky Arrow thread, where he said this:

proemer wrote:I typically use 4800 rpm on the analog gauge, which shows up as 5150 on the Dynon... (take your pick!) for "high speed cruise".


This is no small difference. If my analog tach is reading similarly low, if i’m selecting 5,200 rpm for cruise, it may place me at or above the 5,500 rpm limit for continuous power. I actually re-pitched my prop due to low rpm indicated (less than 5,000 rpm) on takeoff and climb - I now see 5,200 rpm for better performance there, but I think it hurt my cruise performance. My cruise speed and range for any given rpm went down and my fuel consumption increased. If my analog tach is in fact reading low, that explains a lot and re-pitching back to factory specs would be worthwhile.

Leading to these questions:

1) Pete, do you have a Dynon D10A? Did it come set up to show rpm or did you have to modify it? I’ll check my D10A manual to see what’s involved. It may even show rpm somewhere that I just wasn’t aware of.

2) Is the signal going to the analog tach a digital signal? I assume it is.

3) How hard/expensive would it be to replace my analog tach with a digital one? I previously did so in my Tiger and found the improvement in accuracy and readability worthwhile.

4) I know there will be math involved due to the gearbox reduction ratio, but is there an inexpensive device to check propellor rpm? Does anybody have one? I’ll check with our on-field mechanic later today.

Thanks! Bear in mind, my Sky Arrow is EXPERIMENTAL, so anything goes!
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
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MrMorden
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Re: Digital vs. Analog tach

Postby MrMorden » Wed Apr 25, 2018 7:36 am

Wow Eddie, 5200rpm on takeoff roll is pretty high. I see about 5050 when the roll is underway, and I have no problem getting my cruise RPM up into the yellow if desired. Curious: what is your WOT RPM at your typical cruise altitude? I'm not judging, just interested. I like a flatter pitch myself, but when I went much flatter than my current settings I got diminishing returns -- my cruise speed went down a lot faster than I gained climb.

Of course, I'm not in a pusher either.

I'm assuming that if the RPM is really off by that much between two tachs, one of them is faulty or has a wiring issue (poor ground?). I would want to investigate and get them to agree, certainly within 50rpm or so.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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FastEddieB
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Re: Digital vs. Analog tach

Postby FastEddieB » Wed Apr 25, 2018 7:56 am

MrMorden wrote:Curious: what is your WOT RPM at your typical cruise altitude? I'm not judging, just interested.


I think my WOT in level flight gets up to 5,500 to 5,600 rpm.

My SOP is to pull back to 5,200 rpm which results in about 93 to 95 kts.

As an aside, my analog voltmeter reads consistently 1v high, as seen here:

Image

Mentioned only because that makes gauge inaccuracy a more feasible possibility.
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
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Re: Digital vs. Analog tach

Postby MrMorden » Wed Apr 25, 2018 8:35 am

Interesting, I have 5050rpm on the rollout and about 5650 WOT. Maybe there is a prop difference or something about the pusher shielding the prop from some airflow that accounts for the difference.

What is that screw in your GPS dock? Anti-theft measure?
Andy Walker
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2007 Flight Design CTSW E-LSA

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Re: Digital vs. Analog tach

Postby FastEddieB » Wed Apr 25, 2018 8:58 am

MrMorden wrote:What is that screw in your GPS dock? Anti-theft measure?


Fast-forward to about 2:50 below and you'll have your answer!

https://youtu.be/imlMUEf7ekU
Fast Eddie B.
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proemer
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Re: Digital vs. Analog tach

Postby proemer » Wed Apr 25, 2018 9:34 am

Eddie -- The digital is on a D180 and to my info the set up has not been changed since it left the factory. (I'm the 2nd owner)

The difference is usually 200-300 rpm, with the greatest difference in colder weather.

I did notice a capacitor added to the inputs of the analog tac in another Sky Arrow, which I thought was a little suspicious. Maybe it was added to smooth out the display. I had some very shaky evidence that it, too was reading a bit low.

Aircraft Spruce (and others) sells a "prop tac" which a mechanic might have to check the calibration. A "tiny tac" with in inductive pick up on a spark plug wire might also work to check the analog tac. I tried to use a model airplane "prop tac", but it seemed the distance to the front cockpit was too far for it to work. I never did try to set it up closer to the prop.

I would be interested in your calibration results.

Having calibrated a few light aircraft gauges over the years, I've gotten pretty skeptical about any gauge readings... anytime, anywhere, anyhow...

Pete
Last edited by proemer on Wed Apr 25, 2018 10:33 am, edited 1 time in total.

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MrMorden
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Re: Digital vs. Analog tach

Postby MrMorden » Wed Apr 25, 2018 10:18 am

FastEddieB wrote:
MrMorden wrote:What is that screw in your GPS dock? Anti-theft measure?


Fast-forward to about 2:50 below and you'll have your answer!

https://youtu.be/imlMUEf7ekU


Hahaha!
Andy Walker
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2007 Flight Design CTSW E-LSA

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MrMorden
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Re: Digital vs. Analog tach

Postby MrMorden » Wed Apr 25, 2018 10:20 am

proemer wrote:Eddie -- The digital is on a D-100 and to my info the set up has not been changed since it left the factory.


Do you mean a D-120? The D-100 doesn't do engine monitoring. Maybe that's the problem? :D
Andy Walker
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2007 Flight Design CTSW E-LSA

Wm.Ince
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Re: Digital vs. Analog tach

Postby Wm.Ince » Wed Apr 25, 2018 10:32 am

FastEddieB wrote:
MrMorden wrote:What is that screw in your GPS dock? Anti-theft measure?
Fast-forward to about 2:50 below and you'll have your answer!
https://youtu.be/imlMUEf7ekU

That was funny. I saw it before . . . it's still funny.
Kudos to you Eddy, for your composure. Some pilots would freak out.
Just fly the airplane!. . . Right?
Bill Ince
CTSW
Retired Heavy Equipment Operator

proemer
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Re: Digital vs. Analog tach

Postby proemer » Wed Apr 25, 2018 10:35 am

Ok it is actually a D180. I edited the post. Just shows you can't trust what you see on the internet.

Pete

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Re: Digital vs. Analog tach

Postby FastEddieB » Wed Apr 25, 2018 10:39 am

proemer wrote:Having calibrated a few light aircraft gauges over the years, I've gotten pretty skeptical about any gauge readings... anytime, anywhere, anyhow...


Except fuel gauges, of course!
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
FastEddieB@mac.com

proemer
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Re: Digital vs. Analog tach

Postby proemer » Wed Apr 25, 2018 10:56 am

One other way we've used in the past is to do a run-up at night, with a standard fluorescent light illuminating the prop. The fluorescent light acted as a strobe light (sort of) flashing, at 120 times a second... at some rpms the prop will appear to stand still... and calculating the number of blades, and the gear ratio... and maybe something else I've forgotten we could find the actual rpm. (Assuming the 60 hz US ac frequency is stable)

(Caution: un-calibrated, ageing brain, and sloppy fingers contributed to this post... accuracy may vary.)

Pete

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MrMorden
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Re: Digital vs. Analog tach

Postby MrMorden » Wed Apr 25, 2018 11:43 am

proemer wrote:Ok it is actually a D180. I edited the post. Just shows you can't trust what you see on the internet.

Pete


Ah, makes sense now. I think the D180 might be the best unit Dynon ever made, I would like to replace my D100 with one.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

Warmi
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Re: Digital vs. Analog tach

Postby Warmi » Wed Apr 25, 2018 1:26 pm

AVMap makes very handy and relatively cheap mini engine monitor http://avionics.avmap.it/us/products/engibox/ specifically for Rotax.

For $800 you can replace just about every engine related gauge with a handy electronic display which will also record 160 hours worth of engine parameters.

Alternatively, MGL Avionics just came out with their small electronic singles, which also includes a tach monitor:
http://www.mglavionics.com/html/vega.html#RPM-1

For $300 is not too bad .. but again, for $800 you get everything with the EngineBox from AvMap.
Flying Sting S4 ( N184WA ) out of Illinois

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FastEddieB
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Re: Digital vs. Analog tach

Postby FastEddieB » Wed Apr 25, 2018 6:58 pm

Thanks, guys.

I'm trying not to "gild the lilly", so simpler is better.

Step one is to see if my analog tach is off. If not, I'll just leave it.

Other options include an aircraft digital tach, like warmi linked to. I had come across it with a Google search already:

Image

Then again, there are far cheaper non-aviation units out there from Amazon which might work.

Kind of like my OAT gauge!

Image

Though that one above from MGL is awfully nice!

As an aside, I perused the Owner's Handbook from my Dynon D10A and nowhere is RPM mentioned.
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
FastEddieB@mac.com


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