Tach Markings

[Kerbs]

I am ordering a new tach for the Taylorcraft as part of doing the C85 conversion. My previous tach with the A65 just had a red line marked at redline on the tach. I am debating either marking the new tach the same way with just a redline at 2575, or possibly doing a green arc as well. I have also seen a green arc to 2400 and then yellow 2400 to 2575 I believe to indicate a cruise of 2400. Curious if one approach is more common or more original?

Also, with the C85, when do most pull on carb heat assuming a typcial day. With my A65 I always pulled it on at 1,600 rpm or below assuming it wasn't a higher ice risk day.

[LostnSpace]

Originally the tcrafts and all the other planes had operating limitations and the instruments weren't marked cause it was written on the OL's and it stated to be in view of the pilot, also on carb ice, just get it on while the engine is making good power, cause once it starts to loose RPM. the heat go away real fast

[NC36061]

I was taught to apply carb heat before reducing power, and remove it after resuming full power, never heard of referencing the tach.

[KewFlyer]

Kerbs.

I did both; the green and the yellow arc as well as red markers at 2400RPM and at 2550 RPM...just for visual aid. Since I have the metal Senci prop, I can over rev the engine past the 2550.... so the red marker at 2550 helps on Take Off. Engine: C85-12F with 0-200 STC with M-S Carb, dual straight exhausts

Curb Heat. As a general practice, I engage the carb heat minutes after ignition thru taxi to the warm up area. Do the mag/carb heat check and I leave the carb heat in until crossing into the active runway. The same practice with the mixture control. In flight, I will engage the carb heat for a minute or 2 on a regular basis and "prior" to reducing power. I had a few bad experiences with carb ice..one of them in a sunny day.

[Mark Bowden]

In absence of Taylorcraft factory information the next best source is Continental manuals. These are current and supported documents and "legal" information.
They are free on Continental's website and you can sign up for their update services for free by setting up your account based on the engine serial number.

We should all be using it. They also have live bodies in the US available on the phone. The current manual supports A, C series and O-200

Per the currently released manual (also posted in the tech section by Tim) but always get the latest revision from the factory.
For the C-85, rated HP is at sea level, std., day.

Max HP = 2575 rpm 100% hp
Max Rec Climb = 2340, 75% hp
Max Rec Cruise = 2340, 75% hp
Rec Cruise = 2230, 65% hp
Economy Cruise = 2120, 55% hp

Mark

[PA1195]

If you install a new tach and engine try to get the tach's accuracy checked with an outboard tester. Mechanics and those that do prop balancing have them. They're a couple hundred from parts suppliers and mount on the panel top with velcro.

Percent power by only rpm without other info like manifold pressure and density altitude is a guess. Propeller pitch and diameter affects engine load and power output at a given rpm, so non-standard installations or atmosphere may produce different manifold pressure and % hp. Max rpm is still limited to manufacturer's recommendations.

See Section IV below.

Gary

[Mark Bowden]

On RPM for the C-85 from the Continental overhaul manual. Gives 5 min take off power of 2575.

[Kerbs]

Thanks for the info. I remember reading somewhere cruise with the C-85 being 2400 but it may be outdated from the latest factory revision Mark posted. I was going to order a tach calibrated for 2,400 RPM cruise. Am I better off to get one set at 2,300 cruise? What do people use for cruise RPM setting with the C-85? Thanks for all the help.

[Mark Bowden]

The carb heat tach reference can come from some Cessna manuals that say to apply carb heat when the RPM is below the green arc in descent. This is for a Cessna design not all aircraft.

Worst case environmental conditions for carb ice are, when temperatures fall roughly between 50 and 70 degrees F and the relative humidity is greater than 60 percent.

If you keep carb heat on at idle and taxi you are fouling your plugs as the mixture is "more than full rich".

Here is what the Continental manual says about carb heat use.

PRE-TAKEOFF CHECK

4. Restrict ground operations to the time necessary for warm-up and testing

NOTE ... Carburetor ice can form on the ground with the engine idling. Therefore, just before take-off and during the magneto check. Position the carburetor heat to "ON". Leave it in that position until the throttle is advanced for the take-off run, then position the carburetor heat to "cold air". This gives maximum power for take-off. Monitor engine for any indication of ice (roughness or loss of RPM) during climb and add full carburetor heat at the first sign of icing. The correct way to use carburetor heat is to first apply full heat to remove any ice that has formed. Determine the minimum amount of heat required to prevent ice forming, each time removing any ice that has formed by applying full heat.


DESCENDING AND LANDING.

1. The mixture control must be in "FULL RICH" position during descent.
2. If a long glide is made, apply power at short intervals to clear the cylinders and retain engine temperatures in the event that instant power is required.
3. Carburetor heat is available only at engine outputs well above idle. Apply carburetor heat before closing the throttle and place carburetor heat "OFF" before opening the throttle so full power will be available if necessary.

[Mark Bowden]

Attached is a handy probability of carb icing chart based on conditions. Note the chart is in deg C.

carburettor_icing_chart.pdf

[Robert Lees]

If you install a new tach and engine try to get the tach's accuracy checked with an outboard tester. Mechanics and those that do prop balancing have them. They're a couple hundred from parts suppliers and mount on the panel top with velcro.

Digital tach readers are cheaper than that, and can be hand-held from inside the cockpit in flight. You just have to tell the device how many blades.

[PA1195]

Here's a couple of portable tach checkers. Mine is the second one and I loan it out locally to anyone who's curious about their tach's accuracy.

http://www.proptach.com/specs.html

https://www.aircraftspruce.com/catalog/topages/trutach.php

Gary

[3Dreaming]

On RPM for the C-85 from the Continental overhaul manual. Gives 5 min take off power of 2575.

It also gives normal rated power at 2575, so there is no 5 minute limitation. Look at the C-90, and the numbers are different, so it has a 5 minute limitation.

[3Dreaming]

The tach should be marked per the documentation for the airplane. If you are doing a STC for the 85 installation, then you need to mark the tach per the STC.

[N96337]

It also gives normal rated power at 2575, so there is no 5 minute limitation. Look at the C-90, and the numbers are different, so it has a 5 minute limitation.

Exactly!!!