Reference times for Tach Meters

Based on my initial research, I'm using a Tach reference of 2400 RPM.
But, if the plane Tach is a 2500, or 2300, or some other number, my
planning for cruise flight BHP, flight times, and costs gets all
thrown off.

Is your tach accurate? You may find that differences in the tach
=readings= swamp the fine cost issues you are trying to figure. But to
save the most money, plan for lowest RPM consistant with safe and proper
operation. The speed loss will likely be smaller than the RPM drop.

Make a graph of speed vs RPM (if it's a constant speed prop, use the
same power setting). You will probably find that percentage wise, you
get less additional speed than you put in in additional RPM.

Now headwinds should be factored in too - since changing from a GS of
120 to 100 is a smaller percent change (what you're comparing with the
RPM) than going from 100 to 80 due to a 20 knot headwind.

Jose
--
Quantum Mechanics is like this: God =does= play dice with the universe,
except there's no God, and there's no dice. And maybe there's no universe.
for Email, make the obvious change in the address.

On May 14, 11:58 am, Jose wrote:
Is your tach accurate? You may find that differences in the tach
=readings= swamp the fine cost issues you are trying to figure. But to
save the most money, plan for lowest RPM consistant with safe and proper
operation. The speed loss will likely be smaller than the RPM drop.

Make a graph of speed vs RPM (if it's a constant speed prop, use the
same power setting). You will probably find that percentage wise, you
get less additional speed than you put in in additional RPM.

Now headwinds should be factored in too - since changing from a GS of
120 to 100 is a smaller percent change (what you're comparing with the
RPM) than going from 100 to 80 due to a 20 knot headwind.

Jose

Jose:

Thanks for the response. I don't know if the Tach is accurate. Do
you have a good suggestion how I might check?

I'm trying to get the information for exactly the kind of graph that
you are suggesting. Headwinds, of course, can't be calculated in the
abstract, but I do add this consideration when I plan actual flights.

Thanks for the response. I don't know if the Tach is accurate. Do
you have a good suggestion how I might check?

Ask your maintanance guy. A tach check may be part of the annual or the
100 hour. If you have access to a strobe and an accurate flash rate,
you could use that on the prop too. Use a partner, for obvious reasons.
"Kids, don't try this at home!"

Headwinds, of course, can't be calculated in the abstract, but
I do add this consideration when I plan actual flights.

You can figure a break-even point for winds.

You also need to include taxi time in your figuring. On the hobbs, you
taxi at full price. On the tach, it's less than half price. An
unexpected ground delay or early release could also swamp your calculations.

Now, do you actually have a choice, per flight, as to what method you'll
be using (tach or hobbs)? Can you decide after the flight?

Jose
--
Quantum Mechanics is like this: God =does= play dice with the universe,
except there's no God, and there's no dice. And maybe there's no universe.
for Email, make the obvious change in the address.

On May 14, 12:35 pm, Jose wrote:

Ask your maintanance guy. A tach check may be part of the annual or the
100 hour. If you have access to a strobe and an accurate flash rate,
you could use that on the prop too. Use a partner, for obvious reasons.
"Kids, don't try this at home!"

A good question to ask. I called the flight club and spoke with a
young CFI, but knew I wasn't going to get a good answer when he asked
me what I meant by the tach reference speed/time. I'll have to follow
up in person.

Now, do you actually have a choice, per flight, as to what method you'll
be using (tach or hobbs)? Can you decide after the flight?

Unfortunately, no. That would be great, though, wouldn't it?