What is the reference RPM for a tachometer?
Posted by Fair_Intention_4198@reddit | flying | View on Reddit | 16 comments
My club rental works on paying by the Tacho hour, but i’m just wondering what the reference RPM is or how it works
For example, our C172 Reims Rocket POH says cruise RPM range is 2200 to 2600rpm depending on power settings.
What is the likely reference RPM, I can’t find anything on the POH? I’m trying to keep hour building costs as low as possible by selecting the slower power settings if possible and just wondering what the reference RPM would be potentially?
Mispelled-This@reddit
It should be marked on the tach housing, but you’d have to take apart the panel to see that.
A very common value is 2400, though, so just go try that and see if the tach registers exactly 0.1 hours in exactly 6 minutes. If not, it is fairly simple math to figure out the correct value and try again.
Fair_Intention_4198@reddit (OP)
Ah that sounds so obvious now you say it I don’t know how I didn’t think of that 😂.
Thank you i’ll give it a try this weekend
ltcterry@reddit
Go high. Go slow. But not slow flight. Read up on Carson Speed - the fast slow/efficient speed. Then use an idle power glide to descend the last ten miles from 8-10,000 feet.
Fair_Intention_4198@reddit (OP)
In the South of the UK I don’t think you could ever get anywhere near that altitude with the airspace around here 😂.
Also can’t idle decent in a constant speed prop for that long without having some engine problems
bagelb0ss@reddit
It depends on the tach. Usually labeled on the back/sides.
With a stopwatch and watchful eye, you can figure it out while flying. Whatever power setting gives you a 0.1 tick in 6 minutes is your answer.
TravisJungroth@reddit
You could find that with
Where T is the time in seconds for 0.1 tach hours and RPM is what you were running at. Wait for the first tick, start the timer, then start looking at 5:30.
Don’t fixate! Have it be part of your scan or have a passenger do it.
bagelb0ss@reddit
Correct. By using (or having a passenger use) a stopwatch as the tach ticks, the reference can be empirically derived.
rdrcrmatt@reddit
I’ve heard it is usually 2500 rpm.
Like someone else said, run it at different power settings and find the 1:1 time to tach time rpm.
makgross@reddit
What does it matter?
It’s proportional to RPM, so lower RPM is always cheaper.
The minimum involves spending a lot of time on the ground, or flying around at max L/D (around Vy).
fishmousse@reddit
Max L/D would give the lowest fuel consumption right? Idk why you're getting downvoted for giving the most correct answer
flyghu@reddit
You can lower the power to make the tach run slower. But you'll fly slower and take longer to get there. So do the math: 1 unit tach x 1 unit time vs 0.8 unit tach x 1.2 unit time = negligible difference.
Flying isn't cheap. Airplanes mock you when you try to save money.
buzzybootft@reddit
Usually defaults 2400
aftcg@reddit
York be hard pressed to make a difference unless you're flying around at 1500
Ok-Money2811@reddit
Most airplanes I’ve ever flown, tach time is usually the typical normal cruise RPM which is usually at or around 2400.
It’s not in the POH but it might be in the maintenance manual, you can ask the mechanic if you run into him.
tempskawt@reddit
The plane I usually fly has a maximum RPM specified. I thought it was based on that since it’s roughly 20% higher than we usually cruise, and that’s roughly the ratio between tach and Hobbs. I’ve never questioned it past that, though, I’m curious what the real answer is.
rFlyingTower@reddit
This is a copy of the original post body for posterity:
My club rental works on paying by the Tacho hour, but i’m just wondering what the reference RPM is or how it works
For example, our C172 Reims Rocket POH says cruise RPM range is 2200 to 2600rpm depending on power settings.
What is the likely reference RPM, I can’t find anything on the POH? I’m trying to keep hour building costs as low as possible by selecting the slower power settings if possible and just wondering what the reference RPM would be potentially?
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