PA-28-181 Tach meter reference point

[polaris]

I just flew the Piper Archer for 5.2 hobbs, and I saw that it was 5.4 tach. Confused, because I sat idle for at least 0.3 during my trip.

Is this because the tach meter is referenced at 2400 rpm (or something), and I was running it just below redline at 2600 rpm the whole time? I.e. the tach spun a little bit faster than the hobbs?

 

[MichiPilot]

Usually Hobbs time is about 30% more than tach time

 

[MAKG1]

The tach counts prop spins (specifically 144000 of them in each tach hour, using your number), so yes, if you ran it faster than reference, it will count fast.

 

[polaris]

The tach counts prop spins (specifically 144000 of them in each tach hour, using your number), so yes, if you ran it faster than reference, it will count fast.

Thanks. Do you know if reference is 2400rpm for the PA-28-181?

 

[poadeleted20]

Usually Hobbs time is about 30% more than tach time

That's not something you can count on. The difference depends on the power setting you use, the power setting which give 1:1 time, and the flight length and profile (longer flights tend to be close to 1:1 because a smaller percentage of the engine operating time is ground ops either idling or near idle).

 

[poadeleted20]

Thanks. Do you know if reference is 2400rpm for the PA-28-181?

That should be written on the back of the tach and probably in the maintenance manual. It may also be in the TCDS.

 

[TUPilot]

That's not something you can count on. The difference depends on the power setting you use, the power setting which give 1:1 time, and the flight length and profile (longer flights tend to be close to 1:1 because a smaller percentage of the engine operating time is ground ops either idling or near idle).

To add to the that, I have generally found in my PA28-180 that on XC flights of over 1 hour (2400RPM) Hobbs=1.2x tach while for pattern work it is closer to Hobbs=1.4x tach. I'd imagine the -181 is referenced to within 100rpm or so of the 180 given the similarities in airframe and power plant, but I'm not sure. I do know in mine at 2600RPM tach would be greater than hobbs, but I don't run it there.

 

[Bonchie]

I run 75% all the time in the Cherokee I rent and tach is still always lowering the Hobbs, usually be a decent amount.

But yes, if you were running almost to the redline, that's why the tach was so high. You shouldn't run it that hard IMO.

 

[polaris]

I run 75% all the time in the Cherokee I rent and tach is still always lowering the Hobbs, usually be a decent amount.

But yes, if you were running almost to the redline, that's why the tach was so high. You shouldn't run it that hard IMO.

I'm curious as to why people shy away from running it right below redline? The performance charts for 75% power at 7000 ft, for example, says 2640 RPM, which is right below redline.

Is it bad for the engine?

 

[Cpt_Kirk]

I run 75% all the time in the Cherokee I rent and tach is still always lowering the Hobbs, usually be a decent amount.

But yes, if you were running almost to the redline, that's why the tach was so high. You shouldn't run it that hard IMO.

That's not rental power.

I like 2500 on 2700 limited engines. Full mixture.

 

[frcabot]

Usually Hobbs time is about 30% more than tach time

I've never seen this to be the case. It all depends on what the reference speed is set to. A reference speed can be quite low, e.g. 2200 rpm with a max rpm of 2700.

 

[Henning]

I just flew the Piper Archer for 5.2 hobbs, and I saw that it was 5.4 tach. Confused, because I sat idle for at least 0.3 during my trip.

Is this because the tach meter is referenced at 2400 rpm (or something), and I was running it just below redline at 2600 rpm the whole time? I.e. the tach spun a little bit faster than the hobbs?

Yes, that is exactly why.

 

[Henning]

I'm curious as to why people shy away from running it right below redline? The performance charts for 75% power at 7000 ft, for example, says 2640 RPM, which is right below redline.

Is it bad for the engine?

Depends on the engine, but typically no, and when trying to run 75% power in a high performance, especially turboed engine, the higher RPM may actually be kinder to your engine since we are pretty much operating at the bottom of the RPM range for the fuel anyway. Since HP is a factor of time(rpm) * torque (ICP) the faster you let the engine turn, the lower you can keep your ICP. Now there are two ends to this equation, on one end you have efficiency, on the other you have detonation. The lower the RPM and higher the MP, the more efficient the engine is, however when you pass a barrier, you hit detonation, before that you run into high CHTs.

The best way to run an engine for durability and efficiency is to listen to it.

 

[polaris]

Depends on the engine, but typically no, and when trying to run 75% power in a high performance, especially turboed engine, the higher RPM may actually be kinder to your engine since we are pretty much operating at the bottom of the RPM range for the fuel anyway. Since HP is a factor of time(rpm) * torque (ICP) the faster you let the engine turn, the lower you can keep your ICP. Now there are two ends to this equation, on one end you have efficiency, on the other you have detonation. The lower the RPM and higher the MP, the more efficient the engine is, however when you pass a barrier, you hit detonation, before that you run into high CHTs.

The best way to run an engine for durability and efficiency is to listen to it.

Thanks. I figured that "you can't run it that fast" was mostly something for-profit flight school CFIs said to save fuel or build more time. I always followed the POH and not some doofus CFI telling me I have to run a C172 at 2200 RPM during cruise because it is bad for the engine otherwise (this has happened more than once). I understand I don't have to be at 2500 RPM while making circuits around a hold, but I didn't understand why I have to go 2200 RPM back from the practice area.

I do try to lean 50F lean of peak EGT while I run at max RPM.

 

[Bonchie]

That's not rental power.

I like 2500 on 2700 limited engines. Full mixture.

2450-2550 is 75% in a Cherokee unless you get really high.

 

[Bonchie]

Thanks. I figured that "you can't run it that fast" was mostly something for-profit flight school CFIs said to save fuel or build more time. I always followed the POH and not some doofus CFI telling me I have to run a C172 at 2200 RPM during cruise because it is bad for the engine otherwise (this has happened more than once). I understand I don't have to be at 2500 RPM while making circuits around a hold, but I didn't understand why I have to go 2200 RPM back from the practice area.

I do try to lean 50F lean of peak EGT while I run at max RPM.

There's a middle ground between being stupidly conservative and running 2200RPM in cruise vs. coming right to the redline, burning more oil and risking overheating in flight. You never know how good a shape these rentals are in and we all know they are rode rough.

POH on an Archer says 75% is 2640 at 8,000. If you are really that high, then go ahead. I never go far enough to get that high in a rental and I'm in a relatively flat area. The time to climb that high costs too much if there's no real point.

There's also a point where higher RPM settings make the prop spin so fast it becomes less efficient, costing you thrust. Going right up to the redline may actually be costing you performance. How this applies to the Cherokees we fly, I'm not positive.

http://www.cessnaowner.org/articles/13-technical/269-the-myth-of-high-rpm.html

All that aside, whoever rents to you might not continue to rent to you either if they continue to see such high tach to Hobbs readings. Depending on how many renters are in your area and what they charge, that might matter or it might not. I'm in the DC FRZ, so there aren't but three renters within an hours drive.

 

[kgruber]

None of this makes ANY sense with a stock mechanical tach. They are all off, usually over 100 RPM.

To do it correctly, you'll need an optical tach. They are cheap.

 

[polaris]

POH on an Archer says 75% is 2640 at 8,000. If you are really that high, then go ahead. I never go far enough to get that high in a rental and I'm in a relatively flat area. The time to climb that high costs too much if there's no real point.

I agree. Except the rare occasion when you are going east and the difference between 5000ft and 9000ft is winds aloft 130@15 vs. 270@40

 

[Bonchie]

I agree. Except the rare occasion when you are going east and the difference between 5000ft and 9000ft is winds aloft 130@15 vs. 270@40

True. So far haven't went on any long trips because it's not practical in a rental.

I'm looking into a club share now in a nice, modern equipped, 172M. Tired of getting destroyed on Hobbs time just for sitting on the ground. Mines on the master, so it's ticking the moment you start your engine startup flow.

No daily minimums will be nice.

 

[poadeleted20]

I'm curious as to why people shy away from running it right below redline? The performance charts for 75% power at 7000 ft, for example, says 2640 RPM, which is right below redline.

Is it bad for the engine?

It can be if you lean the mixture. Typically, the POH or engine operator's manual tell you not to lean the mixture above 75% power. That's because the engine cooling system is not designed o provide proper cylinder cooling above that power setting with the mixture anywhere less than full rich. You can get away with it if you have all-cylinder CHT instrumentation so you can be sure you are keeping the CHT's below the appropriate levels, but without that instrumentation, you can do serious damage to the engine by leaning above 75% power.

 

[poadeleted20]

Depends on the engine, but typically no, and when trying to run 75% power in a high performance, especially turboed engine, the higher RPM may actually be kinder to your engine since we are pretty much operating at the bottom of the RPM range for the fuel anyway. Since HP is a factor of time(rpm) * torque (ICP) the faster you let the engine turn, the lower you can keep your ICP. Now there are two ends to this equation, on one end you have efficiency, on the other you have detonation. The lower the RPM and higher the MP, the more efficient the engine is, however when you pass a barrier, you hit detonation, before that you run into high CHTs.

The best way to run an engine for durability and efficiency is to listen to it.

The OP is talking about an Archer, with a fixed pitch prop, carburetor, and no turbo or c/s prop. That makes what you said pretty much inapplicable.

 

[poadeleted20]

2450-2550 is 75% in a Cherokee unless you get really high.

Depends on which Cherokee. Some have redlines in that range. And altitude can change that number by 200 RPM. Best to check the book on the plane you are flying for this information and look it up for the altitude at which you are flying. Note also that non-stock props can also change the settings for 75%.

 

[polaris]

If the temperature is 22C (71F)(ISA +15C) at 4000 ft, as it often is in the summer, the POH says the Archer's 75% power is 2600RPM. The POH calls for a "best economy mixture -- 11gph." This means I should be ok leaning it 25-50F shy of peak EGT, correct?

I just wanted to make sure I wasn't unnecessarily damaging the engine by not going full rich in this scenario.

 

[polaris]

Also, another interesting point about this particular Archer I rent:

I typically cruise around 132-135 TAS (adjusted for altitude/temp) in this plane at 75% power. I have cross-checked this number with GPS ground speed with calm winds, and it seems accurate.

The POH says I should only be getting 128 TAS in the absolute best possible scenario.

I have heard in the past that this airplane is uncharacteristically fast for an Archer. It does not have a modified engine, and it is a 1999 model.

Is this cause for alarm? I am just concerned that the RPM is not measured correctly, or something, and I am actually running the engine faster than its redline unintentionally. But then again, even if it were faster than redline, then the prop should theoretically get less inefficient and produce a lower TAS.

 

[Henning]

The OP is talking about an Archer, with a fixed pitch prop, carburetor, and no turbo or c/s prop. That makes what you said pretty much inapplicable.

Sigh, what I said is the fact of how the engine works regardless of configuration. If you cannot control the prop, then MP vs RPM is governed by the fixed pitch of the prop so in theory if you have the correct prop on, you will never run into detonation. We are talking an Archer so we are talking about a 180hp O-360, half a horsepower per cubic inch. That is well below the Maximum Continuous Duty Cycle of any engine out there because the ICPs remain low. At half a HP/CI it is virtually impossible to damage the engine at any combination of throttle and mixture, the engine may not make the full power, however it will not be sustaining damage (unless you are excessively rich, then you are causing long term damage by loading up the valves and stems with carbon and lead.)

This whole rating things by % horsepower is bogus anyway. The percent HP is an arbitrary figure from the maximum the engine configured for, not what it could safely produce in another configuration. Many structurally identical engines come in various power ratings where the highest more than doubles the lowest, however regardless of which you run a WOT, they all say they are running 100% power on their graph for that engine.

 

[Silvaire]

I don't think you're going to get more than 75% out of a Cherokee at 8,000 feet so what are you people talking about here? A standard formula for HP loss is: (Altitude x 0.03 x HP)/1000 so a 180 hp Archer can only make 136 at WOT.