Tach time

[bigred177]

Last weekend on my trip, I put 4.8 on the Hobbs. After reading the tach, it was around 1.2hrs OVER the Hobbs meter. I was not running the engine very hard, probably cruising around 2400-2500 rpm. I know that the tach turns different speends at different rpm but I was under the impression that it would only go up to equal to the Hobbs meter, if calibrated properly. So do I need to have the tach looked at and get recalibrated to count properly?

 

[brcase]

If you look at the tach hours closely a lot of times it will say something like Hrs@2400 RPM. This means @ 1200 RPM it will take 2 hrs to read 1 hr elapsed.

I have seen a tach read slow before, It was slipping internally.

More commonly I have seen Hobbs meters run slow as well.

Brian

 

[Areeda]

Tach times greater than Hobbs are not unusual but 25% greater is odd. I agree with Brian and suspect the Hobbs is running slow, easy to compare it to your watch.

Joe

 

[dmccormack]

A Slow Running Hobbes?

Isn't this the dream of renters everywhere??

 

[poadeleted20]

How old is your tach, and when's the last time you checked its accuracy? I've seen old tachs read as much as 200 RPM low, and some tachs are 1:1 at RPM's as low as 2300 RPM, so you could have been turning as much as 400 RPM over the 1:1 point. Also, some Hobbs meters are hooked to the weight-on-wheels switch rather than the master or an oil pressure sensor, and if so, significant ground time could account for a shortage of Hobbs time, especially if combined with a slow-reading tach.

 

[gismo]

Last weekend on my trip, I put 4.8 on the Hobbs. After reading the tach, it was around 1.2hrs OVER the Hobbs meter. I was not running the engine very hard, probably cruising around 2400-2500 rpm. I know that the tach turns different speends at different rpm but I was under the impression that it would only go up to equal to the Hobbs meter, if calibrated properly. So do I need to have the tach looked at and get recalibrated to count properly?

The tach-hour meter basically counts revolutions so there's nothing to calibrate there. On many GA airplanes the hobbs=tachhours RPM is around 2300 so if you were using 2500 you might see around 8% extra time on the tach than the hobbs which would be 5.2 tach hours, not the 6 you reported. And that much error seems rather implausible, is there any chance you misread something? To get 6 tach hours in 4.2 hobbs @ 2500 RPM would require the tach=hobbs RPM to be 2000 and that's not even accounting for the time spent idling (likely at least another .2-.3 difference between hobbs and tach hours).

You can check the tach hour rate in flight by measuring the time required o advance by one or two tenths (this is easier if the tach reads in 1/100 hr but if you are careful you can do it on a tach with only 1/10 hr resolution).

 

[bigred177]

I'll look over the numbers again. But I'm pretty sure there was at least an hour difference.

Brian: Where might I find that little bit of info? The POH?

Dan: Yes

And Ron, Gismo, and Areeda: Thanks for that information, I will have to check both against my watch next time I go fly.

 

[weirdjim]

Most airports are lit at night by sodium vapor lights or mercury lights. THese lights flicker ("pulse") at the line frequency, 60 times a second. This is 3600 times a minute, but you have two blades on your prop that go around per revolution. Long story short, your prop should appear to stand still at 1800 RPM when only being illuminated by the flashing light.

Poor man's tach strobe.

Jim

 

[Arbiter419]

Most airports are lit at night by sodium vapor lights or mercury lights. THese lights flicker ("pulse") at the line frequency, 60 times a second. This is 3600 times a minute, but you have two blades on your prop that go around per revolution. Long story short, your prop should appear to stand still at 1800 RPM when only being illuminated by the flashing light.

Poor man's tach strobe.

Jim

That's genius! Never thought of that before...

 

[bigred177]

I like it, I'm gonna have to try that next night flight

 

[gismo]

Most airports are lit at night by sodium vapor lights or mercury lights. THese lights flicker ("pulse") at the line frequency, 60 times a second. This is 3600 times a minute, but you have two blades on your prop that go around per revolution. Long story short, your prop should appear to stand still at 1800 RPM when only being illuminated by the flashing light.

Poor man's tach strobe.

Jim

That works, but the "flicker" frequency in the US and Canada is 120 Hz not 60. The prop should still stand still at 1800 RPM although a 3 blade prop will appear to have 12 blades and a two blade prop will show 4. Also, it's not unusual for a mechanical tach to be far less accurate at 2500 RPM than at 1800. At 2400 RPM a 3 blade prop will "stop" with 3 blades showing and a 2 blade prop will stop showing 8 blades. At 2700 RPM a 3 blade prop shows 16 blades and a 2 blade prop shows 8. There's also a barely perceptible stop at 2550 RPM with 48 blades on either prop.

 

[weirdjim]

That works, but the "flicker" frequency in the US and Canada is 120 Hz not 60. .

You are correct. I don't know what the hell I was thinking of with 60 Hz. You get a pulse on the positive half cycle and another pulse on the bottom half cycle to give 120 pulses per second. The remainder of the theory is correct. As they used to tell me back in the old days of Morse Code: QLF ("now try sending with your LEFT foot").

The 1800 RPM stop is a lot easier to see than the 2400 RPM stop, but the 2550 is close enough to the 2400 stop that you really need to see both of them to distinguish which is which. I know there are many more blade stops on the 2550, but sometimes at night difficult to distinguish.

Thanks for the correction.

Jim

 

[gismo]

You are correct. I don't know what the hell I was thinking of with 60 Hz. You get a pulse on the positive half cycle and another pulse on the bottom half cycle to give 120 pulses per second. The remainder of the theory is correct. As they used to tell me back in the old days of Morse Code: QLF ("now try sending with your LEFT foot").

The 1800 RPM stop is a lot easier to see than the 2400 RPM stop, but the 2550 is close enough to the 2400 stop that you really need to see both of them to distinguish which is which. I know there are many more blade stops on the 2550, but sometimes at night difficult to distinguish.

Thanks for the correction.

Jim

I'm perfectly willing to correct you anytime you want.

I believe the 2550 RPM stop would be difficult to see except under perfect conditions which would be viewing a single lamp through the prop arc with no other light sources coming from any direction, especially through the prop. The two are easily distinguishable if you can see both since there would be many more "blades" visible at 2550. I only included that one for folks with a redline around 2600. I should also note that you don't want to spend too much time running the engine at high RPM. This will overheat the engine, it's likely to suck up pebbles and nick the prop, and it makes the airport neighbors angry, especially at night. Also make certain that all the rocks, and wind you're blowing out behind isn't going to affect anything of consequence.

 

[weirdjim]

I believe the 2550 RPM stop would be difficult to see except under perfect conditions which would be viewing a single lamp through the prop arc with no other light sources coming from any direction, especially through the prop.

Respectfully disagree. It could be coming from ANY of the lamps on the field singly or in bunches. Something about light travelling at the speed of light and such. And ground power travelling well beyond the flicker speed of light.

Jim

 

[gismo]

Respectfully disagree. It could be coming from ANY of the lamps on the field singly or in bunches. Something about light travelling at the speed of light and such. And ground power travelling well beyond the flicker speed of light.

Jim

The problem with multiple sources isn't the tim ing of the light's arrival but rather it's direction. When the RPM "stops" the prop with few blades showing the effect is very visible with multiple sources but when you get a beat with 48 blades the contrast between a stopped blade and the space between isn't very great to begin with because each blade image is blocking the light onl a fraction of the time and having light coming at you through the prop from two different directions (or worse yet from behind you) diminishes the contrast further.