Max RPM

Tan

Tan

Pre-takeoff checklist
Joined
Jun 26, 2018
Messages
107
Display Name
Display name:
TheCapTan
On my 1968 Piper Arrow 180 the rpms do not reach 2700 on takeoff. I currently get on my engine monitor 2570 and my tach shows around 2600. My mechanic states that its set to max adjustment on his end. Any suggestions?
 
Tan said:
I currently get on my engine monitor 2570 and my tach shows around 2600.
Click to expand...
Have you check any of these readings against a calibrated indicator? Is the 2700 at sea level? What is your airport elevation?
 
Does your mechanic have an optical tach? Verify that your tach isn't the problem.
 
Tan said:
On my 1968 Piper Arrow 180 the rpms do not reach 2700 on takeoff. I currently get on my engine monitor 2570 and my tach shows around 2600. My mechanic states that its set to max adjustment on his end. Any suggestions?
Click to expand...
How is the performance? Does it get airborne in expected distances and climb at expected rates?
 
Field elevation is 1500. Climb is within book numbers. Last flight temp was around 60F with 800ft min climb till 3500. I haven't checked with a external tach to verify accuracy. But after annual I lost about 30rpm from 2600.
 
Piper Arrow has a constant speed prop. The limiting factor on RPM is the prop governor. The mechanic may have adjusted your governor, using his tachometer, and it disagrees with your two unmatched RPM readings. Now you seem to have three tachometers , and only one, or none, are precisely accurate.

If your airport has sodium vapor flood lights, they flicker at 120 cycles per second, or 7200 per minute. Park your Arrow facing AWAY from a light, chock the wheels, and run the engine with the propeller control set at max. With manifold pressure (the throttle) increase to 1800 RPM, and the image should be stationary. Adjust until it is, and write down the REAL tach reading which is 1800 true RPM. Now increase to 2400 RPM, and the prop should appear stationary again, adjust until it is if stationary, and record the actual indication of your tach at 2400 RPM.

At 1200 RPM, ther3 will be a stationary prop, and at 1600, a stationary image with more the prop lit at two different places.

You can now use proportion to calculate what your tach reads when the engine really IS at 2600 RPM.

Make a placard if appropriate.

This is the easiest and cheapest way to get a precise test of tachometer accuracy, and the light frequency is within less than one hundredth of one percent, on a bad day.

Neon, and most LED lights have a flicker synchronous with the power system, and will give similar results.
 
geezer said:
Piper Arrow has a constant speed prop. The limiting factor on RPM is the prop governor. The mechanic may have adjusted your governor, using his tachometer, and it disagrees with your two unmatched RPM readings. Now you seem to have three tachometers , and only one, or none, are precisely accurate.

If your airport has sodium vapor flood lights, they flicker at 120 cycles per second, or 7200 per minute. Park your Arrow facing AWAY from a light, chock the wheels, and run the engine with the propeller control set at max. With manifold pressure (the throttle) increase to 1800 RPM, and the image should be stationary. Adjust until it is, and write down the REAL tach reading which is 1800 true RPM. Now increase to 2400 RPM, and the prop should appear stationary again, adjust until it is if stationary, and record the actual indication of your tach at 2400 RPM.

At 1200 RPM, ther3 will be a stationary prop, and at 1600, a stationary image with more the prop lit at two different places.

You can now use proportion to calculate what your tach reads when the engine really IS at 2600 RPM.

Make a placard if appropriate.

This is the easiest and cheapest way to get a precise test of tachometer accuracy, and the light frequency is within less than one hundredth of one percent, on a bad day.

Neon, and most LED lights have a flicker synchronous with the power system, and will give similar results.
Click to expand...
That’s some trick stuff right there.
 
Stewartb said:
Does your mechanic have an optical tach? Verify that your tach isn't the problem.
Click to expand...
Optical tachometer,
133.jpg

It doesn't seem right to expect max rpm on take off? I only see 2300 rpm with 0-320 160 hp motor on take off. In this weather my plane climbs real well, so I know I am not down on power.
135.jpg

Then the second way to check rpms is with a dynamic prop balance.
IMG_8681.JPG
 
Buy this $36 optical tach. The previous version works perfectly from the cockpit of my Warrior. The cost will be cheaper than calling in an A&P and just as accurate. It will read precisely 3600 (two-blade mode) when pointed at a fluorescent light.
https://www.amazon.com/dp/B0006N72U2/
 
Gary Ward said:
Optical tachometer,
133.jpg

It doesn't seem right to expect max rpm on take off? I only see 2300 rpm with 0-320 160 hp motor on take off. In this weather my plane climbs real well, so I know I am not down on power.
135.jpg

Then the second way to check rpms is with a dynamic prop balance.
IMG_8681.JPG
Click to expand...
Your TCDS should have an acceptable static rpm range. You the pilot needs to know what that is.
 
OP and others:

geezer said: ↑
Piper Arrow has a constant speed prop. The limiting factor on RPM is the prop governor. The mechanic may have adjusted your governor, using his tachometer, and it disagrees with your two unmatched RPM readings. Now you seem to have three tachometers , and only one, or none, are precisely accurate.

Constant speed prop means the governor will limit the RPM, not anything else.

The real question is whether the governor was adjusted in the annual. Checking and adjusting the governor is one of the steps for a constant speed prop.

The question of tachometer accuracy is separate, and not the underlying problem.
 
If there is not enough governed oil pressure, can't push the prop to low pitch which means no max RPM.
 
There are 2 modes to this.

1. Static

2. Climb -out

Normally a constant speed Governor allows the engine to go to red -line RPM.

In mode 1 ( static) if the engine is not producing enough power you will not make it to red line RPM. OR; the Governor is limiting max RPM. Often you can advance the throttle and see if RPM goes higher before the Governor drags it back down.

In mode 2 ( climb-out ) you should readily achieve Max RPM IF the Governor Stop Is adjusted for proper High RPM.

Another reason is the SLIGHT possibility the the Propeller Low Pitch Stop is set
too high.
 
The Arrow that I flew produced red line RPM about 24 inches of MP. Beyond that MP, thrust increased, but RPM remained the same. This was noted on the runway, preparing to make a short field departure. Thrust was modest before the pitch started to change.

Cessna 172 with the Franklin STC was similar, but more dramatic, as the brakes would not hold it on grass.
 
Which engine analyzer. I've got two and each uses a different method of measuring RPM. The EI uses pulses on the ignition switch end of the p-Leads. The JPI has a magnetic pickup that mounts to the magneto itself.
 
BTW you can verify tach accuracy by running up your engine at night in a lighted area and noting the RPMs where you can "stop" the prop image by synching with 120 flickers/second of line AC frequency.
 
flyingron said:
Which engine analyzer. I've got two and each uses a different method of measuring RPM. The EI uses pulses on the ignition switch end of the p-Leads. The JPI has a magnetic pickup that mounts to the magneto itself.
Click to expand...
Would either method yield less than actual RPM?
 
I would tend to believe the engine analyzer. To be honest, the two tachs in the OP's plane are almost within 1 percent agreement with each other.

I loaned my optical tach to a friend a couple weeks ago with a similar issue...it read within 10rpms of his JPI...mechanical tach off about 100rpms.
 
Tan said:
I currently get on my engine monitor 2570 and my tach shows around 2600.
Click to expand...
The tach is shot. Most tachs use a magnet that spins in an aluminum cup. The magnet is spun by the tach cable from the engine, and the cup is attached to the indicator needle. Eddy current generated in the cup by the moving magnet forms a magnetic field that interacts with the spinning magnet, and the cup gets dragged farther around as the RPM increases.

That magnet get weaker with age, and the tach underreads. In Canada we have a regulation that requires the tach to be checked for accuracy annually, and the error posted on it. The max allowable error is 4%, in the middle of the cruise RPM range. At 2500 RPM, that's a max 100 RPM error.

New magnetic tachs are available but they will just offer the same deterioration over time. They also suffer the jerkiness caused by cable sticking in cold weather or as the lube in it dries up. The better bet is an STC'd digital tach that's always accurate, or entirely dead.

If the JPI is showing 2750 in the takeoff, and the redline is 2700, the mechanic likely adjusted the governor stop to get redline with the airplane static. Not all CS airplanes are spec'd to reach redline in the static condition. The service manual for that airplane should address it.
 
asicer said:
Would either method yield less than actual RPM?
Click to expand...

Not if everything was running right, but the p-lead pickup may read high (or perhaps low) if there's a problem with the mag. The one that fits into the mag itself is watching the mag spin around so it's probably less susceptible to issues. I've seen the kind that electrically couple to the mag spike high on bad contacts.
 
flyingron said:
Not if everything was running right, but the p-lead pickup may read high (or perhaps low) if there's a problem with the mag. The one that fits into the mag itself is watching the mag spin around so it's probably less susceptible to issues. I've seen the kind that electrically couple to the mag spike high on bad contacts.
Click to expand...
The digital tachs that use the P-lead as a source are subject to too much signal in some cases, requiring a resistor in the line to the tach. I encountered that in a EI or JPI (can't remember which) we installed in a 180. The numbers got jumpy. The resistor fixed it. The installation instructions cover it.

If the 50-hour inspections are done on the mags, there is no reason to have bad contacts unless the condenser is failing. Bad contacts are normally found in neglected mags.
 
Dan Thomas said:
The digital tachs that use the P-lead as a source are subject to too much signal in some cases, requiring a resistor in the line to the tach. I encountered that in a EI or JPI (can't remember which) we installed in a 180. The numbers got jumpy. The resistor fixed it. The installation instructions cover it.
Click to expand...

Dan any other choice for a digital tach other then the P-lead connected one?
 
Florida Flyer said:
Dan any other choice for a digital tach other then the P-lead connected one?
Click to expand...
Don't know of any that are STC'd as a replacement for the primary instrument. UMA makes one that is TSO'd and uses a tach sender that fits into the original tach drive on the engine, and that instrument has an analog readout just like the old tach, but has no STCs.
 
Dan Thomas said:
Don't know of any that are STC'd as a replacement for the primary instrument
Click to expand...
There is/was a 3rd party indicator vender who had a AML-STC using a hall effect speed senser installed in a magneto vent port. It was very accurate and replaced the primary indicator as I recall. But it was a bit pricey.
 
You must log in or register to reply here.
Back
Top