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Topic-icon ENGINE WONT DEVELOP 2700 RPM

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2 years 2 days ago #588

I’ve got a problem. . .or maybe not.

Usually, the engine on my 1967 180hp Arrow develops 2700 rpm for take-off and I have to reduce MP (to 25 in.) and rpm (to 2500) after take-off.

Recently, I noticed that at full power for take-off the engine only developed 2500 rpm. It was hot (about 90°) and density altitude was a little higher than normal (3300’ as opposed to 1100’). Earlier this week the DA was lower (2300’). While acceleration and climb were better, the engine still only developed 2500 rpm.

When I have been at higher altitudes (Colorado Springs and Santa Fe), my recollection is that manifold pressure was a lot lower than at my home base, but the engine still developed about 2700 rpm.

Do moderate changes in density Altitude affect rpm or only manifold pressure? If not normal, what should I be looking at as the cause of the rpm reduction?

I’d appreciate any information you can give me.

Thanks

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2 years 1 day ago #590

Hi Frank,

I sent your question to our A&P mechanic, Steve Ells. I'm waiting for a response and as soon as I get one I'll post it for you.

Thanks so much for your patience,

Scott Sherer
N344TB

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2 years 1 day ago #591

Hi Frank,

Here's the response from the Piper Flyer Assn A&P, Steve Ells:
Scott Sherer
N344TB

This member's airplane has a fixed pitch prop. The rpm will be maintained at 2700 if he had a constant speed prop but not with a fixed pitch.
A normally aspirated engine develops rated power at sea level in standard day conditions (59 deg F).
At higher density altitudes engine power falls off. For instance, the 180 HP engine in this airplane generates 180 hp at sea level pressure and temperature.
At 3,000 feet density altitude it develops (all other things being equal) approximately 164 hp; at 2200 DA approximately 168 HP.
A 200 rpm drop (2500 instead of 2700) seems excessive for this small loss in power.
If our member still has a mechanical tachometer it could be the culprit; they do lose accuracy but it usually a gradual process.
Check the carburetor heat door for security and proper operation and to see if the door closes all the way in the cold air position. A little carb heat will reduce engine power and rpm.
It woudn't hurt to do a differential compression test just to make sure the top end of the engine is completely healthy.
Are the pre takeoff mag check rpm drops within limits?? If so, and if the compression readings are above 70 in each cylinder and with a max to min spread of no more than 5 psi, I doubt there's a mechanical problem.
Best,
Steve

Last Edit: 2 years 1 day ago by SCOTTSHERER.

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1 year 11 months ago #592

"This member's airplane has a fixed pitch prop." NO it does NOT....

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1 year 11 months ago #593

Hi,

Thanks for the clarification. I'll get your information back to Steve Ells asap and get his reply.

Best,

Scott Sherer
N344TB

Last Edit: 1 year 11 months ago by SCOTTSHERER.

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1 year 11 months ago #594

Scott;

A slightly amended response:

A normally aspirated engine develops rated power at sea level in standard day conditions (59 deg F).

At higher density altitudes engine power falls off. For instance, the 180 HP engine in this airplane generates 180 hp at sea level pressure and temperature.

At 3,000 feet density altitude it develops (all other things being equal) approximately 164 hp; at 2200 DA approximately 168 HP. This is indicated by a reduction in manifold pressure but the prop should still turn 2700.

If our member still has a mechanical tachometer it could be the culprit; they do lose accuracy but it is usually a gradual process.

Are the pre takeoff mag check rpm drops within limits?? If so, and if the compression readings are above 70 in each cylinder and with a max to min spread of no more than 5 rpm, I doubt there's a mechanical problem with the engine.

So let’s look at some other possible causes.

1. Was any work on the engine done prior to the low rpm readings? Magnetos re timed or new mags installed? Any adjustments to the prop governor high rpm set screw??

2. A constant speed propeller is driven to flat pitch (high rpm) by a force called centrifugal twisting moment—this force is naturally generated during prop rotation. It’s similar to the lift vector generated by wings. Oil pressure from the governor acts on a piston in the prop hub to increase the pitch of the prop blades, thereby reducing rpm. Check the prop governor high rpm screw to make sure it hasn’t moved.

3. If everything looks good, back out the governor high rpm set screw one turn and see if it affects the full power rpm.

Let me know what your find.

Thanks

Steve

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