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Prop RPM on Takeoff

Tcraft938

By Tcraft938
in Vintage Mooneys (pre-J models)

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Tcraft938

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Since the Mooney is new to me with a few hours in it, I was wondering what you see for prop RPM on takeoff, say right around rotation speed?  Are you getting close to the 2700 rpm max while on the ground?  If equipment is important, 62 M20C 180hp new zero time engine, new Hartzell scimitar prop (2 blade)

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jaylw314 Grand Master

jaylw314

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AFAIK, all Mooney's with CS props should max be at 2700 RPM with anything near takeoff power and max RPM.  Only fixed pitch props would be expected to be different depending on speed and altitude.

With a new motor/prop combo, the prop governor probably needs adjustment, which may take a couple of adjustments and test flights 

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Igor_U Mentor

Igor_U

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After installing EI primary engine monitor, it became obvious the old tachometer showed high. I would get only 2650-60 on take-off or in flight.

After an easy an easy adjustment on governor by my IA, I get 2700 before rotation speed.

 

 

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Skates97 Grand Master

Skates97

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2 minutes ago, J0nathan225 said:

Thread Highjack: Anyone getting a little OVER 2700? I usually need to have a half, to full turn out, on the prop to stay at or under 2700 for takeoff.

 

 

How much over 2,700, and is that on an engine monitor or the regular tach?

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kortopates Grand Master

kortopates

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6 minutes ago, J0nathan225 said:

Thread Highjack: Anyone getting a little OVER 2700? I usually need to have a half, to full turn out, on the prop to stay at or under 2700 for takeoff.

 

 

Get it adjusted so it doesn't overspeed - it is a redline, not an optional, okay sometimes or okay for brief excursions :)

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Ragsf15e Grand Master

Ragsf15e

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20 minutes ago, J0nathan225 said:

Thread Highjack: Anyone getting a little OVER 2700? I usually need to have a half, to full turn out, on the prop to stay at or under 2700 for takeoff.

 

 

Having a good engine monitor that shows rpm down to 10rpm is great until it’s not.  Mine will often touch 2710 and turn red about rotation.  I’m pretty sure the original tac needle was about 50rpm thick though so I try to ignore when possible.

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kortopates Grand Master

kortopates

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Having a good engine monitor that shows rpm down to 10rpm is great until it’s not.  Mine will often touch 2710 and turn red about rotation.  I’m pretty sure the original tac needle was about 50rpm thick though so I try to ignore when possible.

Your original tach was never an approved source for setting the RPM limits either. We're required to use a much more accurate source such as an approved optical tach so that RPM can be set accurately not to exceed redline. You really want to ignore it now because you couldn't tell it was overspeeding before? How much error becomes important?

As an example, I just flew with a client who's OEM RPM was reading 120 RPM higher than actual (opposite issue). He wondered why is performance was so far below book. A 50 year old instrument can be pretty far off without getting re-calibrated. 

Modern engine analyzers give us accurate data, generally at the same accuracy test gauges do. Its a shame not to make use of it.

 

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PT20J Grand Master

PT20J

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A prop has high and low pitch stops. When running on the ground at low power, it sits on the low pitch (high rpm) stops. The stops are set so that the prop turns about the rated rpm (2700 in this case), maybe a little less, when the engine is putting out full power at zero airspeed (maximum thrust) . McCauley practice is to set them for a bit less; I think Hartzell does the same. The exact static rpm will vary with engine condition and density altitude. At this point the governor has no effect since it cannot flatten the pitch more than permitted by the stops. As you begin the takeoff roll, the airspeed increases and the rpm tends to increase. At this point the governor takes over and by liftoff you should get 2700 rpm. If not, you can adjust the governor.

Make sure to check the tach for accuracy before making adjustments.

Skip

 

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Ragsf15e Grand Master

Ragsf15e

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3 hours ago, kortopates said:

Your original tach was never an approved source for setting the RPM limits either. We're required to use a much more accurate source such as an approved optical tach so that RPM can be set accurately not to exceed redline. You really want to ignore it now because you couldn't tell it was overspeeding before? How much error becomes important?

As an example, I just flew with a client who's OEM RPM was reading 120 RPM higher than actual (opposite issue). He wondered why is performance was so far below book. A 50 year old instrument can be pretty far off without getting re-calibrated. 

Modern engine analyzers give us accurate data, generally at the same accuracy test gauges do. Its a shame not to make use of it.

 

Possibly you’re right, and I definitely try not to defer maintenance, however if you try to fix every “10ish rpm” issue (.3%) on a 50 year old airplane you’re gonna be chasing your tail with little to gain.  

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carusoam Grand Master

carusoam

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1) real overspeed is bad... bad instrumentation... actual rpm is unknown...

2) Some Mooneys have 2500rpm limits... 200rpm is 30hp for an IO550, or 10%

3) 10% isn’t much of the total hp... but that isn’t the important part... Excess hp is what counts in acceleration, T/O distance, and climb rate...

4) So the extra hp is a factor of about 2X, when the chips are down...  :)

5) O1 - O3 example... 10% more hp... T/O distance goes from 1200’ to 800’ 

6) If your plane accidentally went from 800’ to a 1200’ T/O run... that is 50% more distance....

7) Mechanical tachs are known to have poor accuracy over decades of use... note the oil cup at the back of the tach... you know it’s there right? 

8) Getting a back-up piece of data... use your smart phone app to determine actual rpm and compare...

9) Static rpm is difficult to measure/achieve...Once on the roll, a quick glance at MP, rpm, FF, as airspeed comes alive...

10) Anything more than a quick glance... weeds, and lights will be more of a challenge than exact rpm...

11) So... if 2700 is your redline... get it... and nothing more...

12) Setting the Governor is a mechanic’s job...

13) if the prop’s mechanical limits aren’t set properly... when the gov fails, the prop will overspeed... if the pilot takes out MP trying to avoid the overspeed... the engine produces substantially less power...

14) Stuff I learned about my prop and gov on my M20C... when the shaft plug went missing... fifty cent piece of aluminum, hammered in place...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Tcraft938 Collaborator

Tcraft938

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Thank you everyone and I love"the hijack" a lot of great info. To answer a question from above just before rotation and through most of the roll rpm was 2635 to 2642 range on a digital tach. This was first flight after new engine and prop. I has decided long before the flight if engine instruments good, acceleration good and 2600 rpm or more I would continue and concentrate on flying to safe altitude. Any of those parameters not met, abort takeoff. The one hour circle the airport at 75% or more went well. Airspeed at 4500' was 172mph true oil temp steady at 181 and hottest cylinder 314 lowest 264 not sure if that's a big spread but indicates airflow in the doghouse, right side warmer than left. No oil leaks post flight inspection and used 1/2 qt mineral oil. Only issues is adjust prop rpm after confirm tach is accurate and the fuel flow sensor was inop.  We dont want to ground run much so will get more hours on it before mess with prop unless rpm drops.  Feels good to have the bird flyable again. 

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carusoam Grand Master

carusoam

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While you are at it...

Verify the location of the ship’s CHT sensor and it’s doppleganger....

The high CHT May be real, or... it may be a figment of your instrumentation...  :)

The doppleganger May be a TC hiding under a sparkplug or under the TC well hardware...

Check the tach accuracy with an app... or low cost optical sensor...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Jim Peace Grand Master

Jim Peace

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On 9/4/2019 at 12:20 PM, Tcraft938 said:

 I was wondering what you see for prop RPM on takeoff, say right around rotation speed?  Are you getting close to the 2700 rpm max while on the ground?  If equipment is important, 62 M20C 180hp new zero time engine, new Hartzell scimitar prop (2 blade)

Thanks

I have the same setup except 1000 hours SMOH...I get 2650 rpm soon after full power is applied sometimes it will touch 2700 later on in the takeoff roll or initial climb.

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