Delta betwwen Outside Air Pressure and MAP with Wide Open Throttle and max RPM?

[Fry]

Hello all,

with wide open throtte and max RPM (e.g. in the takeoff roll), I see a delta between the ambient outside air pressure and my manifold pressure of about 1.1 inches Hg. Is that normal, or is it a sign of a clogged air filter or something?

I'm asking because I (subjectively) feel the aircraft performance is a little lower than it used to be. Also, RPM on the takeoff roll is slightly below 2700, going from ~2600-2630 initially up to about 2660-2680 at rotation. The reason cannot be the governor, since in cruise, 2700 RPM is possible (and I usually cruise at 2700 RPM or slightly below that).

Best regards,

Fry

[Hank]

An inch differential is pretty normal, as there are bends and turns between the air intakes up front behund the propellor and where the MP sensor is (shiuld be near where the fhel is introduced into the airflow--my carb or your manifold).

Others will have to comment in your RPM loss, but that would bother me as it extends your ground run.

[kortopates]

Begin by calibrating your MAP indication per the instructions of your engine monitor.

[David Lloyd]

The prop governor keeps your RPM from exceeding 2700, the internal low pitch stops keep your RPM down at low airspeeds.  That is a safety item, to allow the plane to be flown if the prop governor fails. Yours is working properly.

[carusoam]

Begin by collecting data...

As much real data as possible...

 

Take this...  “I'm asking because I (subjectively) feel the aircraft performance is a little lower” - Fry

Then ....  Measure the actual T/O run and climb performance using a Waas source and CloudAhoy app...
 

Followed by comparison to the POH...  take into account the warm summer weather you are having and the DA...

 

You may find that your calibrated seat of the pants you are using... is telling you the truth...
 

Summer DAs are tough on airplane performance numbers... power is down, lift is down, things take longer as they accelerate slower...
 

For fun, swap out the filter for a more expensive newer one... see if the MP changes any? Probably not enough to make you want to keep the new filter....

Definitely compare actual data to POH data to see if anything needs to be double checked... got any JPI data to post?  We haven’t reviewed much engine data lately...

PP thoughts only, not a mechanic...

-a-

 

[PT20J]

Your MAP sounds about right. My prop gets up to 2700 at rotation speed at sea level (factory rebuilt engine with 200 hours on it). Always a good idea to check the instrument calibration before looking into other issues.

[larrynimmo]

With my M20J you can feel the governor adjust the pitch during takeoff roll...I typically open the throttle slowly in my roll and Rpms go up to about 2680 then settle at 2650 on every take off...and it sounds just like I had adjusted the rpm via the blue knob.

[Fry]

ok thank you all! I'll further observe and maybe discuss this with my MSC and will keep you posted!

[Ibra]

Did you try with RAM air open on a quick test (obviously open above 5000amsl), on J it may grab 0.3 or 0.5MP when the prop hits 2700rpm as you roll?
Maybe not? it needs high speed to work above ambient pressure but it will surely help bellow ambient pressure to compensate for air filter 

[Fry]

Sorry, no ram air on a 1995 M20J MSE.

[jlunseth]

Here is a great article John Deakin wrote several years ago that explains it. The title is Manifold Pressure Sucks! https://www.avweb.com/features_old/pelicans-perch-15manifold-pressure-sucks/

Pressure loss through the intake system is about 1 inch.

[Fry]

Thanks! Thinking about it, my "subjective feeling of lower performance" is more related to cruise than to takeoff.

So at the next annual, I will have the MSC double-check my gear doors, cowl flaps and other typical sources of drag.

[David Lloyd]

I'm sure I already did this in the past but don't remember. Noted my manifold pressure gauge before start and then again when going full throttle for takeoff.  About as close to 1" difference as I could tell with the old analog gauge.

[PT20J]

6 hours ago, Fry said:

Thanks! Thinking about it, my "subjective feeling of lower performance" is more related to cruise than to takeoff.

So at the next annual, I will have the MSC double-check my gear doors, cowl flaps and other typical sources of drag.

Also, when you do the next pitot-static check you can have the ASI calibration checked and leak check the pitot system. Usually shops only leak check the static as that's all the regulations require.

Skip

[Fry]

Many thanks for this hint, Skip. Because as it happens, the aircraft is right now in for the pitot static check :-)

[RobertGary1]

Depends on the model. The J gained 3/4” of MP by removing some of the bends in the intake that the F has. The F gains this back with the ram air open. 
 

-Robert 

[RobertGary1]

12 minutes ago, PT20J said:

Also, when you do the next pitot-static check you can have the ASI calibration checked and leak check the pitot system. Usually shops only leak check the static as that's all the regulations require.

Skip

I’ve not seen this. Most shops will detect a pitot leak because there is no way to test the static system without adding vacuum on the pitot (at least without damaging the airspeed indicator).  The bring them up together. 
 

-Robert 

[FloridaMan]

I see about 8 inches higher mp on takeoff

[PT20J]

1 hour ago, RobertGary1 said:

I’ve not seen this. Most shops will detect a pitot leak because there is no way to test the static system without adding vacuum on the pitot (at least without damaging the airspeed indicator).  The bring them up together. 
 

-Robert 

I'm not an expert on this having never done it myself. You are certainly correct that the pressure must be equalized across the airspeed indicator pitot-static ports during the test to avoid driving the ASI way over range and damaging it which could also damage the diaphragm in the gear safety switch. But the test sets have separate systems for leak testing the pitot and static system. My airplane has been recertified every 2 years since it left the factory in 1994, yet I recently discovered a rather significant pitot leak that was apparently created when the Aspen EFD 1000 was installed for a previous owner. Since there is no requirement to test the pitot system for leaks and since there are no log book entries attesting that it has been tested, I don't think it has been. Next time, I'll watch them do the test and see exactly what they do and don't do. 

Skip

[flyer338]

The first time I had the pitot static test done on my electric-gear C model, the shop did it wrong (This was pre-internet and I did not figure it out until years later.) The result of the shop’s lack of knowledge was a burst diaphragm in my gear safety switch. The leak did not prevent the gear from retracting, but it make a change to my indicated airspeed of about 15%. Even as a new Mooney pilot, I noticed.
I did not know there is no requirement to test the pitot system.

My J is in the shop for an Aspen PFD upgrade. I will make sure the shop tested the pitot system before I take delivery.

[Guest]

A leak check of the pitot system should happening during every Annual Inspection as the gear is being swung.  How is your maintainer testing the anti retraction system?

Clarence

[PT20J]

4 hours ago, M20Doc said:

A leak check of the pitot system should happening during every Annual Inspection as the gear is being swung.  How is your maintainer testing the anti retraction system?

Clarence

Not all leaks are equal. The Mooney Maintenance Manual calls for applying pitot pressure to indicate 150 knots and holding the pressure constant while timing any decrease in airspeed indication. The specification is 10 knots or less in one minute. In my case, I measured about 20 knots/min. I noticed that the leak rate slows as the airspeed indication decreases. I did several 4-way GPS tests using the NTPS procedure and calculated 153 KTAS with a TAS calculated by the Aspen PFD a steady 145. So, my error is only about 8 knots at cruise speed. The specification on the gear safety switch is 60 +/-5 knots, and this amount of leakage allows the switch to still be within spec.

I traced my leak to the pitot drain by connecting a known good airspeed indicator to the line coming directly from the wing. Manipulation the drain between tests caused a variation in the results. Of course! I just put the interior back together and it seems the only way to get to the drain to replace the O-ring is through the cabin.

BTW, the procedure for applying pressure to the pitot system in the Mooney manual fails to mention the necessity of covering up the drain hole at the bottom of the pitot tube. If you don't block that, you'll think you have a BIG leak .

Skip

[Fry]

Hello all, thanks for the discussion so far. I'm bringing this topic back up because I have a new idea. On two recent flights, I again perceived slightly lower engine power. I note that on takeoff roll the engine does not reach 2700 RPM (later in the flight, it does - so this is not due to the governor). Also, in cruise at 10k ft or so, the airplane is doing only 140 KTAS or so (well, I'm flying LOP, that _may_ contribute...).

The new idea: maybe my mag timing is retarded? Add to my perceived lower power a really cool running engine. In cruise, with cowl flaps closed the CHTs, are usually around 300-330.

Question to you: is there a way how I as the pilot can verify this hypothesis, without advanced equipment?

According to the logs, the mags have been overhauled in late 2017 (two owners before me) and are due for another overhaul end of 2021. In the work reports of 2017, I have not found a report on checking or adjusting the mag timing.

Thanks for any hints or advice.

PS. The pitot static system has been checked and was/is fine.

[Hank]

3 hours ago, Fry said:

The new idea: maybe my mag timing is retarded? Add to my perceived lower power a really cool running engine. In cruise, with cowl flaps closed the CHTs, are usually around 300-330.

Question to you: is there a way how I as the pilot can verify this hypothesis, without advanced equipment?

Check the data plate on your engine, see if it shows the timing to be 20° BTC or 25°. The former has lower temperatures and slightly less power, leading to what you have described. Seems there was an SB a few years ago to change the timing, and not everyone was happy with the results. 

[takair]

3 hours ago, Fry said:

Hello all, thanks for the discussion so far. I'm bringing this topic back up because I have a new idea. On two recent flights, I again perceived slightly lower engine power. I note that on takeoff roll the engine does not reach 2700 RPM (later in the flight, it does - so this is not due to the governor). Also, in cruise at 10k ft or so, the airplane is doing only 140 KTAS or so (well, I'm flying LOP, that _may_ contribute...).

The new idea: maybe my mag timing is retarded? Add to my perceived lower power a really cool running engine. In cruise, with cowl flaps closed the CHTs, are usually around 300-330.

Question to you: is there a way how I as the pilot can verify this hypothesis, without advanced equipment?

According to the logs, the mags have been overhauled in late 2017 (two owners before me) and are due for another overhaul end of 2021. In the work reports of 2017, I have not found a report on checking or adjusting the mag timing.

Thanks for any hints or advice.

PS. The pitot static system has been checked and was/is fine.

The only way I can think of to verify what the timing is set to is by doing a mag timing check with pretty basic equipment that all mechanics have.  To me, the best way to verify cruise speeds is by comparing equal power settings to other J models.  I would suggest getting a true airspeed reading at 5000 to 7000 feet, 75% power.......approximately 2500 RPm And 100 ROP.  I would expect a J model to be above 160ktas.....J model folks can give better numbers.  If you are making less than 150ktas at 75%, then there is a problem....with something.  Could be rigging, gear doors, engine....etc.