Descent power setting

[rbp]

I usually descend in favorable conditions at 25/24, but I am wondering if anyone is descending at a lower prop settings to keep the airspeed under control

[201Steve]

I usually just make incremental adjustments to stay out of the yellow, conditions permitting. I don’t have a fixed setting. MP, prop, or pitch. Whatever feels right. 

[midlifeflyer]

IMO, there are two reasons to reduce power in the descent in any airplane. Airspeed control and engine cooling. If you are not concerned about either, no reason not to descend at  cruise power.
 

[rbp]

3 hours ago, midlifeflyer said:

IMO, there are two reasons to reduce power in the descent in any airplane. Airspeed control and engine cooling. If you are not concerned about either, no reason not to descend at  cruise power.
 

I'm not sure how this addresses my question regarding prop pitch

[hammdo]

On my C I use 2300 or 2400 RPM and 19 or 20 inch’s MAP to help manage my descent. Keeps me very close to cruising speed… but I do have speed mods…

-Don

[rbp]

38 minutes ago, hammdo said:

On my C I use 2300 or 2400 RPM and 19 or 20 inch’s MAP to help manage my descent. Keeps me very close to cruising speed… but I do have speed mods…

-Don

I'm not sure how this translates to a Bravo?

[hammdo]

I try to use the Van's Key # from the MAPA MSF course -- I believe those are defined for each model and I adjust those number to get to a 'low' or slightly below low cruise setting. Looking at the guide for a C model the 55% power is Key # 43 - so 2400 RPM and 19" (or 2300/20) would be 43

The Bravos (M20M) Van's key # is 50 (low cruise)  so 25/25 or 26/24  -- if that is not working then adjust the 'key #' to say 50% power - around Van's Key # of 47

2400/23 or 2500/22  may even try Van's Key # of 46 (2400/22 or 2500/21) of course I don't know if you have an RPM restriction.

The MAPA guide for enroute decent in a Bravo is 2000 RPM and 25". That is Key # 45...

The Key #'s in the MAPA guide are pretty accurate too...

-Don

[rbp]

1 hour ago, hammdo said:

Van's Key # from the MAPA MSF course

interesting. where do I get this? 

>The MAPA guide for enroute decent in a Bravo is 2000 RPM and 25"

this is exacly what I use, adjusted for the yellow arc and turbulence

[hammdo]

Take the MAPA Safety Course or see if someone has a copy still (after they sold their Mooney) -- it has a whole chapter on settings for all series of the M20 with recommended settings for all the different phases of flight.  I wish they'd sell the manual as the information is great but, I guess its an incentive to take the course.  I have yet to find a PDF version anywhere...

-Don

[hammdo]

@rbp Here at the summary settings:

 

[rbp]

those don't make any sense to me at all

Cruise climb @34/24 @1000FPM and 130KIAS?  no way

En Route Descent @165+ ? redline is 165 at Sea Level. Much lower at an altitude at which you'd do a high-speed descent

[hammdo]

I've never flow an M/Bravo so I can't comment on those number -- hopefully a Bravo/M owner who took the MAPA class can comment...

The #'s for the C (summary like the M's) are pretty accurate -- I have the original MAPA manual (from 1992) and I see those numbers have changed for the M since then...

-Don 

[midlifeflyer]

3 hours ago, rbp said:

I'm not sure how this addresses my question regarding prop pitch

Sorry misread. Could have been better to say

 

6 hours ago, midlifeflyer said:

IMO, there are two reasons to reduce change power configuration in the descent in any airplane. Airspeed control and engine cooling. If you are not concerned about either, no reason not to descend at  cruise power  settings.

You mentioned reducing RPM to control airspeed. Reducing RPM moves the blades to a coarse pitch/less drag setting. The recommendation of many to reduce to 2000 RPM allows for increased airspeed in the descent - let's gravity do its work with reduced drag. The combo of reduced MP and RPM gets an efficient descent. 

Edited September 20, 2022 by midlifeflyer

[jetdriven]

i dont think aerodynamics works that way in this instance. The prop is still pulling the airplane forward.  At 2000 RPM, all else equal, its less HP at the crank which aids descent.

[N201MKTurbo]

I usually descend at cruise power unless it’s bumpy, then I reduce power to maintain cruise airspeed.

[irishpilot]

cruise power and 500 fpm. Used my 530W descent point to hit my target alt at destination and enjoyed the increased speed.

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[Bolter]

If the Bravo has a target minimum CHT like the Ovation, you may need to run at peak EGT to keep the cylinders warm at reduced power in a high speed descent.  Just one more variable to keep in mind.  

[carusoam]

500fpm descent is helpful…

400fpm descent is better for getting the altitude back as forward speed…

Assuming smooth air….

Either way…. You probably need to pull the power back significantly if using 500fpm…

See if you can catch that MAPA class…

Those numbers were generated based on actual CFIIs that fly those planes…

If something doesn’t make sense… it makes a great discussion in that class….

Best regards,

-a-

[Danb]

North east MAPA class in Reading weekend of October 21

[irishpilot]

500fpm descent is helpful…
400fpm descent is better for getting the altitude back as forward speed…
Assuming smooth air….
Either way…. You probably need to pull the power back significantly if using 500fpm…

You do not need to pull power for 500 rpm decent in smooth air. At least not in a FIKI Bravo. It’s pretty self-evident if you need to pull power or not real-time. If you’re uncomfortable going into the yellow or it’s rough, then pull power.


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[carusoam]

15 hours ago, irishpilot said:

You do not need to pull power for 500 rpm decent in smooth air. At least not in a FIKI Bravo. It’s pretty self-evident if you need to pull power or not real-time. If you’re uncomfortable going into the yellow or it’s rough, then pull power.


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IP,

What %hp are you typically using…?

 

I have the added challenge of my throttle set at WOT…. And MP increasing…. Going further LOP on the descent….

Using 400fpm descent typically netted more speed over a longer distance…

The 500fpm descent had the limit of the bumps getting closer to the ground…

The LBs got nice maneuvering speeds….  
 

Best regards,

-a-

[donkaye]

On 9/20/2022 at 10:40 AM, midlifeflyer said:

Sorry misread. Could have been better to say

 

You mentioned reducing RPM to control airspeed. Reducing RPM moves the blades to a coarse pitch/less drag setting. The recommendation of many to reduce to 2000 RPM allows for increased airspeed in the descent - let's gravity do its work with reduced drag. The combo of reduced MP and RPM gets an efficient descent. 

Referring to "Key" numbers:  3 of any combination of MP in inches or RPM/1000 is approximately 10% power when ROP.  As opposed to gliding, turning down the RPM is analogous to shifting down in a car.  The engine is running the prop, not the other way around.  When under power the engine should run the prop for less wear and tear on the engine.  When gliding, as in engine failure, you'd like to reduce drag, and in that case you'd like as little drag as possible and the prop as close to feather as possible.

As mentioned above in another post, it's efficient to go as fast as practical in the descent to make up for the loss of time in the climb to altitude.  So, if it's smooth, I'll descend in the yellow arc, otherwise the top of the green arc or maneuvering speed, if turbulence is a factor.  Regarding the 165 knot limitation in the Bravo, that's with the gear down, and is an irrelevant number in the normal descent, since the gear will be up.  Until in the pattern, the MP should never be set less than 15" to make sure the engine is running the prop.  Slope of descent makes a difference in power management.  A "slam dunk" should be flown differently than a normal descent.  So, for efficiency, I'll descend as fast as practical when VFR, and that means knowing the rate of slow down of our airplane and importantly managing your fastest cooling cylinder head temperature to keep it below the 50°/minute limitation on our engines.  I have it as a data field on my MVP-50 engine monitor.  When I do start reducing power, I'll gradually reduce the MP to 20", then reduce the RPM to 2000.  The MP will increase a couple of inches with reduction in RPM.  I'll be monitoring the fastest cooling CHT to keep it below the 50°/minute  limitation.  Then I will further reduce the MP to 15".  If a faster descent rate is then necessary, I'll add speed brakes, but I leave that to last because of the inefficiency it produces if you add them too soon and then have to add power because your descent rate was too great.  Of course the gear comes down in the pattern, but adding the gear too early is VERY inefficient.

In my opinion this is the way a descent should be managed on all the Mooneys (less speed brakes if a Mooney doesn't have them), no matter what has been written by anyone less familiar with the Mooney aircraft.

[irishpilot]

IP,
What %hp are you typically using…?

29/2400 ROP. I never got my Bravo to run LOP but didn’t really work on doing that, either. Disclaimer - I no longer have that plane.

I use the same planning and descent rate with my Lancair ES (IO-550). I descend 500 fpm and stay LOP until I get to 20” MP and then I enriched as I’m about 5 mi from the airport. Believe it or not, but the fixed gear ES is very slippery and I have to be 15” MP or less to get it slowed down to the white arc.


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