Test Pilot

Flying the M20K 231


 


Ed. Note: We continue our evaluations on the airplanes owned and flown by MAPA members with this report on Mooney's turbocharged alternative to the popular M20J.  The airplane is called the M20K, or the 231.  The 231 is definitely an airplane with more capability and performance than the 201, but it comes at a price of increased maintenance costs and the need for the pilot to stay on top of things when it comes to engine management. When it comes to deciding between a normally aspirated M20J and a turbocharged M20K, tough questions have to be asked about the type of flying to be done if the extra cost and complexity of the turbocharged M20K can be justified.

 

Is the added expense and pilot technique worth it?  Let's go fly an example of a stock 231 loaned to us by Jimmy Garrison and David McGee from the always large and first-class inventory at All American Aircraft here in San Antonio, Texas and find out.

   

 

History of the Model

In 1979, the Mooney factory introduced a turbocharged alternative to the popular M20J 201.  The airplane was called, quite logically, the M20K and its 231 nickname came from the fact that it could supposedly cruise at 231 miles per hour at best altitude and best power.  The airplane was an instant success, especially for those pilots flying out west with high MEAs to contend with and high density altitude airports to use.  However, the extra capability over the 201 did come at a price. 

 

Early -GB Engine Problems

The original 210 horsepower Continental TSIO-360-GB engine ran hot, both in climb and cruise.  Extremely high temperatures are hard on cylinders and the -GB did experience premature cylinder replacements.  The engine also had a fixed wastegate in the exhaust system, meaning careful pilot technique was required to keep from over boosting the engine on takeoff.  If the pilot inadvertently added full throttle for takeoff or a go-around, the engine would over boost, with only a mechanical pop-off valve in the induction system to save the engine from literally coming apart on the runway.  Fixed wastegates also mean high turbocharger speeds (RPM) at altitude, which reduces turbocharger life. 

 

Compared to the absolutely bulletproof Lycoming IO-360 engine installed in the M20J, the TSIO-360-GB engine in the M20K got a bad reputation.  The engine did run too hot (shame on Mooney's flight test department during those days).  But added maintenance costs are just a fact of life with a turbocharged engine compared to a normally aspirated one. 

 

The TSIO-360-LB is a Big Improvement

My very first job in 1983 as Mooney's newly hired engineering test pilot was to certify the 1984 model changes to the M20K.  These changes included the installation of a newer variant of the TSIO-360 engine, the -LB, which replaced the -GB.  The changes between the two engines were minor, but one of those minor changes proved to be a major improvement in engine life and longevity.  The throttle body on the -LB engine was a larger diameter than the one on the -GB.  This allowed more induction air into the cylinders, which helped to cool the engine.  In flight testing the -LB, we found a decrease of approximately 20 degree F on the cylinder and the oil temperature with the -LB installation.  This was huge. 

 

Continental recognized the improvement, so much so that any -GB engine that goes to the TCM factory for an overhaul gets changed to the -LB configuration automatically and the engine's data plate gets changed to reflect the upgraded -LB configuration.  If you're shopping for a 231, look closely at the engine's data plate.  If it says TSIO-360-LB, great.  If it says TSIO-360-GB, you've got an original hot running engine that will simply give you more trouble.  Move on to another one with the -LB engine. 

 

Other Changes to the Airplane Were Minor

Other model changes to the 231 were minor over the years, so there's really no reason to choose a later model airframe over an older one.  Much has been written about the smooth belly incorporated in 1984, but the hype overshadows reality.  I flight tested the smooth belly on the 231 prototype, and I can tell you that it made little difference in cruise speed- maybe a couple of knots.  I know it wasn't enough to change the performance charts in the POH.  However, it did make maintenance easier underneath the airplane since it is a whole lot easier to remove one belly piece than several.

 

The 231 soldiered on until 1986, when a huge model change year and many improvements to the M20K airplane resulted in the M20K 252, a wonderful airplane incorporating an intercooled and variable controlled TSIO-360-MB engine.  This engine was the perfected variant of the TSIO-360 series and it is a good one.  I did all the engineering flight testing of the 252 from first flight to final certification with the FAA, and I can tell you that from the start the 252 was just a wonderful airplane.  Everything worked just right.  But the 252 is such a different airplane than the 231 that it will get it's own pilot report in a future issue of the LOG.

Except for the installation of the LB engine in 1984, airframe changes to the M20K 231 were minor from 1979 - 1985.

 

Cruise Speed Comparison - M20J versus the M20K

First of all, let's get right to the big question. Is there really much cruise performance gain with the turbocharged 231?  To get the accurate facts, let's go to the pilot operating handbooks for the 1979 model M20J and M20K and compare "real world" cruise speed differences from the performance data.  And when I say "real world", I mean the way airplanes are really flown.  Most Mooney pilots cruise at 2500 RPM (or they should for prop efficiency, except in the newer M20M TLS/Bravo and M20S Eagle), so we won't be considering any of that 2700 RPM cruise speed data contained in the M20J POH.  Unfortunately, the cruise performance data in the POH for the 1979 M20J contains only 2400 or 2600 RPM data, so I'll use the 2400 RPM numbers.  The engineering test pilot in those days make the silly decision not to include 2500 RPM cruise data in the1979 M20J POH.  Why not?  The engine is perfectly happy at 2500 RPM and the prop is working at a really good efficiency in cruise.  Anyway, here's the data from the 1979 POHs:

 

Cruise Speeds - 1979 Model M20 J versus 1979 Model M20K

2400 RPM for the M20J and 2500 RPM for the M20K,

% Power as Noted, Economy Mixture Setting,

Standard Day Conditions, Both Airplanes at Gross Weight

Pressure Altitude

M20J % Power/ KTAS

M20K % Power/ KTAS

Advantage M20K

2000 ft

75%/159

75%/158

-1KTAS

6000 ft

70%/160

75%/163

+3KTAS

8000 ft

64%/157

75%/166

+9KTAS

10000 ft

60%/155

75%/169

+14KTAS

12000 ft

56%/151

75%/172

+19KTAS

14000 ft

52%/147

75%/176

+29KTAS

18000 ft

No data

75%/182

A lot

 

 

So the numbers are clear and they don't lie.  The M20K is simply the faster airplane when flown the way we really fly Mooneys.  Cruising your M20J at 2500 RPM will add about 3 KTAS over the 2400 RPM data shown above (and for all you J model owners you should use 2500 RPM in cruise).   But even then, the K model begins pulling away from the J at 6000 feet or so and never looks back, even at non-oxygen altitudes (below 12,500 feet).  Lots of Mooney pilots don't realize just how much faster an M20K is over an M20J, but it's there.  And it's all because of the ability of the turbocharged engine in the M20K to deliver 75% cruise power at increasing altitudes.

 

How About Climb?

An even bigger advantage goes to the M20K, as expected.  Again, from the 1979 POH for both the M20J and the M20K, let's compare full power, Vy climb performan

ce between the two airplanes.  Again, this is for comparison purposes only.  In the real world, it's silly to climb a Mooney to altitude at Vy.  Speed should be Vy plus 10, 20 or even 30 if you're light.  But for our comparison, let's go straight to the POH for the Vy numbers.

 

Climb Performance Comparison - M20J versus M20K
Full Power, Vy, Max Gross Weight, Standard Dry

Density Altitude
M20J Rate of Climb-FPM
M20K Rate of Climb-FPM
2000 ft
920
1060
6000 ft
720
1030
10000 ft
520
1000
12000 ft
420
990
14000 ft
320
970
18000 ft
120
730


The bottom line on climb performance?  The J model is adequate until 10000 feet of density altitude or so, so the difference between it and the M20K isn't so great.  After that, it falls so far behind the turbocharged M20K that it's no contest.  This translates into being able to fly anywhere in the western USA with no sweat in the M20K, where the M20J is marginal above 10000 feet.  This translates into being able to aggressively tackle high density altitude airports in the M20K while the M20J is setting on the ground waiting for cooler temperatures.  This translates into always being able to climb to 10000-12000 feet comfortably in the M20K in the summer to find cool smooth air while the J model is struggling to get out of the heat and bumps.  Climb performance differences like this mean a whole lot.

 

But There's Never a Free Lunch

So, the performance gains of the M20K are significant.  It's faster and it climbs better.  But at what cost does this extra performance come?  Unfortunately, it's not cheap.  The major cost difference, naturally, comes in front of the firewall.  Both engines generally make TBO (2000 hrs for the M20J, 1800 for the M20K), but the M20K will need at least one set of cylinders to make it there.  I have to tell you that most 231 owners we hear from require two sets of cylinders to make TBO.  The TSIO-360-GB or -LB also requires two turbocharger overhauls or replacements to make TBO.  Let's look at these added costs:

 

Additional Cost to Operate an M20K 231 over an M20J 201 From Overhaul to Overhaul

Item

M20J

M20K

Additional Cost M20K

Cylinder Replacement

n/a

6X$1200

$7200

Turbocharger Overhaul

n/a

2X$2500

$5000

Magnetos, Vacuum Pumps

$2000

$4000

$2000

Routine Eng. Maintenance (6 cylinders vs. 4)

$3000

$4500

$1500

Overhaul Cost (FWF)

$2500

$32000

$7000

Total

$22700

 

Take that $22700 and divide it by 1800 hours and you'll come up with the hourly extra cost to operate a 231 over a 201 - $12.60 per hour extra.  From the feedback we get here a

t MAPA, I think that's about right.  If a second set of cylinders is required on the 231 between overhaul, it would run the number up to $16.60 per hour extra.  Again, that sounds about right from what we hear.  Every hour flown on a 231 costs $12.60 more than the same hour flown in the 201.  Keep that in mind when deciding on which airplane it is you want to own and fly.

 

Let's Go Flying

  

For our flight, we found a very nice M20K 231 in All American's inventory to fly.  The airplane was N3901H, a very stock 1980 model.  The engine had been upgraded to the TSIO-360-LB configuration and was without any aftermarket intercooler or wastegate installation, something we'll talk about later.  The only non-standard item on the airplane was the installation of a three-blade propeller in place of the standard two blade McCauley one.  I liked the three-blade prop, the airplane ran very smooth, takeoff and climb was strong and the airplane wasn't affected too negatively in cruise. 

Externally, the 231 looks a little better than the 201 because of the longer nose.  The fuselage on the K model is 25'5" long, the J model is 24'8".  The J model, with the shorter four-cylinder Lycoming engine, looks a little chopped off up front.  Other than that, from the firewall aft the J and the K model are almost identical. 

 

Like all Mooneys, the 231 sets low to the ground.  Bonanzas and Pipers look better on the ramp with their tall stance, which passengers like.  But Mooneys look faster, and that's what really matters the most.

 

A three-bladed prop was installed on our test airplane, N3901H.

 

Interior and Instrument Panel

Instrument panel layout is good in the M20K

Once seated inside, it's typical Mooney, meaning you sit low to the floor with your legs almost straight out.  But the legroom is superb, especially if there is no one behind you allowing you to slide your chair back.  You sit close to the panel in all Mooneys, so if you need reading glasses, have them handy.  The panel is not far from the tip of your nose (making the standard shoulder harnesses in the 231 mandatory in case of a sudden stop).  And speaking of the panel, the M20K has the old shorter instrument panel design that was easy to see over.  The newer Mooneys (M,R,S models) all have the taller version of the instrument panel that give you a submarine effect when trying to see over the nose.

 

The panel layout on all K models is good.  Basic "T" instruments are right in front of the pilot with avionics in the center.  Spillover avionics are located on the right side of the panel along with the engine power instruments.  The engine instruments are pretty far from the pilot, but at least they're on a canted panel for easier viewing.  And the pilot of a 231 needs to be looking at the engine instruments often in flight.  There is a lot of fiddling around with the throttle (manifold pressure gage) and mixture (TIT gage) in a 231, so your eyes are looking at those gages often.

 

Throttle, prop and mixture controls are center and low - just right.  The throttle is the push/pull type while the prop and mixture controls are vernier.  Some 231 owners replaced the push/pull throttle with a vernier.  Don't do this.  It's much easier to regulate manifold pressure with the push/pull control that it is with the vernier on the 231.  Only on the 252 with its much better and smoother engine controlling system does the vernier throttle work at its best.

 

Engine Starting    

The TSIO-360 series engines in the Mooney are some of the easiest to start, hot or cold.  All the engines are equipped with an engine priming system, which squirts fuel directly into the induction system.  An old test pilot once told me to use the electric boost pump for better results in getting the engine started, especially with very cold outside air temperatures.  With the electric boost pump instead of the primer, you're squirting fuel directly into each cylinder instead of into the induction system with the primer.  But I never had any trouble starting the TSIO-360 series engines n the Mooneys.  Throttle open 1/2 inch, mixture full rich, prime with the primer for 3-5 seconds and engage the starter.  Works every time, hot or cold.  One the rare occasion I miss the start, it's generally because I under-primed.  Another 3-5 seconds of priming and the engine generally starts.  If the engine stumbles immediately after firing, be ready with another shot of prime to keep it running until it smoothes down.

The longer nose of the K model makes the airplane look a bit better balanced compared to the J.

 

I can think of only two or three starts where I flooded the engine.  In that case, it's throttle full open, mixture to idle/cutoff, then engage the starter.  When the engine fires, you need to be lightening quick on the throttle reduction while moving the mixture control to full rich.

 

Lean the Mixture on the Ground

Almost every turbocharged Mooney I fly these days have engines that aren't set up properly, meaning that the engines generally run way too rich with the mixture control in the full forward position.  Too much fuel is going through the engine for the power being developed.  At takeoff power, this overly rich mixture shows itself as low TIT indications, low power output and in the worst of cases, black smoke coming from the exhaust.  At idle power, an overly rich mixture shows itself as a rough running engine while taxing.  The engine just doesn't sound right on the ground and taxing around with the mixture control pushed in full rich will quickly foul the spark plugs.

 

Here's how to check ground idle speed and mixture setting.  After a flight and with the engine nice and warm, set the brakes and accelerate the engine to 1500 RPM or so for a few seconds to clear it out.  Then slowly close the throttle to the full aft position.  Idle mixture RPM on the tachometer should be somewhere between 650 and 750 RPM.  With the throttle closed, slowly pull the mixture control aft towards the cutoff position.  Note the tachometer reading as you do this.  The indication should increase slowly from idle speed.  The correct increase, called idle mixture RPM rise, should be about 50-75 RPM. 

 

If it's less, you're a little lean with the mixture setting at idle power.  But if it's greater than 50-75 RPM, it's too rich.  And a mixture setting that is too rich will foul the plugs and cause a stumbling engine on the ground.  So, correct idle RPM is 650-750.  Correct idle mixture rise should be 50-75 RPM.  Check yours.  If either or both or out of these ranges, get your mechanic to reset them.

 

I find most of the 231's I fly set too rich.  So what I do is to aggressively lean the mixture after engine start to obtain a smooth idle.  And I mean aggressive, I sometimes have to lean the mixture control almost 1/2 way out to obtain smoothness at idle.  The key here is to be as aggressive as you want to be with leaning the mixture on the ground.  You can't hurt anything with aggressive leaning during taxi.  Your engine will love you for this. Be good to your engine - lean on the ground.

 

The pilot's field of view is very good over the shorter instrument panel.

Pre-takeoff Checks

CIGAR (Controls, Instruments, Gas, Attitude, Runup) covers it all for me, with the exception of the avionics.  A good abbreviated checklist is generally posted low on the center console.  Use it in the M20K and you won't forget anything important.  Just don't forget to enrichen the mixture before engine runup and takeoff.  Keeping it lean from taxi will result in not enough fuel flow for adequate power for takeoff - the primary reason we didn't include leaning the mixture for ground operations in the POH back in my flight test days at Mooney.  

 

Flaps for Takeoff?

Flaps for takeoff are your option.  There is a takeoff flap setting marked on the flap position indicator.  It's for 10 degrees and setting them there is in the Normal Procedures section of the POH.  But don't think their use is mandatory.  As a matter of fact, I like the way the airplane takes off with the flaps up.  The rotation is a little bit more pronounced and there is one less thing to do (retract the flaps) during the initial climb.  My personal technique is this - unless the runway or density altitude dictate their use (less than 3000 feet for the runway or greater than 5000 feet for the density altitude)), I usually take off in a 231 (or any other Mooney) with the flaps full up.

 

Proper Engine Power at Takeoff

Onto the runway and it's time for the most important function for the pilot in a 231 - properly setting and limiting the throttle to a maximum of 40" manifold pressure.  Remember, with a fixed wastegate control, the pilot becomes critical in setting engine power.  Ram the throttle in and the TSIO-360-GB or -LB will overboost.  Then the pilot will generally get in a see-saw battle with the throttle, increasing and decreasing it throughout the takeoff in a loosing battle to properly set and nail the correct manifold pressure.

 

The secret is a slow addition of throttle during the initial takeoff roll to about 35" or so.  Let the airplane roll for a few seconds and gather some speed, then add the remaining small amount of throttle to get the manifold pressure to the correct takeoff setting of 40".  The final 5" or so of power is best adjusted after the airplane is rolling, not with it static with the brakes on.  So, set 35", let the airplane begin rolling, then set the power to 40".  You'll like this technique.

 

During the takeoff roll, I like to note two other indications of engine power and adjust the controls accordingly.  What I'm looking for is the proper amount of fuel going through the engine at maximum power.  Again, I find almost every 231 too rich.  Correct readings for takeoff are a TIT (turbine inlet temperature) or about 1400 degrees F and an indicated fuel flow of 22.0-24.0 gallons per hour.  More than 24.0 gallons per hour and the engine is set too rich at takeoff power and the power output will be down will be down.  A TIT indication less than 1400 degrees is another indication that the takeoff fuel flow is too rich.  And TIT is generally the more accurate of the two readings.  Don't hesitate to manually lean the mixture in the takeoff roll to obtain 1400 degrees F.  You won't hurt anything and you're getting the TSIO-360-GB or -LB engine set for correct horsepower, critical for good takeoff and climb performance.

 

A Steady Aft Pull on the Control Wheel During the Takeoff Roll

After setting the power, another good hint with the M20K in the takeoff roll is a small, steady pull (about 5 pounds) aft on the control wheel to keep the nose gear light and

the airplane in the proper attitude for takeoff.  Most Mooneys do much better in the takeoff roll with this slight aft pull on the control wheel, because most of the time we're taking off with the airplane near the forward CG limit.  I've actually seen Mooneys get up on the nose gear or begin hopping near rotation speed.  With an aft pull on the wheel, this won't happen.

 

Normally climbs should be done at 40", 2700 RPM, 1400 TIT and 110-120 KIAS.

Climbing to Altitude

After liftoff at 65 KIAS or so, it's gear and flaps (if used) up and accelerate to 110KIAS or so for the climb to 1000 feet AGL.  Power remains full (40", 2700 RPM, mixture set to 1400 degrees TIT which gives 22-24 GPH or so).  Do not reduce power for the climb.  It does nothing for engine longevity and kills a big portion of your climb performance during these critical first few minutes after takeoff.

 

At 1000' AGL, I transition to 120 KIAS for the remaining climb to altitude.  I do not reduce engine power.  The TSIO-360 engines in the Mooney are rated for 210 horsepower all day long.  Reducing power in the climb is a silly waste of performance and an inefficient way to climb to altitude.  In the 231, keep the manifold pressure at 40", the RPM at 2700, the mixture set for 1400 degrees TIT, the cowl flaps full open and the airspeed at 120 KIAS.  Certainly, monitor the cylinder head temperatures and oil temperatures during climb and make sure they don't get within 1/8" or so of redline on the gages.  But at 120 KIAS, they shouldn't, even with an old -GB engine.

 

Our test airplane, N3901H, was climbed in this manner for 20 minutes to altitude on a rather warm day.  We monitored climb performance along with engine temperature readings as the climb to altitude progressed.  Here is the data and performance we obtained.

 

Observed Rate of Climb Data

1980 Model M20K  N3901H

40"MP, 2700 RPM, Mixture leaned to 1400 degrees TIT,

Cowl flaps full open, 120 KIAS

Time

Press. Alt. Feet.

OAT C

Fuel Glow GPH

Rate of Climb FPM

0:00

2000

16

22

--------

1:00

3100

16

22

1100

2:00

3950

17

22

850

3:00

4860

16

22

910

4:00

5700

16

22

840

5:00

6550

16

22

850

6:00

7310

15

22

760

7:00

8100

14

22.1

790

8:00

8850

12

22.1

750

9:00

9620

9

22.2

770

10:00

10440

8

22

820

11:00

11300

6

21.8

860

12:00

11960

4

21.8

660

13:00

12650

2

21.8

690

14:00

13420

1

21.7

770

15:00

14150

0

22

730

16:00

14750

-2

22

600

18:00

16360

-4

22

805

19:00

16920

-6

22

560

20:00

17630

-6

22

710

 

Those are excellent times to climb to altitude for a turbocharged airplane with only 210 horsepower.  Average rate of climb during the test was 868 FPM.  Admittedly, we were light for these tests, about 300 pounds under gross weight with only me on board, full fuel and 50 pounds of equipment.  But keep in mind this test was flown at a constant 120 KIAS, which is Vy plus 20-30 KIAS.  Higher airspeed is the way to climb a Mooney.  Engine cooling is better, you can see over the nose better and more horizontal distance is covered during he climb at the higher airspeeds.

 

How Fast Was the Test Airplane

I tried two different altitudes to evaluate cruise performance using the four-way GPS method (to see a discussion on how to determine the aircraft's true airspeed using this method, see the most recent pilot report on the M20J in the January 2001 issue of the LOG or look for the report on our Web site at www.mooneypilots.com).  One was low, 8500 feet, to look at the airplane's level flight performance at a non-oxygen altitude.  The other was higher, 17,500 feet, to look at performance up where the air is smooth and cool. 

 

The power setting used for these cruise performance tests is one that I have used consistently over the years in stock (non-intercooled) 231's.  Day in and day out, regardless of altitude and OAT, I have found this power setting to give optimum levels of aircraft performance, fuel economy and engine longevity: 

 

Suggested Cruise Power Setting - Model M20K (231) Non-Intercooled Engines

Any Altitude and OAT Manifold Pressure

RPM

Mixture Setting

Cowl Flaps

Approx. Fuel Flow

31" Hg

2500

Peak TIT

+ 50 deg. Rich

Full Closed

11.5-12.5 GPH

 

 

 

From flight testing heavily instrumented airplanes M20K prototypes for several years, I can say that this power setting with the stock TSIO-360-GB or -LB is the best.  The airplane will give you good speeds.  Fuel flows will be reasonable.  And the engine will always be operating within parameters.  Here is how our test airplane performed during cruise using this suggested power setting:

 

Level Flight Cruise Performance   N3901H

31" MP, 2500 RPM, Leaned to Peak TIT + 50 degrees rich, Cowl Flaps Closed

Pressure Altitude

OAT C

Direction

GPS Groundspeed KTS

8500

16

N

144

8500

16

E

172

8500

16

S

179

8500

16

W

156

Average GS/KTAS

180.75

17500 -3 N 171
17500 -3 E 194
17500 -3 S 194
17500 -3 W 164

Average GS/KTAS

180.75

 

How do these number compare to those in the POH?  Here's the data:

Altitude

Predicted POH Cruise Speed

Test Aircraft Cruise Speed

8500

169 KTAS

162.75 KTAS

17500

180 KTAS

180.75 KTAS

 

 

So we missed the predicted book value by 6 KTAS down low but pretty well matched it at 17,500 feet.  But that is to be expected when flying a fixed manifold pressure and propeller RPM setting regardless of altitude.  31", 2500RPM and 50 degrees rich of peak TIT is simply less horsepower at 8500 feet than it is at 17,500 feet.  So, I give up a little cruise speed at the lower altitudes with my fixed cruise power setting.  But I can't stand to go through the silly exercise with turbocharged engines of looking at those power setting charts and adjusting manifold pressure for non-standard OAT and altitude.  Trying to set manifold pressures exactly using the power charts is a silly waste of time.  You can't even read a manifold pressure gage to the nearest 1/10 " Hg anyway (just try to set the manifold pressure in the 231 to 31.8", for example). 

 

So with any turbocharged engine, my personal suggestion is to pick a power setting that you can live with from an aircraft performance and fuel economy standpoint that is within the engine's operating parameters at all altitudes and fly that setting all the time.  For me, I find the best power setting on the M20K 231 (stock) is 31", 2500RPM and 50 degrees rich of peak TIT.  That's the setting I use all the time.

 

How About Aftermarket Intercoolers and Merlyn Wastegates?

These are the two most popular options on 231 airplanes.  Intercoolers are radiators placed in the induction system downstream of the compressor section of the turbocharger.  An intercooler's job is to cool the induction air after it was pressurized (compressed) by the compressor side of the turbocharger.  A really good intercooler installation can cool the induction air by as much as 100 degrees F.

 

So what do you do for power settings if you have a 231 with an aftermarket intercooler installed.  I can tell you one thing you better not do - use the original power setting charts for your engine for setting power.  With an intercooler, a reduction in manifold pressure is required to keep from over-horsepowering the engine.  This reduction is necessary for both takeoff, climb and cruise and technically should vary as a function of the amount of cooling being generated by the intercooler.

 

Again, if you don't make this adjustment downward in manifold pressure settings with an intercooler, you'll be pulling more horsepower than allowed from the TSIO-360 engine by Continental.  Aircraft performance will be up if you fly original manifold pressure settings, but so will fuel flow.  One thing will be down -engine life expectancy.  You can't pull extra horsepower from the TSIO-360 without repercussions to engine reliability and reductions in engine TBO.

 

Here's what I tell MAPA members who have an intercooler installed on their 231 - find a M20K 252 power chart and use it for your engine.  Takeoff and climb your intercooled  231 at 36" manifold pressure, 2700 RPM and 1400 degrees TIT.  Cruise that same airplane at the power setting we designed the 252 to be cruised at, which is  28" manifold pressure, 2500 RPM, Peak TIT plus 50 degrees rich.  Use these settings and your intercooled 231 will be a happy camper and a good performer.

 

So how about airplanes with a Merlyn wastegate installed only.  No manifold pressure adjustments are required.  Fly original POH settings.  This means takeoff and climb at 40", 2700 RPM, 1400 degrees TIT.  Cruise at 31", 2500 RPM and peak TIT plus 50 degrees rich.  The Merlyn wastegate will result in lower turbocharger spinning speeds for a given manifold pressure, but its installation (without an intercooler) will not require manifold pressure reductions.  Only an intercooler installation requires that.

 

Time to Come Down - Keeping the Engine Warm in Descent

Much has been written about descending a 231 and proper engine management in descent.  Many MAPA members go to great lengths to make very gradual reductions in power (like no more than 1" manifold pressure per thousand feet) to make sure the engine stays warm enough during descent.  And that's fine - it certainly won't hurt anything to do this.

 

But engineering flight test data shows a much simpler approach to making descents in the 231 to keep everything warm.  If you'll simply make enroute descents at 20", 2500 RPM and lean to peak TIT, you'll find the TSIO-360 series engines in the Mooneys stay plenty warm and don't run the risk of shock cooling.  20" and 2500 RPM gives good airplane performance for the downhill ride to the airport (expect 150-160 KIAS and 500-800 FPM down).

 

Many pilots ask what is the absolute minimum power for descent in the 231 (or any other Mooney).  In engineering flight test, we often flew with a torquemeter installed between the engine and propeller.  The torquemeter measured torque (power) being developed to the prop at all times.  An interesting observation when flying with a torquemeter was the power setting where the torque went to zero and then negative - in other words, the power setting where the engine was no longer powering the prop, but the prop began to power the engine. 

 

For all the Mooneys I flew, that occurred at 15" manifold pressure. At 15" and below, you could hear the prop start to drive the engine (a whooshing sound) and you would see the cylinder and oil temperatures start to nosedive.  So, when people ask what is the absolute minimum power setting they should use in an enroute descent, I say it's 15".  Stay above 15" in your Mooney in descent and the engine will always be driving the prop and not shock cooling.  Below 15" and the prop will be driving the engine - hello shock cooling.

 

Into The Pattern - Landing Characteristics of the M20K

The K model is a good landing airplane.  Into the pattern, I like to fly the downwind at 100-120 KIAS.  Opposite the touchdown point on downwind, it's gear down and flaps partially down (to the takeoff setting of 10 degrees).  The reason for partial flaps on downwind instead of full flaps is that the M20K has a very strong tendency to get nose heavy with flap deflection.  Running the pitch trim in the nose up direction as the flaps are deploying is an old Mooney trick to counteract this nose heaviness tendency with flap deflection, but the trim won't quite keep up with the pitch change.  So expect a big-time nose heavy condition as the flaps deploy on the M20K.  And be running the pitch trim in the nose up position as the flaps come down.

 

On base, I try to slow at 100 KIAS.  Power setting is as required for the proper descent to the runway.  You can forget the 15" manifold pressure limit to avoid shock cooling at this point - it's time to land.  On base, it's the rest of the flaps down (big nose heavy pitch change, be running that electric pitch trim nose up as the flaps come down).  On base, do a GUMP check to confirm gear down, but don't blindly push the mixture control all the way to full rich.  Most 231 airplanes are set way to rich at idle power, and you'll flood the engine with excessive fuel at this point.  Just set the mixture for smooth idle in the pattern.

 

Final approach is time to decelerate from 100 KIAS to 80 KIAS, with the objective being an airspeed of 1.2Vso as you begin to flare.  As we've spoken about in the past, this is where most Mooney accidents and incidents take place - during the landing flare.  The problem is one of excessive airspeed, and a Mooney that's flared too fast is a floater.  Down the runway it goes, not wanting to land.  The pilot panics, then pushed the wheel forward to plant the airplane on the runway.  You never, ever, push the control wheel forward in a Mooney during the flare.  If you do, sooner or later you're going to 1) bust a propeller, 2) bust a nose gear of 3) lose control and go off the runway.

 

Proper speed to begin flare in a 231 is 70 KIAS.

Proper Flare Speeds in Our Test Airplane

To get accurate target flare speeds (1.2 Vso), we did a series of power off stalls at altitude to determine accurate indicated airspeeds at the stall buffet.  We did these stalls at flaps up, flaps takeoff and flaps full down.  Here is that stall speed data:

 

Indicated Stall Speed Data - N3901H

Mid Weight, Mid C.G., Power Off

Flaps

Gear

Stall Warning

Stall Speed-Vs

Flare Speed-1.2 Vs

Up

Dn

73MIAS/63KIAS

68MIAS/59KIAS

82MIAS/68KIAS

Takeoff

Dn

70MIAS/63KIAS

63MIAS/55KIAS

76MIAS/66KIAS

Full Dn

Dn

65MIAS/57KIAS

60MIAS/52KIAS

72MIAS/63KIAS

 

How did those flare speeds work on a series of landings?  The flaps up number worked great - plenty of energy left for a good flare and even a little float.  Surprisingly, the flaps takeoff and flaps full down flare speeds felt a little bit slow.  It was a gusty day and hot, so maybe that had something to do with it.  Regardless, there wasn't adequate energy left for a comfortable flare using the flare speeds for takeoff and full flaps.  I added 5 KIAS to these speeds and everything worked out really well. 

 

So the bottom line is this.  Try entering the landing flare at 70 KIAS in the 231.  You'll find good energy left for a comfortable flare and short landing distances.  And you'll eliminate the number one accident cause in the MAPA community - excessive airspeed during the landing resulting in the airplane getting up on the nose gear and striking the propeller on the runway.

 

Go-Arounds

One hint with the M20K.  Remember we mentioned the large pitch trim change the  airplane goes through when the flaps are extended?  The wise and experienced Mooney 231 pilot is the one who knows to lead any flap extension with nose up elevator trim through the electric trim switch on the control wheel. 

 

But the situation exists in reverse in the event of a go-around.  Add full power to go around with the flaps down in the M20K and there is a sudden pitch force change in the nose up direction.  And it's significant - almost 20 pounds of push required to keep the nose in the proper climb attitude until the flaps can be retracted or the airplane retrimmed (which seems to take forever).  A 231 pilot needs to be aware of this characteristic in the airplane and plan accordingly. 

 

One thing I do to keep this from happening in IFR is never approach and land with full flaps in the 231 when the weather is really low.  If I miss the IFR approach and add full power to go around with full flaps, I'm faced with a big nose up pitch change to take care of in a very crucial part of the procedure - the initial climb.  So experienced M20K pilots do not use full flaps for landings in IFR conditions.  A go around would be a handful.

 

Other Hints Operating the 231

Here are a few other hints taken from my engineering flight test days with the M20K 231.  The 231 is an airplane that has a primary engine air induction system that is prone to forming ice on the face of the induction air filter. We found this problem after I flew icing tests in northern Canada with our prototype and test M20K in 1984. We found that the original manually activated alternate air door mounted underneath the turbocharger was not an adequate solution to providing the engine a secondary source of induction air.  The alternate air design was okay, it was the pilots who didn't recognize induction system icing quickly enough and didn't pull open the alternate air door in time to keep the engine running. The solution was a new design for the alternate air system that mounted a box on the firewall with a door that opens automatically for alternate air.  The factory made the decision to offer this redesigned alternate air system free to all 231 owners in 1984.  If your airplane has not been upgraded over the years to the firewall mounted alternate air door, get one now. This system is a huge improvement in safety and engine performance.   

 

One other hint from that trip to Canada with the instrumented 231 test airplane - we got lots of airframe ice while looking for engine induction system icing.  We learned more about the handling characteristics of the 231 with airframe ice during that trip than ever before.  We found that the 231 lost airspeed in cruise at the rate of about 10-15 KIAS every 1/2 inch accumulation.  The airplane would handle about 1 inch of airframe ice on the leading edges, but after that the indicated airspeed was down low enough that the airplane wasn't pleasant to fly.                         

 

But the big thing we learned concerned landing with any ice accumulation on the airframe.  We found that landing with ice would cause a significant tail buffet condition if the flaps were used.  There was no buffeting present during he approach and landing with up to 1 inch of airframe ice if the flaps were kept retracted.  The only precaution was to keep the speed up - I held 90KIAS to the landing flare and it seemed to work out okay with 1 inch on the airplane. 

 

But lower those flaps for landing with ice on the leading edges of the wings and horizontal tail and there would be a considerable airframe or tail buffet beginning around 90 KIAS and lower.  So we recommended that any approaches and landings in the M20K with airframe ice accumulation be done with the flaps retracted. 

 

The M20K 231. A superb high altitude flyer.

Summary

So that's a look at the M20K 231.  It's a great airplane that gives the pilot significant levels of improved performance over the M20J 201.  Maintenance costs are higher with the M20K, significantly higher.  Pilot responsibilities are higher to keep the engine happy and well managed.

 

Is it worth the extra cost and complexity to own a 231 over a 201.  If all your flying is 10000 feet or lower, the answer is no.  But if you go above 10000 feet regularly, get a 231 over a 201.  Above that altitude, the differences are definitely worth the extra cost.