M20j 201 top speed

[carusoam]

1) Light weight... empty all the excess cargo... use only the required fuel with safety limits... excess fuel has a tendency to need the tail for balance...   the tail plane is all about drag...

2) Balance... the tail plane supplies the balance... at the cost of drag... so intelligently use the WnB calculations to send the Cg towards the back limit...  we’re trying to eke out a few extra kias, not generate flat spins... 

3) NA engines do lose power with altitude... as the drag is also decreasing with air density... expect 7-8k’to be the sweet spot...of power vs drag... for NA engines...  but... if you remember doing engine break-in runs at 1k’... that is max power, low enough to the ground, you can sense the max speed...

4) Power and ROP go hand in hand... CHT control typically requires plenty of excess FF... Using the blue/white box method would be great cooling, but less power... for a short speed run ROP for max power is probably recommended at 50°F ROP...

5) Read the proper power chart for your engine... look for any limitations on 100% power... there may be a sentence in there... regarding time or altitude... expect CHTs to be trying to climb...

6) Most engines are set to have 2X FF from usual cruise economics... FT, Frpm, and mixture in... lots of FF...

PP thoughts only, not a CFI...

Best regards,

-a-

[PT20J]

12 hours ago, gsxrpilot said:

While it's true that NA engines loose HP with altitude, there is still a sweet spot where the ratio is optimal. My understanding is that that is somewhere around 8000 ft for the J? 

I've never been to FL280 to try for 252 mph in my K but I've seen 238 mph at FL260.

*all figures in TAS

I think you will find that the maximum speed for any power setting will occur at the highest density altitude for which that power is available. For a normally aspirated engine, 100% power is only available at sea level and so the fastest speed will be at sea level. 

The 8000 feet "sweet spot" comes about because that is the highest altitude that 75% power is available which is the recommended maximum cruise power setting. Therefore, the maximum cruise speed would be at 8000'.

Skip

[Niko182]

1 minute ago, PT20J said:

I think you will find that the maximum speed for any power setting will occur at the highest density altitude for which that power is available. For a normally aspirated engine, 100% power is only available at sea level and so the fastest speed will be at sea level. 

The 8000 feet "sweet spot" comes about because that is the highest altitude that 75% power is available which is the recommended maximum cruise power setting. Therefore, the maximum cruise speed would be at 8000'.

Skip

I think that youd be faster at 8500ft doing 75% than you would at sea level doing 100%.

[carusoam]

Keep in mind effects of Temp... so pick a good cruise DA...?

Skip, is this right?

Or too much interpretation?

Best regards,

-a-

[PT20J]

10 minutes ago, Niko182 said:

12 minutes ago, PT20J said:

I think you will find that the maximum speed for any power setting will occur at the highest density altitude for which that power is available. For a normally aspirated engine, 100% power is only available at sea level and so the fastest speed will be at sea level. 

The 8000 feet "sweet spot" comes about because that is the highest altitude that 75% power is available which is the recommended maximum cruise power setting. Therefore, the maximum cruise speed would be at 8000'.

Skip

I think that youd be faster at 8500ft doing 75% than you would at sea level doing 100%.

Nope. See top line on each chart below.

[PT20J]

4 minutes ago, carusoam said:

Keep in mind effects of Temp... so pick a good cruise DA...?

Skip, is this right?

Or too much interpretation?

Best regards,

-a-

Sure, Anthony. The plane flies faster in lower density air (less drag) and density is a function of both altitude and temperature. You are always better off higher if the engine will put out the power you need. The two common ways of achieving this are super/turbocharging and having a more powerful engine derrated at sea level which is how the turboprops operate. The poor  J with its little 200 hp engine doesn't get very high before it starts to run out of power, but it makes up for it by being so efficient. That's why I like to cruise at 8000 -9000 feet ROP. I like to go fast and I figure I already pre-saved on gas by not buying a Cirrus

Skip

[carusoam]

Interesting....

Mooney marketing took a conservative approach with the numbers....

The J could have been called the 203... at less than MGTW.

Best regards,

-a-

[larrynimmo]

15 hours ago, jaylw314 said:

You could have gone even faster if you were at 2700 RPM

Notice how each of the specific power settings increases with altitude, as expected.  Notice the "Full Throttle" line, however, increases with decreasing altitude.  It gets cut off where it meets the 75% power line (at 8000' MSL), but I expect that trend continues down through lower altitudes to where it would meet the (not displayed) 100% power point that can only be reached at sea level.

As I extrapolate this chart, my summary of top speed potential is available at the following temp/pressure altitude relationships

4,000 @100f

5,000@86f

6,000@68f

7,000@50f

8,000@32f

i will experiment with these targeted combinations and see what it delivers 

[philiplane]

the CG needs to be 3/4 of the way back in the envelope to get the book numbers. You should see a 3-5 knot increase at the same weight, simply by moving the CG back. Moving it all the way back will cause the speed to decrease again. Same as flying near the forward limit, you get more drag due to the load on the stabilizer.

[MIm20c]

6 minutes ago, philiplane said:

the CG needs to be 3/4 of the way back in the envelope to get the book numbers. You should see a 3-5 knot increase at the same weight, simply by moving the CG back. Moving it all the way back will cause the speed to decrease again. Same as flying near the forward limit, you get more drag due to the load on the stabilizer.

This could be bad... A wild ride for the pilot trying to get back to the front after hugging the hat rack during a speed run. Bad time for a 256 failure. 

[gsxrpilot]

Should I tell my wife that we'll get there faster if she'll ride in the back seat?

[smccray]

12 minutes ago, gsxrpilot said:

Should I tell my wife that we'll get there faster if she'll ride in the back seat?

All you have to do is put the audio panel on pilot isolate and it will FEEL faster...

[gsxrpilot]

5 minutes ago, smccray said:

All you have to do is put the audio panel on pilot isolate and it will FEEL faster...

Truth is, that's the first button on the whole panel she learned how to use. And after upgrading to the new PMA450b, she immediately wanted to know how to "turn me and the radio, off."

[pinerunner]

On 5/2/2019 at 12:03 PM, jaylw314 said:

Try leaning for best power   That would be a little scary, even if they say the IO-360 has a large detonation margin

At speed, in cruise, and with the nose down getting good cooling you don't need it as rich as during takeoff and in a steep climb where the cooling isn't as good. If I want to go for speed (with my M20E) I level off, keep full throttle and max RPM, and lean from 18 gph down to 17 gph. I keep my eye on the CHT's (especially cylinder number 3 which is the first to go lean as determined at lower power settings) and watch the indicated airspeed go up. The experts say 100 ROP is ideal for maximum power but I don't want to spend time near peak (in the RED box) at maximum power. I'm more interested in not being overly rich and fouling plugs than I am in actually achieving maximum speed. I've only done this sort of thing lately in the colder weather so my CHT's went from about (all this from memory) 270 degrees to just over 300 degrees. 

If I remember I'll be back with better numbers. I've noticed that downdrafts (you trade a little speed to fight them) and updrafts (give you a boost) affect the speed too.  I bet there are a few inflated numbers resulting from that.

[jaylw314]

4 hours ago, gsxrpilot said:

Truth is, that's the first button on the whole panel she learned how to use. And after upgrading to the new PMA450b, she immediately wanted to know how to "turn me and the radio, off."

THAT's what it does???  All this time I've been upset because I thought my ISO button was broken--I thought it would turn my wife's voice off during arguments... 

[PT20J]

6 hours ago, philiplane said:

the CG needs to be 3/4 of the way back in the envelope to get the book numbers. You should see a 3-5 knot increase at the same weight, simply by moving the CG back. Moving it all the way back will cause the speed to decrease again. Same as flying near the forward limit, you get more drag due to the load on the stabilizer.

Curious about this: are you saying that this was the test condition for the 201?  If so, then it’s conservative since full aft CG would have lower trim drag and should be faster. (Trim drag is the induced drag caused by the tail generating a stabilizing down force plus the additional induced drag created by the wing generating additional lift to oppose the tail down force).

Skip

[N231BN]

A supercharged(turbo or mechanical) engine/airplane gains 2% of TAS per 1000ft at a constant horsepower. A normally aspirated engine loses about 3% of horsepower per 1000ft. You cannot go faster by making less horsepower. It sounds good on paper but simply isn't possible. Mooney tried really hard to get the J to 201mph. It would only happen at sea level and it certainly wasn't a typical production aircraft.

[PT20J]

On 5/2/2019 at 9:03 AM, jaylw314 said:

Try leaning for best power   That would be a little scary, even if they say the IO-360 has a large detonation margin

Look at the power setting chart I posted on 5/2 (taken from a 1977 M20J POH). It shows a sea level speed of 201 MPH at 100% power with a fuel flow of 18.4 gph. That's full rich. So you don't need best power mixture to match the "book." From the chart below, best power mixture at 100% power occurs at a fuel flow of about 15.7 gph. This would surely be a bit faster, but Lycoming doesn't recommend continuous operation at this condition. The higher speed will help with cooling, but that's offset by the necessity to close the cowl flaps to cut down cooling drag. It might not hurt anything, but it's pretty deep in the dreaded red box.

 

[Andy95W]

2 hours ago, N231BN said:

A supercharged(turbo or mechanical) engine/airplane gains 2% of TAS per 1000ft at a constant horsepower. A normally aspirated engine loses about 3% of horsepower per 1000ft. You cannot go faster by making less horsepower. It sounds good on paper but simply isn't possible. Mooney tried really hard to get the J to 201mph. It would only happen at sea level and it certainly wasn't a typical production aircraft.

And this quite effectively explains what most of us have realized just by "gut" feeling- our airplanes seem happiest around 8,000 feet or so.  Since most of us (normally aspirated) folks like to fly around 70-75% power, that puts our sweet spot between 8-10k feet.  

Lost horsepower is about 25-30%, true airspeed increase of about 16-20%.