What is your sweet spot altitude (for best TAS)

[ArtVandelay]

Best TAS is sea level.
Best TAS at 75HP is at 8000’.

From the J POH.

What’s interesting is the speeds for at 8000’ for both best power (12.6 gph) and economy cruise (10.8 gph) are the same, both 75% HP.

Oh, their economy cruise is 25° ROP.

Apparently LOP wasn’t an option back in 1977.

[hmasing]

1 hour ago, ArtVandelay said:

Oh, their economy cruise is 25° ROP.

Ooof, that's leaving a lot of good engine hours on the table.  That will shorten the life of your engine considerably.

[ArtVandelay]

Ooof, that's leaving a lot of good engine hours on the table.  That will shorten the life of your engine considerably.

It also means you’re not going to make book speeds if you fly LOP, because they’re not in the book. :-)

[Hank]

8 minutes ago, ArtVandelay said:

It also means you’re not going to make book speeds if you fly LOP, because they’re not in the book. :-)

Doesn't that give you bragging rights for "better than Book speeds"?  

[chrixxer]

On 7/1/2020 at 3:30 PM, carusoam said:

Expect that about 8k’ agl is your sweet spot...

8,000' AGL? So, departing Flagstaff, I'd want to be at ~16,000' MSL? 

[Ragsf15e]

10 hours ago, cliffy said:

So the title of this thread is below

What is your sweet spot altitude (for best TAS)

And I'm going to assume in cruise conditions

It has nothing to do with winds, Ground speeds or O2

The question is - What is the best altitude for TRUE AIRSPEED in a normally aspirated engine Mooney

He flies an E model

I think you are going to find that it is right near where max MP equals 75% HP so you can lean effectively to be near

MAX POWER fuel flow (say 50 to 100 degrees rich of peak EGT ) and MAX RPM ( which can be done in cruise) 

Can anyone guess that altitude?   :-)

I'll wait

The POH for my 68F says the fastest TAS is found at 2500’ MSL on a standard day.  2700rpm, 27.5” mp, 98% hp, 180mph.  Not sure anyone is actually gonna run that and have their engine survive for long.  Sea level and 5,000’ charts show slightly slower at 2700rpm and full power.

For 75%, the same POH shows 171mph at 10,000’msl, 2600rpm, 73.6% power.  That’s about the fastest one I’ve found at 75%power.  If you have cool weather, it’s reasonably fast up high, but loses speed quickly as it warms up and density altitude increases.

In my experience with realistic power settings, ~6,5000-7,500’ is your best bet.

[carusoam]

7 hours ago, chrixxer said:

8,000' AGL? So, departing Flagstaff, I'd want to be at ~16,000' MSL? 

Chrix,

you caught me!

I forgot that some people fly over areas that are above Sea Level...

I’m pretty sure that was an attempt at selecting the best compromise between power and drag for Mooneys with NA engines...

Which typically occurs near a DA of 8k’ msl.

 

of course, I prefer higher altitudes for that extra extended glide safety...  proper Parallel parking a Long Body on a city street is going to be slightly more space constrained than other fine Mooneys...

I prefer things like 12.5k’ MSL and 11.5k’ MSL coming and going from here...

Thanks for pointing that out, and nice to see you... 

By the way...‘94 Os have LOP numbers in the POH... it was acceptable for Continentals at least that far back...

 

Best regards,

-a-

[bonal]

Three way GPS at 8000 produced 148 knots with two on board. But 5500 yielded around 154. If I'm pushing its pretty easy to get it into the yellow arc.

 

Edited July 9, 2020 by bonal

[gsxrpilot]

7 minutes ago, bonal said:

Three way GPS at 8000 produced 148 knots with two on board. But 5500 yielded around 154. If I'm pushing its pretty easy to get it into the yellow arc.

Mooneys love the yellow arc.

[AH-1 Cobra Pilot]

We just took a fairly long trip last week:  https://flightaware.com/live/flight/N468W/history/20200708/1410Z/KBAK/KLNK

My fuel burns for the first three legs were 35.14, 33.5, 32.25 gallons. (I have not refueled for leg #4, but it will be 33-35 gallons.)  I ran ROP at 2500 rpm and WOT at both altitudes, yielding 20.4" at 11,000' and 22" at 8,000'.  Some knucklehead mounted my outside air temperature probe on the engine cowling, so its accuracy is poor, to say the least.

You can judge for yourself...

[chrixxer]

6 minutes ago, gsxrpilot said:

Mooneys love the yellow arc.

Older ones especially; starting with all F models and, in '69, the rest, they raised the top of the green arc.

[bonal]

At 10500 WOT 2500 this calcs out to around 145 knots.  Pretty amazing for 180 horsepower 

[Ragsf15e]

3 hours ago, bonal said:

At 10500 WOT 2500 this calcs out to around 145 knots.  Pretty amazing for 180 horsepower 

No matter what, that’s definitely amazing.  Good speed at 10,500’.  Just be careful trusting the 50 year old pitot system to be accurate.  The poh has a ~4 mph subtraction to get from IAS to CAS and that doesn’t even account for airspeed indicator errors.  The best way is to do a good 3 way groundspeed run and enter it in an online calculator.  If you do this a few times you’ll get a feeling for how close your IAS is, but it’s likely showing faster than actual.  As I said though, impressive no matter what!

[Ragsf15e]

4 hours ago, bonal said:

Three way GPS at 8000 produced 148 knots with two on board. But 5500 yielded around 154. If I'm pushing its pretty easy to get it into the yellow arc.

 

That is good speed... how much power was that?  Greater than 75% I would think?

[bonal]

I used to think the safe number was 47 not 46 so that was 2500 and 22 and since it's a C with only the single EGT I can only go by the lean to rough then rich to smooth so no real idea how rich of peak that is.  I've only ever managed to get it to LOP once what a magical moment. I normally fly much easier like 2400 20 which yields around 135 knots but was fun to stretch it out.  AI is about 2mph fast based on GPS runs. Most likely higher than 75% on those flights. For me it's all based on OAT cause Snoopy likes cold air.  Biggest challenge is keeping things cool up front.

[Tx_Aggie]

On 7/2/2020 at 9:33 AM, Bob - S50 said:

If you really are looking for the BEST (maximum?) TAS, that will always occur at the highest altitude that will allow you to make your desired power.  That is, for an NA airplane, the theoretical altitude for 75% power is about 7500'.  For 70% it is about 9500'.  For 65% power it is about 11,500'.

The actual altitude may be just a bit lower due to pressure loss in the induction system caused by the air filter and friction.  When we used to have a Bracket air filter, we lost about 1 or 2 inches of MP compared to the Donaldson we use now.  That means our best altitude was lower for the Bracket.

That said, on short trips I like 3500 - 5500.  On medium length trips I like 5500 - 7500.  On long trips I like 8500-10500.  However, if the ride sucks I'll go up to 12,500 if that will give me a smooth ride.

Did you really gain 1-2” by simply switching different air filters? Pardon my ignorance but is anything else required or is it as easy as it sounds?

[M20F]

On 4/14/2025 at 1:47 AM, Tx_Aggie said:

Did you really gain 1-2” by simply switching different air filters? Pardon my ignorance but is anything else required or is it as easy as it sounds?

Mooney’s came with a ram air system to bypass the filter.  Engaging it bypassed the filter and added @1” of manifold.  If you have a clogged or inefficient filter it is going to work opposite and take away manifold.  

It isn’t a magic way to go faster.  It is just if your intake is not efficient then you will lose power.  Cleaning that up gets you back to net zero, it isn’t a way to add knots per se. 
 
As for best altitude, it you ain’t 250 then you ain’t nothing. 
 
 

[cliffy]

Only the 200 HP model had the ram air door

[Pinecone]

On 7/1/2020 at 7:40 PM, larryb said:

In my K it's 12,500 for short trips, and 17,500 for trips > 2 hours. Unless the winds aloft are really against me or there are clouds I want to stay out of I'll go for altitude every time. The air is cooler, the ride is smoother, the traffic is less, terrain clearance is not an issue, glide range is better. One just needs a good O2 setup so that O2 usage is not a factor.

Similar with my K.  10 - 12 for short, 16 - 17 for over 2 hours.

Get an Oxygen Concentrator.  My Inogen G5 is good for 2 people to 14 and one person to 18.

[Echo]

Love me some reboot of an old VINTAGE thread.  Flexing by turbo normalized guys flying O2 never gets old on a vintage thread.

7500-8500 is sweet spot for my M20E and my body.

[cliffy]

2 hours ago, Echo said:

Love me some reboot of an old VINTAGE thread.  Flexing by turbo normalized guys flying O2 never gets old on a vintage thread.

7500-8500 is sweet spot for my M20E and my body.

I'll stretch it to 10,500 occasionally in my D model but 9500 and lower is my usual even out here in the west

I cross the west end of the Grand Canyon a  lot so 10.5 northwest bound is the usual for a 1/2 hr crossing it. 

I go around the Rockies not over them E-W

I hate sucking on the tube. 

[Echo]

4 hours ago, cliffy said:

I'll stretch it to 10,500 occasionally in my D model but 9500 and lower is my usual even out here in the west

I cross the west end of the Grand Canyon a  lot so 10.5 northwest bound is the usual for a 1/2 hr crossing it. 

I go around the Rockies not over them E-W

I hate sucking on the tube. 

I will go to 9500-10500 to Cliffy if tops force me up during summer months, but prefer 7500-8500.

[Hank]

I'm generally at 7500-10K when traveling, unless the headwinds are strong. My C runs out of climbing oomph . . .

[0TreeLemur]

The wind and direction of flight usually determine my altitude.

Heading west- usually lower to avoid the winds up high.  At least 4,000 though.  Often 120-130 knots G.S., sometimes 110 (ugh).

Coming back home, 11,500, 2500, 7.5 gph, 160+ knots G.S.

Did non-stop Santa Fe to Tuscaloosa last year in 6.1 hours.

 

[cliffy]

Just for fun 2 years ago I did Tampa (Sun n Fun) to Santa Fe NM at 1500 AGL  all the way

and didn't talk to a sole unless I landed at a towered airport. 

Did 5.3 hr  east bound ONCE but never again. Had 1+15 left in gas. Way too long to sit.