What is your sweet spot altitude (for best TAS)

[Yourpilotincommand]

Stlll trying to find mine (newbie owner).

[carusoam]

Where did you look, the POH?  
 

All normally aspirated Mooneys have a sweet spot...

Where power is still strong...

And air density is thinned...

There is a graph that covers it....

Looks like a big X....

The sweet spot is right in the middle...

A great place where 65% power is available, Wind resistance is minimized, and the red box is non-existent...

The engine can be run at WOT...

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

Use these details to find the data to support your flight in your plane...

PP thoughts only, not a CFI...

Best regards,

-a-

[ShuRugal]

I like anything between 8,000 and 11,000 in my c

[Andy95W]

8,000-11,000 feet, depending on winds aloft.

[Andy95W]

Just now, ShuRugal said:

I like anything between 8,000 and 11,000 in my c

This is great- we posted the exact same altitudes at the exact same time.

[carusoam]

Now...

YourPIC wants to prove his plane is really matching book values...

Need too find the three way speed test to do this efficiently....

Fire up the GPS... go fly fast...

Best regards,

-a-

[Ross Taylor]

Flagstaff field pattern altitude = 8015 feet, so anything above that.   But to be serious, I'm usually above 8k and as high as 12.5

[larryb]

In my J it was 12,500. 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.

[Hank]

In my C, I prefer 7-10 K. Cruising at 9500msl, I often run partial cabin heat year round, after I recover from summer heat during ground ops. 

[ShuRugal]

This is great- we posted the exact same altitudes at the exact same time.

That's too funny.

[211º]

Westbound - 6k or 8k
Eastbound 9k
Southbound 6k
Northbound 7k or 9k

Although I might go higher if the winds are really on my tail. It is a compromise between fast and O2 saturation. I recognize that I am a little more tired after a high flight.


Sent from my iPad using Tapatalk Pro

[gsxrpilot]

3 hours ago, larryb said:

In my J it was 12,500. 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.

If you like 17,500 you'd LOVE FL270.

Eastbound in the K I'm still looking for the best altitude. @Denver98 recently reported still having 80% HP at FL270. And since running LOP at 65% is our SOP, FL290 or FL310 might be even better.

I'll report back.

Even going West, into a head wind, higher is often better. It's not unusual to find the increased TAS at altitude is more than the headwind component. And therefore the higher altitude still yields a better ground speed.

Like @larryb said, you just need good O2 equipment and go.

[S.C.]

I’m with the 7-10K crowd for long cross country’s. Flying my C in Texas in the summer, even for short hops, anything over 5K to get out of the thermals is key for a smooth ride.  With a normally aspirated engine it slogs the last 2K to get to 10K but once there we move right along.  Usually I’ll stay around 7-8K winds determining higher altitudes. 

[Bob - S50]

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.

[Hank]

I think @Bob - S50 hit it, except that it's DA that counts. My screenshot abive shows Altitude of 9500 and DA of 12,000+, so the plane will perform as if at 12,000 msl.

[RLCarter]

The performance charts are based off the mystical Standard Day & Standard Lapse Rate, I like what gives me the best GS and a comfortable (temp & turb) ride. 

[FloridaMan]

FL240

[Tcraft938]

On ‎7‎/‎1‎/‎2020 at 6:34 PM, ShuRugal said:

I like anything between 8,000 and 11,000 in my c

I'm finding the exact same thing in 63C, depending on density altitude.  I would say 7000-8000'msl is best speed, higher than that, speed will slowly start to drop off, but fuel burn also does at greater rate, so I call that the peak efficiency range.  Last week I took it on my first trip, very warm day 90+ on the ground.  Went to 9,500 msl to get above most of clouds and to see the building cumulous and was still in the mid 50's up there.  Density altitude calculated to be 11,400.  At WOT and pull back to just as the MP needle moved, was running 20" MP and 2500 rpm with indicated and true airspeed respectively being 145mph / 173mph or for those in knots 126/150, fuel burn was 9.1 gph.  I experimented with reducing prop RPM to 2400 and the MP went up about 0.5" so speeds were 143/171mph or 125/149kts, but fuel burn dropped to 8.4gph.  I'm new enough to not know how that compares to other stock 63C with only mod being one piece (not 201) windshield, but I was amazed at getting 19 to 20 miles to the gallon.  Also taught me, if willing to go 2 mph slower (which meant about 3 minutes) on this trip, the efficiency increased 7.5%.  So if weather or something requires you to extend things and you're up high, sacrificing a little speed can pay big dividends.  

We did the return in a friend's Beechcraft Sierra.  Nice plane, roomy, but let's just say it was not as happy to go to 9500' as the little C was.  The same trip adjusting for wind conditions took 15% longer in the Sierra and he burned 38% more fuel.  Also interesting was he has a lot more room but less useful load than the C.  He has 842lbs, vs me at 1,020, so his range is further hurt by offloading fuel to carry passengers.  We joked, he can make it fit, he just can't lift it, I can lift whatever I can get in it.  

[Hank]

10 minutes ago, Tcraft938 said:

We did the return in a friend's Beechcraft Sierra.  Nice plane, roomy, but let's just say it was not as happy to go to 9500' as the little C was.  The same trip adjusting for wind conditions took 15% longer in the Sierra and he burned 38% more fuel.  Also interesting was he has a lot more room but less useful load than the C.  He has 842lbs, vs me at 1,020, so his range is further hurt by offloading fuel to carry passengers.  We joked, he can make it fit, he just can't lift it, I can lift whatever I can get in it.  

Our Cs are the best thing going in aviation, the best bang for the buck!

When I flew hurricane supplies to the NC coast, after flying up from Alabama, I was the only SE piston I saw on the sign-in list with over 400 lb useful load. With me and full fuel I have 470 lb, but arriving with half tanks I listed myself at 600 lb for the first trip. The little airplane that could! As you now know, I ran out of space well before load, even unboxing things and cramming them into the nooks and crannies.

[jetdriven]

We flew the whole way from Houston to OSH a couple years ago at 1000’ AGL.  .  Any higher was headwinds and a second fuel stop which would have made us late, after 3 they closed the arrival.   We got there at like 2:30

[jamesm]

On 7/1/2020 at 8:12 PM, gsxrpilot said:

If you like 17,500 you'd LOVE FL270.

Eastbound in the K I'm still looking for the best altitude. @Denver98 recently reported still having 80% HP at FL270. And since running LOP at 65% is our SOP, FL290 or FL310 might be even better.

I'll report back.

Even going West, into a head wind, higher is often better. It's not unusual to find the increased TAS at altitude is more than the headwind component. And therefore the higher altitude still yields a better ground speed.

Like @larryb said, you just need good O2 equipment and go.

Can you even fly at FL290 or FL310  in a GA plane these days?  isn't RVSM equipment  required  isn't something like  FL27 and above?

[kortopates]

12 minutes ago, jamesm said:

Can you even fly at FL290 or FL310  in a GA plane these days?  isn't RVSM equipment  required  isn't something like  FL27 and above?

The highest legal ceiling of the Mooney's is the 252 with a FL280. RVSM starts at FL290, so FL280 is not a problem. RVSM Equipment and training requirements aren't out of reach for Mooney pilots, but our O2 system is really a bigger barrier. Our re-breather type masks that we use in the Mooney is technically only approved to 25K, above 25000' we're supposed to be using diluter demand O2 mask, which are rather expensive at almost $4K https://www.aircraftspruce.com/catalog/pspages/aeroxdoning13-12359.php 

But in fairness, these days we go by the much more important Pulse Ox and virtually all of us monitor our O2 Sat at altitude and would know if our O2 system wasn't keeping up with the altitude, which should keep all safe to the aircraft's certified ceiling. That said though, O2 will be the limiting factor in venturing above into RVSM airspace for us. 

But don't even need any thing special to fly at FL27

[brad]

Fly my 75 F monthly between home base MEV and VGT at 13500 and 14500. Great numbers and good radar coverage.

[gsxrpilot]

As long as your personal O2 supply is sure... altitude is life. 

Cruising in my 252 in the mid 20's, anything within 50 miles is within easy gliding range. I don't even have to calculate the numbers. There is a certain comfort with a glide ring that large.

[cliffy]

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

Edited July 9, 2020 by cliffy