Optimum altitude for crusing small aircraft

Paul Hamilton CFI

Paul Hamilton CFI

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Paul Hamilton CFI
In the process of creating a performance video for entry level students for small fixed pitch reciprocating engines. Stuck with trying to determine the optimum altitude for best cruise performance.

Yes, best angle of attack for best l/d is obvious and as altitude goes up, TAS increases, and fuel consumption goes down. OK. One common myth/legend/rumor is best range goes up to a certain altitude then it goes down after that. Other theories say that altitude does not matter.

I have experimented around and found 8000 density altitude works good for me, but no scientific study involved. Trying to find a reference that would substantiate this or disprove my experience.

Most reliable sources ignore or shirt around the issue. Any help or direction would be appreciated.
 
8k is the sweet spot for any NA piston engine.
 
I was going to say 6000’, I’ll try some more at 8000’ but performance for me started to drop off above 6000’ and GS was much slower due to winds and less HP.
 
My house sits at 6400 MSL and my hangar is at 6800 so the "sweet spot" for my flying is about 9 to 10 k...no, my Arrow is not turbo so true air speed at 10 k is only 140 kts.
 
but what is "best"?
best speed?
best fuel efficiency?
best view?

For me in the rentals I flew and flying in near sea level areas in the east.... so MSL is fairly close to AGL
mostly in the cessna 152, 172, 182 and similar it always felt "best" to me about 3,500 ft to maybe 6,500 ft. Going up much higher than that just felt like an airliner...sort of too high to really enjoy the feeling of flight
but when i flew in the 7AC Aeronca Champ, that thing just felt right around 1,000 ft or so.
BUT
that was all based on enjoyment of flight.... view, time to climb, felt like a reasonable altitude for glide distance, etc... not really so much about performance
 
Paul Hamilton CFI said:
I have experimented around and found 8000 density altitude works good for me, but no scientific study involved. Trying to find a reference that would substantiate this or disprove my experience.
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Most naturally aspirated engines lose about 3% for every 1000 feet of altitude. Max cruising power in most fixed pitch trainers is 75%. Therefore, you’ll hit max cruise power at full throttle at around 8000 feet.
 
mandm said:
I was going to say 6000’, I’ll try some more at 8000’ but performance for me started to drop off above 6000’ and GS was much slower due to winds and less HP.
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Winds can be a factor. If headwinds get stronger with altitude, that may mean you have to fly lower to get best range. The converse is also true. A similar principle applies to airspeed at a given altitude: fly faster into a headwind etc. There are different rules of thumb to estimate that number but the best way to find the optimal flight profile may be to iterate in your favorite planner to find the best combo of speed and altitude. You might want to play with the variables to get the best miles per gallon over the ground. That’s because nobody flies in a Standard Day zero wind atmosphere. Sometimes adverse or favorable winds can be a large portion of a slower aircraft’s cruise speed. It does make a difference.
 
8500-9500 assuming no other factors, and yes you lose power as you fly higher but also have less drag, so cruising speed kinda flattens out between 7000-14000’, losing less than 10 knots.
 
depends on the airplane. With my 140, I'd never bother going above 7000'

cruise prop on the plane?

climb prop on the plane?

etc etc etc
 
In a normally aspirated airplane, the altitude at which you can only make cruise power with the throttle wide open seems to work quite nicely for me.
 
Density altitude of 6,427.84 ft
 
As mentioned, best HOW.

For most fuel efficient, it would be where WOT gives you an indicated airspeed equal to best glide (best L/D) speed.

But for a better balance of time versus fuel used, it would be where WOT gives you an indicated airspeed equal to the Carson speed. Which is 1.316 times the best glide speed.

The WOT assumes normally aspirated.

Also as mentioned, on any given flight there are the winds. Makes no sense to climb to the best altitude and have a 50 knot headwind versus down low with no or light headwind.
 
Winds do seem to pick up to higher speeds at higher altitudes. I don’t mind going up to 12,000’, put on the music and enjoy, you get to log more hours burning 6gph getting almost 20nmpg ground track. But if you have passengers they might not like that extra time in flight.

If I’m at 6000’ full throttle, the fuel flow shows 12gph and 135kts IAS, so typically bring the throttle back to get 130kts IAS 9.5gph. Do people really fly full throttle? Feel like you can bring it back just a little bit and save a ton of fuel, also unsure how accurate the fuel flow rate needle is. It could be off, like another post said when you bring it back it takes awhile for anything to happen as it autoregulates the mixture and starts by leaning the mixture then brings back power.
 
SpecificRangeGraph.jpg OK not the best question but I now have the answer. Was not looking for winds aloft, fuel to climb, inversions, terrain, etc. These are somewhat obvious for any CFI. Was looking for basic characteristics of the engine SPECIFIC RANGE = NM/fuel burn. Thanks to Jane Carpenter Cessna Chart and Rick Henry Piper chart plus my other research, my theory that the engine power drops off faster than the true airspeed (more linear), we can conclude the Specific Range gets better with altitude as shown in the attached spreadsheet/graph directly off the Cessna cruise power chart for a 172. Thanks so much for the help. This will be used as the standard for cruise performance in my upcoming on-line cruise performance course and added to my learn to fly courses.
 
Paul Hamilton CFI said:
View attachment 112849 OK not the best question but I now have the answer. Was not looking for winds aloft, fuel to climb, inversions, terrain, etc. These are somewhat obvious for any CFI. Was looking for basic characteristics of the engine SPECIFIC RANGE = NM/fuel burn. Thanks to Jane Carpenter Cessna Chart and Rick Henry Piper chart plus my other research, my theory that the engine power drops off faster than the true airspeed (more linear), we can conclude the Specific Range gets better with altitude as shown in the attached spreadsheet/graph directly off the Cessna cruise power chart for a 172. Thanks so much for the help. This will be used as the standard for cruise performance in my upcoming on-line cruise performance course and added to my learn to fly courses.
Click to expand...

This is good, hard to find though. I made one with a few different altitudes and engine settings. On older airplanes, I don’t see them matching up exactly. Also confusing for a newer pilot is interpreting IAS vs TAS. Could add a column and add IAS, since the two are determined by formula based upon altitude.
 
mandm said:
Do people really fly full throttle? Feel like you can bring it back just a little bit and save a ton of fuel
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In order to make proper cruise power at altitude, you might have to run WOT. It doesn’t mean you’re burning more fuel by doing so.
mandm said:
also unsure how accurate the fuel flow rate needle is
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Then you need to verify during your post-flight if the fuel you thought you were burning, matches what you actually burned. If it doesn’t, than you need to calibrate the instrument. Seriously, you can’t just be slugging along unaware of these things…
 
There was a nice article in our Cessna 170 newsletter that was touches on the subject of IAS and altitude. To maintain Vy, you will use a lower IAS as you go up. I reckon due to thinner air and whatnot.

9616EF19-2980-4158-A227-83CD61840B16.jpeg
 
Kenny Phillips said:
In a normally aspirated airplane, the altitude at which you can only make cruise power with the throttle wide open seems to work quite nicely for me.
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Right on Kenny. Most engine performance charts show 70% around 7,000.
 
Paul Hamilton CFI said:
View attachment 112849 OK not the best question but I now have the answer. Was not looking for winds aloft, fuel to climb, inversions, terrain, etc. These are somewhat obvious for any CFI. Was looking for basic characteristics of the engine SPECIFIC RANGE = NM/fuel burn. Thanks to Jane Carpenter Cessna Chart and Rick Henry Piper chart plus my other research, my theory that the engine power drops off faster than the true airspeed (more linear), we can conclude the Specific Range gets better with altitude as shown in the attached spreadsheet/graph directly off the Cessna cruise power chart for a 172. Thanks so much for the help. This will be used as the standard for cruise performance in my upcoming on-line cruise performance course and added to my learn to fly courses.
Click to expand...
Good news is, if you ignore certain data, you can make what you’ve got say what you want. ;)

Most of these discussions ignore the lower speeds and higher fuel flows of the climb, which are not necessarily offset by the higher speeds and lower fuel flows of the descent.

so the question remains, best for what?
 
Yes do not worry, this is just one small point in the overwhelming matrix of mixture, IAS/TAS difference, winds aloft, turbulence, inversions, fuel to climb, weight reduction as fuel is burned, weight, short or long distance, comfort of passenger, etc...etc...
One small tidbit that "engine performance decreases faster than true airspeed decreases so you get better fuel economy at altitudes between 7000 and 10,000 density altitude." I need to make sure every statement in my training program is accurate and meaningful. 8000 feet is a rule of thumb I have used through actual cross country flights.
 
"Optimum" altitude depends on how you prioritize various factors. Groundspeed? Comfort? Terrain or weather clearance?

Personally, I like to file for 8,000-10,000 MSL. Often this is above low cloud layers and offers a smooth ride. Westbound flying this high can be slow, however. Usually comfort and an extra fuel stop wins out over higher ground speed and getting beat up in turbulence. Usually the minimum altitude that keeps you above most tops below or low level turbulence will be a good compromise between GS and comfort.
 
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I believe the rule of thumb for naturally aspirated engines is that they lose approximately 5% of their power for every thousand feet of density altitude. Please someone step in to correct my number if I’m off a little bit.

that being said, full throttle is nothing to shy away from, especially when you’re up at altitude
 
There’s a reason you are given a POH and not a general book for all airplanes.

In planes without a mixture control it might be somewhat lower.

Any generalization should point pilots to their approved flight manual.
 
mandm said:
also unsure how accurate the fuel flow rate needle is
Click to expand...

RyanB said:
Then you need to verify during your post-flight if the fuel you thought you were burning, matches what you actually burned. If it doesn’t, than you need to calibrate the instrument. Seriously, you can’t just be slugging along unaware of these things…
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You most certainly can - not every fuel system has a fuel flow sensor. That's why some of use use rules of thumb and gouge based on history and experience for fuel flow. That's why I use 10gph across the board, and stay conservative WRT fuel quantity indications.

Nauga,
and his back-of-the-envelope expansion
 
nauga said:
You most certainly can - not every fuel system has a fuel flow sensor.
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I meant that as needing to be cognizant of knowing your fuel burn, regardless of what your fuel instrumentation is indicating. Hence my recommendation of verifying fuel consumption during a post-flight by comparing actual versus indicated.
 
May the fleas of one thousand camels infest your armpits.
 
2500 to 4500 takes to much gas and time to get higher in my Cessna 150.
 
Really depends on the wind. Sometimes it's best to change altitude during the same flight to get the most out of your plane (GS and range wise). Recent flight from KRAP to near Omaha, started the flight at 7,500 MSL was doing something like 150 GS in a Cherokee. Halfway through the flight, ground speed dropped to 90 and my GPS suddenly went from something like 1 and a half hours to the destination to over 2 hours which would have cut it close with the fuel I had on board. Quick look at windy and winds at altitude and saw that the wind should still be a tailwind at 9,500 MSL so changed to that altitude and GS went up to 120 for the rest of the way.
 
Chrisgoesflying said:
Really depends on the wind. Sometimes it's best to change altitude during the same flight to get the most out of your plane (GS and range wise). Recent flight from KRAP to near Omaha, started the flight at 7,500 MSL was doing something like 150 GS in a Cherokee. Halfway through the flight, ground speed dropped to 90 and my GPS suddenly went from something like 1 and a half hours to the destination to over 2 hours which would have cut it close with the fuel I had on board. Quick look at windy and winds at altitude and saw that the wind should still be a tailwind at 9,500 MSL so changed to that altitude and GS went up to 120 for the rest of the way.
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Slight adjustments to IAS might extend your range, once you find an efficient cruise altitude.

https://www.pilotsofamerica.com/com...r-crusing-small-aircraft.140869/#post-3340895
 
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