I have been playing with some routes for my medium to long cross country trips to be taken this summer, I started to think about the effect of cruising altitude on flight time and fuel consumption. I was a bit surprised that (winds aside) there was almost zero difference in flight time and fuel consumption calculated even if I used wildly different altitudes (between 1500 ft and 9500 ft), then I read about the same thing in @Peter ’s engine management guide. Is it really true that cruising altitude matters this little? I am talking about VFR flights in an old spamcan cruising around 100-110 KIAS.
I’ve seen a similar conclusion in a research paper.
In still air the maximum range achievable varied little with cruising altitude, although the TAS at which it was achieved was greater at higher altitude.
For such a slow aircraft, the wind gradient may be more significant and climbing for better tailwinds or descending for lower headwinds will predominate the calculation.
Going higher can be good for other reasons; clearer radio, more choice in an emergency landing, simpler airspace etc.
Mark_1 wrote:
In still air the maximum range achievable varied little with cruising altitude, although the TAS at which it was achieved was greater at higher altitude.
Assuming the same percentage power and higher TAS, how does that work? Higher engine efficiency at low altitude due to lower rpm? Lost speed due to extended climb period that isn’t regained in descent? This would be interesting to understand.
Understood about winds.
Silvaire wrote:
Assuming the same percentage power and higher TAS, how does that work? Higher engine efficiency at low altitude due to lower rpm? Lost speed due to extended climb period that isn’t regained in descent? This would be interesting to understand.
Take a look at this paper
Pages 16-17 and figure 2 show how theoretical max range is independent of altitude (still air range), but the speed to achieve it goes up with altitude.
I think maybe I understand now. Theoretical maximum range is not the same thing as the more day-to-day practical issue of range while holding constant percent power at two different altitudes.
If engine and prop efficiency were constant over the power range, the range would not vary with altitude.
In reality the efficiency (for sure the engine efficiency) is not constant; the engine has mechanical losses which are a constant HP loss for any given RPM, etc.
What does affect MPG (and I have that in my writeup) is RPM, so if you can cruise at a lower RPM that helps a lot.
I find that all the time the throttle is wide open (which itself means above about 8000ft) the MPG is pretty constant for a given RPM.
Obviously one has to be peak EGT or LOP (most efficient combustion) otherwise you are just burning money 
To get near the operating ceiling one has to go to “best power” (about 150F ROP) so that buggers the MPG by about 15-20%. This is about FL170+. Also you have to go to max RPM of 2575… 2200rpm works only up to about FL140. And 2200 can’t be used at low levels because there is too much MP and the engine doesn’t sound very happy. Also there is a limitation
So… I find 2200rpm works well between about FL080 and FL130 and there I find the MPG is fairly constant.
However in practice I just fly at 2400rpm everywhere below FL170. It’s a bit quicker.
Concerning RPM: I find that each engine/prop/airframe has it’s sweet spot.
As a comparative example: your TB20 has a relatively fat wing that appears to be optimized for about 130K IAS . On the other hand, my Lancair has a fairly low profile that works best @ 150K IAS . Consequently, the sweet spot on the power curve AND prop RPM is lower than that of the Lancair.
That’s not true. Nothing stands in the way of LOP cruise climbs, except a much reduced VSI.
It IS true.
To reach the operating ceiling, by definition you must use the most power available.
For you and for me, that means ROP.
Peter wrote:
If engine and prop efficiency were constant over the power range, the range would not vary with altitude.
That is of course true in principle, if you’re prepared to fly at best L/D. At low levels, this gets a bit boring because it’s a low speed, so in practice, for similar speeds, range increases with altitude.
In real life, the difference in wind at different levels makes a much greater difference to range. So you fly at the level that gives you the best tailwind or least headwind. Most of us would probably trade a tailwind for smooth air.
At low levels, this gets a bit boring because it’s a low speed, so in practice, for similar speeds, range increases with altitude.
That’s an interesting observation i.e. you are saying that as one climbs (non turbocharged scenario) the IAS falls, so one ends up closer to Vbg?
