Effect of altitude on range and endurance - piston engine aircraft

[training wheels]

I've been looking at the AFM for the DA40-180 and noticed that for a particular power setting, the TAS increases with increases in pressure altitude. So would I be right in saying that;

1. Range (nil wind) increases with increase in altitude

2. Endurance is not affected by altitude?

[jb2_86_uk]

Higher altitude = thinner air = less energy required to travel at a certain speed, or for the same energy as at lower altitudes, you can travel faster, ie further for the same time period.

Apologies if you meant something different :ok:

JB

[bookworm]

2. Endurance is not affected by altitude?

This one's a tricky one. There are competing effects.

* The minimum power required has a dependence on TAS, because power is TAS x drag. Thus minimum power required increases slightly with altitude.

* Engines are slightly more efficient at higher altitude because pumping losses are avoided.

Which effect wins depends on the details.

[CiFIcare]

For a normally aspirated piston a/c:

range (NOT allowing for the fuel used during the climb) increases with altitude

endurance (a/c flown at minimum power required to fly level) is greatest at low altitude.

Not familiar with the DA series however, ?some are turbocharged? which will have an effect, also I wonder if the diesel engines respond any differently?

PS wind will affect range but not endurance!!

[411A]

If we look back many years to the piston engine airliner era, we find that, if the weights are identical, and further considering that we are talking about altitudes higher than 10,000 msl, the fuel consumption, for the exact same power (BMEP) setting (and these engines were nearly always cruised at the same BHP, for enhanced engine longivity), will be the same for 10,000 msl as it would be for FL210.
However, the true airspeed will increase at the higher flight level, on the order of two knots per 1000 feet (approximately).
Piston engines (in general) are far less affected with regard to fuel consumption that are turbine engines.

[bjornhall]

2. Endurance is not affected by altitude?

Another aspect is that you use up a lot of fuel while climbing at full power to high altitude. For a flight at low altitude, you can throttle back to a low cruise power setting much sooner after takeoff.

Even so, the difference in endurance for something simple like the C172 is minute; we're talking less than 20 mins on a 6 - 7 hour flight. Either way, that's far longer than my boredom limit... :zzz:

[jb2_86_uk]

Isnt the extra fuel burn during climb generally regarded as being offset by the reduced burn during the descent?

JB

[bjornhall]

Good Q! *thinking* Doesn't that apply more to range than to endurance though?

When looking at the endurance charts for the aircraft I fly (C172R&S), it says the chart allows for the fuel used for engine start, taxi, takeoff and climb, and the time during climb. It says nothing about descent. So maybe the chart would look different if it did...

It would take about 15 mins to descend from 12,000 ft at idle power, and about twice that long to climb up there. You'd use 5 USG for the climb, which is 1.2 USG more than you'd use in half an hour at 55% power at SL. If you were to shut down the engine at 12,000 ft and glide to landing, you'd save the 2 USG you'd have used for about 15 mins of SL cruise. So it seems you're right, it does appear to roughly add up! :8

[bookworm]

If you were to shut down the engine at 12,000 ft and glide to landing, you'd save the 2 USG you'd have used for about 15 mins of SL cruise. So it seems you're right, it does appear to roughly add up!

But you don't shut down the engine, do you? You would run it at idle. My engines (IO-320) burn about 6 USG/hr producing about 90-100 HP, and 2 USG/hr producing no thrust at all.

There's an inefficiency in turning fuel into energy:

a) at low powers, where the engine is relatively inefficient
b) at high powers and low airspeeds, where a richer than optimum mixture is required for cooling

Hence I think there's some offsetting of fuel burn in the climb and descent, but on balance you use more fuel than flying level for the same time.