Why do engine/aircraft manufacturers discourage LOP?

At GA speeds, the biggest factor is headwind or tailwind, driving the flight to be low or high respectively.

And the wind forecast is not all that accurate; I’ve seen 20kt more headwind than forecast, plenty of times. And a slight change of the route can change the wind a lot.

Most non-T IFR tourers (ceiling say 18k-20k) do best MPG at 8k-10k, with not much loss out to say 14k. This is just basic aerodynamics, engine efficiency, and AoA.

Turbocharged planes are different, and more complicated since you have the TIT limit etc which seems to drive a lot of pilots to flying ROP. With a big turbo, the best MPG should be at 25k but I don’t think that’s actually the case, for some reason. TB21 owners have always said they get a lot less range than the TB20.

On any actual flight the MPG is read straight off the fuel totaliser data, and it’s immediately obvious that the wind makes the biggest difference. I find the AR data to be way off reality, and use it just as a route generator.

Airborne_Again wrote:

Autorouter can give you the altitude which is optimal either in time or fuel consumption.

Cool, thx! Last time I used autoroutes that feature has not been there yet… Really interesting results.

Peter wrote:

At GA speeds, the biggest factor is headwind or tailwind, driving the flight to be low or high respectively.

Yes, and: If winds are “normal” for our region, (so like in the example below around 5kt in low levels and 20-25 around FL200) for me it is always amazing how little the effect of altitude to total time is and how big on MPG.

Below a quick example from autoroutes for the route from LOWS to EDFE with in that case headwind: Total time is completely independent from cruise altitude but fuel consumption is almost a factor of 2…

That is for a turbocharged engine, yes?

For non-T, the TAS drops off as you approach the ceiling, and the engine efficiency drops off because the ceiling is reached at (by definition) best power, which is obviously ROP.

This is why the AR model can be way off – if you have to climb, say to get above wx, you spend some time ROP and going pretty slowly. I don’t think there is any way to preplan this aspect which is managed purely tactically when airborne, and this is why an accurate fuel totaliser is needed in practice.

Why are you burning 41 GPH at 18k? That’s a huge amount in a PA46.

Peter wrote:

Why are you burning 41 GPH at 18k? That’s a huge amount in a PA46.

Good catch!
It’s 41 gal (according to that calculation) for the entire flight, so it’s 29GPH.
25 Minute climb at full rich takes the first 15 gal – it’s calculated conservatively but in summer it’s better to at least calculate with full rich to control CHT. But it still looks quite high!

Will have to investigate the autoroutes calculation as soon as I have time…

OK; yes.

Obviously for such models to be any good, one has to enter in some real data, which won’t necessarily be POH data. I did that for the TB20 originally when I was a beta tester for the AR, c. 2013. I just think that on most real flights it is too far off, due to unexpected stuff.

The optimal altitude model should be dead easy if you maintain specific ASI reading otherwise you need to plug 100% power for climb and 75% power for cruise then do load of conversions with wind inputs

I fly NA engines no need for complicated maths, I figure out an easy way if +10kts surface headwind I fly 3000ft, otherwise I fly 8000ft

Airborne_Again wrote:

Autorouter can give you the altitude which is optimal either in time or fuel consumption.

Both Skydemon and Foreflight (if you buy the Performance plan) do that as well, if you have created your plane with a correct model.

Sure, but then you have to fly exactly as per the model

That’s the trick…