With tailwind, invariably the higher you go the better, so long as the engine is running at best-economy (peak EGT or slightly LOP). For non-turbo the best mpg tends to be achieved around 8k-10k (for a “20k ceiling” type of plane) and going to 12k-5k is usually worth it, and above that there is little or no gain in the tailwind or the mpg.
With headwind, it is better to go lower but then you get less “TAS gain”. I find that going down to 5k-8k is worth doing and below that you don’t gain much. All of my 7-8hr flights in the TB20 were into relentless headwinds of 30-40kt so I’ve spent a lot of time playing with this. The limiting factor tends to be that below 10k you are a lot more likely to be in IMC and icing conditions. However it is also better to go to a higher power than best-economy. This stuff is well documented
SD is unlikely to have a good calculation because in Europe this is really an IFR issue. You have to import the GFS model and look at winds at different altitudes, and calculate it versus the aircraft perf model which for a piston aircraft is unlikely to be accurate anyway.
But as usual we have done this before 
Prof Rogers has written a lot on these topics.
Peter wrote:
SD is unlikely to have a good calculation because in Europe this is really an IFR issue. You have to import the GFS model and look at winds at different altitudes, and calculate it versus the aircraft perf model which for a piston aircraft is unlikely to be accurate anyway.
That’s what SD does… Importing the winds at different altitudes and using an aircraft performance model. The performance model is not quite as advanced as that of Autorouter – particularly for turbocharged aircraft – but should be good enough.
Malibuflyer wrote:
Depends heavily on your ride:
For slower planes with NA engines I fully agree that FL50 is typically the sweet spot. For faster planes with Turbo engines in my experience the “as high as possible” is typically true, as long as the climb is less than 20% of the total flight time.Btw: To really optimize this one also has to keep in mind that wind direction also changes with altitude – therefore depending on the specific situation going higher could even reduce the headwind component…
The above summarizes it beautifully.
NA the numbers are easier and just as described by @Peter. In my experience with the C177RG I always got the same cruise TAS between 2000ft and 14000ft regardless of altitude just at different FF’s
However, TN/TC, the altitude/speed gain is so much higher that it complicates the number.
For example, by going down low against the lower headwind you lose so much TAS that the gain is typically minimal, and much more influenced by MEA’s and airspace constraints. On my aircraft , for the same LOP FF of 15GPH, TAS varies from 145 to 190 Kts between SL and FL200 so the 45 kts delta usually offsets the headwind increase with altitude except in the strongest winds.
The key to selecting altitude in a headwind in such case is how far you are flying, so that climb time can be a sufficiently low fraction of total flight time. Also headwind impacts more heavily during climb due to the lower TAS, so I tend to climb at high IAS (120) in a headwind and low (105) in a tailwind. Typically for us above 2 hrs total flight time the higher the better . Of course it also depends on weight and temp: MTOW at FL200 on an ISA+20 day (like two weeks ago between Spain and France) is a waste because of the long time to climb and excessive engine heating so I picked FL180, whereas light and at ISA the gains are immediately evident after a quicker climb to FL200.
Also wind changes direction with altitude and that may drive a lower/higher altitude.
The other key factor, again maybe more so when TC/TN’d because of the higher wind influence at altitude, is routing, especially for longish trips. I had planned (but eventually cancelled) a trip from Mallorca to Billund two weeks ago . That trip of about 1100NM is too long for us unless in some tailwind. On the planned day, the expected tailwind at FL180 was different whether we routed via Paris and the Netherlands or via Northern Italy and through Germany resulting in 0445 vs 0515 flight time for similar routed ground distance.
I find both Autorouter and Garmin Pilot provide quite accurate flight times vs wind if loaded with the appropriate aircraft models. Also SD but the leadtime is only 48 hrs.
With a turbo charged airplane going higher (FL200 fx) usually offset the penalty from increased headwind for a trip of more than a few hours because of increased airspeed from same fuel flow. And its usually much better (smooth air in VMC) to cruise at FL200 than at 5000ft. For a normally aspirated aircraft however there is no speed advantage going the over altitude where manifold pressure drops to result in below 75% power (assuming cruise with 75% power, higher alt. for lower cruise power setting off course). This usually happens around 8000-9000 ft. This applies in no wind condition as well. So going higher than this with an increasing headwind and falling TAS is not the best move and it’s often better to stay at this altitude or below in a non-turbo airplane. I guess most GA pilots know it allready, but the windy app has a great feature to see windspeed and directions at different altitudes.
