So headwind always „wins“, how unfair!
Let’s take an example 400 NM at 160 kts:
- with 0 wind it’s 2 hours 30 minutes
- with 40 headwind kts it’s 3 hours and 20 minutes
- with 40 tailwind kts it’s 2 hours
So with headwind you lose 50 minutes and on the way back you gain 30 minutes comparing to zero wind situation.
This is something most pilots find non-intuitive. It certainly isn’t taught in the PPL, where you use the stupid slide rule to do wind calcs but without developing a practical understanding.
It is in the same non-obvious category as whether a plane “feels” the wind. I have known any number of pilots, and a good number of instructors, who were convinced that a plane flying in a crosswind has got air pushing against one side of it (and all the weird variations). In reality a plane is like a ship; it moves relative to its immediate frame of reference and has no idea about the others.
For a return route, any wind has a net cost – because for any given distance you spend more time in a headwind than you save in a tailwind of the same magnitude.
The only things you can do with wind are second order effects e.g. if you have a tailwind you fly as high as possible (consistent with reasonable engine efficiency) to get even more of it, and if you have a headwind you fly lower down to get less of it. Or if doing a round trip, try to arrange the outbound to be on one side of a high (or low) and the return to be on the other side of it.
Time is distance divided by speed, T = D/GS = Distance/(TAS+Wind)
Obviously, it’s not linear in wind, but rather convex, so you increase average time even if average wind equals to zero, the same happens when the wind is gusty, in other words more uncertainty on GS more average time even if things cancels out…
Ideally, one should fly higher TAS in headwind and low TAS in tailwind to spend similar times in each O/R legs for things to average well for total time
There is however still a net loss due to wind, because any speed above Vbg (strictly speaking Vbg plus a bit for engine efficiency being nonlinear with power) reduces MPG, due to the increase in parasitic drag. So yes while flying a little faster in headwind reduces the “net cost of wind” it doesn’t bring it down to anywhere near zero.
Peter wrote:
It is in the same non-obvious category as whether a plane “feels” the wind. I have known any number of pilots, and a good number of instructors, who were convinced that a plane flying in a crosswind has got air pushing against one side of it (and all the weird variations). In reality a plane is like a ship; it moves relative to its immediate frame of reference and has no idea about the others.
Or the forces acting on an aircraft in a turn. In many places you still see four forces (lift, weight, centripetal and centrifugal force) all balancing each other out neatly. If that was true, the aircraft would still move in a straight line according to Newton’s first law. Once I almost got into a fight with a university physics (!!) student who insisted that the four force-model was correct.
The numbers above are the ones I used to run when flying NA. Similar to @Peter ’s TB20, our former 177RG used to do almost the same constant TAS (in my case 140KTAS) at all altitudes between FL50-FL150, with fuel flow (and winds aloft!) being the major change.
Without giving much thought, I had assumed that since our new steed could do around 200KTS then the impact of wind would not be so much, as shown on the above tables…but I found it not to be as simple as the tables may lead you to believe…
Altitude selection for TC aircraft seems to add a third dimension I had not given much thought to for flight planning.
With a TC/TN aircraft, TAS does indeed change with altitude (that is how you get a faster cruise with the same hp-rated engine, of course), which also tends to be what happens with winds aloft (stronger the higher you fly) .
When flying NA, I used to search for the available altitude on the particular trip with the least headwind (or most tailwind) , sometimes even flying <3000ft for several hours to maximize average GS.
However, for example our typical LOP cruise on our current TC P210 will be around 155KTAS at low level, 175KTAS at FL100 and 195KTAS at FL200, pretty linear, slightly better or worse depending on weight and OAT, but with same FF in all cases around 16GPH.
So when I decide to fly lower in search of say a 20KTS lower headwind, I may end up getting the same cruise GS as higher up with the stronger headwind. Therefore a lot of times the optimum cruise level for longer trips is not as immediately evident. Thank God for flight planning SW (like SD or Garmin Pilot, but I am sure FF also does the same thing).
Even a fourth dimension is added when you factor the time to climb (in our case typically 15 mins to FL100 but 35-40 mins to FL200 again depending, especially the latter, on weight and OAT). Since climbs are done at a constant IAS of 110-120Kt with correspondingly lower TAS (your average altitude during climb is little more than half your cruise altitude) therefore with wind impacting your GS disproportionately more than during cruise as we learnt from the graphs above.
So for comparatively shorter flights of say under 300NM, climb has for us a bigger impact than cruise in the overall trip time vs cruise altitude. Above that it starts waning and cruise wins.
So for short trips, my old “if in headwind fly low” adage from my NA times seems to still work despite the lesser cruise impact. For longer trips, a lot of times it ends up being the same or better to always stay high, unless the winds aloft disadvantage is >30kts.
In practice, I run my SD and Garmin models and figure what seems best. 90% of the times they are very accurate.
