Headwinds

[Bob - S50]

Just thought I'd start a new thread and give people something to think about.

 

Everybody remember the old line about, "if you find yourself fighting a headwind, in order to get maximum range you need to go faster"?  Don't believe it.  I think it was either Mike Busch or John Deakins who points out that is only true if you are starting at L/D max which is about 90k.  Here are some numbers I worked up to prove the point.

 

Let's say I'm cruising along fat, dumb, and happy (my normal state) at 8000', 65% power and I had planned on no wind.  All of a sudden (I told you I was FD&H) I realize I have a headwind and might run a bit short on fuel (forget the obvious that I can land someplace short and get fuel, or change altitude for better winds).  Should I speed up, slow down, or do nothing to maximize fuel?

 

All these numbers are from my POH, 8000', economy leaning ('78J), your mileage may vary.  I take the stated cruise speed and divide by the stated fuel burn to get MPG.  For a 10k headwind I subtract 10k from the cruise speed but keep the burn the same.  I did the same for 20k and 30k headwinds.

 

This first table assumes you use 2700 RPM for all settings because that is what you would need in order to get 75% power.

 

Power 0k HW    MPG      10k HW            20k HW            30k HW

75%   169/10.8=15.6     159/10.8=14.7     149/10.8=13.8     139/10.8=12.9

65%   157/9.7 =16.2     147/9.7 =15.2     137/9.7 =14.1     127/9.7 =13.1

55%   145/8.6 =16.9     135/8.6 =15.7     125/8.6 =14.5     115/8.6 =13.4

 

However, in reality, to get 65% at 8000' I only need 2400 RPM and to get 55% I can use 2200 RPM.  The numbers become:

 

Power          0k HW    MPG      10k HW            20k HW            30k HW

75% 2700 RPM   169/10.8=15.6     159/10.8=14.7     149/10.8=13.8     139/10.8=12.9

65% 2400 RPM   157/9.1 =17.3     147/9.1 =16.2     137/9.1 =15.1     127/9.1 =14.0

55% 2200 RPM   145/7.8 =18.6     135/7.8 =17.3     125/7.8 =16.0     115/7.8 =14.7

 

With either set of numbers, all the way up to 30k of HW, I still get the best gas mileage at the lowest power setting.  Of course the difference in distance depends on the power settings being compared and the wind.  Using the 75% vs 55%, minimum RPM, no wind, 20 gallons can take me an extra 60 miles.  With 30k of HW, it only gets me 36 miles.  If was starting at 65% and slow to 55%, it is only a 14 mile difference... but it will take me 22 extra minutes to fly those extra 14 miles!

 

I may just continue on FD&H... but I won't speed up!

 

Just something to think about.

 

Bob