The excel spreadsheet was pretty but didn't show the
answer to the question. It should not graph the
"difference between this speed and the speed at the
previous MP".
What matters is range, and that is determined by IAS and mpg.
I have to assume the quoted figures are right, in which
case the obvious is proven. The slower you go the greater
the range - in fact the range at 23 MAP (65 knots) is 50%
more than the range at 33 MAP (141 knots). However no
range is lost by increasing the MAP from 23 to 25.
Assuming the fuel burn figures are Imp galls per hour, and
that the tank holds 15 gallons (imp)
at MAP 23 the range is 443 nM which takes 6.8 hours and
at MAP 33 the range is 302 nM which takes 2.1 hours
MAP nM IAS range duration
/gall nM hrs
23 30 65 443 6.8
24 30 88 455 5.2
25 30 108 450 4.2
26 28 120 419 3.5
27 26 125 383 3.1
28 24 129 365 2.8
29 23 131 345 2.6
30 23 133 338 2.5
31 21 136 319 2.3
32 20 139 307 2.2
33 20 141 302 2.1
This analysis shows much more clearly the difference
between the tortoise (at MAP 23) and the hare (at 33 !)
I am sure these figures are with a fixed pitch prop. I am
not sure which engine is shown either. With a CS prop
there is another parameter involved - so 3D graphing is
required :-)
With my 912S with Airmaster I get at 4600 rpm and 25.5 MAP
16 litres an hour and 115 knots.
MAP Burn nM IAS range duration
/gall nM hrs
25.5 3.5 32.7 115 490 4.3
I see that as a fair compromise between speed and economy :-)
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