Originally Posted by
Charles E Taylor
The Rule of 48
This is an old rule, I have copied this from the Van's site.
For normally aspirated engines this gives a good approximation of % BHP
For those unfamiliar with the "rule" it goes like this... take your rpm in 100s and your manifold pressure in inches and add them together. A sum of 48 equals 75% power (such as 24" MAP and 2400 rpm), each reduction of 3 is a reduction of 10% power, so 45 would be 65% and 42 would be 55%.
A table listing all the combinations and settings in between that is used in the cockpit is a handy tool for cruise power management.
All that said, does anyone know how accurate this is? Any empirical data to compare with it?
Charlie
I don't know, but it does look like it'll probably only work on one particular range of engines.
In any case, why on earth try and remember a fiddly "rule of thumb" like that, when all you need to do is copy a few numbers into a table and stick it on your kneeboard - something like power/height/rpm/map/fuel burn - shouldn't take up more than half an A5 sheet for just about anything in the SEP class given you are probably only going to look at maybe 4 power settings, and for most purposes 2,000 / 4,000 / 6,000fpm will cover pretty much any GA cross country.
I did use to own a share in a PA28-161 which had this on a placard on the pilots sun visor, which was a thoroughly sensible idea, but sadly have not seen that bit of common sense very often.
G