I’ve now had the RPM adjusted to be spot on the redline and a quick and dirty test at say 5000ft suggests an extra 100rpm is worth an extra 200fpm.
If this were to be replicated anywhere near the operating ceiling, the ceiling would go up by 1000-2000ft, which would be truly stunning.
The fuel flow is proportional to the rpm so my original belief of getting a third power more HP is clearly wrong, but you do get extra HP in proportion to the RPM, for sure.
Peter – a long time ago, I did the flight test for the first CofA in our ownership of an M20J – i.e. 3yrs after we bought it. It failed to meet “book” climb performance and that’s when we noticed that it was also failing to meet redline by 100rpm. A quick adjustment of the governor and another test yielded the correct result. I recall hypothesising a square law and it seemed to work out.
In many ways I think the 3 year flight test was a good thing. These days with a PA-30 I do a full power single engine climb a couple of times a year to check performance and engine cooling.
you do get extra HP in proportion to the RPM, for sure.
Any reciprocating engine in a mild state of tune (like an aero engine) will have a flat torque characteristic above and beyond the normal operating rpm. As a result, horsepower will vary roughly in proportion to rpm.
Where you see non-linearity is in climb performance with a small increase in rpm/power. A little extra excess power over that required to drag the thing through the air at a given airspeed will go a long way in increasing climb rate. The only limitation there is that the propeller efficiency drops off beyond design rpm. I think its for that reason that my particular plane’s climb doesn’t react greatly to a pilot controlled rpm reduction after takeoff.
Right now I’m chasing tachometer issues again, making this topical for me. New ones don’t seem much better than old ones.
