It appears that there is no specification on this.
For example an IO540-C4 is specified at 2575 RPM.
Now let’s imagine the governor is set just 1% high i.e. 2600 RPM. That is 3% more HP i.e. 7.5HP extra!
Similarly if you are just 1% under, that is 7.5HP under spec, which is going to be worth… how much in terms of say the operating ceiling?
There are phone apps which convert noise to RPM so one can check the tachometer accurately. This has got to be worth doing.
What’s the formula you are using?
Propeller thrust (at a given blade pitch) is proportional to the 3rd power of RPM so, using calculus of small changes, 1% increase is 3% more power.
That’s assuming the engine can deliver the power. The fuel servo will deliver fuel proportional to airflow, which will be proportional to RPM, so you will get only 1% extra fuel if you achieve the RPM increase using the governor alone. So the propeller will adjust its blade pitch so as to absorb only this extra 1%. But the mixture control can increase the fuel flow as desired.
Most aircraft with CS prop that I’ve seen had the RPM set way too low, often 100 RPM or so (sometimes more) below red line. The funny thing is that I somehow feel that in most cases, the engineer did it “for the good of the customer”, saying that “it is better for the engine” ( obviously not since reduced RPM at full throttle yields high ICPs and CHTs).
I have mine set to exactly redline (2700RPM), since I fly from short fields a lot. But to be honest, that’s how it came off the factory, and it was not touched since.
1% more RPM gives 1% more power, if manifold pressure is held constant. Power is proportional to the volumetric flow through the engine, and volumetric flow is proportional to RPM as well as to manifold pressure.
If in doubt, look at the performance charts in the engine manual, and try some RPM numbers.
Peter, it is the “at a given blade pitch” that confuses. Your governor will quickly adjust so that power available corresponds precisely with what the propeller absorbs at the governed RPM.
And remember that almost all analog tachometers read low – typically 30-100 RPM low. Three of the aircraft I fly presently has a tachometer that reads 130-180 RPM low. It is a real problem on a fixed pitch propeller aircraft because when you think you set 70% power, you really have around 80%, and leaning to best power or peak EGT will really heat up the cylinders, and shorten your endurance. On a fixed propeller, the manifold pressure rises as well as the RPM when you increase power, so here you get more like 2% power increase for a 1% RPM increase – in level flight.
By the way, I use a quarts optical digital tachometer (www.proptach.com)) to check tachometers. It is easy and accurate, and the instrument is easily calibrated. I have checked dousins of tachometers. I have not tried using phone apps.
My plane has digital propeller control to set rpm. After the most recent propeller work I had effectively three tachometers on board: the propeller controller, a strobe tach taped to the glare shield, and the mechanical tach. I agree that the battery powered strobe tachs seem pretty good – in this case I got an immediate lock on the propeller through the windshield and it didn’t get ‘lost’.
As expected I found the old mechanical tach read low, and I replaced it. It was important to get the fine pitch stop set correctly because the certified static rpm with blades set full fine is within 50 rpm of red line for my propeller.
Post #6
I would be interested to know how you can assert:
By the way, I use a quarts optical digital tachometer (www.proptach.com)))) to check tachometers. It is easy and accurate, and the instrument is easily calibrated. I have checked dousins of tachometers.
Propeller thrust (at a given blade pitch) is proportional to the 3rd power of RPM so, using calculus of small changes, 1% increase is 3% more power.
All the prop thrust equations I’ve seen showed a squared rather than cubed relationship. I’d also have to imagine as you approach the propeller’s design speed you start to run into diminishing returns if the prop tips are anywhere near the speed of sound, so the graph of the function of static thrust to RPM would be convex down and maybe the graph starting to flatten off by the time you are at takeoff RPM.
I had a brief look to see if there were any graphs of static prop thrust to RPM for a typical aircraft prop, but my google-fu is weak this morning…
I would (and didn’t) ever suggest one should go out of spec on this.
But if there was a tolerance, it would pay to set the governor at the top end of it, for best all around performance – especially at altitude.
Strangely enough, all enquiries I have made confirm that there is no tolerance. It is just (for mine) 2575. But anybody with any education will know this is silly because there must be a tolerance. It’s just that Lyco don’t appear to publish one.
Reading the above posts I am thinking it isn’t a cubed relationship once the engine control systems are taken into account. But you will at least get a linear relationship i.e. 1% for 1%.
BTW one US engine shop which builds both certified and Exp engines advises me that their Exp clients routinely run the 2575 rpm engines at 2900 rpm. There is no apparent loss of engine longevity, as there is with say increasing the compression ratio which does give you a big HP increase for no extra fuel.
