In my 1/2 A planes it seems that all the headroom the governor needs is about 10% above the static start governed rpm. In a typical flight using perhaps 80% of batery capacity I do not see any lap time "sag" durung the flights.

I'd like to learn how this compares to larger planes like the Super Clown I am now setting up.

Any thoughts?

To make this more clear Alan Hahn's SC runs a 10-7 at 8500 rpm. I have a lower kv motor and plan on a 10-10 prop. Just comparing pitch speed I get the required rpm is 5950. With my batteries I can just get 6500 rpm at full throttle. Is this enough rpm (headroom) to complete a flight?

Ok, now to describe what is in the plot.

There are two curves, the straight line red curves are the power input (volts*amps) from the battery vs the rpm you are running. The highest one is for wide open throttle (WOT) for a 4s battery near the end of a flight when the pack voltage has dropped to 13.8V---this value was taken from my own data on a real flight.

The lower red curves are for 90%, 80%, and 70% of WOT.

The violet curves are the theoretical curves for power out as a function of rpm. To match them with the red curves---look at the high rpm where the power goes to zero (this rpm is the "no-load" rpm. The no-load rpm is approximately just Volts*kV.

With that over, look at the vertical red line at about 8000 rpm. This I believe represents my minimum (~270 watts cruise) and max (440 watts--vertical climb) input power needs (from the battery). Again, these power inputs are what my data recorder tell me I am pulling from the battery during a typical pattern flight.

The minimum input power value falls roughly halfway between the 70% red line and the 80% red line. So from this I imply I am flying level laps with about 75% of full throttle near the end of my flight. The top point of the vertical red line is just touching the 80% line.

So from this, it appears that the governor is only changing the throttle from 75% to 80% of WOT. Notice this change appears small, but the power difference is 270 watts to 440 watt, 170 watts, or 163% of the level flight power.

So all that is saying that a simple 5% change in the throttle can make a much bigger change in power input (and output).

From this I figured I could drop my kV for the next motor. Which is what I did. The current Scorpion 3020 has a kV of 780 rpm/volt. So now I am running closer to wot, but also that means the current bursts from the battery are a little lower (but for a longer time), which should be treating my battery a little more kindly.

If I can get my ICE 50 ESC to work (tomorrow I hope), it tells you what the % of throttle is.

So hope I didn't bore you (and others) to death.

To actually answer your question, you would need two things. One would be the characterization of your motor (DriveCalc probably has it in its database).

The other is to really know what rpm you need. I found (from my original Super Clown experience) that simply scaling by pitch underestimated the actual rpm that you need to fly the level lap speed you need. Or in clearer language, a 10-10 prop needed more than 7/10 times the 10 x 7" rpm. How much of this was due to propeller dynamics, and how much to the actual pitch being off on a particular propeller never was obvious to me.

Basically you need the same thrust to get the same lap speed. This doesn't appear to scale exactly with pitch (even when the prop shape is identical).

Also I will note that level lap speed isn't the entire story. When you pull the nose up and the airspeed drops, the lower pitch prop will generate more thrust (if rpm is kept constant by a governor). This may or may not be important--depending on how fast you fly your level laps.

Alan

Jim, you not need to know "%". I do it just opposite. If you know rpm for your prop, an voltage of your discharged battery under load, then you can run some motocalc and check if your motor can handle your prop at voltage of your discharged battery.

Prop torque in figures reachs torque on ground, so it is good estimation. You have also remember that old battery will go deeper and deeper with voltage under load as its internal resistance is higher and higher with age, so it is good to calculate volge of completaly flat battery.

I agree with Igor.  I try to run a RPM that is about 75% of max RPM figuring battery voltage and motor Kv.

Thanks everyone!  It appears from your comments my 10% rpm overhead may or may not be enough (marginal at best).   Also the target rpm may not be enough.  I will proceed with flight testing though and let you know what I find out.  This really helped my understanding.