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What follows is my technique for setting up THR curves for ESC's in normal mode (non-governor). There are several brands of ESC available, each with different requirements for calibrating to your receiver. Please follow your specific ESC instructions to do this. I'm going to talk about how I determine the THR curve points to start with. One important technical note to keep in mind is the differnces between electric motors and nitro motors in regards to how they respond to load changes. Nitro motors have a very direct inverse response to load change for a given throttle setting. If you raise pitch, the motor slows down. If you take load off, the motor speeds up. In fact, if THR curves are set incorrectly, a suddenly unloaded nitro heli can overspeed the head dangerously.
Electric motors on the other hand, by their very nature, are somewhat self regulating. The motor derives its torque by passing a current through armature windings (which can be stationary or rotating, our brushless motors have stationary armatures) that produces a strong magnetic field that interacts with a field force (which can be produced by permanent magnets, or field windings, again this can be rotating or non-rotating). Our brushless ESC's produce an alternating 3 phase current that when applied to the armature field windings, produces a rotating armature field, by design. This pulls the rotor (with permanent field magnets) around and around. The higher the frequency, the faster it spins.
Now here's a key part to note...as the rotor spins, it starts to produce a voltage that is opposite of that applied to the armature. It's acting like a little generator.
This Counter EMF (electromotive force) is smaller in value than the terminal volatge applied to the motor. Since it's like a little generator, the faster you spin it, the more Counter EMF it makes. Now since it's opposite in polarity to the terminal voltage aplied by the ESC, there's a difference in voltage (Vterminal -EMF= Varmature). Since the armature windings have a fixed resitive(impedance) value, the current pushed through it drops off as the EMF gets bigger. Eventually, the motor will reach a speed where the voltage to the armature and the current it produces will produce power equal to the load on the motor and it will accelerate no more.
So, if you load up an electric motor, it will tend to slow down, produce less EMF, allowing more armature voltage, and thus more current to flow to try to meet the demand of the load. So the more the motor slows, the more it tries to keep up, until another equilibrium is reached.
So if you unload an electric motor, it will only speed up so much..it will not overspeed out of control
If you load an electric motor, it will automatically draw more current and will not underspeed out of control.
So the Take Away is, you don't need to have dramatic THR curves for your ESC. Just add a little more THR to high pitch ends of the curve. Even if you just use a flat line, head speed will not drop that much under high pitch load maneuvers.
So on to the method:
It's kind of like the above discussion on Governor mode ESC's, but what we want to find out, is the MINIMUM load THR percentage required to get the head speed we want.
Once you have your heli and ESC setup spinning in the right direction, you're ready to set up your thr curves.
Make
your pitch curve for normal and thr hold a flat 0 degrees across the board.
Then make your normal throttle curve a straight line ramp from 0 to100%.
Then make sure thr hold is flat 0 percent across the board(for quick shutdown).
Now when you spool up, the pitch stays constant and zero. This will be the minimum load seen by our motors. Let it settle down to some reasonable speed to complete whatever slow spool up your ESC does.
Then, with an optical tach, slowly increase throttle until you get the head speed you want. Then WRITE down the thr esc
percent displayed on your radio.
If you want more speeds (idle 1, 2, 3,etc.) just keep jacking up the thr to see what percentage it takes. Write it all down or you will forget!
Now once you've got all the data you want, turn everything off and re-program your radio with the standard normal, using 0, x,x,x+2,x +5 (x =
your first speed %), the idle 1 using x+5,x+2,x,x+2,x+5, idle 2 using y+5,y+2,y,y+2,y+5, etc. (if you are not using standard 3D pitch curves, adjust the THR points according to your zero pitch point)
The idea is to add just a little more throttle to the higher pitch ends of your curve. It's a very, very shallow "V." I recommend starting with an extra 5% max and then fly to see if it bogs. You can add or subtract in small increments until you're pleased. Also note that since you're doing your zero pitch spool ups on the ground, the aerodynamic properties will be different out of ground effect, and you may need to shift the whole curve up a few percent once airborne (be sure to have a friend help with the tach!)
Don't forget to reset your pitch curves for thr hold and normal.
Now when you start up, quickly take the stick to the zero pitch point, wait for spool up to complete, then I like to flip to idle
1 (I use the same pitch for all flight modes, -6 to +10 deg), then go fly.
Once you're comfortable with how it spools up, you can keep thr hold on, flip to idle 1, set your collective to zero pitch, then take
thr hold off, and wait for spool up. This works great all the time.
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