Join Date: May 2011
Location: Riverside, CA
This is a pretty big topic with lots of misinformation out there, but the gist of what you said is true.
A brushless speed controller is most efficient when operating at 100% throttle. At 100% throttle all it has to do is apply the full battery voltage to the corresponding windings as they magnets swing by.
When operating at less than 100% throttle, It has to apply a lower average voltage to the motor. It In order to do this it uses what is called pulse width modulation or PWM. What this does is rapidly switch full voltage on and off in such away that if you average the time that it's on and the time it is off you will get a reduced average voltage. (this is the same kind of concept that a switching BEC uses to drop the voltage with the high inductance motor replacing the traditional IC filtering in order to keep a steady current).
The problem with having to switch many times rapidly is that each time the switch goes from off to on the current drawn from the battery is momentarily converted to a heat rather than going to perform useful work on the motor.
How inefficient this is can vary greatly from ESC to ESC and is not easily predictable. For example a certain ESC might be really inefficient at 95%, have pretty decent efficiency at 86% and absolutely horrible efficiency at 65%.
Now, a lot of people hear that "100%" is the most efficient and because they equate 100% throttle with 100% power, they reason "Wow, 100% power and it's the most efficient. Everyone should run 100% flat throttle curve". You'll see this recommendation all over the place. But I think it is misguided.
100% is definitely the most efficient for a given headspeed. No question about that. If you hover a 450 size heli at 3000RPM and 100% throttle it will be more efficient than if you hover the same heli at 3000RPM and 80% throttle with a bigger pinion. You will see much more battery drain at 80%.
On the other hand running 80% 2500RPM will typically use up much less battery than running 100% 3000RPM on the same pinion, even though technically it is less efficient. This is important because some people misunderstand this point. Efficiency means how much battery capacity each WATT of power is costing you. Sure, running at 80% throttle is costing you more battery capacity per watt, but if you are using less watts overall (due to the lower headspeed) then your flight time will increase even though your efficiency has decreased.
Now, while it is true that 100% throttle is most efficient setting on the ESC, is it true that 100% gives you the most power? Absolutely not. Anyone who says otherwise does not understand how brushless motors work. So let's try to understand the basics.
In order to spin the rotor we need a force or "push". In things that rotate we call this "torque". Where is the torque generated? It is generated when a current goes through the coils in the motor causing a magnetic pull or push. Motors have what is called a Kt constant (similar to Kv) but tells us how much force will be generated for a given current (or amps).
Now lets keep following back. How do we generate a current? The ESC applies a voltage across two of the motor leads. And depending on the resistance/inductance of the motor we will get some current. Without voltage there is no current, without current there is no torque, without torque there is no force to accelerate the motor.
Let's complicate things a bit now. We understand now that a voltage causes a current which causes a force that makes the motor spin. What we haven't talked about is that the reverse is also true. If you spin the motor by hand, it causes a current which ends up causing a voltage on the wire leads (even if no battery is plugged in). In other words, the motor is also a generator.
How much voltage will it generate? Well, in ideal conditions you could calculate it using the motor Kv. For example if you took a a long string and wrapped it around a 3500Kv motor many times over and then yanked it really hard (lawn mower starter style) and due to your super human strength you were able to spin it at 44,100 RPM you would temporarily generate (44,100/3500 = 12.6Volts). That means you could charge a 3 cell lipo with it (though I wouldn't recommend this as your arm would get really tired after a while). You'll often see this voltage referred to as "Back EMF".
So why is this important? Well, in order for the ESC to be able to speed up the motor and apply rotational force (torque), it needs to be able to overcome this back EMF. For example, if that same motor is spinning at 35,000 RPM it will be generating 10Volts of Back EMF. This means that the ESC must output 10 volts just to push back against the volts generated by the motor. So in this case if the ESC outputs 10volts and the motor is generating 10Volts in the opposite direction, the effective voltage is 0 and hence the current is 0 as well. No voltage, no current, no current no torque, and then motor does not speed up.
But what if we do want to speed up? Well to speed up we need torque, to get torque we need current and to get current we need voltage. So say the amount of torque I needed required 12 amps of current, and that worked out (due to motor inductance/resistance) to require 2 volts. Well, the ESC could not just output 2 volts, it would have to output 12 volts (10 to overcome the back EMF and an extra 2 to generate the necessary 12 amps. The motor would start speeding up but as it speeds up the back EMF also would increase. Eventually the back EMF will reach 12 volts, at which point it will equal what the ESC is outputting, and the current will be 0 again and the motor will stop accelerating.
So what does this have to do with 100% throttle curves? Well, there is a recurring myth out there that brushless electric motors are self-governing. What is usually said here is that they will draw the right amount of current necessary to offset any load you put on it.
While this is technically true, people often misinterpret it. Yes, it is true. whenever a load is put on the motor, the motor will automatically draw enough current to counteract the torque generated by that load. But lets explore what this means in practice. Especially as it relates to 100% throttle.
Say you are on a fully charged 3 cell battery happily hovering at 3000 RPM in your 450 pro. Now in an ideal world the ESC is outputting 12.6 volts and the motor is generating 12.6 volts back EMF and so your RPMS are steady (this is not possible in reality because there is always air resistance and frictions on bearings, gears, etc). All of a sudden we give it full collective creating a lot of air resistance and a huge amount of torque in the opposite direction of rotation. What happens?
Well, we said the motor is self-governing and will try to generate enough torque to balance out all that drag. So we're good right? The motor will magically draw the current it needs and we'll have enough power to plow right through the maneuver? Not so fast. Let's trace it back. In order to counter act the drag caused by our maneuver we need the motor to generate torque in the opposite direction. If you have been following you'll know that in order to have torque we need current (AMPS) to flow through the engine coils. . Well, current doesn't come out of nowhere. What do we need to generate current? That's right! Voltage.
So let's take our previous example and pretend we need 2 volts in order to generate the right amount of current and consequently enough torque to counter all that air resistance. That seems easy enough, 2 volts doesn't sound like much. But wait! The engine is generating 12.6 volts back EMF, which means we'll need at least 14.6 volts in order to counter the back EMF and have 2 volts left over to generate the current we need. The problem is that the ESC is already passing 100% of the battery voltage (12.6) and simply cannot get to 14.6 volts.
So what happens? Well... we don't have enough torque to fight the air resistance, so the head speed is going to start dropping. As the headspeed starts dropping the amount of back EMF generated by the motor also drops. So say we dropped 100RPM and now the motor is generating only 11.6 volts back EMF. What does this mean? Well the ESC is still outputting 12.6 volts but the back EMF is only 11.6 so we have 1 volt available to generate current and some amount of torque. It is still not enough to counteract the load we're putting on the rotor but at least we're fighting it now.
So the headspeed keeps dropping. Say it drops down to 2800RPM. And now the back EMF generated by the motor is 10.6 volts. 12.6 - 10.6 = 2 volts. So now we can generate enough current to counter the load. This means we will stop slowing down at 2800RPM.
But how do we get back to the 3000RPM we were happily hovering at? Well as long as we are applying full collective and putting a load on the heli it is simply impossible. We're using the 2 volts we've got just to keep it from slowing down any further. In order to speed it up we'll need more than 2 volts, but there is simply no where to get that from. Until we stop applying full collective and get back to a hover, we simply will not be able to get our head speed back. Physics will just not allow it.
So that's the secret. While the motor will eventually draw enough current to counter the load, in order for it to do so it MUST necessarily drop RPMS, and there is no way around it if the voltage is fixed as is the case when using a straight throttle curve.
So now let's analyze what happens when you are hovering at less than 100% throttle. Say you dropped down a pinion and are now hovering at that same 3000RPM at 84% throttle and same full battery.Now the ESC is operating at 84% of it's full voltage or around 10.6 volts. Because we are in a hover at stable RPMS (in our ideal physics world where there is no resistance in hover) the motor must be generating 10.6 volts of back EMF and everything is in balance, nothing speeding up or slowing down.
Now we apply the same full collective and, again we need 2 volts to generate enough current to counter the added drag of the blades at full pitch. Now it's a different story. Because the ESC is only operating at 84% of max voltage, we can simply increase the voltage to 100% or 12.6 volts. This gives us enough to counter the 10.6 volts of back EMF plus the 2 volts needed to generate the current for the torque necessary to counter the load. What's great now is that because we can simply increase the voltage, there is no need to drop the RPMS. If we apply the 100% fast enough, we won't see any drop from our 3000 hover RPM.
So as you can see, when running at 100% throttle, we will always bog proportionately to the load we put on the blades. There is physically no way to load the heli even a little without bogging by a corresponding amount.
So what does this mean in terms of your initial question with respect to gearing? In general your understanding is correct. As a general guideline you want to run your esc's within the 80-100% range. This is just a a rule of thumb, and the optimal range will vary from manufacturer to manufacturer. For example some ESC's might handle 65-75% throttle without breaking a sweat, while another ESC will catch fire if you run it at 75% for an extended period of time.
What this means is that if you are not getting your desired headspeed within that 80-100% range, then you should go up or down a pinion to get to where you want to be, rather than further reducing the throttle.
If you are interested only in efficiency, then you want to get the headspeed you want at 100% flat throttle curve. However, you will trade off performance if you do this. If you are interested in best performance you should stay away from flat throttle curves altogether and go with a slight V-curve. The optimal V-curve will vary depending on your ESC and motor setup. A good starting point is something like a 100-90-100. But again, this can vary quite a bit.
The way to get the best performance is using a good governor. With a governor you want a pinion/headpseed combination that will make it so that the governor resorts to 100% throttle only under extreme conditions. This is where data logging like with the Castle Ice ESC's or eagle tree loggers can help you optimize your setup. If you are running a governor and it is hitting 100% a lot throughout the flight, it means you are using too small of a pinion for the headspeed you are running.
On the other hand, if you are using a pinion that is too big for the head speed you run then you will notice the ESC overheating. If your ESC is regularly coming down real hot then you should probably go down a pinion.
I know that this answer was very long and more than you asked for. But I'm hoping it will help other people who may be as confused as I was about this stuff when I started in this hobby.
Last edited by dtabuenc; 04-04-2012 at 10:51 PM..