Wednesday, October 3rd, 2012 10:42 am GMT -6 Wednesday, October 3rd, 2012 10:42 am GMT -6Wednesday, October 3rd, 2012 10:42 am GMT -6
 
Speed controls - BEC - ESC

Here I get into specific recommendations on the settings to use when programming our model airplane ESCs.


 

 

 

Introduction

What follows are my specific recommendations for the settings of a modern speed control. There are always exceptions, but the settings I use and recommend should work fine for most model airplanes.

Start Mode: Soft

The start mode says how quickly the motor reacts when you move the throttle stick up from being stopped. I always use a soft start with my smaller models, which translates into an RPM ramp up of about one second.

Large propellers have more inertia than smaller ones. Helicopter blades have tons of inertia. The more inertia, the slower the speed ramp up or acceleration should be. I would use at least two seconds for a giant scale model and five seconds or more for a helicopter.

An electric motor that starts up suddenly can exert an enormous amount of torque on the propeller and its mount. You do not want either of these coming loose.

I can see a start mode of normal or hard being necessary for something like a 3D model airplane. You need very fast and crisp throttle response to control these models. But for everything else, go easy on the acceleration.

Brake: On

You should fly with the brake turned on. This stops the propeller when the throttle is moved to 0%.

When the motor is not running, there is actually less drag on the airplane if the propeller is stopped rather than allowed to freewheel. Right now I am about 90% convinced of this. One day I am going to have to research this some more so I can be 100% certain. I will write an article about it then.

From my experience, you are also less likely to break the propeller if it is stopped. You could get lucky and have the propeller stop horizontally. But even if it does not, do not underestimate the inertia of a freewheeling propeller.

Battery Type: LiPo

Make sure the battery type is not set to something nutty like NiCad.

I have yet to see a speed control that supports LiFe (A123) batteries. The problem with this cell technology is in telling the speed control what cut-off voltage to use. More about this below.

Number of Cells: <actual number>

Modern speed controls tend to have an “auto detect” setting for the number of battery cells. This can be very dangerous, specially with the higher cell count packs. The problem is that partially discharged packs can easily confuse the speed control. If it miscounts the number of cells, then the cut-off voltage will be way off. The result will be a damaged or destroyed battery pack and a bad day at the field.

Speed controls sound off a series of beeps equal to the number of detected battery cells. Always listen and make sure the count is right. But the system is not foolproof. Again, with the higher cell count packs, it is easy to miss a wrong count. Did I just hear five or six beeps?

Cut-Off Voltage: High or 3.2 volts

The cut-off voltage is the minimum voltage per cell that the speed control will let the battery pack reach. When the battery pack gets to this voltage, the power to the motor is cut-off. This is done to prevent damage to the battery pack. You should never let a LiPo battery cell go below 3 volts. I recommend to set the cut-off at 3.2 volts per cell. This will probably be a setting of “high”.

For LiFe (A123), use a cut-off voltage of no lower than 2 volts per cell. It will take an entire article just to talk about the different ways to use a LiPo BEC with these cells, so I will not say anything else here.

Cut-Off Type: Soft-Cut

When the battery pack reaches the cut-off voltage, do you want the power to the motor to be cut-off all at once, or do you want it to be ramped down over a few seconds? In practice, this setting appears to make little difference. Having an extra 5-10 seconds of limited power just does not make that much of a difference to me. If I’m in trouble, it is not enough to help me much. But sure, go ahead and use a soft-cut setting.

This is an area where there is room for innovation in speed control technology. How about pulsing the power on and off quickly a couple of times, and then making sure the pilot has a full minute of at least 50% power?

I suppose that with the new telemetry systems, this feature is on its way to becoming obsolete.

Timing Mode: Auto

Do not change the timing mode of the speed control without testing. It is extremely easy to lower the efficiency of your power system in a misguided attempt to make it better.

First, measure the maximum RPM of your motor. Then change the timing mode setting and measure the maximum RPM again. Only keep the new setting if the RPM increased.

A decrease in RPM with a timing mode change means that the motor is less efficient. That extra energy that could have gone into turning the propeller faster is now going into heat. A lose-lose situation.

Speed controls have gotten very smart. I would bet that a setting of “auto”, if available, will work just fine.

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