Dave Lilley
Posts: 109
Joined: 6/4/2003
From: Schertz,
TX, USA
Status: offline
|
You have to be careful when talking about electric motors and power. For example, bigger does not always mean better, and power does not always mean thrust. As you probably know, you can spin a small prop at high RPM, and it still may not fly your plane. Brushed direct drive brushless motors are often good at turning a small prop at high RPM, but that does not provide enough usable thrust. Often, a larger prop, even if it is turning fewer RPM, is needed. Therefore, sometimes gearing a smaller brushed motor is the better choice than using a large direct drive brushed motor.
I have brushed motors that can turn an 11” prop at a reasonable speed when geared, but only a 7” prop when set up for direct drive. A brushed motor that can turn the same 11” prop will be much heavier and pull more amps. If the plane could even hold the larger brushed direct drive motor, it is safe to assume the plane would need higher capacity batteries to compensate for the higher amp load and to bring the flight times back to an acceptable level. Higher capacity batteries often (but not always) means more weight, and this could also increase the weight and relative wing loading. If your not careful, you could quickly end up with a plane that flies marginally or not at all.
You may notice that I keep mentioning brushed motors in my examples. There are many options in higher turn brushless motors that have enough torque to turn large props at reasonable amp loading. They often are fairly efficient and very easy to setup, since they do not need gearboxes. They can sometimes be lighter. If you don’t mind setting up a gearbox, there are even more options in geared brushless motors. (Gearboxes are not that hard to set up.) They are usually more efficient than the direct drive higher turn brushless motors, and they nearly always more efficient than the geared brushed motors.
Gearing any motor also decreases the relative amp draw when compared to a direct drive motor that can turn the same prop. There are two major down sides with geared motors. The first is that with increasing gear ratios, you will need larger props to get the same amount of thrust. The second downside is that higher gear ratios lower the prop speed, which limits the top speed of your plane. (If you are not mindful of this, you could end up with a plane that barely flies faster than its stall speed.) Finally, a geared motor sometimes needs more cells than a comparable direct drive motor. A thing to consider with geared motors is that there is a sweet spot. This is the point where geared motor can turn a prop that can be used on the target plane, the necessary batteries (number, type, and size) will fit in the target plane, the amps are low enough for the batteries you want to use, and the pitch speed high enough for more than just cruising.
When considering geared and direct drive motors, I find it useful to consider the following questions.
What size prop can the plane use?
(This is usually limited by the type of plane (sport, sailplane, scale, slowflyer, etc) and how much clearance there is for the prop.) I will often target the prop size used in a comparable glow version of the plane, or to a known plane of relative size and type. If those comparisons aren’t available, I try to target the prop size to the relative scale of the plane and the scale speed that I want for the plane.)
What is the target weight of the plane?
(I try to target the final flying weight under a comparable glow version of the plane. With NiMH cells, this is rarely possible, but with the new LiPoly cells, it is finally becoming a reality.)
Does your plane need higher speed, higher thrust, or both?
(For fast planes, I go with a direct drive motor. For scale or slower flying planes, I often use with a geared motor. If you need both, it often costs more money.)
How much room is available for the power system? (motor or motor/gearbox)
Some geared systems are long. I have planes that could not use a particular brushless system because there was not enough room in the plane. The same can be true for large direct drive motors, which often have more girth.
Do you want to use a more efficient brushless motor or a less expensive brushed motor?
I hope I have not overwhelmed you with options, but I just wanted to point out some of the differences and some of the things you may need to consider. Even for very experienced electric modelers, it is often it is very helpful to use a software program such as ElectriCalc or MotoCalc to help determine motor, gear drive, prop, and battery choices. They are not perfect, but they can let you test a large number of power systems to get a relative idea of what may work.
If you answer the questions I posted, I can run the numbers and provide a few choices.
< Message edited by Dave Lilley -- Jun 18 2003 3:22PM >
|
Printable Version
All Forums >> Electric Aircraft Universe >> Brushed/Brushless motors, speed controls, gear drives >> Use One Speed Controller For Two Motors? 
Use One Speed Controller For Two Motors? - 
Use One Speed Controller For Two Motors? - 
Use One Speed Controller For Two Motors? - 