Exploring the 4.5 Ohm Didel 6mm Motor

Since I first flew my Quick Junior with the 4.5 ohm pager motor in August of 2003 it is quickly becoming "the" pager motor to use. Properly geared and propped, it generates good power without getting hot. And it is a very good match for the discharge capabilities of the E-Tec 90mah LiPoly cell. Consequently it is worth exploring this motor in more detail.

Let's start with a quick review of the this motor's characteristics. Early on in my use of this motor, before it was widely available, I sent one to my friend Roger Carignan. He was interested in using it direct drive in a super micro 10g version of his pusher configuration Pinky, while I was more interested in using it geared in my Quick Junior. Roger enhanced Joachim Bergmeyer's formulae for finding motor constants for a given motor (see the February 2003 Inside Story article). Roger's version improves the calculation of rotational power loss based on two measurements of no load conditions, rather than one. I don't want to go into more detail here. It's enough to say that Roger measured motor constants for this motor and then I did the same with my equipment and got virtually the same results.

The graph below shows motor efficiency and power, in Watts of output, for the 4.5 ohm pager at various RPM's. The dotted red line shows power output which peaks at 0.64W and 25,519 RPM. The solid blue line shows efficiency which peaks at 52% and 33,818 RPM.

Table 2 below shows a variety of static tests for the 4.5 ohm pager. The first two rows give the predicted measures for max efficiency and max power and the corresponding motor RPM. These are useful for interpreting the results of the various static tests. In general we will want to choose prop and gear combinations that have a motor RPM that lies between the max power and max efficiency RPMs. If the ET-90 LiPoly cell is used we have plenty of capacity, so efficiency will not really be an issue, and we should try to find prop and gearing choices where the motor RPM is closer to the max power RPM.

For each test the amps, watts, thrust, and prop and motor RPM are given under the static measurements heading. For each test the predicted amps, efficiency and power are also given under the predicted motor measures heading. And, for the efficiency and power the percentage of the max efficiency and max power at the top of the table are also given.

The Falcon PU04 propulsion set now comes with the Didel 4.5 ohm motor. It has a gear ratio of 6.5:1, and will soon come with a folding KP00 69mm prop. So a limited set of tests for this gearing are included. However, since the 6.5:1 and 6.7:1 ratios are so close, I present more tests for the 6.7:1 gearing, but they really apply to the PU04 with its 6.5:1 gearing as well.