Consumption Resumption

I have to admit I just sat here for about 5 minutes trying to think of a good word that rhymed with presumption and assumption. Anyway, as the title might suggest, I'm bringing back our earlier discussion of power consumption. When I left off last time I noted that slowing down by about 30% can more than double our flight range. This got me thinking, what other small tweaks can we make to get big gains; the low-hanging fruit, if you will.

I started to think about motors and propellers and their efficiency. With that in mind, I pulled up eCalc, a great online tool for calculating all sorts of things about your aircraft, motor, propeller, and ESC. The big thing I was interested in was efficiency of the motor/propeller combo. I set out to model what we currently fly, an MK3638 motor with an 11x6 propeller. Here is the graph I got:

As you can see, the efficiency of this motor/propeller combo (the blue line) peaks at about 90%. It's 87.8% to be exact. It also has a nice thrust-to-weight ratio of 0.66:1. Those number are both quite good. The only problem is that the 87.8% efficiency doesn't happen until you draw 24.1 A. This is a problem because we generally cruise at 5-10 A. As you can see in the figure, the efficiency in that range is pretty bad in that range (well lets just face it, for an electric system its abysmal). At 10 A the efficiency is about over 77% and in the butter-zone, 7.5 A, it drops to less than 75%.

So, what is the diminution solution? Enter, the Tiger MT-2814-10. This motor spins at 770 RPM/V, just like the MK3638, but the way it is wound does something interesting with it's efficiency curve:

Again, the peak efficiency of this setup is quite good, 86.2%. It also has a decent thrust-to-weight ratio of 0.60:1 (with a change to a 12x6 propeller). The icing, though, is that it's 86.2% efficiency peak falls right in the middle of our cruise range, 8.8 A. This means that with a simple motor/propeller change we should gain more than 10% efficiency at cruise. For such a simple change that is a pretty impressive gain.

So, we ordered the motor and a propeller and for $60 we have (hypothetically) added 10% efficiency. I got it mounted up yesterday and spend a bit of time today balancing the propeller and motor. It is now ready to fly and, as an added benefit, it puts almost no vibration into the airframe (look here for a bench test after mounting and balancing which shows that there is no longer any "jello" in the video). Now, all we need to do is get it airborne and see how it goes.