Up! Up! and …what?

Please mount propeller appropriate to rotational direction (10×8 using 3S and 8×8 using 4S no payload). Improper propeller can damage motor and electronics! 

What?

Flight time with my new six rotor drone was peaking at just under five minutes, which really isn’t all that long. At least, it seemed that way to me. So I decided to replace the 4 amp hour battery with a five amp hour battery. It was heavier, but the greater power should produce a longer flight time. That’s what all of the 3S and 4S stuff in the instruction manual was alluding to. However, it was all Greek to me and I might as well have been trying to decode Sanscrit as understand what the manual was talking about. I couldn’t help but think of the old Bud Abbott and Lou Costello comedy routine “Who’s on First.”  Except, of course, I was not laughing.

I do this hobby to learn about new things and so I tenaciously sat down to my PC and went to Mr. Know-It-All: the Google home page. Two hours later I had assumed the fetal position and began to suck my thumb. Considering that I’d found at least a hundred different links which appeared to offer some daylight to the darkness that was my ignorance, a fair majority of them turned out all to lead to the same page. The rest of them were the plaintive cries of those asking the very same questions I was.  I pulled myself up by my bootstraps, ate six Lorazepam relax-o-matic pills and doggedly returned to the fray.

Using a larger propeller offers a greater disk area and increases the effect of wind on the aircraft. However, a ten inch prop almost doubles the lifting surface of the props and consequently significantly increases the load on the motors. This can lead to overheating and damage. Therefore, when using a larger capacity battery it is better to utilize a smaller propeller to reduce the motor load.

What?

Not only was I stunned that I mostly understood what I was reading, I was stunned to find that I was the one who’d written it. This was my translation of the sentence that begins this blog posting. I gave myself a standing ovation and then sat back down, realizing that although what I’d decoded made sense, I still had no idea why a greater reserve of power would have so significant an effect on the aircraft. How on earth could having a more powerful battery have anything to do with propeller size? As I created my earthbound robots, a bigger battery led to longer running times between recharging, it didn’t require me to change the wheels. Actually, a larger battery often meant that I could use larger wheels and increase the overall speed of the robot. Obviously, this was one of those quirky little math conundrums –like dividing by zero in quantum mechanics.

Let’s see here. We take the number of magnetic poles within the motor and divide it by the number of rivets in the Brooklyn Bridge, divide that by this morning’s breakfast (remember to carry the 1) and the product will be the quotient of the closed set of integers that result in prime numbers and prime roast beef. It was time for more Lorazepam. Okay, let’s try this again.

The larger the propeller the slower it will turn and the lower the voltage required to turn it. However, as we know from Electronics for Dummies, the amount of current for a circuit is inversely proportional to the voltage. Thus, the bigger the prop, the slower it turns, the greater the current and therefore the greater the heat generated. Ergo, reducing the propeller diameter when increasing the available power keeps the voltage the same. Come to think of it, I already knew that. I just didn’t think of it the right way. This is why brownouts are so hard on computers. As the voltage in the wall socket goes down, the more current the computer draws, eventually reaching a point where it gets too much current and the resulting heat blows up the processor. That’s why we use battery backup systems; not to make the computer run during a power outtage, but to maintain the voltage in spite of reduced line voltage and power spikes –which are deadly to computers. It also permits the computer time to properly shut down when the power fails. See? I actually already knew this. I was just thrown off by all this aviation engineering terminology. See, all engineers of the different disciplines create their own ways of saying the same thing so that the uninitiated will think there is a special magic for a given area of technology. Boat engineers use terms like bow and stern and port and gunwale when they just mean front, back, left and side. If people just used standard lay expressions for everything then everyone would realize they’re qualified engineers. Of course, that would lower the salaries of the various and sundry disciplines and so, say, theoretical physicists, for instance, are going to argue for gobbledegook.  Of course, the worst offenders at this sort of thing are lawyers, who aren’t engineers at all. They just created a weird language to appear high falutin.

Okay, this is great. Now, if I could only figure out why a 3S is a 3 cell and a 4S is a four cell, I would have the secret of why the initial ‘S’ stands for Clark Kent.