21-Nov-2009 Tags: General

In order to better understand how motors behave under severe loads, I figured it'd be interesting to subject a motor to increasing coil temperatures and record what happened along the way. Some motors are made with high-temperature components, and others are not. Does this provide an advantage? Should we treat the motors made with low-temp components the same way we treat the motors made with high-temp components? This first round of tests will establish the test methodology, and the baseline for motors made with high-temp components. And assuming the motor still worked after the test, I wanted to use it to take a motor to a very high temperature so that I could accurately check my understanding of motor thermal parameters and also build a more accurate thermal model. This can be done at low temperatures, but because the parameters involve exponentials, the surest way to know your parameters are accurate is to get as far away from the room temperature case as possible and check the math. This also lets us understand what a high-temp failure mode might look like on this particular motor.

The candidate motor was a Scorpion 2208-30. There are a few reasons for picking a small motor instead of a big motor. First is cost. Small motors are cheaper. Second, the power supply I have doesn�t have enough oomph to kill a big motor. Third is the time constant. Big motors have larger time constants, so everything takes longer. ...more...

23-Sept-2009. Tags: General

Previously I'd looked at estimating in flight cruise speed of a flying wing based on predictions from PeakEff. The conclusion there was the in-flight speed and vertical performance of the model closely matched the predicted speeds. And the predicted speed substantially exceeded the static pitch speed of the model. In the case of the wing, a higher KV motor was used, with a nearly square prop.

In this post, I take another look at cruise estimates. This time, however, it's with a larger 3D bird, a symmetric airfoil, and a dramatically different P/D on the prop. ...more...

13-May-2009. Tags: RC

In my quest to go a bit faster, I wanted to cover the plane that I CNC'd with a plastic cover to help hide the wires. As previously discussed, the plane was designed such that everything could be buried. In CAD, this is fairly easy because you can build a plane with a certain wing span, and then see how things fit. If a corner pops up above the top surface, then you grow the wingspan a bit, and the center section automatically gets a bit thicker. And after a few iterations and sculpting, you can hide whatever you wish. That's how I arrived at the current wingspan of about 34". ...more...

10-May-2009. Tags: RC, CNC

Today I flew my first CNC airplane. I wish it would have been a bit more satisfying, but in the end it was like any other flight. Yesterday was to be the real maiden, but after an impressive 20 foot climb out, it dipped south and headed straight in. Not enough reflex in the wings. Motor broke off, taking a big hunk of Home Depot foam with it, and the battery slipped out of an opening I have yet to identify. I thought it was worse than it was. I fixed it today in about 10 minutes. After thinking about it all night and a few test flights on a previous build of the same plane (sans equipment), I determined there wasn't enough reflex. ...more...

26-Apr-09. Tags: CNC

I've long wanted a CNC machine. There's an intersection between three interests I have, guitars, woodworking, and RC, that could benefit from such a machine. While perusing the CNCZone site, I ran across this thread describing a bolt-together CNC machine. I'd started a CNC machine a few years back. It was a smaller machine using a dremel, and my hope was that I could bootstrap my way up from a fairly inaccurate machine to a more precise machine. But my interests were pulled in other directions, and other projects around the house started taking more time.

But I still kept thinking about how much use I could get from a CNC machine. ...more...

21-Mar-2009 (Revised 3/27/2009). Tags: General

Often I think we tend to calculate the pitch speed of the prop, and we just assume the model will fly close to that speed. But I've always had a sinking sense that when my model was supposed to be flying at 65 MPH it indeed was not. And last summer while doing some work with a GPS-equipped PDA on my Easystar I found that pitch speed turned out to be a rather poor predictor of flying speed and that often thrust often mattered more. But why? ...more...

Sunday, 1-Mar-2009. Tags: General

When operating at partial throttle, the ESC is required to chop, or modulate the duty cycle of the motor drive to achieve reduced RPM. To the motor, this appears as if it is being driven from a reduced voltage. The frequent question asked is "what is the efficiency hit to run at partial throttle operation?" As this study will reveal, the efficiency reduction at partial throttle is indeed real. However, it's small and in many cases can be offset by reduced copper losses in the motor such that the net loss at higher partial throttle settings is nearly zero compared to full throttle....more...

17-Feb-2009. Tags: General

APC provides very clear guidance on the maximum RPM for their propellers, but many other manufacturers do not. The table below lists the limits used for calculations on the PeakEff website. If a propeller exceeds the RPM threshold, it is flagged as a "dangerous RPM". On the Motor/Prop graph, this means that the prop name is shown in red if the "show dangerous props" checkbox is selected. On the "Pick a Motor" page, props that exceed the safe RPM are excluded from consideration....more...

15-Feb-2009. Tags: General

There's a lot of information contained on a PeakEff Motor/Prop graph. I wanted a graph that could quickly provide a lot of information and show tradeoffs, and I didn't mind if the graph looked overly busy and complicated. After working on several simpler presentation formats, I stumbled across a graph Bruce Abbott on RC Groups had shown for some motors he had tested. In these charts, Bruce overlaid props on top of the current and efficiency curves, and that format really resonated with me. So, I used his presentation as a starting point and built from there, adding in thermal information, as well as estimations for every prop that made sense thanks to a deep database of bench-derived prop constants. ...more...

24-Jan-2009. Tags: General

There's a lot of debate about just how hard a brushless motor can be pushed, and the answer depends on a lot of factors. But the primary measure for determining this is to know how much "waste heat" your motor is producing. When PeakEff makes a recommendation of how much power your motor can produce, it's based on two pieces of information. The first is the motor efficiency, which is known with great accuracy because it's based on user-entered data. The second is based on the weight of the motor. The assumption is that heavier motors can dissipate more power. But is that accurate? Let's take a look. ...more...

20 January 2009. Tags: General

It's fairly intuitive to understand how much thrust is required for an airplane to hover. A 500 gram airplane, for example would need 500 grams of thrust to hold a hover. In this case, it's a simple matter to measure the static thrust of the power system on the bench, since both the bench and hover condition occur at 0 m/s. But how much thrust is required to zip upwards at various rates of climb? That question gets a bit more tricky because as the velocity increases from zero, the thrust of the prop decreases. ...more...

25-Jan-2009. Tags: General

Dr. Kiwi from RCGroups was kind enough to share his extensive database of motors, which I've added to the PeakEff database. These did not come with Io data, but I've been working on ways to estimate Io, and also analyzing the error associated with the estimation. Where Io has been estimated, you'll see it noted in the trial notes. I'm still scanning for import bugs, so if you see anything weird, please send mail to MattTay@PeakEff.com ...more...

19-Jan-2008 @ 1 AM. Tags: General

I started PeakEff a few years ago as an exercise to sharpen my understanding of motors and electric flight, and also because I wanted to learn a bit more about writing web based applications. I learned a lot about motors, and also writing web-based apps, but apparently I didn't learn enough about writing web-based apps because at some point just after PeakEff started working, something really bad happened and my ability to update the site failed. Badly. There was enough stuff to learning about moving databases back and forth, and the amount of SQL I had to learn to try and resurrect things just became too much. But it worked and was reliable, and while it looked like crap, it did what I wanted. And so the site moved like a ghost ship through the ether, and as long as I paid the annual hosting bill, it just kept going. ...more...