PopularRC

The lost power generated as heat inside a battery, during use, is equal to I^2*R, where R is the Internal Resistance of the battery.
This lost power causes the temperature of the battery to rise and these higher temperatures significantly impact battery life.

Using an internal resistance meter from NQRC , I determined the internal resistance of four 1000 ma-hr size LIPO batteries.

The conditions of the test were:

1. Battery fully charged
2. Battery at room temperature (78f)
3. Internal resistance measurement taken using a 0.5 ohm load, approximately 8 ampere discharge rate.
4. Each cell in the battery was measured independently, one at a time.
5. Measurements were taken after approximately 30 seconds stabilization time.
6. Battery dongle contacts were cleaned until readings wer reasonably stable. ( contact resistance can be several milliohms)

The four LIPO batteries tested were all 3 cell 1000 to 1050 ma-hr sizes of four different brands, and unknown history.

Battery A is a relatively new TURNIGY NanoTech from Hobby King.
Battery B is a ZIPPY brand from Hobby King, this battery is well used and at least two years old.
Battery C is an EXCALIBER brand from BMKdesigns and is well used and is about the same age as Battery B
Battery D is a RHINO from Hobby King, of unknown vintage but not new.

Battery A    34, 33, 34 miliohms per cell.
Battery B    68, 63, 61 miliohms per cell.
Battery C    89, 83, 88 miliohms per cell.
Battery D    37, 33, 35 miliohms per cell.

Both battery age and battery technology are variables here, but one thing is very clear , batteries B and C will generate two to three times more heat during use than batteries A and D.

At a 10 amp discharge rate, Battery A would generate 9.9 watts of wasted power as heat inside the battery and Battery C would generate 26.4 watts of wasted power.

I know this has all been covered (somewhere in vastness of the internet) but I wrote a super simple application to write my Date and Time TAG.txt file for me. I thought I would share it for those that are interested.

I have had the Spy Gum camera since the early days and still use it. However a good friend of mine game me a brand new Keychain camera (green tinge to the lens) and I am thrilled — so much easier to place on the planes. Anyway, after dinking around with different Date Time files (none worked including the one in the instructions that came with it). I found one that worked:

TAG.txt

[date]
2010/05/26
23:07:10

You can download the app here: http://popularrc.com/KeyChainDateTime.zip

Source:

Imports System.Text
Imports System.IO

Catch ex As Exception
MessageBox.Show("Error: " + ex.ToString())
End Try
Me.Close()
End Sub
End Class

I noticed a series of links to video clips today showing up on my Twitter feed from the AMA — Burt Rutan at the 2010 AMA Expo — Cool!   I was riveted by his home video and photos of his research and development.  As well as some of his RC building and testing.

Read more…

A few days ago HobbyKing indicated they are experimenting with a Soft Magnetic Composite (SMC) alloy as a brushless motor stator material.

The following is a twitter feed from the HobbyKing blog.

“17/02/2010 11:30:41 AM
SMC Somaloy material for stator builds. No other R/C motor producer has ever used such an advanced and efficient material for producing stators. We’re currently prototyping the new material for solid stator production which greatly increases motor efficiency. The stators are extruded from molten Somaloy material to create a material with iron losses far lower than a standard Stator motor and with less wire for the same Kv and voltage. Our initial tests have shown amp draw to be far lower than is needed with a standard stator motor. We wont have the exact numbers until March when we dyno test motors side by side, but from looking at propeller size differences we are calculating around a 20% increase on an already very efficient motor. THIS COULD POSSIBLY BE THE MOST EFFICIENT R/C MOTOR EVER BUILT.”

………………………………….

Here’s a link to an article from Hoganas, the producer of Somaloy materials which offers some insight into using these materials as motor components.

http://www.hoganas.com/en/News-Center/Published-Articles/Advances-in-Soft-Magnetic-Composites—Materials-and-Applications/

The results of HobbyKing’s comparison should be interesting and potentially marks a significant step in brushless  motor development.

The following two quotes are from the FMA LIPO Handbook, Volume two, page 8.

…the determinant of (LIPO) cell life and performance is the temperature the battery cells reach during discharge.

A (LIPO) cell run continuously at the maximum C rating will lose capacity to 80% in as little as 25 charge cycles. The same cell with maximum current bursts less than 10 seconds and average current of half the maximum allowable discharge rate can last 500 cycles.

…………………………………………………..
Battery life  (number of cycles until capacity degrades to 80% of  rated capacity)  is important to electric aircraft pilots:

A $35 battery that only lasts 25 cycles results in a cost of $1.40 for each flight, while the same battery at reduced temperature during flight could result in a cost of only seven cents per flight.

Every battery will have a ‘final flight’. It’s better to have fewer of these.

Although it’s important to provide maximum battery cooling during flight, it’s more important to reduce the heat being generated during flight.

Factors contributing to temperature increase during discharge are many, but include:
Ambient temperature
Air flow rate over the battery.
Battery shape.
Power being delivered by the battery.
Physical and chemical design of the battery.

Some of these factors relate to removing heat from the cell, others relate to the generation of the heat energy within each cell.

Let’s use a simplified circuit model and describe each cell as consisting of a ‘perfect LIPO battery’ in series with a lumped resistance represented by a single resistor we will call ‘Internal Resistance’. (At the low discharge frequencies involved let’s consider reactive components to produce lower order effects.)

So every electron delivered by the ‘perfect battery’ must flow thru the series ‘Internal Resistance’ resistor and as this current flows, energy is dissipated in the form of heat. If we know the value of the internal resistance we can make an estimate of how much power is ‘lost’ in the form of heat during the discharge of each battery cell.    It turns out that the value of  ’Internal Resistance’ is easily determined.    Application of ohms law under two discharge conditions is all that’s required.

There are a couple of ways to measure d.c. internal resistance: (both are applications of ohms law)

1. Measure the voltage of a battery with no load, then connect a resistor of known value and measure the current flow thru the load resistor.

InternalResistance = (noloadbatteryvoltage / currentinampswithresistorattached) minus the value of the load resistor in ohms.

Here’s a video of measuring Internal Resistance using this method.

I.R. Measurement

2. Measure the battery voltage and battery current under two different load conditions.
InternalResistance = (Voltage1 minus Voltage2) / (currentinamps1 minus currentinamps2)

Even simpler, a couple of readily available commercial battery chargers measure and display Internal Resistance.

If we use a 3 cell LIPO battery with an internal resistance of 8 milliohms per cell, the overall internal resistance is 24 milli ohms (.024 ohms). Let’s say the current supplied by the perfect battery is 40 amps. Power lost to generating heat is I^2*R and equals 38.4 watts. That’s a lot of heat generated inside the battery. (Some soldering irons use only 15-20 watts of power). That 38.4 watts will never get to your motor.

That ‘perfect LIPO battery’ inside each cell maintains the same output voltage over the life of the battery, but we know that Internal Resistance increases during the battery’s lifetime. As the battery grows older, more and more power is lost inside the battery due to the increasing value of the internal resistor until only 80% of the capacity of the battery can be delivered to it’s terminals, the rest is lost as heat. Monitoring cell internal resistance levels over the life of a battery can be a useful tool as an indicator of electrical age, which may also differ from cell to cell.

We also know that as batteries are made smaller and lighter, the internal resistance tends to go up due to fewer and smaller parallel circuit paths.

When we buy a battery we look at size, weight, number of cells, capacity and something called C rating. C rating is the manufacturer’s estimate of maximum amperage before the battery reaches some temperature that results in loss of lifetime recharge cycles at some unknown rate. That’s close to meaningless and in almost all cases it is not verifiable. We are expected to accept the word of the battery sellers  and in most cases it’s not clear what they are claiming. Even if all the C parameters were defined, you would come close to destroying the battery trying to verify the sellers claims.

Internal Resistance measurement is not destructive and the resulting value relates directly and easily to the power lost as heat within each battery cell. Being an easily verifiable battery parameter the use of Internal Resistance as a quality and aging indicator could put in perspective the many exorbitant and close to meaningless “C” ratings advertised today.

Internal Resistance is easily measured and easily used to determine heat in watts generated within the battery during discharge at any current level.

Internal Resistance can be reduced by making the battery heavier and larger, but small size and light weight are two attributes that are highly valued in any battery used to power a model aircraft.   Internal resistance can also be reduced during the chemical and physical design phase of LIPO battery development and if the manufacturing process consistantly reproduces the design, a battery that loses less power to that always present Internal Resistor can be produced.

When buying a LIPO battery , some Quality Factors to consider that don’t make a lot of sense include:

1. Manufacturer’s/Packager’s poorly defined C rating claims.
2. Claims of lifetime in terms of number of cycles, without extraordinary data to support them.
3. Price
4. Generally, any recommendation that’s not verifiable.

Consider  instead:   size, weight, capacity,  verifiable data,   internal resistance  and expected cost per flight.

“…the determinant of (LIPO) cell life and performance is the temperature the battery cells reach during discharge.”

Spektrum DSM2 interference detector trials:
When I read that the latest version of the Spektrum AR7000 could output a datastream quantifying any failures of the transmitter to receiver 2.4GHz DMS2 link, the idea that this could be used to detect areas of particularly high noise began to take shape. Loss of signal by the receiver occurs when the signal to noise ratio becomes so small that the signal from the transmitter can no longer be discerned from noise (unwanted signals). Both the signal and noise strengths vary with distance from the source.
When the AR7000 DSM2 receiving system encounters a marginal signal to noise ratio it calls this an antenna fade (A) and switches to a second receiver/antenna combination within milliseconds. If the second receiver/antenna combination fails this is also called an antenna fade (B) and then the subsequent loss of information is called a frame (data packet) loss and if several frame losses sequentially occur then the system declares a hold. The AR7000 receiver contains an output data stream that reports the number of antenna fades for receiver A and receiver B, the total number of frame losses and the number of holds, in real time. This datastream can be inputted to a monitoring device, in this case I used an EagleTree V3 datalogger. You could also remote the datastream back to an FPV pilot/observer or a remote data dashboard.

Spektrum says that several antenna fades (50 or so per flight) and several frame losses (20 or so per flight) are normal for a typical flight and if the number of antenna fades exceeds 500 per flight then you should reconfigure the receiver locations in the plane, etc.

The plan was to attach an AR7000 with one remote receiver to the fuse of a slowstick so that there was minimal blockage from motor, esc, battery etc during flight.

Next , fly to different predetermined locations near the field using time as the locator , then after the flight,  analyze the data looking for areas of high antenna fades and frame losses.

Here are a couple of pictures of the SS fuse and a shameless plug for BlueSkyRC’s nifty new plywood stick motor mount.

SlowStick Fuse showing two part receiver and data logger.

New motor mount from BlueSkyRC

Well , like many ideas, this one didn’t pass the experimental stage. On the first flight there were zero antenna fades recorded during a 20 minute flight around Nichols park in Gilbert, AZ. I checked the system using the range check function, it was working, antenna fades and frame losses could be generated but only at a distance of several hundred feet with the range check switch on (transmitter power output greatly reduced). (flying your plane with the range check switch on is highly risky).
On a second try I actually did record two antenna fades during a 20 minute flight but they were at the very edge of my visual ability to determine the attitude of the plane.
A third flight several days later produced zero antenna fades in 15 minutes.

This idea began to resemble a solution that was looking for a problem.

I’m sure that signal blockage by large brick batteries, perhaps some carbon fiber and a large motor can occur, but for park flyers, the DSM2 link between transmitter and receiver is extremely strong and reliable. In larger planes, common sense in positioning receiver(s) is obviously required.

In this case, adding a second redundant receiver was not necessary, the primary receiver was openly exposed and did not fade within visual range.

The next time one of my planes falls from the sky, the last thing I will suspect as the cause is radio link failure due to either low signal strength or high levels of interference. DSM2 technology is indeed robust and impressive.

First of all, I want to thank Jim Wagoner for sharing the great article posted the other day on how to prepare files for a laser cutter.   I have learned much from Jim and actually pickup on QCad because of his recommendation.  I have to admit, I have tried AutoCad and a few others, but keep coming back to QCad — perhaps because it is the one that I have the most time on. I have had as much fun designing radio controlled airplanes as I have had flying them.

One of the things that slow down development on a drawing your aircraft are the precise placements of lines and intersects or a simple thing like equally spacing 1/8th inch square sticks around a curved fuselage former.   I plan on doing up some demo videos here that might be helpful to others on aircraft design here in the future.  Including scale design from drawings.

In the mean time, I have found some videos that cover some cool little tricks that are not always obvious to the QCad user.   Though this author of this video is a bit dry I do find the content quite useful and helpful.

Laser cutting has made the art of model airplane construction easier. It can give the builder the quality and precision to construct an accurate model. Why do I say can? Well, preparing the file for cutting is just as important as a good glue joint. The time spent with your file is worth the end result. Once you have your file, it can be used to cut another set of parts just as accurately as the first or, you can mod it for your next project.

Ok, you just spent your vacation from work designing your dream plane on your favorite CAD software. Now what? What needs to be done? There are a few things that a builder needs to do to prepare their parts to be cut. First thing check to make sure your cutting service you choose can cut the size and thickness you need. Various services have different size cutting tables so see if there are any limitations.

To prepare your file you should remove any text from the drawing and ensure that all lines are on the same layer. (We’ll cover labeling your parts later) Also ensure that all lines are of the “continuous” type. Files should be saved in the industry standard “.DXF.” Contact your laser cutting service to see if they require any special instructions that need to be followed with your file.

For our example here we will be using a set of wing ribs for a sport plane based on the Ugly Stick. I will be using Qcad for this exercise but feel free to sub in your favorite software. The goal is to have the parts to be cut, laid out on the material to be cut. Some things to consider when doing this is grain orientation of the wood and reducing waste.

The ribs here are to be setup to be cut on a sheet of balsa 3”x36”. The first thing I do is copy the rib to a blank page into Qcad. I then create a second temporary layer to draw the 3”x36” balsa sheet on, to use as a reference. This is done so you can visually see that the parts actually fit on the sheet. Create this virtual Balsa sheet in the 00 or home position.

Now that we can see where the ribs go, we need to copy and paste multiple copies into position on our “balsa”. For this model we need 8 of the R1 ribs. Again take note of the grain and try to save material where you can. Leave 1/8” minimum around the sheet perimeter and between the parts.

Once the ribs are in position, the “balsa” on our temporary layer can be removed. Save your file and it’s basically ready to be sent to the cutter. Remember one sheet one file. If you have two different sheets to cut make two files.

Lets touch on labeling parts. What I do is setup the text in another layer on the Cad drawing. This will be on another layer separate from the ribs. With the software I’m using it’s easier to label the part first, adjust the power on the laser, then cut the parts out. There are services that can label and cut in one pass. Again check with your laser cutting service to see what they require..

There are a few things we will look at now. Some services require tabs be left on your parts so they won’t fall out of the sheet when cut. Check with your cutting service and adjust your file as needed. They typically would like two per part about 1/8” running with the grain. Some services will cut without tabs if you don’t want them though.

One question I get is what if the part has to be exact size? Whenever something is cut out, be it with a laser, router, tablesaw or any cutting tool. There has to be allowance for the kerf of the cutter or the part will be the wrong size. The laser I use has an .011 kerf. The software for the cutter allows tool offset to compensate for the width of the cut. That way if you have a 1/4” spar in a rib, you can design your rib with 1/4” cutout to accept the spar. Once cut out, the spar will be a good fit in the rib. So when designing your aircraft make your parts the exact size needed.

Now that the laser cutting service has your file, what do they do with it? For the software and cutter we are using, the file needs to be converted to G-code. This is the format the machine needs to cut the parts. For this I use Deskam2000. This is where the offset for the kerf is set if needed, speed of the laser, adding text and various other settings if needed. Once the file is saved as a .dnc, the file can be loaded into the computer controlling the laser and the cut can be made.

The next few photos show the laser cutting process of the ribs that were
prepared in our example.

I hope this has shed some light on what it takes to prepare your file for cutting. The first time you try it it might seem a little involved. Once you do it a few times though you’ll have all your parts ready to be laser cut in no time.

Jim Wagoner

jtechlaser.com