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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.

Safety Considerations When Flying RC Aircraft at Nichols Park

1. Flying should always occur west of the concrete divider at the top of the hill.

2.  Spectators and stashed gear at the top of the hill should always be on the east side of that concrete divider.

3. All take-offs , Landings , Low Passes and Walk Ons should be announced loudly by the pilot to alert other pilots and spectators.

4. When landing from the west, plan on stopping several feet in front of the concrete divider.

5. High velocity low passes should be at least 15 feet from the concrete divider.

6. The maximum number of planes in the air ,  being flown from the top of the hill , should be 4.

7. Before maiden flights and after rebuilding or modifying an aircraft:

…..a.  Have a fellow pilot double check the setup and structural integrity of your plane.

…..b.  Take off from the bottom of the basin.

8. Fly within the boundaries of Nichols Park.

9. Do not fly over people, houses, automobiles or streets.

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.”

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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.

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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.

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.

Less than 20 years after the Wright brother’s first flight, Henry Woodhouse published a textbook on Aeronautic Engineering.  Excerpts from this book that relate to German WWI war planes have been reprinted and offered for sale for less than $10 at this site:

If you ‘re interested in modeling WWI aircraft, this collection contains details of construction that can be used to improve the accuracy of your models.  Other details , like the configuration of the  control linkages may not be of much modeling help but will be interesting to WWI era airplane history buffs.

The Fokker Single Seater Biplane D-7 makes a good example of the contents of this collection.   Photographs and diagrams are abundant and the specific topics covered include:  Airfoil, Wing Construction, Struts, Fuselage, Tail, Undercarriage, Engine and mounting, Radiator, Petrol and Oil Systems, Throttle control and Fabric and Dope.

The advancement in aviation in its first two decades is amazing and exemplified in the contents of this publication.

This book is well worth the reasonable price and if you have even a glancing interest in early aviation I recommend it.

I acquired a short kit for a Dr1 from the designer, David Payne,  at BlueSkyRC a few weeks ago and the build is about complete.
This is a 31 inch (top wing span) 1/9 Scale model of the famous tri-wing WW1 fighter. As a scale model it has the same number of wing ribs as the original and your first impression when inspecting the kit is ‘holy cow that’s a lot of ribs’.    ( phrase acquired at Joe’s BBQ)
Framing the plane is straightforward but takes some time, I found myself searching through sheets of laser cut balsa looking for individual parts.  Speaking of laser cut parts, these were excellent, having been cut by master laser cutter,  Jim Wagoner at JTechlaser.
David has a build thread on RCGroups and combining the photos online with his well drawn plans  provides the direction required to successfully complete the building of this plane.  This is not a kit for the first time builder, but with a grand sum total of 2 different short kits and a couple  full  kits of experience I was able to get the pieces glued together to complete the framing.  The cowl is located and attached with two carbon fiber eighth inch rods and two pairs of magnets.  Binding for the metal wires for the cabanes and later the landing gear was braided kevlar fishing line, saturated with thin CA.
The rudder and elevator servos are located with mini-connector adjustment access from the open cockpit and use push rods.  The aileron servos were glued to the cover plates and wind up inside the top wing with the servo arm protruding thru the plates.

OK,  so now I’ve got 4 wings, a fuselage, 4 control surfaces, a balsa/plywood cowl,  etc, and it’s time to decide on which full scale Dr1 this model is going to look like.  The decision was made,  a blue and white Dr1, registered in Germany as PH-EBF, which is a replica of Dr1  flown by LT von Raben, Jasta 7, 155/17.   Here’s the one picture I could find of that plane actually in the air, among a dozen or so ground shots including one of the plane displayed indoors in a museum in Belgium.

The Dr1 aircraft preceded color photography, so the actual colors are often disputed.  History indicates that the blue color was based on a fabric sample that was mistakenly assumed to be from LT von Raben’s Dr1 but probably came from a later Fokker DVII that replaced the Dr1.  Nevertheless, I really liked the blue and white scheme , so my plane is a model of above PH-EBF which is a replica of LT von Raben’s Dr1, which was probably red and white rather than blue and white.

Here’s the firewall and motor mount , note the carbon rods, magnets and tapers on the motor mount to give a little down and right thrust.

Building this plane has been a learning experience, a lot of fun, and because of the well executed design and quality laser cutting produced a good looking scale model Dr1.  Will it fly? —  report coming up in a few days.

Link to Callie Graphics

This book  is a very interesting read, I  highly recommend it to anyone browsing the PopularRC site.  It’s a non-academic discussion of the things that cause planes to behave the way they measurably do. Whether you’re an RC pilot or model designer/builder this book will keep your interest because it’s chock full of useful information presented in an easy to read style.

In the section on airfoils, Carlos states that airfoils are overrated (I always suspected that) and then goes on to discuss the Kline-Fogleman airfoil, in which the stall and lift characteristics are changed by simply introducing ’steps’ into the airfoil, fascinating stuff, and a good example of the contents of this book.

For less than $20 you can buy a copy of this 200 page paperback from RCadvisors.com and they will throw in a 6 month membership to their online design assist calculators. Alternatively, it’s available at Amazon.

http://www.rcadvisor.com/book

Hyperion has claimed 5C charge rates for their G3 LIPOs for several months now.
Most Chinese LIPO factories are now supplying data showing their own LIPOs are also 5C charge capable. Hobby King will change their battery labels as soon as they use their current labeled product.

The new Hobby King labels will specify the following acceptable charge rates:
TURNIGY 20,25,30,40C : 5C Charge Okay
Rhino 20,25,30,40C : 5C Okay
Flightmax 30,40C: 4C Okay

Seems as if these batteries have been capable of being charged at rates greater than 1C for some time since no known new technology has come into play.