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created  by Kevin Kelm

Tech Talk - ERCA
Tech Talk  
Questions, Answers, Discussions about Technical issues related to R/C flying
Submit your input to the  webmaster   (Please put Tech Talk in the subject line)

Warbird Primer  by Khoi Tran

   Part 1         Part 2 
Passed along by Khoi Tran

Throttle Safety Gadget  (AKA Rubber Band)
Passed along by Warren Edwards

Trimming Flying Wings (and delta wings)
Passed along by Jim Corbett


Passed along by Rich Richardson

rc Aero Products   2 Stroke Gas Engine Articles, Formulas and Information Charts


Lithium Polymer battery voltages balancing

Awhile back there was a discussion at the field about Lithium Polymer battery voltages balancing. I researched the subject via Google and found a wide range of expert advice and specs.  Below is a spattering of the info found….

Max voltage/cell
Minimum voltage/cell

  Imbalance of one cell of greater than;
  0.1V difference from lowest to highest cell voltage
  0.06V compared to average of other cell(s) in the pack.

My Opinion - Max 4.1v Min 3.0v Bal 0.1v between highest and lowest.

Best Battery Brand ? - I have no opinion, I have tried MANY.  Some same-brand, same-size packs purchased from the same-place at the same-time have behaved differently.

Worst Battery Brands - I could list a few, but none of those are for sale anymore!

What works best for me - Not running the pack down until the ESC cuts it off.  Using approximately 70% of the battery capacity before recharging appears to have doubled the life expectancy of the packs I have used.

If anyone else has experiences or opinions on LiPo packs, PLEASE share them with us.
One of the GREAT things about this HOBBY is folks share information.

Jim Corbett

Digital Servo FAQ

Q. What are the advantages of digital servos?

A-1. Instantaneous response
The digital micro processor is 10 times faster than an analog servo, this results in a much quicker response from the beginning with the servo developing all the rated torque 1 degree off of the center point.

A-2. Programmability
Using Hitec's proprietary programmer and servo test device, model HFP-20, Hitec digital servos can be programmed for, direction of rotation, center and end points, failsafe option, speed and dead bandwidth adjustment. This is great for matching sets of servos for deadband width, center and end points in giant scale aircraft applications and for reversing a digital servo when two are used on a "Y" harness.

A-3. Enormous Standing Torque
The standing torque of a digital servo is 3 times that of its analog counterpart.

Source: http://www.servocity.com/html/digital_servo_faq.html

I have three recent products that deserve some attention:
1) First, my Desert Aircraft DA-85 is an 85cc gas engine that I bought used from a guy on RCUniverse.com.  I was worried that it may never arrive, and I was ecstatic that it finally did arrive and appeared to be in the condition that was promised.  Unfortunately, I could not get it to run consistently.  We tried flushing out the carb and putting it back together, but that did not help.  I decided I did not want to mess with it any further and I sent it off to Desert Aircraft for service.  I was notified a little over a week later that they had replaced the cylinder, rings, and piston with updated parts and the engine was now ready to be shipped back.  My quick calculations had me thinking I might owe them $250 at least, maybe more!  While I prayed for $250 to be the bill the nice guy on the line told me that it was going to be free and to expect it in a couple days.  When it arrived, I put it in my trusty 40% Giles G202 and attempted to start it.  It only took a few flips while choked before it showed signs of life and a couple more produced a running engine.  Is it really that easy?  Apparently, yes!
Just wanted to share a good experience I had with an American company, Desert Aircraft.  By the way, the "new" engine runs excellent and weighs less than my 3w85xi, which has found a new home on my old 95" Extra 260.
2) Hobby King / Hobby City / (insert next company name here) has been flooding the market with cheap products that are often times a really good price on a really good product.  I bought an electric motor recently that really impressed me, (it was a 750W, I believe, but not certain on that) and their lipo batteries seem to work great.  However, what I bought was the four-chargers-in-one unit that they sell for $99.  I have been very impressed with how it operates.  You can charge just about every type of battery we use (NiMh, NiCd, LiPo, LiIon, Pb), and then some, including the LiFe (A123)'s.  It's a very versatile, powerful charger that can replace the last four chargers you have bought, most likely.  It also comes with several cords and adaptors that you will likely use.  I can't remember the name of the charger, but if you look through their online catalog, you're bound to find it.  If you think that's too much for you, try for their more standard single charger unit that now costs somewhere around $20.
3) One last product you may be interested in:  The Wike IBEC (Ignition Battery Elimination Circuit) availavle from TroyBuiltModels.com
What is it?  Well, if you have a gas engine that uses a spark plug, then you most likely have an electronic ignition module that requires power, meaning an extra battery.  That extra battery weighs several ounces, (6-8 oz. I'm hearing from others).  So, you can eliminate that battery and it's switch by replacing them with an IBEC.  It is a neat little device that plugs into a spare receiver channel and provides the power your electronic ignition system needs.  The power is now coming from the receiver battery, which on gas planes is usually one of two giant batteries.  In addition, it has an ignition kill feature built into it so you can turn the power to your ignition on or off with a switch on your transmitter.  You can tell if the ignition is "hot" if the little red LED (included) is illuminated.
I was skeptical and nervous about such a device seeing as we've always been told so much about keep anything radio-related as far away as possible from anything ignition-related.  In my set-up, the IBEC wire was routed within an inch of a servo wire.  I decided to try it and see if it would pass the range tests... with the engine ON.
Well, I tried it, expecting to see glitches.  I really tried.  I did the range test on my Futaba 10CG past the 90-100 ft they recommend.  I went to about 150 ft.  The controls were solid, as if I were standing only five feet away.  I had to look down and wait for the beep to be sure that I was really in power down mode.    No glitching at all.
Time to fly.  The engine was new and it was very windy, but I still put that new engine and the IBEC through their paces in my 40% Giles 202.  Again, no glitches, no problems.  And the ignition kill worked perfectly.  There was about a 1/2 second delay between the time I flipped the switch and the ignition stopped...well...igniting.
That's three good products in a row for me, which has never happened before!
-Patrick Willis-

HX12K Servos

I would like to post these servos on the news letter for anyone interested. I ordered 6 of these from HobbyKing in HongKong for $59 w/2-3day air post. What a good deal for pylon racing planes. I wouldn't put them in a 1.20 sized plane but they would surely be good for general flying.
            $7.99 215oz MG/BB High Speed Digital Servos 5-7Volt, 2oz standard size case

HX12_BBMGHS_Serrvo     HX12_BBMGHS_Servo

I researched these and found they work smoothly after the case has been opened and a little more lithium grease added to each gear.


I found some web pages that list servos specs for futaba, hitec, JR servos that I think will be helpful for members to reference to at our web site.  Sometimes you come across a servo and wondering the speed and torque on the little beast.  It's nice to be able to go to our web site and pull up the data instead of spending time googling and look.  It may be good to put in Tech Talk section of our web site.  I will update my own excel spreadsheet that has the latest servo offering specs that these older listings do not have.  It's nice complement to see list that include newer servos from my file too.
Here ya go:
http://www.servohut.com/hitec_servo_specifications.htm for Hitec
http://www.radiosouthrc.com/Futservochart.htm  for Futaba
http://www.radiosouthrc.com/JRservochart.htm for JR


This would be a good aid to go with the servo charts.  It calculates torque needed to operate control surfaces.

Hey guys,
I found a cool video that shows you how to correctly set up 28 degrees above TDC on your gasser.
Check it out:   http://www.youtube.com/watch?v=GBXFpxWg7vY
And if a young grasshoppa is wondering about: How do you get your hands on that nifty degree wheel?
Aahh, you have much to learn...
Here ya go:   http://www.tavia.com/free_degree_wheel.html
 Print it out and tape it to some hard paper and you're set.

NGK CM6 spark plugs

Are you guys geting tired of getting ripped off from NAPA locally, paying $7 for NGK CM6 spark plugs?
If so, stand back and be amazed...

I found boats.net web site through RCU where you can get genuine ngk plugs for less than $2 each.
I've just bought 20 plugs to last me for years.

Check it out. 

These plugs are replacement for DA, DLE gassers.


Just a FYI, our shop sells Yamaha & Honda parts also. We sell these plugs for $2/each. If i knew people where wanting these i would have them in stock.

Drew Gauderman
RC Manager
Cottage Grove Yamaha & RC

QUESTION    January 18, 2010

Does anyone have a handle on all those different motor naming configurations? Lots of numbers and what do they mean?  MARK GIESSEN


Hello fellow aeronauts,
After an eight year hiatus, I am amazed and somewhat bewildered at the improvements in electric flight technology. For me, Lipo battery ratings seemed a bit misleading when I was trying to pick the correct pack for an airframe.
I searched many websites looking for a simple rule of thumb for selecting a pack of which FMA Direct proved to be very enlightening . They seem to have a lot of information for an electrical novice like myself and I felt it was worth sharing. In paraphrasing ,voltage aside, the mAh rating is only meaningful for a constant rate of drain like a low current flashlight or a transmitter. Meaning, my new 950mAh Lipo Battery has a discharge rate of  0.950Amps for one hour (950 x .001 to convert to Amps). This would be fantastic if the flashlight scenerio could power my plane. But as we all know, the throttle thumb is dynamic and tends to move making a constant discharge rates impractical.
As I studied the foil incased cells, I noticed that the manufacture had added a new bonus number that would surely fill me with confidence.
25C.  What the heck is "C"?  Once again back the web to discover the heart of the mystery.  As it happens, “C” means multiple of discharge rate. (I never did find why “c” was used, I guess it made the industry happy.) This meant that my new lipo could produce a theoretical current of  0.950 x 25 = 23.75 amps. Okay, now at least I know that can burn up my 18AMP ESC without to much effort.
A couple of things for my rule of thumb immediately came to mind , one get a bigger ESC or never pull more than 18amps through my existing one, and two, the motor /propeller combination shouldn’t exceed 23.75 amps at full power.
A fun fact, which I stumbled upon, is a battery phenomenon called cell depression. This occurs when the energy is sucked out of a battery pack so fast that it can not chemically react fast enough to keep the voltage up to it's rating. The motor rpm slows down. Simply said, if my airplane has a brisk take off and runs out of power after few seconds of full throttle sky boring, and then, when throttled back regains power, it’s likely to be cell depression.
(This sure seemes a lot more complicated than those wonderful little fuel burners I loved so much. After all, if power was an issue I'd just up size the engine and add nitro – problem solved!) Anyway, being armed with the fundamental concept of Lithium Polymer Amperage rating spared me from spending extra cash on ESC’s and motor issues. I am currently testing a few motor-prop combinations for Amp usage.   MARK GIESSEN