From the Lab - December 1998 - RC Groups

# From the Lab - December 1998

### Steve shows off how...well...dumb he is.

Flying in the Winter

According to the calendar, winter is approaching. The temperature today went above 60F (15C), so I am not so sure. But if it is winter where you are, you may want to make some preparations so that you can continue flying. See the November 97 and December 96 From the Lab columns for some suggestions on flying in the winter.

Some Thoughts on Charging Nicad Batteries

How and when you charge your flight batteries is probably determined by when and where you fly. If you fly at a club field where there are rules about the number of planes in the air at any time, you probably have to wait your turn to fly, so you have plenty of time between flights to charge your batteries, and probably only need one battery pack. This article is not for you.

If, like me, you go to the field to get a number of flights in a short time, you need a number of battery packs and and it makes sense to charge them in advance. I have three batteries for most of my planes, and I try to gang charge them the night before. This works OK, but the batteries do not perform as well as when they are charged just before flight. The real problems come in when for some reason (such as the weather...) I cannot do my planned flying, and the batteries sit in a charged state for days or even weeks. I have found that even if I 'top off' the packs by putting them on the charger and re-peaking them, they just do not compare to freshly charged packs. For high current applications (such as the Jerry) the decrease in battery pack performance is even more obvious. Re-peaking, while a nice idea, does not appear to do much for the performance of the battery. Even if your charger tells you it is putting something into the battery, most of it is probably wasted as heat.

The decrease in performance appears to be a combination of decreased voltage and increased internal impedance (resistance), which causes even lower voltage as current is drawn from the battery.

The solution to this problem is to charge the packs as close to flight time as possible. If they are going to be left for several days before use, you might as well discharge the packs, and recharge them when you need them. I use my Robbe/Aveox Power Peak Infinity charger to discharge packs to 0.75V per cell at 5 Amps.

The following table of experimental measurements of loss of capacity versus time between charge and discharge came from the Australian magazine Radio Control Model News, August 96. The pack used for testing was a 7 cell pack of Sanyo 1000SCR cells.

 Effects of Delay between Charge and Discharge Delay Before Discharge Minutes Discharge Current Amps Discharge Duration Seconds Average Voltage Capacity Amp-Minutes Watt Minutes 40 15 245 7.83 61 480 20 15 257 7.85 64 504 10 15 269 7.91 67 532 5 15 271 7.91 68 536 3 15 273 7.90 68 539 0 15 281 7.96 70 559

The discharge current was the same for all of these tests. The 'Discharge Duration Seconds' is the time to discharge the pack (at 15A) to a fixed cutoff voltage, with longer meaning more capacity. The average voltage is the average of the battery voltage over the discharge time, with higher meaning more power available. The capacity in Amp Minutes and Watt minutes are measures of battery capacity, with higher meaning more capacity - more power available. If you are unsure of these measurements, see the January 97 From the Lab for more details.

The above table is clear: any delay can cause a significant loss of capacity, and at 40 minutes, the capacity loss is about 13%. This will obviously be different for different types of packs, but this is worth considering for any nicad pack.

Oh Yeah? Well I Am Dumber Than You Are!

I, like pretty much everyone else, am getting older. At least I think that is what is happening. There are three things that happen to you as you get older. The first is called CRS, which stands for "Can't um er ah, oh whatever". I don't think I can remember the others. But the problem with not remembering things means that it is really easy to do dumb things.

Dumb thing number 1:

Several weeks ago, I drove to my usual before-work flying field (which is right next to where my daughter goes to day care), got out of the car, and started putting together my plane. I reached into my field bag for my transmitter.. and it was not there! My transmitter tray, which usually stays in the back of the car, wasn't there, either. Now, what happened to it? At first, I tried to convince myself that it was at home, but I could not remember having taken the transmitter out of the field bag the night before when it was in my workshop, and the bag did seem kind of light that morning. As I thought about it more, I realized that I must have never put it back into the car after flying the previous morning. I must have left it at the field, which is actually a city athletic field. I assumed it was gone for good, and I would have to replace the transmitter, tray, and stopwatch.

As I was about to get in the car, I noticed a sign on a nearby fence, made from what appeared to be the top of a pizza box. It said:

GLIDER PILOT
CALL PEABODY
PARK DEPT
531-5427
ABOUT CONTROL PANEL

A nice person from the park department had noticed the transmitter, which I left on top of a large rock, taken it to their office, and left me a sign to let me know. Talk about nice people! Actually, the Park Department people are so nice that they often leave the sprinklers off in the morning in the area where I usually launch and land. I went over to the Park Department offices and retrieved my transmitter and tray, and thanked them for their good deed.

I did find myself wondering how or why they noticed the transmitter, and I found out pretty quickly when I turned it back on for about 5 minutes. When I left it at the field, I must have left it turned ON, and the battery discharged until the low battery beeper started. The Hitec Prism 7XF low battery alarm is continuous - once it starts, it beeps until you turn the transmitter off, which is a pretty good idea. Someone must have heard the beeper! If the transmitter had been my JR XF783, they never would have heard it, because it beeps just once as the voltage goes below 9.0V.

Dumb thing number 2:

After a few flights on my Multiplex Twin Star, I got rather brave and decided to try extended inverted flight. Check out the 'Flying' section of the review in this month's E-Zone for the details on this dumb idea.

Dumb thing number 3:

I have recently started flying a Hobby Lobby/Graupner Cumulus 2000, which is a Cumulus 97 with ailerons. There are servos out in the wing at each aileron and 24" (60cm) cable extensions. On a recent flight, the motor started to sputter - literally turn on and off, as if it were an erratic glow engine. I had never seen this sort of behaviour before, and I was rather baffled by it. Could it be interference from the servos out in the wings? With the motor off, the plane flew fine, and with the motor on at partial throttle it seemed to stop sputtering. I finally landed the plane, and checked out the motor on the ground. It ran fine, so I relaunched, and the problem started again.

When I landed again, I discovered the problem. The nose cone catch pin was not completely in the mating hole, so the nose cone was held on only by friction. The Cumulus uses a slip-on nose cone with internal motor power connectors. When the nose cone is removed, the connector to the motor is separated. With the nose cone only held on by friction, it could move forward a little when the motor was on, and would settle back when the motor was off. Each time I turned the motor on, the propeller would pull the nose cone forward a little, and aerodynamic drag would pull the rest of the plane back a little, and the connectors would separate. As soon as they separated, the motor would stop, and the nose cone would settle back onto the fuselage, causing the motor to restart.

The solution, of course, is to back out the (threaded) catch pin a little so that it more solidly catches on the nose cone. But this could have been a disaster! If the nose cone came off in flight, the plane would be completely out of balance - and completely powerless.

So, those are some of the dumb things I have done recently. Can you top them? Why would you want to?

Printed Word Watch

If you find this month's review of the Twin Star interesting, you may also want to read about other planes in the Multiplex Pico line.  Electric Flight International December 98 [ http://www.traplet.co.uk ] has a review of the Multiplex Teddy

Copyright

This document is copyrighted (c) 1998 by Steven Kranish, and may not be copied or used in other forms of publication (electronic or paper) without written permission from the author. I will probably grant permission, but I would like to know about it, so go ahead and ask.

Contacts

If you have any questions, please feel free to contact me at skranish(at)ezonemag.com

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