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Member postings for John Cole

Here is a list of all the postings John Cole has made in our forums. Click on a thread name to jump to the thread.

Thread: Peter Vivian's letter, Dec. RCM&E
17/11/2008 19:21:00

PV's letter in All Write describes control problems with an electric plane which he had converted to Futaba 2.4 GHz.  His post-crash investigation showed (as I read it) aileron controls being moved (to the left) when the motor was run full-power.  He emphasises the need for power-on checks before flying, and I support that fully.  He also makes some comments about aerial siting.

 However, I can't understand what he describes: as least not with a correctly-working set.  If the signal is correctly decoded by the Rx then the flying controls (such as aileron) will not be affected when the motor runs flat-out.  If the signal is not correctly decoded then the Rx should go into fail-safe; the flying controls will freeze in their last good position and the motor should go to the fail-safe setting: 20% of full power is the default, but for electric models I set it to (essentially) Zero - by using the throttle-cut button during set-up.

 So why is he seeing what he describes?  Is it anything to do with 2.4 at all?  Maybe it's a duff servo affected by vibration?  Maybe the voltage from his BEC is dropping at full motor power and this is causing strange results (but why on just one channel?).  Maybe he's inadvertently programmed-in throttle-aileron mixing (well it's worth asking!).  What problems have others found?

His comments on (Rx) aerial: the Futaba recommendations are a sound way of ensuring that at least one of the two aerials can "see" the Tx but I've found the system tolerant of proximity to ESC, battery wires. motor wires and minor bits of metal.  Certainly I would not expect ANY of those to create a problem during the take-off run when the plane is close to the Tx.  What have others found?

Thread: New charger
17/11/2008 16:38:00
If you have a lowish-capacity LiPo charger designed for 12V input, the cheapest way to mains-power it is to buy a PSU mass-produced for what are variously called TFT or LCD screens (flat PC display screens).  My charger takes a max of 3.7A input (for 2A output).  I bought a 5A PSU for £4, brand-new from China, via Ebay (search for TFT power supply).  It arrived in one week, carriage £5.  These are switched-mode smoothed and stabilised devices, and will run from 110V to 250V mains input.  I saved £1 by electing for a Continental 2-pin mains plug!  The 12V output is delivered through a jack-type socket, usually centre-positive.  I cut this off and wired the output to a 2-pin polarised socket (JST-EHY), and inserted a similar plug/socket in the charger input lead.  This means I can continue to use the croc. clips with a 12V battery, but if I unplug them and connect the charger to the power supply I can't mistakenly use incorrect polarity in powering the LiPo charger. 
Thread: The October Grand Prize Draw
27/10/2008 14:04:00
As my Yamamoto trainer was built in 1974 I am in need of a new one.  A WOT would do fine.   JSC
Thread: LIpo's for tx
21/10/2008 13:57:00
Note: the GREEN ones (lower capacity - 2100 mAH) are the ones with improved charge retention.
20/10/2008 19:32:00

Don't overlook the new-generation NiMH cells I referred to earlier: 2000mAH capacity and good charge-retention, and inexpensively available in an 8-cell AA flatpack (with Futaba plug).

 The LiFePo4 cells don't seem yet to be available in AA format (you could then use 6 in a 2P 3S config and remain within the Tx pack-space); ordinary Li Ion cells are, but not this new chemistry.

These new cells seem to be aimed at power tools where their ability to accept charging- and discharging-abuse will be every useful, and the AA size may be too low-capacity for that application.

20/10/2008 18:09:00

18650 are half the capacity of 26650, and the latter weigh 70 gm per cell (210 gm total).

The bigger cells are 26 mm dia and 65 mm long.  18650s are 18 mm dia and 65 mm long (the original Li-ion packaging size).  You could of course use 6 of these smaller ones to get 2000 mAH capacity: 2P 3S.

Not a good shape-match to AA.  Would I think need to be external.

20/10/2008 15:24:00

Would it be better (for 8-cell Txs) to consider A123 LiFePo4 cells, with nominal voltage 3.3V and working range 2.0 to 3.5V?  A three-cell pack would give a voltage range of 6.0 to 10.5V so the max voltage would be within the NiMH max (I see 11.3V after a full charge) and the safe minimum well below the Tx cut-off (for me 8.4V)?  A123 ANR26650M1 cells offer 2300mAH.  See:

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6444 and

http://www.a123systems.com/#/products/p1

Thread: The October Grand Prize Draw
02/10/2008 15:27:00
It's my turn to win this month so no-one else need bother entering!
Thread: The great Ohm's Law debate
26/09/2008 11:18:00

Timbo: I don't think you were necessarily correct as I said in my original posting.  Most of the posts which related to the original subject focussed on a simple resistive load, and in that simple case (as Ohm said) if you increase the voltage you'll increase the current - and thereby exhaust the battery quicker if it's of the same mAH capacity.

But you were talking about feeding a Rx and servo system.  The bulk of the current draw for a model helicopter will be to drive the servos, as they work a lot in a helicopter.  The bulk of the current draw for the servos is to drive the servo motors to move the blades.  If you increase the voltage, these motors will draw more current.  BUT they will move more quickly to the target position.  So they will draw more current for a shorter period.  So they won't necessarily exhaust the battery more quickly.

If the servo motor efficiency does not change much between 4.8V and 6V then we can ignore the ELECTRICAL issues and look at the WORK the servos do per unit of flight time (the average WORK rate).  If that increases at 6V then you need a battery with more ENERGY stored in it.  Whether the WORK rate increases (averaged over the flight, not just when the servos are moving) depends not on electrical issues but on what "friction" they are working against.  If it is an aerodynamic-drag  (i.e. viscous) load then this will increase with servo speed.  If it's straight friction it will not.

To take the extreme case of straight friction (i.e. no increase in mechanical load at higher servo speed) with no change in servo motor efficiency at 6V, and ignoring the "electronic" current for the Rx and the servo electronics then the energy capacity of the battery needed for a given flight time at 6V is the same as 4.8V, as the increased current draw will be exactly offset by the reduced movement time.  BUT if you add a cell (from 4 to 5, sticking at the same mAH capacity) you have INCREASED the energy / work capacity by 25% already!  So maybe a smaller cell-capacity would do!

Thread: The September Grand Prize Draw
25/09/2008 10:36:00
Try again...
Thread: Ohm's Law?
13/09/2008 17:36:00

Peter: going back to some earlier posts of yours on brushless motors and the pulsed DC used to drive them:  I seem to remember reading somewhere that the potential for radio interference was greater from the two battery leads to the ESC than from the three power leads from the ESC to the B/L motor, and it was therefore desirable to site the Rx (and aerial) further from the battery lead than the motor lead.  The reason given for this was that "the AC current from the ESC to the motor is smoother than the DC feed to the ESC".

Your explanation and diagrams make me feel this reasoing is specious.  Do you agree?  Mind you, specious reasoning does not mean the conclusion is wrong!  So comment on that too please.

Thread: LIpo's for tx
09/09/2008 16:24:00
Timbo: I don't know the power source for a DX6i, but I've seen some references that suggest it just takes 4 cells.  If that's so then a Vapex Rx pack might fit (they do them flat and square). and recharge with a (reversed-polarity) Rx charger?
08/09/2008 17:58:00

Myron: in an early note you said "For those who dont know by the way the 2.4 6EX  tranny uses twice the power of its predecessor  Nobody told me that before today & 'tis from a reliable source. "

I have previously said the Futaba 6EX 2.4 GHz Tx has a current drain of 170 mA, so should give more that 4 hours off the standard 700 mAH pack (and showed this is actually the case with mine; still going at 4 hours 30 muinutes).  I have since found that the current drain of the predecessor (6EXAP) is 250 mA.  So it's gone DOWN not UP, and they use the same capacity NiCd pack.  In both cases the info I am using is in the Futaba manual.

Thread: Futaba 6EX exchange programme
07/09/2008 20:50:00
My replacement too has a 7* serial, but it has an I sticker, and the Ripmax recall notice says if it has an I sticker it's OK.
Thread: LIpo's for tx
06/09/2008 19:11:00
If the problem is in the battery pack or the charger then a replacement won't help as neither of those is changed.  However, it will rule out a Tx fault.
06/09/2008 15:02:00

Myron: I think something in your set-up is faulty.  Yesterday I showed you that the theoretical working time of the 6EX-2.4GHz was over 4 hours.  Here's a practical result.  I charged mine overnight, and left it switched on today.  The charger puts out 70 mA so for a 1.4 overcharge it needs 14 hours from flat.

On switch-on it showed 11.1 V.  After 15 mins it was down to 10.6 V.  After 1 hour 10.1 V.  After 2 hours 10.0 V.  After 3 hours 9.8 V.  After 4 hours 9.6 V (the stated nominal).  After 4 hours 30 mins it was still showing 9.3 V when I switched it off.  So this type of Tx has good duration if all is working correctly (I appreciate some e.g. glider pilots might like more, and you can get NiMH packs with more than 3 times the capacity).

I suggest you check to see what's wrong before you move away from Nickel.  Four obvious possibilities:

1. Faulty Tx with excess drain (or faulty voltage sensor): extremely unlikely

2. Faulty charger.  Unlikely but easy to check if you can try another charger (make sure it's also rated at 70 mA; the old Futaba ones were not).

3. One or more faulty cells in the pack.  Possible.

4. Cell-cell connector spot-welding faulty.  That's what I found in my last dodgy pack.  You need to disassemble the pack to find the fault, and then if you can be bothered to fix it the pack's really only fit to run e.g. a spare Tx used to drive a simulator.

Hope this helps.

Thread: 2.4 GHz tranny black ribbon
05/09/2008 21:51:00

Erfolg: you say "SMAE was seen by many as irrelevant to them, I suspect the BFMA may be similar".

BMFA is the current operating / trading name for SMAE Ltd.  See the back cover of the Handbook.

They thought that by changing the name they would fool people.  Well...

Thread: LIpo's for tx
05/09/2008 17:33:00
The standard NiCd in a 6EX-2.4 GHz Tx is 700 mAH.  The stated current drain is 170 mA.  So with the standard NiCd it should transmit for four hours before packing up, if fully charged.  See also my posting above about 2100 mAH Vapex high-retention packs if you want more.
05/09/2008 16:45:00
Tmbo: if the operating voltage is c. 3 V why do we put 9.6 V packs in the Tx?
Thread: Ohm's Law?
04/09/2008 16:51:00

Seems to me you are now saying that our brushless motors ARE A/C motors and that the non-A/C bit is that we drive them with interrupted-DC from our ESCs!  Otherwise they wouldn't run off three-phase A/C, I think. 

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