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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: LIpo's for tx
04/09/2008 15:30:00

If you want SOME of the advantages of a LiPo (capacity-retention in storage, and higher capacity than the supplied battery) with none of the risks why not try "hybrid" NiMH: mentioned in RCM&E a while back (as tagged cells, I think) but now cheaply available as Tx and Rx packs from Vapex Technologies.  Look for the ones with Green cells (confusingly sometimes obscured by red shrinkwrap, as in the case of the AAA Rx pack); the correct ones are shown as 2100 mAH capacity (AA), and the cells are marked Instant.

 Rx 4.8V pack £6.45 with free postage; Tx 9.6V pack £10.85 (also seems to be free postage, but doesn't say so):

http://www.vapextech.co.uk/acatalog/copy_of_High_Power_Model_Control_Batteries.html

Thread: Ohm's Law?
04/09/2008 14:46:00
PR: so what would happen if I connected my prop-equipped model brushless motor across a (low-voltage) three-phase 50 cycle supply (and gave it a flick to get it going, as you used to have to, with old synchrous AC electric motors)?
Thread: What speed controller?
04/09/2008 14:37:00

I understand that at least some Turningy motors are sold (possibly cheaper!) as KeDa units by GiantCod, for instance, a cracking 220 W 1050 kV unit including lots of spares / accessories for £10.40:

http://www.giantcod.co.uk/product_info.php?products_id=402408&osCsid=9c8440871bc48a8e54e355d2b7f5a06d

04/09/2008 11:31:00
I guess a simple soldered-in fuse is a lot cheaper than a high-current ammeter!  Put them in parallel and add the capacity up to match (a bit less than) your ESC?
Thread: Phoenix Rainbow
02/09/2008 14:01:00

Al's hobbies are listing it:

http://alshobbies.com/shop/search.php?Desc=phoenix+rainbow

Thread: Whatever happened to..........
02/09/2008 13:52:00

Eric: from the very battered transfer on my +1 looks like the kit was produced by Rojair.

Thread: Model Restraints
01/09/2008 19:54:00

I enclose photos and a drawing of a lightweight compact (folding) model restraint, suitable for trike and taildragger 'planes. A secure model restraint is a major factor in safety in the pits. Most are large and heavy. If you fly electric (and you just take model, Tx and a couple of LiPos to the field), the normal model restraint is disproportionately large and heavy. You don't need one for a small electric model? My colleague found he did. He selected the wrong model program on his Tx, with reversed throttle direction. When the motor started up "cutting" the throttle actually gave full power, and more than a little excitement as the "people-slicer" rushed round and round! So this is what I knocked up from a few scraps of wood, a few screws and glue to stop them rotating, a piece of tough string, a big nail (which acts as a ground anchor and would restrain a very powerful model; it restrains a sport-40 easily) plus two fencing staples to retain the big nail when it's folded up.

http://www.modelflying.co.uk/sites/3/images/member_albums/26009/restraint_unfolded.JPG

And here it is all folded up; it goes in your pocket (if you have big pockets).
http://www.modelflying.co.uk/sites/3/images/member_albums/26009/restraint_folded.JPG

The upper arms slope back over the tail plane leading edge, restraining it.

http://www.modelflying.co.uk/sites/3/images/member_albums/26009/restraint_in_use.JPG


As you can see here! The nail would be pushed right in, if this picture were at the field. The string loop on the nail is to pull it out of the ground.Yes, I know the model looks tatty - but it is 30 years old.Here is how it's put together:
http://www.modelflying.co.uk/sites/3/images/member_albums/26009/restraint_diagram.JPG

Thread: LiPo pack individual cell voltages
01/09/2008 19:27:00

I've just started with Electric Flight.  I've bought various bits, including an inexpensive digital cell-voltage checker; this plugs into the balance connector and tells you the individual cell voltages for up to 4 cells in series.  When I compared its results with those from my digital multi-meter (DMM) I found it was not very accurate, and showed slight out-of-balance when my DMM found the cells to be the same to within 1 mV.  But it was not easy to use the DMM on the balance plug.  The cell-checker has five wire pins sticking out, at 2.5 mm spacings, and is designed to plug into any JST-XH balance connector (or adapter, if you were persuaded to buy a LiPo with manufacturer-specific connectors on it).  As the pins are not shrouded you can use it to check 2, 3 or 4S packs in one go, or 5S etc. by moving it along the connector (a shroud would foul on the balance connector unless it was 4S / 5 pin.  That's why connectors are shrouded - to stop you connecting two different JST-XH types together, with exciting results).  That gave me an idea: I took what is usually simply called a JST 2-pin connector (charger end) - of the type used on small LiPo packs and small models, and which is correctly called a JST-EHY connector.  The charger end has 2 pins at 2.5 mm spacing, shrouded.  Cut 5 mm off the shroud and it will connect across any single cell on the JST-XH balance connector: plug into any pair of adjacent "pins".  I then wired the cut-down 2-pin plug to two off-cuts of fine copper tube (cut from a failed central-heating boiler thermostat) as this was just right the right diameter to plug my DMM probes into, and fixed them into a small softwood offcut for convenient handling.  Now I can plug these DMM probes into it and use my DMM by just inserting the 2-way plug into my JST-XH balance connector, cell by cell.

 But what if I find the cells are out of balance? I soldered a 4.8 V 750 mA torch bulb to wires which were then soldered onto two short lengths of bike spoke (same diameter as my DMM probes), and applied heat-shrink tube.  To bring down the high cell a bit, just plug the torch bulb connectors into the copper-tube sockets and put the 2-pin plug across that cell via the balance connector.  The bulb shines brightly using up the capacity at about 700 mAH per hour (at about 4 V).  But how long should I leave it connected?  If the battery pack is charged to between 3.7 and 3.9 V then it loses about 5 to 10% capacity per 0.1 V (less if above 3.9 V).  So if you saw a difference of 0.1 V in a 2250 mAH pack then the high cell needs taking down between 112 and 225 mAH.  That takes between 10 and 20 minutes.  So check it after 5 minutes!

Thread: Looking after your LiPo cells
01/09/2008 19:24:00

I used a different chart, from an online book on all kinds of cells and batteries:

http://www.buchmann.ca/default.asp

The Li (Ion, not LiPo  but it's similar chemistry) capacity chart is at:

 http://www.buchmann.ca/chap2-page6.asp    Click on the link to see the chart, then page down.

It says that almost all the useable capacity is between 4.0 and 3.5 V.

Loads of other interesting stuff in this book.

Thread: Ohm's Law?
01/09/2008 19:16:00

Kevin Annells assumes a constant-power situation when he contradicts Tim Mackey. What TM says is absolutely correct in describing a simple case: when a voltage (potential difference) is applied across a resistive circuit, increasing the voltage (PD) will increase the current (in direct proportion to the voltage (PD) increase).  Or as Ohm put it: V = I *R,  or Potential Difference (volts) = current (amperes) times resistance (Ohms!).

I think KA is probably wrong in his assumptions, because if you change the voltage fed to a radio system it does NOT necessarily take constant power when the servos move.  It's not an electrical issue.

In electrical systems, power is quoted in Watts.  Power is the product of current and voltage (= V * I). Power reflects the rate of using energy and doing Work. The energy or Work capacity of a battery is the product of its voltage and its current-capacity (the latter is how long it will deliver a certain current and is measured in ampere-hours). So the energy (or Work) capacity of a battery will increase if you increase its voltage OR if you increase its current capacity.  If you do both you get a double effect.

A radio system is not simply a resistive load, and the peak consumption is when the servos are moving (and they move against a modest force-load, using electrical energy and doing Work).  The Work they do is the product of the force-load and the distance moved.  The power they generate (equals the power they use, divided by their electrical efficiency) is the RATE at which they are doing Work (Work per unit time).  When you increase the voltage, the servo moves faster but it gets to its target position quicker (so it's working for a shorter time).  It's using more Watts but for a shorter period.  So if the forces are not speed-related its energy consumption does not necessarily increase.  Whether they are or not is not a question of Electrics but of Mechanics and Aerodynamics!  If the quicker servo movement increases the physical load on the servo (as it would if it were e.g. moving a paddle through the air, as air resistance increases with the square of the speed) then it will need a bigger battery as the Work done would also square, requiring an increase in both battery voltage and Ampere-hour capacity.  Working in a vacuum against just speed-independent friction forces it would not: the work done in a constant length flight might stay the about same so a smaller Ampere-hour capacity might suffice, as the increased volts increases the energy or Work capacity at constant Ampere-hours.

In the real world: put a big one in. Model helicopters don't fly well in a vacuum!

Thread: The September Grand Prize Draw
01/09/2008 17:26:00
Yes please.
Thread: Whatever happened to..........
01/09/2008 17:23:00
Eric: about the "Plus Three".  You are referring to the Plus-series of slope-soarers.  Plus 1 (well, +1 really) was the slightly-swept 72" foam wing 2-channel trainer. +3 was the aileron-equiped aerobat (60"?) and the +2 was sort-of in-between.  I've just been to look at my old +1 to see if the battered transfers say who it's from (Dave Worrall?) but no sign.  Bought mine in mid-seventies. Mine flew from Winter Hill too! 
Thread: What speed controller?
01/09/2008 16:43:00

There are two references (about ten and thirteen postings back) to a supplier's "ESC guarantee", and a statement that this may not be legal.

It's not that common for a retailer to offer a guarantee, as it's not really necessary.  A guarantee is normally a way of the manufacturer taking direct responsibility for their product, so a claim does not have to go step-by-step up the chain:  retailer / wholesaler / manufacturer: in English law your claim for a defective product is against the person you bought it from (and his is against whom HE bought it from...).  There are several laws covering this but the original one if the Sale of Goods Act (originally 1893, latest major overhaul 1979).  This act requires that what you sell is Fit for Purpose and of Merchantable Quality, or you are in breach of contract.  These two aspects are not defined, but are what a reasonable, informed and responsible consumer would expect - and their meanings depend on what you're buying and what you are paying.

There's nothing in law stopping a guarantee spelling out restrictions; it's perfectly legal (in the sense that it's not a criminal offence to do it).  Indeed, all the guarantees I can think of do just that!  However if the retrictions are not reasonable, they would fail in court.

Merchantable Quality does not mean perfect.  If I buy a box of matches, I can expect a few to snap as I light them, but only a few.  The product does not need to last for ever: I can only expect each match to light just once.  However, I can expect a lightbulb to light every time I switch it on, for many hours use: provided I use it in a sensible way and keep within its voltage limit.

I use the supplier referred to and have been very satisfied with the goods he's sent me (decent quality at rock-bottom pocket-money prices).  I shall keep buying from him - including ESCs.

Thread: Futaba 6EX exchange programme
31/08/2008 10:48:00
But the Bind is supposed to be a one-time process, of course, which you do BEFORE installing the Rx.
Thread: What speed controller?
30/08/2008 10:59:00
I would add they also "come in to their own" when you want to move to 4- or 5- (or even 6-) S packs, as there's more voltage drop from the e.g. 18.5 V of a 5S pack to the 5.0 or thereabouts delivered to the radio stuff.  A linear BEC throws away (1 - 5.0/18.5) = 73% of the power and delivers only 27% to the radio (a switch mode unit essentially does not).  So if the radio takes 5 W a larger amount of heat than this will be generated in the BEC (= 5 W * 73/27) = 14.6 W.  More than this really because of inefficiency.
29/08/2008 19:25:00
See my post above about Hobbywing Guard 40 ESC / Giantcod, and all for less than twenty quid.
26/08/2008 09:58:00

What kind of ESC: I've just seen a type from Giantcod (the Guard series by Hobbywing, who manufacture for several brand names) which connects to both the power leads AND the balance connector and cuts motor power if ANY cell goes below a pre-set limit, not just the overall pack).  Seems an excellent idea to me so I bought one: the 40 A version (to power a motor which should take 20 - 25 A).  Also has a switch-mode 3 A BEC which will work with  5S LiPos (6S for the bigger ones), though the Giantcod generic notes don't make this clear (but it's in the Hobbywing instruction sheet).

http://www.giantcod.co.uk

Thread: Closed-loop linkages
26/08/2008 09:40:00
Servo movement: put it on a switched channel and you may see more, as the Tx trim and EPA use some of the potential movement.  I wanted to allow for as far as the servo could possibly travel.
25/08/2008 17:56:00

1. My servos move more than yours.

3. (yes 3 not 2) Yes an intermediate arm allows you to take the tension off, with a push-rod between the servo arm and the intermediate arm.  It also lets you have the arm on the control surface LONGER than the servo arm, by matching the length of the control surface arm to that of the intermediate arm - and maintaining the rectangular geometry you mention in 2 above ('required' to avoid binding forces).

2 (yes 2 not 3) The reason why I would LIKE to have a control surface arm (or pair of horns) LONGER than the servo arm is that otherwise the angular deflection of the control surface will exactly match that of the servo.  That may be OK for a rudder but I prefer a bit less for the elevator. 

The reason why I started all this was to establish how significant the binding issue was, and to see if there was a geometry which gave less angular deflection at the control surface than at the servo, did not introduce binding forces and was slack-free at all angles.  There is!

Thread: PET Sheets
25/08/2008 14:41:00

Gemma: Try looking for a trade-name product such as Mylar sheet.  Mylar is PET film (as are Hostaphan and Melinex).  You'll also see it referred to as polyester film / sheet.  It's the same basic material: the difference between Mylar, pop-bottle PET and the resin to make polyester trousers (as worn by Electric Flyers) is simply one of molecular weight (how hard it's been cooked!).

Try: http://www.stencilwarehouse.com/acatalog/Stencil_Material.html

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