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Member postings for Mike Blandford

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

Thread: Understanding motor sizing?
07/10/2017 23:45:18

A general feature of changing to a longer motor is several parameters change at the same time.
If you just make the stator longer, but keep the same number of turns of wire, then the kv will reduce (longer wire in the same amgnetic field) and the resistance will increase (longer wire). So to keep to the same kv, the number of turns would be reduced (lower resistance), which then allows a larger diameter wire to be used (again reducing resistance).

Suppose you have a motor with a kv of 1000 and a resistance of 20mohms (milliohms), using 10 turns of wire. Now create a new motor with a stator of twice the length. The kv woud reduce to 500, so you would need to reduce the number of turns to 5. If you use the same diameter wire, the resistance would be about the same the first motor as you have half the number of turns, but each turn is about twice as long. However with fewer turns, you can fit a larger diameter wire in, one that gives twice the cross section in fact, and this would halve the resistance.

So you end up with a motor of the same kv (1000), but the resistance is now only 10mohms. For the same heating effect in the windings, this longer motor could handle a 41% increase in current, resulting in a (potential) 41% increase in torque.

To use John's example where the motor was 50% longer, this would reduce the winding resistance to 2/3.
Suppose the original resistance was 52mohm. At 40A, you would lose 2.08 volts across the windings which would heat at 83W, and bo operating at about 80% efficiency. The longer motor would have a resistance of 35mohm. At 47A, only 1.65 volts would be lost (giving an extra 0.43 volts towards turning the motor, 430 rpm with a kv of 1000).
Only 77W is now lost as heat, and the efficiency is 84%.

A note regarding using a longer motor. I use a "2832" (long) motor in a WOT4. Originally I had this mounted "backwards" so the stator was bolted to the firewall and the prop/spinner was mounted on the rotating part. This meant the prop/spinner was a large distance from the fixed mounting, and at high revs i got significant vibration and I couldn't get rid of it no matter how well I balanced the prop/spinner. Eventually I pushed the shaft through the motor, built a ply box onto the firewall and mounted the motor "forwards" with the rotor behind the mounting and the prop/spinner in front. This completely solved the vibration problem. If you wish to mount a motor "backwards" I would therefore suggest a shorter, but larger diameter motor would be better.

Mike.
 

Edited By Mike Blandford on 08/10/2017 00:01:43

Thread: Totally drained NiMh pack - can it be resuscitated, and how?
06/10/2017 18:34:38

Stewart: I'd like to see a reason for your statement.

In my experience, NiMh (and NiCd) cells are quite resilient, unlike lipos that really don't like being over discharged.

If my suggested method is followed to check the pack, I can't see any problems with then using the pack. In fact, I think I would prefer it to a brand new pack that hasn't been tested for capacity and discharge capability.

Mike.

Thread: Who wants a Warbird Replics Hurricane?
05/10/2017 23:22:25

I've had a look at some 3-views of the Hurricane, and it looks to me that the aileron span should be less than that shown on the plan, but the aileron chord should be greater, reaching the slots already in the wing. Unfortunately, making the ailerons have a shorter span would mean the torque rods supplied won't be long enough.

I think I shall go for a more scale span, and fit individual servos in the wing. This will also then allow me to program aileron differential, rather than create it mechanically. The torque rods will then be used for the flaps I intend to fit.

Mike.

Thread: Understanding motor sizing?
05/10/2017 23:14:01

Torque is directly proportional to current, and my understanding is that given two motors with the same kv value, they will provide exactly the same torque for the same current, a larger motor will not provide a greater torque for the same current.

A larger motor will, as you say, have a lower resistance, thus you would only need a lower voltage to obtain the same current. If you use the same voltage, then, due to the lower resistance, you will get a higher current and that gives a higher torque.

For the same mechanical output, the larger motor will, in general, have a higher efficiency and so be the better choice.

If you select a larger motor that has a lower winding resistance, then you may also choose to go for a (slightly) lower kv value. This will give a higher torque for the same current allowing you to use a larger propellor, which itself will tend to provide greater efficiency.

Mike.

Thread: FrSky Taranis - user chat
04/10/2017 23:12:28

If you flash using the STM bootloader method with DFU-UTIL, then you write both the bootloader and the main application at the same time. So if you choose the wrong firmware, both are wrong.

If you use the trims bootloader, you only flash the main application, so that bootloader remains working, which is a good reason for using it rather than the STM bootloader.

For me, I always use this bootloader as it is common to all my transmitters (Taranis, 9XR-PRO, and 9X with 9Xtreme and AR9X/SKY upgrades).

Mike.

04/10/2017 19:21:41

I do advise using the bootloader accessed using the trims, rather than the dfu-util method if at all psossible.

It provides many more options for flashing. This includes the option of putting the firmware file on the SD card and flashing it from there. This method would easily recover the problem currently being discussed.

Mike.

Edit: When trying to flash the firmware, use the "Read/Write" menu. You should see a small dialog box appear asking for the name of the file to flash, along with a checkbox "Check Hardware Compatibility". Untick this box and select the file you wish to write.

Edited By Mike Blandford on 04/10/2017 19:28:05

04/10/2017 18:12:44

You could try just flashing from a command prompt.

Go to the directlory where you have companion installed, then just type:
dfu-util -a 0 --dfuse-address 0x08000000 -D x9dp_rom.bin

where "x9dp_rom.bin" is the name of the file you are trying to flash.

You will either need to put the file you wish to flash in the Comapnion directory, or enter the path to it as part of the filename.

Curious as to why you don't start the Taranis in bootloader mode normally (both horizontal trims held inwards at power on), rather than flash using the STM bootloader?

Admittedly, having flashed the wrong version you have overwritten this bootloader with a wrong version, but if you use this bootloader normally, then even if you flash the wrong firmware this bootloader will still work.

Mike.

Thread: Totally drained NiMh pack - can it be resuscitated, and how?
04/10/2017 16:59:46

I suggest charge at c/10 for 12 to 14 hours, then put the pack on a charger with a discharge function and check the pack provides something close to its rated capacity.

Recharge at c/10 for 2 hours, then test in a model that servos drive OK (so testing with a battery that is only charged to 1/5th capacity.

If this is all OK, then charge normally (peak detect) and the battery should be fully ready.

I've use flight power packs (NiCd and NiMh) that have been fully discharged then recharged (pre-lipo days) without any problems.

Mike.

04/10/2017 13:31:21

NiMh cells handle a c/10 charge rate even when fully charged. You may charge at that rate for 24 hours or more quite safely. This is the method to use to ensure a NiMh pack is "balanced" i.e. all cells are fully charged.

Over time a pack of NiMh cells will become unbalanced as some cells self discharge at a (slightly) higher rate than others. Every so often a NimH pack should be charged at c/10 to get all cells fully charged.

Mike.

Thread: Who wants a Warbird Replics Hurricane?
01/10/2017 11:55:40

hwing.jpg

Here's a shot of the outer wing panel.

The plan shows:
Panel root chord 314mm
Panel tip chord 200mm
Aileron length 374mm
Aileron cutout at tip 42mm (including fixed balsa framing)
Aileron chord 374mm from tip 55mm (including fixed balsa framing)

The actual panel is:
Panel root chord 314mm
Panel tip chord 182mm
Slot at tip is 50mm from trailing edge
Slot 374mm from tip is 68mm from trailing edge

So if I use the slot as the start of the fixed framing, the aileron will be quite a bit larger than is shown onthe plan.

Mike.

30/09/2017 16:59:32

Anybody else building at present?

In the absence of any replies to my questions, I'll use 3.0 inch wheels.

I'm looking at the wings now. The outer panels have a slot cut lengthways in them. Is this the line to cut fully for the Ailerons, and then take the torque rods?

The plan shows ailerons a bit narrower than these slots give, but then the panels also taper a bit more than the plan shows.

Mike.

Thread: Understanding Watt Meters
28/09/2017 20:15:34

"Lower kv motors have more winds of thinner wire with greater resistance. You can't win".

You potentially win by lower losses, due to lower current, all the other wiring, the ESC and the battery. Also, if you can use a physically larger motor, then you get the extra windings without needing thinner wire. A larger motor may well be only 30 to 40gm heavier, but everything runs that much cooler, and with some aircraft you need the extra weight in the nose anyway!

Mike.

28/09/2017 15:20:09

"Multiplying the amps by the generated voltage to get the output power is surely just measuring the heat generated again, not the mechanical turning force?"

The mechanical turning force (torque) is directly proportional to the current flowing.

The generated voltage (dynamo/alternator effect) opposes the current flow. We are forcing the current against this voltage, so there is no heat generated by this, it is exactly the mechanical power. The heat is generated in the windings, and the amount is the difference between the generated voltage and you battery voltage, multiplied by the current.

A "perfect" motor would have zero resistance in the windings (and no mechanical losses!). In this case, if you apply 12V to a motor with a kv of 1000 rpm/volt, the motor will run at 12000 rpm. It will be generating 12V to oppose the applied voltage.

A "real" motor is a "perfect" motor with a resistor in series. In my examples, the motor has a resistance of 0.066667 ohms. With 30A flowing, this resistance drops 2.0 volts, so the "perfect" motor sees only 10V, so spins at 10000 rpm.

Yes, you do need to measure the motor rpm to obtain the generated voltage (called back e.m.f.).

With all motors, in one respect current is your enemy, unfortunately you need current to generate torque!
Losses due to current flowing are proportional to current squared, double the current then you get four times the losses. Where possible, it is better to use a motor with a lower kv and use more cells at a lower current.

Mike.

27/09/2017 15:44:14

A motor has a "kv" value (rpm/volt). What this specifies is the voltage the motor generates when turning. So, if you have a motor with a kv value of 1000 rpm/volt, and it is turning at 10000 rpm, then the motor generates 10 volts (acting like a dynamo).

When the motor has a voltage applied to it to make it turn, the motor still generates this voltage, which is in the opposite sense to the applied voltage. The mechanical output power, in watts, is simply the current flowing multiplied by this generated voltage.

There will also be some mechanical losses. These may be estimated by finding the current taken by the motor when it runs without a load (propellor in our case). The mechanical losses are aproximately the no load current multiplied by the generated voltage.

Here is an example, take a motor with a kv of 1000. Test it with no load (or use the manufacturers sepcification) to find the no load current. Let us assume this is 2 Amps.

Now we run the motor with a load. A wattmeter shows an input voltage of 12V and a current of 30 A giving an input power of 360 Watts. The motor is also measured to by running at 10000 rpm, so is generating a voltage of 10 volts.

Of the 30A, 2A is overcoming the mechanical losses, so 28A is left providing the output power. The output power is therefore 28A multiplied by 10V (generated voltage) or 280 Watts, at an efficiency of about 78% (280/360).

2A (no load current) times 10V (generated voltage) is 20W so this is the power loss due to mechanical losses.

Of the 360W going in to the motor, 280W is useful output power, 20W is mechanical losses leaving 60W as electrical losses. This value may also be calculated by (applied voltage-generated voltage) mutiplied by current = (12-10)*30 = 60.

Most of this is heat in the windings of the motor.

Suppose we load the motor with a larger propellor so it only spins at 9000 rpm. The wattmeter now shows 12V (what a good battery I'm using!) and 45A, so 540 watts in.

The generated voltage is now only 9V, so the output power is 387 watts ((45-2)*9), mechancial losses are 18W (2*9) and the electrical losses are 135W. The efficiency is now around 72%, but notice how much hotter the motor wil become with 135W instead of 60W of heat generated in the windings.

Mike.

Thread: Servo reverse... Be careful
24/09/2017 20:36:19

I believe the "correct" place to reverse a servo is in the OUTPUTS menu.

As you have discovered, reversing in the INPUTS menu doesn't reverse the trim.

Reversing in the mixer is OK provided you only have a single mix on the channel. With more than one mix, you would need to make sure you reverse ALL the mixes.

Reversing the servo in the OUTPUTS menu is common to all mixes and the trim. This is, in fact, the "Servo Reverse" option!

Mike.

Thread: Transmitters for DSMX indoor receiver/servo modules
24/09/2017 20:31:01

For information:

My understanding is that full power (100mW, 20dbm) DSM2 is no longer permitted to be sold in the EU as new equipment (unless the product was manufactured before the regulations cane into effect).

Any 2.4GHz equipment that transmits at a power level of 10dbm (or less) is not covered by the regultaions. If you have a Multi module, and wish to use DSM2 for indoor use only, then you may build the firmware for it (usually necessary anyway as they ship with very old firmware) with the power of DSM2 reduced by 10dbm. THis reduces the output power by a factor of 10, but will only reduce the range to about a third of full range.

Mike.

24/09/2017 15:11:31

When binding, the receiver sends back the protocol(s) it can accept, together with the number of channels, and the refresh rate (11 or 22mS). The module then stores this information.

Depending on the protocol used between the module and the radio firmware, it may also be possible for the bind information to be stored in the radio, so the module doesn't need binding when you change to another model that requires a different configuration.

An Orange module flashed with the Multi protocol will do this, provided the radio is using a relatively recent revision of ersky9x or openTx.

Mike.

23/09/2017 13:04:14

The Turnigy 9XR-PRO is only £44.21 from the Eu warehouse at present, with the iRangeX IRX4 module at £28.05 from Banggood.

Mike.

Thread: Who wants a Warbird Replics Hurricane?
23/09/2017 11:25:45

Also, any suggestions for a suitable spinner? I'm just building a motor mounting box for the electric motor I intend to use.

Mike.

Thread: Transmitters for DSMX indoor receiver/servo modules
23/09/2017 09:14:17
Posted by Geoff Sleath on 22/09/2017 21:46:29:

A club-mate has just bought an E-Flite Convergence VTOLfoamie and found it wouldn't bind to the Orange module he has in his Taranis and had to buy a new transmitter to use it. He got a DX6e which binds and flies it OK.

I ported the Multi (4-in-1) code to run on the Orange DSM modules. If the module is flashed with this, then it is very likely to then work with the Convergence. Many "bind'n'fly" models don't work with the Orange module as supplied but then are fine when the module is flashed with the Multi code.

Details on how to flash the module are on this thread: **LINK**.

Mike.

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