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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: See if you can help me with this
29/08/2018 18:52:38

I disagree regarding to using the motor power rating. You should use the motor CURRENT rating as the limit. Regardless of the voltage, the heat generated in the motor is current squared times the winding resistance. Exceed the current rating and the motor will overheat.
Note also that many motors show a "headline" current limit, but closer inspection reveals this is limited in duration (often 10 or 60 seconds). Look for the continuous current rating.


28/08/2018 19:01:31

An electric motor is driven by the current, not the voltage. A larger prop needs more current then a smaller one at the same RPM.

Giving a motor a voltage specification is a bit arbitrary, it is the current that really matters, in particular the maximum current. If you are using a 3S battery, and are at the maximum allowed current, then going to a 4S battery requires you to fit a smaller prop to keep the current at that value, however, since power is voltage times current, you will have more power.

When a motor rotates, it acts as a generator (its called an alternator in your car!). Specifically, if say the kv of a motor is 900, then it means if it rotates at 900 RPM, it generates 1 volt. This happens even if the cause of the motor rotating is a battery. So, if you apply 10 volts to this motor, it will rotate at 9000 RPM (with no load). In practice there are some losses so the motor will rotate a bit slower. If you put a load on (fit a prop), the motor will then run slower.


Thread: Please Help Me Out With The Arithmetic On This One Gentlemen!
22/08/2018 18:47:04

The limiter, according to the rules I just read, is set for 1750W-mins, so that 40 second power was running at around 2.6kW. On 10 cells, this is 75A, and less than 1Ah. I note that F5B people mention they are sometimes running at 100C discharge rate, and using pre-heated batteries (to reduce internal resistance).


21/08/2018 12:47:10

I tried to find some data on the specified motor, but it has been supersceded by a PO-3547-800. That motor is specified as having a maximum current of 29A continuous (36A peak for 10 seconds).

The original question described a flight of 2 minutes cruising followed by 4 minutes of full power, after which the battery (2200 mAh) was "flat".

Such a battery could (theoretically) provide 2.2A for 60 minutes, which is only 33A for 4 minutes, so I reckon the motor is only taking around 25 to 28A (at full throttle), which would be appropriate for the specification of the replacement available. The Ammeter the OP shows could, at least, give an indication if this is what is happening in practice.

Personally, I doubt the motor is really an 800W motor, other purple power motors of the same size, but different winds, have ratings of 710W and 580W. I would also suspect these ratings are at the peak current specified, so are only for 10 seconds anyway.

My take is the OP needs full power for 5 minutes, and, perhaps 4 minutes at "cruising" power. I'd estimate cruising power would be around 10A (from my telemetry log files where I have flight pack voltage and current logged).

4minutes at 10A is 40 Amp-minutes. 5 minutes at 25A is 125 Amp-minutes, a total of 165 Amp-minutes. 265/60 gives 2.75 Ah so a battery of 2800 mAh should be OK, I'd use 3000 or 3300mAh as the higher capacity may well have a lower internal resistance, so providing a higher voltage (and hence higher current) at full throttle.

I would also recommend checking what the maximum current rating of the motor actually is.


Thread: Power Safe for any receiver
19/08/2018 13:19:03

Try these (should be available with different number of connections) **LINK**

Preci-Dip part 801-87-036-10-001101 is a 36 way version where you may "snap off" however many connectors you need. Clearly, these may be available from elsewhere (found them at Digikey as well as Farnell).

I measured the (square) pins on a FrSky Rx (X8R) as 0.6mm (digital caliper), and the pdf file for these shows 0.635 as the square pin size that fits. For round pins it shows 0.7mm to 0.9mm. 0.635mm square is 0.898mm across the diagonal, so I reckon a 0.5mm square pin (diagonal 0.7mm) would also fit, so should be fine for 0.6mm.


Edited By Mike Blandford on 19/08/2018 13:19:35

Thread: Stabilisation
18/08/2018 19:57:16
Posted by Robin Mosedale 1 on 18/08/2018 18:21:12:

This confused me Mike until I realised that they only work with FRSKY tx's.

See the opening post

Posted by Mike Blandford on 18/08/2018 14:38:08:

For FrSky, the S6R and S8R stabilising receivers are only a couple of pounds more than the X6R and X8R. You may configure them with the stabilising function turned off, so I reckon you might as well always get them.

They may be configured from the Tx (assuming you are using ersky9x or openTx).


It was aimed more at BEB!


18/08/2018 14:38:08

For FrSky, the S6R and S8R stabilising receivers are only a couple of pounds more than the X6R and X8R. You may configure them with the stabilising function turned off, so I reckon you might as well always get them.

They may be configured from the Tx (assuming you are using ersky9x or openTx).


Thread: Finding records of WWII service men.
17/08/2018 12:39:19

Definitely look into getting his service record. My wife got those of both her parents and I got my fathers record. It will probably cost £30.

They can make interesting reading, although sometimes the writing is difficult to read and you need to look up acronyms!


Thread: Power Safe for any receiver
17/08/2018 12:28:06

Very nice and a good idea.

A thought on Rx brownouts. You could use a schottky diode feeding the power to the Rx, then include a (large value?) capacitor on the Rx side to keep supplying power to the Rx if the power rail does dip.

As always, adding this makes everything larger and more complicated, but may be useful if you are concerned about brownouts.


Thread: EU. LBT
11/08/2018 19:37:15

Interesting! I don't have any official DSM transmitters, only 4-in-1 and Orange modules (flashed to the Multi protocol).

Checking on a 4-in-1, I see what appears to be two short transmissions, a bit over 1mS each from the power voltage dip, every 22 mS, so as long as the power isa bit under 100mW, then they are under 10% MU. However, in 11mS mode there are FOUR such transmissions in 22mS, so I reckon the power would need to be under 50mW to be under 10% MU.

Of course, a "real" DSM transmission may be less than 1mS in duration.


Edited By Mike Blandford on 11/08/2018 19:38:24

11/08/2018 09:33:10

Does anyone know if DSMX does LBT, and if it doesn't, what does it do to be legal in the EU?

I'm not aware of a different version for EU and rest of the world use.


Edited By Mike Blandford on 11/08/2018 09:33:28

09/08/2018 13:35:29

Some information on FrSky "D" series receiver. The "D" protocol works with the Tx sending a packet over the air every 9mS, for 3 periods, then the Rx sends a packet back (telemetry) in the 4th 9mS period. By looking at the receiver voltage (that dips while the Rx is transmitting), I see the Rx only transmits for 6.7mS, so only 6.7 mS every 36mS.

The power is, I understand, less than 60mW. I just looked at the FCC test report and this indicates 58.7mW for the DJT module. I'm unclear if this is with the 2db antenna. The Rx, without a 2db antenna may well be lower than 60mW. The range of the telemetry to the Tx will be the same as the control as the 2db antenna on the Tx will provide the same gain when receiving.

According to my calculations, if the Rx only transmitts at 53mW, then it will be under the 10% occupancy.


Thread: AXI 2826 bearing replacement. Advice needed.
07/08/2018 22:58:10

Once you have removed the circlip, the bell and shaft should slide out, no need to undo the grub screws. The magnets are very strong, so it does need quite a pull. Be very careful when sliding the shaft back in, the pull is so strong, it goes together "fast" and can trap part of a finger!


06/08/2018 23:49:40

The Rx (X8R) servo pulses are always at 18mS. Some of the D8R receivers had an option where you press the bind button on the Rx for 6 seconds to switch the outputs between 18mS and 9mS.

On openTx selecting 8 channels (9mS) simply sends channels 1-8 to the XJT every 9mS. The XJT still sends all 16 channels, but since channels 9-16 are always in the centre position, changes on channels 1-8 will always be sent before channels 9-16, so you get lower latency.


06/08/2018 22:39:35

In ersky9x, I have a mode available where all 16 channels are sent to the XJT module every 9mS. The XJT actually monitors all 16 channels and doesn't always send the channels in order. Each RF frame, sent every 9mS, contains 8 channels, nominally 1-8 or 9-16. However, each channel position has a bit to indicate whether it is from 1-8 or 9-16, so the first channel in the frame is either 1 or 9, the second is either 2 or 10 and so on up to the last channel that is either 8 or 16.

What the XJT does is to note the "last value sent" on a channel, then when it is about to send a frame it looks to see if channel 1 or channel 9 has the larger difference from the "last value sent", and sends that channel. This means that sometimes channel 1 is sent in consecutive RF frames. I have seen this happen when using a logic analyser on the SPI bus to the CC2500 RF chip on the XJT.

By sending all 16 channels to the XJT every 9mS, this gives the XJT all the information to get channel changes sent as early as possible, so minimising latency. I believe openTx may have or get this at some point.

To make best use of this, you really need to use SBUS as that also outputs frames every 9mS. For minimum latency, you therefore need to use the 9mS PWM output and so a digital servo.

I have some changes (unpublished) to my arduino SBUS decoder that try to make use of the 9mS update rate. Basically, it normally sends pulses only every 18mS, but if it sees a change on a channel, and a pulse is not normally sent in that 9mS period, it sends a pulse, then returns to an 18mS period. I still need to do some more testing on this.


05/08/2018 11:25:19

I have an Arduino project that is a SBUS decoder here: **LINK**

By default the output rate is 18mS (for analog servos). There is an option to switch to 9mS.


Thread: Electric setup conundrum
02/08/2018 14:51:22

Peter, if the ESC has a BEC, and is then powering the receiver and servos via the red wire, connecting a battery to the receiver as well is then providing two power sources in parallel, with NO protection to either.

It the receiver battery is lower in voltage than the BEC output, then the BEC will be charging the battery, with no current limit.

If the receiver battery is higher in voltage than the BEC, then the BEC circuit will be powered in reverse. These days, many BEC circuits are "switch mode" operating in "step down". This generally means there is a MOSFET transisistor that switches the flight battery voltage through an inductor directly to the BEC output. Such MOSFETs have a (parasitic) diode that would conduct the opposite way to normal (i.e. pass the receiver voltage back to the flight battery. One result of this is if the receiver battery is connected, but the flight battery is not connected, then the receiver battery will power the ESC. If you advance the throttle at this point, then the motor will start, powered from the receiver battery. There is likely to be enough power for the propellor to injure someone, particularly if they thought the motor wouldn't start because the flight battery is not connected. The amounr of current flowing to the motor may also be enough to overload the copper tracking in the receiver, thus causing damage to the receiver.

If you don't have a "power sharing" circuit to prevent these events happening, then I strongly recommend the red wire is disconnected from the ESC.

If the ESC is an "opto" type, then it doesn't have a BEC and the red wire does need to remain connected.


Thread: New Drone Laws from 30/5/2018
25/07/2018 13:41:32

Well done to all from me as well.

I does seem that my Qudratwirl (4-rotor autogyro) is still limited to under 400 feet (not that I think I'd see it that well above that anyway, and I mostly flew it indoors). Does an autogyro have "lifting rotor or rotors", even though it/they are not powered? I would think so!

Is the Tony Nijhuis Harrier restricted to 400 feet as it has 4, lift generating "rotors"?

Note, I'm not being negative, we have an excellent result, just thinking about clarifying things. Clearly, if in doubt, keep below 400 feet.


Thread: The 2018 Transmitter Survey!
23/07/2018 09:53:03

Andy, what is it about the "Taranis" programming you don't get on with? I'm guessing you mean openTx programming model. ersky9x open source firmware also runs on the Taranis and may be easier to follow:



Thread: New Drone Laws from 30/5/2018
20/07/2018 23:21:05

For interest I put a vario in my aerobatic model (the one in my avatar, 52 inch span) and flew it today while logging the telemetry data. The model is quite light (3.5 pounds I think) and has around 700W available!

For much of the flight I was comfortably below 400 feet, but I did fly in level at around 100 feet then pull up into a vertical roll (single roll) and then do a stall turn. The height peaked at 500 feet, and took only 5 seconds to go from 100 to 500 feet.. Later in the flight I pulled a nice large loop and that peaked at 401 feet, again taking only 5 seconds to go from 70 feet to 401 feet..


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