MU13 Charlesworth

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John Vella
Posts: 110
Joined: 20 Mar 2017, 22:09
Location: UK

Re: MU13 Charlesworth

Post by John Vella » 09 Nov 2018, 20:06

Thanks for the advice. I will replace with twisted triwire. No point taking a chance. Regards John.

John Vella
Posts: 110
Joined: 20 Mar 2017, 22:09
Location: UK

Re: MU13 Charlesworth

Post by John Vella » 05 Dec 2018, 13:13

Next the making of the aileron. It is faced with 1.5mm medium balsa from 48 inch stock. The control horn is glass fibre mounted in a box of 1.5mm ply between the ribs. The aileron is fitted and top hinged on 5 "Robart" metal pinned hinges. The wing tip and tip aileron profiles were further blended. The 0.4mm ply facings were added top and bottom, and the 0.4mm cap strips which blend into TE gussets. This scale feature adds considerable strength to a fragile point of the wing. The balsa facings were worked to give a small gap at the top and V section underneath with room for the fitting of an aerodynamic seal. The spoiler box frame is built from 2.6mm sq spruce strip and 4mm section balsa rib doublers. The "letter box" 0.4mm ply surround is glued on. The leading edge will but upto the wing D box 0.4 ply sheeting. The Next job will be the ply root facing rib to fit and blend in with the fuselage. Some more reprofiling of the ribs then the fun of wing skinning begins. I will be using J G' s method (See Ghost Squadron excellent video). Regards John.
Attachments
20181120_152505.jpg
Aileron fitted and hinged
20181120_153440.jpg
Wing tip blended
20181120_152852.jpg
Wing root awaits facing ply rib
20181201_152947.jpg
0.4mm ply facings and cap strips fitted
20181201_152026.jpg
View of aileron with small top gap.
20181201_152503_001.jpg
Tip profiled
20181201_213151.jpg
Spoiler box frame
20181205_104831.jpg
Letterbox surround fitted
20181205_105804.jpg
The LE will but upto the wing D box 0.4mm sheeting.

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Chris Jesshope
Posts: 36
Joined: 24 Jan 2017, 16:13
Location: Shropshire

Re: MU13 Charlesworth

Post by Chris Jesshope » 08 Dec 2018, 10:03

Hi John, Robbie,

just a comment on the twisted pair issue. There is no crosstalk in a 3 wire flat cable as you only have one signal wire. By definition, to get crosstalk you need more than one signal. The two other wires are constant potential, ground and VDD and hence can not contribute any EM interference to the signal wire. The 3 wire cable may be better for longer runs as it tends to have a larger x-section than the flat and hence less resistance and smaller voltage drop, especially on powerful servos.

Keep up the good work John.

Chris

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Peter Balcombe
Posts: 697
Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: MU13 Charlesworth

Post by Peter Balcombe » 08 Dec 2018, 10:44

Chris,
I understand that twisted wires are principally used to reduce Mutual interference from external radiated noise sources.
The idea is that the twisting ensures that all wires pick up the same amount of interference, rather than some being partially shielded from noise coming from any given direction.
The presence of similar noise on all wires means that the differential signal between the wires (which is usually what the device at the end of the wires is interested in) is largely unaffected by the interference picked up.
This type of wiring was often backed in particularly noisy environments with differential signal receivers which were able to virtually totally ignore any interference common to both signal & ground lines.

Note that twisted wires (for Mutual Interference reduction) & ferrite rings (for Conducted interference reduction) were common in 35MHz days when the long aileron etc. servo runs approached 1/4 wavelength, but this is no longer the case with 2.4GHz equipment as the wavelength is 1000 times shorter.
Modern equipment also tends to have for better out-of-band filtering performance.
Peter

John Vella
Posts: 110
Joined: 20 Mar 2017, 22:09
Location: UK

Re: MU13 Charlesworth

Post by John Vella » 08 Dec 2018, 17:17

Hi Gents what you say makes perfect sense to me. Thanks and on the subject of interference I would be much more concerned about mobile phones on the flight line. I gather they have to be very close to the transmitter to have any affect, however I keep mine in the model park when flying and not on my person. Regards John.

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Chris Jesshope
Posts: 36
Joined: 24 Jan 2017, 16:13
Location: Shropshire

Re: MU13 Charlesworth

Post by Chris Jesshope » 11 Dec 2018, 01:39

Peter, John,

There are two issues here, one is crosstalk, one interference due to external magnetic fields. There is, as I said, no crosstalk issue as we have power, ground and one signal wire in the cable.

External interference from magnetic fields is generally an issue in very long signal cables, e.g. telephone wires, where signals are attenuated significantly and hence become susceptable to noise. And yes in such cases differential signalling is used as well as twisted pair. However induced noise is unlikely to be any higher than a few mv and differential noise, i.e. the difference on two wires due to their position relative to the magnetic field, will be less.

Differential signalling detects the change between signals, +/- to -/+ so can be used to detect very weak signals propageted over long distances. Baseband as is used in servos detects a range, e.g. 4 to 5v as high and 0 to 1v as low. The frequency is too low for impedance missmatch to have a significant attenuation of signals therefore there is not going to be a problem. Yes 35MHz was worse as servo wires could make good aerials but even here You are talking belt and braces.

John, if phones are going to have any effect its goingto be throughthe receiver not the servo wires and at that only a statistical one as the signals are spread spectrum with unique codes to separate signal from noise.

Having said all that it certainly can not harm to use twisted cables.

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