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Member postings for Brian Parker

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

Thread: The May issue Nijuis free plan teaser
20/03/2008 16:43:00
Count me in!
Thread: 4-stroke exhaust length
19/03/2008 20:52:00

Richard,

If despite the above sound reasons for caution, you still wish to proceed with a 'straight exhaust system. Then it is possible to tune the exhaust length to give the required scale performance using an engine test bed/stand, a rev. counter and monitoring the temperature of the engine and exhaust, (after first having noted the results with the standard exhaust system). The fuel mixture will need resetting at each alteration in exhaust length and also the idle performance (or lack of) noted.

The following may also be helpful.

At the begining of  the exhaust stroke a 'slug' of high pressure exhaust gas, caused by its initial inertia, starts a high-speed pressure wave  that travels down the exhaust until it reaches the end of the pipe. At this point a reversal takes place and a reflected low pressure wave (negative pressure) travels back up the pipe. If this reaches the port at the correct time (at/near TDC) it augments extraction of exhaust gases and the incoming inlet charge.

As a straight exhaust system has a natural frequency of vibration.The time taken for a wave to travel down to the end of the pipe and back to the port is constant. Hence the limited beneficial speed range. Altering the natural frequency of the system, by altering the diameter or length of pipe will alter the usable maximum and minimum speed.  Short and/or small diameter pipes raise frequency. For more flexibility and lower peak power a longer pipe is better. Exhausts can also be double the optimum length to achieve maximum/required  power for a more practical length.

Cutting the end of the pipe at an angle will have the same effect as shortening the pipe. Useful for 'fine' tuning.

I should add that I have never tried this on model aero engines, but have extracted maximum power from motorcycles many years ago and can vouch for the power increase and for the 'music' of a well-tuned exhaust.

At your own risk.

Thread: Complete-a-pack kits
11/03/2008 19:05:00

Sabre,

If you are interested in building from plan then the C.A.P. HP Harrow plan is available from the Traplet website. Plan No. MW2124  (£12). A  set of Cowls, Spats. Turret and Canopy are also available for about £20.

Thread: Battery packs won`t charge
06/03/2008 11:30:00

Chris,

It's all in the configuration of the 7805 as a constant current source.

In my original posting I emphasised that the centre leg (common) of the 7805 is NOT connected to negative, but to the battery side of the output resister. This use of the 7805 is to provide a  constant 5 volt drop across the resister.  It is not used as a conventional voltage regulator.

I have used this circuit for charging nicads for many years for both single cells and upwards to 8 cells, even PP3 type. It is based on an old data sheet (probably from RS), It is usable from 1.2V (single cell) to 12V(10 series cells) with the 7805 and sugested PSU.

I actually use a plug board arrangement to select the required charge current from various resisters wired in series, parallel and series/parallel. Not ideal but like Topsy, "it just grew".

05/03/2008 10:24:00

Chris,

I'm still unable to post my sketch of the above to the site, but if you are interested then you will see from the description there are only 4 components costing under £1 in total.

The Resister/Current values in the description are using a 7805 regulator. Thisgives us our reference voltage...... V(ref)   i.e. 5Volts. The circuit has a quiescent current of 5mA.... I(q).

Then using Ohms law .......  I(out )= V(ref) / R + I(q)

The diode used( IN4001) across  the regulator has it's cathode to the 'in' leg.

Use a 1microfarad capacitor.

A PSU of 15volts or over will allow charging of single cells up to 10 cells (12volt battery). I use the high wattage resisters to keep things cool. The heatsink is a piece of aluminium bolted to the regulator and components are soldered direct, veroboard not needed due to low component count.

Two or three full charge/discharge cycles using the above circuit should restore your batteries to full capacity provided they  have not been too 'overcooked'.

Thread: 4-Max power package prize draw entries!
04/03/2008 19:03:00
I'm in!
Thread: The March Grand Prize Draw
03/03/2008 15:41:00

Count me in.

Thread: Battery packs won`t charge
03/03/2008 12:01:00

Have tried to paste sketch of the above, failed twice!

How do you paste/insert without going through the gallery?

02/03/2008 21:05:00
Last sentence should read All available from Maplins or RS.
02/03/2008 21:02:00

Nicads are robust but not bullet proof. They should be fully discharged before charging at rates above C, but if cells in a series nicad are discharged to below one volt at a higher discharge rate than C/5 there is a danger of a cell becoming reverse charged due to its low internal resistance. Not good for the cell!! (If necessary fully discharge with a suitable bulb).

The maximum charge rate for indefinite charging is C/8. The battery is fully charged when 150% has been returned, i.e. around 12 hours. A constant current charger must  be used for indefinite charging. The current remaining consant throughout the time the battery is charging/connected (use an ammeter in the circuit).

High charge rates of up to 10C are permissible (check battery spec.), providing the charge period is controlled as appropriate. A constant current charger is recommended over a constant voltage unit.

DO NOT subject a nicad to indefinite charging at a high rate unless the current is reduced to the continuous rate for the battery once charged.

 A simple 'home brew' continuous current charger can be built for a few pence using a 3 leg voltage regulator from the L78/TS78 range plus a suitable resister on the output leg, 82 ohm will give 66mA, 22 ohm for 250 mA, 10 ohm for 500mA etc. Use 5W resister for 22 and 10 ohm. Connect a diode across the input and output legs. Link the centre leg (common) to the battery side of the resister. A tantaium capacitor from 'common' to input leg and that's all. You will of course need a suitable power supply/source(PSU) at 12-15 volts. DO NOT connect the PSUground/negative to the L78 'common' but to battery only. Mount the L78 on a heat sink, the resisters will also get a little warm. Don't forget the ammeter.

All available for Maplins or RS.

Thread: Help tracking down a battery
02/03/2008 14:21:00

Nicads in parallel can have problems with reverse charging in service due to their low internal resistance.

Thread: Pick a plane 2008
27/02/2008 18:40:00

Bristol Beaufighter

Westland Lysander

 Hawker Sea Fury

Thread: The February Grand Prize Draw
03/02/2008 15:24:00
Count me in.
Thread: JP Mini Aviator free prize draw
15/01/2008 14:18:00
I'm in.
Thread: Pick a plane 2008!
13/01/2008 15:41:00

My vote would be for..

       i)  Bristol Beaufighter

      ii) Lysander

     ii) Sea Fury

Thread: JP Mini Aviator free prize draw
08/01/2008 14:59:00
I'm in!
Thread: Pick a Plane: shortlist
19/04/2007 19:08:00
My three votes are..

1. Avro 504
2. Stuka
3. Islander
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