(and other things of an electric modelling nature - RULES OF THUMB)
On Sunday 6th November, the BEFA Technical Workshop was held at Leamington
Spa. This is an opportunity for members of the British Electric Flight Association
to buy electric stuff from the traders in attendance, and also take in the
3 lectures organised by the BEFA committee.
Traders this year included;
Modellhaus,
Gordon Tarling,
Puffin Models,
Inwood Models,
West London Models,
Importechnic,
Somosa,
Hillcot Electronics,
FVK Models,
Free Flight Supplies,
… and a few others which regrettably I have forgotten!
The lectures were interesting as usual, the three topics being;
1. Lithium Battery Technology by Simon Shelden, Amberjac Projects Ltd
Primarily this covered the areas where the Saphion 2 cell technology (using lithium-vanadium phosphate) excels over the current LiPoly cells which use Lithium-cobalt dioxide. All of the advantages are in the area of safety. LiPoly cells have no tolerance at all for abuse through overcharging, over discharging, or mechanical damage. In many instances the abuse will result in spectacular self-ignition. The fire cannot be extinguished until the cell (and those connected to it) has been reduced to ash. While this is happening your model is totalled, possibly your car or house as well! I don’t want to alarm anyone, but if you are using LiPolys be very careful!
The Saphion 1 (using Lithium-iron phosphate, and available now from Overlander)
is about as safe as NiCad cells, as is the Saphion 2.
The Saphion 2 will need (yet another) different charger, as it requires a higher
charge voltage than the other existing LiPoly cells. Initially, charger manufactures
like Schulze whose products can be software updated easily, are being contacted
to make them compatible.
The down side of the Saphion cells (1 and 2) is they are sacrificing about 15% energy density over the other LiPoly cells – which maybe isn’t that bad!
2. Composite Construction Techniques, by Mike Woodhouse of Free Flight Supplies
This lecture illustrated how to tackle lightweight wing building using carbon
fibre. Mike had some examples of moulded leading edges and trailing edges from
carbon cloth, and some of the jigs he needs to produce to build the wing parts.
It is a special construction area which individuals need to work at over time,
but typically a 4 foot wing for a free-flight model or glider can be built
and covered for a weight of about 60 grammes. The rigidity of the structure
was impressive.
3. BEFA Technical Support, by Alan Bedingham
Alan is the BEFA committee member who dispenses advice and technical support on electric aeroplane matters. His talk illustrated the value of using “rule-of-thumb” calculations and also cheap software programmes to arrive at a good starting set-up for an electric project.
I have mentioned some “rule-of-thumb” calculations in an earlier page, and Alan included the following helpful gems:
Based on a sub-C size NiCad (24mm diameter x 43mm high, 2oz.);
1 cell can deliver 30W;
For general flying, 1 cell per 50 square inches of wing area is required;
For sport/aerobatics, 1 cell per 35 square inches of wing is required;
-this one is useful for converting a glow model to electric-
Rule of 1000,
Take the recommended glow motor size (cubic inches) and multiply by 1000, to
give a (minimum) power input requirement in Watts.
E.g. a 40 sized model (0.4cu.inch) requires (0.4 x 1000 = 400W of input power)
Alan went on to demonstrate the use of “electricalc” computer software,
which ties together motor and battery combinations with model aircraft sizes
and drag parameters, to give an indication of the viability of the model set-up.
Current consumption, minimum and cruise flight times, climb rate and maximum
altitude are some of the indicators returned by the calculations.
While only costing about £25, there are also on-line calculators available
for free use which work very well.
Try the calculator at www.brantuas.com/ezcalc/dma1.asp It
is only concerned with the power set-up (motor, prop, gear ratio, battery
choice) so you need
to use the “rule-of-thumb” calculations about the model in addition.
However it is free, and contains a reasonably comprehensive pull-down list
of motors and batteries, so you don’t have to worry about all the constants
required.
4 – Calculations based on a typical MOTOR/GEARBOX assembly
As a practical example, I was recently requested
to identify and suggest typical applications for a Motor/Gearbox assembly
which a club member had in his possession - see supplied photograph below:
The important parameters are; Kyosho AP36, with a 3:1 speed reducer fitted.
The motor is a “can” style of the same mounting dimensions and
overall sizes of the ubiquitous Graupner “540” or “Speed
600” range, which I use a lot. This Kyosho is a little more refined and
exhibits exposed brush gear (better cooling), and there is an indicator to
facilitate timing the motor should you want to fiddle with this! Most ferrite
magnet motors like this need suppression capacitors fitted, and 2 can be seen
on this example, the tops of the encapsulated components folded over the brush
holder structure at the extreme right of the photo.
Power wires are connected to the terminal spades with connectors. For the current
this motor could be drawing (20 – 30A) I would recommend the wires are
soldered directly to the terminals. Wire size should be at least 1.5mm² for
the same reason – to minimise resistance and resultant power loss.
The gearbox is a single stage speed reducer of simple construction. The output
shaft here has a propeller adaptor fitted.
While very simple and effective, unless the gearbox face plate contains mounting
holes the mounting of this assembly can be difficult. Many are of this format
however, and mounting may have to be undertaken by clamping a band around the
motor can body. This can be ok, but remember the propeller torque has to be
countered by the clamp, as does the thrust.
Using the on-line calculator- (see below for this example)
Select the motor
This displays the motor constants and from the Kv value of 3521 rpm/Volt this
is a ”hot” motor compared to a Graupner speed 600 8.4V motor
at 1890 rpm/V. The gearbox with a 3:1 reduction is well matched, allowing
the motor to run fast where it has a good chance of being at best efficiency,
while the propshaft will be turning at 1/3 speed allowing a reasonable sized
prop for a good climb rate.
Select the battery
In this case I will choose a subC NiCad discussed in the “rules-of-thumb”,
above. The particular choice I will make is the Sanyo 2000SCR, 7 cell pack,
to keep the power train within the capability of cheap battery charger technology.
The designation 2000 denotes the capacity in mAh, and SCR indicates it is capable
of fast charge and discharge. The constants which appear in the boxes confirm
this, with a cell resistance of only 4m?. Also useful is the weight, which
can be added to the weight of the empty model. (The data for the motor does
not include weight, but the motor will weigh approximately 7oz. and the gearbox
another 2oz.)
Gearbox and Prop
Enter a gear reduction ratio of 3.
With “Master Airscrew” selected (this will pull in specific constants
for the shape of MA propellers) enter 12” diameter, 8” pitch and
we’ll have a look at the results… (Leave the speed controller resistance
at 0.01 ohms)
Calculated Results
What I look for here is current, Watts in, static thrust, and prop pitch speed.
Current is a bit high at 37A. Keep this size of motor to a maximum in the range 25 – 30A.
Watts in is 270W. If we wish to fly at a minimum of 50W/lb, we can build a model which weighs 5lbs. Not bad but look at the full throttle duration. Only 3 minutes 16 seconds!
Static Thrust – is 44oz. This needs to be at least 1/3 the model weight, preferably nearer ½ to give a decent climb rate. The model here could be 5.5lbs (see Watts in).
Prop Pitch Speed – is 43mph. Remember the
relationship between stall speed and wing loading? Vstall=3.7 x sq root(L)
Assume the wing loading for this model will be in the region of 20 oz/ft².
Stall speed will be approximately 17mph, and to be safe we must be capable
of flying at twice this speed, 34mph. So we are well in hand here!
I would suggest that this configuration will give a workable solution which can be throttled back a lot.
Yet another “rule-of-thumb” says that
the power train will weigh no more than half the total model!
So power train weighs; (motor+gearbox+prop+speed controller+battery)
Approximately from the data this is; (7+2+1+1+14) =25oz.
Therefore the covered airframe and servos must be built for 25oz.
If I try another prop to reduce the current draw for better efficiency…
With a 10 x 6 prop the figures become;
Current – 23A
Watts in – 182W
Static Thrust – 32
Prop Pitch Speed – 42mph.
This is good – keep the model weight below 3 ½ lbs. An airframe
under 30oz will dictate a nice model of wingspan in the region of 50” will
work very well. (A Piper Cub in 1/8 or 1/7 scale?)
Static thrust is well over ½ the model weight so no problem with take-off
and climb-out.
Plenty of speed is available.
Full-throttle performance is 5:10 (cruising for 8 – 10 minutes probably)
So, if you follow these few rules, and always build lightly, a large range of models could be built around this power combination. There is plenty of scope to experiment with prop sizes, in particular one between 10 x 6 and 12 x 8.