Large electric model were unthinkable only a few years ago. Today they are growing
in popularity. There are not very many of these represented on the LMA circuit.
A spectacular one though is the Tony Nijhuis' Lancaster. This refreshing article
and photographs were to sent to me by Tony, describing his creation of the first
electrically powered model to go through the Over 20kg scheme. In fact, (as
of July 2005) this is the largest, electric powered, model in the world!
Scale: 17%
Wingspan
17 ft
Power
AXI-4130-16 motors, 20 cells per motor. 3.5KW input power.
Weight
50lb
Like the majority of modellers, when the whether turns
colder, thoughts of winter building project start to take a grip...an overwhelming
urge.....its a bit like animals needing to hibernate or birds flying south for
the winter...I know for me its the most sensible time to build with little distraction
in the form of gardening or DIY. So come November time its time to put the thinking
cap on. My models seem to follow a certain historic pattern... firstly building
Lancaster's and secondly building them bigger. So the 2004 contribution to the
air force was not much of a surprise to many. However, to finally decide on
another Lanc. I have been somewhat round the houses.
Before deciding on the subject, what was clear in my mind was the model was
going to be a big multi-engined model and powered by electric. So why make it
electric? Well there are two reasons, the first is the pure challenge of going
into the unknown with this size of model and secondly the cleanliness and ease
of electric power over glow or petrol powered. I'm not adverse to IC powered
models and have quite a few lining the workshop walls including a 11ft B-17.
Every time the model is flown and then cleaned, a little more paint will become
sticky, a little more balsa will become fuel soaked.....you know the sort of
thing. It can be quite disheartening to see your best paint job dulled by adding
fuel proofer and even worse when you find the fuel proofer is not doing what
it says on the tin!...have touched a raw nerve yet?.
I think also the success of the small 11ft Electric Lancaster and last year's
15ft Spruce Goose was also a catalyst for keeping the electric theme going.
With both of these models the secret to their success was keeping them as light
as possible. This also allowed the use of cheap brushed motors and in the case
of the Spruce goose it was purely the number of motors (8 in total) that provide
such an impressive thrust to weight ratio.
Having been so impressed with the flying characteristics of both these models,
which have a lot to do with their size, the logical option seemed to be to go
larger.
Yes that's right, this is the 6th Lancaster I have built/owned so you could
say I'm a bit of a Lancaster nut.
This latest offering is yet another scaled-up of the very successful 72" RCM&E
plan. Having drawn the original Lancaster on CAD it really wasn't too difficult
to scale-up the plan to the size I wanted. What, of course, had to be worked
out was how the airframe would break down into manageable parts for building
and transportation. In total, there would be nine parts, the wings in four parts,
the fuselage in two and the tailplane in three parts including detachable fins.
Once that had been figured out, and I was happy with the plan, a template CAD
drawing was done and then emailed to SLEC for them to cut out using their CNC
machine. In order to build the model, the plan had to be printed and this
was no mean feat! By arranging the plan to achieve the best use of paper it
still required a 20m roll of 1m wide paper just to print it!
During the design stage, discussions took place with various suppliers of electric
motors. This proved invaluable and it soon became apparent that conventional
brushed motors were not going to be suitable due mainly to the size of project
and also the efficiency of these motor. I had originally calculated that 50W/lb
power to weight had to be achieved for a model such as the Lancaster to work
successfully. This equated to 2.2kW of input power based on 44lb (20kg). This
rule of thumb has worked well to date, but I had some concerns since as models
get larger the power to fly the model needs to be increased.
The alternative to brushed motors was of course brushless. This led me to speaking
with John Emms at Puffin Models. Now I have to say brushless is still a bit
of a mystery, and trying to compare sizes of brushless motors to brushed 'can'
size ie 400, 600, 700 etc, it was soon clear there was no relationship and each
manufacture has there own form of specification and numbering. So its is more
important to be able to speak to a knowledgeable person and in this case John
Emms had the answers. Puffin Models are the UK distributor for the AXI motors
which I have to say have so far proved to be excellent both in quality and performance,
and will also not break the bank. Because my original plan was to use 16-cells
per motor, the AXI 4120-14 seemed to fit the bill based on an input power of
650W. One of the main advantages of brushless motor is their efficiency which
can be any where between 80 to 95% depending on the load they are put under.
With brushed motors if you achieve 60% you are doing very well indeed. I was
also advised when selecting speed controllers it pays to go for one with a higher
current handling capacity than the motor will pull. The reason for this is the
reassurance this will give knowing the current draw is well within the handling
capacity of the controller and therefore the chance of failure through overload
is much reduced. The type that were finally selected was the Jeti 75NC, again
supplied by Puffin Models and all next day delivery. These really are a quality
speed controller.
So with the power system sorted, it was time to get on with building. Now for those who have read other articles of mine will know I like to build quickly. I usually have a short span of attention and often lose interest after a couple of months of work on one model. If things are dragging on I find to break the monotony I try and fit another smaller and simpler model in.
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Shown here is the construction of the fuselage. You can clearly see lightness being "built-in" from the start.
Work began on the big Lancaster the day after New Years Day in 2004 and for
10 weeks solid doing approximately 40 hours of evenings and weekend work a week
the model was ready to test fly. As with all my pervious electric models, I
like to test fly the model 'uncovered' so I am absolutely sure the model is
right and more importantly the power set-up was going to work. For an electric
model of this size both John Emms and myself were entering un-chartered territory!
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Here are two photographs of the wing construction.
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Here is a nacelle. Simple, yet perfectly fit for purpose. |
Being reasonably confident the model was well under weight It was only the day
before the first test flight that I thought would check the weight of
the model. I was in for a bit of a shock! Weighing all the pieces separately
and tallying up I found the models weight to be nearly 43lb I have to say I
was mortified but, nothing stood out as any one single cause. The weight of
the cells motors radio had been taken into account so it seemed it was the effect
of lots of small items not weighing much singularly, but together they conspired
to make the difference. I did have to add 2lbs of lead to the nose, which for
this size of model was not too much of a concerned. However, I did have a concern
about the power requirement. The finish weight would be heading towards 50lbs
and reducing my target power to weight 50W/lb.
The only positive point was the model just crept under the 20kg limit and therefore
I could go and test fly without the exception paperwork in place. So with
good weather set for Sunday 28th March, the Lanc was put on charge ready for
an early start.
Basic construction was completed in the lounge!
Fortunately, belonging to the Hastings MFC means I have
access to a first class flying site, all safely set out, not a single tree within
the flyng zone and a 150m long grass runway, which is second to none in the
southeast. The Lancaster takes about an hour to put together and even though
the model was unfinished I have to say it still looked pretty impressive. Powering
up the model for the first time and taxing out revealed the same excellent ground
handling characteristics as the smaller 11ft version. Although the power seemed
fine on the ground it was very noticeable the prop which although being 14"
in diameter only just cleared the bottom of the cowls. I was not happy with
this and bigger props would have looked so much better and been more efficient.
However, this was not going to stop the moment of truth. The Lancaster was lined
up into gentle 5-knott wind and the throttle progressively opened. Slowly she
started to roll and after 10m or so the tail was off the ground and she balanced
perfectly on the main wheels with no elevator correction. Because the power
is uniform across the motors there is no tendency for the Lancaster to swing
or do anything other than run straight down the runway. The maximum ground speed
seem to occur at about 50m or so at which point I started to feed in up elevator.
Progressively more elevator was used and after some 70m she was airborne and
climbing in a very scale like manner. The wheels were retracted and the nose
immediately rose and the model noticeable picked up speed. The climb-out continued
through a right-hand turn and by the time she had completed one circuit about
150ft or so had been achieved. All the controls felt fine, but I was holding
in quite a lot of up elevator to maintain level flight. At this point the elevator
stick was neutralised and the Lancaster nose pitch down quite suddenly. Full
up-trim had to be applied to maintain level flight but all the other main control
surfaces required only the smallest of adjustments before the model was flying
'hands-off' and at a very scale speed. By the second circuit I felt very comfortable
and knew the model was every thing I had hoped for. After some six minutes of
gentle circuits I call a landing and lined her up. The model doesn't have or
need flaps and power has to be applied all the way to the ground. Backing off
to about half power, the Lancaster descended slowly and gracefully to about
3m of the threshold of runway. At this point I cut the motors and the model
noticeable slowed and began to wonder to the right slightly. This caught me
unaware slightly and I had to power up again to drive the Lancaster forward
and on to the runway. The final touch down was an bit out of shape and as a
result one of the undercarriage legs broke away and the model unceremoniously
ground to a stop 'one wing low'. Never-the-less, I was absolutely delighted
with this first outing and it was now full speed ahead to finishing the model
ready for the model show at Sandown.
After test flying any new model I away like to analyse the flight in my mind.
My immediate thoughts turn to the power system. Although the power seemed adequate
there were a number of things niggling me. Firstly, the size of the props which
if larger would be more effective. Secondly, the finished weight may add another
10lbs and could the current power system cope. Thirdly, the nose weight will
probably rise to 4lbs by the time the finishing is applied to the tail and one
thing that greaves me is giving ballast weight a free trip round the sky.
So first thing Monday morning I got on to John Emms to ask whether it was possible
to increase the prop diameter and squeeze another 10% of additional power. The
answer was not favourable and it soon became clear the only option was to re-motor
and go for the AXI-4130-16 (the largest AXI motors currently available) and
as a result increase the cell count from 16cells to 20cells per motor. Fortunately,
the battery compartment in the inner nacelle are caverness and could easily
accommodate the extra cell. The other benefit, which soon became apparent, was
the additional cells would reduce the necessity for the nose weight. In short,
each new motor using a 16"x10" prop on 20 cells would give an additional
200W in power (800W in total over the four motors) which, equates to a whopping
30% increase power at the same current draw. This would give the Lancaster an
incredible input power of 3.5kW.
Once again John Emms came to the rescue and replaced the motor after first checking
the original Jeti speed controllers were up to the job, which, fortunately they
were. Both the AXI 4120 and 4130 have the same shaft size and mounting points
so no modification had to be made. Once again the motors being a little heavier
would further reduce the need for nose weight.
Feeling now very confidence about the model and the modified power system, it
spurred me into action to get the model finished. I needed to keep the finishing
weight as light as possible so I was tempted to film cover, add some detail,
and paint to a similar standard to that of the 11ft Lancaster. But, then a passing
comment ....'yes the smaller Lancaster was nicely finished, but what it really
needed was a bit more detail... you know just a bit of turret detailing and
maybe just a rivets? I have to say he was right. The bigger the model
the more detailing is required. Fortunately, I have a mass of Lancaster books
and pictures collected over many years so knowing the detail was not a problem,
it was just the length of time it might take to complete it.
Although the model was test flown, the canopies and turrets had not been made
and this required vacuum formed patterns to be made. This took about a week
or so to do and once made the cockpit detailing could be started. The model
was also ready for covering. Because the detailing was now an important feature,
cover with a Solarfilm material was really not an option, so I decided to use
a lightweight glass cloth (17grm /sqm), but instead of using epoxy, I used a
2-pack flooring varnish (Sadolin PV67) which dries in about 30minuites. One
of the main benefits of glass cloth is it does give the model protection against
'hangar rash', which for this size of model being maneuvered around the workshop,
is very likely indeed.
Prior to the covering being applied, the model was given a coat of sanding sealer
so the varnish does not sink in to the wood. The glass cloth is applied dry
and smoothed out over the airframe. The varnish is only then applied, using
a 4" foam mini-roller This method is really quick and because of the short drying
time, you can re-work the model after and hour or so. After three long evenings
the model was covered. Just to give the finish that extra surface hardness,
a thinned coat of epoxy was applied to the glass cloth. At this point windows
were cut into the side of the fuselage.
Prior to any further detailing the model was given a light coating of grey primer
to pick out any anomalies with the covering. All being well it was time to move
on to the riveting detail. Now the simplest way to apply dome rivets is by using
PVA glue, but the thought of hand doing 100,000 or so individual rivets filled
me with dread and to be honest I had better things to do like watch repeats
on tele or wash the car again.....It seemed a compromise had to be reached.
So I took a piece of hard wood about 1/2" sq by 12" long. At 1/2" intervals,
small holes were drilled along its length. In each hole was pushed a 1" long
panel pin and epoxied into place. The sharp point of the pins was then linished
to a flat end and there you have your multi-rivet maker. To make a suitable
glue tray I cut a length of 22mm plastic pipe down the centre, glued plastic
piece to each end to enclose and hey presto, a glue trough. To achieve a good
dome rivet you will need to thin the PVA glue with water slightly. To make a
line of rivets just dip the rivet 'comb' into the trough of glue by no more
than a few millimetres, and carefully 'rock' the comb from one end to the other
and lift vertically away.....and there you have it 30 plus rivets instantly.
Because the glue grips the comb pins the comb can be used another couple of
time before being recharged with glue. So assuming you know where the rivet
line go and applying at a reasonable pace you could do 200 a minute, 12,000
in an hour.
It took me only two evenings (about 8 hours in total) to complete the model
and although I had misgiving after the first hour or so and wished I had never
started, I carried on and by the time I had finished it was clear that the time
spent was going to make the single biggest visual impact on this model. I have
since not been disappointed.
Carrying on with the detailing, bomb-aiming lights were added and these were
angled to achieve convergence at 10ft! 1/6th the scales drop height of the full
size. A working bomb rotating and drop mechanism was fabricated, and to complete
the detailing, navigation lights and retractable landing lights were installed
into the wing.
Applying the final colour scheme was straightforward using Flair Spectrum paints,
thinned to a 50/50 mix using cellulose thinners, and spray applied. To finish
off, the paint was given a light rub down using 1200 grade wet 'n' dry. This
nicely revealed the tops of the rivet heads and that evocative Lancaster look.
To avoid damage, I decided to make the turrets and observer dome removable.
This also allowed me to do this detailing last of all. Because the turrets are
such a large noticeable feature on the Lancaster the detailing had to be convincing.
So armed with all sorts of bits and piece, I spent a week just detailing the
front and rear turrets and the observer dome. Once again the visual impact certainly
brought another dimension to the overall realism of the model.
Endeavoring to keep the model as light as possible, I managed to source a pair
of lightweight 9" wheels from Len Gardner. The wheel covers were home and made
from a mould which was then vacuumed formed in plastic. The oleo legs were from
J. Perkins and the retracts units were the Ripmax 'Giant' spring/air. These
retracts are supposable designed for model up to 15kg so with a model approaching
30kg (60lbs) they needed to be strengthened somewhat. This was done by making
a saddle plate from 2mm thick aluminium to the profile of the retract housing
which supported the rather vulnerable plastic.
After 8 flights amounting to just over an hours worth of flying the Lancaster
received exception and use ready for public display. Although the one hours
test flying did seemed onerous when starting out it does allow your confidence
to build in readiness for the show season. It also allow those teething problems
to get sorted which it seems as the model gets larger, the more gremlins there
are.
Although the size of model has entered uncharted territory and is the first
electric model to come within the LMA 20kg scheme, the Lancaster has lived-up
to all my expectation and I hope to push the boundary of electric flight to
greater limits.
When displaying the model at Sandown I did have a hard job of convincing one
or two modelling enthusiast the model was indeed electric. A question which
was often asked was what's the comparison cost of electric package when compared
to IC. Well strangely, if you bought four OS120 4strokes or equivalent petrol
engines, you may well have to find another £100 or so. Surprised? I know I was.
When going to large or giant scale one of the great benefits of electric is
the batteries become useful ballast. In essences putting 5lb of lead in a 100"
model is not uncommon. Even the best and lightest builder such as Richard Rawel
had to concede and put 10lbs of lead in his beautiful 1:3 scale spitfire, almost
15% of the overall weight. So the weight of conventional NiNH or nicads for
large format models must be considered an advantage. The Lancaster carries over
12lbs of batteries and no ballast. I think that says enough.
What this all means is far from electric models of this size being heavy, they
can be and often are lighter than their IC counterparts. The only slight drawback
is the flight times which are currently around the 7 minute mark. Although for
display flying this fit nicely into the usually 10min slot, to increase duration,
I will either have to increase the number of cell to increase the voltage on
to the motors or wait for a higher capacity (which is seaming less likely with
the advent of Lithium technology) or go in the direction of lithium cells, but
we are talking serious money there!
This page was last updated on 24/6/05.
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