"Big Boys Toys?"
Plan B
At the beginning of my summer
holidays, 2010, I had a mad rush of blood to the head and decided to go the
whole hog with the HP0/7 and fit diesels. This decision goes back a couple of
years when the BMFA free flight scale rules for i.c. power models were changed.
Up to that point Co2 and electric powered models were permitted to compete in
the same contest. They are now excluded and can only compete in their own class.
This was to be expected as the improvements in electric powered flight have come
on in leaps and bounds in the last 5 years. This I know only to well as I was
fortunate enough to win the Superscale Trophy in 2007 with the electric powered
O/7. So you could say the exclusion was as a result of my success and my own
fault. However I was somewhat motivated by this exclusion, enough to make an
immediate purchase of two PAW 049 r/c engines to install into the O/7 and prove
a point.
Having proved the reliability of the DH10 over the last two seasons, I made the
time in my holidays to convert the model in a headlong rush to get it ready for
the Nats in only 3 weeks.
As can be seen from the photo the two engines were fitted on radial mounts and
the throttles worked by push rods connected to torque rods through the existing
tubes to the fuselage centre section. The fuel tanks, behind the engines, are
cut down team race tanks with their tops cut off and moulded clear plastic lids
fitted.
In the centre section is a modified fuel cut off timer fitted with a cam that
releases the spring loaded arm on the torque rods to cut the engines at the same
time by closing the throttles.
The small yellow knob on the arm is a vernier adjuster to help balance the
engine throttles but in practice this has not proved very practical. More
practice needed.
These engines have proved to be far too powerful and run the 7x3 props at 8400
rpm, flying speed, with the throttles almost closed. Starting the engines singly is fine but to
start both with the cowls on is terrifying as you have run one on
full power until the other engine starts and then throttle back. It will take a
lot more tinkering to perfect this.
At the Nats, on a flat calm Friday evening, I made some low powered trimming
flights and it showed promise. It got too dark before I could give it a
full flight.
Then we had a howling gale for three days over the whole off the Nats. "Oh
Gosh"!!!
On the Saturday evening a few hardy souls had a go with predictable results and
few came away unscathed.
I was fortunate to find a slightly calmer 30 seconds of airtime and the 0/7
performed beautifully. Unfortunately it took me too long to start both engines
and I went outside the time permitted in competition so did not qualify with my
one and only flight. I was however more than pleased with the model being safe
and stable in flight. Try again next year if I don't break it or the fuel
residue eats it first.
The biggest bonus was to go home with 2 intact models unlike a lot of
competitors this year.
Now
that electrics have advanced to a point where battery capacity and duration is
sufficient, the motors powerful enough and the total weight relatively light a
viable set up is now possible. Switch on, check that the props sound synchronised,
let it go. It could not be easier. Famous last words. The smell of a diesel's exhaust is one of
the sniffing pleasures of aeromodelling but the "keep clean electrickery" is a huge
bonus. All the work that one puts into building a twin is not rotted by the
intrusion of fuel and you could expect it to last for many seasons.
The De Havilland DH10C, 1/18th scale 66'' wing span
with twin electric motors.
There were a number of potential problems, but each new model has these and they
provide stimulus for the little grey cells. Being a twin engined aeroplane one
needs plenty of tail area in the prop wash. The tailplane is huge so no problem
there,
but the fin/rudder is not directly in line with the engines so I am relying on the fact
that the very large area (8'' high) presented by the fin/rudder will be
sufficient. Time will tell. (Model has proved to have no vices at all except for
the fact that the tail feathers are very sensitive to the smallest of
adjustments)
The tyres have a very distinctive tread, similar to the front tyre of a tractor,
so I will have to swat up on moulding rubber.
There is a huge angle of incidence on the wings and tailplane to deal with so it
is possible that the model will fly very nose down. I assume the real aeroplane
did.
Start Date 1st May 2006
I usually start with the tail
surfaces and this is no exception. I find this gives me a good intro' to the
build as it is straight forward. There is nothing innovative about these
surfaces and are standard sandwich construction with just a tweak up on the wood
sizes as they are relatively large. I've had to adjust the notoriously difficult
DH fin/rudder shape from the drawings as the leading edge was found to be too
rounded when viewing the photographs. Note the rectangular hole in the rudder.
On some 10s it was found that the balance was too powerful so they simply cut a
hole to reduce it's effectiveness. The
next step was the huge tailplane. Same build as the fin/rudder.
April 28th 2007. It has taken some time but the wings are almost complete. The
wing roots, wing dowels and interplane strut anchorages will be fitted when the
fuselage is complete. Wing construction is in my usual way plus I've used carbon fibre for the
first time for the leading edge, trailing edge and rib caps. I am very impressed with the improvement over balsa as it is very straight and taught.
The rib caps seem to make the delicate 1/32nd ribs bullet proof.
A note on the construction of the trailing edge. The carbon is very thin and
there is not much to adhere too so I fitted a light 1/32nd strip of balsa on the
inside edge. The rib caps must go from the very outside of the leading edge to
the trailing edge to lock it all together. You will notice a very fine balsa
strip on the trailing edge. This is to create an edge on the top surface so that
the covering maintains a nice sharp contour and does not stick to the full width
of the trailing edge.
As previously mentioned the tyres on the 10 are of a particular and unusual
shape and appear to resemble the front tyres on a farm tractor. In the photo is
the finished balsa tyre that I turned with an electric drill. I made up 4 x 3mm
hard balsa rings of the appropriate size and the centre one being a disc to
which I cynoed a dowel to hold in the chuck of the drill. Once I had shaped the
tyre I removed the centre disc and finished the inner shape by hand. It is
finished with 3 coats of sanding sealer. I am going to mould the
tyres in rubber. We will see what happens. This proved a lot more difficult to
do than I thought. Out of 20 attempts I managed to get 2 usable, but not
brilliant, tyres. The cost of the raw materials was outrageous so I will be wary
of this next time and keep to a more usual round plain section tyres.
Things that make you go
Hmmm!?!?
A lot of thinking spent on this bit as it is critical to get it right. The lower
inner wings support everything i.e. motors, undercarriage and top wing. With the
experience gained in building the Handley Page I have applied very similar
techniques but tried to reduce weight where possible. The H.P. is over
engineered and consequently rather heavy.
The fuselage is now beginning to take shape. As you can see the two basic sides
are made and are loosely held together by the wing spars.Inner wing spars are double the size being two
laminated together. The thinner rear spar has 18 swg piano wire re-enforcements
going through the centre. Trailing edge is a 25mm wide .8mm balsa with carbon
fibre trailing edge.
November 2007. Model weighs
18ozs in the photo but there is a lot of work to do. You can see that it is all
wing and it should float well. The motors will be brushless and powered by li-po
batteries. Using the "new" technology, the weight of the electrics
will be half of the Handley Page power set up.
Sunday June 15th 2008 and the Model
is structurally complete and ready to cover. I will allow myself a week of it
sitting on the dinning room table, just to look at, before covering all that
work up.
April 2009. It's been a long
time in coming but here it is in pre test flight guise weighing in at 3lbs 3ozs
The wings and ailerons were finished some months ago and left separate. When it
came time to fit them all together I found to my horror that warps had crept in
on two wing panels and all the aileron ends had bowed in. A little work to
correct this and then once put together each panel was pinned down flat on 7 3/8
square balsa strips, rib wise, to give an air gaps under the panels. These
panels were then heated with a hair dryer and left to cool, twice a day, for
five days to help stabilize the tension in the covering. It appears to have
worked but I'm keeping a weary eye on them.
You will notice the tail feathers have been painted and some test colouring has
been going on. I am very happy with the Flair PC10 but the grey has proved to be
a real headache. No colour photos of the real thing exist and every magazine
impression of the colours are different so I am using colour photos of the DH9
at the Imperial War Museum as reference. The grey on this aircraft is nearer a
light blue. I reasoned that they know what they are doing and who am I to argue
with the experts. So light bluish grey it is. But I can't help
wondering??????
Test flew the crate on Easter
Sunday. The conditions
where perfect. To
start with the 10 was as stubborn as a mule. I was starting to think that years
of build were going down the drain. It kept dropping a wing and turning in
directly after launch. I had a long think about it. Increased the dihedral a tad
and put 2ozs of ballast in the nose. Seemed a bit better. I then increased the
down thrust on the motors to 3 degrees. Seemed to want to go. A touch of down
elevator and it gave up wanting to self destruct. Hooray. Now it was getting
promising. With the advantage of the programmable electrics I was able to
increase power very gradually over several powered glides. And then it happened,
level stable flight for a full large right hand circuit. Magnificent, if I say
so myself. I keep increasing the power over many more flights so that it now has
three distinct phases. Climb, cruise and descend to land. I got it to about 50
ft with large right hand, rock steady, circuits. Whoopee! There is some damage
to repair but it could have been worse. I will take it all apart and finish it
off now that I know it is "safe"
Finished
June 2009
Handley
Page 07
A commercial conversion of the 0400 WW1 bomber
I have kept the Handley Page electrics as
simple as possible. (See update below) The two motors are straight drive 480s with 7x3 propellers.
The batteries are 7x 500mah SanyoAR cells. These are installed, wrapped in foam,
in the cabin space as seen in the photo. This box is re-enforced with thin ply
to resist the inertia of the batteries exiting through the bulkheads or cabin
sides on landing. The switch is located on the battery box front bulkhead and the actuating
lever exits the bottom of the fuselage so that it is easy to turn on from a hand
launch. The switch is a 20amp toggle switch from a car accessory shop. A blade
fuse (20amp) is used for safety as the amperage pulled by the two motors is quite high.
The wiring is quite hefty. Do not skimp
on this as thin wiring will cause resistance and reduce your power dramatically.
The total weight of the electric package is one pound. This sounds
alarming at first but I have not had to use any ballast in the nose to correct
the C of G and the large wing area of the model carries it very well. Initial
test flights where a disaster and I nearly wrote it off. I
could not get any sort of consistent low powered flights. Back to the drawing
board. I removed the engines and batteries and set them up on a test rig. This
set up was tested repeatedly. This was worthwhile as I discovered that the two
motors, they where purchase years apart, were miss matched by a large margin
even though they were apparently the same. Test before installation. Common
sense really but we all have our up-cocks at times. I purchased two new motors
and they balanced perfectly and produced a slow beat of two to three a second. Sounds
good in the air as it drones overhead. This characteristic beat is essential on
twins as any trimming adjustment cannot consistently make up for the miss-match
of two odd motors. I know, I've tried.
A note on engine thrust lines. The HP is set up with 2 degrees down thrust on
both motors and 2 degrees right thrust on the port engine and 3 degrees right on
the starboard. I was advised by an experience radio twin flyer that the extra
degree on the starboard motor is to counteract the tendency of the motors torque
to pull the model to the left being on the outside of the left turn. (I have
read this last sentence several times and edited it endlessly. I trust it makes
sense)
Update
The HP is now fitted with an electronic timer that works a speed controller.
This enables me to control the duration and power of the motors to a very accurate
degree. I have also fitted a 3 cell, 1200 mah li-po battery . I can now fly a
whole competition on one charge. Easy Life.
Photo
Tony Dowdeswell 
A pair of real twins by Bill Dennis.
Handley Page 0/400 Bomber and an Armstrong Whitworth Argosy. Both powered by
Mills .75s
The
Airspeed Ferry was sarcastically given the alias of Airspeed Hoover by fellow
competitors as it had a very vacuum cleaner like sound and not the rhythmic beat
normally associated with a multi. This, I assume, was due to the motor/ prop
combinations being different. It was fitted with 3 motors. The two outer motors
were 360s with an Olympic gearbox. I tried a straight drive at first but was
unable to obtain enough thrust to fly well. The nacelles were redesigned to take
these units, a very tight fit. I was able to replace 6x4 props with 8x6. The
upper motor was a kyosho racing motor straight drive to a 7x4 prop. The battery
pack was, if memory serves correctly, 7x 600mah AA nicads cells.
