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Composite-ARF Yak 55 (2.1m/83")
Motor Installation
The new Composite-ARF Yak 2.1m design
allows so many different choices of power unit
that it's impossible to provide mounting bulk-
heads & hardware for all of them! Therefore,
depending on your choice of gas engine you
may need to modify the milled plywood parts we
have included, or even make a few of your own.
The original prototype was flown with a DA-50,
probably the top end of the power range that
anyone could possibly want ... and it has lots of
vertical performance! There are many other gas
engines that would be suitable, and ready-to-fly
weight with these will be in the range of 7 - 8kg
(15.5 - 17.5 Lbs).
DA-50 (gas engine installation)
Installation of gas or methanol engines could vary considerably, depending on your motor and
exhaust system choice, but here we show a typical set-up of a DA-50 and MTW TD75 mini-pipe.
The method shown here can be modified to suit your preferred set-up.
The integral black fibreglass firewall and exhaust tunnel is factory installed in the fuselage and
jig aligned, so that the front face the front face of the firewall has, nominally, 0 degrees side
thrust. However the manufacturing process does not allow us to set the up/down thrust exactly,
and this could be as much as +/- 1 or 2 degrees from the wing incidence. The exact incidence
of the front face of your firewall, after you have glued the plywood parts into the fuselage, is eas-
ily checked using an incidence meter to determine the angle in relation to the wing roots. Of
course when you install the engine the thrustline required is easily adjusted by using a stack of
washers between the back face of the motor stand-offs and the front of the plywood firewall.
The firewall consists of 2 milled 3mm plywood sheets. The larger one must be glued to the inside
of the fibreglass firewall, and the smaller one onto the front face, with the black fibreglass part
sandwiched in between. With the DA-50 and carbon Mejzlik 22 x 10 prop the correct thrust line
is approx. 2 degrees right thrust, and 0.5 - 1.0 degrees 'down-thrust', but this can vary a little
depending on your motor/prop combination.
We recommend that you accurately mark, and drill, the engine mounting holes in the 2 plywood
firewalls before they are glued in to the fuselage. The dimensions shown here suit the DA-50.
To give the correct side and down-thrust, and still have the spinner centred on the nose of the
fuselage, the motor needs to be mounted offset from the centreline of the fuselage. Cover the
front plywood firewall with masking tape and draw a horizontal line on it between the two short
straight parts on the sides. Now mark a vertical centreline exactly bisecting the horizontal line at
90 degrees. Mark another horizontal line 2mm above the first one - this is the horizontal centre
of the motor to allow for the 0.5 degree downthrust.
To account for the 1.5mm right thrust the centre of the motor needs to be offset 5mm to the
(pilots) left side - so mark another vertical line 5mm to the right side of the first vertical centre-
line. Now mark the positions of the 4 mounting bolts equally spaced either side of the motor cen-
techsupport@composite-arf.com
(above) The 2 firewalls are CNC milled from
3mm aircraft-grade plywood. The larger one is
glued to the inside of the fibreglass firewall,
and the smaller one is glued onto the front.
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