Cooling
In operation the 2
nd
stage radiates a lot of heat. The engine produces 100kw of heat per second, most
goes straight out the exhaust after giving up energy to the propshaft but some is retained by the 2
nd
stage parts, meaning it gets very hot in the immediate area inside the cowl at the front of the airframe.
Allow plenty of propeller and forward flight airflow to be directed into the cowl area via openings and
vents just as you would a petrol or glow engine and this will keep the area well ventilated and help
prevent damage to surface finishes and/or paintwork. The ideal exit for this air is via side or lower vents
and a large clearance around the exhaust outlets of at least 6mm/1/4” is ideal to allow good outflow and
afford a limited amount of exhaust cooling. Be careful not to close off vent flow by the use of large
spinners or backplates.
It is important as noted earlier to minimize this hot cowl air reaching the engine intake as it raises the
engine running temperature significantly (an extra 1C at intake = approx 4C extra EGT), which is
detrimental to reliability and sensitive electronics are housed in the front of the engine.
Any required side or down-thrust for the powerplant is usually built into the firewall, so once the engine
unit has been positioned to place the propeller shaft in the centre of the cowl, the engine clearance hole
and bolt mounting holes can be cut out using the template. Protect any exposed wood using fuel proofer.
Allow a small clearance between the end of the cowl and the back of the propeller. Add a spacer to the
mounting position on the firewall if required, do not add a washer behind the propeller.
Exhaust extensions
As is also for the Heli engine, in normal operation the engine exhaust flow is continually expanding as it
leaves the 2nd stage. Adding exhaust ducting beyond the exhaust outlets inhibits the exhaust expansion
and causes local restrictions to the free flow, increasing the back pressure on the engine. This reduces
the power available to run the engine and has to be made up by burning more fuel. This therefore
significantly increase the running temperature and spoils the starting and running characteristics of the
unit, so is strongly discouraged.
However, where the standard exhaust outlet does not quite reach beyond a particular cowl or where you
want to turn the exhaust slightly to the rear for scale effect or streamlining, short simple Xicoy approved
push-on exhaust stubs are acceptable provided they cause minimal disturbance to the engine flow. It is
the longer lengths or significant changes in direction which cause back pressure and must be avoided.
Availability of approved exhaust extensions from various manufacturers will be noted on the Xicoy
website.
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