The needle valve on the carburetor meters the amount of fuel mixed with the air. Too
much fuel (too “rich”) causes the engine to slow down and lose power. Too little fuel
(too “lean”) causes the engine to over-heat, slow down and lose power.
Vmax
engines operate best within a needle valve range of about +1/4 to -1/4 turn.
Proper compression is needed for a reliable idle as well as maximum power.
Compression is adjusted on
Vmax
engines by inserting or removing thin copper
washers under the glow plug. If compression is set too high, the fuel/air mixture
ignites too soon. This does not affect top-end power appreciably, but the engine may
stop abruptly while idling. If compression is too low, the engine may not produce
enough power for normal flying.
Compression is affected by air density. Anything that increases or decreases air
density increases or decreases compression. The elevation of your flying site and
the local temperature both affect air density and have a major influence on
compression.
As elevation and air temperature increase, air density decreases. To maintain the
same compression at high elevations and air temperatures, the volume in the
cylinder above the piston must be reduced slightly by removing washers from under
the glow plug.
For instance, if you fly at a high elevation (5000 ft or 1524m at Denver, Colorado,
USA), you will use fewer washers under the glow plug than at a lower elevation (700 ft
or 213m at Lafayette, Indiana, USA). Also, if you fly when the outside air temperature
is 75°F (24°C), and the temperature drops to 40°F (4°C) you may need to add
additional glow plug washers before flying again. The large temperature drop will
otherwise affect engine idle because the cold, dense air increases compression.
When air density decreases, less fuel and air enter the engine on each piston stroke.
This means that the engine will not produce as much power at high elevations or high
temperatures where the air is thin. Lift produced by the main rotors depends upon air
density in a similar way. The performance of all aircraft (including full size
helicopters) degrades considerably at high elevations and on hot days.
Hint:
The compression rules for
Vmax-6
engines are to start with 15% nitro fuel and two
washers under the glow plug. Add an additional washer for each 5% of nitro above
15%, and add an additional washer if the outside air temperature drops below 50
°
F
(10°C). Remove one washer for every 2000’ (600m) above sea level. For example,
with 25% nitro fuel start with four washers. If the air temperature is 40
°
F (4.4°C) add
one washer. If flying at 3000’ (900m) remove one washer.
Hint:
Vmax-7
engines generally prefer high nitro (25%) and high compression, so use two
to three fewer washers on the
Vmax-7
than on the
Vmax-6.
Op er a tor's Guide
Glow-Fuel Engines
4-2
LITE MA CHINES
