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Valach VM 120 B2-4T Instructions
11
Propeller
The best RPM range for the VM-120 is 4500 to 5000 RPM on the ground.
Suitable Fiala-2-blade Propellers are: Suitable Fiala-3-blade Propellers:
28x14“ 30x12“
30x14“ 32x12“ 27x14” 28x12”
With the smooth power stroke of the VM-120, it usually is not required to use 3-blade
propellers. Their efficiency is about 5 % less. You would experience a noticable reduc-
tion in performance, when using a 28x12“ 3-blade instead of the 30x12“ 2-blade prop.
Hint:
Drill the mounting screws of the propeller from both sides. Start from the backside
and than go on on the front side. This will make your holes more precise.
Tighten the propeller screws the right way
Insert all six prop screws and tighten the screws diametrically opposed, until they have
the same torque.
The M10x1 Nuts are
not
intended to mount the propeller on the engine. They can
become loose, fly around and cause serious injuries.
Safety instructions, the right use of propellers
Do not stay in front or to the side of the propeller longer than needed. Make absolutely
clear to spectators, that they have to stay behind the propeller area.
Check the propeller before every flight. Do not repair propellers or use damaged
propellers.
Check every (!) day, before the first start, if the six M5 prop screws are tightened.
Check the length of the M5 prop screws, before you attach the propeller. On the
one hand, they have to be long enough to form a reliable screw joint, on the other
hand they do not have too be to long, to prevent them from bottom out in the blind
hole. Keep in mind, especially wood props shrink a little, every time you tighten
the prop screws. Even if you tighten them up several times, the screws have to be
short enough.
For a (hopefully) better understanding: The friction between prop hub an the backside
of the propeller holds the prop. The friction is achieved by the contact pressure of the
six screws. If the friction is not sufficient the propeller can move a bit, as a result the six
screws are loaded in bending mode with every revolution. Although this movement
is very small at the beginning, the screw will not withstand this alternating stress. The
screws will “sheer”, as most modelers call it. The truth is, that these screws will break
because of fatigue in the same manner as a spoon, that is bent by the clever Uri Geller
a few times.
