RPM Tuning
Tuning the engine rpm can be a problem.
The preferred head speed for flight is 1550-1600 (hovering) to 1800 (3D).
There are various rpm measuring devices available, but they are expensive and
only the most expensive ones are capable of measuring the in-flight rpm.
Furthermore, these expensive ones use a hole you have to look through and then
you turn a button until your blades look as if standing still.
This is of course not practical unless you have a friend available checking your
rpm while you are flying, and furthermore he has to be very quick to measure temporary effects such
as quick decents.
Here I propose to measure the speed by just listening to the pitch of the engine.
Fly the heli and remember the pitch of the engine at the point you want to measure.
Then land the heli, find the frequency of the pitch you remembered by comparing
it with the tuning fork (or whatever pitch meter you have) and adjust your throttle curve.
Alternatively, you could remember or play the target pitch during flight.
This pitch gives you the engine RPM (table below),
which can be converted to rotor RPM with the appropriate
table, depending on the gear ratio of your machine.
A cheap mouth organ or flute (toy shop!), or a tuning fork may come in handy
for those who have no absolute hearing. Tuning forks usually
give a standard 440Hz 'a', toy instruments may have a pitch indication
but you'd better check its accuracy.
For the following tables I used 'reine stemming', assuming that you use a tuning fork.
pitch of engine | engine Hz | engine rpm |
a | 220 | 13200 |
bes | 238 | 14280 |
b | 248 | 14880 |
c | 264 | 15840 |
cis | 275 | 16500 |
d | 297 | 17820 |
es | 317 | 19020 |
The pinion to main gear ratio is 9:86, so you can use the following table:
pitch of engine | rotor rpm |
a | 1380 (way too low!) |
bes | 1491 (just too low for hovering) |
b | 1554 (low end for hovering) |
c | 1658 (high end for hovering) |
cis | 1726 (low end for 3D) |
d | 1865 (3D) |
es | 1990 (too high!) |
For the Raptor 46/50, the main gear ratio is 10:85, and in that case
you can use the following table. You can see that the engine should
sound a full tone lower as compared to the Raptor 30.
pitch of engine | rotor rpm |
g | 1397 (way too low!) |
as | 1489 (just too low for hovering) |
a (220Hz) | 1552 (low end for hovering) |
bes | 1676 (high end for hovering) |
b | 1747 (low end for 3D) |
c | 1863 (3D) |
cis | 1941 (high end 3D) |
Just for information, for the raptor 30 the tail drive pulley (fixed to the main gear)
to tail gear ratio is 41:9 (4.556:1).
Setting the main gear at 1, we get a ratio of (86/9):1:(41/9) which
would better be written as 86:9:41 for
engine/main/tail, or approximately 10:1:5 if you like.
Many web sites state a gear ratio of 1:9.56:4.56,
corresponding to an order main/engine/tail.
For the raptor 46/50, the pinion gear has 10 instead of 9 teeth,
and the main gear 85 instead of 86. This gives a ratio
(85/10):1:(41/9) or approximately 17:2:9.
© W.Pasman, 10/2/2