National Free Flight Society

SEN 2746

  1. Engine vibration
  2. A Dozen ?
  3. Balance
  4. One, six, twelve, … or the real elephant in the corner

Engine vibration

From:Klaus Salzer

Low vibrations do not need 6 cylinders (though I agree, the sound would be great!)
2 Cylinders are enough for a nearly vibration free 2-cycle motor…
if they are opposed, fire simultaneously, and they are on the same axis. This is NOT the standard way to build a boxer engine, as in most boxers the conrods act on the same crankpin (so firing alternatively), and for ease of construction and to reduce side forces the cylinder axis are staggered. Yet the boxer engine is a fairly smooth running device, as shown by many 4-cylinder boxers in cars (like the old beetle!).

Something also nearly vibration free is the combination of 2 small single cylinder engines arranged opposite to each other and driving a single prop via a central gear. I remember having seen such an arrangement in an old modeling magazine (MAN?) many years ago.

And to return to the beginning of the discussion:
Of course there is an energy loss due to the reversing of the piston. Otherwise there would be no sense in using aluminum, thin walled and milled out shapes – even if these also help in reducing vibrations.

A Dozen ?

From:Pete Reinhart
SEN 2745 Six cylinders just won’t serve.
To sound really ,really right and be smooth, really smooth running, it can only be a V-12!
Twelve Cox 010 cylinders on the right crank and you have it.

Editor’s comment  by my calculations .010 is .1638 cc , so dividing it into 2.5 we get 15.2….
So how about a 15 cylinder radial ?


From: gilbert morris
Peter, your points are well taken. Balancing reciprocating engines is as
much an art as it is a science, particularly the single cylinder engine
typical of F1C. The counterweight movement is rotational whereas the
piston’s is reciprocal and they are in  offset planes so there are bending
moments as well. These are pesky considerations as well as wind resistance
and friction which I am disregarding. Also fuel explosions are a whole
different consideration. But none of these distract from Sir Issac Newton’s
#2 law of F=MA (force = mass x acceleration) that unequivocally dismisses
piston reversal as nearly effortless even with only one cylinder.

One, six, twelve, … or the real elephant in the corner


The recent discussion about Newton, force, energy, balance and more is very entertaining and instructional. It shows the wide range of skill, knowledge and humor/humour we have in our community. And while it has some non-serious elements that could be classed as anti-pandemic there is real information that is always of use.  The good news is that we are not likely to see anyone show at field with  V-12 in the front of their F1C because I’m guessing that even though we have seen pictures of some very small multi-cylinder engines they probably don’t offer any performance advantage for a F1C model.  But at the risk of being accused of going after the ever persecuted F1C class where the performance, cost and complexity seem to be reducing participation faster than the other classes, I am surprised that the FFTSC/CIAM have not banned VP props for this class.  I’ve seen pictures but one on the field yet.