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Connecting Rod Balance in Lycoming aircraft engines requires that:

lycoming connecting rod

(1) the linear and angular acceleration is identical between rods of equal mass, and

(2) the moment of inertia is identical between rods.

The requirement for equal linear and angular acceleration of the connecting rods is met by the symmetrical movement the engine provides to the rods. If all rods have the same center-of-mass and that mass is of identical amount, then by Newton's Third Law, acceleration forces caused by translation motion will equal.

The requirement of identical moment of inertia (rotational inertia about an axis of rotation) means that the way the mass is distributed with respect to the axis of rotation is important. All rods need to have identical radius of gyration. This is achieved by having identical mass placed at the identical distances from the axis of rotation (piston end).

For a more through discussion of connecting rod and engine balance is found in the Sky Ranch Engineering Manual.

Equation for instantaneous piston acceleration

for calculating connecting rod stress

Using the following notation:

r=crank radius
n=conrod radius/crank radius
x=piston displacement
o=crank angle from TDC
w=do/dt
a=dw/dt=d^o/dt^2

x         = r*((4n^2-1)/(4n) + COS(o) + (1/4n)*COS(2o))

dx/dt     = (-SIN(o) - (1/2n)SIN(2o))*w*r

d^2x/dt^2 = (-COS -(1/n)COS(2o))*w^2*r
            + (-SIN(o) - (1/2n)SIN(2o)*a*r

The highest inertial stresses will be tensile at TDC, ie o=0. Don't forget though that there are gas pressures (highest at TDC) which could cause the connecting rod to buckle in compression. You will have to use the euler buckling formula for this:

Euler_Force = (PI^2*EI)/l^2

E=young's mod
I=2nd moment of area
l=conrod length

You need to check this in direction of rotation and at 90 degrees to this.

Why didn't he bail out? Parachute Sense US Navy 1944

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