Lets do the Maths on it,

F=Ma (basically means force is equal to the rate of change of momentum)

Wiki tells me that "violent rain" (blackadder would suggest a hat) is >2"/hr but we'll pput that in sensible units because this is 21st century so 1.38*10^-5m/s.

The projected upper surface of a

GA plane I will estimate at ~15m^2.

Rate of accumulation * Area of aircraft

1.38*10^-5 * 15 = 2.08*10^-4m^3/s (this is the volume of rain that falls on the aircraft every second)

Volumeetric rate * Density

2.08*10^-4 * 1000 = 0.208kg/s Mass of water hitting the aircraft per second

The velocity of a rain drop is ~10ms^-1

Mass flow rate * velocity = momentum

If we assume that all the rain stops moving when it hits the airframe and so the momentum becomes effectively 0 then the change in momenum and hence force becomes:

0.208 * 10 = 2.08N

This is less than the difference of whether you remembered to go for a leek before you left or not.

Hope that helps.

Fred