Originally Posted by morsesc
A fellow builder said he called Van's about this and was told that 2" blocks were used to raise the mains only if using a single scale for the nose wheel. After wheel pants, paint and carpet, my 12 is due for another wt and bal. Need to find out for sure.
Lines of communication must have gotten crossed somewhere, because the above is totally wrong. How would someone at Van's now how thick the scale being used was?
When computing the weight and balance of an aircraft based on weights obtained at the locations of each of the wheels, the airplane must be positioned at the same pitch attitude that was used by the manufacturer when they derived the C.G. range limits.
The RV-12 PAP specifies that the airplane be weighed in a level attitude when measured at the canopy side rails.
This can be done one wheel at a time, but it is usually not as accurate.
Regardless of how you choose to do it, the pitch attitude needs to be adjusted because the normal static pitch attitude with the airplane sitting on all 3 wheels is tail low. To get it level it requires the main wheels to be shimmed up about 2" (the reason it calls for the 2" blocks).
If you are weighing one wheel at a time, it would require 2" blocks under each main wheel, plus blocks equal to the scale height, under each wheel not on the scale.
Another method for leveling the airplane can be to remove air from the nose tire, but the reason the blocks are specified is that you need to make the nose tire almost completely flat to get the fuselage leveled at the cockpit side rails.
Which weighing method used does not matter, but the pitch attitude the airplane is in while doing so is critical if the final results are going to mean anything.
As already mentioned, remember to close the canopy after getting it leveled on the scales (it does make a difference).