Weights and Balance. Now that a lot more builders are getting
finished or getting closed to being finished the issue of weight and
balance has become the topic on the Internet web site. As all of you
pilots know an airplane must be in balance to be able to fly. Being “out of
balance” can be a dangerous thing. Simply described from my Radio
Control Model days the general rule of thumb was to hold your model
airplane by the wingtips at approximately 1/3 of the width of the wings
from the leading edge. If the plane is properly balanced it should stay
level as if it were in level flight. If the tail drooped then it was referred to
as tail heavy and vice-versa it would be nose heavy. Either of these
conditions would make the plane very difficult to fly if it would fly at all.
The CG or Center of Gravity on the Loehle P5151 is at 67 ½” from the
“0” bulkhead which is the bulkhead directly behind the spinner. However,
the plane will fly fine if between 62” and 73” which is called the CG
range. It is no coincidence that the wing box spar is located in exactly
this same range. The Loehle factory has it’s own method of calculating
the CG of the P5151 and here is how it works.
  You must make all measurements with the plane set up as if in level
flight. The tail is raised and placed on a bench, saw horses or something
substantial. Calculations are made once with the plane empty and then
again with the pilot in the cockpit the later referred to as the Gross CG
calculation. Since you probably won’t be flying the plane with no pilot, the
gross CG is of most importance. Two important measurements by a
tape measure are required. Using a plumb bob or level you will first
need the distance between the “0” bulkhead and the center of the axles
on the main landing gear. Mine was 57”. Next you will need to use a
plumb bob from the axle on the tail wheel to the floor. The next
measurement is from the center of the main landing gears to the tail
wheel axle plumb line. Scales are then placed under each of the three
wheels and weights recorded. Here is the formula:

Wheel Base x  Tail Wheel weight
        Total Weight                              =  CG in inches aft of vertical datum













Here is an example of the actual weights and measurements on my
P5151 which has no modifications. I’m using the standard Rotax 582
with electric start and small car battery located just ahead of the wing
leading edge.  I’ve also added approximately 70 lbs in the nose to get
within CG limits.

Tommy’s P5151 (pilot 170 lbs)

Vertical datum =  57.0 inches
Wheel Base =   169.5 inches

Right main gear =  372 lbs
Left main gear =    376 lbs
Rear tail wheel =     78 lbs
 Total gross weight =  826 lbs


Gross CG calculations

169.5 inches  x  78 lbs
      826 lbs                      =    16.0 inches  +  57.0 inches vertical datum =  
73.0 inches CG

As you can see my CG with me in the plane is actually right on the aft
CG limit.


Here is another example from Ken Lebbins plane:
Ken has the Rotax 582, long range fuel system, a 174 lb pilot and 52
pounds of extra ballast in the nose.

Vertical datum =  57.5 inches
Wheel Base =    171.3  inches

Right main gear =   403 lbs
Left main gear  =    398 lbs
Rear Tail wheel =     79 lbs        
  Total gross weight  880 lbs

His CG is 72.88 inches





David Waters has a stock built P5151 with the Rotax 503. He added
electric start, battery and lead ballast totaling 70 lbs to get within CG
range.

Vertical datum =  57 inches
Wheel Base  =    169 inches

Right main gear =  408 lbs
Left main gear  =   410 lbs                               
Rear Tail wheel =   67 lbs
  Total gross weight  885 lbs

His CG is  69.79 inches




You may have noticed that all three examples had to use extra ballast in
the nose to get their planes within the CG range. From my own personal
experience I can tell you that I was concerned about tail weight from day
one and did everything I could to keep the weight down aft of the main
box spar.  However I still had to add 70 lbs to the nose. While the design
of the Loehle P5151 is still a winner it may need some minor
adjustments. I see two possible solutions. Move the wings aft two inches
and /or move the pilot forward a few inches.  Moving the wings aft by two
inches would seem to change several factors. It would change the ARM
of the engine weight plus it would change the CG range from 62” and
73” to 64” and 75”. After all the plane should balance at the same point
on the wing chord.  While I can’t tell you that this is what you need to do,
it might be something you may want to discuss with an aeronautical
engineer.