|
Hi Alan,
I made all my Legacy's CG calculations from the firewall joggle.
I wrote in my POH that the firewall joggle is assigned the arbitrary value
of fuselage station (FS) 65.0, regardless of the actual distance from the
propeller spinner. I used 65.0" instead of the 64.625" number in the
factory POH because since both the 65.0" and 64.625" are arbitrary numbers, I
started with a round number. (Besides, perhaps due to my finance
background, I don't like to use numbers that imply more precision than is
actually there. (Sure, I know that 64.625" is the round number of 64-5/8",
but who uses fractions to compute CG?))
I could have just as easily assigned the value of 0" to the firewall
joggle, but then I would have to deal with negative numbers forward of the
firewall joggle and positive numbers aft. That would just make this more
difficult than it needs to be.
Once I defined the firewall joggle as FS 65.0, I measured the distance
between the center of the nose wheel and FS 65.0 and the distance between the
center of the main gear and FS 65.0. I measured the distance between
the leading edge of the wing at butt line (BL) 27 and FS 65.0". From
there, the rest is straight mathematics from the factory POH.
Using this method, I don't think it makes any difference where a builder
sets the reference point. So long as they use actual
measurements of the distance between the reference point and the wheels and
leading edge of the wing at BL 27, the mathematics will be
identical.
I backed into the CG of the baggage area by weighing the airplane
with and without weight in the baggage area and then computing the actual
CG. In my case, the baggage CG is 144.5" (with a firewall
reference point of 65.0"). That was with the weight loaded into the
approximate center of the baggage area.
This part isn't related to your question, but so long as we're talking
about CG, I compute my Legacy's CG with and without fuel. My empty Legacy
has a forward CG and burning fuel moves the CG forward. I can
theoretically take off with the CG within limits and then be too far forward on
landing. My zero fuel CG calculation checks to make sure I'll still
be in limits on landing in the worst case situation that I land with empty
tanks.
To eliminate the need to compute the baggage effect on CG for every flight,
I made up the following table that shows the minimum and maximum
allowable baggage weight for various combinations of pilot weights. Note
that this is based on forward CG considerations; many of these combinations
would result in exceeding max gross weight of 2,400 pounds with full
fuel. However, if the amount of fuel is managed to stay within the max
allowable gross weight, the following baggage weights are acceptable.
Here's the page from my POH:
Acceptable Baggage Loading, in Pounds
|
Combined Wt of Pilots, Any
Fuel |
Baggage, with Oxygen Bottle |
Baggage, without Oxygen
Bottle |
|
150 |
30-160 |
38-160 |
|
175 |
20-160 |
28-160 |
|
200 |
12-160 |
20-160 |
|
225 |
2-160 |
10-160 |
|
250 |
0-160 |
0-160 |
|
275 |
0-150 |
0-160 |
|
300 |
0-130 |
0-140 |
|
325 |
0-100 |
0-110 |
|
350 |
0-75 |
0-85 |
|
375 |
0-55 |
0-65 |
|
400 |
0-30 |
0-40 |
Note: The baggage
weights are the minimum and maximum weights for the combined weight of the pilot
and copilot. For example, a pilot
weighing 175 pounds would have to carry at least 20 pounds of baggage in
addition to the oxygen bottle, or 28 pounds if not carrying the oxygen
bottle. The weight of the baggage
should be roughly centered (forward-aft) in the baggage area. Moving the bags aft is better than
moving them forward. Although
heavier baggage is acceptable, the maximum recommended is 100 pounds. The weight of the pilots includes
gear they carry and wear and center console glove box contents. The maximum weight of instrument panel
glove box contents is three pounds.
The key factor in this table is the forward CG upon landing,
assuming the worst case situation of zero usable fuel. If additional fuel is assured on
landing, each gallon of fuel eliminates the need for 1.5 pounds of baggage. For example, a 175 pound pilot would
have no minimum baggage requirement if the usable fuel upon landing was assured
to be at least 20 gallons.
*The empty weight of the airplane includes all avionics, radios,
instruments, two gallons of unusable fuel in each wing, eight quarts of oil in
the engine, a headset, and a two-pound toolkit behind the seat.
Happy landings,
Dennis
Legacy, 220 hours
|