Chippy CoG limits anomaly
 

I don't want to make too big a deal of this but if someone knows the answer I would appreciate it.

I've gone through an old copy of the DHC-1 Chipmunk pilots' manual and created a spreadsheet W&B calculator.

I've come across a small anomaly. In the manual a graph is given of C.G -(minus) Inches forward of datum vs Fuel in aircraft - Gallons

It then shows slopes for CREW ONE (FRONT SEAT) and CREW TWO

Though I can get close to the graph in the spreadsheet my slopes are too shallow.

Now it might sound not a big deal, but the fun bit is that at the right combined crew weights one **could** find that the aft CoG limit is exceeded when fuel is burnt.

Obviously this is just an exploration of the pilot's notes and I won't be taking off in a DHC-1 without checking the real W&B figures for the aircraft. Just wondered if anyone can tell me how this graph would have been originally calculated so I can trace the anomaly.

The copy I have is a 1969 version with a red cover, and the graph is exactly as you describe. Depending on which version you have they either show the pilot and passenger as 170lbs each, or 200lbs each including parachute.

The slopes in the graph just show the C of G change as the fuel burns with one person [200lbs] or two people [2 x 200lbs]. The assumption is that the pilot and passenger both weigh 200lbs as per the example in the table.

If you look at the graph the far left of the graph for two crew [i.e. full fuel] this equates to the calculation shown in the table. In the graph for the 22 and 22A this shows it right on aft limits with all useable fuel burnt. The slope equates to the arms as shown in the table.

Hope this helps. Feel free to PM me if you have any further questions.

http://img534.imageshack.us/img534/432/chippy.jpg

Yep that's what I'm looking at.

Having slept on the problem, I've a feeling that the published graph includes oil burn as well as fuel. My spreadsheet version starts at the correct full fuel position (corresponding to 18 gallon) but ends 'high'. The only way I can see why this would be is that something else forward of the CoG is disappearing and I can only guess that is oil.

I'll calculate the start and finish positions as a cross check and see what I can come up with.

Many thanks for the help.

Half a quart an hour looks like what they may have used for the figures, and is about what they use unless you turn her upside down :-)

Careful which figures you use, as the 22A (shown) has bigger fuel tanks.

ifitaint..