Weight and CG Envelope Development Question

[rockitdoc]

RV14A:
Van's published Forward CG limit = 82.08"
Van's published Aft CG limit = 88.24"

My empty weigh is 1305lbs
Gross weight is 2050lbs
Empty CG = 81.61"

So, my question is:

Does this data create a rectangular CG (x-axis) and Wt (y-axis) envelope with corners at (82.08", 1305 lbs), (82.08", 2050 lbs), (88.24", 1305 lbs), and (88.24", 2050 lbs)?

If so, why do other airplanes have more complicated envelopes shaped like trapezoids, ie, C172's and so forth?

 

[N890GF]

The trapezoid shape is because of limits of nose wheel weights when landing. so it might possible to have a trapezoidal shape for a -14A if there is a nose gear weight limit. I think the vans W&B documentation talks about nose wheel weight limits (RV-7), but I don't remember seeing a trapezoidal shape in any of the documents.

 

[AeroEngineer]

There are many limitations that could cause pieces of the rectangle to be cut out.

If the weight is near the max and the CG is near the forward end...

• ... the nose gear loads will be at their maximum (like George said), so the nose gear strength on some airplanes will mean that the airplane should not be at max gross AND at max forward CG at the same time.
• ... the elevator and the horizontal stab will be working their hardest at keeping the nose up at the stall speed, so, cutting off that corner of the rectangle means the airplane could have a slightly smaller tail.
• ... the elevator and horizontal stab will need to apply lots of down-force to balance the airplane, meaning that the loads on the wings (during any given maneuver) will be at their max... so, some airplanes may prohibit that combination so that the structure can be lighter / so that the horizontal and elevator can be smaller.

If the weight is near the min and the CG is near the aft end (i.e. opposite corner of the rectangle)...

• ... the load on the nosewheel might be too low for steering (in airplanes that steer with the nosewheel rather than with the main gear brakes).

If the weight is near the max and the CG is near the aft end...

• ... the loads on the main gear during landing will be at their max, so, cutting off that corner of the envelope allows you to have slightly lighter main gear.

I'm sure there are others but these are the first that came to mind. (If you look at CG envelopes on jetliners, they usually have three or four corners of the rectangle cut off, not just the top left).