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W&B question for J400

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Thread transferred May 2018

W&B question for J4001 year 5 months ago#421

·         Steve Bennett

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I've just been running over some weight and balance calculations with a new member of our group and was asked a question I didn't really know the answer to, so.....

The POH of the J400 specifies a linear increase from 500kg to 700kg MAUW applicable from 00mm to 200mm aft of datum CoG. The question is why exactly?

I had assumed it was linked to elevator authority, however, it seems this forward of 200mm restriction does not apply to the J430. I was under the impression the 430 was just a different main wing therefore suggesting the same elevator set up as the J400.

Any insights would be appreciated


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W&B question for J4001 year 5 months ago#428

·         Doug Smith


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Hi Steve,

Those J400 limits sound a little bit out of date. I'll see if I can get someone to send you a new copy.

Typically, including for the J400, the forward limits are determined by stall speed and/or stall handling requirements. As you say, elevator authority.

The J430 is essentially a J400 with a bigger wing, and a bigger wing flap. The bigger wing (and particularly the bigger flap) produce more pitching moment (tending to push the nose down) and so the elevator has to work a bit harder to get the nose up, which you need to do to fly slowly or stall. So we do get more lift from the bigger wing, but the tail has to push DOWN harder to make the most of it... and that down force has to be carried by the wing, so the wing has to work a bit harder. It's a trade-off and basically the CG envelopes get tailored to produce the best aircraft handling for the category.