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In many of the POHs, in the W&B section it is indicated that there is a moment change due to gear retraction. For example : Moment change due to gear retraction +780 in-lbs.

Two questions :

  1. If I understand correctly, to simulate a gear retracted scenario I only need to add 780 to the total moment (sum of moment for the empty weight, crew, passengers, baggage etc...) while calculating the W&B. Is that correct?

  2. In the majority of the POHs the C.G. envelope limits are based on gear down (fwd and aft).I saw one aircraft where it states the aft C.G. for both gear down and gear up - does it mean there is a different C.G. envelope for each case?

The aircraft in question is CJ-6A, please note the footnote in the image below

enter image description here

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  • $\begingroup$ What poh are you talking about? $\endgroup$
    – Ron Beyer
    Jan 6, 2019 at 15:49
  • $\begingroup$ Sorry , I'm not clear with your question.I mean the aircraft operating manual if that's what you mean. $\endgroup$
    – Yuval
    Jan 6, 2019 at 15:54
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    $\begingroup$ I know what a POH is, I'm wondering exactly which one you are referring to? For example I can't find any reference to a moment change in the POH for a 172-RG. I also don't think you would add anything, you need to move it, the weight isn't changing, the CG is moving. $\endgroup$
    – Ron Beyer
    Jan 6, 2019 at 16:03
  • $\begingroup$ I see it now, the CJ-6A, I didn't see it at the end of your second question before... $\endgroup$
    – Ron Beyer
    Jan 6, 2019 at 16:04
  • $\begingroup$ You have a many aircraft with moment change due to landing gear retraction.I will send you few examples if you need.The moment is added to the total sum of moments and then devided by total aircraft weight to get the loaded aircraft c.g $\endgroup$
    – Yuval
    Jan 6, 2019 at 16:10

1 Answer 1

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Yes the +780 inlb is the increase in the total moment aft of datum (I'm assuming it's the firewall) due to the center of gravity of the gear strut moving aft when it retracts. It's like you had a lead weight at your feet and you moved it a foot or two aft. Some retractable gear airplanes don't account for this but the Nanchang does seem to have a very long nose gear with substantial mass, so it looks like they decided they needed to take its movement into account.

The aircraft's CG envelope is a function of pitch stability at the aft limit and tail authority at the forward limit at various loadings and doesn't change. You are concerned with the actual CG location relative to the limits and there shouldn't really be two envelopes, just two CG location values for any particular load case, a gear up one and a gear down one, that reflects the 780 in/lb influence of the nose gear's movement on the CJ-6.

Since gear retraction moves the CG aft, your main concern is that the actual CG location respects the potential two worst case conditions; that is, aircraft CG gear up is within the aft limit, and aircraft CG gear down is within the forward limit.

That's how it should work and if there are actually two CG envelopes, it suggests that there are aerodynamic effects of gear extension on pitch stability and/or tail authority that were uncovered in flight testing, and that go beyond just the CG of the nosewheel moving forward. But in that case you would just expect to see a CG envelope that is more restrictive than it would otherwise be, to cater to the worst case possibilities, not two separate sets of forward and aft limits.

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  • $\begingroup$ Thank you for the detailed and clear answer John. $\endgroup$
    – Yuval
    Jan 6, 2019 at 17:13
  • $\begingroup$ I edited the original question and added a photo of the c.g. envelope of the CJ-6A .The wording at the bottom state the aft c.g. when landing gear is up.This raised my question.Can you explain why they pointed the aft c.g. with gear up? further info - I also have the aft c.g. with gear down and also the %MAC of change due to landing gear retraction (which is the difference between %MAC aft gear down to %MAC aft gear up).Last point - the envelope in the picture is according to aft gear down. $\endgroup$
    – Yuval
    Jan 6, 2019 at 17:20
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    $\begingroup$ That seems very weird to me. You would normally take all the variables into account to come up with an aft CG limit to cover the worst case loading and gear configuration, which would be gear up, because if you're at that limit in flight, lowering the gear moves it forward and all's well so who cares. I think maybe the Chinese/Russians (it's a Yak knockoff) were thinking too hard. $\endgroup$
    – John K
    Jan 6, 2019 at 19:18
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    $\begingroup$ Pure speculation, but could it perhaps be to allow flight with an extra heavy passenger or cargo in the rear seat subject to the limitation of not retracting the gear? Performance on the Yak doesn't improve all that much when you retract, anyway. $\endgroup$
    – pericynthion
    Jan 6, 2019 at 21:59
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    $\begingroup$ That's an interesting thought. Tandem airplanes that solo from the front tend to have to deal with a large CG movement between 1 and 2 up because all the load variation is aft, so there could be a back seat weight restriction that can be increased if you leave the gear down. $\endgroup$
    – John K
    Jan 7, 2019 at 1:50

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