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I am working on the design of an aircraft (2 m long, wingspan = 3 m). The platform is longitudinally stable but it is very sensitive to the variation in CG location. For example, 1) CG_location = 100 mm from wing L.E, the static margin is about 15% of MAC. 2) CG_location = 105 mm from wing L.E, the static margin is about 8% of MAC. 3) CG_location = 110 mm from wing L.E, the static margin is about -1% of MAC. (Unstable: Cm vs Alpha curve slope becomes positive)

So, effectively while manufacturing the aircraft or while dropping the payload the CG of my aircraft should not vary more than 1 cm, otherwise it would become completely unstable. Taking into any manufacturing errors, the platform CG might not be exactly as expected and hence having such a small margin creates issues.

How should I deal with this issue? Can I have any suggestions on how do decrease the sensitivity of the longitudinal stability to the CG location? Thanks in advance.

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It sounds like it has a horizontal tail, so you basically need to increase the tail volume which will allow you to move forward CG limit forward, allowing you to move the median CG forward and expanding the overall range. So just make the horizontal tail a bit bigger.

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    $\begingroup$ Or increase the tail arm. $\endgroup$ – Koyovis May 30 at 0:43
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    $\begingroup$ Tail volume is area times arm. You can increase one, both or all three. Making the surface bigger is easiest. $\endgroup$ – John K May 30 at 1:04
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A picture of the design would be helpful, and also a caveat on the legality of "dropping something from a plane".

Now, in addition to what John K suggested, a real quick fix would be to lengthen the fuselage a full meter aft and fore as well. This increases the torque of your tail (I would make it bigger too). This not only gives you a wider CG range, it also slows down the pitching movement, making the plane more controllable. Particularly if extreme range is not absolutely necessary, go ahead. Lowering the aspect ratio of the wings (making them wider) also will help.

As a point of interest, many people are flying quad rotors around these days with possible plans for package delivery. Like a miniature Chinook helicopter, they can tolerate a very wide range of CG fore and aft simply by adjusting power to the rotors.

But for an aircraft, look to the wing, the fuselage, the tail, or all. Any payload, just like real cargo carriers, needs to be centered at CG as much as possible. A moving or sliding adjustable cargo floor may help your design.

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    $\begingroup$ Really, he has the choice of making the tail boom longer, the tail larger, or both. Any one will increase tail volume (area x arm). $\endgroup$ – John K May 29 at 20:07
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Using your numbers, it looks like your MAC is about 71.4mm and NP is about 110.7mm aft of LE. That means a 1cm shift in CG is equivalent to 14% MAC, which sounds about right. That is, longitudinal stability is not sensitive to CG location.

For a small drone like this, keep in mind that CG isn't as sensitive to manufacturing as you may believe, unless you have a superb payload to weight ratio. If that's the case, you can increase the tail volume as others suggested to shift the NP aft (thereby increasing the static margin).

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  • $\begingroup$ 10 mm CG shift is 16% MAC in the OP's numbers. That makes MAC = 62.5 mm. This is very narrow for a 3 m wingspan (aspect ratio = 48). But if this is true, then indeed, there is nothing particularly wrong. With the NP already almost a chord aft of the wing, the tail volume must already be quite big. With the arm about 1.5 m (of the 2 m length), damping will be enormous (and downwash small), so I bet the model will be flyable even with very low static stability. $\endgroup$ – Zeus Jul 2 at 7:31
  • $\begingroup$ Thanks for catching this. Yes, this is indeed an enormous aspect ratio...(larger than the Rutan Voyager). Is this intended by the OP? $\endgroup$ – Jimmy Jul 2 at 14:27
  • $\begingroup$ That's the question... I don't know. It's not impossible, but it's significantly more than the champion-level gliders of comparable size. $\endgroup$ – Zeus Jul 3 at 1:08

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