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I'm working on a design having a 3.2-metre wing with a constant chord section in the middle and taper on the sides. With a tail volume of about 0.68, the neutral point is at 60% chord from the leading edge. I want to choose a static margin of 12.5, which puts the CG at the middle of the chord at the midsection. Also, the main spar is twice farther from CG than the false spar in this configuration which should put more load on false spar than main spar. Bringing CG to 33 percent of chord requires a TV of 0.4 which I think is considerably less from what other aircraft in the same class use.

My question is - Do aircraft usually fly with CG at 50 percent of chord and would it be a good idea to reduce the TV to less than 0.4 to bring CG where I want? to reduce TV, should I keep the same tail area and reduce the length of boom? Also, should I change the spar locations to suit the CG location?

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  • $\begingroup$ Add some weight to nose if you can. $\endgroup$
    – Robert DiGiovanni
    Sep 13, 2021 at 16:55

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Tail volume has very little to do with CG, and it is rather silly to have it so far back that the tail has to lift.

Unless there is a specific reason to put CG at 50% chord, put it at 25% for starters. You can always move the CG.

Tail volume (specifically horizontal) is there as needed not only for trim in flight, but also for directional stability and stall control.

It needs to be big enough for that too. Also keep in mind that weight further forward increases the leverage of the tail, allowing for smaller size.

Now the "smallness" will be more related to that which is needed for pitch control.

It may be best to reference aircraft of your dimensions and weight to improve your design. If in doubt, leave the TV a bit large, as this will result in a very slight increase in fuel consumption. This is generally not a problem for models of that scale, and makes the plane safer to fly.

Only airliners that use millions of gallons of fuel a year need to "cut it" any finer.

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  • $\begingroup$ I'm confused because I have been reading articles that say tail volume affects neutral point which should affect the cg location. $\endgroup$
    – Mridul
    Sep 13, 2021 at 17:18
  • $\begingroup$ @Mridul The wing also helps with stability if CG is further forward (as does any fuselage aft). There is very little reason to have CG further back. Why not put it near the main spar? So, you see, there are 2 ways to handle NP: weight forward or bigger tail. Which will you choose? $\endgroup$
    – Robert DiGiovanni
    Sep 13, 2021 at 17:28
  • $\begingroup$ Don't confuse NP with net CP (center of G and Lift). NP is further back for stability. $\endgroup$
    – Robert DiGiovanni
    Sep 13, 2021 at 17:32
  • $\begingroup$ I calculated neutral point of the aircraft in Xflr and used the static margin of 12 to get a CG location. If I put CG at 25 percent chord, will it not make the aircraft too nose heavy? $\endgroup$
    – Mridul
    Sep 13, 2021 at 17:46
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    $\begingroup$ NP is the aerodynamic center of the airplane's whole horizontal footprint. Static stability is basically a weathervane effect about the C of G, being the pivot, and the NP being the aerodynamic center of the weathervane. Trim is another thing; it's the creation of a force balance between opposing pitching moments from wing (wanting to pitch ND) and tail (trying to pitch NU) that allows the airplane to be statically stable at various selectable angles off the wind (angle of attack). CG is both the "pivot" of the weathervane, and is also a component of the ND pitching force required for trim. $\endgroup$
    – John K
    Sep 13, 2021 at 20:19

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