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From what I understand, the CG of full-sized fixed-wing airplanes need to be forward of the wing's center of lift for stability. But the CG of a helicopter must be directly below the rotors since it doesn't have a horizontal stabilizer to counteract the pitching moment.

So, where is the CG in a tiltrotor airplane (the kind that has its engines on the wings, such as the V-22 or the AW-609)?

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    $\begingroup$ "From what I understand, the CG of a fixed-wing airplane needs to be forward of the wing's center of lift for stability."-- not accurate-- for example, many free-flight model airplanes have lifting tails, meaning that the CG is behind the center of lift of the wing itself. $\endgroup$
    – quiet flyer
    Commented Mar 12, 2021 at 1:08
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    $\begingroup$ @quietflyer I was mostly thinking about full-sized aircraft. I've edited the question to make that more clear. $\endgroup$
    – HiddenWindshield
    Commented Mar 12, 2021 at 2:43
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    $\begingroup$ Looking at photos of the V22 Osprey, it appears that the rotors are pretty much aligned with the quarter chord of the wing, where on most aircraft the CG is located. So, wouldn't it make sense that the CG is in a small range right there? How much detail are you looking for? $\endgroup$
    – Michael Hall
    Commented Mar 12, 2021 at 2:52
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    $\begingroup$ Why were you thinking that? (Because it is, after all, a fixed wing...) $\endgroup$
    – Michael Hall
    Commented Mar 12, 2021 at 16:04
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    $\begingroup$ Related: How do VTOL aircraft like the V-22 Osprey not tilt? $\endgroup$
    – user14897
    Commented Mar 13, 2021 at 0:13

2 Answers 2

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enter image description here

The c.g. limits change in forward and hover flight, so loading the aircraft takes special care. On the Bell XV-15, the range of c.g. was around 16 inches. For comparison, the internal cabin was 157 inches long. Based on the fuselage stations below and the image above, the c.g. limits were very close to the rotors in helicopter configuration.

For the control method in-flight, see: How do VTOL aircraft like the V-22 Osprey not tilt?

Commentary: how much forward ballast (see image below) an aircraft like the V-22 has -- I don't know (classified).

enter image description here

Maisel, Martin, D. C. Borgman, and D. D. Few. "Tilt rotor research aircraft familiarization document." NASA TN X-62 407 (1975).

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If we take the horizontal stabilizer as part of the fixed "wing" arrangement in horizontal flight, then for trim the CG and center of lift must always align exactly (although the rotor/propeller thrust line may affect that slightly). Stability is a bit more complicated and is not relevant to your question.

The trick with a titrotor is to arrange it so that when the rotor tilts up to become a rotary wing, its center of lift still aligns with the CG.

You have to be real careful when loading up a tiltrotor like the V-22, to get the CG right in between the rotor hubs. Cyclic pitch control of the rotor can shift the center of lift a little, but not by very much.

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  • $\begingroup$ This is pretty vague and generic. Where is the actual CG? $\endgroup$
    – Michael Hall
    Commented Mar 12, 2021 at 17:03
  • $\begingroup$ @MichaelHall I am sorry if "You have to be real careful when loading up a tiltrotor like the V-22, to get the CG right in between the rotor hubs" is not clear. Perhaps you can suggest something clearer? $\endgroup$
    – Guy Inchbald
    Commented Mar 12, 2021 at 18:10
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    $\begingroup$ I didn't say it was unclear, just that it is about as useful as "you have to be real careful to get the CG right in between the fore and aft limits." But then I didn't like the question either, because it seems intuitively obvious. (Unless someone is looking for actual numbers.) But if your answer is accepted I will delete my comments... $\endgroup$
    – Michael Hall
    Commented Mar 12, 2021 at 19:29

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