How does the V-280 Valor maintains its Center of Gravity (CG) when it is converted from helicopter (hover) mode to plane (flight) mode?

Question

Picture source:

The above pictures are showing three modes of V-280 Valor: hover or VTOL/hover mode, transition mode from VTOL to flight, and flight/air plane mode. As we may see, there is a CG shifting in that conversion, its CG must be shifted backward to compensate the weight of the two rotors and their supporting cases. I guessed its CG is exactly at the rotors' interconnecting shaft. But when the rotors are tilted forward, the CG must be shifted backward as the load-factor of the rotors/propellers are changed significantly. Indeed, during the tilting forward, the front side load maybe supported by the thrust generated by the rotors/propellers. But after they are inline to the fuselage, no more thrust can compensate. As we know, the tilt rotor of V-280 Valor is different than tilt rotor of V-22 Osprey, which with the V-22, the engines themselves also involved, they are tilted backward-up so the CG remains in its location.

My question then, how does the V-280 Valor maintains its Center of Gravity (CG) when it is converted from helicopter (hover) mode to plane (flight) mode?

Answer 1

First an foremost, the CG does shift, I strongly believe that there is not counter-action to this. Furthermore I speculate that this is a clever design feature in order to satisfy two needs in flight (however I cannot proof this...)

1. In Hover flight, the CG is exactly under the rotor shaft, therefore no cyclic trim input is needed which reduces the stress on the shaft and also on the actuators as the aircraft can hover without the need for constant cyclic input.
2. In forward flight, the CG shifts forward, as you already noticed in your question. This has the added benefit that the forward CG adds more longitudinal stability in forward flight. Infact this is needed, as the V-280 would otherwise be unstable in forward flight, which would vastly complicate the flight control system.

This CG-shift is probably already the case for the V-22, it is just not that obvious as the entire nacelles rotate.

Unfortunately I could not find a good reference quickly, but you might be interested in this dissertation, on page 55 the trim values for a V-22 like aircraft is given.

Answer 2

The center of gravity does not need to be exactly under the rotor shaft in helicopters or tiltrotors. This would be difficult to manage, and to mitigate the problems involving non aligned CoG and lift, helicopters and V-22 Osprey, as well as most likely the V-280 Valor use swash plates that transfer the center of lift of the rotor disk(s) as per commands from the pilot or flight computer.

During transition, before the forward speed is sufficient to create balancing force from the tail assembly, as the rotor is tilted forward the balance of CoL vs CoG is maintained by shifting the CoL as necessary via the swash plate.

Also note that during transition the lift must be increased in the beginning stages if level flight is to be maintained. As the rotor tilts forward, the vertical component of the lift decreases, and it must be increased before forward speed is sufficient for the main wing to produce lift. This is also done by the swash plate by incresing AoA of the blades.

There is speculation in the web about V-280 not using a swash plate, but instead conteolling the AoA of each blade individually by electric motors, but I was unable to find any authorative source for this.

Answer 3

how does the V-280 Valor maintains its Center of Gravity (CG) when it is converted from helicopter (hover) mode to plane (flight) mode?

It doesn't. And there's no need for the CG to be maintained fixed among helicopter and aeroplane configuration.

1. In helicopter configuration, CG should be as much as possible centered between and under the two rotors in order to minimise stress on the rotor shafts and this limitation is for sure valid also for the V-22 and the V-280. The limits within which the CG must lie is called CG envelope and depends mainly on the weight, being the envelope smaller at higher weight. For example for the R44 it looks something like that (source; on x-axis distance from the nose; on y-axis weight):

2. In transition phase the two rotors have to be rotated forward just of a very small angle to gain forward speed, with a relevantly small forward shift of CG as well. Just as an example, main rotors of conventional helicopters are tilted forward of some 3 to 5° and that's enough to make them reach forward speeds as high as 300kmh (200mph). This small forward CG shift has anyway a smaller impact than if it was toward the back, as can be seen from the previous plot.

3. Finally, in aeroplane configuration CG shifts are less of a problem than in helicopter configuration since CG shifts are compensated by the horizontal stabiliser, just like any conventional aeroplane. And just like any conventional aeroplane, CG must lie within the limits of the longitudinal stability, as well explained for example in this answer.

---

By the way, since combustion engines are quite heavy (as much as some 10% of the empty weight) and since in the V-22 they rotate too during transition phase, I would actually expect a bigger CG shift in the V-22 compared to the V-280, where only the two relatively light rotors rotate.