@Graviman
Apologies for slow response Doug - put this down to my being off with flu...
Hope you are feeling better.
Don't underestimate the potential requirement for complex hydraulic accumulators to power rotor mast tilt actuators though. I am of course thinking about the need to recover from engine failure during transition which would require a quick and absolutely failsafe reaction.
While longitudinal stability is greatest at a 45-degree mast tilt angle as discussed earlier, this trimmed flight condition also has zero load on the conversion actuator. To understand this behavior, first think of hover in which the tilt actuator must push the tailboom up to horizontal. Then think of cruise in which the tilt actuator must pull the gearbox and engines underneath the wing. At some point during conversion the actuator will have zero load as it crosses over from pushing to pulling, and a static stability analysis reveals that this point is in the neighborhood of a 45-degree conversion angle. So, if the conversion actuator were to completely fail in forward flight, static trim analysis indicates that the aircraft would tend towards a stable flight condition at a 45-degree tilt angle. Again, this doesn't fully address your concern, nor does it address dynamic stability, but it does provide a good starting point for failure mode/autorotation analysis and flight testing.
what sort of assumptions are you making about structural loads?
We developed a set of V-n diagrams based on representative military certification limits for helicopters and fixed-wing aircraft in the cargo/utility class. These diagrams were then augmented in order to meet the agility requirements for cargo/utility class aircraft and gust load requirements. I have more detail in a restricted access report produced under subcontract by Army Research Labs. While it is my intent to openly publish all data from our work, this particular data never quite made it into a publicly released conference paper.
Are there any particular control aspects that will need significant deviation from the stick and lever control? Does the lever simply become a prop pitch control in forward flight?
The 1:10 scale functional flight demonstrator controls work as follows. The rotor has 3-axis angular rate stabilization so that stick inputs generate pitch/roll/yaw moments in the reference frame of the rotor (in the RC world this is known as CCPM flybarless electronic stabilization). During a 90-degree conversion we swap rotor roll and yaw controls in proportion to the conversion angle (both conversion and swap are controlled by the same knob on our transmitter). Other than that, all controls are as you would expect of a fixed wing airplane. We increase collective in airplane mode for increased cruise speed, but so far have not found any need to lock out rotor cyclic or pedal while in airplane mode. Of course, XV-15 (and I suppose V-22) phase out cyclic during conversion and we could do the same if appropriate.
Also are you seeking to minimise any flapback forces on the rotor during operation above transition?
I had been warned that flapback could be an issue during conversion of our 1:10 scale model, but no. The pilot reported that in forward flight the aircraft handled like a conventional airplane at all mast angles. As you may have noticed in the flight video, we performed conversion while in a banked turn. Our mid-sized MTR Scaled Demonstrator (9400 lbs GW) has a rigid unarticulated hub design, but larger designs will likely require an articulated rotor and thus need a control schedule to fly the rotor into axial inflow airplane mode.
@Shawn Coyle
Thank you sir for your kind observation.