Flight tests of a Mono Tiltrotor (MTR)

[gdbaldw]

@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.

[gdbaldw]

Vertiflite Magazine (of the American Helicopter Society) - Summer 2011 - has an illustrated feature article on the Mono Tiltrotor (MTR), available here .

[Lonewolf_50]

Doug, much obliged for the update. You folks are doing some wonderful stuff, hope the funding/investment stream can be found.

Advantage over the unmanned KMAX (for a maritime VERTREP mission) appears to be speed. How does payload comparison stack up, (if you can say) at this point?

[PPRuNeUser0211]

looks like a good concept to my mind. Interesting article and great to see the scale demonstrator actually flying.

The things that spring to my mind are failure modes. Whilst someone has already brought up a total loss of power in the transition to forward flight, which would be an area of concern, what about the tilt actuators failing in the forward flight configuration? I would imagine that would make the landing somewhat tasty? (As an aside, how does the osprey deal with a landing in "plane" mode?)

Also, whilst you say the aircraft has a fairly good lift/drag ratio, how well do you think it will glide in the forward flight configuration with the proprotor dangling out the front?

Anyhow, best of luck with the project, early day, but I hope someone provides the cash to look at it further, as it looks interesting!

[gdbaldw]

@Lonewolf_50

...hope the funding/investment stream can be found.

While staying under the radar screen over the past few years, all studies and functional flight demonstrations were paid through small Government R&D contracts. However, as is highlighted in the Vertiflite article, DoD policy effectively restricts available prototype funds to Government contractor controlled IR&D, otherwise known as Government dollars paid as an overhead line item on production contracts. As you might expect, producer's interests are not necessarily aligned with this project.

Advantage over the unmanned KMAX (for a maritime VERTREP mission) appears to be speed. How does payload comparison stack up, (if you can say) at this point?

An initial study performed in 2004 shows the advantage increases with mission radius. For up and down missions, nothing beats helicopter, be it intermeshing, coaxial, dual, or with tail rotor. When range is needed, helicopters burn more fuel and transport less payload than this design. Specific performance numbers are in the Gov't reports on our website.

@pba_target

...what about the tilt actuators failing in the forward flight configuration?

My 3 DEC post touches on this; the actuator caries no load at approximately 45 degrees tilt in trimmed forward flight. We've actually experienced this in-flight failure mode. Early in our functional tests of a small RC demonstrator we had a structural failure resulting in complete loss of the conversion servo, and loss of one swashplate servo. Remarkably, the RC pilot was able to fly in to a flaired landing. Fixed wing controls in forward flight compensated for partial loss of rotor controls, and the powered rotor was able to soften the landing.

(As an aside, how does the osprey deal with a landing in "plane" mode?)

As I understand, the Osprey operational procedure for total power loss is a conventional airplane mode glide landing, with rotors destroyed on impact and theoretically the blade fragments being tossed away from the aircraft. Obviously, this has yet to be tested or demonstrated.

I hope someone provides the cash to look at it further...

A very senior Government official initiated an effort to do just that. Unfortunately, the rotorcraft pre-acquisition train is carried by production inertia. has no central leadership, and its prototypes are funded through producer IR&D (undirected Government overhead dollars).

Doug