Tilt my rotor

There's this thing that's starting to turn up on the news and the cover of Time magazine and movies like Transformers. Its the Bell-Boeing V-22 Osprey, and its the world's first operational tiltrotor. A tiltrotor is essentially a conventional aeroplane, except with normal engines replaced with oversized turboprops that can tilt up to vertical and act as helicopter rotors, thus making the thing capable of vertical take-off. Tiltrotors are- in theory at least- supposed to combine the flexibility of a helicopter with the range and speed of an aeroplane. I've got a feeling that some of the compromises involved reduce the actual performance to something less than spectacular. But I'll leave that for another post. Today I want to start from the beginning, and talk about one of the first tiltrotors, the Bell XV-3.

The XV-3 was the first experiment in tiltrotoring, being built in 1955 to iron out the kinks in the concept. The testing program was dominated by aeroelastic troubles, the combination of traditionally built wings and the relatively new art of helicopter rotor design creating some interesting maths at a time when people were still using analog computers. Playing around with the XV-3 solved the first round of problems with the tiltrotor concept, paving the way for the V-22- now seeing action with the Marine Corp in Iraq- and its in-development civilian counterpart, the BA609.

There is however, one vital difference between the XV-3 and the V-22. The XV-3 had a single engine located in the fuselage, driving the rotors through a long drive-shaft through the wings to the rotors at the tips, while the V-22 has the engines at the tips, tilting with the rotors. At first glance the tilting engines might seem like the better option. It sounds simpler, it eliminates the weight of the drive shaft, and after all it is found on the production model of a military aircraft. But here's the problem, the V-22 also has a drive-shaft between the rotors, so that if one engine fails the other can power both rotors. So suddenly the superiority of the tilt-engine-pod isn't so clear.

Mounting the engine on the wing tip and tilting it presents other problems too. The control wiring, and more importantly fuel piping (and probably some hydraulic lines too) have to pass through the pivot, crowding a lot of complex joints that add weight into a tight space. Cantilevering the engines off the wing creates structural problems. At least on land, the wing root has to take all that load, which means it has to be stronger, which means it has to be heavier. The majority of tiltrotors have been investigated by the military. Military rotor craft usually have some kind of armoured floor, to protect the cargo, passengers, and whatever else is in the fuselage. With tilt-engine-pods, that does not include the engines. With the engines mounted on the fuselage driving the rotors through drive shafts it does. Also, mounting the engines on the fuselage decreases the angular momentum of the craft in roll, providing a nice increase in maneuverability.

It is claimed that since the drive-shaft on a tilt-engine-podded craft is for emergency use only, it can be lighter, but such a drive-shaft still needs to carry sufficient power to keep the thing aloft, which is not inconsiderable. It is also worth mentioning that since the shaft only need operate for short periods (probably one half of a full range return journey), you can cut down a lot of the weight needed to give the thing a reasonable service life. I have to wonder though, whether the weight saving is as significant as is claimed.

So far I've failed to mention the 100-pound gorilla in the corner of the drive-shaft showroom, the horrible horrible vibrations. Having a spinning shaft running the length of a fairly flexible wing tends to shake the assembly to pieces in a pretty short length of time. This is probably why the tilt-pod-rotor configuration was chosen in the first place. But here's the thing, since the XV-3 testing was shut down, several things happened. Stiffer materials such as carbon fibre were developed, as was sophisticated computing capable of analysing vibrational modes in tiltrotors. Long drive-shafts have been used in other aerospace applications, such as helicopter tail rotors (the Sikorsky H-53 being a particularly high-powered and successful example), and I believe some of the tandem rotor choppers (like the Chinhook) have to use drive-shafts to power the front rotor.

So I have to wonder if the move away from centre mounted engines driving tip rotors through drive-shafts is the best possible direction for tiltrotor aircraft. I suspect that I may have oversimplified the above arguments a bit (I neglected mentioning gearboxes and swashplates on the assumption that they were the same for each type, which may not be the case), but I really think that centre mounted engines have a distinct advantage over tilt-engine-pods that is worth pursuing.
What is interesting to me is why centre mounted engines have been essentially forgotten for most tiltrotors. I suspect a large part of it is the protracted development of the tiltrotor in general which has been going on since the mid-fifties, but is only now seeing production. But a more significant if subtler effect lies in the fact that the tiltrotor concept was too far ahead of its time. The inability to properly analyse the vibration modes of the XV-3 led to the adoption of tilt-engine-pods in test aircraft-namely the XV-15, the precursor to the V-22- to simplify the engineering and results from testing. The development of fairly successful drive-shafts went unnoticed by the tiltrotor's designers, who were probably focused on other things. The interesting thing is that although it is only one aircraft, the V-22 has been in development for so long that it is now the face of tiltrotor aircraft, and so the tilt-engine-pod is seen as the defacto choice of engine placement. What happened then, was that the practical application of an idea grew not out of a thorough and complete analysis of all possible designs, but as an iteration of a well known design. That design was originally analysed as it was the easiest possible design to analyse. The V-22- a combat aircraft- shows the pedigree of a research aircraft. I wonder then if this is proof that the tiltrotor is undergoing proper evolution, rather than a simple progression of designs. The anatomy of the tiltrotor has retained those phenotypes that while not optimal, in the past provided an advantage, and so were selected.
I kind of cheated you here, because I want to talk about evolution, not aeroplanes. I think the really important thing to realise is that this needn't be so. Human beings possess both memory (of the XV-3) and some damn good computers, something that DNA does not. We are able to revisit and consider our failures, rather than carrying on blind to our history. Our flying machines therefore are not limited to random mutation and survival of the fittest, and should be all the better for it.