There have been several articles regarding the Sikorsky S-97 Raider, which achieved first flight this week. 

FoxtrotAlpha’s got a good backgrounder on this aircraft, its history and future.

There were some nay sayers in quite a few comments that I hit on a couple of the articles that pooh poohed the coaxial rotor as a limited solution for high speed vertol aircraft.

I think the mistake these folks have is confusing this machine with the older coaxial rotor machines like the Kamov KA-50 below (“Russian Air Force Kamov Ka-50” by Dmitriy Pichugin)

A quick scan of the two pictures, focusing on the rotor mast and then the blades, will show you that there are a lot of differences in the aerodynamics.

Helicopters are speed limited because the blades are moving in respect to the air passing the aircraft. On one side the advancing blade adds to the air speed and at the tip can easily move towards the supersonic where air becomes in-compressible and aerodynamics change radically (which is why the blade tips on high performance helicopter blades are swept like a fighter wing) On the retreating side the blade can quickly reach stall speed and loose lift..

Coaxial rotors have the advantage of putting more energy into the air in a smaller rotor disc. Because the length of the rotor blade has a large impact on the tip speed this reduction means that the aircraft can fly faster before hitting the above limits.  Also since one blade on each side is advancing and the other retreating lift is symmetrical even if the retreating blade looses lift, meaning the aircraft can fly faster.  And indeed the X-2 demo aircraft Sikorsky built as a tech demo before the S-97 hit something like 300 miles per hour while a conventional chopper maxes out at about 150.

The principal difference between the KA50 and S-97 is the type of blade control. The S-97 has a so called rigid blade, which does not have a flapping hinge at the rotor head. The hinge is part of the  conventional blade control system allowing the blades to flutter somewhat as the lift changes through the blades rotation (you can see in the picture of the KA50 that the blades are at various incidences to the path of flight, partly because of the turn but also because of this ‘flapping.’)  The more advanced though simpler and more rugged rigid blade system on the S97 is based on advance composites and aerodynamic control theory.

So why does it matter, why do we need faster helicopters?

Simply put speed up to a certain point is always a winner because it means that for the same cargo load you can move more material in a shorter period of time. It also means you spend less time in any particular point in space which in a military context means you’re less of a target. Fast and being able to land anywhere and hover behind cover are all very interesting to the military.

The other fast vertical take off aircraft, the jump jets like the F35 and the Harrier or the tilt rotor V22 Osbrey are really optimized for vertical take off and landing and fast transit, the jump jets have no real hover capability and the Osprey is a big and somewhat clumsy helicopter. The S97 is much more of a blended capability and its simpler and cheaper than a jump jet or tilt rotor. Sikorsky is hoping that they can convince the DoD to forgo doing too much specmaniship and competitive development and move forward with the coaxial rotor machine for the next generation of vertical lift air mobility platforms.

Of course right now the outlook for anything new is pretty bleak and Sikorsky is probably struggling to figure out where to take the technology they have developed. A typical ‘innovators dilemma the world of modern military acquisition.