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DESIGN: UniCopter ~ Power Train - Mast - Static for Absolutely Rigid Rotor

Outside Helicopter

"The term static mast is an odd one, since the rotor shaft is often called the mast, and the Hughes/Apache design is technically a mast support system. A static collar/bearing the wraps around the rotor shaft, and that supports all the rotor bending and control loads. It allows the main transmission to float below the support, and keeps the gearbox housing nice and light because all those rotor loads are carried by the "static mast". It also allows the transmission to be changed without disassembling the rotor and controls, kind of like when the magician takes his vest off while leaving his jacket on.

Generally, the extra weight of a transmission housing is not as much as the extra weight of the static mast if a normal low offset rotor is used (see some back posts about hinge offset). Also the static mast needs an extra mast bearing to keep the collar from rubbing on the spinning shaft.

Comanche has a very high offset main rotor, so the mast collar structure is a good idea on it.

BTW, any torque developed by the engines must be restrained by the main transmission. The static mast is of no help there. The torque limits on the typical Bell are associated with engine failure tolerance, I think, and the Black Hawk torque limit for the L model is due to the need to keep the torque at the level that qualified all the rotor structure when the 701C engines were introduced. Nothin to do with mast structure in either case, I think."

~ Nick Lappos

The mast is stationary and bolted to the airframe, the rotor head is mounted via bearings to the static mast and the M/R driveshaft goes from the XMSN thru the hollow mast to drive the rotor head- all dynamic rotor system loads are imparted on the very sturdy static mast the driveshaft only has to deal with the forces to rotate the rotor system.

> Since the AH-64 was originally designed by Hughes, I imagine its static-
> mast is similar in concept to those in the other Hughes/MD-series
> helicopters, but not having worked on MD-500s or AH-6 variants, I can't
> swear to that.

Yes, conceptually, the AH-64 is a scaled-up version of the H-6 idea. The RAH-66 Comanche has a static mast. All other Army helicopters have rotating masts.

> The downside of a static mast is extra weight. If you design a mast strong
> enough to take the torque from the engine you have enough strength to hang
the
> airframe from the same part.

I am not so sure the weight of the static mast is so significant when it allows the transmission to be less complex as it is no longer a structural member which lifts the aircraft. The transmission under a static mast can be built much lighter and still provide the same horsepower rating. An aircraft with a static mast was also the first helicopter to lift a load of over 1 1/2 times it's own emtpy weight. That aircraft was the Hughes 369HS or commonly called the 500C.

> Another downside is the bearings for the
> swashplate are also larger so gains are iffy at best. I would feel better
> knowing the extra material was in one rotorshaft rather than two parts on
the
> edge of the strength envelope.

Take note of the aircraft which have TQ vs. Airspeed limits applied as a result of excessive mast loading/bending. These include the B206 series, some of the UH-1 family and I believe even the UH-60 has some new limitations because of this.

I have over 3000 hours under a static mast and 6000 under a non-static mast, I have to say I like riding under a static mast much better.

Real Rigid Rotor

Weight saved by no teetering hinge etc. go into blades and hub.

Static mast

Hughes 369HS Hughes MD series

From LZ: Using a static mast is a great idea as this is one of the best designs incorporated in the design of helicopters. Hughes or their successors hold however I believe the patent and it may entail getting a license to use it.

An advantage of the static mast is that oscillating, fatigue producing stresses are contained in the non-rotating structure where they are less critical.

UniCopter

The masts will be integral portions of the final transmission housings.

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Last Revised: November 25, 2002