Originally Posted by
212man
I am interested to know why helicopters cannot also use EMAs as I'm sure the control loads and input frequency for an F35's flying controls must be comparable to that of a helicopter MR, and this would take away the need for the primary hydraulic pumps and the TR Servo hydraulic lines (with their attendant vulnerability).
For comparable weight vehicles, rotorcraft use an order of magnitude more hydraulic power and fixed wing aircraft. There is an AHS paper on the 12,000 lb 609 tilt rotor that shows it requires more hydraulic power for flight controls then a 138,000 lb MD 90 commercial jetliner. For conventional helicopters, the hydraulic power requirements are about 5 to 1 compared to fixed wing aircraft. So no, comparisons to the high performance F-35 fighter actuator power requirements do not extend to rotorcraft.
About a decade ago an attempt was made to install EMAs on a EH-101 for main rotor control. The HEAT project was appropriately named, since one major problem that halted further research was the inability to keep the actuators cool. Realize that with hydraulic actuator‘s, the cooling system comes for free.
Recently, under US Army FVL funding, Karem is developing an EMA rotor control using similar roller screw actuator configuration as HEAT. One significant difference however with the Karem design is the addition of pressurized oil circulation to each actuator to provide both lubrication and cooling. So the Karem EMAs will require hoses and pumps, same as hydraulic actuation ;-)
The biggest problem with EMA usage for rotorcraft swashplate control is the high probability of a jam failure mode. With linear hydraulic actuators, loss of fluid results in the actuator failing free to be back driven. This makes it easy to employ architectures with multiple cylinders either in parallel or series to provide redundancy. With EMAs, either rotary with gears or linear with screws, jam failure modes are very probable, and difficult to mitigate without adding considerable complexity and weight.
Remember on airplane you can have one actuator per surface, and multiple surfaces for redundancy. On a rotorcraft it is difficult to have multiple rotors for redundancy.
Hope my explanation makes sense.