Shawn C,
The following requirements are excerpted from CFR Parts 27 and 29 for Normal and Transport Category Helicopters....
§ 27.1323 Airspeed indicating system.
(a) Each airspeed indicating instrument must be calibrated to indicate true airspeed (at sea level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.
(b) The airspeed indicating system must be calibrated in flight at forward speeds of 20 knots and over.
§ 29.1323 Airspeed indicating system.
For each airspeed indicating system, the following apply:
(a) Each airspeed indicating instrument must be calibrated to indicate true airspeed (at sea level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.
(b) Each system must be calibrated to determine system error excluding airspeed instrument error. This calibration must be determined—
(1) In level flight at speeds of 20 knots and greater, and over an appropriate range of speeds for flight conditions of climb and autorotation; and
(2) During takeoff, with repeatable and readable indications that ensure—
(i) Consistent realization of the field lengths specified in the Rotorcraft Flight Manual; and
(ii) Avoidance of the critical areas of the height-velocity envelope as established under §29.87.
These requirements must be met in order to obtain FAA certification. If this necessitates the use of specifically shaped pitot tubes (a la MD900, S76, S92, et al) then so be it. Some models I've tested required custom designed pitot tubes to comply with the requirement, while another was able to demonstrate compliance with an off-the-shelf AN5816 pitot tube.
HT