Military detailed specifications typically required hot high OGE hover performance. This is diametrically opposed to maximum forward flight speed. The Blackhawk called for OGE performance at 4000 ft and 95º F. To achieve the OGE specified hover performance Sikorsky incorporated 18º of equivalent twist in the main rotor blades. This effectively power limited the forward flight speed to approximately 155 kts. Rotor system drag drives up the power required and becomes the limiting factor in forward flight. Similarly the CH-53E incorporated main rotor blade equivalent twist of 20º to achieve hot and high OGE hover performance. At 137% torque (approx 13000 HP) the CH-53E is capable of 192 kts Vh. This was demonstrated for the US Navy in 1994to substantiate data for a new generation transport helicopter. By comparison, the CH-53A with 11ºof equivalent twist was capable of this same airspeed at 7500 HP. The MH-53E in another example of just how much rotor system drags plays in the equation. Sikorsky built the MH-53E Sea Dragon with large fuel tanks effectively doubling the flat plat area of the fuselage while only reducing the maximum Vh speed by 3 kts.
