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Airspeed
We know from our triangle that airspeed is going to be a function of available power and altitude we are will to trade off. For this discussion, trading off altitude will not be as important. However, available power will be an important aspect. In addition, the second primary factor will be the amount of surface area the helicopter has, and how much drag is associated with that surface area.
For this reason, the 2 sections of the .air file that are of most interest to us when it comes to airspeed are:
1402: Main Rotor
1404: Fuselage Area and Drag
The configuration of the main rotor will dictate the amount of lift/thrust that the rotor system will generate. Any surplus of thrust beyond that which is required to keep the aircraft in the air (at a desired altitude) can be converted to airspeed by simply lowering the nose of the helicopter.
So we can increase/decrease our airspeed limits by increasing the amount thrust/lift generated by the rotor system as a result of manipulating the following params:
1. Main Rotor CLMin
2. Main Rotor CLMax
3. Main Rotor Blade Length
4. Main Rotor Blade Chord
5. Number of Main Rotor Blades
The amount of surplus lift/thrust available from the rotor system will be counter-acted by the fuselage area and drag. Fuselage frontal surface area and drag will dictate how much resistance is encountered when the aircraft is flying straight and level with its face into the wind. Making it the primary factor in cruising airspeed.
Fuselage side surface area and drag will dictate how much resistance is met as the side of the aircraft encounters airflow over the fuselage. This will dictate how efficiently the aircraft can move through the air while in turns. If you desire an aircraft that bleeds off a lot of airspeed in its turns, this is an area to look at.
Things to Consider
Achieving higher airspeeds by drastically reducing fuselage surface areas or drag coefficients will result in an aircraft that does not like to bleed off airspeed very easily.
Achieving higher airspeeds by drastically increasing the amount of surplus lift/thrust in the rotor system will result in an aircraft that has to be flown in a very nose-down attitude to generate given airspeeds.
Therefore, harmony between the rotor system and surface area/drag parameters should by sought. With the goal being an aircraft that cruises at a natural attitude, but is also capable of bleeding off airspeed well when the power is reduced, and the nose is raised to compensate for the resulting descent.
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