The ability to increase speed rapidly is important for many types of operations. The maximum level flight acceleration capability is primarily a function of the excess power available. At hover, the maximum acceleration is achieved when the maximum available rotor thrust is tilted until the vertical component is equal to the gross weight.

For this situation, the acceleration is:

where the maximum thrust is taken as equal to the maximum hover gross weight from a hover ceiling plot such as Figure 4.35. From hover, the equation can be used for accelerations rearward and sideward as well as forward. The acceleration capability in forward flight varies from the hover value to zero at Vmix. For speeds between hover and Vmix, the acceleration capability can be computed using the forward flight charts of Chapter 3 with the following procedure:

• Assume that CT/o = Cw/g.

• For even values of tip speed ratio, Find 0O at C2/omax. avaiL t0 main rotor.

• Find Отру from charts.

• Calculate


COS Ct’ppp

• Find new 0O at Cg/Gm, x lvii]..

• Find f/Ah corresponding to Ct/g and 0O.

• Convert tip speed ratio into forward speed and dynamic pressure, q.

• Calculate the acceleration capability from the equation:


Figure 5.14 shows the acceleration capability of the example helicopter at its design gross weight and at sea level standard conditions.

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