Example of hover characteristics
Corresponding to CL/a and CD/CL characteristics for fixed wings, we have CT/qand Cp/CT for the helicopter in hover. An example has been evaluated using the following data: blade radius, R = 6m blade chord (constant), c = 0.5m blade twist, linear from 12° at root to 6° at tip number of blades, N = 4 empirical constant, k=1.13 blade profile drag coefficient (constant), CD0 = 0.010 The variation of CT/s with q is shown in Fig. 3.8(a). The nonlinearity results from the CT term in Equation (3.28). The variation of CP/s with q is calculated for three cases:
• k=1.13, Equation (3.48), • k=1.0, Equation (3.46), the simple momentum theory result, • Figure of merit M = 1.0, which assumes k=1.0 and CDo = 0.
Over the range shown (Fig. 3.8(b)), using the factor k=1.13 results in a power coefficient 0–9% higher than that obtained using simple momentum theory. The curve for M = 1 is of course unrealistic but gives an indication of the division of power between induced and profile components. (Rotor performance characteristics are sometimes plotted as CP/s versus CT/s. This type of plot is known as a hover polar.)
Corresponding to CL/a and CD/CL characteristics for fixed wings, we have CT/qand Cp/CT for the helicopter in hover. An example has been evaluated using the following data: blade radius, R = 6m blade chord (constant), c = 0.5m blade twist, linear from 12° at root to 6° at tip number of blades, N = 4 empirical constant, k=1.13 blade profile drag coefficient (constant), CD0 = 0.010 The variation of CT/s with q is shown in Fig. 3.8(a). The nonlinearity results from the CT term in Equation (3.28). The variation of CP/s with q is calculated for three cases:
• k=1.13, Equation (3.48), • k=1.0, Equation (3.46), the simple momentum theory result, • Figure of merit M = 1.0, which assumes k=1.0 and CDo = 0.
Over the range shown (Fig. 3.8(b)), using the factor k=1.13 results in a power coefficient 0–9% higher than that obtained using simple momentum theory. The curve for M = 1 is of course unrealistic but gives an indication of the division of power between induced and profile components. (Rotor performance characteristics are sometimes plotted as CP/s versus CT/s. This type of plot is known as a hover polar.)
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