The GEM in Forward Flight

In forward flight power must be supplied to overcome the drag of the GEM in addition to sustaining the weight of the machine. Although the

The GEM in Forward Flight

The GEM in Forward Flight

Fig. 12-5. Effect of height, disk loading, and jet angle on power loading.

available data are somewhat conflicting, there is evidence that the thrust augmentation factor does not vary radically with forward speed. Thus the power required in forward flight is approximately equal to the sum of the power calculated to hover and the product of the drag and the forward speed. The drag is equal to the sum of the parasite drag and the momentum drag. Hence it is important for an economically feasible, high-speed GEM to be aerodynamically clean.

The momentum drag is not so high as one might think. If Mj is the mass flux through the machine, the momentum drag might be calculated as the product of the forward speed and M;. However, experimental data suggest a thrust recovery of this momentum somewhat similar to the jet flap. What

little data are available in this regard show the momentum drag to be approximately half of the product of V and My The effect of forward speed on the lift augmentation factor is presented in Fig. 12-6, taken from the work of Higgins and Martin reported in Ref. 4. We cannot really generalize on these curves because the results must depend

The GEM in Forward Flight

Fig. 12-6. Effect of forward speed on augmentation ratio: D = 16 in., h/D = 0.1, all slots 0.06 in.; elliptic planform has same base area as circular planforms.

on the height of the machine above the ground, the ratio of jet velocity to forward speed, and the external shape of the machine. However, these data, as well as those obtained by others, show no serious deterioration of A with forward speed and even suggest the possibility of some increase of A with V.

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