**3.3 Results and discussion**

48 Autonomous Underwater Vehicles

model 1 model 2 model 3 model 4

> model 1 model 2 model 3 model 4

= (9)

0 2 4 6 8 10 12 14 16 18 20 angle of attack α(°)

0 0.5 1 1.5 2 drag coefficient *C*<sup>D</sup>

The drag of the hybrid glider will be increased because of the drag generated by the rudders and propeller compared with the legacy gliders in the glide mode. The range in the glide mode will be decreased because of the drag of these parts . The ratio of drag on the propeller and rudders to whole drag is illustrated in Fig. 11, where we find that the ratio changed as the angle of attack increases, and the values is within the range of 10%~35%. Compared with the legacy gliders, the range of the vehicles with the same configuration as PETREL will be decrease 10%~35%. The Lift to Drag polar curves of the four concrete models are shown as figure 12. The model 3 and model 4 have the bigger lift than the model 1 and model 2 when the drag coefficients from the figure 3-9 is less than 0.5, but the lift of model 1 and model 2 increases greatly when drag coefficients gets bigger than 0.5. Due to the drag of the vehicle need overcome by the variable buoyancy *B* in the end and there is equation (9), so the net buoyancy supplied by the buoyancy driven system and glide angle should be

> *B D* sinθ

is the glide angle, *D* is the drag of the glider.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Fig. 12. The lift-drag polar curve of four concrete models

θ

taken into consideration.

Here *B* is the net buoyancy,

Lift Coefficient *C*L

The drag ratio of rudders and propeller

Fig. 11. The drag ratio of rudders and propeller

lift coefficient The orthogonal experiment shows that glide efficiency is most significantly influenced by the chord length while stability of the vehicle is most remarkably affected by the sweep angle.

Further numerical calculations based on four specific models with the attack angle in the range of 0°-20° indicate that location of the wings mainly affects glide stability but has little influence on glide efficiency.

When the vehicle glides at about 6° attack angle it has the maximum ratio of lift to drag. The range of the hybrid glider with the same configuration as PETREL will be decrease 10%~35% compared with the legacy gliders.
