Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Figure 16.6 shows the elevator effect on the drag, lift, and pitch when varying the control angle and the wind speed. The pitching moment clearly increases when the deflection angle increases, as expected. This is clear for all the speeds, but the effect with respect to the AoA changes with the speed. At lower speeds, the trend is generally decreasing with AoA. This is reasonable because the lift at the elevator decreases the pitch. However, at higher speeds the pitch increases for negative AoA and decreases for positive AoA. The total lift generally decreases with the elevator angle, because the elevator lift decreases. The drag trend is not
Figure 16.5 Decode-1 baseline wind tunnel results (control surfaces in neutral positions) under varying wind speed. (a) Lift coefficient. (b) Drag coefficient. (c) Side coefficient. (d) Pitch coefficient. (e) Roll coefficient. (f) Yaw coefficient.
Figure 16.6 Decode-1 elevator effectiveness with varying deflection angles and wind speed. (a) Lift coefficient at 15m/s. (b) Drag coefficient at 15m/s. (c) Pitch coefficient at 10m/s. (d) Pitch coefficient at 15 m/s. (e) Pitch coefficient at 24m/s.
so clear. Here, the general effect seems to be to shift the minimum drag point to higher AoA and higher values. There are two main effects involved: The elastic effect on the tail due to the higher load at higher AoA is to change the actual AoA. Furthermore, the wake coming from the wing and the fuselage changes the flow impacting the tail. This could be due to the combination of the elevator and wing lift, because the wing lift increases with the AoA and this effect is opposite to the elevator one, being more effective at higher speeds. Furthermore, in these results, drag plays a role too, because the moment axis is not the wing centerline, and therefore the behavior is more complex.
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