# Modeling and Analysis of a Generic Internal Cargo Airdrop System for a Tandem Helicopter
## 4 Trim Analysis
### Case of Helicopter Flight Operation in the Process of Internal Cargo Airdrops  
Simulation of CH-47B tandem rotor helicopter flight operation in the process of internal cargo airdrops at the flight velocities of 0, 40, 80, 120, and 150 knots were conducted. Figure 7 shows the simulation results of the main parameters related to the internal cargo. The detailed simulation results of the thrust, cyclic and pedal controls, and the pitching and rolling attitude angles are presented in Figures 8-11. The trim deviations relative to the initial trim state are also plotted in order to analyze the effects of the internal cargo airdrop on helicopter flight. According to the simulation of helicopter flying from 0 knots to 150 knots, the total airdrop time is between 2.6 and 9.3 s, the maximum backward moving velocity of the cargo reaches by 4.7 m/s at hovering, and the maximum rotational angle of the cargo around the point of tipoff reaches by 54.48 deg at the flight velocity of 150 knots. The increase in flight velocity will constantly decrease the helicopter pitching attitude angle, thus increases the total airdrop time and decreases the backward moving velocity of the cargo. Theoretically, the shorter the total airdrop time, the better, because the influence of the airdrop operation on the flying helicopter will be smaller and smaller. However, considering the actual influence of the high speed of the moving cargo on the worker's safety, there might be a limit of the moving velocity of the cargo. Thus, tandem rotor helicopter flying at a velocity between 80 and 120 knots might be acceptable in the practical application. Figure 8: Simulation results of the thrust control. Figure 7: Simulation results of the main parameters related to the moving cargo. Figure 9: Simulation results of (**a**) the longitudinal cyclic control and (**b**) the lateral cyclic control. As seen in Figures 8-11, in order to keep the helicopter flying smoothly during the internal cargo moving backward, the pilot should constantly decrease the longitudinal cyclic control and increase the lateral cyclic and pedal controls and slightly hold the thrust control. Once the cargo begins to rotate around the point of tipoff, the thrust control should be decreased softly. Note that, after this moment, we need to pay attention to some of the other details, as follows:  
1. The longitudinal cyclic control should be continuously decreased when the helicopter flies at the velocity of less than 120 knots, and be decreased firstly and then increased when the helicopter flies at the velocity of 150 knots. 2. The lateral cyclic control should be continuously increased when the helicopter flies at the velocity of less than 40 knots, and be decreased when the helicopter flies at the velocity between 80 and 120 knots, and be decreased firstly and then increased when the helicopter flies at the velocity of 150 knots. 3. A slight adjustment of the pedal control should be made in order to trim the helicopter. The complex changes in the forces and moments of the backward moving cargo acting on the helicopter might be contributed to these changes in the thrust, cyclic, and pedal controls, especially the changes in the cyclic control. Considering the obvious reverse travel control of the longitudinal and lateral cyclic controls after the cargo rotates around the tipoff, it is not suggested to conduct the internal cargo airflow operation when the  
Figure 11: Simulation results of (**a**) the pitching attitude angle and (**b**) the rolling attitude angle. Figure 10: Simulation results of the pedal control. helicopter flies at a high velocity such as 150 knots. This may increase the pilot's control load and may even degrade the helicopter's flying qualities. An additional interesting analysis can be conducted as follows:  
1.