## 3 Experiment Design  
The aim of the developed system is to study the effects of incorporating haptic feedback into the controls of a fixed wing UAV on the handling capabilities of operators during different phases of  
Figure 6: (**a**) Control stick acceptable range, (**b**) restriction mechanism for limiting haptic device to 1 DOF.  
Figure 7: Experimental setup.  
flight. To this end, an altitude hold task was designed and validated on graduate students enrolled in the Computational Engineering postgraduate program at National University of Sciences & Technology, Islamabad. The altitude hold task was chosen on the basis of the findings from previous studies [19] which show that haptic feedback has a significant effect on performance when it is related to pitch rotation (up and down movement) as compared to yaw or roll rotations.  
Fifteen graduate students enrolled in the Computational Engineering program participated in the study. None of them had any previous experience in interacting with force feedback haptic devices. A list of instructions on how to operate the haptic device was given to every user. To acclimatize the users with the use of haptic forces, they were asked to use the haptic device in an application available as an example in the OpenHaptics SDK. The application allowed them to interact with objects in a virtual environment and experience force feedback on collision. The use of the application before the study enhanced user confidence in haptic device control. The experiment on each user consisted of three testing scenarios: flight control in the absence of force feedback, flight control in the presence of realistic force feedback, and flight control in the presence of exaggerated force feedback, i.e., twice the force computed by the mathematical model. These three force conditions were chosen to evaluate if accurate haptic modeling always results in better UAV control. Each scenario was tested for 2 min 30 s. The total experimentation time was 120 min with approximately 8 min for each user. In each scenario, the task was to increase or decrease an altitude to a height specified by the acoustic command and hold that altitude until the next command (30 s altitude hold time). The user had to move the haptic device backward and forward to increase and decrease the altitude of the UAV, respectively. The three testing scenarios were run in a random order for different users, and to remove bias, they were not informed about which scenario was being played.  
There was a total of five acoustic commands: (1) Hold altitude at 1000 ft. (2) Increase altitude to 1200 ft. (3) Increase altitude to 2000 ft. (4) Decrease altitude to 1200 ft. (5) Decrease altitude to 1000 ft. With each command followed by a silence of 30 s. The specified altitudes were selected to check the effect on operator performance, whether it is easy to hold an altitude if the difference is larger (e.g., between 1200 ft and 2000 ft) or if it is easy if the difference is small (e.g., between 1000 ft and 1200 ft). Such a design of the altitude variation task allows for the analysis of different phases of flight against various time-related criteria. We were also interested in seeing the effect of increasing and decreasing altitudes on undershoot and overshoot.  
Moreover, in order to evaluate the effect of haptics on altitude hold subjectively, all the participants were also asked to fill out a questionnaire after performing the experiment, which gave us a quick insight into the individual's perceived performance. Figure 7 shows the experimental setup, consisting of a high resolution graphic display (altimeter on right and attitude meter on left), earphones, and a Touch 3D Stylus haptic device with maximum force of 3.4 N, which is sufficient for this experiment.