Note: see TigerDrone Design Report for full explanation of the design
In designing this drone, we wanted to keep simplicity in mind while still fulfilling all of the requirements. At first, we thought of a design similar to that of a Lockheed P-38. This plane has a fuselage, giving us room to store the payload and equipment, and two booms connecting the tail, which we planned on connecting vertical propellers to. However, after reconsidering the design, we decided that a blended wing body design would be much more sturdy and suitable for our purposes.
Choosing the blended wing body configuration allows us to save weight compared to a traditional wing and fuselage design because no fuselage structure is needed -- the wing structure encompasses the entire fuselage. The structure is can also be lightened in this configuration because the wing can be more easily elliptically span-loaded with weight so that in cruise, the weight distribution equates the lift distribution over the wings. Additionally, this configuration allows us to generate more lift compared to a traditional configuration because the entire wetted area produces lift.
One problem that is often encountered with this design is that it may be difficult to control. However, control issues are not an issue in this case because the aircraft is equipped with a feedback control system that keeps it stable using the various propellers and control surfaces.
We used a MH78 airfoil cross section for the wings and most of the body because of its common use in other flying wings like hang gliders. We then modified this airfoil for use in the center section of the drone, where the cross section needed to be thicker to incorporate the payload and equipment. Having a flying wing gave us sufficient space to embed our vertical propellers into the body of the drone. Embedding the propellers, as opposed to having them separate from the body itself, makes the drone much more aerodynamic. Putting the propellers outside of the drone would add a significant amount of drag, so having them inside means that the airflow is not disrupted. We also decided to put our forward pusher propeller in the back of the drone, between the aft vertical propellers. This propeller will serve to allow the plane to fly in forward flight, with roll and pitch control offered by use of differential power from the four vertical propellers. Because these four vertical propellers will not control the yaw, we added rudders on the vertical tails to control this direction.
Vertical tails with a symmetric airfoil cross section were also added for a variety of reasons. We needed to have a vertical stabilizer in order to keep our drone straight during flight. It was also necessary to add the rudders to the tail in order to control the yaw. Some sort of landing gear was also needed so that the drone was not landing on the curved bottom of the body. Since the landing will be completely vertical, we chose to use the vertical tails as our modified landing gear. These two tails, along with a front strut, will serve as the landing gear that the drone will be able to slowly land onto. Wheels may also be added onto the strut and tails for a more traditional landing.
It is possible for all of the equipment to fit into the center hatch section of the drone:
The payload also fits in the front hatch section with easy accessibility from the nose, allowing for the payload to be accessed by simply opening a small section. This simplicity means that anyone can retrieve the payload and it is not necessary to have any tools or equipment on hand to retrieve the medical supplies. There is also a hatch at the bottom of the cargo bay so it can be accessible from below (and can easily be swapped for downward facing sensing equipment).
The batteries can placed outboard of the propellers to have better span-loading, and the exact position may be changed chordwise depending on the weight of the payload. The batteries are kept in place with straps inside of the hatch compartment. Easy access to the batteries means that a 20 minute turnaround time will not be a problem for emergency missions.
As is further explained in the Structure section, modularity was very important in design. The wings and body are separate pieces so that no part is larger than 2 meters in any dimension. The wings slide and are bolted on to the body, and the entire body is less than 25kg with the payload.
In order to avoid catastrophic failure, it is necessary to include a parachute to let the drone fall safely to the ground and not risk the safety of civilians. This parachute will deploy from the top section of the drone and is stored in the center hatch area. Having the propellers embedded into the body of the drone is also an added safety component. This means that there are no propellers spinning without protection while the drone is on the ground, which could be a safety hazard to bystanders who are close to the drone.