Expert: Paul Soderman - 3/1/2008

Question
Sir,
I am currently a third year mechanical engineering student in the University of Mumbai. We, a group of eight students have chosen to design and fabricate an UAS as our final year project. I shall be designing the propulsion system for the aircraft. We plan to make a small to medium size aircraft with an gross weight of 70 kg (approx.) and a tentative payload capacity of 10 kg. Could you please help me decide the parameters on the basis of which I should design or select the propulsion system for the aircraft.
Regards,
Sagar

Answer
Sagar
This is a nice system analysis problem. Given your mission requirements (airspeed, altitude, payload, range), the propulsion system must be sized to provide the power and thrust to a) accelerate the aircraft during takeoff to flight or, b) equal the cruise drag, whichever is greater.  The weight and mass can be estimated and the drag calculation will depend on the aircraft shape and the lift required. In cruise the thrust equals drag, and the aircraft power required is drag times velocity.  Once the power required is found, you will search for the size and type of powerplant that will satisfy the requirement and fit the aircraft geometry.  Of course engine power must be somewhat greater than aircraft power required because efficiency cannot be 100 percent.  The powerplant will change the aircraft weight, lift required, and drag so the process must start again. The fuel required to supply the engine for the desired range also affects weight and geometry.  The calculations will depend on altitude because of the density factor and engine performance with altitude.
Like any system problem you may discover secondary factors that are also important such as controlling engine torque by aerodynamic forces, ground clearance, climb requirements, etc.  Your team will need some knowledge of aerodynamics, aircraft design, control, and propulsion systems. Best of luck.
Paul