Propulsion system

 

As endurance is established as a main requirement, the propulsion system must be one of the first elements to be evaluated. Considering the state of art, a solar powered propeller aircraft would attain the best endurance performance, reaching in some cases more than 30 days of continuous flight. Unfortunately due to structural and weight limitations, these kind of planes allows a very limited payload, so has been discarded its use.

After consider the rest of propulsion systems, the turboprop seems to be the best option to achieve the adequate compromise between endurance, payload and flying altitude.

The weight of a turboprop engine is about half that of a comparable piston engine and have some advantages over a turbojet. For example: The amount of power available for propulsion is largely independent of the forward speed of the aircraft, so that more power is available during the initial stages of the takeoff run. The engine can be run under more efficient and economical conditions at low and medium altitudes, and retains these two qualities at low aircraft speeds. With the use of interconnected engine and propeller controls, the power response to throttle movement is more rapid than that of a turbojet engine and operations can be conducted from shorter runways.

Two commercial turboprop models had been considered for been the power plant of the UAV, PT6A (made by Pratt & Whitney Canada) and TP10 (made by Honeywell). Below tables with its performance are shown using the data available on internet. Despite the usual units in bibliography are horsepower and pounds, I have preferred to use SI units for coherence with the rest of data and calculations.

Model	SP (Kw)	Cp (kg/Kwh)	m* (kg/s)	Length (m)	Diameter (m)	Weight (kg)
PT6A 36A	559,3	0,359	3 - 4,5	1,575	0,483	150
PT6A 66	633,8	0,62		1,778	0,483	213
PT6A 135	559,3	0,585		1,778	0,483	156

Performance data and dimensions of some PT6A mdels

Model	SP (kw)	ESP (Kw)	SFC (kg/Kwh)	m* (kg/s)	Weight (kg)
TPE331-10	670	704	0,35	3,5	190

Performance data of TPE331-10 turbprop

For the Honeywell TPE331-10 also are shown a geometrical sketch and charts of fuel consumption and shaft power depending on the true air speed and altitude.

From these data can be drawn some interesting conclusions:

• Considering a long range cruise speed of 80 m/s (150-160 Knots) at an altitude of 12000m (above 40000 ft.) the approximate fuel consumption is 63 kg/h, which means around 1500 kg of fuel to fulfill the 24 hours goal. This figure is only a rough approximation and aerodynamic data will be necessary to obtain more accurate results, but suggests that an initial consideration of 1800 kg of fuel (1500kg plus 20%) may be appropriate for first calculations.

• With an aerodynamic efficiency factor of 30 for clean configuration (somewhat higher than in commercial jets but far less than in sailplanes) and assuming steady level flight the maximum weight of the airplane can be expressed as:

For the TPE331-10 engine, with a propeller efficiency of 0.8 and supposing long range cruise speed and same altitude, the weight limitation due to maximum shaft power, would be of 6400 kg. Although there are more restrictive factors such as wing loading, 6000 kg for MTOW was one of the first references in order to extrapolate the weight data of the similar aircrafts.

• Besides the relevance of the engine performance over the design also has to be considered its effect on operations. One of the most clear is that limitation in altitude is established by the engine operation limit, at 45000 feet, 5000 feet less than expected. On other hand, to allow an easy comprehension of the range of speeds that are achievable with this engine graphs of power against airspeed are showed below: