This essay has application in the EMS world. Selection of a fixed wing aircraft for EMS operations depends on many factors. In the end, it is probably runway length that is the determining factor. Whilst the higher speed, range, and lower maintenance cost of a jet for EMS work might be an attraction, the loss of so many smaller regional airports would prohibit the carriage of many patients flown today. Read on….
Liberty U – AVIA 305 Week 4 Essay
The choice of propeller or turbine aircraft in short or long-range operations.
This essay will consider the factors in deciding which type of aircraft to use for short or long-range flights. Traditionally, propeller aircraft have been used for short-range flights and turbine engine aircraft for long-range flights.
Some of the factors include short field performance, range, and fuel used during a climb.
Short Field Performance – Climb Angle
Many of the regional airports have shorter runways than the major international airports. As stall speeds are lower for propeller driven aircraft, the speed at take off (Vr) is consequently lower. Therefore shorter take off runs are possible.
The maximum climb angle for a power producing aircraft is the stall speed, unlike thrust producing aircraft where maximum climb angle is at maximum lift/drag (L/D) ratio. For this reason airports with significant obstacles in the climb-out path are only suitable for propeller aircraft.
Comparison of power required curves.
See the fig. 8.1 below. Of particular interest is the shape of the two total power required curves (Pr). For the jet, the curve is relatively unaffected by increasing altitude, and yet the efficiency greatly increases. On the other hand, the power required curve shifts to the right for propeller driven aircraft with increasing altitude.. Indeed, it may be that Vmax may be reached before the maximum endurance speed is achieved in some aircraft. In short, propeller aircraft fly lower and slower than their jet counterparts.
Efficiency changes with altitude
Fuel consumption of power-producing aircraft is roughly proportional to the power produced, instead of the thrust produced. Range and endurance performance are functions of fuel consumption, and so the power required to fly the aircraft is of prime importance. There is little significant increase in efficiency for propeller driven aircraft with increasing altitude. Whilst endurance does increase with altitude, specific range remains little effected.
Consider the time and distance to height for either aircraft to climb to the most economical, or best range height. All other factors being ignored, the extra fuel burned to climb higher by the jet will make it less efficient over shorter range, where cruise may only be possible at height before it is time to descend for landing. As the propeller aircraft cruises more efficiently at lower altitude, less time and fuel will be consumed, and consequently more time at cruise altitude will be the norm.
Maintenance Cost Comparisons
R Babikian et.al. (2001) studied the costs and efficiencies of turboprop, regional jet, and turbine aircraft. This fascinating study revealed details of the average maintenance costs of each type, reproduced below. It has to be noted that the percentage maintenance cost for turboprop aircraft is 20.5% as compared to 15.8% for RJ’s and 7.5% for large jets. For this reason an increasing use of RJ’s has had a major impact on the cost efficiencies of short-range transportation.
Choice of aircraft for particular routes is a complex and involved process. Many factors are involved, some beyond the scope of this essay. However, the basic premise is that propeller aircraft are more cost effective over short range.