# Aircrafts

The resistance forces acting on aircrafts are similar to those discussed for marine vehicles and consist mainly of pressure and viscous forces. Pressure forces act normal to the surface and arise from differences in air pressure along the body. Viscous or shear forces are surface resistances close to the airplane body due to the fact that it moves in a viscous fluid. The magnitudes of these resistances vary widely depending on the airplane’s shape, size, speed, and altitude (1). Normally, the resultant aerodynamic forces are resolved into two components: that perpendicular to the flight path (lift) and that parallel to the flight path (drag). During cruising, the engine must be able to overcome drag forces. Maximum power is needed during takeoff and increases with the lift and the rate of the climb.

Example: To ferry a space shuttle from Edwards Air Force Base in California to its launching site at NASA’s Kennedy Space Center in Florida, it is bolted on the back of a modified Boeing 747-400 jumbo jet. The volume of fuel used for this journey is huge, as it will take one gallon of fuel to travel only one length of the plane (231 feet). Because of many refueling stops along the way, the trip takes two days to complete. Calculate the amount of fuel it takes to complete the 2,600 mile journey.

Solution: The fuel efficiency of the Boeing 747-400 is around 0.2 mpg. Because of the heavy payload and loss of much of the aerodynamic advantages, with the space shuttle piggy-backed, the fuel efficiency drops to only 231 feet per gallon (0.04 mpg). For the 2,600 mile trip, 2,600 mi/(0.04 mi/gal) = 65,000 gallons of fuel is needed. The Boeing 747-400 can carry 32,750 gallons (215,000 liters) of fuel, which is about half the required fuel. For safety reasons, planes carry at least twice the amount of fuel required to complete a trip. This means the plane has to make at least 3 or 4 stops for refueling.

## References

(1) Schaufele, R. D., The Elements of Aircraft Preliminary Design, Aries Publications, Santa Ana, Ca, 2000.

(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

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