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The drag on an airplane taking off from a floating runway

 

JANG WHAN KIM and WILLIAM C. WEBSTER

 

Department of Naval Architecture and OffShore Engineering, University of California, Berkeley, CA, USA

 

Abstract: The motions of an infinitely long, two-dimensional runway subjected to the dynamic moving load imposed by an airplane taking off are investigated. The runway is assumed to be floating in an inviscid fluid and is initially at equilibrium before the plane takes off. The deformation of the runway resulting from the take-off is wave-like and moves in the same direction as the plane. The maximum drag occurs when the plane catches up with the first wave. Three different runway configurations were considered: a baseline and ones which were ten and one hundred times more flexible than the baseline. For these runways, the added drag to the aircraft was very small, ranging from 1% for the stiffest to 10% for the most flexible runway.

 

Key words: floating runways, take-off, dynamic motions, drag

 

Address correspondence to: J.W. Kim, Department of Ocean Engineering, University of Hawaii, 2540 Dole Street, Holmes Hall 408a, Honolulu, HI 96822-2303, USA

Received for publication on March 4, 1997; accepted on Nov. 25, 1997

 

Introduction

 

As a result of the need for more runways to handle the ever-growing airplane traffic, and the lack of land near urban centers, airport planners have been forced to locate major metropolitan airports further and further from population centers. The remote locations require all passengers to spend several hours in local ground transportation at each end of an airplane flight, often significantly increasing the transit time between the start and end of the journey. Many metropolitan areas are located near significant bodies of water, for instance rivers, lakes, bays, or harbors. Geotechnical conditions or environmental restrictions often prevent the use of landfill to create airport runways in these bodies of water. In these cases, the use of floating runways adjacent to existing land-based airports may provide an answer for the expansion of airport facilities without destroying the convenience of their metropolitan location.

Floating runways pose many unique design problems due to their huge size and special usage. Runways for large commercial jets need to be about 4000m long and as much as 50m or more wide. Their great length implies that these runways must be flexible and able to respond to dynamic loads. Floating runways installed in exposed ocean settings will respond to ocean waves. In the setting of a protected harbor, lake, or river, these responses may not be important. The use of barge-like structures where the runway deck and the buoyancy unit are combined into one simple floating structure is a simple and economical choice. These structures also respond to waves, but as shown by Mamidipudi and Webster,1 this response occurs predominately on the periphery of the platform, not near its center where presumably airplanes will be landing and taking off. In protected waters, it is presumed that the exposure to significant waves will be a rare event.

This paper focuses on barge-like floating runways exposed to a different type of dynamic loading, the regular taking off and landing of heavy airplanes. Airplanes are usually at their heaviest during take-off. In addition to their passengers and cargo, they also have a full load of fuel, which may be as much as one-third of the gross weight of the airplane. Because a floating runway is flexible and receives buoyant support from the water, the runway will deflect from the airplane's weight and will form a dish-like“dent”around the aircraft. When the aircraft begins its take-off, it accelerates down the runway. This moving load causes a corresponding dynamic response of the flexible runway. That is, the dent will change and will also progress like a wave down the runway.

 

 

 

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