Title page for ETD etd-05312002-160944


Type of Document Dissertation
Author Magill, Samantha Anne
URN etd-05312002-160944
Title Compound Aircraft Transport: Wingtip-Docked Flight Compared to Formation Flight
Degree PhD
Department Aerospace and Ocean Engineering
Advisory Committee
Advisor Name Title
Schetz, Joseph A. Committee Chair
Durham, Wayne C. Committee Member
Grossman, Bernard M. Committee Member
Mason, William H. Committee Member
Telionis, Demetri P. Committee Member
Keywords
  • Wingtip-Docked
  • Compound Aircraft Transport (CAT)
  • Close Formation
Date of Defense 2002-05-08
Availability unrestricted
Abstract
Compound Aircraft Transport (CAT) flight involves two or more

aircraft using the resources of each other; a symbiotic

relationship exists consisting of a host, the mothership aircraft

and a parasite, the hitchhiker aircraft. Wingtip-docked flight is

just as its name implies; the two aircraft are connected

wingtip-to-wingtip. Formation flight describes multiple aircraft

or flying objects that maintain a pattern or shape in the air.

There are large aerodynamic advantages in CAT flight. The

aforementioned wingtip-docked flight increases total span of the

aircraft system, and formation flight utilizes the upwash from the

trailing wingtip vortex of the lead aircraft (mothership) to

reduce the energy necessary to achieve and/or maintain a specific

flight goal for the hitchhiker and the system.

The Stability Wind Tunnel (6 X 6 X 24 foot test section)

at Virginia Tech, computational aerodynamic analysis with the

vortex lattice method (VLM), and a desktop aircraft model were

used to answer questions of the best location for a hitchhiker

aircraft and analyze stability of the CAT system. Wind tunnel

tests implemented a 1/32 scale F-84E model (hitchhiker) and an

outboard wing portion representing a B-36 (mothership). These

models were chosen to simulate flight tests of an actual

wingtip-docked project, Tom Tom, in the 1950s. That project was

terminated after a devastating accident that demonstrated a

possible "flapping" motion instability. The wind tunnel test

included a broad range of hitchhiker locations: varying spanwise

gap distance, longitudinal or streamwise distance, and vertical

location (above or below wing) with respect to a B-36-like wing.

The data showed very little change in the aerodynamic forces of

the mothership, and possibilities of large benefits in lift and

drag for the hitchhiker when located slightly aft and inboard with

respect to the mothership. Three CAT flight configurations were

highlighted: wingtip-docked, close formation, and towed formation.

The wingtip-docked configuration had a 20-40 percent performance

benefit for the hitchhiker compared to solo flight. The close

formation configuration had performance benefits for the

hitchhiker approximately 10 times that of solo flight, and the

towed formation was approximately 8 times better than solo flight.

The VLM analysis completed and reenforced the experimental wind

tunnel data. A modified VLM program (VLM CAT) incorporated

multiple aircraft in various locations as well as additional

calculations for induced drag. VLM CAT results clearly followed

the trends seen in the wind tunnel data, but since VLM did not

model the fuselage, has assumptions like a flat wake, and is an

inviscid computation it did not predict the large benefits or

excursions as seen in the wind tunnel data. Increases in

performance for the hitchhiker in VLM CAT were on the order of 3

to 4 times that of the hitchhiker in solo flight, while the wind

tunnel study saw up to 10 times that of solo flight. VLM CAT is a

valuable tool in supplying quick analysis of position and planform

effects in CAT flight.

Modifications to a desktop F-16 dynamic simulation have been developed to investigate

the stability of wingtip-docked flight. These modifications

analyze the stability issues linked with sideslip angle as seen by

the Tom Tom Project test pilot, when he entered docking maneuvers

with 5 degrees yaw to simulate a ``tired pilot". The

wingtip-docked system was determined to have an unstable aperiodic

mode for sideslip angle greater than 0.0 degrees and an unstable

oscillatory mode for sideslip angle greater than 2.0 degrees.

There is a small range of sideslip angle that is a stable

oscillatory mode, sideslip angle between 0.0 and 2.0 degrees. The

variables, altitude and speed, yield little effect on the

stability of the system. The sensitivity analysis was

indeterminate in distinguishing a state driving the instability,

but the analysis was conclusive in verifying the

lateral-longitudinal (roll-pitch) coupled motion observed by test

pilots in wingtip-docked flight experiments. The parameter with

the largest influence on the instability was the change in pitch

angular acceleration with respect to roll angle.

The aerodynamic results presented in this study have determined

some important parameters in the location of a hitchhiker with

respect to a mothership. The largest aerodynamic benefits are seen

when the hitchhiker wingtip is slightly aft, inboard and below the

wingtip of the mothership. In addition, the stability analysis has

identified an instability in the CAT system in terms of sideslip

angle, and that the wingtip-docked hitchhiker is coupled in

lateral and longitudinal motion, which does concur with the

divergent "flapping" motion about the hinged rotational axis

experienced by the Tom Tom Project test pilot.

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