Article
Experimental Control of Two Connected Fixed
Wing Aircraft
Collin Carithers and Carlos Montalvo *
School of Mechanical Engineering, University of South Alabama, Mobile, AL, USA;
crc1421@jagmail.southalabama.edu
* Correspondence: cmontalvo@southalabama.edu
Received: 23 August 2018; Accepted: 24 October 2018; Published: 28 October 2018
Abstract:
This paper investigates the design and flight test of two fixed wing aircraft connected at the
wing tips. Connecting multiple aircraft introduces flexible modes into a typically rigid body system.
These flexible modes make manual control of the entire system extremely difficult if not impossible.
An autopilot system that seeks to keep this aircraft system wings level and a constant pitch angle is
investigated here. The autopilot system is shown to work in an example simulation for a two body
aircraft connected at the wing tips. An experimental aircraft system is also designed, built and flown
with reasonable success proving the implementation of said controller on a real system.
Keywords:
controllability; multi-body; dynamics; reconfigurable; simulation; robotics; mobile;
modular; adaptation
1. Introduction
Fixed wing aircraft performance is governed by aerodynamic efficiency. One way of increasing
aerodynamic efficiency and thus range and endurance is to fly in the wake of another aircraft. Birds
have used this effect before the first analyses on this flight method were conducted [
1
]. The concept of
flying in the trailing vortex has been a source of research for quite sometime [
2
] with problems such as
time-delay effects [
3
] or self spacing issues [
4
]. Although numerous work has been done on formation
flight with benefits categorized, it is a potentially dangerous maneuver and quite costly in terms of
control to ensure station keeping in such a turbulent flow regime [5–7].
As such the topic of joined aircraft has been studied as opposed to formation flight [
8
].
The performance benefits are still similar to formation flight in that range and endurance are increased.
Joining aircraft at the wing tips reduces the effect of wing tip vortices and increases lift on each aircraft
thus increasing aerodynamic performance. The flagship project for connected aircraft occurred post
World War II codenamed Project TipTow [
9
]. This experiment involved three aircraft. The center
aircraft was a B-29 while two F-84s docked with both wingtips on the B-29. One of these flight tests
exhibited an unstable oscillation which resulted in a failure and a pilot death. Not much work outside
some windtunnel experiments and trade studies have been conducted since that project [
8
,
10
] however
the Distributed Flight Array (DFA) offers the first successful flight using vertical take and landing
aircraft [
11
,
12
]. The DFA is a set of ground rovers that contain one main rotor with no cyclic control.
Thus, each individual ground robot cannot fly independently. Once the individual robots link together,
the thrust from each rover can be controlled independently to ensure that the attitude and altitude of
the conjoined flight array is stabilized.
The task of connecting multiple singular independent bodies has been investigated for quite some
time. Resource management of multiple UAVs has been investigated for hostile territories showing
the increase in efficiency by using multiple aircraft [
13
,
14
]. Modular or morphing robots have created
interesting research problems including the development of reconfiguration algorithms [
15
,
16
] to
Aerospace 2018, 5, 113; doi:10.3390/aerospace5040113 www.mdpi.com/journal/aerospace
