General Flight Dynamics Modelling

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					                  Rotorcraft Simulation Challenges

General Flight Dynamics Modelling & Simulation
The flight dynamics of a helicopter are governed by many complex phenomena, a
number of which are still quite poorly understood. Conventional high-fidelity
helicopter models, developed for pilot-in-the-loop simulation applications, predict on-
axis responses reasonably well, but the prediction of off-axis responses or ‘cross-
coupling’ is less than satisfactory. In general, further development of motion
platforms and vibration tables/seats are required to provide pilots with improved
vestibular feedback. The reproduction of realistic audio cues are particularly
challenging for helicopter simulations. Helicopter visual systems require a wide field-
of-view of very high resolution graphics, with photo-realistic textures for hovering
flight close to the ground and other obstacles. In a helicopter hovering close to the
surface, small details such as waves on water, or the branches of a tree moving in the
wind or downwash are easily observed. These small details may provide subtle cues
to the pilot and add to the overall level of realism of the simulation. It is for these
reasons that high-fidelity simulation of rotary-wing aircraft is arguably much more
difficult to achieve than for their fixed-wing counterparts.

Helicopter Aerodynamics
The helicopter is characterised by an abundance of aerodynamic interactions which
have a powerful effect, usually adverse, on the flight dynamics of the vehicle. A
principal source of these interactions is the main rotor wake interacting with the
airframe and the tail rotor disc. The main rotor wake also interacts with the ground
and itself, in the so-called vortex ring condition. Capturing these effects in a high-
fidelity model is as important as it is difficult. Some aircraft exhibit a loss of tail rotor
effectiveness when the main rotor wake becomes entrained in the tail rotor.
Introducing accurate models of the onset and development of the vortex ring state
would benefit pilots by teaching them to recognise when they have entered or are
about to enter this potentially lethal flight regime and to allow escape strategies to be
developed and practised. Brown/white-out conditions occur when the main rotor
wake interacts with lose particles on the ground (sand or snow) creating a cloud
around the vehicle, reducing visibility to zero close to the ground. At present it is not
possible to practice operations in these conditions using a simulator.

Helicopter Operating with Slung Loads
This is the practice of using helicopters to carry loads externally and is employed in
both civil and military applications. When attached an external load effectively acts
like a pendulum changing the stability of the helicopter/load combination. Modelling
challenges associated with simulating slung load operations for both training and
control law design applications include: modelling the aerodynamics of the load and
the dynamics of sling/load. The dynamics of the load and its effect on the helicopter
are of particular concern during manoeuvring flight. The combination of the
helicopter and load can become very unstable during certain wind/manoeuvre
conditions compelling the pilot to jettison the load.
Tilt-Rotor Aircraft and Unconventional/Hybrid Configurations
The last few years have seen a significant amount of progress in the development of
the tilt-rotor aircraft concept for both military and civil applications. Much
experience has been gained through the introduction into military service of the
Osprey V-22. Civil tilt-rotor aircraft appear an attractive prospect to alleviate
congestion at busy airport hubs as short/medium range transport solutions. Civil
search and rescue is another application where tilt-rotor aircraft would provide
benefits through faster transit times to and from search areas, with longer periods on
station. There are several challenges associated with tilt-rotor modelling and
simulation including: correctly modelling the behaviour of the aircraft during
conversion from helicopter mode to aeroplane mode and modelling the strong
aerodynamic interaction between the rotor wake, airframe and ground surface during
operations close to the ground or deck of a ship. The modelling and simulation of
autogyros, commonly used for sport or recreation purposes, is still and area where
little work has been done, particularly considering their poor safety record.

Ship/Helicopter Dynamic Interface Simulation
This covers both military operations from the decks of naval ships and civil
operations from stationary and floating off-shore oil installations. Simulators can be
used to help define the optimum deck layout, markings and lighting requirements for
new or existing ships or oil rigs. The clearance of a military helicopter to operate
from a particular ship is a lengthy, difficult and expensive procedure, culminating in a
Ship-Helicopter Operating Limits (SHOL) envelope, defining the peak ship motion
and wind strength/direction conditions under which it is safe for a pilot to attempt a
landing. If these clearances could be carried out within the safe and repeatable
confines of a flight simulator the potential savings would be significant. In order to
achieve this goal, accurate models of the ship/helicopter dynamic interface are
required, including a model of the ship’s motion on the sea and the turbulent airflow
around the ship’s superstructure, commonly known as the airwake. Current state-of-
the-art dynamic interface simulations use time-accurate airwake models derived from
off-line computational fluid dynamics simulations of the ship. These simulations are
uncoupled since the presence of the helicopter and its rotor downwash has no effect
on the ship’s airwake. Research is ongoing to develop a method of dynamically
computing real-time fully coupled ship airwakes. Realistic visual representation of
the sea surface and water wake, particularly at the point where the sea meets the
ship’s hull is still challenging. In nature the surface of the sea looks very different
under various conditions and this is easily observed from a helicopter in a hover
alongside the deck of a ship. Sea spray is also picked up by the helicopter’s
downwash, re-circulated and deposited on the windscreen reducing the pilot’s
visibility. A further application where modelling the effects of rotor downwash on the
sea surface is important, is when the helicopter is dipping a sonar buoy, from a low
hover over the sea. Other areas that are neglected in most current simulations include
the effects of the ship’s funnel efflux or exhaust, particularly important for oil rig
operations where exhausted flumes can have a dramatic impact on helicopter
performance, and the inclusion of a dynamic flight deck officer providing guidance
signals to the pilot. In many cases a pilot will only encounter dynamic interface
phenomena, such as airwake, when they perform their first ever landing at sea.
Current training simulators are poor environments in which to prepare pilots for the
experience. Several luxury yachts and other vessels are now being fitted with, less
than adequate, helipads and pilots with little or no maritime experience are operating
from them without specific training.

Unmanned Autonomous Vehicles (UAVs)
Relatively few unmanned rotorcraft concepts have matured into operational products,
compared to fixed-wing UAVs. However, simulation still plays an important role in
the design and development of rotary-wing UAVs, and in the training of the operator
or crew members. One important advantage of compact vertical take-off UAV’s,
such as the Fire Scout, is that they are ideal platforms for operating in a maritime role.
The same advances in simulation technology that would benefit ship/helicopter
dynamic interface applications (e.g. ship motion and airwake modelling) would also
benefit the development and clearance of UAVs designed to operate from ships. The
robustness of mini and micro-UAV concepts operating in urban areas might also be
enhanced, if simulations modelled the effects of gusts around building in the urban

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