STABILIZATION OF HELICOPTER SLING LOADS
The focus of my proposed dissertation relates to a challenging application of nonlinear
control theory: nonlinear control of under-actuated systems. This statement contains
the objectives of the research to be undertaken in preparation of the dissertation
the relation of the issues raised by this application to the engineering and science
literature on the nonlinear control of under-actuated systems, anti-sway control, and
active vibration suspension, and
the rationale for the proposed methodology to be carried out in the dissertation research.
Helicopters have vertical take-off and hovering capabilities. They can reach places where
ground vehicles and other types of aerial vehicles can not. Therefore they are highly valuable in
aerial transportation and as aerial cranes. Both of these modes of helicopters require carrying an
external load suspended beneath the helicopter, called sling loading. However, use of these two
modes of helicopters are limited due to the danger and difficulty of carrying a load that can
freely swing beneath the helicopter. Even though helicopter automatic flight control systems
(AFCS) are used widely on-board helicopters, , there aren’t any AFCSs that can stabilize
slung loads to the best of the author’s knowledge. Hence the motivation of this thesis is to
stabilize helicopter sling loads.
This chapter of the thesis is dedicated to
a brief overview of helicopter sling load transport and issues,
importance of the stabilization of the helicopter sling loads,
statement of the sling load stabilization problem,
literature survey of relevant stabilization problems.
1.1 Helicopter Sling Load Transport and Issues
Helicopters are being employed in rescue operations, emergency medical services,
disaster relief, fire fighting, detection and assessment, construction, power line, heavy lifting,
forestry, and petroleum support operations. In all of these operations and many other similar
applications, an external load is lifted, transferred, and landed/positioned by helicopters (Figures
1-8). Helicopters offer the following advantages due to their vertical take-off, land, and hovering
capabilities: heavy, oversized or bulky cargo can be handled, areas that are inaccessible by any
other mode of transportation can be reached, and transfer time is faster than any other mode of
transportation. They are used as aerial ambulances, aerial search and rescue units, aerial fire
engines, and aerial cranes. They allow human beings to accomplish tasks on air. Therefore,
helicopters are very critical in saving lives, and habitat, as well as saving many industries time
and money by providing a way of aerial reach. This is especially true since we have skyscrapers
all around and we don’t have a good means of rescuing people when a disaster such as a terrorist
attack (September 11) or a fire on a high-rise building hits us. However, the helicopter sling
loading is not fully utilized due to the free rotational motions of the sling load.
Figure 1.1 (a) A Helicopter, filling its bucket to extinguish a fire nearby (left); (b) Helicopters are used in high
rise rescue operation (right.)
Figure 1. 2 (a) A helicopter, transporting cargo to a ship (right,) (b) Fire fighting helicopter (left.)
Figure 1. 3 Helicopters in construction.
Figure 1. 4 The picture depicts a helicopter that is being used in offshore cargo transportation or
Helicopter sling loading is a mode of helicopter transportation, where one or more loads
are suspended beneath the helicopter from one or more provisions on the helicopter using one or
more lines of cables (Figure 1. 5, and Figure 1. 6b.) This is the common way of carrying external
loads with helicopters. Sikorsky’s CH-54A S64 Tarhe, Figure 1. 6a, is specifically designed to
carry heavy loads and is an exception to that.
Figure 1. 5 Helicopter sling loading with more than one cable/provision.
Figure 1. 6 (a) Sikorsky’s CH-54A S64 Tarhe crrying a military vehicle, (b) Helicopter carrying a Heli-Basket
through a cable (single line sling loading).
The single line configuration causes the suspended load to have free rotations (3
rotational degrees of freedom, one being the twist of the cable) with respect to the helicopter. As
can be seen from Figure 1. 6, the cable that carries the load may have a large angular deviation
from the vertical. This motion of the suspended load arises when the helicopter accelerates,
decelerates or maneuvers. Because the suspended load oscillates freely (lightly damped,) the
pilot has to stabilize the suspended load most of the time, and compensate for the effects of the
oscillations on the helicopter. The pilot, whose job is already challenging and vitally critical,
now has to fly the helicopter in a way that would not disturb the suspended load oscillations. One
element that helps the pilot keep his craft and the payload stable is the length of the cable. The
cable length is usually kept fairly long (20-200 feet) to decrease the natural frequency of the
suspended load system. This decreases the effectiveness of the operation, whatever it may be,
and furthermore can be very dangerous. For example, the following excerpt from Arthur
Negrette’s article issued on the February 1st of 1999’s Aviation Today is alarming: “…During
the flight to reposition the helicopter, the 206 reached an altitude of 50 to 60 feet AGL when the
aluminum mesh basket caught on a nearby fence…” Hence there are other tasks that are added to
pilot’s list of survival when the helicopter is carrying a suspended load such as are clearing the
obstacles that are on the load’s path, The pilot needs to watch for obstacles and maneuver the
helicopter so that the load can clear the obstacles. He is supposed to perform that action while the
load is far down beneath the helicopter. Generally, it is advantegous to carry the suspended load
as close to the helicopter as possible. That way, the pilot is not looking hundreds of feet down to
identify any obstacles there might be.
Other tasks that the pilot may be required to perform is to position the load on a target
location by estimating the relative position of the load with respect to the location. Moreover,
there are other people that are affected by the way the load is carried by a helicopter. During the
lifting of the external load, a ground crew hooks the load and guides or stabilizes it, mostly by
means of ropes or sometimes even with their hands. During the landing or positioning of the
external load, the crew maneuvers the load above on the target location. These tasks are
dangerous and require clear communication between the pilot and the ground crew. Once the
stabilization of the suspended load is accomplished, precise positioning of the load on a target
location is inevitable. In conclusion, there are many issues related to the stability of the
helicopter sling loading that makes the very critical and important uses of helicopters, such as
rescue from high rise buildings, or aerial crane type operations cumbersome and even dangerous.
Yet another issue that carrying a suspended load with a helicopter raises is the
certification of the helicopter load system. When a load is suspended beneath the helicopter, the
helicopter and suspended load system has to go through extensive flight tests before the
helicopter is allowed to fly . The flight envelope of the helicopter usually gets smaller due to
the added lightly damped suspended load system. There has been some effort to reduce the time
and cost of flight tests that are carried to certify the helicopter load system . Cicolani et. al.
analyzes the flight test to identify the key dynamic parameters such as aircraft stability margins,
handling-qualities parameters and load pendulum stability. They also compare the helicopter
suspended load simulation results with the flight test data to identify the weaknesses of the
model. The goal of this work was to both revise the cumbersome flight tests and use validated
simulation models. Cicolani et al.  emphasizes the reduced gain margin near the pendulum
frequency (at about 1.5 rad/s) in the lateral axis (roll axis) for the helicopter and the suspended
load system at hover. This results a loss of helicopter response at that frequency. They also
mention the significant reduction of the attitude bandwidth, maneuver response, of the system
with the addition of a suspended load. Being a lightly damped system, a suspended load can
cause a helicopter to be unstable or to lose altitude if the flight test envelope of the helicopter is
not revised. Therefore increasing the damping of the suspended load system, or in other words,
stabilizing the motion of the suspended load would increase the stability and the flight envelope
of the helicopter.
Considering the need to carry suspended loads with helicopters, in a way that is more
secure and efficient than the current means, and also considering the instability that a suspended
load may add to the helicopter, we asked the following questions to ourselves:
How can we stabilize a helicopter sling load?
Should there be a stabilizing control mechanism that works independently from the control
of the helicopter?
What is the methodology to stabilizing the helicopter sling loads?
How would the solution of this problem advance nonlinear control and control of under-
actuated systems fields?
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The disturbance is in the form of acceleration or it may be taken as 2nd order
nonholonomic constraint then go to the non-holonomic system control literature. Check
the vibration control of acceleration input systems, like earthquakes…
Might want to try shaping the control input, notch filter to see the effect of it.