Tilt Rotor UAV
Agarwal A.
Faculty of Mechanical Engineering, National University of Singapore
ABSTRACT
In this paper, I shall first describe the aim of the project and the reasons why the particular
design of the helicopter was chosen, along with a brief description on the working of this class of
helicopters. Then, I shall present a brief history of this class of helicopters, from the first built in
1929 to the present day versions. This is followed by a description of the prototype built, and an
analysis of whether the two concepts of ground effect and overlapping rotors have a major effect
on the performance of the helicopter. Finally, I look into possible improvements.
INTRODUCTION
The aim of this project was to make a prototype of an unmanned aerial vehicle to be used for
surveillance purposes. Since this would require carrying a heavy load, in the form of surveillance
equipment, the tandem rotor design was chosen.
Tandem rotor helicopters have two horizontal rotors instead of one horizontal and one vertical
rotor, as in the conventional helicopter. In a conventional helicopter, the angular momentum due
to the large horizontal rotor is neutralized by the horizontal force generated by the vertical tail
rotor. In the case of a tandem rotor helicopter, the two horizontal blades rotate in opposite
directions and hence cancel out the angular momentum due to each other, while both generating
lift. This can be seen in the figure below. Due to the shape of the counter rotating blades, though
they rotate in opposite directions, both generate lift. Therefore, in tandem rotor helicopters, as
both the rotors are used to generate lift, they are ideal for lifting heavy loads.
HISTORY
The first successful tandem rotor
helicopter was built in Belgium by
Russian born engineer Nicolas Florine,
in 1929-1930. This was before the first
helicopter with the configuration most
popular now, a single horizontal rotor
and a vertical tail rotor, was built. The VS-300 was the first such helicopter, built in 1940. It had
a main horizontal rotor and three auxiliary tail rotors. Two of the three were horizontal, and one
was vertical. In the later VS-300A, only the vertical rotor was retained out of the three auxiliary
rotors, making it similar to the standard configuration on most helicopters today. However,
Florine’s machines were destroyed during the Second World War.
In 1945, the Piasecki Helicopter Corporation built
a tandem rotor helicopter called the P3V – Dogship.
This was three times larger than any other helicopter
flying at that time. It was first built to satisfy Coast
Guard requirements. This aircraft was called ‘Flying
Banana’ because of its shape, but was very successful.
This led to further designs from the same company
like the XHJP-1, in 1948, which was the first overlapped tandem rotor helicopter and the H-21,
in 1952, which was used extensively by the U.S. army.
The only other company to manufacture tandem rotor helicopters in the United States was
Bell Helicopter, which manufactured the XSL-1 during the 1950s. In Britain, Bristol Helicopters
built the Type 173 in the 1940s and Type 192 in 1958. In 1952, the Kamov Company built the
Yak-24 tandem rotor helicopter, which was not very
successful and discontinued due to technical problems.
In 1956, Piasecki’s corporation became The Vertol
Company, and finally Boeing Helicopters. Further designs
from the company were the Vertol 107, a civilian helicopter,
and the CH-46 and CH-47, both successful military
helicopters. These started flying in the early 1960s, and are
still in use. The photo shows the CH-47 Chinook.
THE PROTOTYPE
The following is a table of the components of the helicopter, along with the weight distribution.
Component
Multiplex Permax 400 6V Motor
Multiplex 400L Gearbox
Master Airscrew (Electric Only Series) 8.5 x 6
in Rotor
Aeronaut Carbon Elektro 8.5 x 6 in Rotor
Futuba RX – R116FB
Balsa Wood Frame and Perspex Supports
Supporting Aluminium Rods
Total
Electronic Speed Controller (50 amp)
Futuba TX – T6XA
9V Battery Pack
Weight (g)
70g x 2 = 140g
10g x 2 = 20g
40g
40g
20g
20g
10g x 8 = 80g
360g
Overboard
Overboard
Overboard
Gearbox
Rotor
Balsa Wood
Perspex Support
Motor
Aluminium Rod
Signal Receiver
The rotors chosen were each 8.5 inches across and the motors were Multiplex Permax 400
motors. The gearbox was purchased along with the motor to fix onto the body of the helicopter.
Four aluminum rods were used to support each motor along with Perspex supports. At the top of
the helicopter, where the rods met the wooden frame, Perspex bits were stuck to the frame to
prevent it from being ruptured due to the vibration of the rotors.
A larger piece of Perspex was used in the middle for each motor, holding the motor in place,
as it was glued to the aluminum rods. At the bottom of the helicopter, a piece of Perspex was
stuck to the underside of the wooden base to hold the aluminum rods and prevent rupture of the
wood due to vibrations, as well as to provide support for the motor. As the rear rotor had to be
raised to allow an overlap of the two rotors, the motor at the rear was placed on the signal
receiver for the remote control system.
Tests were conducted for the motors, using a spring balance and each motor was found to
generate a lift of about 2N. This may not have been a correct representation of the lift that the
motors would generate once fitted on the helicopter due to two main factors.
Firstly, the rotors of the helicopter were arranged such that they partially overlap each other.
However, according to Leishman (2006), the induced power of partly overlapping tandem rotors
is found to be higher than that of two isolated rotors, because one of the rotors must operate in
the slipstream of the other rotor, resulting in higher induced power for the same thrust.
This is a negative effect and hence should be minimized. However, if the rotors are separated
to a larger distance, it would require making the helicopter body longer, which would decrease
performance in terms of the added weight of the material and any extra support required due to
the increased dimensions. For tandem rotor helicopters like CH-46 and CH-47, the d/D ratio is
approximately 0.65, giving κov of about 1.13. In the case of the prototype,
Secondly, as the lift was measured by hanging the
motors upside down, the ground effect was not taken into
consideration. According to Leishman (2006), because the
ground must be a streamline to the flow, the rotor
slipstream tends to rapidly expand as it approaches the
surface.
From the figure, it can be seen that due to the friction
between the surface and the air, there is a decrease in the
slipstream velocity, and so the air forms a cushion
underneath the hovering helicopter. Therefore, as a result of this effect, it is found that the rotor
thrust increases for a given rotor power.
Leishmenn (2006) also provides a graph to measure the ground effect. For the prototype built,
the readings from the graph are as shown.
Rotor
Rotor Radius (R)
Height from
Ground (z)
Front Rotor
Rear Rotor
10.75 cm
10.75 cm
7 cm
9 cm
Thrust Ratio, TIGE/TOGE
Cheeseman &
Hayden
Bennett
1.17
1.35
1.09
1.21
Therefore, taking the over lapping effect and the ground effect according to Cheeseman &
Bennett into account, the thrust generated is,
The result of these effects is thus a difference in lift generated by the two rotors.
IMPROVEMENTS
Along with building the prototype, I simultaneously worked on the Parallax Boe-Bot kit and
completed the tasks laid out in it, learning more about microcontrollers and the various devices
that can be controlled using a microcontroller. The devices used were servo motors, LEDs, an
infra-red receiver, a piezo-speaker, a SONAR device and whisker wires. Programs were written
in BASIC and the devices were controlled using a microprocessor. A transceiver set was also
used to send signals from one microcontroller to another, hence enabling wireless
communication.
Currently, the prototype is only capable of vertical takeoff and landing. The most important
improvement for the helicopter would be to enable it to be maneuvered. Yaw motion can be
controlled by making the two propellers rotate at different angular velocities. A difference in
angular velocities would mean a difference in angular momentum, making the body rotate about
a vertical axis passing through its centre of mass. This would require controlling the two motors
separately and hence the use of two speed controllers. Another advantage of controlling the two
motors separately would be that the difference in lifts calculated above can be negated by
varying the rotation speeds.
Another improvement to the design would be to incorporate the BASIC Stamp unit into the
helicopter body. This will allow sensors in the form of infra-red or SONAR devices to be
mounted on the helicopter, which will help to automatically avoid collisions. These devices can
also be used for purposes like measuring the height of the helicopter above the ground. Also, if a
transceiver is fixed to the Stamp, another Stamp can be used for wireless communication
between the two using a laptop, instead of a remote controller and an electronic speed controller
as used now. However, these improvements would be at the cost of adding weight to the
helicopter, and hence might require different motors.
Bibliography
Cantrell, P. (n.d.). Ground Ecffect. Retrieved August 2008, from Helicopter Aviation:
http://www.cybercom.net/~copters/aero/ground_effect.html
Leishman, J. G. (2000). A History of Helicopter Flight. Retrieved August 2008, from
http://terpconnect.umd.edu/~leishman/Aero/history.html
Leishman, J. G. (2006). Principles of Helicopter Aerodynamics. New York: Cambridge University Press.