Abstract. The objective of this project is to develop a Camouflaged Miniature Air Vehicle

(MAV) inspired by an eagle. MAV’s and UAV’s (Unmanned Aerial Vehicles) which are used for

military applications flying at low altitudes are often detected and destroyed since they take the

form of small aircraft. MAV’s and UAV’s camouflaged like birds are difficult to detect and look

natural in any territory. An MAV camouflaged like an eagle looks like a gliding native bird. In

this project, biologically inspired flight is used as a framework to improve MAV performance

since they operate in a similar flight regime to birds. Eagle is one of the most maneuverable and

aerodynamically efficient bird, found in most regions of the world, has high gliding ratio and can

reach high velocities. Aerodynamic advantages of wing with slotted winglets and gliding of eagle

have been discussed and have been adopted in the design. A fixed wing MAV has been modeled

similar to an eagle which inherits its aerodynamic properties and avoids detection by imitating in

appearance. A radio controlled prototype is built with Styrofoam, reinforced with glass fiber

using hand layup technique. The wing form, fuselage and tail have been modeled similar to the

eagle. The model was flown, exhibited good flight stability, gliding properties and has taken the

form of an eagle in appearance.

Keywords: Camouflage, Eagle, Slotted wing tips, gliding, MAV Nature inspired design

Introduction

Miniature Air Vehicles and Micro Air Vehicles which are used for reconnaissance, surveillance

and other military applications may need to loiter at significant altitudes to avoid detection, and

thus require complex sensors to observe the target far below. MAV’S and UAV’s flying at low

altitudes are often detected and destroyed since they take the form of small aircraft.

Miniature Air Vehicles camouflaged like birds are difficult to detect and look natural in any

territory. Eagle being one of the most maneuverable and aerodynamically efficient bird which can

soar for miles, It has high gliding ratio and can reach high velocities. Eagles are found in most

Camouflaged Miniature Air Vehicle - An Eagle Inspired

Design

N Sai Srinivas*, Shiva Kumar**, Ch Naveen Reddy*, K Shanmukha Prasad^

*PG Student at JNTUH College of Engineering Hyderabad (Autonomous) Kukatpally, Hyderabad, Andhra

Pradesh, India

**Research Scholar at JNTUH College of Engineering Hyderabad (Autonomous) Kukatpally, Hyderabad,

Andhra Pradesh, India

^B Tech Graduate from Symbiosis Institute of Technology and Science, Shameerpet RR dist Andhra Pradesh

E-mail: srinivas.think@gmail.com

regions of the world, an MAV which looks like an eagle flying above, would be natural and it can

fly at low altitudes to get better data and return un noticed .Since the MAV can fly at low

altitudes high resolution cameras and sensors need not be used as in case of UAV’s Flying at high

altitude.

A Camouflaged Air Vehicle mimics a bird in appearance; it closely imitates bird during flight. A

camouflaged MAV which are designed to appear like an eagle would be difficult to differentiate

from a native bird.

Aerodynamics of fixed wing MAV is simple and efficient than a flapping wing or a rotary wing

MAV. Flapping flight is a far more complicated process than gliding. During flapping flight, the

bird’s wings systematically change shape. Flapping involves up and down movement of the

wings. During the down stroke (or power stroke), the wings move downward and forward.

During the upstroke (or recovery stroke), the wings move upward and drawn in toward the body

to reduce drag. During flapping flight, the wings also change their angle of attack depending on

the stroke. Flapping flight is basically rowing in the air with the added complication that lift

needs to be generated as well.

A rotary wing MAV has an advantage of hovering but range and flight time are less compared to

fixed wing MAV.

About The Eagle Aerodynamics

Eagles being one of the most aerodynamically efficient birds, it reaches high velocities and stays

airborne for hours. Wingspan of a typical eagle Varies from 60 cm to 210.

Salient features of eagle:

a) High Lift to Drag Ratio Wings

b) Slotted Wing Tips with sweep

c) Variable Geometry wings

d) Pointed nose

Figure 1: An eagle with fully stretched wings.

High-speed bird wings, common to bird species like swifts, swallows, falcons, shorebirds etc. are

built for speed, but require a lot of work to keep the bird airborne. The long and cumbersome,

high-aspect ratio wings of these birds may not get them into the air quickly or easily, but these

wings are perfectly designed for soaring long distances with little effort. In contrast, the short

rounded elliptical game bird wings of a grouse, turkey, pheasant or quail can get them off the

ground in a heartbeat, but the energy that it takes to lift that heavy body off the ground doesn’t

last long. The slotted, high-lift wing of hawks, eagles, swans and geese provides the extra lift that

is needed to keep their large bodies airborne or to carry heavy prey.

Birds such as eagles and osprey, which soar and glide. When soaring, the wings are fixed and

rigid and act like those of conventional aircraft. For these fliers, flapping is restricted to limited

operations, such as take-off, landing, and stabilization.

Eagles have high lift wings with “pin” feathers at the ends that produce slotted wingtips.

Biologists found that the pin feathers worked to reduce drag during gliding flight, as well as being

used to provide roll control, in the same manner as ailerons on aircraft. Variable geometry wings

aircrafts has taken inspiration over eagles (such as the golden eagle that can reach the speed of

320 km/hr during a dive when they hold their wings close to their body in order to minimize the

air resistance.

Gliding and Soaring of Eagles

Birds usually flap their wings to generate both lift and thrust. But if they stop flapping and keep

their wings stretched out, their wings actively produce only lift, not thrust. Thrust can be

produced by gravity force while the animal is descending. When this happens, we call them

gliders. Eagles and other soaring birds appear to hang in the air effortlessly, gaining height with

barely a twitch of a wing. Eagles with high lift-to-drag ratio wings generate a lot of lift without

producing much drag. (Fig. 2)

Figure 2: An Eagle while gliding with its wings fully stretched .

To maintain level flight, a flying animal must produce both lift and thrust to balance the gravity

force in the vertical direction and drag in the horizontal direction respectively. Because gliding

occurs with no active thrust production, an animal always resorts to the gravity force to overcome

the drag. In gliding, the animal tilts its direction of motion slightly downward relative to the air

that it moves through. When the animal tilts downward, the resulting angle between the motion

direction and the air becomes the gliding angle. The gliding angle directly controls the lift-to-drag

ratio. The higher this ratio, the shallower the glide becomes. The lift-to-drag ratio increases with

the Reynolds number, a parameter proportional to animal size and flight speed. Large flying

animals fly at high Reynolds numbers and have a large lift-to-drag ratio. For example, a

wandering albatross, with a wing span of over 3 meters, has a reported lift-to-drag ratio of 19,

whereas the fruit fly, which has a span of 6 millimeters, has a lift-to-drag ratio of 1.88

Soaring flight is a special kind of glide, in which the bird flies into a rising air current. Because

the air is rising, the bird can maintain its height relative to the ground without the need of

flapping its wings. Instead of using gravity, Soaring uses energy in the atmosphere, such as rising

air current.

The Physics of Gliding Flight in Birds

A steadily gliding bird, i.e. non-powered flight with fixed wings at a constant flight speed,

converts potential energy to counteract the aerodynamic forces. It will glide at a certain horizontal

forward speed and sink speed. The sink speed (Us) will depend on the bird’s weight, wing

morphology and body shape (streamlining). Sink speed is calculated as:

Us U sin, (1)

where U

is the angle of the glide path in relation to the horizontal (Fig.3). To standardize airspeed

measurements, all speeds refer to equivalent airspeeds (UUtrue√/0), where Utrue is the true

01.225kgm–3 is the standard air density at sea level). At equilibrium gliding, the resultant of

lift and drag balances the weight of the bird (mass gravity; mg). The lift component of the total

force is directed perpendicular to the glide path. Note that lift in this case does not refer to the

force keeping the bird aloft, but the force generated perpendicular to the wing surface (true lift).

Lift is then given by:

L mg cos . (1)

Drag of the bird is directed backwards, parallel to the glide path and perpendicular to lift and is

calculated as:

D mg sin. (2)

Figure.3. Forces acting on a bird in equilibrium gliding. The glide path is inclined at an angle to the

horizontal and the resultant of the vectors lift (L) and drag (D) is equal and opposite to the weight of the

bird (mg). Total speed (U) involves the velocity component sink speed (Us), which is directed downwards

and perpendicular to the horizontal.

Drag calculated in this manner represents the total drag of the bird. Drag of a flying bird is

usually separated into three components: parasite drag, profile drag and induced drag. Parasite

drag consists of friction and form drag of the body, caused when propelling the birds’ body

through the air and is calculated as:

Dpar 1

2 Sb CD,par U2, (4)

Where is the air density, Sb is the body frontal area and CD,par is the parasite drag coefficient.

Profile drag is the drag generated when moving the wings through the air and is calculated as:

Dpro 1

2 Sw CD,pro U2, (5)

Where Sw is the wing area and CD,pro is the profile drag coefficient. The third component is

induced drag and is due to the downwash induced by the wings and tail of the bird when creating

lift. Induced drag is calculated as:

Dind 2kL2 πb2U2, (6)

Where b is the wingspan and k is the induced drag factor. k indicates how much the wing deviates

from an elliptical lift distribution. This factor is typically set to 1.1 (e.g. Pennycuick, 1975;

Pennycuick, 1989; Pennycuick, 2008; Rosén and Hedenström, 2001)

Profile drag is the component of aerodynamic forces that has proved most difficult to measure

(Pennycuick, 2008). Since the calculation of Dind is relatively well established and Dpar could be

estimated from the wake (see below), Dpro could be estimated by subtraction as:

Dpro D – Dind – Dpar. (7)

COEFFICIENTS OF LIFT AND DRAG: In order to make lift and drag of a particular bird

comparable to those of others, the forces may be converted into dimensionless coefficients. These

coefficients control for size of the bird, the flight speed and the air density. Lift coefficient is

calculated as:

CL 2L SwU2, (8)

and drag coefficient is calculated as:

CD 2D SwU2. (9)

Effect of multi / slotted winglets of an eagle

Eagles have high lift wings with “pin” feathers at the ends that produce slotted wingtips. It was

found that the pin feathers worked to reduce drag during gliding flight, as well as being used to

provide roll control, in the same manner as ailerons on aircraft the potential of multi-winglets for

the reduction of induced drag without increasing the span of aircraft wings. (Fig 4) shows an

eagle with slotted wingtips.

Figure 4: slotted wing tips of an eagle

A substantial increase in lift curve slope occurs with dihedral spread of winglets set at zero

incidence relative to the wing. Dihedral spread also distributes the tip vortex. Whitcomb showed

that winglets could increase an aircraft's range by as much as seven percent at cruise speeds.

The blended winglet of eagles reduces drag by eliminating the discontinuity between the wing tip

and the winglet.

Split wingtips of eagles are found to be highly effective. Vance Tucker, a biologist with an

aerodynamics background, demonstrated that the tip slots of soaring birds reduce induced drag

and increase the span factor of the wings. Researchers found that slotted tips of bird wings can

have a strong effect on the aerodynamic yawing moment due to sideslip and, thus, on yaw

stability. The sweep of slotted wing tips is of primary importance. It causes an aerodynamic

yawing moment of considerable magnitude, being much larger than the same wing without sweep

in the slotted tips would produce.

Yaw Stability -The ability to generate an appropriate yawing moment in case of a sideslip

disturbance is essential for the inherent stability with respect to this axis and, thus, for overall

flight stability. The required level of aerodynamic moments for yaw stability in birds is a subject

of recent research (Sachs, 2005a). While in aircraft yaw stability is primarily provided by the

vertical tail, there is no such device in birds so that they have to rely on other mechanisms. Thus,

the wing gains an increased importance as a means for producing a yawing moment large enough

for efficient stabilization. Therefore, any wing feature which augments the ability to generate

stabilizing yawing moments has a positive effect on flight stability

Design and fabrication

The main motive of the project is to make a MAV which looks like an eagle, inherits the

aerodynamic properties and mimics its flight as a fixed wing aircraft. By choosing eagle as base

airframe the MAV will have added advantage of longer flight time and better gliding

characteristics compared to a regular aircraft with the same wing span

Eagles flap their wings to generate both lift and thrust. But if they stop flapping and keep their

wings stretched out, their wings actively produce only lift like a fixed wing aircraft but not thrust.

For the fixed wing MAV a thrust source is required to maintain flight for long duration. An EDF

(Electric Ducted Fan) system is used for propelling the model since, No moving parts like

propeller should be visible outside, the EDF unit is placed inside the fuselage and necessary

intake and exhaust vents are provided.

The wing is the most critical part of the Camouflaged MAV. Apart from generating lift, the wing

is responsible for the yaw stability. As discussed earlier, eagle wing has very complicated profile

with multiple winglets at the end. It is the largest part of the MAV hence it should resemble an

eagle in every aspect. The wingspan of the model is taken as 110 cm which is the average

wingspan of an eagle found in Asia. Remaining components are scaled appropriately. Wing is

fabricated using high density foam. The foam is reinforced with glass fiber cloth and carbon fiber

tube is added along the spar to give added strength to the wing. The slotted wing tips/winglets are

made of aluminum and are attached to the main wing.

The fuselage with a head and beak in the front is made of foam, balsa wood and glass fiber

the control surfaces design are scaled from an eagle. Fuselage is made hollow since it has to

accommodate the propulsion system, battery, control system and payload. Fuselage is constructed

using Styrofoam, balsa wood and fiberglass reinforcement is used in stressed areas on the

fuselage.

Horizontal stabilizer or tail of the aircraft is hinged at the exhaust of the EDF. Vertical stabilizer

is not employed since the model attains yaw stability from slotted wing tips.

The Legs and nose made out of balsa wood. A Li-Po battery (Lithium polymer) with 11.1 volts is

used as power source. Special patterns are made on the surface for the plane, model is painted

with brown and black paint making eagle like textures.

Commercially available Ratio Transmitter and receiver are used for vision based control of the

model which has a range of 1000m. Micro servos were used to activate the control surfaces i.e.

ailerons and elevator.

Figure 5: Styrofoam Model of the MAV

Flight Test

We have successfully flown the Camouflaged MAV in multiple flight experiments,

demonstrating stable and controlled flight. First flight lasted for around 11 min. The second flight

test lasted much longer with 14 minutes flight time. The MAV had a stable flight and exhibited

good yaw stability in spite of the rudderless tail. The model showed good gliding properties.

Figure 5 shows the images of the MAV during flight and (Fig.6) shows image of an eagle. The

two flyers i.e. the eagle and the Camouflaged MAV were identical.

Figure 6: Camouflaged MAV during flight tests captured at different altitudes.

Figure 7: Eagle captured from different altitudes

A comparison can be made between eagle and MAV from (Fig.5) and (Fig.6) it can be concluded

that the MAV has taken the form of an eagle in flight. Both eagle and the MAV look identical

during flight

Conclusion and Future Work

A Camouflaged MAV mimicked from an eagle was successfully developed and flown; test flight

results showed that a MAV camouflaged like a bird can deceive visually. Slotted wing tip and

rudderless tail have been adopted successfully in the model. The method of camouflaging

MAV’S inspired by birds has shown positive results and can be adopted by MAV’S and UAV’S

for secret operations.

An Autonomous control system can be used for the guidance and control of Camouflaged MAV

airframe. Nature inspired designs like eagle; hummingbird, swift albatross etc can be

implemented in conventional aircrafts and MAV’s. In future models flapping wing and variable

wing geometry can be adopted which comes under different regime of aerodynamics and requires

extensive study and research.

References

[1] Aerodynamics Of Gliding Flight In A Falcon And Other Birds By Vance A. Tucker

And G. Christian Parrott ,Department of Zoology, Duke University, Durham, North Carolina

[2] An Aerodynamic Analysis of Bird Wings as Fixed Aerofoils by Philip C. Withers

Department of Biology, Portland State University, P.O. Box 751, Portland, Oregon 97207

[3] Effect of slotted wing tips on yawing moment characteristics

Gottfried Sachs, Mochammad Agoes Moelyadib

[4] Performance Analysis Of A Wing With Multiple Winglets M. J. Smith, N. Komerath,

R. Ames, O. Wong, School of Aerospace Engineering, Georgia Institute of Technology, Atlanta,

Georgia And J. Pearson, Star Technology and Research, Inc., Mount Pleasant, South Carolina

[5] An Aerodynamic Analysis of Bird Wings as Fixed Airfoils by Philip C. Withers

Department of Biology, Portland State University, P.O. Box 751, Portland, Oregon 97207

[6] Aerodynamics of Gliding Flight in Common Swifts P. Henningsson* and A.

Hedenström

Department of Theoretical Ecology, Lund University, SE-223 62 Lund, Sweden

[7] Nature Inspired Design Application over aircraft design & geometry

Authors: Vincent Audoire, Sahire Dogru, Chi-Ju Chiu M. Young, The Technical Writer’s

Handbook. Mill Valley, CA: University Science, 1989.

[8] Chapter 2 Aerodynamics of Flapping Flight by Joel Guerrero

[9] Gliding Birds: Reduction of Induced Drag by Wing Tip Slots Between the Primary

Feathers

Vance A. Tucker Department of Zoology, Duke University, Durham, NC 27706, USA