13-09-2012, 05:35 PM
DESIGN AND DEVELOPMENT OF A MICRO AIR VEHICLE (μAV) CONCEPT: PROJECT BIDULE
Paper_mAV_Bidule.pdf (Size: 396.19 KB / Downloads: 134)
Abstract
This paper presents an analysis of the concept of Miniature Air Vehicles and Micro Air Vehicles
(respectively mAV and μAV) mainly carried out through the development of a mAV prototype
called "Bidule", which was first successfully flown in 1998. The final objective of the project is to
reduce the size of this miniature demonstrator to a micro sized vehicle. This paper presents the
initial work done to achieve this goal with a particular interest in the aerodynamics of the present
vehicle. In keeping with the context of a vehicle’s size reduction, a preliminary quantitative
approach of the concept shows that the tiny size of mAVs and μAVs creates a strong coupling
between the different design fields and requires a high degree of integration. Therefore, reducing
the size of an aerial vehicle has major consequences on the performance, especially in terms of
endurance. Conserving the operational capabilities of the "Bidule" then requires considering the
interactions between the different design parameters, with a particular attention being paid to the
wing loading and a high maximum lift. The wind tunnel testing of a propelled model of the "Bidule"
prototype shows that the basic design benefits from the prop-wash effect in terms of increased lift.
In the context of size reduction to the present vehicle, the results suggest keeping the idea of a wing
body immersed in a propeller slipstream, providing that the destabilising effects due to the power
system and the prop-wash can be kept to an acceptable level.
Introduction
The concept of micro-sized Unmanned Aerial Vehicles (UAVs) or micro Air Vehicles (μAVs) has
gained increasing interest over the past few years, with the principal aim of carrying out surveillance
missions. The primary payload of these tiny aircraft (~15 centimetres or 6 inches wingspan) is
usually a miniature image sensor. Operating in an approximate radius of 600 metres from the
launch point, μAVs are used to acquire real-time visual information for a wide range of
applications. According to DARPA (Defense Advanced Research project Agency) in reference 1,
μAVs are “six-degree-of-freedom aerial robots, whose mobility can deploy a useful micro payload
to a remote or otherwise hazardous location where it may perform any of a variety of missions,
including reconnaissance and surveillance, targeting, tagging and bio-chemical sensing.”
Aerodynamics
The equation of the power suggests that increasing the lift-to-drag ratio would be done preferably by
increasing the lift, rather than focusing on a drastic drag reduction. The small size and low speed of
μAVs also result in an unusually low Reynolds numbers. As the main constraint with the flight
platform size mainly restricts the wingspan, high chord length might be achieved in the design
process to increase the wing area. As a result, μAV configurations often have a low aspect ratio
involving fully tridimensional aerodynamics. Moreover, at these low Reynolds numbers, the
propeller efficiency is highly degraded.
Structure
In order to reduce the wing loading, the most critical parameter to work on is the weight of the
airframe for a given wing area. In addition, the vehicle’s high surface area-to-volume ratio limits
the available volume for the payload. This is especially true for flying-wings, as the proportion of
the empty room for the payload becomes a relatively flat volume and may be divided by structure
spars and ribs. A high degree of integration is necessary for μAVs, as size and functional
complexity have contradicting constraints.
Propulsion
The difference of power and energy density between electrical and thermal power sources is known
to have a great influence on a vehicle’s endurance. The difference of power density (usually
expressed in W/kg) between an electric battery and combustible fuel is one of the major differences
that strongly influence the vehicle endurance. Internal combustion engine powered vehicles also
benefit from the weight reduction due to fuel usage. The present "Bidule" is a twin engine
configuration. From an endurance point of view, each motor of a twin engine configuration requires
less power than a single engine solution to maintain flight conditions. However, it is not yet clear
whether the addition of important losses due to the low terminal efficiencies (also due to the low
Reynolds number) would be lesser for a twin engine configuration than that for a single engine, with
regard to energy penalty.
Stability
Attention is to be paid to the longitudinal stability margin and the lateral manoeuvrability. The
stability margin needs to be rather high for two reasons. Firstly, the drone must be able to fly
smoothly to achieve exploitable observations; secondly, the vehicle is expected to fly at low altitude
in a turbulent atmosphere, and thus should be stable enough to be as unaffected as possible by gust
perturbations. The lateral manoeuvrability should allow tight turns.
Synthesis
The aerodynamics is strongly affected by the Reynolds number drop, which increases the drag and
produces fully tridimensional phenomena. This low Reynolds number also influences the
propulsion system global efficiency. When dealing with endurance, the most critical parameters are
apparently to minimise the weight in order to decrease the wing loading, and to maximise the lift in
order to increase the L/D ratio. The power density and energy density also seem to have a great
importance, but investigations into this still need to be done. Attention has also to be paid to the
number of engines, especially to account for the effect of mechanical losses.