Preliminary+design - The University of Sydney

Preliminary design
The focus of preliminary design is to determine approximate dimensions,
weight, power, load limit and other physical characteristics for an optimum
aircraft, which best maximise scores for competition. The main areas of
concern were wing area, span and empennage size. The entire routine is
modelled based on the conceptual design, with carefully considering the
specific requirement of the competition.
1. Critical design parameters and sizing trades
Based on results obtained in the conceptual design phase, the critical design
parameters are revised to reflect on the performance parameters considered
most influential to the overall score. These include main wing dimension,
power plant, ménage dimensions and load factor limit.
 Wing area
Since the weight of the structure is approximated in the conceptual design
stage, and the payload weight is specific by the competition organisation, the
total weight at level flight can be found. Since no vertical acceleration applies
during level flight, and fuel consumed can be neglected in comparison with
the total weight, thus,
L  W  4.5  9.8
Equation 1
From the definition of lift produced in flight:
L  1 / 2V 2 SC l 
Equation 2
From the studies of existing RC planes, it is sensible to assume the plane flies
at 200m, =1.22kg/m^3, V=25m/s, and the aircraft flies at an angle of attack
at 5 degrees.
The lift coefficient of the whole aircraft can be found from the wing tunnel test.
In order to find out the minimum area required, it is assumed that CL is at
maximum. The MAX (Cl) equals to the Cl of the Clark Y aerofoil. From the
NACA full-scale wind tunnel test result, we found the lift coefficient at the
same flow condition is approximately equals to ---------. Due to the box-wing
configuration from the wing tunnel testing conducted by Mississippi state
university, the Cl can be doubled if the distance of separation is greater than
the chord length. Therefore, the minimum plan form area required is about
1.01 m^2. Due to vortices and flow separation the maximum Cl would
decrease and it would result in increasing the plan form area. Assuming the
efficiency of the box-wing to be 0.9, the minimum plan form area required is
approx. 0.22m^2.
Figure 1
The lift curve slope depicted in Fig. 5 shows that the nonplanar configurations have
much greater slope than monoplane allowing the nonplanar wings to produce the
same amount of lift at much lower angles of attack
 Wingspan
The wingspan can be found from the aspect ratio of the plane since the
minimum plan form area is known. Using results from regression method,
existing bi-planes have a typical aspect ratio of approx. 6.8. Since the
efficiency factor of box-wings is approx. 5% hight than that of bi-planes, a
reasonable aspect ratio of 6.5 can be used. Hence the span of the main wing
equals to 17.1 cm.