Preliminary Design:

After completion of conceptual design phase, preliminary design phase began to determine the sizes of different aircraft components according to different requirements. An iterative process was utilized to reach final preliminary design. To design the airplane, the prime considerations were the ability of airplane to develop enough lift, being able to perform moves under mission statement (Horizontal loop etc.) and being able to accommodate bottles of 500 ml standard size.

4.1 Design and Analysis Methodology

4.1.1 Wing

The airfoil that is chosen is MH114, for its ability to produce greater lift and enough drag.

MH-114’s camber also helps to produce negative moment needed for aircraft’s stability.

Wing area is chosen to provide enough power and wing loading to meet mission targets.

Flaperons are used that aids in controlling bank of airplane as well as in increasing lift.

4.1.2 Fuselage:

The fuselage is made as streamline as possible to reduce the effects of drag.

Several holes are introduced in the structure of the fuselage to reduce weight while not compromising too much on structural integrity.

Enough hollow space provided in the fuselage to carry 500 ml standard bottle as well as the speed controller, motor and battery.

4.1.3 Propulsion:

Three major components of a propulsion system are batteries, motor and propeller.

Batteries are selected to provide enough power for takeoff and perform each move within the designated time. To reduce weight and get good performance minimizing number of high performance batteries is important.

Propeller is selected to reduce take off distance and maximize cruise speed.

Motor is selected to ensure that power loading of aircraft is matched and in order to meet performance requirements.

4.1.4 Tail:

Conventional tail configuration is chosen since in this type elevator is directly attached to fuselage and therefore provides decent yaw and pitch control characteristics.

Rudder is fitted to vertical tail to help control yaw of airplane about the vertical axis. Elevators are also fitted to control pitch of airplane.

4.2 Design and Sizing Trades:

Airplane was designed in such a way that maximum performance was attained with least possible weight of aircraft, so the aircraft is cost efficient. In this year’s competition, there are no limitations to the maximum size of airplane. However, minimum size of the plane is restricted by allowed take off distance, payload size, and the desire to produce enough lift. Being able to perform various maneuvers efficiently during mission was also a major factor in determining design and size of aircraft. All these factors or constraints were taken into account while designing the aircraft.

4.3 Mission Model:

Four maneuvers categorize mission performance:

4.3.1 Takeoff:

Installation of flaps will help in takeoff by increasing lift and limiting the power required from motor.

4.3.2 Climb:

The motor will need to operate at maximum power during climb. The safe altitude of flying is determined by the judges and the pilot.

4.3.3 Cruise:

The aircraft operate at full throttle to complete the rounds within designated time.

4.3.4 Turns:

There are two types of turns: horizontal 360 turn and vertical 360 turn. Ailerons will help aircraft to roll for performing horizontal turn and elevators will help in performing vertical turn.

Mission model is simplified and therefore each maneuver is subjected to uncertainties.

Three different uncertainties found during the mission are:

1. Power: Current was assumed to be constant for power analysis and in reality both current and voltage varies during the mission and they cause uncertainties.

2. Weather conditions: Change in weather may also affect performance of an aircraft. In not so windy areas like Islamabad and Topi, it is safe to fly the plane. Strong winds pose danger to the structure of aircraft due to its light weight and also increase uncertainties in the time required to complete the maneuvers.

3. Interference drag: This is a drag which is found between the aircraft’s components and it is very difficult to calculate it as compared to drag on each component separately. Therefore, interference drag may increase uncertainty in the total drag calculated.

4.4 Airfoil selection:

Different airfoils are for different flight conditions. There are three major types of airfoil:

1. Symmetrical airfoil (No camber)

2. Semi-Symmetrical airfoil (Cambered)

3. Flat bottomed airfoil

An airfoil can be further characterized according to maximum thickness, maximum camber, location of maximum thickness, position of maximum camber etc.

An airfoil is needed that can help aircraft perform mission maneuvers with and without payload as well as that can create negative restoring moment for stability of aircraft. A semi-symmetric airfoil which combines the advantages of both symmetric and flat bottomed aircraft, most closely meets above requirements.

Semi Symmetric airfoils NACA 4412, MH-114 and EPPLER-422 were analyzed and compared according to their lift and drag characteristics. A suitable Reynold number of 500000 was selected to analyze the airfoils.

The best performing airfoil is one with greatest Cl (max) and Cl/Cd ratio. Eppler-422 has highest Cl at the given Reynold’s number than the other two airfoils. However, Cl/Cd ratio for Eppler 422 is less than that of MH-114, which reduces it efficiency as compared to MH-114 because Eppler 422 experiences more drag per unit lift. So a compromise is made and MH-114 is selected because it has higher Cl/Cd ratio even though its Cl is less compared to Eppler-422. MH-114 is also better candidate than Eppler-422 for completing the vertical 360 degree loop because it is less cambered than Eppler-422. Moreover, MH-114 is easier to fabricate than Eppler-422 because MH-114 has a smaller camber.

Other special characteristic of MH 114 as shown by above graphs are that it gives lift at zero angle of attack as well when angle of attack is negative. Stall is reached at about 15 degree angle of attack.

4.5 Lift and Drag analysis on 3D wing:

A CFD numerical analysis was performed on the 3D wing of our aircraft using ANSYS fluent. Reynold number was kept at 600000. A fine mesh (consisting of cells and nodes) was made on the surface of 3d wing with inflation layer. Different boundary conditions were applied on the wing’s surrounding. Flow was assumed to be turbulent and K-epsilon turbulence model was used to calculate results. Numerical iterations were performed using ANSYS at each node of mesh cells till the values of Coefficient of lift (Cl) and Coefficient of drag (Cd) converges to a constant value. Values of Cl and Cd obtained were:

Cl = 1.3586 Cd = 0.30758

Lift force comes out to be 55.88 lb

Drag force comes out to be 12.65 lb

4.6 Stability:

As shown in the figure below, the max lift occurs at about 22% of chord length from the leading edge. The location of maximum lift force almost coincides with the CG of our airplane and therefore this will ensure that out aircraft will be able to increase and maintain its altitude easily. For a straight line flight any moment or forces about CG disappears. Otherwise, there would be unsteady moments generated and aircraft will not fly in straight line.

Static stability determines the ability of aircraft to restore its original angle of attack after any disturbance or change in motion path. Cases of static stability considered are:

4.6.1 Longitudinal stability:

It is concerned with CG movement in vertical plane when the aircraft is nose down or nose up. MH-114 airfoil allows aircraft to come back to its original straight line path by producing negative restoring moment through its positive camber.

4.6.2 Lateral stability:

To allow aircraft to restore its original straight line path after rolling motion, a small dihedral angle is introduced in aircraft’s wings. This dihedral angle helps aircraft to produce restoring moment.

4.7 Flight Mission Performance

Following values are expected flight conditions during our flight.