Aerodynamic and Stability Analysis of the Safat 01 Aircraft

This paper aimed to predict aerodynamic & stability data for the Safat01 aircraft. Because of its economic and time efficiency, the Digital DATCOM program has been used to predict the stability and control derivatives for the subsonic, low angle of attack (less than 15 degrees) flight regimes. The DATCOM aerodynamic results were very close when compared to CFD. The effect of the nominal centre of the gravity position was considered. According to the computed stability results, the aircraft has been found statically stable and possessing longitudinal dynamic stability. This agrees with the flight tests results conducted on the aircraft which showed stable behaviour.


Introduction
HIS research targets to perform an aerodynamic analysis of the Safat01 aircraft (Fig. 1).It involves determining the aerodynamic qualities of the aircraft.Digital DATCOM has been chosen to be a design tool of the Safat01 to estimate the aerodynamic data and stability derivatives.In addition to money and time saving, the results of DATCOM at low speed are close to wind tunnel tests and usually provide an acceptable level of accuracy [1].

Safat01 aircraft specifications
The Safat01 is a light, single-engine, monoplane aircraft with a high -strut -braced wing of the USA 35B airfoil section.It is used for primary training, and has the following specifications given in Table 1, Table 2 and Table 3.

Aerodynamic and static stability results
The aerodynamic and static stability derivatives results are displayed in Table 4.

Lift
The lift varies linearly with the angle of attack till it equals 15 degrees -where the lift coefficient reaches its maximum valuethe stall begins and the non-linearity appears due to flow separation.The value of the lift coefficient at zero angle of attack is 0.469 and the maximum lift coefficient is 1.69; the two values are the same for DATCOM and CFD.The DATCOM result is shown in Fig. 2. The ground effect on the lift coefficient is to increase lift when the wing is near the ground and takes advantage of the favorable interaction between the wing and the ground.When the ground factor (wing height above ground/wing span) is near the ram value of 0.1, the ground effect is too high because the wing is very close to the ground and the trailing edge creates a sealed envelope and modifies the pressure field.The effect is decreased as the aircraft is far away from the ground; the lift variation with the ground effect is given by DATCOM and shown in Fig. 3.

Centre of pressure movement
The centre of pressure does not occupy a fixed location.As the angle of attack is changed, the pressure at each point on the airfoil changes.Therefore, the location of the centre of pressure changes as well.The centre of pressure moves forwards as the angle of attack increases.
However, at the point of stall, the centre of pressure moves quickly backwards, causing a nose down pitching moment.The result is obtained by DATCOM and shown in Fig. 4. When the lift coefficient is zero, an airfoil generates no lift but a conventionally cambered airfoil generates a nose-down pitching moment, so the location of the centre of pressure is an infinite distance behind the airfoil.

Drag
The drag varies with the angle of attack linearly till the stall angle of attack because of its dependency on lift.After the stall angle of attack, the drag coefficient keeps rising due to flow separation.The variation of drag coefficients with the angle of attack is the same for DATCOM and CFD and equals 0.28 per deg.The various parts contributions to the drag coefficient are shown in Fig. 5, from where it is obvious that the wing is the main contributor.The ground effect is to decrease drag by decreasing the induced drag, when the wing is in the area of the ground effect due to a part elimination of wing tip vortices.The variation of the drag coefficient with the ground effect obtained by DATCOM is in Fig. 6.

Longitudinal static stability
The results obtained from DATCOM show a positive longitudinal stability according to the stability criteria.The value of zero lift moment about the centre of gravity 0.0577 is positive quantity and the moment's curve slope is negative and equals 0.0138, which satisfies the longitudinal static stability conditions.The main contributor to longitudinal static stability is the horizontal stabilizer by the largest positive value to balance the pitch down caused by the wing and other aircraft parts.The value of the moment curve slope is within the range for most aircraft, between -0.3 to -1.5 per rad [5].The obtained results and various aircraft parts contributions are shown in Fig. 7.

Directional static stability [9]
The aircraft showed stable behaviour in directional stability.The value of n c β equals the desirable value obtained (0.00057) and close to that of CFD.

The effect of the centre of gravity location on longitudinal stability [10]
The effect of the movement of the centre of gravity towards the trailing edge on longitudinal static stability is to increase the zero lift moment coefficient and decrease the moment curve slope due to the reduction in the static margin.The trim angle of attack of the aircraft increases with the centre of gravity (c.g.) forward shift.The moment curve became approximately flat with the movement of the centre of gravity towards a neutral point.The rate of the change of the pitching moment coefficient with respect to the pitch rate decreases with the centre of gravity forward shift; it is directly affected by static margin reduction.The larger the negative value of q m C , the higher a stabilizing impact on longitudinal dynamic stability.The values of q m C for the whole centre of gravity range are within the range.For most aircraft, it is between -0.087 to -0.523 per deg [5] and the rate of change of the rolling moment coefficient with pitch is also reduced as demonstrated in Fig. 10.All these results are demonstrated in Table 5.The whole studied centre of gravity range showed that nominal locations from 23% to 28% are permissible (Fig. 8), because the stability criterion is still satisfied and q m C is also within the typical value for most aircraft.

Longitudinal dynamic results
For the specific aircraft parameters, the longitudinal dynamic roots are 1,2 6.1384 14.3744 The roots are complex pairs, which indicates that the response is periodic.The real part is negative, which means that the amplitude of the periodic variation will decrease with each oscillation (the damped mode).This type of motion is called subsidence [6].The first couple of roots indicates a short period oscillation (SPO) with heavy damping.In this period, the angle of attack and the pitch rate change rapidly whereas the velocity remains approximately constant.Within this short time, the angle of attack is nearly restored to its initial undisturbed value and remains so thereafter.The second couple of roots indicates a long period & low damping (long period oscillation).In the long period oscillation, there are changes in velocity while the angle of attack remains almost constant.The long period oscillation persists after the SPO has died down and influences the changes in velocity while the angle of attack remains almost constant.The total energy of the aircraft is nearly constant during this motion and there is an exchange between the kinetic energy and the potential energy of the aircraft.As the flight speed decreases, the aircraft loses kinetic energy and gains potential energy.It reaches a crest when the velocity is minimal.Conversely, when the velocity increases, the altitude decreases and the flight path shows a minimum value with the maximum velocity.

Conclusion
The DATCOM program has been used to estimate the stability and control derivatives for the subsonic, low angle of attack (less than 15 degrees) flight regimes for the Safat01 aircraft.The resulted stability and control derivatives have been used to estimate the aircraft longitudinal dynamic stability.According to the computed stability results, the aircraft has been found statically and dynamically (longitudinally) stable.This, however, agrees with the flight tests results conducted on the Safat01aircraft which showed stable behaviour [7,8].

Recommendations
Although this research has identified a data generation technique, to assure stability and controllability in the ranges of low angles of attack, the following research is still needed: -Improving the accuracy results by other techniques to reinforce the obtained results and enhance the accuracy.-Effects of aeroelasticity should be taken into account [11,12,13].

Table 4 .
Aerodynamic and static stability resultsCFD result

Figure 2 .Figure 3 .
Figure 2. Variation of the lift coefficient with the angle of attack

Figure 4 .Figure 5 .
Figure 4. Variation of the centre of pressure (cp) location with the angle of attack

Table 5 .
Effect of the movement of the centre of gravity (c.g) on longitudinal stability