Hybrid buoyant (HB) aircraft in which 50% of gross takeoff mass is supported by “free of cost aerostatic lift”, are a new arena to boost up the tourism and agricultural industry by leveraging on the new merger of lighter than air and heavier than air technologies. Due to non-availability of historical trends for HB aircraft, which are required to begin with traditional method for aircraft design, it is quite difficult to estimate its aerodynamic and stability characteristics. In the present research work, correlation of the geometric and buoyant properties of the swimming animals with the HB flying vehicles has been done to link the existing modern knowledge of aerospace with the biological sciences. Fineness ratio, location of maximum width and buoyant independent drag of a California sea lion are found to be the three quantities which are common with hybrid buoyant aircraft. Based on the existing fundamental relationships used for aircraft as well as airship design, a new conceptual design methodology for such aircraft is proposed with the help of two design examples. Pugh concept selection charts have assisted to rank the population of different concepts of such aircraft. Driving factors of such design concepts have been reviewed along with the selection of figure of merits. The diffused lift technology in HB aircraft seems to have eradicated the separate requirement of the heating mechanism for the lifting gas. A methodology for system design for consistent aerostatic lift is also proposed. The focus of this research work is not on the degree of “exactness” of the potential designs being considered at conceptual level, but rather to get the first- hand knowledge of the aerodynamics and static stability characteristics. Existing analytical relationships for the skin friction drag and Munk-Multhopp’s relationships for the estimation of pitching moment are revisited and potential issues related to their derivation are also elaborated. For the conceptual design work, Aircraft Digital DATCOM is used for a hybrid lift aerial vehicle. XFLR software along with the CFD results of the fuselage are used for HB aircraft, designed for STOL application. New analytical relationship for the estimation of the neutral point of a HB aircraft is derived. A first order approximation of the power-off stick fixed neutral point is done by using the computational results of the fuselage along with the panel method results for the lifting surfaces. The value so obtained is then compared with that obtained from the steady state simulations of the clean configuration of a two seater HB aircraft for which the SIMPLE scheme is employed for pressure velocity coupling along with the k-ω SST model. CFD results under predicts the slope of pitching moment as well as the static margin. Irrespective of the difference of flight and wind tunnel’s Reynolds number, a good comparison of results is obtained. However, from the controllability point of view, it’s negative sign can be made positive by designing an elevator for constant pitch down position for the level flight, moving the wing to further aft position or by increasing the anhedral angle of the canard. A chaotic behavior in the overall lift, drag and yawing moment is observed due to the dorsal fins. An increase in the aerodynamic coefficients is also observed when the configuration is tested after removing the dorsal fins. Moreover, an increase in the lateral stability is also observed when the canard is given a small anhedral angle. The developed databank of aerodynamic and static stability derivatives will be highly beneficial for the future design work of such aircraft after applying the Reynolds number corrections.

This research investigates the effect of permeability on the aerodynamics of airfoils and wings. The aerodynamic performance of these airfoils and wings were studied experimentally in the IIUM-low speed wind tunnel. From the literature, it appears that a comprehensive experimental study on permeable wings and airfoils is needed. The research comprises two main objectives; the first of which is to investigate the aerodynamic performance of permeable wings and airfoils using experimental and simulation methods (CFD). The second objective is to investigate experimentally the effect of permeable wingtips on the flow field over the wingtip and its effect on the wake vortex flow downstream using particle image velocimetry (PIV). A permeable thin flat plate, representing a thin symmetric airfoil, as well as a finite wing of the same cross section is used. Permeability is introduced by using a honeycomb structure. The experiment was performed for a range of different porosity values. The results are presented in terms of lift slope versus permeability. The lift slope reduces as the permeability increases for both wings and airfoils. The behaviour/trend of the lift slope is similar to the analytical results available in the literature. The effect of permeability on the aerodynamic center is plotted as well. As the permeability increases the aerodynamic center moves towards the impermeable region. The investigation on the applicability of the standard equation for calculating the lift slope of a wing from an airfoil is applied to permeable wings and airfoils. The result shows that this equation is applicable to both conventional impermeable as well as permeable wings and airfoils. The CFD work is carried out on a thin symmetric airfoil using NACA008 as its cross section. The results of the variation of the lift slope with permeability show a similar behavior as in the experimental study. The results of permeability from CFD shows that a low value of permeability reduces the drag coefficients and thus increases the lift to drag ratio by a large amount. The effect of directional porosity of wing tips on the flow field on the wing surface and in the nearfield of the wing is investigated through PIV. The PIV experiment was performed on seven models of wingtips including the base model. An impermeable wing with a NACA 653218 section was used in this study. Directional porosity is used in five wing tip configurations and one wing tip was made of a honeycomb structure. Configurations 4 – 7 have the highest porosity and the porosity direction in configurations 4 and 5 is 90º, and configurations 6 and 7 have a directional porosity of 95º and 100º, respectively, the directional porosity angle is measured from the chord line., have the highest effect on flow vortex downstream and the reduction in vorticity can reach up to 90% and reduction in tangential velocity can reach up to 74%. These directional porosity wing tips have a great impact on the flow field over the wingtip surface as shown by studying the flow field over the upper surface of wingtips using PIV measurements. These configurations have a porosity perpendicular to the chord line. Configuration 5 has the highest impact as it has the highest porosity value. Configurations 2 and 3 result in a lesser effect on vorticity and tangential velocity as they have porosity inclinations of 30º and 45º respectively. The PIV results over the upper surface of the wingtip show a high disturbance of the flow on the upper surface which results in a reduction in wake vortex downstream. The aerodynamic performances of permeable wingtips were obtained as well and they show a negligible reduction in lift but increase in drag coefficients in some configurations can reach up to 18% at angles of attack [10º - 15º]. These permeable wing tip configurations can be used to alleviate the wake vortex as they are not add-on devices and they are easy to deploy. Thus, this research investigated the behavior of permeable airfoils and wings and compared their behavior with analytical results. It also verified the applicability of the standard equation of calculating lift slope of wing from airfoil lift slope for permeable wings and airfoils. The research also introduced new directionally permeable wingtips which have high impact on vorticity reduction downstream in the near wake field. Last, it investigated the flow behavior over the porous wingtip surface to investigate its role in wake vortex strength reduction downstream.