Progress in Inverted Joined Wing Scaled Demnostrator Programme

The Institute of Aviation is leading the consortium dedicated to investigate properties of the inverted joined wing aeroplane configuration. CFD analyses, wind tunnel tests and flight tests of scaled demonstrator were undertaken to perform this task. This paper will present a summary of results achieved so far with particular attention put on CFD, wind tunnel and flight test results. Joined wing configuration is considered as a candidate for future aeroplanes. It is an unconventional aeroplane configuration consisting of two lifting surfaces similar in terms of area and span. One of them is located at the top or above the fuselage, whereas the second is located at the bottom. Moreover one of lifting surfaces is attached in front of aeroplane Centre of Gravity, whereas the second is attached significantly behind it. Both lifting surfaces join each other either directly or with application of wing tip plates (box wing). This concept has many possible advantages like induced drag reduction and weight reduction due to the closed wing concept. Unfortunately it is much more complicated to design than conventional aeroplane due to the strong aerodynamic coupling and static indeterminacy. Therefore it was not possible to build successful, safe and efficient aeroplane in this configuration before computer aided design systems became available and even its early versions were not powerful enough. Large meshes are necessary to describe it accurately enough for precise CFD analysis, so very capable computers were required and unfortunately unavailable. On the other hand potential weight reduction comes from the static indeterminacy of the joined wing configuration. Once again powerful computers were necessary to analyse it with FEM method with satisfactory accuracy. Moreover, static indeterminacy causes significant manufacturing problems due to tight tolerances required to assembly the joined wing with no random internal stresses. All these difficulties can be resolved with application of modern CFD and FEM software, increased computing capabilities and prototyping capabilities based on computer controlled machining. All of them are currently available, so attempts to design a joined wing aeroplane are more frequent. However in most cases researchers concentrate on configuration where front wing is attached at the bottom of fuselage and aft wing is installed either at the top of the fuselage or at the top of the vertical stabilizer. Our previous experience lead to the conclusion that joined wing aeroplane could fly much better in upside down position. The most probable reason of this fact comes from the interaction between wings. Front wing wake is very close to the aft wing if gap between wings is too small. It becomes even smaller at high angles of attack if front wing is located below aft wing. As a result aerodynamic advantages are diminished. They may be recovered if aft wing is installed high at the top of the vertical stabilizer, however this requires strong stabilizer which decreases potential weight reduction. Configuration, with front wing above aft wing should work in the opposite way, thus delivering expected advantages, providing that fuselage is reasonably high. Our recent CFD analyses confirm, that joined wing aeroplane L/D grows together with increasing gap between wings. Moreover, assuming the same gap, configuration with front wing above aft wing provides not only greater maximum L/D, but also greater L/D in wider range of angles of attack. In particular L/D at high angles of attack is greater in this configuration which suggests advantageous flight endurance. Configuration with front wing below aft wing is advantageous only at low angles of attack assuming that aft wing is installed at the top of the vertical stabilizer. However, as mentioned before, weight advantage should be reduced in this case due to the increased loads of vertical stabilizer. As can be seen from this result, final conclusion is not clear yet, which was the motivation to undertake our current project. Multicriterial optimisation, performance, stability and control analyses of the joined wing unmanned aerial vehicle (UAV) are a main goal of the this project. They are being verified by wind tunnel and flight tests. It is believed that it will allow for collecting an extensive database of knowledge concerning joined wing aeroplane configuration. Most of analyses and optimisation in this project are conducted for UAV since this allowed for building inexpensive real flying test-beds. Three UAVs are to be tested in this project, one with wing span of 1,2m and two with wing span of 3m. Two of them are currently flying. Flight test campaign will last to the end of November 2015. Proposed paper will present its results obtained before the deadline for full paper submission. Moreover additional CFD analyses, flight simulations and wind tunnel tests are planned. They are to be compared with flight test results. Results of these analyses and comparisons will be presented as well.