Introduction and goals
After two DLG projects with Andrey Yakovlev and Jochen Reuter, Andrey approached us again to realize an F5J project. Since I am quite a purist when it comes to soaring, I was not very enthusiastic to design a glider with an engine. After having studied the competition rules however I must admit that the focus in F5J is very much on pure thermal flying. In addition to that the boundary conditions for the glider design differ significantly from those in F3J and F3K which made the design an interesting new challenge.
Starting the design process we discussed which attributes the new glider should have to achieve best results at highest competition levels. We agreed to focus on the following goals:
– Glider specifically designed for F5J
– One design for all weather conditions (only layup variants)
– Capable of handling FAI wind limit situations well
– Nice control harmony in all three axes
– Good visibility
– Vice free flight characteristics
With the above targets in mind we chose a conventional glider configuration with a Supra-like cross-tail. To realize best flying characteristics and thermal sensitivity we focused on keeping the extremities light and inertia as low as possible. To achieve this goal and due to the good experience with the Flitzebogen F3K designs, the Rohacell solid core construction technique was chosen for the wings and tails. Based on these building technology and material properties, iterations could be performed to define wing span and wing area for optimal system performance in all flight phases (weighted flight phases where: fast glide, cruise, climb, and floating, ordered by weight). The resulting wing came out close to the span limit of 4m with thin airfoils that perform well at high speed/low wing loading but also generate enough lift to climb well in thermals when the glider is ballasted. The resulting low volume of the Rohacell helps to keep the weight of the core down. Since the glider must not withstand high bending loads (as in winch launch or two men towing) the weight of the spare caps is still moderate.
Based on the design iterations the wing geometry was set to 3.948m wing span, 74.3dm^2 wing area both resulting in an aspect ratio of 20.97 (and finally we are in the same range as the old 15m class – 21.4 is the aspect ratio of the LS6 I like to fly). The chord distribution matches the optimal load distribution very closely. In addition to that the wing planform allows to maintain a straight spar to decouple torsion and bending loads as well as keeping 25% chord flaps perfectly along the span. For ease of transportation a three piece wing configuration was chosen to fit standard size model boxes.
Nine airfoils along the span were designed to match Reynolds number and lift requirements as well as best L/D in glide mode also at low all up weights. The wings relative thickness ranges from 7.48% at the root to 6.49% at the tip. As stated above the thin wing helps to handle low Reynolds numbers for super light layups but also keeps pressure drag low to perform best in fast cruise. It would be technically possible go even thinner on the outer wing, but the current design guarantees safe handling qualities with positive flap settings in tight turns without having to use washout which would hurt high speed performance. The wing lower surface is smooth for fast cruise and the upper surface is smooth for best L/D at slow cruise. Our flight tests have shown that even very skilled pilots are not always able to keep the optimal speed constant for each flap setting during all flight phases. This is why the airfoils were designed to perform well over a wide angle of attack range and hence speed range for each flap setting.
For optimal sizing of the tails classical handbook methods as well as AVL were used. The tail design is based on experience with F3J and F3K designs, but credits again go to Mark Drela for the tail design of the Supra which was a real eye opener for me in 2007. The Prestige, the latest design for F3J I was involved in back in 2010 followed the Supra tail configuration, but incorporated a hinged elevator on a pylon mount for the sake of easy construction and very precise pitch control around the elevators neutral position. For the Sense I used the Prestige tail configuration with the main improvement being the use of solid core construction technique resulting in very light and durable tails.
For the elevator a high aspect ratio planform was chosen which makes for a steep CL over alpha slope providing very effective longitudinal damping. Three non-symmetrical airfoils with no dead band effect and capable of working the flow field behind the wing at low Reynolds numbers were designed.
In contrast to the elevator, a moderate aspect ratio planform was chosen for the vertical tail. This results in a lower gradient of the lift slope curve and hence a wide range of possible side slip angles without stalling the vertical tail. In addition to that the airfoils used on the vertical tail operate at higher Reynolds numbers and are designed to achieve good maximum lift at slow speeds while maintaining low drag at cruise flight conditions. The generous area of the vertical tail was chosen because the high wing span, the fuselage and tails all build up inertia around the yaw axis which the vertical tail has to fight against.
Further aspects for the generous sizing of the vertical are directional stability of a potentially very light airframe. In addition to that it is difficult to spot the flight attitude correctly flying at a distance due to the slim lines of the Sense. A large rudder helps a lot in this regard when flying far away.
Generally the fuselage layout was designed to fit all components with the least wetted area possible. Another important aspect was to go for a short nose to keep the inertia around the yaw axis as low as possible. The short and relatively slim nose also results in little wetted are with partly turbulent boundary layer which will result from the hub and propeller tripping the boundary layer. For the same reason the tail boom diameter was kept minimal in size. The boundary layer on the rear part of the fuselage will be tripped by the hub, propeller and wing and hence it will be fully turbulent flow. For fast cruise flight conditions the skin friction coefficient on the fuselage surface with a fully turbulent boundary layer is approximately six times greater than it would be the case with all laminar flow.
Since there is only one span variant of the Sense, the tail boom length could be chosen in combination with the tail sizes to reach the goal of nice control harmony on all three axes.
Benjamin Rodax, 2018