“Two in one” glider concept for unlimited aerobatics and FES-assisted cross-country flying – part 1

The “two-in-one” idea has recently attracted the attention of many designers, and there is no reason why it should not apply to aviation. This idea can be used in various aviation branches and, among others, in recreational ones, i.e. flying only for fun. Such activity may involve aerobatics as well as cross-country flying. Often, for economic reasons, a glider or motor glider is chosen for this purpose, wherein a motor glider would be preferred since it facilitates the return home in the event of thermals disappearance (Kozuba et al., 2021). However, many pilots prefer aerobatics, or one may like both aerobatics and cross-country flying. In such circumstances, quite different gliders are needed, e.g. MDM-1 Fox (Margański & Mysłowski) with a load factor of n = +9/−6 and maximum glide ratio L/D = 30 or Diana −3 (DIANA SAILPLANES - SCANDINAVIA AB, 2024a) with n = +5.3/−2.65 and a maximum glide ratio L/D = 50. Unfortunately, both gliders are expensive, and from the air club’s point of view, the idea of having “two in one” would be attractive for economic reasons. One example of such a solution is the SZD 59 single-seat glider (SZD Allstar Glider). It can be reconfigured from an aerobatic class glider to a utility one by attaching wing tips with winglets. In the case of the former, allowed max. load factor of n = +7/−5 and maximum glide ratio of L/D = 36, and for the latter n = +5.3/−2.65 and L/D = 40.

To facilitate home return in case of the disappearance of thermals or to allow for self-lunching and in the same time, bearing in mind the climate changes, the combustion engines used so far are being replaced by electric ones heaving several advantages over the former ones (Keskin et al., 2019). Nevertheless, the electric drive has some limitations, as discussed in Joshi et al. (2022) and Hepperle (2012). When choosing an electric drive, several configurations can be considered. Some gliders are equipped with retractable motors behind a cockpit, e.g. Taurus Electro (PIPISTREL), AOS-71 (Marianowski et al., 2017) or AS-34 Me (Alexander Schleicher), which allows to restore smooth configuration in gliding flight. On the other hand, nonretractable propulsion systems can offer the possibility of energy regeneration in flight (Galvao, 2012) and have a simpler design than the former. Distributed propulsion systems can be also considered (Kuźniar and Orkisz, 2019) as well as hybrid ones (Kalwara et al., 2021). Recently, special attention has been pain to gliders equipped with Front Electric Sustainer (FES), e.g. LAK-17C FES (LAK, 2024), Silent 2 Electro (LAK, 2014), Discus2c FES (Schempp-Hirth) and Diana 2 FES (DIANA SAILPLANES - SCANDINAVIA AB, 2024b). They can be self-launchers (LAK, 2024 and LAK, 2014), but they must be equipped with relatively high, retractable landing gear; otherwise, they need towing (Schempp-Hirth and DIANA SAILPLANES – SCANDINAVIA AB, 2024b) due to insufficient ground-propeller clearance not allowing for engine starting during take-off.

So far, despite the still dynamic development of gliders and electric motor gliders, to the best of authors’ knowledge, none of the offered gliders combines a two-seater unlimited aerobatic glider and a utility cross-country FES-equipped one, i.e. meets the two-in-one glider concept.

This paper presents the way the two-in-one glider concept was attained. The idea was to redesign the existing PW-6 utility glider (ZS Jeżów), manufactured by ZS Jeżów, to obtain a two-seater aerobatic glider that could be easily converted to a utility glider equipped with FES to facilitate home return in case of thermals decay. The additional challenge was minimize changes in manufacturing tools to make this two-in-one concept attractive from the economic and sustainability points of view. Important requirements were that the glider should comply with CS 22 regulations and that it could be flown by one or two occupants in both utility and an aerobatic version, allowing for a load factor n = +7/−5 in the latter case. Obviously, instead of redesigning an existing glider, a two-in-one project can be started from scratch. However, this scenario would not comply with assumptions of the economical and sustainable concept. This paper presents some relevant design features, load considerations, aerodynamic and flight mechanics aspects of such a design.

The presented concept derives from the glass/epoxy PW6 glider. It was intended to facilitate basic training for student pilots who would soon fly PW-5 world-class gliders solo. To achieve “two-in-one,” the original PW-6 design was modified as described below. The objective was to be achieved by developing an electric motor glider based on the PW-6U design that could be easily converted to aerobatic and vice versa, by simply removing or adding some components. An important requirement was that an instructed air club pilot would be able to make such a replacement. Another important issue was that, for economic reasons, existing tooling should not be modified or modified to a minor extent if necessary, at the lowest possible costs.

The evolution of PW-6U design is presented in Figures 1(a)-1(c). Essential modification of the PW-6U consisted in:

• an airframe modification; and

• equipping the glider with a FES of LZ Design (LZ Design, 2024), activated in flight to facilitate home return in the event of thermals disappearance.

The considered FES consisted of a 30 kW brushless direct-current engine powered by a 4.8 kWh pack of lithium-ion batteries and driving a 1.2-m-diameter foldable two-blade composite propeller. It was expected to enable a climb of 1.7 m/s in the case of a two-person crew and 2.5 m/s in the case of a one-person crew, or safe home return from a distance of approximately 70 km. The airframe modification consisted of increasing the wingspan, adding winglets, elongating the fuselage by 50 and 200 mm at 2 m and 4.45 m from the nose, respectively, and adding two bulkheads, facilitating the installation of the engine bed and battery pack. These modifications made it possible to obtain two configurations: aerobatic (PW-6A), Figure 1(b), and utility (PW-6AM-W), for cross-country flying, Figure 1(c). The locations of the main parts of the drive unit are shown in Figure 2a. The FES and battery pack could be attached and detached depending on the need. After detachment the engine, the streamlining of the front part of the fuselage was maintained by replacing the engine with a specially designed fairing that could be attached to the fuselage, using the engine bed. The battery pack was secured in a specially designed case that could be detached from the fuselage and removed through a hole in the fuselage crown after dismounting the fairing covering the hole and unlocking the battery case, Figure 2b and c. The scope of wing modifications is schematically marked in Figure 1(b) and (c), by shading interchangeable wing sections. In the utility version (PW-6AM-W), the wing tips used in the aerobatic one could be replaced with the 900-mm-long wing sections ending with winglets. A popular approach for the winglet design engages Munk’s theorem that analyses the induced drag in the Trefftz plane (Blackwell, 1976). The design process of winglet for a glider should take into account the influence of the profile drag to ensure that the benefits of reducing induced drag are not overcome. The efficiency should be considered for a whole speed range (Kubryński, 2003). Another challenges are associated with a deterioration of the directional stability and stall characteristics (Kubryński, 2003). The PW-6-W winglet shape was optimized using the methodology described in Kubryński (2003) and Kubryński and Więcko (2022) which resulted in an improved L/D ratio.

Changes in the operating conditions, airframe geometry modification and installation of a drive unit resulted in changes in the in-flight loads compared to the PW-6U ones. The selected important wing and fuselage loads are presented in Figures 3–5, illustrating the issue. The plots represent master load envelopes of the in-flight load envelopes (envelopes of envelopes) for wings and fuselages of PW-6U, PW-6A and PW-6AM-W gliders. The purpose of constructing the master envelopes was to facilitate the appropriate selection of airframe parts that should be differently strengthened depending on the location. The way the master envelopes were constructed is explained with the help of plots in Figure 4(a). For example, in the case of PW-6AM-W wing and positive Tn force, at first, all load cases that should be considered according to CS 22 and relevant to the vertical wing load were analyzed. Next, the important load cases were selected, and the corresponding load vs wingspan plots were constructed. In the case of this example, they were the distributions of Tn force. Then, the dominant sections of these load curves were selected, marked AB, BC, CD and DE, shown in Figure 3(a). Based on this distinction, the resulting load envelope was constructed, it is plotted with a dashed line. In this example, sections AB, BC, CD and DE correspond to the flight conditions defined next to the plots. One repeated the same procedure for the PW-6U and PW-6A wings. The envelopes obtained this way are presented in Figure 3(b) in blue (PW-6 AM-W), red (PW-6A) and black (PW-6U). As before, the dominant envelope’s sections (representing the locally highest load) were selected (marked AB and BC). Their combination plotted schematically with the dashed line, produced the sought master envelope for the positive Tn.

Figure 3c and d, present envelopes of the bending moment Mn due to Tn and the torque Mt for the wings of gliders under consideration. The dominating sections of the important envelopes were marked with capitals.

The plots in Figure 4 represent the PW-6A and PW-6 AM-W fuselage loads acting in the x-z plane (vertical) and x-y plane. Unfortunately, the load envelopes of the PW-6U fuselage were unavailable. The envelopes of Txz and Mxz are shown in Figures 4a and b, respectively. Figure 4c and d, show the corresponding results in y-z plane. As previously, the limits of the dominant envelope sections are marked with capital letters.

The next stage is to evaluate the performance of the PW–6 after the mentioned modifications to confirm the feasibility and attractiveness of the two-in-one concept. The following configurations were considered:

• PW6-A configuration without the winglets and without the FES;

• PW6-AM configuration without the winglets and with installed FES;

• PW6-A-W configuration with the winglets and without the FES; and

• PW6-AM-W configuration with winglets and with installed FES.

It should be noted that configuration with FES can flight in both glider and motor glider mode.

The velocity polar was calculated with the use of the PANUKL package (Goetzendorf-Grabowski, 2020a) which is in-house software that can be classified as low order potential tool. In the case of the motor glider mode, the impact of the propeller on the aerodynamic characteristics was neglected in the numerical model. The friction drag was estimated using analytical correction (Raymer, 2006) applying the equivalent skin friction coefficient.

From the point of view of aerodynamic computations, two different geometrical models were built (Figure 6). The aerodynamic computations were repeated for each geometry using different reference point that corresponds to the appropriate positions of the center of gravity. The numerical model without the winglet contains 7,070 panels, while the numerical model with the winglet consists of 8,798 panels. The mesh on the fuselage, horizontal tail and vertical tail is identical for both models. The difference in number of panels is associated with a bigger wingspan and more complex wing geometry of the model with the winglets. The reference values used to determine the aerodynamic coefficients are associated with the wing geometry, and the basic geometrical data are presented in Table 1.

A verification study was conducted to assess the quality of the numerical model and to evaluate the impact of model simplifications on the obtained results. The aerodynamic characteristics obtained from PANUKL were evaluated by comparing the flight polar calculated for PW6-A with data of PW-6-U presented in the glider manual (WSK PZL-ŚWIDNIK S.A., 2003). For the purpose of this analysis, the mass of PW6-A was assumed to be 546 kg, which corresponds to the mass of PW6-U. Figure 7 left shows a very good compliance with the results. Due to limitation of a potential flow that cannot be used to model a flow separation, therefore, results close to a stall speed cannot be properly predicted.

The comparison of lift to drag ratio of PW6 with the regular wing (without the winglet) against configuration with the winglet is presented in Figure 8. The maximum lift to drag ratio for glider without the winglet was equal to 33.97 while it increased to 37.78 due to application of the winglet.

To estimate performance, the trim computations were done with use of the SDSA package (Goetzendorf-Grabowski, 2020b). The detailed mathematical model implemented in the SDSA is explained in (Goetzendorf-Grabowski et al., 2011). The most important assumptions are that the glider is modeled as a rigid body, the impact of the propeller downwash on aerodynamics is neglected, but the position of the thrust vector with respect to center of gravity is taken into account when the trim conditions are determined. The results of performance encompass the speed hodograph (flight polar), range and rate of climb. Figure 7 right shows comparison of the flight polar calculated for PW-A and PW-A-W against configuration with FES. All presented results are for the maximum mass. It was assumed that the sink rate is denoted as a negative speed.

The range of the powered flight for altitude equal to 1,000 m is presented in Figure 9 left. The calculations were performed for both minimum and maximum flight masses. It was assumed that 88% of the battery power is delivered to the motor. The climb rate results in case of the flight with the maximum mass at altitude equal to 1,000 m are presented in Figure 9 right.

Inspection of the plots in Figures 3 and 4 indicated that the wings of the “two-in-one glider” are dimensioned with PW-6AM-W loads along their external parts (section BC and DC) and with PW-6A loads along their internal parts (section AB and DE).

The plots in Figure 4 concerning the fuselage loads acting in the x-z plane lack the corresponding PW-6U load envelopes. Such a deficiency was not crucial because the lateral dimensions of the fuselage mold cavities did not limit possible changes in the thickness of the fuselage skin. This feature made the geometry of the fuselage mold cross-sections independent of the fuselage loads as opposed to the spar molds, which cavities limited possible changes in the flanges’ cross-sections, Figure 10. In this case, the relation between the wing loads of PW-6U and its modified versions was essential since it could affect the range of the tooling modification and the related costs.

The comparison of the gliders’ master envelopes showed that the differences in the in-flight loads of PW-6A and PW-6UW, except for the wing sections y> Y_B, Figure 3, were such that the PW-6A loads were dimensioning ones for both glider versions. The load on the wing section y>Y_B was higher in the case of PW-6AM-W due to the additional load generated by the attached wing sections and winglets. However, the in-flight loads of PW-6A and PW-6AM-W were significantly higher than those of PW-6U. Such differences indicated the need for a significant increase in the strength of certain cross-sections of the PW-6U structure, which was impossible using the old tooling in the case of the wing spars due to insufficient mold cavities. A possible solution to the problem was a replacement of the glass reinforcement with carbon. Inspection of the plots in Figures 3–5 indicated that in none of the load cases under consideration, this ratio exceeded the load ratios, proving the validity of the mentioned material replacement concept and eliminating the need for mold redesign except for the fuselage elongation.

Examination of the Figure 7 right shows that the minimum sink rates are similar for all considered glider configurations. The configurations PW-6A-W and PW-6AM-W are heaver but have better aerodynamics due to implementation of the winglet (Figure 8) that can compensate the overall performance. In case of thermal decay, the motor glider mode can be used to return home, in case of the maximum mass the range was estimated to 52 kilometers while for the minimum mass can be increased to 76 kilometers (PW6-AM configurations, see Figure 9 left). In the future, those results are going to be verified in the flight tests. The climb analysis confirmed that with the selected engine it is possible to obtain the climb rate of 2.4 m/s for maximum mass (see Figure 9 right).

The authors investigated the possibility and consequences of modifying the existing PW-6U two-seater glider to obtain one at-home reconfigurable glider suitable for unlimited aerobatics and cross-country flying supported with FES to facilitate home return in case of thermals decay. It was found that the airframe structure was dimensioned by the loads assigned to the aerobatic version, except for the outer parts of the wings. That was dimensioned by loads assigned to the utility version, caused by detachable wing sections ending with winglets. It was also shown that there was no need to modify the existing tooling, except for the molds needed to produce the skins of the modified fuselage.

Based on the aerodynamic and performance results can be concluded that none of modifications deteriorate the performance.

To the best of the authors’ knowledge, only two glider designs are conceptually similar to the one presented in the paper: SZD-59-1 Acro (SZD Allstar Glider) and DG1001 eNEO (DG Aviation). However, neither of them offers the possibility of at-home reconfiguration and is suitable for unlimited aerobatics with two occupants in a cockpit.

The second part of this manuscript is under the preparation and is going to address stability.

Funding: This work was supported by funding from ZS Jeżów: ITLIMS.022.88.2022.