Flap hinge

This section describes the design of the flap hinge. At first, the design process of the hinge is explained, followed by the actual design of the hinge.

4.3.1

Design process

The design procedure that was presented in figure 3.2 contains a double arrow between the boxes ‘Blade + Clamp’ and ‘Kf lap’. This double arrow represents an iterative process, which will be explained into further detail.

The design requirements state that the ratios between the different mode frequencies of the demonstrator should give a representation of a real helicopter RBS; the rigid and the first elastic flapping mode frequencies should be in the same range with respect to the operation frequency Ω as they are in table 2.1. To realize the desired flapping frequencies, an added rotational stiffness is necessary in the flap hinge. The effect of this added rotational stiffness on the elastic body frequency of the blade is determined in appendix A. It appears that adding a rotational stiffness in the hinge leads to a decrease of the bending frequency, while increasing the rigid body frequency.

With the method described in the appendix, it is possible to calculate the elastic body frequency when the blade parameters and the rotational stiffness are known. To get both the rigid and elastic body frequencies in the range of the desired frequencies, the blade parameters can be changed. The iterative process of figure 3.2 is schematically shown in figure 4.3. For the initial blade design, the required rotational stiffness for the rigid flapping frequency is calculated. With the stiffness known, the resulting elastic flapping frequency is calculated using the method of appendix A. If this fulfills the design requirements, the required rotational stiffness is known and the flap hinge can be designed. If the result is not good enough, the parameters of the blade are changed until the requirements are fulfilled.

Initial blade design 𝑓𝑓𝑟 𝐾𝑓𝑙𝑎𝑝 𝑓𝑓𝑒𝑙,1 Satisfying result?





Figure 4.3: Iterative process to realize the desired frequencies

As can be seen in this figure, only the length of the blade was changed when the desired frequencies were not realized. This is done because the width of the blade does not have an effect on the natural frequen- cies, while it is favorable to use a standard thickness for the blade; a thickness of 4mm is used. Following this process, the required rotational stiffness and blade length were found to beCF ≈2.2Nm/deg and L= 551mm.

4.3.2

Hinge design

There are different ways to generate a rotational stiffness in the flap hinge. It can be done by placing a torsion spring directly on the flap axis. However, if small angles are assumed, the torsional stiffness can also be generated by placing a linear stiffness at a certain distance from the flap axis. See figure 4.4. It is decided to generate the rotational stiffness by using an external stiffness at a certain distance from the flap hinge. When the rotational stiffness is generated at the outside of the system, it is easy to replace it when other blade configurations that require a different stiffness are used. Besides, when an external stiffness is used, it will be easier to control the distance between the hinges; a quick investigation of standard torsion springs that could be used shows that relatively large torsion springs are needed to realize the required rotational stiffness in the hinge. This automatically means that the hinge itself, and thus the distance between the flap and lag hinge, will become large as well. The distance between these hinges will quickly become around 15 to 20% of the blade length. This is highly unfavorable; the different axes are all close to the root in a fully articulated helicopter rotor, so to better represent a helicopter RBS, it is favorable to keep the distance between the hinges as small as possible.

The solution that was eventually used involves a torsion spring that is located outside the hinges. The spring that is used for this is connected to the flap hinge at one end and the lag hinge at the other end. It was ordered at Lesj¨ofors, a spring supplier company that produces and supplies a large selection of

Flap axis Linear stiffness Blade Extension arm 𝐾[N/m] 𝐶_𝑓[Nm/deg] Flap axis Torsional stiffness Blade

Figure 4.4: Two ways to generate a rotational stiffness in the hinge

Figure 4.5: Torsion spring end design

standard springs, including torsion springs [29]. It was possible to customize the design of the standard springs to be able to fit them in the design. For the spring that was selected for the flap hinge, the leg length was changed. Besides, the springs’ end design was customized to be able to fit it more easily in the design, see figure 4.5. The legs of the left spring are in the original configuration, while the right one has the customized configuration that is used in the demonstrator.

The torsion spring that was used for the flap hinge is a customized SF-VFR 8503, with a spring rate of 0.488Nm/deg [29]. The arm that was used to connect the torsion spring was arbitrarily chosen to be 50mm. With these parameters, the leg length that was required to realize a rotational stiffness of CF ≈2.2Nm/deg was calculated to be 23.5mm. The derivation of this required leg length is explained in appendix B.

At this stage of the design process of the flap hinge, it is known how the torsion spring is going to be implemented in the hinge. Besides, the hinge should contain a clamping section to connect the blade. These are the two most important characteristics of the hinge, so the rest of the design of the hinge can now be done. Figure 4.6 shows the final design of the flap hinge. The bottom section is the part that clamps the blade, the black dashed line is the flap axis and the arm in the front is the part where the torsion spring is connected.

With the blade, the clamp and the flap hinge designed, the design of the first subsystems of the demon- strator is finished. Figure 4.7 shows the assembly as it is at this stage of the design process. The end of the torsion spring that is not connected to anything will form a connection to the lag hinge.

Flap axis

Blade connection Connection with

torsion spring

Figure 4.6: Design of the flap hinge Connection

with lag hinge

Flap torsion spring Flap axis

50mm 23.5mm

Figure 4.7: Assembly of subsystems 6 to 8