Parametric study of structural model characteristics

One question that was raised during this study regards the possible effect of the Gurney flap actuation mechanism on the structural dynamics of the blade. For that reason a parametric study was conducted taking into account different structural properties of the blade as well as adding extra weight which corresponded to the actuation mechanism. The aim was to quantify and qualify the Gurney effect, as well as, to compare it against the effect that a blade-tip design study may have on the structural properties of a given blade.

First of all, a parametric study was taken place to check how several variables in the model used in NASTRAN affect the final deformation of the blade. The first parameter investigated was the sectional area of the aerofoil used. A first attempt was made with the main aerofoil of UH-60A helicopter as its blade was almost as long as the W3 MR blade. The area of UH-60 A wasAs=0.01886m2_{. However,}

the area of NACA23012M was more than 4 times bigger withAs=0.08127m2_{. It is proved that the}

sectional area of the blade is too small comparing to the other dimensions and even a big change hardly affects the deformation of the blade. An other parameter that should affect the deformation of the blade and it is not specified is the stiffness of the blade. Data provided by PZL Swidnik considered the product of stiffness with Young modulus E or with Shear modulus G as far as torsional stiffness is concerned. If the material used for the blade is supposed to be the fibre glass and is considered to be unique along the blade then a small study on the literature could give a good approximate value of E, G and n which is the Poisson ratio. As a result a final study was conducted considering these parameters. First of all, the Young modulus was decreased by 2 order of magnitude but the Poisson ratio remained the same, which showed that it affects the deformation of the blade to great extent and especially the pitch of the blade.

For the completeness of the study the effect of different tip designs on the aeroelastic response of the blade was studied using the S-76 blade. The details of the model are presented in the Appendix D. Four different tip designs are used, a rectangular, a tapered, a swept, and a tapered-swept as presented in Fig. 4.23. Basically, what changes between the four designs is the mass distribution, the torsional inertia of the tip segment, and the location of both elastic axis and centre of gravity at the tip. The comparison

4.5. AEROELASTIC CALCULATIONS CHAPTER 4. HOVER CASE

of the modes up to 125Hzare presented in Figs. 4.29-4.31. It is to be noted that the different designs did not alter the characterisation of the modes and the frequencies were shifted by less that 1% compared to the baseline tip design. This outcome shows that even such differences in the design, which lead to significant changes on the aerodynamic behaviour of the blade, will not affect a lot the aeroelastic response of the blade. Figs. 4.25 -4.28 present the properties used in NASTRAN for three different blades, the S-76, the W3-Sokol, and the UH60A blade to get an insight of the different parameters used in the models.

CHAPTER 4. HOVER CASE 4.5. AEROELASTIC CALCULATIONS 30 deg 35 deg 3.1 in 1.86 in 3.1 in 5 % R 5 % R 3.1 in 1.86 in 5 % R 0 deg Tapered tip (0.6 c) (0.6 c) 6.31 deg _{18.63 deg} 5 % R 3.1 in 3.1 in 20 deg 20 deg 0 deg 20 deg

Swept tapered anhedral tip

Swept tapered tip (I) Rectangular tip (II)

(III)

Swept tip

(IV)

(V)

4.5. AEROELASTIC CALCULATIONS CHAPTER 4. HOVER CASE

Figure 4.24: Structural model of S-76 blade.

CHAPTER 4. HOVER CASE 4.5. AEROELASTIC CALCULATIONS

Figure 4.26: Flapwise moment area of inertia of different blades used for static computations.

4.5. AEROELASTIC CALCULATIONS CHAPTER 4. HOVER CASE

Figure 4.28: Torsional constant of different blades used for static computations.

Figure 4.29: Spoke diagram for S-76 blade, comparison between rectangular and tapered-swept tip design.

CHAPTER 4. HOVER CASE 4.5. AEROELASTIC CALCULATIONS

Figure 4.30: Spoke diagram for S-76 blade, comparison between rectangular and tapered tip design.

4.5. AEROELASTIC CALCULATIONS CHAPTER 4. HOVER CASE

According to the study conducted by Hoerner[107]on the Aerodynamic shape of the rotor blade tips it was found that the shape of the tip leads to a different formation of the generated vortex. For the case of a rounded tip he observed based on water tunnel experiments that the vortex is generated above the tip. This vortex was also captured during both rigid and aeroelastic calculations and it is presented in Figure 4.32. As shown the vortex starts to form close to the trailing edge of the tip and has been fully formed above the blade before leaving the blade tip. According to Hoerner this type of blade it may lead to a decrease of the effective span of a fixed wing up to 20% of the chord.

Figure 4.32: Vortex generated at the blade tip of W3 Sokol in hover.

Finally, the effect of the additional mass of the Gurney flap actuation mechanism on the aeroelas- tic response of the blade was tested by distributing an additional 10% of the total mass of the blade at the sections where the Gurney flap was located. Next Figures present the spoke diagram of the clean blade

CHAPTER 4. HOVER CASE 4.5. AEROELASTIC CALCULATIONS

and a comparison against the fully instrumented blade is given in Table 4.2. Again, the mode shape char- acterisation was not altered by the Gurney flap mechanism, while the frequencies of the given modes were decreased up to 1.6%. As a result it seems that the uncertainty due to the Gurney flap is of the same order of magnitude with the one introduced due to the different tip shape designs.

Figure 4.33: Spoke diagram for W3-Sokol blade. Circles are used for blade with Gurney flap.

Mode Shape Clean blade frequency (Hz) Blade with Gurney flap frequency (Hz) Difference (%)

1stchordwise 3.2738 3.2551 −0.57 1st flapping 4.7496 4.7249 −0.52 2ndflapping 12.3107 12.1162 −1.58 2ndchordwise 19.7262 19.5171 −1.06 3rdflapping 21.763 21.4257 −1.55 4th_{flapping 33.479 33.1141 −1.09} 5thflapping 47.973 47.3254 −1.35 3rd_{chordwise 50.1519 49.8109 −0.68} 1sttorsional 64.9486 64.3511 −0.92 2ndtorsional 84.5769 83.3505 −1.45

4.5. AEROELASTIC CALCULATIONS CHAPTER 4. HOVER CASE