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Structural modeling and validation of an active twist model rotor blade

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Abstract

DLR has been researching on active twist rotor blade control for at least 15 years now. This research work included the design and manufacturing of model rotor blades within the blade skin integrated actuators. As a main subject, numerical benefit studies with respect to rotor noise, vibration, and performance were carried out with DLR’s rotor simulation code S4. Since this simulation code is based on a modal synthesis, it uses the natural blade frequencies and mode shapes to model the blade dynamics. Both, natural blade frequencies and mode shapes, are computed in advance employing a finite element beam model of the blade. Each beam element possesses certain structural properties that are derived from an ANSYS model for certain cross sections of the blade. Since model rotor blades are built for wind tunnel testing, they are highly instrumented with sensors and therefore vary in their structural properties along span. Modifications in the structural properties due to the instrumentation are not included in the ANSYS model. However, to account for these variations, two experimental methods have been developed. They allow the determination of the real values for the most important structural blade properties such that the structural blade model is improved. The paper describes the experimental methods, as well as the development of an advanced structural blade model for rotor simulation purposes. It shows a validation of the structural blade model based on the measured non-rotating and rotating frequencies.

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Notes

  1. Referring to [22], the term voxel “is an artificial word composed of volume (x) element. It is used to discretize a volume and is represented as a unit cube with a single value. This is analogous to a pixel in two-dimensional”.

Abbreviations

c :

Blade chord (m)

e :

Distance between center of gravity and shear center (m)

\(e_F\) :

Offset of blade hinges from rotor center (m)

\(EI_y\) :

Lag-bending stiffness (\(\mathrm{Nm}^2\))

\(EI_z\) :

Flap-bending stiffness (\(\mathrm{Nm}^2\))

\(f_m\) :

Correction factor

F :

Force (N)

GJ :

Torsional stiffness (\(\mathrm{Nm}^2\))

\(I'_1\) :

Mass moment of inertia per unit length about the shear center of the cross section in chordwise direction (kgm)

\(I'_2\) :

Mass moment of inertia per unit length about the shear center of the cross section in flapwise direction (kgm)

\(I'_{10}\) :

Mass moment of inertia per unit length about the principal axis of the cross section in chordwise direction (kgm)

\(I'_{20}\) :

Mass moment of inertia per unit length about the principal axis of the cross section in flapwise direction (kgm)

\(m'\) :

Mass per unit length (kg/m)

m :

Mass (kg)

n :

Control frequency normalized to the nominal rotor rotational frequency

M :

Torsional moment (Nm)

r :

Radial blade coordinate (m)

R :

Rotor radius (m)

x, y, z :

Coordinates with the x-axis along the undeformed position of the shear center (m)

\(\Delta\) :

Difference

\(\epsilon\) :

Relative error

\(\eta ,\xi ,\zeta\) :

Coordinates of the cross section of a finite element with respect to shear center (m)

\(\vartheta\) :

Elastic torsion angle (rad)

\(\theta _{tw}\) :

Pre-twist (rad)

\(\omega\) :

Natural frequency (rad/s)

\(\Omega\) :

Rotor rotational frequency (rad/s)

\(\Omega _0\) :

Nominal rotor rotational frequency (rad/s)

CG:

Distance of center of gravity from leading edge (m)

FA:

Fairing

i :

ith radial station

R:

Root

SC:

Distance of shear center from leading edge (m)

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Authors and Affiliations

  1. German Aerospace Center Institute of Flight Systems, Lilienthalplatz 7, 38108, Brunswick, Germany

    Frauke Hoffmann

  2. Institute of Composite Structures and Adaptive Systems, Lilienthalplatz 7, 38108, Brunswick, Germany

    Ralf Keimer & Johannes Riemenschneider

Authors
  1. Frauke Hoffmann
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  2. Ralf Keimer
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  3. Johannes Riemenschneider
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Corresponding author

Correspondence to Frauke Hoffmann.

Additional information

This paper is based on a presentation at the German Aerospace Congress, September 16–18, 2014, Augsburg, Germany.

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Hoffmann, F., Keimer, R. & Riemenschneider, J. Structural modeling and validation of an active twist model rotor blade. CEAS Aeronaut J 7, 43–55 (2016). https://doi.org/10.1007/s13272-015-0173-0

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  • DOI: https://doi.org/10.1007/s13272-015-0173-0

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