A feasibility program determined that it would be possible to significantly reduce the cost of manufacturing wind generator rotors by making them of cast urethane. Several high modulus urethanes which were structurally tested were developed. A section of rotor was also cast and tested showing the excellent aerodynamic surface which results. A design analysis indicated that a cost reduction of almost ten to one can be achieved with a small weight increase to achieve the same structural integrity as expected of current rotor systems

Fertis, D. G.

Ross, R. S.

English

NASA Technical Reports Server

RESEARCH OF LOW COST WIND GENERATOR ROTORS
Demeter G. Fertis
University of Akron
Akron, Ohio 44325
and
Robert S. Ross
Concept Development Institute,
1790 Stoney Hill Drive
Hudson, Ohio 44236
Inc.
ABSTRACT
This feasibility program determined that it would be possible to signifi-
cantly reduce the cost of manufacturing wind generator rotors by making
them of cast urethane. Goodyear developed several high modulus
urethanes which were structurally tested at the University of Akron.
A section of rotor was also cast and tested showing the excellent aero-
dynamic surface which results. A design analysis indicated that a cost
reduction of almost ten to one can be achieved with a small weight in-
crease to achieve the some structural integrity as expected of current
rotor systems.
IN TRODUC TION
Recently NASA has been given the task of trying to find some way that
wind energy could be harnessed at a large enough scale to become sig-
nificant as a world energy source. In preliminary investigation NASA
has determined that large size windmills might be one solution to this
problem. In their studies, however, they find that the cost of the initial
installation is considerable and if the overall system is to be economically
feasible, the initial cost of the equipment must be reduced. These first
costs can be divided into three major components; 1) the rotor blades
themselves, 2) the tower that supports the equipment, and 3) the elec-
trical and electro-mechanical tie-ins. In this particular program CDI
has proposed a solution for reducing the cost of the rotor blades.
In the first NASA installations and systems, rotor blades were based on
either propeller or airfoil design and fabrication techniques. These
provided workable blades, but at a relatively high cost. It appeared
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that an entirely new approach would be required if a significant cost re-
duction was to be expected. CDI, therefore, looked to new materials
and fabrication techniques as the solution to this problem. Since ure-
thanes had a wide capability of structural properties, and they could be
cast in low cost molds, where they generate their own internal heat, it
appeared that these may be likely candidates for rotor components.
Through CDI, therefore, NASA funded a program to examine the physical
properties of the urethanes both experimentally and analytically and to de-
termine if full scale rotor blades could be made using this technique. The
program involved not only physical testing of specimens, but also the
fabrication of a model airfoil and a preliminary study of a full scale blade
design to determine the practicability of the approach. The majority of
the work was performed at the University of Akron and the Goodyear Tire
& Rubber Company Research Laboratories.
MATERIAL EVALUATION
Urethanes are relatively new materials and, therefore, very little is
known regarding their physical properties and structural behavior as a
construction material. Two specific types of urethanes, microcellular
foam and rigid foam, were selected for evaluation on this program. The
properties examined were: density, tensile strength, compressive
strength, shear strength, bearing strength, impact strength, creep re-
sistance, fatigue strength, temperature effects and Poisson's ratio.
Typical other behaviors examined were: hysteresis, modulus of elastici-
ty, repeatability, experimental and analytical similarity, etc.
Using specimens 3/4" x 3/4" x 30" and three and four point loading
systems, a large number of tests were conducted and data collected
on load-deflection properties. The data proved to be very linear and
creep was not significant for the most interesting formulations. Table
4 summarizes some of the properties obtained. Temperature effects
were also briefly examined from 20°F to 150°F and appear to be within
practical limits.
Fatigue tests were also conducted on tensile specimens at stress levels
of Z200 psi at 5 and 10 cycles per second and went 2,000,000 cycles
before being taken off the test machine. Strain gages tests were also
conducted and they verified values of E of about 450,000 psi were
achieved.
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The test program indicated that the urethanes were a good structural
material which behaved in a predictable engineering fashion and could be
expected to perform as analytically shown similar to standard materials.
ROTOR DESIGN
This portion of the program consisted of two major areas: I) The design
of a test section to permit the casting of a urethane blade component
which would demonstrate the structural capabilities of the material in
a typical rotor blade configuration; and 2) the preliminary design of a
full scale rotor blade that would permit the determination of a reason-
able weight estimate.
For the purpose of this program a test section 7' long with a chord of
30" was planned. With a thickness of 15%, this represented a section
of the 125' Mod O system at 80% of the radius. The model was made
in two halves and cemented together. Two models were made and
tested, one all urethane, and the other containing some typical reinforc-
ing bars. This was to illustrate how readily metal could be used to vary
the structural properties as needed.
The models were mounted in a cantilever fashion and tested to destruction
in bending. Before this, however, they were tested in torque and were
equipped with an accelerometer to check the natural frequency. As
expected, the unit with reinforcing rods increased the stiffness of the
test specimens. The surface was extremely smooth and did not require
additional machinery.
The blade sections actually broke at stress concentration points at the
root and indicated that the predicted structural properties were achieved.
The blade was very stiff in torsion and was only tested up to 4 ° when
20,000 in-lbs was applied.
FULL SCALE ROTOR DESIGN ESTIMATES
Using design goal structural properties supplied by NASA on full scale
preliminary designs of a 125' radius blade was made, assumAng that it
ware to be cast urethane with metal stiffeners. It turned out that the
259
stiffeners criteria was the most critical and a blade using this technique
would be slightly heavier than the present Mod 0 design to meet this
structural goal. Since no attempt was made to optimize the designs,
it is not known if the same weight could be achieved.
For casting purposes, the blade mold and fabrication costs were esti-
mated and it was determined that costs in the vicinity of $2 to $3 per
pound were not out of line, depending, of course, on the number of
blades to be built. Size was not a limit in the technique considered.
It is now necessary to build and whirl test a set of blades as well as es-
tablish more precisely all the design parameters.
DISC USSION
Q. What would be the weight of a MOD O sized blade constructed from
urethane ?
A. Preliminary estimates of blade weight vary from 2,750 to 3,300 pounds_
which is slightly more than the present blade weight. Judicious design,
however, could possibly bring it close to the same weight.
Q. Do you have data and/or plans to obtain data on fatigue properties of
your urethane foam samples?
A. Samples of urethane were cycled at repeated tensile stress levels of
2,200 psi at speeds of 5 and 10 cycles per second and reached
2,000,000 cycles before being taken out of the machine. No visible
signs of failure was presen t,
Q.
A.
Are properties you showed for an is.tropic material or did they
include the high density of the skin which would form at the mold
interface ?
The properties we showed did not include the ability to create a skin
density at the mold surface greater than the rest of the specimen.
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Research of low cost wind generator rotors

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