Transcript of "Helicopter rotor blade design process"
Helicopter Rotor Blade 1
Running Head: HELICOPTER ROTOR BLADE DESIGN PROCESS
Helicopter Rotor Blade Design Process
Mersie A. Melke
Embry-Riddle Aeronautical University
Daytona Beach, Florida
Department of Distance Learning
Instructor: James MacKay
September 30, 2008
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Rotor blades of helicopters have to possess stiffness and strength that keep structural stresses
within functional limits (Edwards, Davenport, 2004). The geometry should produce aerodynamic
forces. The aerodynamic and structural loads developed by the rotation of the blades fluctuate
throughout the rotational cycle, inducing fatigue (Edwards, Davenport, 2004). In addition,
environmental effects need consideration (Edwards, Davenport, 2004). Consequently, using an
appropriate material for helicopter rotor blades constitutes the study of variable requirements.
The design process of helicopter rotor blades regarding strength analysis and material selection is
the topic of discussion of this paper. Additionally, this paper shows historical and current rotor
blade material compositions to have a practical perspective of helicopter rotor blade design.
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Structural Loading of a Rotor Blade
The rotor blades of a helicopter are airfoils that provide aerodynamic forces when
exposed to a relative motion of air across their surface. The rotational motion of the rotor hub
initiated by the helicopter engine develops this relative motion, as well as forward, sideward and
backward flight. While developing aerodynamic lift and drag forces, structural loads occur on
the blades along their span and across their chord. In order to discuss these loads, one must first
visualise the three dimensional geometry of the blades.
Structural loads that act on rotor blades are three-dimensional. A combination of these
structural loads is also possible. Due to their length, bending moments that vary along the span of
the rotor occur on the blades. Because of these bending moments, the top section of the blades is
compressed and tension forces act on the bottom during rotation. These forces are directly
proportional to the lift generated by the blades and the radial distance from the rotor hub.
Bending forces vary during the rotational cycle of the blade. This is because when a rotor blade
advances in to an incoming flow of air and when the same blade retreats from the relative flow of
air, the lift load produced is not equal. As a result, a variation in bending forces on the blade
develops. Due to the above dynamic manifestations, the rotor blade flaps about the horizontal
plane of its rotation and cyclic loads also occur at the same time.
Another load is the force added during the feathering motion of the blade. Feathering
refers to the twisting movement of the blade about its span wise axis. Twisting of the blades by
the collective control in order to change rotor blade angle of attack is intentional. There is also a
tendency for the blade rotation to force the blade to twist to a pitch angle of zero due to rotational
inertia (Watkinson, 2004). During autorotation, a non-powered vertical decent, the rotor blade
has three aerodynamic regions, which are the driven region, the driving region, and the stall
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region. Force vectors are not equal in each region because rotational relative wind is slower at
the blade root and increases continually toward the blade tip (FAA, 2000). The above-mentioned
twisting forces affect the strength of the blade.
The rotor blades on a parked helicopter droop down along their span. A deflection of the
blades is possible by lifting the tip.. Rotating blades are not in static equilibrium and they should
accelerate towards the hub if they are to follow a circular path. This requires an inward or
centripetal force. Appendix A shows that if the blade bends upwards, the downward component
of the centripetal force will balance the lift at some coning angle, and the resultant will be a
horizontal force only allowing no further bending to takes place. Appendix A also depicts the
reaction of the blades at the rotor head. The force from the coning blade has an upward
component, which is the lift, and an outward component due to the rotation.
In hovering conditions, if the blades are properly balanced and all have the same coning
angle, the horizontal forces cancel in the rotor head and only lift results. Adjusting all of the
blades to the same angle needs a process called tracking which ensures that the collective control
applies the same pitch to each blade. Unbalanced or untracked blades result in vibration.
“Accurate blade balancing and tracking is important” (Watkinson, 2004).
Available Materials for Rotor Blade Manufacturing
From a structural point of view, a blade is essentially a continuous load-bearing member
with clearly defined principal stress directions (Edwards, Davenport, 2004). In order to bear the
types of loads described above, the materials used must carry these loads. At the same time, the
material should exhibit a resilient character that would with stand the cyclic nature of the loads.
Wood was a raw material for making blades. This turned out to be a poor choice of materials due
to two reasons. One reason was that wood had a character to ingest moisture when exposed to an
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environment laden with water vapour. Consequently, the rotor blade made of wood lost their
rotational stiffness, which induced vibration. Another reason was that after twisting, wooden
blades permanently changed their cross sectional geometry affecting aerodynamic loading
scenarios. This meant that wooden blades are not appropriate for cyclic loads. For this reason,
metals replaced wood as a possible candidate for rotor blade manufacturing.
Consider appendix B, which depicts the physical properties of the candidate materials
able to make rotor blades. Aluminum, stainless steel and titanium are limited to specific strengths
of 0.17–0.23 Mega Pascal (Mpa) and specific stiffness of around 0.025 Giga Pascal (Gpa). “The
metal with the best mechanical properties is titanium but its use is controlled because of its cost
and availability” (Edwards, Davenport, 2004). However, composite materials, for instance; glass
fibre and epoxy, aramid fibre and epoxy and carbon fibre and epoxy have specific strengths of
0.22–0.94 MPa and specific stiffness of 0.01–0.11 GPa. Another reason for choosing composites
to make rotor blades is the weight advantage available as compared to metal blades. Composite
materials also have the advantage of making variable cross sections across the span of the blade,
which will accommodate the variable aerodynamic loading discussed earlier. Thus, metals as
compared to composites are not a viable choice.
Corrosion is a problem with any metal aircraft structure especially for stressed
components such as helicopter blades. Metal blades are susceptible to stress and crevice
corrosion (Edwards, Davenport, 2004). The problem is particularly severe when operating near
the sea. Salt water is corrosive to aluminum. Therefore, stainless steel and titanium are used.
“However, these materials are difficult to process and fabricate, especially titanium, which has a
high melting point” (Edwards, Davenport, 2004). Composite materials are not prone to corrosion
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problems, although prolonged exposure to moisture demands special resins for preventive
Composites when exposed to fatigue loads do not fail immediately. This means a crack in
the matrix that makes up the composite material will lead to a loss of stiffness of the blade. This
character helps operators to notice and take preventive action for the wear and tear of blades by
appropriate inspections and maintenance procedures. “The Civil Aviation Authority (CAA) in
the United Kingdom (UK) and Federal Aviation Authority (FAA) in the United States of
America (USA) approved operating life for metal helicopter blades are around 1500–3000 hours.
The CAA or FAA approved operating life for composite blades is unlimited, although most
operators inspect their blades at regular intervals, typically after 10,000 hours” (Edwards,
Although composite materials have the aforementioned advantages over metals, they do
have disadvantages of their own. One disadvantage is that composite blades are susceptible to
erosion. Erosion, particularly in the tip region is a major problem in composite blades. However,
metals do not have such kind of eroding behaviour. As a result, in areas of the rotor blade where
erosion occurs like the leading edge, using metals as a cover for a composite blade helps in
bringing out the advantages of both materials. “Titanium is the favourite material for the inboard
region of the blade but for the more complex shaped tip region, more advanced metallic
materials are employed such as electro-deposited nickel”(Edwards, Davenport, 2004).
Material Content of Sample Helicopters Rotor Blades
In this section the topic covered, are rotor blades of contemporary and
non-contemporary helicopters and the material or materials used to make them. From this, one
would be able to identify a trend in the usage of material types by helicopter manufacturers.
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Discussion of rotor blades installed on Bell Textron Helicopters and that of Boeing helicopters
ensues, to get a practical perspective.
Bell’s first all composite rotor blade was initiated in 1972. The 214B and 214ST
helicopters installed derivatives of this design. This blade had a D spar arrangement as shown in
appendix C. This D spar arrangement had axially wound fiberglass spar caps wound around a
main attachment pin on the inboard end (Dobyns, Rousseau, Minguet, 2000). As a result, the
fiberglass functioned as a load path for primary loads on the blade such as bending.
Aerodynamic contours of these blades were the same as the ones for the preceding metal blades
with part number M214A. Consequently, in 1975, full use of the design flexibility of composite
blades employed, developed a blade with non-linear variation in airfoil section, thickness, chord
and twist (Dobyns et al.). The M412 helicopters were the once that used this blades.
Boeing started using composite rotor blades in the mid 1960’s. The rotor blades on the
tandem helicopters CH-46 and CH-47 and their later variants were the initial composite material
applications by Boeing. These blades mostly employed fibreglass with epoxy cured at 250
degree Fahrenheit (Dobyns et al.). The main load bearing structure is a “D” spar similar to the
one shown in appendix C that transitions into a pin wrap type design at the root end. The trailing
edge is a minimum core over a nomex core. The Boeing-Sikorsky built RAH-66 which is a
reconnaissance and attack helicopter employs a rotor blade with a carbon and epoxy tube spar
with leading and trailing edges formed from honeycomb with fibreglass face sheets for the outer
surface (Dobyns et al.).
Rotor Blade Design Considerations
Until now, the topic was loads acting on rotor blades. In addition, examples of practical
rotor blades installed on helicopters elaborated on the material of choice for rotor blade
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manufacturing. As a trend, one can see the use of composite materials on rotor blades as
becoming functionally acceptable. Manufacturing advantages of composite materials over metals
is one of the reasons for choosing composite materials over metals. The life of the blades also
has important implications on operating cost and must be maximised to ensure economic
viability. The dominant effect on the life of blades results from fatigue but in-service conditions
such as corrosion and erosion also have damaging secondary effects. Improvements in these
areas lead to increased reliability and reduced maintenance. However, in order to understand the
design process of rotor blades, one must integrate load analysis of both static and cyclic loads,
material choice based on the preceding load analysis and aerodynamic performance analysis of a
rotor blade. The following paragraphs will outline design function requirements of rotor blades.
“A combination of high specific strength and controllable specific stiffness is desirable in
main rotor blade” (Edwards, Davenport, 2004). This is necessary in order to separate the forced
vibration of the rotating blades from the natural frequencies in the airframe. A summed
frequency of the rotor blade vibration and the natural frequency of the airframe will cause
instability and as a result uncontrollability of the helicopter. In addition, as a supplement to the
stiffness consideration of rotor blades appropriate vibration dampers and isolators will be part of
the rotor head design. Because of the cyclic loads experienced by the blades, fatigue strength
coupled with enhanced strain properties is desirable in a rotor blade material (Edwards,
Operating conditions of the helicopter is another design consideration of a rotor blade.
Whether it’s a dusty environment or an icy one, the rotor blade should be able to withstand and
develop the necessary lift force for the type of maneuver done with in the performance limit of
the helicopter. The blades have to be resistant to foreign body impact. The tip of the blade, which
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is at the maximum radius, travels at the maximum achievable rotational speed or angular speed.
Because of particles in the air, the tip is prone to abrasion. The erosion effect is particularly
intense at take off and landing when the down draft from the rotor picks up loose materials from
the ground (Edwards, Davenport, 2004). Abrasive resistant materials are therefore necessary for
all the leading edges of the blades.
Another environment related design consideration should be the ability to conduct
lightning strikes with out structural damages. Rotor blades should be fitted with electrical
conductors along their span to properly discharge sudden build-up of electrical power through
them. De-icing of blades is a design consideration for helicopters operating in icy weather.
Heating or de-icing of leading edge of the rotor blades prevents the formation of ice and as a
result damage of helicopter structure and engine by ice thrown off the blades.
Still another design consideration for rotor blades is the cost. “The main requirement is to
achieve a functional design at minimum cost” (Edwards, Davenport, 2004). In order to
comprehend the cost involved, considering the cost of ownership is essential. The term cost of
ownership encompasses the engineering hours that go in to the design of the blade, the
manufacturing hours cost, the raw material purchasing cost, the machinery lease for
manufacturing cost and the maintenance necessary to keep the blade in functional condition
when the blades are working. In order to optimise the cost of ownership rotor blade designers
and manufacturers must ensure the decrease of initial acquisition costs by mechanizing or
automating the production process (Edwards, Davenport, 2004).
Rotor blade design is an interdisciplinary function, which involves the compromise and
consideration of the variables that build the design process. While planning for the production of
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the blades the factors discussed above, need consideration. These factors could supplement each
other and may contradict each other. For instance to design and manufacture a rotor blade with
10,000 flight hours life time, it would require compromising between two factors which are the
acquisition cost of the raw material to be used and the intended life time of the blade. The
conciliation of these factors will always be with in the boundaries of the regulations dictated by
governing civil aviation authorities like the FAA.
In addition, rotor blade design is also an evolving area of research. Blades installed on
helicopters are also data source to improve the efficiency, environmental effects and durability of
the blades. The tapered tip rotor blade shown in appendix D minimizes noise, while the same is
true of the British Engineered Rotor Planform (BERP) tip (Conlisk, 2001). These kinds of rotor
blade design are products of research and development processes, which involve repetitive
prototype advancement tested both in the laboratory and on actual helicopters. The governing
factor in this particular scenario is the availability of the market that would absorb and raise
profit for the designers of the blade.
In summary, one can say that in order to develop a rotor blade design process the analysis
of loads and stresses alone is not adequate. Similarly, separate analysis of cost of raw materials,
manufacturing processes, market study or environmental effects will not suffice. A simultaneous
analysis of all the variables mentioned above is a necessity. Compromise of design factors within
the governing civil aviation regulations is also required. Therefore, simultaneous analysis of
relevant factors and compromise within governing regulations constitute the appropriate tools to
develop a rotor blade design process.
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Conlisk, A.T. (2001). Modern helicopter rotor aerodynamics. Progress in aerospace sciences, 37,
419-476. Retrieved September 15, 2008, from
Dobyns, A., Rousseau, C.Q., Minguet, P. (2000). Comprehensive composite materials.
Helicopters application and design, 6, 223-242. Retrieved September 15, 2008, from
Edwards, K.L. and Davenport, C. (2006). Materials for rotationally dynamic components:
rationale for higher performance rotor-blade design. Materials and Design, 27, 31-35.
Retrieved September 11, 2008, from
Federal Aviation Administration (2000). Rotorcraft Flying Handbook. (FAA-H-8083-21)
Washington, DC: Author.
Watkinson, J. (2004). The Art of the Helicopter. Burlington MA: Elsevier Butterworth-
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Inward acceleration of the blades makes them rotate. At (a) the blades cone upwards until
the resultant of the lift and the blade tension is perfectly horizontal. At (b), depicted is the force
balance at the rotor head. The tension in the upwardly coned blades cancels in the horizontal
direction leaving only a vertical component to balance the weight of the machine.