Propeller blade of a fiber-reinforced plastic material

A propeller blade of fiber-reinforced plastic with a combined erosion, icing and lightning protection system has a metal shroud fitted to its leading edge, the shroud enclosing a cavity and accommodating a heating conductor at its interior. A ground line of the heating conductor is connected to ground and serves as a lightning conductor.

BACKGROUND AND SUMMARY OF THE INVENTION 
This present invention generally relates to a propeller blade constructed 
from a plastic material such as a fiber-reinforced plastic material, and 
more particularly to an arrangement for protecting the structural 
integrity of a plastic propeller blade as well as indicating the 
occurrence of foreign object damage (FOD) to a leading edge of the 
propeller. 
Fan blades of turbo engines and propellers have previously been 
manufactured from composite materials including fiber-reinforced plastic 
materials. These materials give great strength and low weight, but are 
detrimentally effected by a high sensitivity to rain and particle erosion 
(e.g. insects). Further, such composite fan blades and propellers are 
easily damaged by impinging large-size foreign bodies, such as chunks of 
ice, rocks, birds, etc. (FOD). 
The above-noted drawbacks apply particularly to the slim, sharp leading 
edges of such composite-material propellers or fan blades, all the more so 
with increasing relative velocity between the propeller and the impinging 
particle. At the same time, the need to maintain high aerodynamic 
efficiency does not permit any changes in profile of the leading edge. 
A known practice to protect, e.g. wooden propellers of composite-material 
helicopter rotor blades, has been to provide them with a metal coating or 
cladding on the leading edge. However, while FOD to translucent and 
impact-resistant fiber materials, such as glass fiber reinforced plastic 
materials and aramide fiber-reinforced plastic materials, is indicated by 
the resultant surface delamination, even the most severe internal FOD 
often remains invisible in carbon fiber-reinforced materials and therefore 
are quite difficult to detect. However, in the known practice of 
protecting propeller blades with a metal coating or cladding, no provision 
is made for indicating severe FOD to the structure of the blade, i.e. give 
early warning without sacrificing the protective functions. 
Icing risk is another problem afflicting propellers. Ice buildup on the 
blades greatly impairs aerodynamic propulsion, and the high-velocity ice 
particles hurled off the propeller additionally jeopardize the airframe 
structure. Also, considerable engine imbalances may result. 
Several known deicing methods exist, such as by pulsating inflatable rubber 
air cells (Goodrich anti-ice boot), by chemical deicing through alcohol or 
glycol spray (see FAA Advisory Circular 20-117), by hot bleed air from the 
engine , or by electrical resistance heating conductors fitted to the 
airfoil. 
Another problem with plastic propeller blades is posed by the fact that 
such propellers or profan rotors are among the structures most probably 
struck by lightning. Carbon fiber-reinforced plastic materials are 
especially sensitive to lightning damage owing to their partial electrical 
conductivity in that the fiber is conductive while the plastic matrix is 
insulating. 
Therefore, it is an object of the present invention to provide a propeller 
blade constructed from a composite material with protection from erosion, 
FOD and icing in combination with lightning protection in a complemental, 
redundant arrangement. 
It is a further object of the present invention to provide a propeller 
blade constructed from a composite material with a metal shroud at the 
leading edge of the propeller which is spaced from the plastic airfoil of 
the blade by a cavity having electrical conductors. 
The combination of features provided in accordance with certain preferred 
embodiments of the present invention provides a number of advantages. For 
example, the metal shroud spaced apart from the plastic airfoil of the 
propeller blade proper protects the airfoil from the erosive effects of 
small particles or rain drops. When a large-size body impinges, as perhaps 
a bird, the metal shroud is dented to absorb sufficient impact energy to 
prevent damage to the plastic profile behind it. The deformation is 
recognized also at surface inspections, enabling the damaged blade or 
metal shroud to be replaced. Deformation of the metal shroud prevents the 
blade from being weakened structurally; it merely impairs the aerodynamic 
flow around the blade at this point. Severe deformation of the metal 
shroud suggests possible internal FOD to the plastic propeller and 
therefore further inspection of the blade. 
The cavity provided between the metal shroud and the plastic airfoil can 
advantageously be utilized for ducting hot deicing air, which is diverted 
from the compressor or consists of fresh air routed through an exhaust gas 
heat exchanger, so that icing of the critical airfoil leading edge is 
prevented. Provision for blowing off the air from the propeller blade can 
be made at the joints of the metal shroud or at the blade tip. 
Also providing deicing features is the advantageous provision of an 
electrical heating conductor connected to the metal shroud to conduct 
heat. This serves to effectively heat the metal shroud at a stagnation 
point area, which is especially jeopardized by icing, while simultaneously 
protecting the temperature-sensitive plastic material, the air space 
between the metal shroud or heating conductor and the plastic airfoil 
serving to provide thermal insulation. 
Additionally, the metal shroud advantageously discharges current when 
lightning hits the plastic propeller blade, so that the flash will not 
stress current-sensitive composite fiber materials. Serving to discharge 
the lightning current is also the grounded ground return of the electrical 
heating conductor. 
In a further advantageous feature of preferred embodiments of the present 
invention, a second metallic inner shroud is conformally fitted to the 
plastic propeller blade surface and is connected to the outer metal 
shroud. This inner shroud is preferably arranged near a hub area of the 
propeller blade and gives additional protection of the plastic airfoil in 
the event of FOD and permits improved fixation of the outer metal shroud. 
Preferably, the inner shroud is welded to the outer metal shroud and 
bonded to the plastic airfoil. The weld joint is preferably roller seam 
welded or alternatively spot welded. 
In an advantageous embodiment of the present invention, the metal shroud 
has a number of sections arranged along the direction of blade span. This 
helps to offset thermal expansion of the metal shroud and reduces the risk 
of fatigue fracture as a result of different moduli of elasticity of metal 
and plastic, respectively. This also enables the replacement of individual 
sections upon damage. 
Preferred materials for the metal shroud are titanium, titanium alloys, 
INVAR steel and maraging steel alloys. 
In a preferred embodiment of the spanwise edge of the metal shroud and the 
plastic airfoil, the adhesion of the edges under centrifugal load is 
improved by giving them a zig-zag or circular arc shape. 
In an advantageous feature of preferred embodiments of the present 
invention, the heating conductor takes the shape of a coaxial conductor 
having an inner conductor, an insulating layer enveloping it, and a 
metallic shroud tube. This permits the heating conductor to be 
advantageously connected to the metal shroud by brazing or other process. 
The shroud tube also serves to increase the metallic cross-sectional area 
and so reduces the conductor resistance to improve the current discharge 
when lightning strikes. 
In a preferred embodiment of the present invention, the heating conductor 
is brazed to the metal shroud in sections, and at intermediate sections is 
curved inwards to form expansion bends. This serves to reduce mechanical 
and thermal stresses caused in the heating conductor by differences in 
temperature and thermal expansion between metal and plastic. In one 
embodiment of the present invention the heating conductor is attached by 
regularly spaced heat conduction lugs, which improves the dissipation of 
heat to the metal shroud and advantageously stiffens the shroud. 
Deicing/heating of the radially outer propeller areas is not invariably 
required, because the high peripheral speed prevents icing. In a further 
feature of preferred embodiments of the present invention, therefore, the 
heating conductor is provided only along a portion of blade span and is 
eliminated wherever continuous heating is difficult for lack of space in 
the sharp-edged outer zone of the propeller airfoil or where the heating 
conductor is not durable enough to safely withstand the increased flexural 
vibration in the upper blade section. In an advantageous embodiment, the 
heating conductor is then looped with forward and return lines running 
parallel to produce uniform heating of the metal shroud. 
In a further feature of the present invention, the radially outer tip of 
the plastic propeller blade is clad with a metallic protective cap which 
electrically conductively connects to the metal shroud and the shroud tube 
of the heating conductor. This cap forms a flash entry or exit point, 
which is especially threatened by destruction through lightning, and it 
additionally protects the blade tip of the plastic propeller from 
mechanical damage. The conductor area of the lightning protection 
provision is preferably increased by installing one or several metallic 
conductor strips to run the length of the blade from the protective cap to 
the propeller hub and grounding them. In a preferred embodiment, 
electrically insulated flat metal conductors are formed, bonded and 
embedded into the pressure side of the airfoil below an erosion protection 
skin extending over the entire propeller blade. 
Given suitable design and selection of material these lightning conductors 
are a little less durable than the plastic propeller blade. This provides 
a substantial advantage, considering that fatigue failure of the lightning 
conductors can be detected by electrical resistance test between the blade 
tip and the hub to give a simple maintenance criterion for further 
airworthiness checks on the propeller system. 
The advantages provided by the propeller blade protection system of 
preferred embodiments of the present invention, therefore, include its 
deicing capability and simultaneously, its redundancy with respect to 
multiple lightning strikes, its suitability for testing by simple visual 
inspection and simple electrical resistance check, and the ease with which 
damaged individual components can be replaced. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
In FIG. 1, a cross-sectional view of the leading edge of a propeller 1 
taken near the propeller hub shows a U-section metal shroud 2 fitted to a 
plastic airfoil 13 to form an aerodynamic wing contour. The metal shroud 2 
is connected to the plastic airfoil 13 through a number of rivets 14 
spread over the entire extent of the metal shroud 2 and the propeller 
airfoil 1, respectively. The leading edge of the plastic airfoil 13 is 
surrounded by an inner shroud 6 bonded to the plastic airfoil 13 and 
attached to the outer metal shroud 2 by spot, roller seam or other welding 
methods. Attached to the inner wall of the metal shroud 2 preferably by 
brazing is also a heating conductor 5 to provide a thermally conductive 
connection. 
In FIG. 2, the same propeller blade 1 as in FIG. 1 is shown in 
cross-sectional view near the blade tip, where the profile exhibits a 
substantially sharper edge. Unlike the section near the hub (FIG. 1), this 
area has no inner shroud. Also, the metal shroud 2 is bonded to the 
plastic airfoil 13. The heating conductor 5 is attached to the two inner 
surfaces of the metal shroud 2 facing one another in V-arrangement. 
In FIG. 3, an oblique view of the leading edge of propeller blade 1 in 
schematic arrangement shows how the metal shroud 2 around the forward 
portion of the plastic airfoil 13 is attached by rivets 14. The metal 
shroud 2 shown in FIG. 3 has a number of partial sections composed at 
parting joints 15. The heating conductor 5 has an inner conductor 7, an 
insulating layer 8 surrounding it, and a coaxial shroud tube 9 brazed to 
the metal shroud 2. In the area of the separating joint 15, the heating 
conductor 5 exhibits expansion bends 11, so that at these points the 
heating conductor 5 is not connected to the metal shroud 2 but is bent 
into the interior of the cavity 3. According to one embodiment, the 
joining edge 17 is bow-shaped. 
FIG. 4 illustrates an alternative or additional embodiment for attaching 
the heating conductor 5 to the metal shroud 2 by way of thermally 
conductive lugs 12 preferably arranged in the near-hub area of the metal 
shroud 2. 
FIG. 5 illustrates an embodiment featuring a looped heating conductor 5 to 
heat the blade leading edge in the area of the blade root 10 only. 
The metal shroud 2, which is heated by the heating conductor 5, can 
additionally be heated also by hot air injected into the roughly indicated 
cavity 3 through a hot air duct 16 in the interior of the propeller blade 
1, the duct being shown in schematic arrangement in FIG. 5. 
In FIG. 6 the propeller blade 1 is illustrated in a perspective view and 
has a sectional metal shroud extending along the length of the leading 
edge of the propeller, the blade tip 18 being covered by a metallic 
protective cap 19. The cap 19 is connected to the heating conductor 5 at 
the interior of the metal shroud 2 and to a number of lightning conductors 
20 routed along the pressure side 21 of the airfoil below the outer skin. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only, and is not to be taken by way of 
limitation. The spirit and scope of the present invention are to be 
limited only by the terms of the appended claims.