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Propulsion and propellers

Y. Hong, ... R.G. Wang, in Marine Composites, 2019

13.2 The characteristics of composite propeller

13.2.1 The structural characteristic

The propeller is mounted on the tail of the hull and consists of a few blades and a hub. The hub is a truncated cone that is connected to the tail shaft to transfer the load. The propeller blade is a typical curved surface part with a helical surface shape, which is determined by the value of a series of cylindrical sections. All blades are fixed and distributed around the hub, forming overlapping relationships in space. In general, a fairing (that is, a hub cap) is mounted on the rear end of the hub, which forms a streamlined body with the hub to reduce water resistance. The side of the propeller blade seen from the rear of the ship is called the pressure surface, and the other side is called the suction surface. When the propeller is rotating, the edge of the blade first entered the water is called the leading edge, and the other side is called the trailing edge, as shown in Fig. 13.1.

Fig. 13.1. The component names of propeller.

For the traditional metal propeller, the structural characteristics of the propeller are only affected by the material properties, the geometrical characteristics of the blade, and the connection type of the blade and the hub. The geometrical characteristics are determined by the geometrical parameters of the blade, such as diameter, disk ratio, pitch, rake, skew, chord length, etc., and the connection type depending on the design is the fixed pitch propeller or the variable pitch propeller. However, the structural characteristics of the composite propeller are very different from those of the metal propeller.

For the composite marine propeller, the rich material and flexible molding provide a good foundation for the diversification of composite propeller structures. The diversified design is mainly reflected on the following aspects: (a) From the point of view of selecting materials, the blades can be prepared from the carbon fiber-reinforced composites with good comprehensive performance, or the blades also can be prepared by selecting many different types of the fiber-reinforced composite according to the design requirements. (b) From the point of view of designing the internal structure, the propeller blade can be designed as the laminated composite structure (Fig. 13.2) (Lin et al., 2009), or it can also be designed as the sandwich composite structure; (c) from the point of view of determining the connection type, the blade and the hub can be integral molding or can be separately molded and then assembled (Fig. 13.3). When the blade and the hub are molded separately, the composite blade and the metal hub may be used, or the composite material blade and the composite material hub may be used.

Fig. 13.2. The laminate molding of composite propeller blade.

Fig. 13.3. The assembled composite propeller.

13.2.2 The working characteristic

In general, we can regard the propeller blade as a part of the wing at a certain angle of attack and the inflow velocity. As shown in Fig. 13.4, it has a small angle of attack between the wing and the flow direction. At that time, the fluid velocity above the airfoil is greater than that below the airfoil. The cross section a–b is greater than the cross section a′–b′, the pressure P2 above the wing is less than the static pressure P1, the cross section b–c is less than the cross section b″–c′, the pressure P3 below the wing is greater than P1, so it forms a pressure difference between the upper and lower surfaces of the wing. This pressure difference, together with the frictional force generated when the fluid flows through the wing, synthesizes a total fluid force R. Divide the total fluid force R into two components: a component X parallel to the direction of fluid flow, to prevent the forward movement of the wing, which is called resistance; another component Y perpendicular to the direction of fluid flow, called lift, which forms the propeller thrust.

Fig. 13.4. The working principle of the propeller.

After understanding the working principle of the propeller, we further discuss the force and torque acting on the propeller blades. It should be noted that the propeller generates the thrust by fluctuating water, due to the relationship between the force and the reaction force, the flow is also affected by the propeller to obtain an additional velocity (commonly referred to as the axial-induced velocity) that is opposite to the thrust direction. At the same time, the circumferential induction velocity is also obtained in the rotation direction of the propeller. Therefore, the flow around the propeller should be discussed, in addition to considering the forward and rotation speeds of the propeller, but also the axial induction speed and circumferential induction speed should also be considered.

Fig. 13.5 shows the fluid velocity polygon of the blade element at any radius. It can be seen that the relative velocity acting on the blade element is the result of the synthesis of undisturbed flow velocity Vp, circumferential velocity u, axial-induced velocity cu1, and circumferential-induced velocity cu2. The synthetic speed acts on the blade element with a certain angle of attack. According to the working principle of the propeller, we can see that the fluid force will be generated on the blade element, that is, the lifting force dY and the resistance dX.

Fig. 13.5. The speed polygon of blade element.

Fig. 13.6 shows the force polygon of the blade element at any radius. According to the force polygon, the thrust dP(dP = dYT − dXT = dY cos β − dX sin β) and rotation resistance dQ(dQ = dYQ + dXQ = dY sin β + dX cos β) of the blade element can be obtained. Integrating from the hub to the blade tip in the radial direction, and multiplied by the number of blades, the total thrust and torque of propeller can be obtained.

Fig. 13.6. The force polygon of the blade element.

As far as the composite propeller be concerned, the principle of hydrodynamic action is the same as that of metal propellers. In the same inflow case, the initial thrusts and torques generated by the composite and metal propellers are the same exactly, but because the composite material has good elasticity. The deformation of the blade element at any radius is larger, as shown in Fig. 13.7, resulting in the effective angle of attack decreasing and the hydrodynamic characteristics of composite propeller also changing.

Fig. 13.7. Comparing the force polygon of the composite and metal blade element at any radius.

13.2.3 The difference between the composite and metal propellers

Based on the above analysis, the great difference in the structure and working characteristics is exhibited between the composite propeller and the metal propeller, which leads to the need to consider more details in the design and analysis methods of composite propeller and provides a good opportunity to improve the performance of the propeller.

In general, metal propellers are designed primarily around their functional requirements (hydrodynamic, cavitation, noise). By changing the profile of the propeller blades and controlling the inflow around the propeller, the hydrodynamic design requirements under the design conditions are achieved, while meeting the design objectives of high efficiency and low noise. However, metal propellers are often limited by the nature of the metal material. In order to meet the requirements of propulsion efficiency at the design condition, metal propellers often have to reduce the propulsion efficiency at other operating speed. At the same time, because of the larger density and the poor damping performance of metal, the performance of the metal propeller to further improve is also restricted to a large extent.

In the case of composite propellers, the nature of the composites determines the research difference between the composite propeller and the metal propeller, and it also shows the potential advantages of the composite propeller:

(1)

Compared with the traditional metal propeller, the deformation of the composite propeller blades is much larger, so the inflow angle on the blade element at any time will change greatly, which causes the thrust of the blade to change greatly. This situation shows that the composite propeller has a great hydro-elasticity, so the design of composite propeller must take into account the effect of fluid-solid coupling.

(2)

The hydro-elasticity can be effectively utilized in the design of composite propeller, to achieve the maximum operating efficiency of the propeller in multiple working conditions.

(3)

Considering the structural complexity of the composite propeller, the strength of the composite propeller must be analyzed and evaluated.

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Smart composite propeller for marine applications

H.N. Das, S. Kapuria, in Marine Composites, 2019

9.2.2 Domain and mesh for flow analysis

Four different propellers of the same diameter are analyzed here. As the size of the propeller is same, similar mesh and domains are used for all the cases. A suitable domain size is considered around the propeller to simulate ambient condition. A circular cylindrical domain of diameter ~ 4D and length of ~ 7D is used for flow solution. A multi-block structured grid is generated for the full domain using ICEM CFD Hexa module. Around 1.2 million hexahedral cells are deployed to descritize the domain. Necessary boundary conditions are set. For the present solution, standard scheme is used for pressure and a SIMPLE (Semi-Implicit Pressure Link Equations) procedure is used for linking pressure field to the continuity equation.

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Hybrid wood composites – integration of wood with other engineering materials

M.P. Ansell, in Wood Composites, 2015

16.3 Rotor blades

The blades for early propeller-driven aircraft (e.g. aerofoil blades for the Wright biplane) and helicopters (e.g. main rotor and tail rotor for Sikorsky YR-5A) were manufactured from solid wood or laminated wood. During First World War mahogany was the preferred timber for aircraft propellers but walnut, oak, cherry and ash were also employed. Wooden fan blades have also been used extensively in large wind tunnels (Young et al., 1991) comprised of Sitka spruce and compressed birch veneer composite (Hydulignum) at the hub (butt) end of the blade. Laminated Douglas fir/epoxy blades were a later development. Young et al. present fatigue data for Sitka spruce in the form of S-N curves and constant life diagrams. The fatigue properties of wood composites for turbine blades are also reviewed by Ansell (2003). In 2010, the wind tunnel at the General Motors Aerodynamics Laboratory in Warren Michigan was thirty years old and during its life it has been used to measure drag in motor vehicles and the aerodynamics of skiing and the performance of yachts. The wind tunnel (Figure 16.7) is the world's largest with a diameter of 13.1 m and the six blades (Figure 16.8) are laminated from spruce.

Figure 16.7. General Motors Aerodynamics Laboratory © General Motors.

Figure 16.8. Fan with six Sitka spruce blades at the General Motors Aerodynamics Laboratory © General Motors.

Hoffmann Propeller (2015) variable pitch propellers are currently manufactured for use in hovercraft (e.g. Griffon Hoverwork, Southampton), light aircraft and airships. In the same manner as some wind tunnel blades, the hub end of the blade is made of densified hardwood and the rest of the blade utilises spruce lightwood.

The blade is clad in fibre-reinforced epoxy to provide torsional stiffness. To protect the wood from erosion and the marine environment, the composite is coated with several layers of polyurethane lacquer.

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Anodization of magnesium (Mg) alloys to improve corrosion resistance

S.A. Salman, M. Okido, in Corrosion Prevention of Magnesium Alloys, 2013

8.6.2 Aerospace industry

Magnesium fuel tanks, propellers and engine crank-cases were used in German aircraft in the early 1930s. The trend continued through the 1940s, when the B-47 jet bomber had a total of 5.5 tonnes of magnesium sheets, extrusions and casting. Magnesium was intensively used as a skin material on the wings and tails, jet engine pods, cowlings, gun enclosures and doors. At present, commercial aircraft have very few magnesium parts. Helicopters contain some cast components, such as gearboxes, seats and pedals. There are cast and wrought magnesium parts in satellites. The military industry uses much more magnesium, including in wheels, radar antennae, radar bases, covers and missile elements (Czerwinski, 2008). The DOW 17 and HAE anodizing coating do not confer the required protection properties (Manoj and Nai, 2010). By contrast, the Tagnite coating provides superior corrosion resistance for magnesium sand castings, die-castings, extrusions and forgings, and has therefore been used on a production basis in the aerospace industry for more than a decade. Many applications require paint and Tagnite is an excellent base for all kinds of paint. In the aerospace industry, the coating is used for gearboxes, transmission cases and covers, and sump and oil pump housings (Tagnite, 2011). Keronite treatment is now approved by FAA, US Army and US NavAir as a surface pre-treatment for housings. A durable coating with long service intervals could be obtained, after combining with a class-leading topcoat system (Keronite, 2011).

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Separations and Analysis

J.W. Canary, ... S. Mortezaei, in Comprehensive Chirality, 2012

8.30.2.2.2 Metal-free propeller sensors for ditoptic organoammonium derivatives

Suzuki et al. fabricated metal-free molecular propellers that are capable of supramolecular chirality sensing (Figure 6).26 Each of the secondary terephthalamide host 16a-H with four aryl blades undergoes a conformational change from a nonpropeller anti-form to a propeller-shaped syn-form upon complexation with ditopic guests such as p-xylylenediammonium derivatives (R,R)/(S,S)-17 (chirality generation). Through the transmission of the point chirality attached on the nitrogens in the chiral guests to the mobile helicity in 16a-H, the propeller-shaped host in the complex is biased to prefer a particular handedness (chirality enhancement) (Figure 6(A)). Although chiral guests with simple point chirality such as (R,R)/(S,S)-17 exhibit only very weak CD activity, complexation with the dynamic propeller host 16a-H results in much stronger chiroptical signals (chiroptical enhancement). The chirality generation and biasing protocol was successfully applied to a neurotransmitter, (−)-phenylephrine 18, acting as a chiral ditopic guest (Figure 6(B)).

Figure 6. (A) Continuous changes in the CD spectrum upon complexation of achiral host 16a-H (4.2×10−4 M) with (R,R)/(S,S)-17; (a) 1 equivalent (4.2×10−4 M), (b) 2 equivalent (8.3×10−4 M), (c) 4 equivalent (1.7×10−3 M) of (R,R)-17 (blue line), (d) 1 equivalent (4.2×10−4 M), (e) 2 equivalent (8.3×10−4 M), (f) 4 equivalent (1.7×10−3 M) of (S,S)-17 (red line), and CD spectra of (g) (R,R)-17 (black line; 2.7×10−4 M), and (h) (S,S)-17 (black line; 2.5×10−4 M): (B) Continuous changes in the CD spectrum upon complexation of 16a-H (2.8×10−4 M) with (−)-phenylephrine 18; (a) 1 equivalent (2.8×10−4 M), (b) 2 equivalent (5.5×10−4 M), (c) 4 equivalent (1.1×10−3 M) (green line), and (d) CD spectrum of 18 (black line; 1.8×10−4 M). All spectra were measured in CH2Cl2 at room temperature. Reproduced with permission from the American Chemical Society.

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Safety and precautions

Andrew J. Jefferson, ... Hom Nath Dhakal, in Repair of Polymer Composites, 2018

5.3.3 Safety around airplanes

In addition to the earlier discussed items, it is vital to be conscious of propellers. During taxing, do not consider that pilot can observe the maintenance operators. The operators should position where the pilot can notice. Moreover, jet engine inlet and outlet may be very hazardous areas. In operation, there must be no smoking anywhere close to an aircraft. During maintenance, be conscious of fluids that may be harmful to the skin. Be sure to allocate gap stuck between support equipment and the aircraft. All things in the region of maintaining aircraft should be stored back correctly.

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Case studies: failures of polymer composites due to in-service factors

In Failure Analysis and Fractography of Polymer Composites, 2009

Description of the failed component

This final case study on impact is by the author and Meeks, and is the failure of a propeller from a light aircraft.4 The pilot reported that, after a touch- and-go landing, the plane exhibited massive vibration and, subsequently, an emergency landing was performed. The two propeller blades; the left (Fig. 10.19) and the right (Fig. 10.20) were examined in detail. The blades consisted of a wooden core with a glass-fibre/epoxy woven sheaf. Both tips exhibited abrasion damage to the composite sheath with splitting and material loss to the wooden core (Fig. 10.20). This damage generally extended over the first 110mm from both tips and was consistent with the blade tips dragging or clipping the ground. However, the appearance of the right and left blades were very different; both faces of the right blade (Fig. 10.20) still had most of the composite attached whilst the lower face (Fig. 10.19a) of the left propeller blade was missing and the upper face (Fig. 10.19b) was found away from the rest of the aircraft. The polyurethan leading edge protection strip on the left blade had been greatly stretched and plastically deformed, whereas the strip on the right blade did not exhibit such damage and had torn close to the tip. Furthermore, on the left blade, adhesive tape on the leading edge had been mechanically cut half-way along the length of the blade. Between the tip and the cut along the leading edge the tape had a jagged edge, where it was apparent it had been torn. The equivalent tape on the right blade was continuous and did not exhibit any such damage. Finally, close to the tip of the left blade, there were two adjacent sites of blackened debris between the adhesive tape and the composite (Fig. 10.21).

10.19. Left blade from the failed propeller: (a) lower face with GFRP skin missing and (b) upper face with detached GFRP skin; the location of samples A and B are indicated.

(Courtesy of AAIB)

10.20. Tip of the right blade of the failed propeller (as viewed from the upper face) with the location of SEM specimen C indicated.

(Courtesy of AAIB)

10.21. Debris found between the adhesive tape and the skin on the tip of the left blade (×40).

(Courtesy of AAIB)

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Composites, Joining of

F.L. Matthews, in Encyclopedia of Materials: Science and Technology, 2001

1.2 Special Configurations

Nonstandard configurations are reserved for particular applications, e.g., wedge joints are used for securing turbine blades or propeller blades to the turbine disc or propeller hub, respectively. As the rotational speed of the device increases so does the corresponding radial force on the blade and the constraint afforded by the wedge produces an ever tightening joint. The same principle is encountered in the wedge grips of tensile testing machines.

Another special configuration, which has been used for securing helicopter rotor blades to the rotor hub, is the "race track" or "wrapped" joint. Here, continuous fibers are positioned around the hole (Fig. 2) in which the securing pin is located, the forces on the joint being such as to load the fibers in tension, an ideal uni-axial stress state.

Figure 2. "Wrapped" joint.

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Composites, Joining of

F.L. Matthews, ... K.Y. Benyounis, in Reference Module in Materials Science and Materials Engineering, 2016

2.2 Special Configurations

Nonstandard configurations are reserved for particular applications, for example, wedge joints are used for securing turbine blades or propeller blades to the turbine disk or propeller hub, respectively. As the rotational speed of the device increases so does the corresponding radial force on the blade and the constraint afforded by the wedge produces an ever tightening joint. The same principle is encountered in the wedge grips of tensile testing machines.

Another special configuration, which has been used for securing helicopter rotor blades to the rotor hub, is the 'race track' or 'wrapped' joint. Here, continuous fibers are positioned around the hole (Figure 2) in which the securing pin is located, the forces on the joint being such as to load the fibers in tension, an ideal uni-axial stress state.

Figure 2. 'Wrapped' joint.

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Structural Integrity Assessment—Examples and Case Studies

R.J.H. Wanhill, in Comprehensive Structural Integrity, 2003

1.04.2.1 Case Histories

The De Havilland Comet was the first commercial jet transport, entering service in 1952. The aircraft's performance was much superior to that of then contemporary propeller-driven transports. Apart from its speed, the Comet was the first high-altitude passenger aircraft, with a cabin pressure differential almost double that of its contemporaries (Swift, 1987).

Within two years of entering service, two of the fleet disintegrated while climbing to cruise altitude. Comet G-ALYP was lost on January 10, 1954. Modifications were made to the fleet to rectify some of the items that might have caused the accident. However, Comet G-ALYY was lost on April 8, 1954. The fleet was then grounded. Extensive investigations followed, including most importantly a full-scale repeated pressurization test on an aircraft removed from service, registration number G-ALYU.

The test aircraft had accumulated 1,231 pressurization cycles in service. It was tested in a water tank to minimize damage in the event of failure. After 1,825 test pressurizations, the pressure cabin failed during application of a proof cycle at 33% higher loading. The failure showed evidence of fatigue cracking that began at the aft lower corner of the forward escape hatch (see Figure 3). Additional investigation of wreckage from Comet G-ALYP also showed evidence of fatigue, in this case commencing from the right-hand aft corner of the rear automatic-direction-finding window (see Figure 4).

Figure 3. Probable failure origin of test aircraft Comet G-ALYU: stress distribution obtained after repair (source Swift, 1987).

Figure 4. Probable failure origin of service aircraft Comet G-ALYP: stress distribution obtained from repaired test aircraft, Comet G-ALYU (source Swift, 1987).

The test aircraft was repaired and strain gauges applied to the outside surfaces of several escape hatches and windows. Results for the service and test failure locations are also shown in Figures 3 and 4. Swift (1987) pointed out that out-of-plane bending would have caused the inside principal stress to be significantly higher, which could well have contributed to the early fatigue failures. This out-of-plane bending would not have been considered in a design analysis for the Comet, nor indeed for subsequent commercial jet aircraft (Swift, 1987). However, a full-scale test effectively accounts for it.

Swift (1987) described the Comet pressure cabin structure in more detail, in order to bring out some further important aspects of the service failures. Figure 5 shows the basic pressure shell structure and the probable origin of failure for Comet G-ALYP. The basic shell structure had no crack-stopper straps to provide continuity of the frame outer flanges across the stringer cutouts. The cutouts, one of which is shown in Figure 5(b), created a very high stress concentration at the first fastener. In the case of the probable origin of failure for Comet G-ALYP, the first fastener was a countersunk bolt, as shown in Figure 5(c). The countersink created a knife-edge in both the skin and outside doubler. The early fatigue failure may thus be attributed to high local stresses (Figure 4), combined with the stress concentrations provided by the frame cutout and knife-edge condition of the first fastener hole (Figures 5(b) and (c)).

Figure 5. Details of the probable failure origin of service aircraft Comet G-ALYP (source Swift, 1987).

Once the fatigue crack initiated in Comet G-ALYP, its growth went undetected until catastrophic failure of the pressure cabin. Obviously, this should not have happened, but Swift (1987) provided an explanation from subsequent knowledge. He showed that the basic shell structure of the Comet could have sustained large, and easily detectable, one- and two-bay cracks if they had grown along a line midway between the positions of the frame cutouts. In other words, the basic shell structure would have had adequate residual strength for these crack configurations. However, neither one- nor two-bay cracks would be tolerable if they grew along the line between frame cutouts. For these cases, crack-stopper straps would have been needed to provide adequate residual strength.

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