How do you make a GE90 fan blade
I doubt the advances would be able to overcome the basic fundamentals behind the cause of rotor lock though in the particular operating environment (Mach 3+). Rotor, or core, lock is an issue even today, though rare.
https://www.ntsb.gov/investigations/...ts/AAR0701.pdf
https://flightsafety.org/wp-content/...l06_p44-49.pdf
Information gathered by NTSB during public hearings on the CRJ200 accident indicates that core lock has occurred in engines other than the CF34; however, the engine types were not specified in the public docket.
https://flightsafety.org/wp-content/...l06_p44-49.pdf
Last edited by megan; 15th Jul 2017 at 04:57.
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Here is an article that gives some insight into the composite outer fan casings GE Aviation is using in their newest engines and the key reasons for using the technology. The fan blade tips do not rub on the composite fan casing, there remains an abradable shroud ring just as there is on earlier engines where the fan casings were metal and the fan blades composites.
https://spinoff.nasa.gov/Spinoff2006/T_1.html
Also, core lock doesn't involve the fan of the engine. For the most part, core lock at high altitudes can be avoided by maintaining a speed above a designated minimum speed and not allowing the engine core to stop rotating. In the classic Pinnacle Airline's Canadair CL-600-2819 accident, examination of the engines did not exhibit evidence of core lock as originally believed. As I recall, the aircraft speed at altitude dropped below the designated minimum speed resulting in an unsuccessful relight in the time and altitude remaining. Today in commercial engines, there is a great deal of sophistication in clearance control systems to prevent uneven cooling of turbine components, the classical core lock cause.
https://spinoff.nasa.gov/Spinoff2006/T_1.html
Also, core lock doesn't involve the fan of the engine. For the most part, core lock at high altitudes can be avoided by maintaining a speed above a designated minimum speed and not allowing the engine core to stop rotating. In the classic Pinnacle Airline's Canadair CL-600-2819 accident, examination of the engines did not exhibit evidence of core lock as originally believed. As I recall, the aircraft speed at altitude dropped below the designated minimum speed resulting in an unsuccessful relight in the time and altitude remaining. Today in commercial engines, there is a great deal of sophistication in clearance control systems to prevent uneven cooling of turbine components, the classical core lock cause.
I doubt the advances would be able to overcome the basic fundamentals behind the cause of rotor lock though in the particular operating environment (Mach 3+). Rotor, or core, lock is an issue even today, though rare
As long as thermal shock is avoided, the design of the engine assures that blades, discs, and cases expand/contract at the same rate and proper clearances are maintained. Accel/decal rates - especially at altitude - are carefully tailored to avoid thermal shock. It's also why there are mandatory warm up and cool down requirements for starting and shutdown. On of the worst things you can do to a modern turbine engine is to shut it down from high power...
Some are better at this than others - Pratt and Whitney really lagged in this regard (resulting in a rather expensive redesign and retrofit of the PW4000/94" HP compressor about 15 years ago).
I suspect the "Blackbird" rotor lock was due to it's dual role Jet engine and ram-jet.
At very high altitudes you wanted lots of bleed air bypassing the burner-turbine so as to ram the afterburner and let that push you along. The bleed air in this case would have been very cold and washed over the outside of the turbine cases.
Eventually you need the gas generator as you slowed down to land. Without it you would have to pick a suitable airfield somewhere in your glide path
At very high altitudes you wanted lots of bleed air bypassing the burner-turbine so as to ram the afterburner and let that push you along. The bleed air in this case would have been very cold and washed over the outside of the turbine cases.
Eventually you need the gas generator as you slowed down to land. Without it you would have to pick a suitable airfield somewhere in your glide path
The "bleed air" from the six High pressure Compressor ducts was injected directly into the exhaust ahead of the afterburner, where it accumulated at the rim of the case, cooling the exhaust(and the engine case)..this bleed was not cold, but was hotter than the external passive bleed.
I think it also served to "attenuate" the ram effect, preventing the "choke" that is a prerequisite of a ram engine? It worked contra the ram effect. It was this choke that prevented triple sonic. The bleeds were a novel approach, they really made the J58 the phenomenal design it turned out to be....
Oh, rotor lock. This was determined to be the (non engine related) cause of rudder hard over, no? The actuator cylinder cooled faster than the piston, and locked the rudder in place?
I think it also served to "attenuate" the ram effect, preventing the "choke" that is a prerequisite of a ram engine? It worked contra the ram effect. It was this choke that prevented triple sonic. The bleeds were a novel approach, they really made the J58 the phenomenal design it turned out to be....
Oh, rotor lock. This was determined to be the (non engine related) cause of rudder hard over, no? The actuator cylinder cooled faster than the piston, and locked the rudder in place?
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Below is the patent P&W was granted for the J58 engine design which was used.
https://www.google.com/patents/US3344606 - Robert B Abernethy
In the patent, the objective of the pipes taking air off the compressor was to improve surge margin of the compressor due to ram air temperature rise:
https://www.google.com/patents/US3344606 - Robert B Abernethy
In the patent, the objective of the pipes taking air off the compressor was to improve surge margin of the compressor due to ram air temperature rise:
At high supersonic flight speeds, conventional turbojet engine performance deteriorates primarily because of ram air temperature rise. As a result the thrust output drops because of insufficient airflow, compressor tolerance to surge is poor, and low compressor efliciency occurs resulting in high fuel consumption. Also, the compressor blades are subjected to high stress from the combination of high rotational speed and flutter from rotating stall in the front stages.
In addition to the thrust restriction, low surge margin and inefficiency problems just discussed caused by point L iiow restriction, the compressor front stages, which are operating in stall or close to stall, are subjected to flutter fatigue from the cyclic separation and attachment of the air stream lines passing there over.
My solution to the compressor flow blockage problem is my recover bleed air engine wherein air is ducted from the compressor middle stages into the afterburner during supersonic, maximum power (point L) flight conditions, thus bypassing the flow restriction that exists in the rear compressor stages under such flight conditions. In the afterburner, the bleed air is brought up to the same energy level as that of the air that flows thru the turbojet.
In addition to the thrust restriction, low surge margin and inefficiency problems just discussed caused by point L iiow restriction, the compressor front stages, which are operating in stall or close to stall, are subjected to flutter fatigue from the cyclic separation and attachment of the air stream lines passing there over.
My solution to the compressor flow blockage problem is my recover bleed air engine wherein air is ducted from the compressor middle stages into the afterburner during supersonic, maximum power (point L) flight conditions, thus bypassing the flow restriction that exists in the rear compressor stages under such flight conditions. In the afterburner, the bleed air is brought up to the same energy level as that of the air that flows thru the turbojet.
Below is the patent P&W was granted for the J58 engine design which was used.
https://www.google.com/patents/US3344606 - Robert B Abernethy
https://www.google.com/patents/US3344606 - Robert B Abernethy
by now I forgot how the heck we diverted this discussion so far from the thread title
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by now I forgot how the heck we diverted this discussion so far from the thread title 
.. but isn't wandering off thread half the fun in Tech Log ?
.. but isn't wandering off thread half the fun in Tech Log ?
Abernethy was the P&W engineer who gave life to the J-58. Extract of a speech he made to A-12/SR-71 and J-58 folks at a reunion.
Thrust Produced
Mach 2.2 - Inlet 13%, Engine 73%, Ejector 14%
Mach 3 - Inlet 54%, Engine 17%, Ejector 29%
Graphic of thrust contributions by components, including spike drag.
My first job was to create a thermodynamic computer program to calculate the performance of the J58. My computer analyst and I worked day and night on this in the winter of ‘57-58. This was the second engine computer “deck” every created and I learned a lot from it. I quickly understood that the J58 would never get to Mach 3 as designed. At about Mach 2.5 the exhaust pressure was equal to the inlet pressure, the compressor was deep in surge, and there was no cool airflow for the afterburner liner that would therefore melt. This is not good. About this time Norm assigned me to head up the compressor development. I knew nothing about compressor theory so I returned to East Hartford for education from their computer expert, Jim Fligg. I had to analyze data from the compressor rigs that Les Churchill was testing in the Willgoos Laboratory.
With a little knowledge of compressor aerodynamics I could see at Mach 3 the front stages would be deep in stall from too low airflow and the rear stages were choked preventing the airflow to increase. The same problem exists when starting the engine and P&W solution was to open “start” bleeds. This brought the front stages out of stall and bypassed the rear stages. In October 1958 the solution for all these J58 problems was clear to me. Bypass the bleed air around the compressor at high Mach number into the afterburner and it would solve the surge problem, provide cool air to afterburner and increase the mass flow and thrust significantly. Actually it converted the engine into a partial ramjet with capability above Mach 3! I called it the Recover Bleed Air engine on my patent.
Norm Cotter did not exactly leap for my solution so I wrote him memo after memo for seven months trying to convince him. One of the problems was that if you open the bleed valves at high Mach number there would be a hiccup in the airflow which could unstart the inlet and possible destroy the aircraft. My solution was to open the valves at a lower Mach number, around 2.0 where there would be no bleed flow and no hiccup. Here is my report to George Armbruster, the senior project engineer on the J58 dated April 1959. George was later loaned to the CIA for the Glomar Explorer project to raise a Russian submarine from the Pacific floor.
I finally convinced Norm on a Monday in April 1959. Norm immediately convinced Bill Brown who called his buddy, Kelly Johnson, on a crypto phone and explained the concept of the Recover Bleed Air engine, emphasizing the Mach 3+ capability. Kelly flew to Washington and had funding for the Blackbird by weeks end. The aircraft flew with J75s about just about two years later from a top secret base, now known as Area 51 north of Las Vegas. We called it the “sandpatch.” About a year later they flew with the J58-D20 engines.
The Navy J58 Engine became The Recover Bleed Air Engine JT11-D20
About this time Bill Brown came screaming after me, “I figured it out, Recover Bleed Air, RBA, are your initials! We won’t call it that anymore!” Much to my surprise I was transferred to the RL10 project, the other Florida project team. I never found out why but I suspect Kelly Johnson thought I knew too much. For example the flight condition I picked for my report was Mach 3.2, 90,000 feet.
My other recommendation was to add a variable inlet guide vane to the JT11-D20 design as it would help the airflow schedule and keep the front compressor blades out of flutter. Bill Brown said, no way, that’s a GE fix, but later he understood the need. Here are the performance improvements, bleeds open compared to bleeds closed. I predicted the net thrust change but not the installed thrust improvement. The increased airflow really helped Kelly’s inlet performance.
Recover Bleed Air Benefits Bleeds Open to Bleeds Closed
• Airflow Increase +22%
• Net Thrust Increase +19%
• Installed Thrust Increase +47%
• Installed Fuel Consumption -20%
• Increased Afterburning Temperature
• Reduced Blade Flutter
With a little knowledge of compressor aerodynamics I could see at Mach 3 the front stages would be deep in stall from too low airflow and the rear stages were choked preventing the airflow to increase. The same problem exists when starting the engine and P&W solution was to open “start” bleeds. This brought the front stages out of stall and bypassed the rear stages. In October 1958 the solution for all these J58 problems was clear to me. Bypass the bleed air around the compressor at high Mach number into the afterburner and it would solve the surge problem, provide cool air to afterburner and increase the mass flow and thrust significantly. Actually it converted the engine into a partial ramjet with capability above Mach 3! I called it the Recover Bleed Air engine on my patent.
Norm Cotter did not exactly leap for my solution so I wrote him memo after memo for seven months trying to convince him. One of the problems was that if you open the bleed valves at high Mach number there would be a hiccup in the airflow which could unstart the inlet and possible destroy the aircraft. My solution was to open the valves at a lower Mach number, around 2.0 where there would be no bleed flow and no hiccup. Here is my report to George Armbruster, the senior project engineer on the J58 dated April 1959. George was later loaned to the CIA for the Glomar Explorer project to raise a Russian submarine from the Pacific floor.
I finally convinced Norm on a Monday in April 1959. Norm immediately convinced Bill Brown who called his buddy, Kelly Johnson, on a crypto phone and explained the concept of the Recover Bleed Air engine, emphasizing the Mach 3+ capability. Kelly flew to Washington and had funding for the Blackbird by weeks end. The aircraft flew with J75s about just about two years later from a top secret base, now known as Area 51 north of Las Vegas. We called it the “sandpatch.” About a year later they flew with the J58-D20 engines.
The Navy J58 Engine became The Recover Bleed Air Engine JT11-D20
About this time Bill Brown came screaming after me, “I figured it out, Recover Bleed Air, RBA, are your initials! We won’t call it that anymore!” Much to my surprise I was transferred to the RL10 project, the other Florida project team. I never found out why but I suspect Kelly Johnson thought I knew too much. For example the flight condition I picked for my report was Mach 3.2, 90,000 feet.
My other recommendation was to add a variable inlet guide vane to the JT11-D20 design as it would help the airflow schedule and keep the front compressor blades out of flutter. Bill Brown said, no way, that’s a GE fix, but later he understood the need. Here are the performance improvements, bleeds open compared to bleeds closed. I predicted the net thrust change but not the installed thrust improvement. The increased airflow really helped Kelly’s inlet performance.
Recover Bleed Air Benefits Bleeds Open to Bleeds Closed
• Airflow Increase +22%
• Net Thrust Increase +19%
• Installed Thrust Increase +47%
• Installed Fuel Consumption -20%
• Increased Afterburning Temperature
• Reduced Blade Flutter
Mach 2.2 - Inlet 13%, Engine 73%, Ejector 14%
Mach 3 - Inlet 54%, Engine 17%, Ejector 29%
Graphic of thrust contributions by components, including spike drag.
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Understanding The Complexities Of Bigger Fan Blades | MRO Network
Understanding Complexities Of Bigger Fan Blades
How engine fan blades are becoming bigger and bigger—and their impact on the overall aircraft
This summer, the largest jet engine ever built was being put through its paces in Peebles, Ohio.
Peebles—GE Aviation’s boot camp for new jet engines—has seen the GE9X, which is destined to power Boeing’s next-generation 777X passenger aircraft, undergoing tests during the past few months. The new engine has had a lot thrown at it, including 160-hr. icing tests. Meanwhile, engineers have gathered reams of data from more than 50 test points that will be used to inform subsequent manufacturing and design.
The engine’s components have been under analysis for much longer than just this summer, however. Testing of GE9X components began six years ago, including evaluation of the fourth-generation carbon-fiber fan blades and fan case, 3D-printed fuel nozzles and its special new lightweight materials, known as ceramic matrix composites.
The combination of these components “decreases engine weight, boosts efficiency and will also decrease fuel burn,” says GE. Airlines apparently agree: Nearly 700 GE9X engines are on order.
As development of the GE9X illustrates, jet engines have been getting bigger. In tandem with this trend, fan blades also have been growing. In fact, thinking big is a good way to approach the capabilities of a modern jet engine: Each fan blade carries a load of air when sucked into the engine that is equivalent to nine double-decker buses and swallows a huge amount of air—in fact, a squash court’s worth of air—every second.
SHAPE TRANSFORMATION
Over the years, fan blade designs have been transformed from flat plates to complex, three-dimensionally curved shapes. That has been made possible thanks to the advances in computational fluid dynamics (CFD) modeling. Today, extensive computer modeling simulates the flows through the fan itself and determines how efficiently the blades pump air through the fan. The aim is to attain optimum efficiency. The fewer the blades, the cheaper the engine. The desire is for high aerodynamic efficiency while minimizing the number of blades required. The latest engines have fewer fan blades than older ones because the aerodynamics have improved.
Over time, the engine diameter also has increased. This is because the diameter of the fan—the length of the fan blades—has grown in relation to the core diameter of the engine. What is known as the engine bypass ratio always is being boosted to achieve higher propulsive efficiency. The core is the high-temperature, high-pressure turbo-machine that sits behind the fan blades and generates the power to drive the fan. It contains the higher-pressure compressors, the combustor and the turbines.
On modern high-bypass-ratio engines, most of the air passes around the outside of the core, which means most of the thrust is generated by the fan, not the core. Typically, the bypass ratio on today’s modern engines is at least 10:1. “That means 10 times as much air goes through the fan and straight around the outside of the core through the bypass duct, as opposed to the core. Consequently, around 90% of the thrust is generated by the fan itself,” says Simon Weeks, chief technology officer at the UK Aerospace Technology Institute.
Bigger fan engines generate thrust much more efficiently than older engines that have lower bypass ratios. Fan blades necessarily grow to pump a much bigger volume of air. The actual surface area of the blades has increased with the chord. The latest turbofan blades have very wide-chord blades, for example. The hollow, titanium wide-chord fan blade, pioneered by Rolls-Royce and introduced in the 1980s, set new standards in aerodynamic efficiency and resistance to foreign-object damage. “Designed specifically for high-bypass turbofans, the breadth of these blades sets them apart from the narrow and less efficient earlier equivalents,” says Rolls. Some fan blades have very large surface areas and resemble paddles rather than blades.
One of the compromises with high-bypass-ratio engines is that aerodynamic drag tends to increase. The casing of the engine and nacelle grows in diameter—and higher in surface area, optimizing the overall architecture of the aircraft so the bypass ratio does not adversely affect aerodynamics. The bigger the engine, the more complex the flow of air and the bigger the potential to interfere with the aerodynamics of the wing. CFD, along with some physical wind-tunnel testing, is used extensively to try to understand what is taking place.
STRENGTHS
Over the decades, hollow titanium fan-blade technology has served engine-maker Rolls well, from the RB211 to the Trent family of engines. Now Rolls is examining the possibilities of producing a carbon-titanium family of fan blades, a technology still in the development phase.
Rival GE, meanwhile, continues to rely on carbon-fiber composite fan blades, which it introduced on the GE90 engine in 1995. Today, large fan blades are manufactured at GE using a carbon-fiber tape-layer process, in which engineers layer strips of tape to make up the shape of the blade. Meanwhile, for the CFM56 and Leap engines, GE partner Safran has been developing a 3D woven technology in conjunction with Albany Composites. Engineers pump resin into, and consolidate, a 3D carbon-fiber shape.
Rolls has other strengths. Historically, it has pushed development of and manufacturing with hollow titanium fan-blade technology further than many of its competitors. Rolls boasts what is said to be a unique superplastic formation process for producing hollow titanium fan blades. They are made from a sandwich of three sheets of titanium that is selectively diffusion-bonded and then super-plastically formed into a complex 3D shape in a mold at high temperature.
“You inflate the blade in the mold, and that expands the blade out to fill the mold form, the full 3D shape. At that temperature, you also make selective bonds within the titanium, so you end up with a bridged structure inside the blade itself. This gives you additional mechanical strength,” explains Weeks.
GOING TO THE BIRDS
Innovations such as these have enabled Rolls to use titanium fan blades for a long time, and their strength is a key attribute. A main reason is that fan blades must be able to cope with bird strikes.
“It is not uncommon for fan blades to go for years without repair,” says Sam Rice, sales and business development director for engines at aircraft spare parts specialist AJW Group. “However, when we do have to send them to our network of shops, the reason is often foreign-object damage—particularly bird strikes.” Following this type of damage, fan blades will be subject to nondestructive testing to ensure there is no blade-cracking, measurements to make sure they still have the correct shape and strength-testing to ensure they can still perform correctly.
Weeks says “one of the main challenges [for fan-blade technology] is tolerating ingestion of birds.” Aero engine-makers must ensure the powerplant can continue to operate safely after bird strikes. In the unlikely event that a fan blade comes off because of a severe impact—with a very large bird, for example—engine-makers must demonstrate that the loss of a fan blade can be tolerated. That is one of the reasons why manufacturers deliberately detach a blade during a test to ensure an engine can continue to operate safely.
In part to help with the bird-impact problem, manufacturers are migrating toward what might be described as a carbon-fiber composite-metal hybrid.
Carbon-fiber composite fan blades tend to feature thin titanium edges, which give them the impact-resistance required for a bird-strike. Birds are not the only threat from the environment. “On composite blades, you need a metallic edge to protect the carbon-fiber composite, to deal not just with bird strikes but also erosion from the dust that is in the air, rain, snow and hail,” says Weeks. These factors “predominantly affect the leading edges of the blades, particularly toward the tips, which are the fastest moving parts of the blades,” he explains.
Carbon fiber is used for the blades because of its light weight. But whereas it is easier to shape thin titanium blades for aerodynamic performance, composites tend to have thicker cross sections. “With titanium, you can make thinner blades, which means the aerodynamics are slightly better. You have to lay down many layers of composites to form a fan blade, and it is quite challenging to form aerodynamically efficient complex 3D shapes with sharp curves using composites,” Weeks stresses.
As in the case of the GE9X, a composite carbon-fiber fan-casing can be employed if the blades themselves are carbon fiber. Composite fan blades just require a composite engine casing, rather than a titanium engine-casing, which also helps to cut engine mass dramatically. A carbon-fiber casing can be used because the impact on—and potential for damage to—the casing in the event of inflight fan-blade loss is much less than for a titanium blade.
For metallic blades that experience foreign-impact damage, additive manufacturing can be used to build up a whole new section, machine it back, shape it and restore the blade to its original shape. “Ultimately, additive manufacturing may be used to create the blades themselves,” says Rice of AJW. Engineers would take advantage of the technique’s design flexibility to produce new forms with superior aerodynamic properties.
The UK’s Aerospace Technology Institute, meanwhile, is studying new techniques for repairing composite fan blades. Weeks concludes: “Composite blades typically need less maintenance than metallic ones. You find that composite structures are quite robust and can take a lot of abuse, but you do need to be able to inspect them, and impact damage is not always as apparent as it is on a metallic blade.”
Understanding Complexities Of Bigger Fan Blades
How engine fan blades are becoming bigger and bigger—and their impact on the overall aircraft
This summer, the largest jet engine ever built was being put through its paces in Peebles, Ohio.
Peebles—GE Aviation’s boot camp for new jet engines—has seen the GE9X, which is destined to power Boeing’s next-generation 777X passenger aircraft, undergoing tests during the past few months. The new engine has had a lot thrown at it, including 160-hr. icing tests. Meanwhile, engineers have gathered reams of data from more than 50 test points that will be used to inform subsequent manufacturing and design.
The engine’s components have been under analysis for much longer than just this summer, however. Testing of GE9X components began six years ago, including evaluation of the fourth-generation carbon-fiber fan blades and fan case, 3D-printed fuel nozzles and its special new lightweight materials, known as ceramic matrix composites.
The combination of these components “decreases engine weight, boosts efficiency and will also decrease fuel burn,” says GE. Airlines apparently agree: Nearly 700 GE9X engines are on order.
As development of the GE9X illustrates, jet engines have been getting bigger. In tandem with this trend, fan blades also have been growing. In fact, thinking big is a good way to approach the capabilities of a modern jet engine: Each fan blade carries a load of air when sucked into the engine that is equivalent to nine double-decker buses and swallows a huge amount of air—in fact, a squash court’s worth of air—every second.
SHAPE TRANSFORMATION
Over the years, fan blade designs have been transformed from flat plates to complex, three-dimensionally curved shapes. That has been made possible thanks to the advances in computational fluid dynamics (CFD) modeling. Today, extensive computer modeling simulates the flows through the fan itself and determines how efficiently the blades pump air through the fan. The aim is to attain optimum efficiency. The fewer the blades, the cheaper the engine. The desire is for high aerodynamic efficiency while minimizing the number of blades required. The latest engines have fewer fan blades than older ones because the aerodynamics have improved.
Over time, the engine diameter also has increased. This is because the diameter of the fan—the length of the fan blades—has grown in relation to the core diameter of the engine. What is known as the engine bypass ratio always is being boosted to achieve higher propulsive efficiency. The core is the high-temperature, high-pressure turbo-machine that sits behind the fan blades and generates the power to drive the fan. It contains the higher-pressure compressors, the combustor and the turbines.
On modern high-bypass-ratio engines, most of the air passes around the outside of the core, which means most of the thrust is generated by the fan, not the core. Typically, the bypass ratio on today’s modern engines is at least 10:1. “That means 10 times as much air goes through the fan and straight around the outside of the core through the bypass duct, as opposed to the core. Consequently, around 90% of the thrust is generated by the fan itself,” says Simon Weeks, chief technology officer at the UK Aerospace Technology Institute.
Bigger fan engines generate thrust much more efficiently than older engines that have lower bypass ratios. Fan blades necessarily grow to pump a much bigger volume of air. The actual surface area of the blades has increased with the chord. The latest turbofan blades have very wide-chord blades, for example. The hollow, titanium wide-chord fan blade, pioneered by Rolls-Royce and introduced in the 1980s, set new standards in aerodynamic efficiency and resistance to foreign-object damage. “Designed specifically for high-bypass turbofans, the breadth of these blades sets them apart from the narrow and less efficient earlier equivalents,” says Rolls. Some fan blades have very large surface areas and resemble paddles rather than blades.
One of the compromises with high-bypass-ratio engines is that aerodynamic drag tends to increase. The casing of the engine and nacelle grows in diameter—and higher in surface area, optimizing the overall architecture of the aircraft so the bypass ratio does not adversely affect aerodynamics. The bigger the engine, the more complex the flow of air and the bigger the potential to interfere with the aerodynamics of the wing. CFD, along with some physical wind-tunnel testing, is used extensively to try to understand what is taking place.
STRENGTHS
Over the decades, hollow titanium fan-blade technology has served engine-maker Rolls well, from the RB211 to the Trent family of engines. Now Rolls is examining the possibilities of producing a carbon-titanium family of fan blades, a technology still in the development phase.
Rival GE, meanwhile, continues to rely on carbon-fiber composite fan blades, which it introduced on the GE90 engine in 1995. Today, large fan blades are manufactured at GE using a carbon-fiber tape-layer process, in which engineers layer strips of tape to make up the shape of the blade. Meanwhile, for the CFM56 and Leap engines, GE partner Safran has been developing a 3D woven technology in conjunction with Albany Composites. Engineers pump resin into, and consolidate, a 3D carbon-fiber shape.
Rolls has other strengths. Historically, it has pushed development of and manufacturing with hollow titanium fan-blade technology further than many of its competitors. Rolls boasts what is said to be a unique superplastic formation process for producing hollow titanium fan blades. They are made from a sandwich of three sheets of titanium that is selectively diffusion-bonded and then super-plastically formed into a complex 3D shape in a mold at high temperature.
“You inflate the blade in the mold, and that expands the blade out to fill the mold form, the full 3D shape. At that temperature, you also make selective bonds within the titanium, so you end up with a bridged structure inside the blade itself. This gives you additional mechanical strength,” explains Weeks.
GOING TO THE BIRDS
Innovations such as these have enabled Rolls to use titanium fan blades for a long time, and their strength is a key attribute. A main reason is that fan blades must be able to cope with bird strikes.
“It is not uncommon for fan blades to go for years without repair,” says Sam Rice, sales and business development director for engines at aircraft spare parts specialist AJW Group. “However, when we do have to send them to our network of shops, the reason is often foreign-object damage—particularly bird strikes.” Following this type of damage, fan blades will be subject to nondestructive testing to ensure there is no blade-cracking, measurements to make sure they still have the correct shape and strength-testing to ensure they can still perform correctly.
Weeks says “one of the main challenges [for fan-blade technology] is tolerating ingestion of birds.” Aero engine-makers must ensure the powerplant can continue to operate safely after bird strikes. In the unlikely event that a fan blade comes off because of a severe impact—with a very large bird, for example—engine-makers must demonstrate that the loss of a fan blade can be tolerated. That is one of the reasons why manufacturers deliberately detach a blade during a test to ensure an engine can continue to operate safely.
In part to help with the bird-impact problem, manufacturers are migrating toward what might be described as a carbon-fiber composite-metal hybrid.
Carbon-fiber composite fan blades tend to feature thin titanium edges, which give them the impact-resistance required for a bird-strike. Birds are not the only threat from the environment. “On composite blades, you need a metallic edge to protect the carbon-fiber composite, to deal not just with bird strikes but also erosion from the dust that is in the air, rain, snow and hail,” says Weeks. These factors “predominantly affect the leading edges of the blades, particularly toward the tips, which are the fastest moving parts of the blades,” he explains.
Carbon fiber is used for the blades because of its light weight. But whereas it is easier to shape thin titanium blades for aerodynamic performance, composites tend to have thicker cross sections. “With titanium, you can make thinner blades, which means the aerodynamics are slightly better. You have to lay down many layers of composites to form a fan blade, and it is quite challenging to form aerodynamically efficient complex 3D shapes with sharp curves using composites,” Weeks stresses.
As in the case of the GE9X, a composite carbon-fiber fan-casing can be employed if the blades themselves are carbon fiber. Composite fan blades just require a composite engine casing, rather than a titanium engine-casing, which also helps to cut engine mass dramatically. A carbon-fiber casing can be used because the impact on—and potential for damage to—the casing in the event of inflight fan-blade loss is much less than for a titanium blade.
For metallic blades that experience foreign-impact damage, additive manufacturing can be used to build up a whole new section, machine it back, shape it and restore the blade to its original shape. “Ultimately, additive manufacturing may be used to create the blades themselves,” says Rice of AJW. Engineers would take advantage of the technique’s design flexibility to produce new forms with superior aerodynamic properties.
The UK’s Aerospace Technology Institute, meanwhile, is studying new techniques for repairing composite fan blades. Weeks concludes: “Composite blades typically need less maintenance than metallic ones. You find that composite structures are quite robust and can take a lot of abuse, but you do need to be able to inspect them, and impact damage is not always as apparent as it is on a metallic blade.”
