Air-headed Aviator
Member
I work as a Line Technician at an airport, and that allows me to observe manned aircraft of various types in action. Recently one of the local maintenance shop had the engines of a business jet down and I thought that was a great opportunity to compare the dimensions of a real world turbofan engine with those recommended for our Remote Control analog, the Electric Ducted Fan.
For a long time EDFs confused me due to their perceived contradictions. They're associated with fast and powerful RC "Jets" but are not typically the powertrain of choice for peak performance with an electric setup, along with being extremely inefficient. As I researched fans and how they worked, I learned something that provided an epiphany: fans are fundamentally the same as propellers. Both units are fundamentally wings turned 90 degrees to the path of flight. As they spin they produce lift, which in this case is thrust.
Part 1: How Thrust
Just like a wing, thrust from a fan or prop depends primarily on the face of the disc. Lift is created when a foil moved through a fluid reduces the relative pressure of the fluid on its top by forcing it to move faster thanks to greater relative curvature. The faster a fluid like air moves the lower its pressure becomes. As a consequence the fluid pressure below the foil is now higher. Air wants to equalize differences in pressure in its general area, and as quickly as possible. The shortest distance to this low pressure zone however is blocked by a wing, or prop, or fan. So instead the high pressure air exerts a force on the lifting body, creating lift or thrust. The way fans defer to propellers is in their ability to produce significant pressure differences. Because of their small diameter fans can be spun incredibly fast, and thanks to their multiple blades the low pressure zone at the fan face is much lower than that aft. However in order to capitalize on that advantage fan units need a variety of additional elements to props: ducts, thrust tubes and inlets.
A partially assembled PW306A Turbofan
The front face of a Turbofan engine on a Business jet
Part 2: The Parts of a Fan
The reason fans require so many elements to be effective is inherent to their compact size. A fan is a lot like a low aspect ratio wing of small area. Such wings have a high amount of induced drag that develops at the wing tips. A duct acts a lot like winglets on a wing, helping to reduce the drag of the fan and increase the thrust returned. Thrust tubes are used to help keep the thrust produced laminar and increase its speed. The more laminar a flow is the more energy that can be extracted from it. In the same vane the longer high pressure fluid is restricted from a lower pressure zone, the faster it picks up speed, which also results in increased thrust (though if restricted to long, backpressure picks up). Inlets are potentially the most underrated aspect about Fans. In addition to helping make the air that meets the fan as laminar as possible, inlets help in reducing the low pressure zone ahead of the fan. Once again, as a fluid accelerates its pressure decreases, and a fluid can be forced to accelerate when it travels through path with a change in cross section. The inlet is that change in cross section, and combined with the fan's sucking force helps increase that pressure drop. The driving force of a fans thrust is its ability to drop pressure ahead of it; Many turbofan engines (whom 80% or more of their thrust is produced by the fan) will feature plenty of obstruction behind the fan in the form of the driving turbojet core, but a clear and open path in front of the fan.
Part 3: RC's Achilles Heal
Its my opinion that issues with inlet design is a major contributor to the lack of thrust and efficiency seen with EDF Jets. Most scale jets, and even several sport jet designs are emulating the inlet of aircraft intended to fly at high subsonic speeds. Inlet effectiveness is very sensitive to the speed the aircraft is flown at. The faster the aircraft is intended to fly, typically the deeper the inlet would be until it meets the fan face. Airframes originally belonging to supersonic or high subsonic aircraft will have inlets too deep for economical thrust at lower subsonic speeds. This I believe is why so many scale jets have such low efficiency despite the amount of power applied, among other things. These craft are flying slower than 100mph with elements meant for 6 times that! Under a certain fan diameter many scale RC jets need to add cheater holes to help reduce the pressure of the inlet ahead of the fan face, but this still denies the advantage of an inlet that enhances the thrust, working only as a band-aid.
A Freewing Vulcan High Performance EDF
Part 4: Lessons from the Ramp
While scale EDF Jets have limited flexibility in optimizing the fan housing, custom designs can be made to follow dimensions that maximize the capability of its powertrain. And the dimensions that would maximize the powertrain I think are modeled by manned business jets. Business Jets are turbofan powered aircraft that fly at relatively low subsonic speeds to that of any military jet. Thus they experience similar rules of aerodynamics as our toy planes. I've seen a large variety of business jets in my short career; some slow, some a little faster, and yet the vast majority of those craft featured similar dimensions for their engines.
Finally coming back to the maintenance engine I got to explore, this was a PW306A Turbofan engine from a Gulfstream G200 Galaxy Business Jet. The general dimensions of the turbofan are as follows:
The PW306A disassembled on an engine rack
When researching recommendations for sizing the parts of an EDF for an RC plane the criteria are as follows:
A 64mm EDF with a poster board thrust tube
No criteria exists for inlet depth. When you quantify the relationships of the PW306's dimensions with each other, some interesting ratios come up. The exit nozzle's ultimate area is closer to that of the fans total area minus the area of its spinner. Other research focusing on turbofans revealed that there's a high focus on fan swept area, which is like the actual area of the fan face made up by blades. The thrust tube of the engine (more accurately full engine installation) is more accurately equal to the circumference of the fan (100.7 inches, which is 96% accurate). The inlet depth itself is equivalent to 70% of that of the fan diameter. When double checking how consistent these attributes are on other business jets, I found it constant for the fan/exit relationship and fan/thrust tube length relationship to exist. The inlet depth relationship varied only one of two ways: there were the "shallow" variations at 70% of the diameter, and the "deep" variations at 114% of the fan diameter [for all aircraft within the same speed range]. Either relationship was seen on both large and small aircraft, and with any or the same engine brand. Based off these measurements the updated criteria for EDF installations could be as follows:
Part 5: Theory in Practice
Whom would I be if I didn't practice what I preach though? I already have one EDF design running off a single 64mm 10 blade fan, running on a 4 cell 2200. While most single 64mm EDFs get about 3.5 minutes flight time on modest power off 2200 mAh my design manages 5.5 minutes flight time on mixed throttle. Its also relatively heavy for its wingspan. Another design is on the workbench, this time using two 64mm EDFs to run on 6 cell batteries. The aircraft these motors were taken from also sees only around 3.5 minutes of mixed throttle flight time on 2200 mAh. Time will tell if I can extract better powertrain performance from this design as well
A custom RC EDF design
A custom Twin EDF design in progress of completion
Part 6: Additional Details and Conclusion
Other considerations towards the design of EDF installations include a few other elements. The path to the fan ought to be as smooth as possible. Many Business jets even have their inlets lined with tiny holes to make it act even "smoother" to the air. In model building making smooth inlets that change directions can be very difficult and costly (often why real business jets have such straight forward engine nacelles) so bends and corners can reduce fan performance. The lips of subsonic inlets are often very thick and rounded. This assists in the angles at which the fan can be operated at without encoring stalled air, retaining thrust even if the aircraft is at high angles of attack. An EDF can be given all of these elements to improve the thrust is produces but at best its efficiency and capabilities will only be on par to propellers, never exceed. The reasoning for this is based on a fans main advantages over props: Efflux and Pressure differential. Efflux refers to the rate at which mass exits a nozzle. Fans create a higher efflux than props can, which contributes to their ability to accelerate an aircraft faster as long as sufficient thrust exists. However, the greater in difference between the speed of the aircraft and the efflux of the engines are, the worse the whole systems efficiency is. Its essentially wasted energy not being delivered to the aircraft. Efficiency for real turbofan aircraft can be recouped the higher and faster they fly. Starting around 400 mph, forward flight starts to reduce the pressure of incoming air into the inlet simply from the act of forward motion. This greatly reduces how much fuel is needed to make sufficient thrust and greatly improves system efficiency. However as long as a turbofan is operated below that ideal zone, it can experience greater inefficiency. EDF RC airplanes very rarely crack 150 mph... An EDF will essentially always operate in a state of greater inefficiency than what's ideal. It looses out on the high speed efficiency gain from high mach cruise and retains only the Efflux advantage.
The main interest in exploring what qualities can enhance the capabilities of an EDF aren't entirely motivated in making them "better" than propellers necessarily. EDFs exist in the RC world primarily to make scale jets more convincing for the hobbyist; providing a more authentic experience. Outside of scale designs though, looking for an optimized way to use EDFs is about potentially finding the niches Ducted fans can serve, and what experiences they can provide with as little drawbacks as necessary.
For a long time EDFs confused me due to their perceived contradictions. They're associated with fast and powerful RC "Jets" but are not typically the powertrain of choice for peak performance with an electric setup, along with being extremely inefficient. As I researched fans and how they worked, I learned something that provided an epiphany: fans are fundamentally the same as propellers. Both units are fundamentally wings turned 90 degrees to the path of flight. As they spin they produce lift, which in this case is thrust.
Part 1: How Thrust
Just like a wing, thrust from a fan or prop depends primarily on the face of the disc. Lift is created when a foil moved through a fluid reduces the relative pressure of the fluid on its top by forcing it to move faster thanks to greater relative curvature. The faster a fluid like air moves the lower its pressure becomes. As a consequence the fluid pressure below the foil is now higher. Air wants to equalize differences in pressure in its general area, and as quickly as possible. The shortest distance to this low pressure zone however is blocked by a wing, or prop, or fan. So instead the high pressure air exerts a force on the lifting body, creating lift or thrust. The way fans defer to propellers is in their ability to produce significant pressure differences. Because of their small diameter fans can be spun incredibly fast, and thanks to their multiple blades the low pressure zone at the fan face is much lower than that aft. However in order to capitalize on that advantage fan units need a variety of additional elements to props: ducts, thrust tubes and inlets.
A partially assembled PW306A Turbofan
The front face of a Turbofan engine on a Business jetPart 2: The Parts of a Fan
The reason fans require so many elements to be effective is inherent to their compact size. A fan is a lot like a low aspect ratio wing of small area. Such wings have a high amount of induced drag that develops at the wing tips. A duct acts a lot like winglets on a wing, helping to reduce the drag of the fan and increase the thrust returned. Thrust tubes are used to help keep the thrust produced laminar and increase its speed. The more laminar a flow is the more energy that can be extracted from it. In the same vane the longer high pressure fluid is restricted from a lower pressure zone, the faster it picks up speed, which also results in increased thrust (though if restricted to long, backpressure picks up). Inlets are potentially the most underrated aspect about Fans. In addition to helping make the air that meets the fan as laminar as possible, inlets help in reducing the low pressure zone ahead of the fan. Once again, as a fluid accelerates its pressure decreases, and a fluid can be forced to accelerate when it travels through path with a change in cross section. The inlet is that change in cross section, and combined with the fan's sucking force helps increase that pressure drop. The driving force of a fans thrust is its ability to drop pressure ahead of it; Many turbofan engines (whom 80% or more of their thrust is produced by the fan) will feature plenty of obstruction behind the fan in the form of the driving turbojet core, but a clear and open path in front of the fan.
Part 3: RC's Achilles Heal
Its my opinion that issues with inlet design is a major contributor to the lack of thrust and efficiency seen with EDF Jets. Most scale jets, and even several sport jet designs are emulating the inlet of aircraft intended to fly at high subsonic speeds. Inlet effectiveness is very sensitive to the speed the aircraft is flown at. The faster the aircraft is intended to fly, typically the deeper the inlet would be until it meets the fan face. Airframes originally belonging to supersonic or high subsonic aircraft will have inlets too deep for economical thrust at lower subsonic speeds. This I believe is why so many scale jets have such low efficiency despite the amount of power applied, among other things. These craft are flying slower than 100mph with elements meant for 6 times that! Under a certain fan diameter many scale RC jets need to add cheater holes to help reduce the pressure of the inlet ahead of the fan face, but this still denies the advantage of an inlet that enhances the thrust, working only as a band-aid.
A Freewing Vulcan High Performance EDFPart 4: Lessons from the Ramp
While scale EDF Jets have limited flexibility in optimizing the fan housing, custom designs can be made to follow dimensions that maximize the capability of its powertrain. And the dimensions that would maximize the powertrain I think are modeled by manned business jets. Business Jets are turbofan powered aircraft that fly at relatively low subsonic speeds to that of any military jet. Thus they experience similar rules of aerodynamics as our toy planes. I've seen a large variety of business jets in my short career; some slow, some a little faster, and yet the vast majority of those craft featured similar dimensions for their engines.
Finally coming back to the maintenance engine I got to explore, this was a PW306A Turbofan engine from a Gulfstream G200 Galaxy Business Jet. The general dimensions of the turbofan are as follows:
- 32 inch wide fan diameter
- 24 inch wide exit diameter
- 97 inch long outer casing or "thrust tube"
- and a 22.5 inch deep inlet
The PW306A disassembled on an engine rackWhen researching recommendations for sizing the parts of an EDF for an RC plane the criteria are as follows:
- thrust tube exit should be between 90%-80% of the fan area
- thrust tube length should be around 4 times that of the fan diameter
- inlet area should be 100% of the fan area.
A 64mm EDF with a poster board thrust tubeNo criteria exists for inlet depth. When you quantify the relationships of the PW306's dimensions with each other, some interesting ratios come up. The exit nozzle's ultimate area is closer to that of the fans total area minus the area of its spinner. Other research focusing on turbofans revealed that there's a high focus on fan swept area, which is like the actual area of the fan face made up by blades. The thrust tube of the engine (more accurately full engine installation) is more accurately equal to the circumference of the fan (100.7 inches, which is 96% accurate). The inlet depth itself is equivalent to 70% of that of the fan diameter. When double checking how consistent these attributes are on other business jets, I found it constant for the fan/exit relationship and fan/thrust tube length relationship to exist. The inlet depth relationship varied only one of two ways: there were the "shallow" variations at 70% of the diameter, and the "deep" variations at 114% of the fan diameter [for all aircraft within the same speed range]. Either relationship was seen on both large and small aircraft, and with any or the same engine brand. Based off these measurements the updated criteria for EDF installations could be as follows:
- inlet area should be 100% of the fan area
- inlet depth should be between 70% and 114% of the fan diameter
- thrust tube length should be equal to the fan circumference
- thrust tube exit should equal the fan swept area: total fan area minus spinner area
Part 5: Theory in Practice
Whom would I be if I didn't practice what I preach though? I already have one EDF design running off a single 64mm 10 blade fan, running on a 4 cell 2200. While most single 64mm EDFs get about 3.5 minutes flight time on modest power off 2200 mAh my design manages 5.5 minutes flight time on mixed throttle. Its also relatively heavy for its wingspan. Another design is on the workbench, this time using two 64mm EDFs to run on 6 cell batteries. The aircraft these motors were taken from also sees only around 3.5 minutes of mixed throttle flight time on 2200 mAh. Time will tell if I can extract better powertrain performance from this design as well
A custom RC EDF design
A custom Twin EDF design in progress of completionPart 6: Additional Details and Conclusion
Other considerations towards the design of EDF installations include a few other elements. The path to the fan ought to be as smooth as possible. Many Business jets even have their inlets lined with tiny holes to make it act even "smoother" to the air. In model building making smooth inlets that change directions can be very difficult and costly (often why real business jets have such straight forward engine nacelles) so bends and corners can reduce fan performance. The lips of subsonic inlets are often very thick and rounded. This assists in the angles at which the fan can be operated at without encoring stalled air, retaining thrust even if the aircraft is at high angles of attack. An EDF can be given all of these elements to improve the thrust is produces but at best its efficiency and capabilities will only be on par to propellers, never exceed. The reasoning for this is based on a fans main advantages over props: Efflux and Pressure differential. Efflux refers to the rate at which mass exits a nozzle. Fans create a higher efflux than props can, which contributes to their ability to accelerate an aircraft faster as long as sufficient thrust exists. However, the greater in difference between the speed of the aircraft and the efflux of the engines are, the worse the whole systems efficiency is. Its essentially wasted energy not being delivered to the aircraft. Efficiency for real turbofan aircraft can be recouped the higher and faster they fly. Starting around 400 mph, forward flight starts to reduce the pressure of incoming air into the inlet simply from the act of forward motion. This greatly reduces how much fuel is needed to make sufficient thrust and greatly improves system efficiency. However as long as a turbofan is operated below that ideal zone, it can experience greater inefficiency. EDF RC airplanes very rarely crack 150 mph... An EDF will essentially always operate in a state of greater inefficiency than what's ideal. It looses out on the high speed efficiency gain from high mach cruise and retains only the Efflux advantage.
The main interest in exploring what qualities can enhance the capabilities of an EDF aren't entirely motivated in making them "better" than propellers necessarily. EDFs exist in the RC world primarily to make scale jets more convincing for the hobbyist; providing a more authentic experience. Outside of scale designs though, looking for an optimized way to use EDFs is about potentially finding the niches Ducted fans can serve, and what experiences they can provide with as little drawbacks as necessary.
