The Truth Behind Installing EDFs!?

[Air-headed Aviator]

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:

• 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 rack



When 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 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:

• 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

With these attributes theoretically the best potential thrust can be extracted from an EDF, and also the best potential efficiency.

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.

 

[Mr Man]

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:

• 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 rack



When 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 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:

• 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

With these attributes theoretically the best potential thrust can be extracted from an EDF, and also the best potential efficiency.

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.

Interesting points.

 

[quorneng]

My own experience in designing and building EDFs is that for maximum thrust the inlet should have a constant and free flowing area that is 1.2 times the fan swept area.
To achieve this with a scale size inlet, without using inefficient cheat holes, means the EDF has to be significantly smaller than would normally be considered which in turn requires the plane has to be built really light.
The only benefit of this lightweight approach is that both the EDF and battery can be as much as 50% lighter too.

The other point to consider is that by simply raising the cell count to achieve the required thrust is a double edged sword. Not only is the battery heavier for the same capacity but the thrust/watt figure of the EDF falls away rapidly as the RPM rises.
In many ways a low power but light EDF is not commercially popular (everybody wants thrust regardless) so options are limited.

An example of this is the initial 40mm AEO EDF was very light but had relatively feeble thrust as it could only use a 2s LiPo. As a result it was pretty quickly replaced by 4s versions than generate over twice the thrust but weigh double and need fours time the energy to do it. Is it actually a "better" EDF?

 

[telnar1236]

In terms of how EDFs work, they're more similar to props than to real jet engines. While they have a fairly high pitch speed and spin faster compared to most RC props which gives them a flatter thrust curve, EDFs still will produce significantly less thrust as they fly faster due to the fan being unable to accelerate the air as much. EDFs, like props, suck at generating static pressure, which is why efficient ducting is so critical.
Personally, I tend to go for inlet areas of around 1-1.1x FSA since I tend to like speed and good kinematics more than static thrust, but anything from 1 to 1.3 or 1.4x FSA is fairly reasonable. I'm not as much of a fan of very light planes since there is just something cool about a heavy airframe sliding around a turn, and I'm not quite as averse to cheater holes, although they do come with an efficiency penalty, but otherwise all I'd add to what Quorneng said is that a shorter duct is also generally more efficient.

 

[quorneng]

The other aspect of EDF ducting to consider is no so much the length of the inlet ans exhaust ducts (even non scale EDFs tend to follow a conventional jet layout) is which is the most sensitive to length?
At model sizes the duct surface boundary layer thickness is significant and is more or less constant in both that have the same type of surface. On this basis it could be argued that the effect of the boundary layer is more marked on the smaller diameter exhaust than the inlet suggesting that for minimum total duct losses the inlet should be longer than the exhaust. Taking this concept to the extreme the exhaust should be as short as possible, just to achieve the appropriate nozzle reduction with a suitably sized inlet duct providing all the remaining length. EDF at the back?
The printed duct for my Depron Folland Gnat

The exhaust has a modest 95% nozzle so is short! All the inlet duct including the bifurcated part is set to give 1.2xFSA. It also follows my principle of using a "small" QX50 EDF allowing the true scale inlets to have a combined area of 1.2xFSA.

Much of this is the result of my own logical deduction with only trial and error proof so maybe not every one's cup of tea but with a wing loading and thrust to weight matching a good park flyer it hand launches like one too!

 

[quorneng]

It flies pretty nicely too!

 

[quorneng]

I mentioned that the good g/w thrust efficiency of a low powered EDF can be an advantage but only if a scale airframe can be built light enough.
One of my first true EDFs used an early AEO 40mm EDF. I recognised that although it was light and with a 2s power limitation a DH Venom was a good scale subject. Unlike the even earlier Vampire it used a more powerful Ghost engine so had a rather bigger exhaust duct and slightly enlarged wing root inlets.

This meant true scale inlet and exhaust would be possible if the plane was big and light enough. It would have a span of 30" (760mm)
For robustness a twin boom layout is not ideal for any RC plane but the resulting short fuselage does benefit the installation of an EDF.
Although the tip tanks were permanent on the Venom I did manage to find a picture of one flying without them.
By placing the EDF right at the back of the short fuselage it was possible to create an efficient inlet duct rather than the truncated version required by the layout of the Ghost engine.
A bit of a labour of love as it was all constructed using lots of bits of 2mm Depron sheet.

The full duct including that in the wing roots.

Being relatively small and light it was built as a "one piece" plane relying entirely on strength and stiffness of its 2mm Depron skin and everything but the LiPo permanently built in to save weight.

Not exactly a scale colour scheme but I was more concerned to make it visible!

To my amazement it flies very well such that after a few flights it was obvious that the 850mAh 2s could be replaced with a 1400mAh.

Even with this battery it weighs just 202g and the longest flight achieved so far is 12 minutes giving an average draw of just 50W!

 

[quorneng]

The success of the DH Venom made me wonder if the same "light but big" principle could betaken further. How about four of the same 40mm EDFs in a Concorde.
Not quite a daft as it sounded. A delta is strong for its weight, gives plenty of wing area and can be stable at high angle of attack provided there is sufficient thrust to over come the drag.
Its size would be determined by the EDFs actually being the exhaust ducts. It would make Concorde 7' 6" long but with just a 32" span. The long inlet ducts but without the complex internal ramps would give an area of 1.2 FSA for each EDF.
This suggested the EDFs would give close to their maximum possible thrust. The big question was going to be could the substantial airframe be built light enough to give sufficient to fly and with sufficient excess thrust to actually climb.
To get an idea of the likely thrust I built a scale "test" duct with two EDFs again all built in 2mm Depron.

It gave pretty close to twice the "free air" figure of a single 40mm EDF it seemed reasonable to proceed but little did I know how long it would take!
To cut a long story short everything was to be 2mm Depron and designed to be as light as possible regardless of the effort involved.
The closely spaced wing webs on the lower wing skin.

One top skin added. The wing actually incorporated Concorde's complex wing twists and bends based on this famous photo.
The nose and tail sections of the fuselage were built using a "half shell over the plan" technique. When fully skinned they were added to the wing. The top of the fuselage over the wing was left open to install the wiring and battery. As an EDF delta the fin was fixed with no rudder.

Note the ESCs placed just ahead of the EDFs. However the first low power "glide" showed it was tail heavy and the long wires from the ESCs to the battery quickly wrecked the ESCs. After much testing they were moved so the heat sinks were on the underside of the wing just ahead of the intakes. This placed the ESCs much closer to the 3000mAh 2s LiPo in the fuselage but required many openings and repairs to be made to the wings.

After well over a year it eventually looked like this.

It is perhaps worth noting that the underside of Concorde fuselage is square so giving an adequate finger hold for hand launching.

At 18oz and a thrust of just 8 oz it was never going to be a "rocket" and the 3000mAh LiPo is only allowing 750mAh for each EDF so duration is limited but it does fly although it has zero crash resistance!

Given that Concorde is strictly speed limited at a low manoeuvring altitude it is quite realistic. Unusual for an EDF.
Worth all the effort?
Just about but I will never do anything like it again not least as those particular EDFs and big sheets of true 2mm Depron are no longer available! 😥

 

[LitterBug]

Beautiful! I remember the only time Concorde flew out of our town. Definitively different sound than anything else that has ever flown over my house about 15 miles west of the airport.

 

[quorneng]

I actually decided to build my Concorde after seeing it fly over my house on one of its rare visits to Manchester when it was doing "jollies" over the Bay of Biscay. A noise quite unlike anything else!
I rather wish I had taken advantage of one of those although still expensive trips particularly with my late dad who had a jolly with his sister in an Avro 504 in 1926 at Hendon for his 18th birthday and with a picture. It would have meant going from a rotary engine biplane to supersonic in his lifetime!

 

[Piotrsko]

So what's a "jollie" ?

 

[quorneng]

A jolly.
To be taken on, and possibly pay for, an "experience" ride in a plane with no intent or opportunity to touch the controls.

 

[Piotrsko]

Thank you. So it is the equivelant to an American Young Eagles flight experience.