Okay, now this is really keeewwwll.
As far as I'm concerned, you just told me that eventually I'll be able to re-gen all the way down...enough to go back up, some fraction, for another dive.
Yeehaw!!!!!!
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Electric planes
- Thread starter doug
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As far as I'm concerned, you just told me that eventually I'll be able to re-gen all the way down...enough to go back up, some fraction, for another dive.
Yeehaw!!!!!!
JRP3
Hyperactive Member
If it helps, I'm an Aerospace Engineer...
1. Aircraft on descent still typically operate at some higher-than-idle thrust--meaning they're still putting energy into the system. So...a first order comparison against today's technology suggests there's little opportunity to take energy out of the system during descent via regen.
2. Controlled airspace is just that--there are regulated limits on airspeed and descent rates, partially for safety and partially for passenger comfort, so bouncing off #1 there's not a lot of regen opportunity while still adhering to current regulations. In the far future when every aircraft is electric we could see a bit more of a dive into the destination which would facilitate regen a little more, but at some point you hit the practical limit of passenger comfort. People don't typically like the sensation of falling.
3. The ability to generate more energy on descent is actually pretty low value added. During that phase of flight an aircraft is headed to its destination where it can charge up again. There's not a whole lot of reason to design appreciable regen into that flight mode since electrons on the ground are always going to be cheaper than electrons in the air. The aircraft would also still have its required reserve, so its not like you can really make the safety argument that the regen would facilitate more flight time.
Thank you for the more detailed, expert explanation which confirms my previous assumptions.
JRP3
Hyperactive Member
When you say regen replacing flaps do you mean steerable motors? What happens you want to change flight angle when under power and accelerating, you can't use regen at that point.
Hey, take it easy with the name calling! First and foremost I'm just another Idiot On The Internet.
I assumed it was a more colloquial reference to air brakes and/or spoilers. Obviously, a fan in regen mode has nothing to do with supplementing/replacing flaps that increase lift at low speed.
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LargeHamCollider
Battery cells != scalable
I think you might be confusing "flaps" with control surfaces like the elevator.
Flaps are there for two primary reasons.
1. Increase lift
2. Increase drag
Reason number 1 is less important than it used to be for GA aircraft. It used to be the case that many aircraft were operated off short runways and so very low touchdown/rotation speeds were required, very low touchdown/rotation speeds meant that either high lift devices like flaps or very low wing loading were needed. Now most runways (but not all) are way longer than a GA aircraft needs, reducing the need for extra lift and slower speeds.
But reason number 2 is as important as ever, extra drag is still very helpful in many situations, the most common of these being a high approach. Personally if I was flying patterns in an electric plane with re-gen I would use the re-gen on every approach, if I was the only one in the pattern this would extend my flying time by 5-10% if I recall the numbers given at CAFE correctly. Too lazy to run the numbers myself right now but it wouldn't be that tough.
We got that.
If it helps, I'm an Aerospace Engineer...
Slightly more serious, you're right that electric aircraft regen is possible, but there are some basic limitations to the concept. Aircraft typically don't have a lot of 'wasted' energy that can be recovered by regen, because they operate in an explicitly balanced dynamic environment. You fall out of the sky if your basic Thrust vs Drag and Lift vs Mass equations don't square up. Its not the same as a car, where the vehicle has [presumably] already fallen out of the sky and is resting safely onto the ground.
Random thoughts:
1. Aircraft on descent still typically operate at some higher-than-idle thrust--meaning they're still putting energy into the system. So...a first order comparison against today's technology suggests there's little opportunity to take energy out of the system during descent via regen.
2. Controlled airspace is just that--there are regulated limits on airspeed and descent rates, partially for safety and partially for passenger comfort, so bouncing off #1 there's not a lot of regen opportunity while still adhering to current regulations. In the far future when every aircraft is electric we could see a bit more of a dive into the destination which would facilitate regen a little more, but at some point you hit the practical limit of passenger comfort. People don't typically like the sensation of falling.
3. The ability to generate more energy on descent is actually pretty low value added. During that phase of flight an aircraft is headed to its destination where it can charge up again. There's not a whole lot of reason to design appreciable regen into that flight mode since electrons on the ground are always going to be cheaper than electrons in the air. The aircraft would also still have its required reserve, so its not like you can really make the safety argument that the regen would facilitate more flight time.
4. As LargeHam notes, the real benefit to aircraft regen could be in displacing other aircraft systems, like spoilers and airbrakes. Some obvious short term barriers aside, removing those systems removes mass, complexity, and cost, and that's A Good Thing.
Just because aircraft descend with some thrust today, that's just a side effect of how turbofan/turboprops work.
Actually turbofan flight idle thrust at very high altitudes is just a few % of take off thrust at sea level, for all practical purposes its nearly zero.
But as the descent continues and air density increases, idle thrust increases due to the minimum fuel flow required to avoid a compressor stall.
With electric propulsion, thrust can go to zero at any time, and can be reverse too like you concur.
Assuming descent at 4000fpm all the way from FL510 cruise to the ground, including deceleration at approach, we're talking about around 15 minutes of nearly continuous regen.
Comparing breaking from 100 mph to a stand still is quite wrong. Its more akin to driving a Model S from 15000ft to sea level with cruise control regulating regen to keep the speed at 65mph.
Jet aircraft routinely descend at a range of vertical speeds. From 3000fpm down to about 1000fpm at cruise descent.
If a more aggressive descent rate were a real problem, then spoilers/speed breaks wouldn't be used. But they are used, producing over 5000fpm descent rates. If you have flown in a commercial jet, you have felt the effects of full flight spoilers applied.
I contend passengers would actually prefer a faster and more aggressive descent as it gives them more time in a quiet/stable cruise and less time with their fasten seat belts lights on and perhaps flying through some turbulence.
Like I said, ATC loves fast descent (and climb) rates.
How do you think ATC handles F15/F16/F18/F22/F35s flying through controlled airspace ? Do you think they force them to limit their climb/descent rates ? After all those aircraft don't have a whole lot of endurance at low altitudes.
The way it works is when ATC clears you to climb/descent, they ensure that vertical area is free right now, so the faster you get down, the faster the area you're descending through is clear for others. Same thing on the way up. What might happen is the next sector up/down can't take you or will take you but can't give you a climb/descend yet. Another great reason for regen, that energy that was stored can be converted back into thrust again if the aircraft is stuck.
You're very true that be ability to regen is of limited advantage just to save kWhs of charging. But more important is saving charging TIME. Even more important is the simple fact that the first generation of medium range electric aircraft will be very tight in their range capacity. Furthermore, regen is also good to slow down from a 250kt pre approach speed down to a stable 100kt ILS. Spoilers aren't just wasteful, by dumping a lot of lift and adding a lot of drag they abruptly change the vertical speed. Regen would make it far more practical to delay slowing down to approach speeds in the last few minutes of flight (if there's room ahead of course).
Normal turbine aircraft flying involves transitioning from 280-300 knots indicated cruise descent to 120-150 knots approach speeds. Being able to reliably and safely doing that faster is a desirable trait for a commercial aircraft. Regen helps there.
Another interesting possibility is that even if 1:1 Thrust to Weight ratio isn't possible (vertical take off / landing), about 40% as much thrust and vertical tilting of the engine then the wing can be designed without flaps/slats at all and with wings optimized solely for cruise speeds, which can further reduce drag and increase range/energy efficiency.
Considering how cheap electric motors are, the Musk idea of VTOVL isn't stupid at all.
Look at the Osprey wings. Quite tiny, as they're designed just for cruise.
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JRP3
Hyperactive Member
Yes but from what you describe if you are close on range you would be better off with a shallower glide angle than going into a steeper descent to try and capture regen, since regen itself is not 100% efficient. Better to use your forward momentum directly instead of converting it into electricity through the motor-inverter-battery pack then back out from the pack-inverter-motor. Just as coasting in a car is better than regen.
Etna
Member
Another aerospace engineer here. My specialties are aircraft design and performance.
We can kill the idea of regen for an airplane descending. Let me put it this way:
What the airplane is doing in a descent is trading altitude for range, ideally at the most efficient speed which is near the speed for maximum lift to drag ratio. If you use regen during the descent, the propeller that drives the generator is creating torque, it will work against the airflow and create drag. Increasing drag will steepen the descent slope for a given airspeed, in other words the distance covered during the descent will be shortened. You cannot maintain the same slope angle and fly at a lower airspeed because your drag due to lift has increased. More drag slows you down even more, and you end up stalling the airplane. So the slope has to steepen.
The energy that you will need to cover that lost distance will always be more than the energy you gained by regen during the descent, as there is a loss when you transform energy (propeller efficiency, generator efficiency, etc.).
So basically the concept of regen in a descent is a concept of trading things with a loss, you can do it if you want but it makes no sense. You will reach the airport with less energy than if you did not use regen. So you are better off not trading anything.
For a car driving down a mountain road, regen will reduce speed (since the slope is constant) and thus reduce drag and thus energy loss. Therefore you can charge your battery in a descent since a slower car always has a lower drag force, contrary to an airplane.
The only condition where regen might make sense in an airplane is by using regen instead of speedbrakes when they need to be deployed. But speedbrakes are used only during poorly planned and inefficient descents, so that condition should actually be avoided anyway.
We can kill the idea of regen for an airplane descending. Let me put it this way:
What the airplane is doing in a descent is trading altitude for range, ideally at the most efficient speed which is near the speed for maximum lift to drag ratio. If you use regen during the descent, the propeller that drives the generator is creating torque, it will work against the airflow and create drag. Increasing drag will steepen the descent slope for a given airspeed, in other words the distance covered during the descent will be shortened. You cannot maintain the same slope angle and fly at a lower airspeed because your drag due to lift has increased. More drag slows you down even more, and you end up stalling the airplane. So the slope has to steepen.
The energy that you will need to cover that lost distance will always be more than the energy you gained by regen during the descent, as there is a loss when you transform energy (propeller efficiency, generator efficiency, etc.).
So basically the concept of regen in a descent is a concept of trading things with a loss, you can do it if you want but it makes no sense. You will reach the airport with less energy than if you did not use regen. So you are better off not trading anything.
For a car driving down a mountain road, regen will reduce speed (since the slope is constant) and thus reduce drag and thus energy loss. Therefore you can charge your battery in a descent since a slower car always has a lower drag force, contrary to an airplane.
The only condition where regen might make sense in an airplane is by using regen instead of speedbrakes when they need to be deployed. But speedbrakes are used only during poorly planned and inefficient descents, so that condition should actually be avoided anyway.
Some pages back, Pipistel was mentioned.
They do have an energy recovery system on their electric populsion system - they've coined the name 'prop-mill' to describe it.
The propeller has wider blades than normal to allow it to be used as a generator.
What is the scope for enregy recovery?
As a former glider pilot, we would normally only use the air brakes (2 metre long fences that stick out of the top and bottom of the wings) on the final descent, maybe some on the downwind to make the final turns at the right place, but generally 50% or so on the final approach to hit the aiming point and stop at the wing catcher. There is little regen scope, even in a slippery sailplane.
Any airliner I've been in recently, starts the descent about 1/2 hour out, so will descend 10km in 300+km. This seems to be no accident and allows the engines to be 'idled' (at something like 40%) while the descent glide is occurring. The no-engine glide ratio of the aircraft is something like 15 or 20 to 1*
Pipistrel seems to be in a niche, where the compromise to the electric propeller (more blade area means more drag) is more than offset by the energy it can recover as a training aircraft doing takeoff and landing circuits all the time**. The approach procedure must be a bit odd.
* Transat Flight 236 (Airbus A330) ran out of fuel afer a leak on a flight from Toronto to Lisbon and managed to glide 120km to the Azores. The glide ratio was around 15:1 and very lucky for the passengers, the captain was also an experienced glider pilot.
** Aircraft propellers will always be not very good at energy recovery because of the 'slip' (slip stream velocity) - the propeller must accelerate (or decelerate) the air going past it to produce thrust (or recover energy). A larger diameter propeller will be better at energy recovery, but more lossy at cruising speed, heavier and may present handling issues on the ground, such as striking the runway. The propeller-slip concept is similar to that in the induction motor in a Tesla, but the propeller system much worse with, between 5 and 50% or more slip, depending on speed and thrust.
They do have an energy recovery system on their electric populsion system - they've coined the name 'prop-mill' to describe it.
The propeller has wider blades than normal to allow it to be used as a generator.
What is the scope for enregy recovery?
As a former glider pilot, we would normally only use the air brakes (2 metre long fences that stick out of the top and bottom of the wings) on the final descent, maybe some on the downwind to make the final turns at the right place, but generally 50% or so on the final approach to hit the aiming point and stop at the wing catcher. There is little regen scope, even in a slippery sailplane.
Any airliner I've been in recently, starts the descent about 1/2 hour out, so will descend 10km in 300+km. This seems to be no accident and allows the engines to be 'idled' (at something like 40%) while the descent glide is occurring. The no-engine glide ratio of the aircraft is something like 15 or 20 to 1*
Pipistrel seems to be in a niche, where the compromise to the electric propeller (more blade area means more drag) is more than offset by the energy it can recover as a training aircraft doing takeoff and landing circuits all the time**. The approach procedure must be a bit odd.
* Transat Flight 236 (Airbus A330) ran out of fuel afer a leak on a flight from Toronto to Lisbon and managed to glide 120km to the Azores. The glide ratio was around 15:1 and very lucky for the passengers, the captain was also an experienced glider pilot.
** Aircraft propellers will always be not very good at energy recovery because of the 'slip' (slip stream velocity) - the propeller must accelerate (or decelerate) the air going past it to produce thrust (or recover energy). A larger diameter propeller will be better at energy recovery, but more lossy at cruising speed, heavier and may present handling issues on the ground, such as striking the runway. The propeller-slip concept is similar to that in the induction motor in a Tesla, but the propeller system much worse with, between 5 and 50% or more slip, depending on speed and thrust.
Why are you talking about props when I'm talking about fans ?
Aren't those very different from each other ?
Props are quite thick. Props are bona fide rotating wings.
Fans are much thinner shape. Much higher RPM.
I'm certain some first principles thinking will be required to do electric aircraft propulsion right.
Remember I'm not talking about regen as a means to stop the aircraft on the ground. Landing will likely expend energy rather than recover.
Regen applies to anywhere from full cruising speed to somewhat less than cruise, but still over half of cruising speed, still a LOT of airflow flowing through the fans to be decelerated. With the fan blades inside the fan nacelle, airflow there is quite different from props too.
There's a clear trend towards larger and larger turbofan blades. Wouldn't that increase the surface area that could generate thrust and do regen, in exchange for lower RPMs ? Wouldn't that be negative from what you explain ?
Either way, there are several differences between designing props for piston aircraft, turbo props, turbo fans, super sonic fans. A properly designed electric fan propulsion should be quite different from modern fossil turbofans. There might not be nearly the same advantage of having just 2 or 4 engines since there's no hot section and no turbos and just a single low pressure fan stage. Worst case a single fixed gearbox to adapt electric motor spinning speed to optimal fan (low pressure blade) spinning speed.
There's a lot of first principles engineering that needs to be done for optimal electrical aircraft propulsion systems. What's on the text books will change quite a bit.
To make a parallel, nuclear design textbooks don't mention fluid fuel reactors, or using Thorium as a nuclear fuel. Completely different design technique for those.
I could be completely wrong, but I wouldn't take aerospace engineering textbooks as gospel considering there are ZERO electrical propulsion systems on aircraft today.
But you could be right and I could be completely wrong.
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Etna
Member
Your guessed correctly, you are completely wrong.
Where to start.
Props and fans are the same thing. The turbofan is simply a shrouded propeller that has the ability to be a lot more powerful and produce a lot of thrust for a given fan diameter. This is possible because there are many more propeller blades (solidity factor) and the duct prevents the air captured in the inlet to attempt a side escape from the violent pull of the propeller blades.
In regen mode, and again pointing out it wouldn't be useful anyway, a wind milling turbofan can produce barely enough electricity to keep the lights on in the airplane. Most of the air encountered by the fan face will simply flow around the nacelle, instead of through it. If there were just two or four blades in the nacelle the air might have considered going through it to please you and do some work. As far as turbofans are concerned, regen is completely hopeless.
The best propulsive efficiency is obtained via propellers as they exist today. If the electric motor is super powerful then a turbofan might make sense, but the energy density of batteries is not there yet to make it work.
An electric motor produces torque at a given RPM (i.e. power) just like any other engine, and that is translated into propulsive thrust via a propeller. Why do you think that torque produced by electricity would be any different than torque produced by a piston or a turbine engine? How would the propeller know the difference? It's the same torque!
One thing that can be different on electric airplane design is the ability to have more engines with little weight penalty for redundancy and/or control purposes, especially in the case of Vertical TakeOff and Landing vehicles. But that will always be less efficient than a single propeller.
Where to start.
Props and fans are the same thing. The turbofan is simply a shrouded propeller that has the ability to be a lot more powerful and produce a lot of thrust for a given fan diameter. This is possible because there are many more propeller blades (solidity factor) and the duct prevents the air captured in the inlet to attempt a side escape from the violent pull of the propeller blades.
In regen mode, and again pointing out it wouldn't be useful anyway, a wind milling turbofan can produce barely enough electricity to keep the lights on in the airplane. Most of the air encountered by the fan face will simply flow around the nacelle, instead of through it. If there were just two or four blades in the nacelle the air might have considered going through it to please you and do some work. As far as turbofans are concerned, regen is completely hopeless.
The best propulsive efficiency is obtained via propellers as they exist today. If the electric motor is super powerful then a turbofan might make sense, but the energy density of batteries is not there yet to make it work.
An electric motor produces torque at a given RPM (i.e. power) just like any other engine, and that is translated into propulsive thrust via a propeller. Why do you think that torque produced by electricity would be any different than torque produced by a piston or a turbine engine? How would the propeller know the difference? It's the same torque!
One thing that can be different on electric airplane design is the ability to have more engines with little weight penalty for redundancy and/or control purposes, especially in the case of Vertical TakeOff and Landing vehicles. But that will always be less efficient than a single propeller.
I realize this quote is from a previous post, but I think it sums up the discussion so far:
On the flip side, bouncing between contextually unrelated nuggets of detail makes it difficult to identify the point you're actually trying to make. Doubly difficult is navigating through your points that support electric airplanes over conventional but don’t maintain any specific relevance to the regen conversation (which, near as I know, is the topic du jour…).
I think what this discussion really comes down to is speculation on the magnitude and value of the benefit from aircraft regen—which, to be sure, is a fun thought experiment and I applaud you for pursuing. From a first principals perspective though--an Elon-ey approach, if you will--one should be able to communicate even the most difficult concepts with fundamental concepts and a manageable word count.
From a first principals perspective, they’re the same thing: Lifting bodies in relative motion to an air mass, transferring energy to or from that air mass.
Props came into the discussion specifically with Large Ham’s contribution, and specifically since a company apparently thinks there’s a legit reason to have regen on their products…but as noted above its all the same thing anyway. The grey list of pros and cons for aircraft regen just shifts this way or that depending on whether you’re talking about a GA trainer or a long haul heavy.
I’m not sure that logic closes. The landing roll of a would be one of the more straightforward places to implement aircraft regen. It’s really a basic equation: Plane lands with energy E, plane needs to dump E before the end of the runway. Only in a situation where the plane’s energy shunting systems can’t collectively convert enough of E to not crash at the end of the runway would you need to add even more [negative] energy to balance out the equation.
It’s not obvious what you’re suggesting here, but I’ll assume you’re not making the “we should strap a windmill to the top of our Teslas” analogy.
Bottom line, any flight phase that is a net consumer of energy has zero opportunity for regen. Basically the only phase that has any opportunity for regen is descent [through landing], where—efficiency and practicality aside—you could theoretically trade speed and/or altitude for regenerated electrons. But, you still have the first principals problem of Lift=Weight and Thrust=Drag. There’s only so much energy you can take out of the system period, regardless if its through regen or some other form of energy shedding.
To be clear, I’m a fan of taking the perspective that opportunities>>roadblocks. ’m not trying to tell you that regen won’t work, I’m just trying to help you bound the regen possibilities. @Etna has provided some detail on why a fan won’t regen well. From a more first principals perspective @Etna’s point is simply that, like most things in the universe, the air mass is going to take the path of least resistance. In the case of an aircraft engine that path happens to be mostly around the fan, not mostly through it. That goes for any phase of flight, by the way, including on the ground. In the top level energy equation, the drag component of an aircraft motor will likely be much higher than any realistic regen component.
Even so, from the thought experiment perspective I’m willing to throw a magical cloak of The Future over that and assume there’s some technology advancement to maximize energy recovery. And with that in mind, you could be on to something with the time component of flight. If, for instance, you could shave 5 minutes from the descent phase of every flight, what’s that worth? Even if it meant more overall energy consumption, would that be A Good Thing for aircraft? Possibly. If that energy was 100% electric, renewable, and effectively limitless? Likely.
Then again, the above 5 minute concept isn’t exclusive to electric engines. Current aircraft could easily cruise longer and descend faster, saving that 5 minutes, but they don’t. Consider why.
On the flip side, bouncing between contextually unrelated nuggets of detail makes it difficult to identify the point you're actually trying to make. Doubly difficult is navigating through your points that support electric airplanes over conventional but don’t maintain any specific relevance to the regen conversation (which, near as I know, is the topic du jour…).
I think what this discussion really comes down to is speculation on the magnitude and value of the benefit from aircraft regen—which, to be sure, is a fun thought experiment and I applaud you for pursuing. From a first principals perspective though--an Elon-ey approach, if you will--one should be able to communicate even the most difficult concepts with fundamental concepts and a manageable word count.
From a first principals perspective, they’re the same thing: Lifting bodies in relative motion to an air mass, transferring energy to or from that air mass.
Props came into the discussion specifically with Large Ham’s contribution, and specifically since a company apparently thinks there’s a legit reason to have regen on their products…but as noted above its all the same thing anyway. The grey list of pros and cons for aircraft regen just shifts this way or that depending on whether you’re talking about a GA trainer or a long haul heavy.
I’m not sure that logic closes. The landing roll of a would be one of the more straightforward places to implement aircraft regen. It’s really a basic equation: Plane lands with energy E, plane needs to dump E before the end of the runway. Only in a situation where the plane’s energy shunting systems can’t collectively convert enough of E to not crash at the end of the runway would you need to add even more [negative] energy to balance out the equation.
It’s not obvious what you’re suggesting here, but I’ll assume you’re not making the “we should strap a windmill to the top of our Teslas” analogy.
Bottom line, any flight phase that is a net consumer of energy has zero opportunity for regen. Basically the only phase that has any opportunity for regen is descent [through landing], where—efficiency and practicality aside—you could theoretically trade speed and/or altitude for regenerated electrons. But, you still have the first principals problem of Lift=Weight and Thrust=Drag. There’s only so much energy you can take out of the system period, regardless if its through regen or some other form of energy shedding.
To be clear, I’m a fan of taking the perspective that opportunities>>roadblocks. ’m not trying to tell you that regen won’t work, I’m just trying to help you bound the regen possibilities. @Etna has provided some detail on why a fan won’t regen well. From a more first principals perspective @Etna’s point is simply that, like most things in the universe, the air mass is going to take the path of least resistance. In the case of an aircraft engine that path happens to be mostly around the fan, not mostly through it. That goes for any phase of flight, by the way, including on the ground. In the top level energy equation, the drag component of an aircraft motor will likely be much higher than any realistic regen component.
Even so, from the thought experiment perspective I’m willing to throw a magical cloak of The Future over that and assume there’s some technology advancement to maximize energy recovery. And with that in mind, you could be on to something with the time component of flight. If, for instance, you could shave 5 minutes from the descent phase of every flight, what’s that worth? Even if it meant more overall energy consumption, would that be A Good Thing for aircraft? Possibly. If that energy was 100% electric, renewable, and effectively limitless? Likely.
Then again, the above 5 minute concept isn’t exclusive to electric engines. Current aircraft could easily cruise longer and descend faster, saving that 5 minutes, but they don’t. Consider why.
Regen on the ground will be quite problematic using the fan cause the airflow is quickly reducing. Half speed = 1/8 as much kinetic power from the airflow (which is proportional to v³). By the time the aircraft touches down its likely at 1/4 of cruise indicated airspeed or 1/64th of power from the airflow. Like you observed correctly, regen means slowing down air passing through the fan, so it takes fast airflow to do powerful regen.
On the other hand if instead of trying to recover a large volume of kWh in a few seconds (which would be a tiny % of total potential + kinetic energy the aircraft has at cruise to regen), the electric fans can be a bona fide reverse thruster which would briskly slow down the aircraft, saving break pad wear. In fact by substantially reducing fan speeds far below windmilling speeds, an electric fan can generate a far stronger breaking force than fossil turbofans (which do a piss poor job of reversing airflow backwards while the whole engine is trying to produce forward thrust).
On the other hand maybe having electric engines in the landing gear system, which could be the primary propulsion system for taxing, could substantially help on takeoff and then actually regen effectively to stop the aircraft. That's a far more logical option, but current landing gear designs don't leave room for putting an electric motor there. Perhaps a radically different design approach could provide a totally different landing gear design. A very interesting idea, as the higher the acceleration possible on the ground the faster the aircraft could take off, which allows for a thinner wing which could have better L : D (lift over drag) for climb, cruise and descent. But this will depend a lot from the mass of such a solution. Weight is hugely important in aircraft design.
I am a private pilot and a junkie of transport aircraft (with several thousands of flight time in MSFS/FlightGear and several other jet flight sims). In fossil aircraft the flight idle fuel flow varies hugely from FL410 to sea level. In fact, a turbofan consumes as much fuel idling on the ground as it consumes cruising near its service ceiling. Think about that ! On the way down, gliding is massively useful before the air gets dense, OAT (outside air temps) rises and the thermal efficiency of the engine drops like a rock (substantially increasing idle fuel flow). Above 20000ft gliding is the way to go, but below 10000ft, its more important to land quickly and shutdown the engines than to squeeze that last minute gliding capability. Plus speeds are limited to 250KIAS at 10000ft and below. Fossil jets loose a lot of efficiency at slower airspeeds.
Electric jets on the other hand should have insignificant propulsive efficient effect from varying OATs. Colder temps should mostly help reduce battery cooling requirements.
On the other hand if instead of trying to recover a large volume of kWh in a few seconds (which would be a tiny % of total potential + kinetic energy the aircraft has at cruise to regen), the electric fans can be a bona fide reverse thruster which would briskly slow down the aircraft, saving break pad wear. In fact by substantially reducing fan speeds far below windmilling speeds, an electric fan can generate a far stronger breaking force than fossil turbofans (which do a piss poor job of reversing airflow backwards while the whole engine is trying to produce forward thrust).
On the other hand maybe having electric engines in the landing gear system, which could be the primary propulsion system for taxing, could substantially help on takeoff and then actually regen effectively to stop the aircraft. That's a far more logical option, but current landing gear designs don't leave room for putting an electric motor there. Perhaps a radically different design approach could provide a totally different landing gear design. A very interesting idea, as the higher the acceleration possible on the ground the faster the aircraft could take off, which allows for a thinner wing which could have better L : D (lift over drag) for climb, cruise and descent. But this will depend a lot from the mass of such a solution. Weight is hugely important in aircraft design.
I am a private pilot and a junkie of transport aircraft (with several thousands of flight time in MSFS/FlightGear and several other jet flight sims). In fossil aircraft the flight idle fuel flow varies hugely from FL410 to sea level. In fact, a turbofan consumes as much fuel idling on the ground as it consumes cruising near its service ceiling. Think about that ! On the way down, gliding is massively useful before the air gets dense, OAT (outside air temps) rises and the thermal efficiency of the engine drops like a rock (substantially increasing idle fuel flow). Above 20000ft gliding is the way to go, but below 10000ft, its more important to land quickly and shutdown the engines than to squeeze that last minute gliding capability. Plus speeds are limited to 250KIAS at 10000ft and below. Fossil jets loose a lot of efficiency at slower airspeeds.
Electric jets on the other hand should have insignificant propulsive efficient effect from varying OATs. Colder temps should mostly help reduce battery cooling requirements.
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Etna
Member
Your landing gear ideas have been looked at, and the current landing gear designs do actually allow the presence of electric motors for "eTaxi" as some call it. Airbus, Honeywell, L-3 and other companies have attempted and actually demonstrated this but most have given up (except WheelTug for example) because it has proven uneconomical. Instead of on-board integrated systems, autonomous electrically powered tugs will more likely become ubiquitous around airports in the not too distant future (the soon famous TAAG or "Tesla Autonomous Airport Tug"). At least the aircraft won't have to carry around the weight penalty of the tug, which sounded like a bad idea in the first place.
All the other statements in your above message are not backed up by physics or facts, including the one in bold. We can continue this discussion privately if you want to learn more since this is more about basic aeronautic knowledge rather than electric airplanes.
All the other statements in your above message are not backed up by physics or facts, including the one in bold. We can continue this discussion privately if you want to learn more since this is more about basic aeronautic knowledge rather than electric airplanes.
I, for one, am finding this discussion fascinating, so I hope it does not go private, even if it gets moved to another thread. And it does have some relevance to electric planes, since an electric plane will need to obey basic aerodynamics. I'm learning stuff here.
Etna
Member
Oh, sure ok then.
Turbofans as thrust reversers are very effective in terms of deceleration capability (turboprops even more so). They are inefficient from the fuel consumption point of view but it does not matter as this lasts only a few seconds for an entire flight. So it is not as bad as it sounds.
A thinner wing is not always desirable from the aerodynamic standpoint and the drag created by its thickness is a tiny fraction of total drag. A thinner wing is always much heavier so you are actually trying to get the wing as thick as you can get away with. There is no need for novel landing gear designs since landing gears are generally not the drivers of wing thickness.
A turbofan does not consume more on ground idle than in service ceiling flight cruise. It was true for some old engines (and that notion sticks around in some books) but not anymore, at least on the turbofans I have been working on, ground idle is about one third of ceiling flight cruise (or one sixth if you shut one engine down on the ground). Still it is more than what people expect.
Flight idle fuel flow does not increase massively as the aircraft descends below 10000 ft., and there is no need to urgently dive to the ground and shut down the engines. A continuous gentle descent at idle is still the most efficient way.
As a matter of fact, more and more ATC services are implementing optimized descent profiles that allow the airlines to fly in idle from a properly timed top of descent to final approach. It rarely works 100% but it saves a lot of fuel to just try to get there.
The 250 KIAS limit below 10000 ft is immaterial to best glide speed which is typically below 250 KIAS for lightly loaded airliners (like they should be before landing).
Turbofans as thrust reversers are very effective in terms of deceleration capability (turboprops even more so). They are inefficient from the fuel consumption point of view but it does not matter as this lasts only a few seconds for an entire flight. So it is not as bad as it sounds.
A thinner wing is not always desirable from the aerodynamic standpoint and the drag created by its thickness is a tiny fraction of total drag. A thinner wing is always much heavier so you are actually trying to get the wing as thick as you can get away with. There is no need for novel landing gear designs since landing gears are generally not the drivers of wing thickness.
A turbofan does not consume more on ground idle than in service ceiling flight cruise. It was true for some old engines (and that notion sticks around in some books) but not anymore, at least on the turbofans I have been working on, ground idle is about one third of ceiling flight cruise (or one sixth if you shut one engine down on the ground). Still it is more than what people expect.
Flight idle fuel flow does not increase massively as the aircraft descends below 10000 ft., and there is no need to urgently dive to the ground and shut down the engines. A continuous gentle descent at idle is still the most efficient way.
As a matter of fact, more and more ATC services are implementing optimized descent profiles that allow the airlines to fly in idle from a properly timed top of descent to final approach. It rarely works 100% but it saves a lot of fuel to just try to get there.
The 250 KIAS limit below 10000 ft is immaterial to best glide speed which is typically below 250 KIAS for lightly loaded airliners (like they should be before landing).
How does a turbofan reverse thrust? It's a powerful deceleration, but I always wondered how they do it. Do they reverse the fan somehow, or use some kind of baffle to direct the exhaust forward?
With a turboprop I presume they can adjust the prop pitch so it's pushing backwards.
I've read the fanjet entry on How Stuff Works, and couldn't make sense of it.
With a turboprop I presume they can adjust the prop pitch so it's pushing backwards.
I've read the fanjet entry on How Stuff Works, and couldn't make sense of it.
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