I was so confused by the title - I thought jets were fairly efficient at ~40%-50% of theoretical maximum and turbofans can't be that far behind. It would maybe make sense for a single prop aircraft.
I think there are startups making a similar sort of engine for general aviation. It's good to see that there is development of the same idea for commercial aviation.
This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.
This means that the battery is quite small and light, having only enough charge to take off and get to altitude.
I suspect that this system probably improves safety as well, if architected properly. If one or both of the gas engines fail, so long as they are not seized, that electric motor can still provide some power for diversion.
>This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.
Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?
IDK if anyone really answered -this- question properly, so I will.
> Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?
Well, it kinda depends. Where Hybrids get the biggest MPG boost is in city driving. The stop and go traffic lets you use Regen braking and go quite some time without the engine kicking back on while still moving forward.
I still don't understand Honda's system enough to speak well on it, but I can speak to THS because it seems to be the cheapest to do and is most proven on the road.
The general parameters for a THS type system on a Toyota Prius or Rav4, or a Ford Maverick/Fusion/Escape is a 2.0L or 2.5L (at least in modern US examples) engine paired to a simple planetary gearset containing a power split device. It is a single speed (At least in the cheap configurations, however that simplicity is possibly close enough to be viable for air usage vs a reduction gear.) That's part of why they tend to have fairly large engines, the valve timing magic gives them at least a bit more HP to not be too bad on the highway.
> but insufficient for acceleration
Going back to the modern cases, the engine is typically sized large enough to give some acceleration even on the highway. Not always great but usually enough.
At least as far as the non-plug-in hybrids, the 0-35MPH can be surprisingly peppy.
The bigger magic (again, at least as far as THS) is it makes it easy to just run the engine at the 'most optimal RPM' for certain tasks, excess energy gets piped to the battery or back out through the system, this does also help reliability tho, because you can then design the reliability of the engine around certain RPM ranges...
That's such a fantastic video. I never totally grasped why hybrids were so much more efficient, because my naive assumptions about how they worked were so simplistic. The real-time graphs he showed were excellent for making his points.
TC is filled with these sorts of videos. If you have time to burn then they are basically all this quality. His interests are also just wildly all over the place. From Christmas lights to dishwashers to coffee machines you just don't know what the next video will be.
No it’s not. The majority of power comes from the engine. It drives the electric motor mechanically, using it as a transmission. It is not just charging the battery.
Sorry, I meant in the low speed, high acceleration regime (maybe easily confounded with takeoff?). There the engine will turn one motor to generate electricity, which will then power the second motor, like a series hybrid.
No, that's just wrong: at no point is (say) a Toyota Hybrid burning gas to generate electricity to immediately turn another motor that actually moves the wheels.
What do you imagine happens if the battery is low and the car wants to start from a stop? The engine can't shift down to first so it would just stall out if you tried to use it to drive the driveshaft directly, and there's no other source of electricity to put into the motor driving the wheels.
Chevy Volt was still a parallel hybrid. The gasoline engine was used for driving the wheels for highway cruise because it was more efficient. I think the range extender version of the BMW i3 was a pure serial hybrid though
Honda's recent hybrids (CR-V, Civic, Accord) are like this too. Mostly series, but the engine can directly drive the wheels via a single overdrive gear for better cruising efficiency.
The fuel burn of take-off and climb substantially lightens the aircraft for cruise. Electric batteries have no such effect, you're carrying all that dead weight for the rest of the flight. This reduces the passenger or cargo capacity of the aircraft, which reduces potential revenue.
On an airplane, most of the energy in cruise is spent overcoming parasitic drag, not induced drag. It’s spent pushing the airmass out of the way as it moves forward, not creating lift to stay aloft.
For that reason, a change in weight does not significantly change cruise fuel usage.
Weight is still precious, but that’s because airplanes’ load are more often weight constrained than volume constrained, and capital and operating costs are such that you want to maximize the load.
> you're carrying all that dead weight for the rest of the flight
If you're recharging the batteries for extra go-arounds during landing, they are as dead weight as the fuel you would otherwise reserve for that purpose. And if you have 30% more efficient engines, meaning less fuel and smaller engines, it's possible you could come out ahead, weight-wise.
> what if you need two go-arounds
I assume that a go-around requires less sustained power output than a full climb from takeoff, so you will probably get more than one go-around anyway, and we don't know how much over-capacity they're designing for. In any case, any design will require tradeoffs in safety, and having more engine-out capabilities might improve safety enough to overcome the higher risk with go-arounds.
Not saying this project is will work out or that you're even wrong necessarily (this could be the equivalent of a concept car for Pratt & Whitney).
Due to various penalties, wind resistance. gear down and aircraft configuration. A go-around consumes a huge amount of fuel, not as much as climbing to cruise but its alot
So, for a Dash 8-100, at 13,000 kg, disregarding drag, engine efficiency, etc, to take-off and climb to 1000m and accelerate to 150 knots (77 m/s), you will need:
- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude
- 0.5 * 13000 * (77)^2 = 38.5 MJ, to accelerate to your climbing speed.
Total: 127 + 38.5 = 166 MJ, or about 46.11 kWh
For a go around, re-accelerating from 1.3 * stall speed (85 knots / 44 m/s) to your climbing speed, and going to your missed approach altitude of 1000 m, you will need:
- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude
- 0.5 * 13000 * (77^2 - 44^2) = 26 MJ to accelerate back to your climbing speed.
Nice to see some numbers. So for the peak load situations, a Dash 8-100 would not require a battery bigger than that a short range BEV ("city", though in reality the short range BEV use case is more for the rural equivalent of stuff that would be walkable in a city setting). And that's even before considering the energy contributed by the fossil fuel engine.
"13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude"
Presumably quite a bit of that would be harvested back during the descent that follows. The conventional engine would still need some excess power (relative to cruise load) to fill the gap left by drag and imperfect circle efficiency of the electric motor/generator, but mass x altitude is stored energy, not lost. (I'm still talking about the "what if we need a second abort" of the root post)
You don't do a full climb after a go-around, so the heights are not equal, and the mass is less since you've expended fuel. You also retain some kinetic energy but I assume that is closer to a negligible effect.
Also, PE = mgh is probably an not a great formula for energy cost of takeoff/go-around, as there are probably large costs it ignores (gravity loss, less efficient engine use, maybe less efficient turbines?).
For your source 2 I have no rebuttal so will have to defer to you, but would ask for an explanation.
Not a pilot, but on approach for landing you bleed off a lot of energy. For a go-around you need to reverse your descent and build up enough energy to fly away again. Take-off/Go-around tends to be the same throttle setting, AFAIK. Of course it also depends on how early you decide to throw away the approach and go around. Doing it at 1000 feet is different from bouncing it off the runway.
but the point they were making is that it inevitably takes less energy to get to a level flying state (in similar weather conditions) due to fuel consumption.
so, unless the pilot is fighting weather it would make sense that equal throttle levels and equal pitch plans in equal weather conditions would require less and less fuel burn until the tanks are empty.
an IFR missed approach can have you climb quite high, especially in areas with serious terrain. Example: https://aeronav.faa.gov/d-tpp/2607/00346IZLZ17R.PDF airport is at 4400 feet over sea level, but missed approach says: climb to 13,000. Also, some go arounds will lead you to have to divert to an alternate airport, getting there may require climbing high to clear terrain or gaining required engine efficiency to fly the distance.
> And the mass is less since you've expended fuel
In our theoretical aircraft with batteries, mass is the same.
> You also retain some kinetic energy but I assume that is closer to a negligible effect.
Would imagine these are significantly more useful on heavier and faster aircraft - surely the weight and whatnot to retract move them is less worth it for smaller planes ?
They are commonly used on small single-engine fighter jets as well. They typically cannot be retracted once deployed, and free-fall using their own mass, so there is no actuation system weight to account for.
Energy density of liquid fuels cannot be beat by batteries, so this is not competitive if you are looking to maximize cargo. However, there are plenty of short haul flights: private jets, island hopping, regional routes where you need to move little mass.
Easy: you don't try the second landing approach before the battery is sufficiently recharged to contain enough energy for the second abort. Chances are this does not take any longer than going through the pattern anyways.
The saving is not just the dead weight of the bigger engine you'd need to do take-off, climb and abort without electric assist, it's also the fuel saved during cruise from running an engine that is completely designed for efficiency at cruise load instead of for some compromise between cruise efficiency and sufficient peak power for start and abort.
Exactly. A hybrid passenger car can tolerate unpredictable power output that may come with an auxiliary power setup that may or may not be available when stronger dynamics are called for.
A plane doesn’t have this luxury and needs predictable output. The fossil fuel engine either needs a sacrificial “overboost” mode for emergencies (at the cost of wear/long-term longevity), or has to be sized for full power at the ultimate cost of efficiency.
So theoretically if the electric motors fail (or battery is dead) an engine could be sized and designed smaller (for cruise efficiency) but have some sort of boost mode that still ensures safety ? At the cost of increased maintenance or wear or something if it must be used
This is very clever: instead of focusing on electric only flight, just make the existing engine fly in the most efficient window while the electric engine buffers the flight profile.
I kind of wonder how the orders of magnitude work out for solar and wing area.
Solar cells are actually quite thin and could be almost like paint on the wings of aircraft. I wonder if the energy generated vs required is even ballpark. There is plenty of sun at 30,000 feet during the day.
Man this is a middle-schooler level napkin math question.
Take a Boeing 777, it has a wingspan of about 60 meters, and I'll ballpark an average upper wing chord of about 7 meters, for a total upper flat area of about 420 square meters.
High quality modern but standard single sided solar panels can do about 220 watts per square meter is full sunlight (around 22% efficiency tested at a 1000w/m^2 irradiance).
So that is 92,400 watts at full power.
92kW is less than the peak power of a Nissan Leaf. 92kW is 123 horsepower.
The two GE turbofans of a 777 are generating something like 40-50 MW of shaft power during cruise. MW.... megawatts. 50-60,000 horsepower.
Plastering the wing surface of a commercial plane with solar panels would make up less than a quarter of a percent of the total power it uses to produce thrust at cruise, at best case with them fully-lit.
Fully solar sailplanes do exist (NASA's Helios prototypes are an example) but that isn't anything close to a 'normal' aircraft with any appreciable payload/passengers.
Cheers- I guess I'll add, the turbofans that all modern airliners use are almost always referred to by their 'thrust' and you ofter see a lot of published numbers of takeoff thrust and such - it is harder to find numbers at cruising seed and altitude, and then in reality what you need to know is the actual power needed to generate that thrust.... there is some complexities there but in general the turbines generate what is known as 'shaft horsepower' which is a good stand in number we're looking for to compare.
The real takeaway is that power is power and energy is energy and regardless of how it gets to do the 'pushing' of the air, if you want to use solar energy (power, at any given moment, not integrating over time) those are the numbers you are comparing. It's all just unit conversions, at least when you're attempting such napkin math. Anything more - taking into account the actual systems, losses, efficiencies, etc.. just makes it all worse, not better.
A cool exercise is - given the 777 wingspan I esitmate and the power output from solar of said size... what kind of current aircraft use piston-driven engines with similar horsepower? Assuming we had motor inverter electronics and an electric motor that was 100% efficient, you could imagine trying to build a similar weight aircraft of that size that has such a wingspan.
This is why you wind up with the only solar powered aircraft out there being superlight, high altitude craft with super high aspect wing surfaces- maximizing wing area to weight / lift capability such as:
https://en.wikipedia.org/wiki/AeroVironment_Helios_Prototype
(note the largest of those prototypes and the tiny amount of power output they were making. All that wing area and it would have barely been able to carry a single passenger as payload and cruise a day without battery power)
Rtx has an interesting patent [1] on this that highlights some of the novelties of this setup vs a traditional hybrid (planetary motor/generator like in a Prius):
It's a boost-only motor, it doesn't/can't harvest energy on descent.
The patented solution (transient smoothing under auto-throttle control) puts electric motors on both the low spool and high spool, then uses a power-splitting algorithm to route high-frequency thrust changes to the electric motors while keeping fuel flow nearly constant on the thermal engine (turbine). The turbine cruises at a steady operating point with tight compressor/turbine clearances and the electric motor smooths out the spikes that are normally there with turbulence and load changes. Benefits: lower fuel burn, longer turbine life (fewer blade-rub risks from speed variation), and smoother ride quality since the auto-throttle bandwidth improves. This setup (based on the various cutaways and photos so far) seems to be only a single 1MW motor so it only runs on the low spool but can still help modulate the turbine decently in the same way it does in the Koenigsegg Regera's hybrid electric setup, that removes the need for a flywheel because the electric motor can smooth out the gas motor's inherent lumpiness.
Also disclosed in a previous press release [2], it's only a 200kWh battery so at 1MW peak boost (cited load during takeoff/ascent) it would only run for ~10-15 minutes at the beginning of the flight.
Seems most of the savings are due in part to not using as much fuel during takeoff (~20% of a 1-hour flight's fuel) but also in large part to the under-sizing and optimization of the thermal turbine to keep it running in it's peak efficiency zone for more of the flight (~10% of a 1-hour flight's fuel).
Curious how the safety margins work here - if the battery is depleted on takeoff (aborted takeoff) or there's an issue that requires descent-then-reascent, if the batteries can't be replenished in-flight there could be a power deficit in that window where you'd normally have 2+2MW of gas turbine power for the plane and now you only have 1+1mw of gas turbine power.
Why not just use the larger turbine with the efficiency benefits ? Or does downsizing the turbine save on so much weight that it makes a big enough difference?
The larger turbine (like a PW100 1.8MW in the dash-8 that this demonstrator is replacing) is at peak efficiency (~0.30kg/kWh output SFC) near full load, then it cuts back to 50% power while cruising where it also drops into a less-efficient SFC rate (~0.36) then down again to 20% (~0.45) for descent vs the "always at 100%" 1MW version which stays pretty much pinned at 0.30kg/kWh sfc sweet spot during all of the flight except descent where it also drops back and takes an efficiency hit.
Yes but then you're erasing some of your fuel efficiency gains on take-off by running a significantly less efficient APU to charge it back up. Also I think the APU is a built in unit designed entirely for in-flight loads (like an alternator on a car) and not intended to provide the kind of energy you'd need to even moderately recharge this pack after takeoff/ascent.
Says that it's boost-only in all the literature I could see, so it can only add power to the prop not generate with it. Regardless, you don't really get into a position where you're harvesting energy in a plane - you just use less power while you're descending. Unlike in a car, most of a passenger jet's flight time is at speeds where drag (which squares with speed) basically means you'd have to nose down at a very aggressive angle to actually pick up speed without the engines providing thrust. The plane's engines are almost always under some kind of load until it is on the tarmac and slowing down so there isn't any opportunity to "regen" during a normal flight.
Yea that makes sense. Can't be hitting stall speed. Although maybe there's an argument for less load / wear on the turbine motor if the electric motors could be involved during landing ?
Or maybe you keep your downsized turbine motor running closer to peak efficiency even during landing and increase the mechanical energy harvesting to your APU or alternator thing or whatever to control speed and use that to charge a small supercapacitor. In the event of an aborted landing the supercapacitor could provide a boost of thrust to your electric motors. This would keep wear on the battery lower and also allow it to dump energy at its peak efficiency
I'm not sure how that would help. If you're descending it's probably to land, and then you can recharge the batteries with electricity from the ground. That would be more efficient (and cheaper) than burning fuel to charge them.
1300 HP electric engine power, now that's an achievement.
I do wonder if prop engines can act as "windmills" (similar to turbine engines, which often in accidents still have been found to provide a bit of hydraulic power), which means regenerative braking could be used instead of speedbrakes.
There are times where the intent is to rapidly shed energy and systems/procedures designed for that (spoilers, speed breaks, slipping the aircraft). I suspect though that implementing any kind of efficient regenerative capability during these times will be something that is developed later/as an afterthought initially. That being said dismissing the idea out of hand is probably wrong. If this system is used during takeoff and climb and would be needed for landing/go around then I imagine they would want minimum energy levels in the batteries so there is probably going to be some system to tap the generator to give them an ability to have minimum energy for landing. Why tap the generator if you can scavenge along the way or just build in robbing power from the prop as the connection to get that power. This system probably naturally beefs up the generator substantially on these aircraft to give them essentially this exact capability. Hmmm.. as I type, the more I think it may happen very quickly honestly. But I'm not an aerospace engineer so this is all a guess!
There would never be a reason to use windmilling props in flight for electrical generation. Even when descending, airliners like the Dash 8 need at least some forward thrust in order to maintain control and stay on the glide slope. They only use reverse thrust for a few seconds during the landing roll.
I've seen references to this capability in reporting elsewhere. It's less useful in an aircraft than a car because you can gradually descend, reducing power proportionally as you're preparing to land. But I'm guessing this is part of where the claimed 30% improvement in efficiency comes from.
After they land, they usually sit around for a while. Not really super important to charge the battery on decent I wouldn't think. Would be interesting to get rid of the battery entirely (or reduce the size) and have a beefed up magsafe plug that was attached during lift off, and came unplugged once it was at cruising altitude.
This is about replacing a turboprop with a slightly smaller turboprop and electric motor/generator+battery. The turboprop burns Jet-A, not leaded aviation gasoline.
If you care about leaded fuel, the culprit now is general aviation - small airports, Cessnas, not commercial flights. If you go work on it, let me know, I'll help you!
Not “30% fuel efficiency”, but an improvement of 30%.
FTA: “The project aims to demonstrate up to 30% improved fuel efficiency for a typical 250-nautical-mile regional turboprop mission”
I was so confused by the title - I thought jets were fairly efficient at ~40%-50% of theoretical maximum and turbofans can't be that far behind. It would maybe make sense for a single prop aircraft.
30% improvement makes much more sense.
HN title mangling strikes again.
I think there are startups making a similar sort of engine for general aviation. It's good to see that there is development of the same idea for commercial aviation.
This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.
This means that the battery is quite small and light, having only enough charge to take off and get to altitude.
I suspect that this system probably improves safety as well, if architected properly. If one or both of the gas engines fail, so long as they are not seized, that electric motor can still provide some power for diversion.
>This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.
Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?
IDK if anyone really answered -this- question properly, so I will.
> Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?
Well, it kinda depends. Where Hybrids get the biggest MPG boost is in city driving. The stop and go traffic lets you use Regen braking and go quite some time without the engine kicking back on while still moving forward.
I still don't understand Honda's system enough to speak well on it, but I can speak to THS because it seems to be the cheapest to do and is most proven on the road.
The general parameters for a THS type system on a Toyota Prius or Rav4, or a Ford Maverick/Fusion/Escape is a 2.0L or 2.5L (at least in modern US examples) engine paired to a simple planetary gearset containing a power split device. It is a single speed (At least in the cheap configurations, however that simplicity is possibly close enough to be viable for air usage vs a reduction gear.) That's part of why they tend to have fairly large engines, the valve timing magic gives them at least a bit more HP to not be too bad on the highway.
> but insufficient for acceleration
Going back to the modern cases, the engine is typically sized large enough to give some acceleration even on the highway. Not always great but usually enough.
At least as far as the non-plug-in hybrids, the 0-35MPH can be surprisingly peppy.
The bigger magic (again, at least as far as THS) is it makes it easy to just run the engine at the 'most optimal RPM' for certain tasks, excess energy gets piped to the battery or back out through the system, this does also help reliability tho, because you can then design the reliability of the engine around certain RPM ranges...
The Prius uses a planetary gear set to blend power of the engine with that of the two motor-generators.
Both the engine and motors are used at all speeds. Particularly during highway acceleration the entire assembly rotates in the same direction.
This is a great related watch if you have some time to kill: https://youtu.be/KnUFH5GX_fI
That's such a fantastic video. I never totally grasped why hybrids were so much more efficient, because my naive assumptions about how they worked were so simplistic. The real-time graphs he showed were excellent for making his points.
TC is filled with these sorts of videos. If you have time to burn then they are basically all this quality. His interests are also just wildly all over the place. From Christmas lights to dishwashers to coffee machines you just don't know what the next video will be.
The Prius is in series, or something like it.
No it’s not. The majority of power comes from the engine. It drives the electric motor mechanically, using it as a transmission. It is not just charging the battery.
Sorry, I meant in the low speed, high acceleration regime (maybe easily confounded with takeoff?). There the engine will turn one motor to generate electricity, which will then power the second motor, like a series hybrid.
No, that's just wrong: at no point is (say) a Toyota Hybrid burning gas to generate electricity to immediately turn another motor that actually moves the wheels.
What do you imagine happens if the battery is low and the car wants to start from a stop? The engine can't shift down to first so it would just stall out if you tried to use it to drive the driveshaft directly, and there's no other source of electricity to put into the motor driving the wheels.
Hybrid cars are mostly parallel hybrids. Only the Chevy Volt comes to mind as a serial hybrid.
Chevy Volt was still a parallel hybrid. The gasoline engine was used for driving the wheels for highway cruise because it was more efficient. I think the range extender version of the BMW i3 was a pure serial hybrid though
Honda's recent hybrids (CR-V, Civic, Accord) are like this too. Mostly series, but the engine can directly drive the wheels via a single overdrive gear for better cruising efficiency.
The BMW i3.
And infamously the ill fated Fisker Karma
The fuel burn of take-off and climb substantially lightens the aircraft for cruise. Electric batteries have no such effect, you're carrying all that dead weight for the rest of the flight. This reduces the passenger or cargo capacity of the aircraft, which reduces potential revenue.
And what if you need two go-arounds?
Don’t conflate airplanes with rockets.
On an airplane, most of the energy in cruise is spent overcoming parasitic drag, not induced drag. It’s spent pushing the airmass out of the way as it moves forward, not creating lift to stay aloft.
For that reason, a change in weight does not significantly change cruise fuel usage.
Weight is still precious, but that’s because airplanes’ load are more often weight constrained than volume constrained, and capital and operating costs are such that you want to maximize the load.
> you're carrying all that dead weight for the rest of the flight
If you're recharging the batteries for extra go-arounds during landing, they are as dead weight as the fuel you would otherwise reserve for that purpose. And if you have 30% more efficient engines, meaning less fuel and smaller engines, it's possible you could come out ahead, weight-wise.
> what if you need two go-arounds
I assume that a go-around requires less sustained power output than a full climb from takeoff, so you will probably get more than one go-around anyway, and we don't know how much over-capacity they're designing for. In any case, any design will require tradeoffs in safety, and having more engine-out capabilities might improve safety enough to overcome the higher risk with go-arounds.
Not saying this project is will work out or that you're even wrong necessarily (this could be the equivalent of a concept car for Pratt & Whitney).
Due to various penalties, wind resistance. gear down and aircraft configuration. A go-around consumes a huge amount of fuel, not as much as climbing to cruise but its alot
> I assume that a go-around requires less sustained power output than a full climb from takeoff,
No.
Source 1: PE = mgh
Source 2: am pilot
So, for a Dash 8-100, at 13,000 kg, disregarding drag, engine efficiency, etc, to take-off and climb to 1000m and accelerate to 150 knots (77 m/s), you will need:
- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude
- 0.5 * 13000 * (77)^2 = 38.5 MJ, to accelerate to your climbing speed.
Total: 127 + 38.5 = 166 MJ, or about 46.11 kWh
For a go around, re-accelerating from 1.3 * stall speed (85 knots / 44 m/s) to your climbing speed, and going to your missed approach altitude of 1000 m, you will need:
- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude
- 0.5 * 13000 * (77^2 - 44^2) = 26 MJ to accelerate back to your climbing speed.
Total: 127 + 26 = 153 MJ, or about 42.5 kWh
Nice to see some numbers. So for the peak load situations, a Dash 8-100 would not require a battery bigger than that a short range BEV ("city", though in reality the short range BEV use case is more for the rural equivalent of stuff that would be walkable in a city setting). And that's even before considering the energy contributed by the fossil fuel engine.
"13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude"
Presumably quite a bit of that would be harvested back during the descent that follows. The conventional engine would still need some excess power (relative to cruise load) to fill the gap left by drag and imperfect circle efficiency of the electric motor/generator, but mass x altitude is stored energy, not lost. (I'm still talking about the "what if we need a second abort" of the root post)
You don't do a full climb after a go-around, so the heights are not equal, and the mass is less since you've expended fuel. You also retain some kinetic energy but I assume that is closer to a negligible effect.
Also, PE = mgh is probably an not a great formula for energy cost of takeoff/go-around, as there are probably large costs it ignores (gravity loss, less efficient engine use, maybe less efficient turbines?).
For your source 2 I have no rebuttal so will have to defer to you, but would ask for an explanation.
Not a pilot, but on approach for landing you bleed off a lot of energy. For a go-around you need to reverse your descent and build up enough energy to fly away again. Take-off/Go-around tends to be the same throttle setting, AFAIK. Of course it also depends on how early you decide to throw away the approach and go around. Doing it at 1000 feet is different from bouncing it off the runway.
but the point they were making is that it inevitably takes less energy to get to a level flying state (in similar weather conditions) due to fuel consumption.
so, unless the pilot is fighting weather it would make sense that equal throttle levels and equal pitch plans in equal weather conditions would require less and less fuel burn until the tanks are empty.
> You don't do a full climb after a go-around,
an IFR missed approach can have you climb quite high, especially in areas with serious terrain. Example: https://aeronav.faa.gov/d-tpp/2607/00346IZLZ17R.PDF airport is at 4400 feet over sea level, but missed approach says: climb to 13,000. Also, some go arounds will lead you to have to divert to an alternate airport, getting there may require climbing high to clear terrain or gaining required engine efficiency to fly the distance.
> And the mass is less since you've expended fuel
In our theoretical aircraft with batteries, mass is the same.
> You also retain some kinetic energy but I assume that is closer to a negligible effect.
Negligible indeed.
> you climb quite high, especially in areas with serious terrain.
Interesting, thanks.
> In our theoretical aircraft with batteries, mass is the same.
The fuel that's expended during cruise reduces the mass.
Time to invent regenerative air brakes, like fold-out windmills.
https://en.wikipedia.org/wiki/Ram_air_turbine
And besides that, there is the looping parachute APU.
https://patents.google.com/patent/US9452721B2/en
Would imagine these are significantly more useful on heavier and faster aircraft - surely the weight and whatnot to retract move them is less worth it for smaller planes ?
They are commonly used on small single-engine fighter jets as well. They typically cannot be retracted once deployed, and free-fall using their own mass, so there is no actuation system weight to account for.
Energy density of liquid fuels cannot be beat by batteries, so this is not competitive if you are looking to maximize cargo. However, there are plenty of short haul flights: private jets, island hopping, regional routes where you need to move little mass.
"And what if you need two go-arounds?"
Easy: you don't try the second landing approach before the battery is sufficiently recharged to contain enough energy for the second abort. Chances are this does not take any longer than going through the pattern anyways.
The saving is not just the dead weight of the bigger engine you'd need to do take-off, climb and abort without electric assist, it's also the fuel saved during cruise from running an engine that is completely designed for efficiency at cruise load instead of for some compromise between cruise efficiency and sufficient peak power for start and abort.
Exactly. A hybrid passenger car can tolerate unpredictable power output that may come with an auxiliary power setup that may or may not be available when stronger dynamics are called for.
A plane doesn’t have this luxury and needs predictable output. The fossil fuel engine either needs a sacrificial “overboost” mode for emergencies (at the cost of wear/long-term longevity), or has to be sized for full power at the ultimate cost of efficiency.
So theoretically if the electric motors fail (or battery is dead) an engine could be sized and designed smaller (for cruise efficiency) but have some sort of boost mode that still ensures safety ? At the cost of increased maintenance or wear or something if it must be used
On anything but very short flights most of the fuel is spent on cruising.
I would classify 290 miles as a very short flight, that's like 1-2 hours or something?
Valid.
Perhaps it's not all negative: the electric portion could give a pilot a bit more glide than the gas portion dies.
This is very clever: instead of focusing on electric only flight, just make the existing engine fly in the most efficient window while the electric engine buffers the flight profile.
Like a big-boy prius.
I kind of wonder how the orders of magnitude work out for solar and wing area.
Solar cells are actually quite thin and could be almost like paint on the wings of aircraft. I wonder if the energy generated vs required is even ballpark. There is plenty of sun at 30,000 feet during the day.
Man this is a middle-schooler level napkin math question.
Take a Boeing 777, it has a wingspan of about 60 meters, and I'll ballpark an average upper wing chord of about 7 meters, for a total upper flat area of about 420 square meters.
High quality modern but standard single sided solar panels can do about 220 watts per square meter is full sunlight (around 22% efficiency tested at a 1000w/m^2 irradiance).
So that is 92,400 watts at full power.
92kW is less than the peak power of a Nissan Leaf. 92kW is 123 horsepower.
The two GE turbofans of a 777 are generating something like 40-50 MW of shaft power during cruise. MW.... megawatts. 50-60,000 horsepower.
Plastering the wing surface of a commercial plane with solar panels would make up less than a quarter of a percent of the total power it uses to produce thrust at cruise, at best case with them fully-lit.
Fully solar sailplanes do exist (NASA's Helios prototypes are an example) but that isn't anything close to a 'normal' aircraft with any appreciable payload/passengers.
So, no need to wonder.
I wish I could upvote you more than once. I now realize my middle-school math studies were sub-par :)
yeah, I was thinking of the solar planes. I also didn't know that "horsepower at cruising speed" was something you could look up. obvious now, thanks!
Cheers- I guess I'll add, the turbofans that all modern airliners use are almost always referred to by their 'thrust' and you ofter see a lot of published numbers of takeoff thrust and such - it is harder to find numbers at cruising seed and altitude, and then in reality what you need to know is the actual power needed to generate that thrust.... there is some complexities there but in general the turbines generate what is known as 'shaft horsepower' which is a good stand in number we're looking for to compare.
The real takeaway is that power is power and energy is energy and regardless of how it gets to do the 'pushing' of the air, if you want to use solar energy (power, at any given moment, not integrating over time) those are the numbers you are comparing. It's all just unit conversions, at least when you're attempting such napkin math. Anything more - taking into account the actual systems, losses, efficiencies, etc.. just makes it all worse, not better.
A cool exercise is - given the 777 wingspan I esitmate and the power output from solar of said size... what kind of current aircraft use piston-driven engines with similar horsepower? Assuming we had motor inverter electronics and an electric motor that was 100% efficient, you could imagine trying to build a similar weight aircraft of that size that has such a wingspan.
This is why you wind up with the only solar powered aircraft out there being superlight, high altitude craft with super high aspect wing surfaces- maximizing wing area to weight / lift capability such as: https://en.wikipedia.org/wiki/AeroVironment_Helios_Prototype (note the largest of those prototypes and the tiny amount of power output they were making. All that wing area and it would have barely been able to carry a single passenger as payload and cruise a day without battery power)
Aptera for airplanes would be cool. I doubt you could generate enough power in-flight, but maybe after sitting outside for a few days
More like an Edison Motors hybrid system.
Rtx has an interesting patent [1] on this that highlights some of the novelties of this setup vs a traditional hybrid (planetary motor/generator like in a Prius):
It's a boost-only motor, it doesn't/can't harvest energy on descent.
The patented solution (transient smoothing under auto-throttle control) puts electric motors on both the low spool and high spool, then uses a power-splitting algorithm to route high-frequency thrust changes to the electric motors while keeping fuel flow nearly constant on the thermal engine (turbine). The turbine cruises at a steady operating point with tight compressor/turbine clearances and the electric motor smooths out the spikes that are normally there with turbulence and load changes. Benefits: lower fuel burn, longer turbine life (fewer blade-rub risks from speed variation), and smoother ride quality since the auto-throttle bandwidth improves. This setup (based on the various cutaways and photos so far) seems to be only a single 1MW motor so it only runs on the low spool but can still help modulate the turbine decently in the same way it does in the Koenigsegg Regera's hybrid electric setup, that removes the need for a flywheel because the electric motor can smooth out the gas motor's inherent lumpiness.
Also disclosed in a previous press release [2], it's only a 200kWh battery so at 1MW peak boost (cited load during takeoff/ascent) it would only run for ~10-15 minutes at the beginning of the flight.
Seems most of the savings are due in part to not using as much fuel during takeoff (~20% of a 1-hour flight's fuel) but also in large part to the under-sizing and optimization of the thermal turbine to keep it running in it's peak efficiency zone for more of the flight (~10% of a 1-hour flight's fuel).
Curious how the safety margins work here - if the battery is depleted on takeoff (aborted takeoff) or there's an issue that requires descent-then-reascent, if the batteries can't be replenished in-flight there could be a power deficit in that window where you'd normally have 2+2MW of gas turbine power for the plane and now you only have 1+1mw of gas turbine power.
[1] https://patents.google.com/patent/US20250296689A1/en
[2] https://www.aerospacetestinginternational.com/news/h55-deliv...
Why not just use the larger turbine with the efficiency benefits ? Or does downsizing the turbine save on so much weight that it makes a big enough difference?
The larger turbine (like a PW100 1.8MW in the dash-8 that this demonstrator is replacing) is at peak efficiency (~0.30kg/kWh output SFC) near full load, then it cuts back to 50% power while cruising where it also drops into a less-efficient SFC rate (~0.36) then down again to 20% (~0.45) for descent vs the "always at 100%" 1MW version which stays pretty much pinned at 0.30kg/kWh sfc sweet spot during all of the flight except descent where it also drops back and takes an efficiency hit.
I’d imagine the apu could recharge it during flight at least somewhat.
Yes but then you're erasing some of your fuel efficiency gains on take-off by running a significantly less efficient APU to charge it back up. Also I think the APU is a built in unit designed entirely for in-flight loads (like an alternator on a car) and not intended to provide the kind of energy you'd need to even moderately recharge this pack after takeoff/ascent.
I wonder if a supercapacitor could be integrated into this setup
Only AI stories show up now, so they had to call it an Ai craft I guess.
This is why it makes sense to carbon price the private jets. On a smaller plane you can innovate much faster.
Love a gearbox those guys over at P&W. Can't get enough gearboxes.
I wonder if during descent instead of cutting power alot to control speed it can instead use the energy to charge the batteries.
Says that it's boost-only in all the literature I could see, so it can only add power to the prop not generate with it. Regardless, you don't really get into a position where you're harvesting energy in a plane - you just use less power while you're descending. Unlike in a car, most of a passenger jet's flight time is at speeds where drag (which squares with speed) basically means you'd have to nose down at a very aggressive angle to actually pick up speed without the engines providing thrust. The plane's engines are almost always under some kind of load until it is on the tarmac and slowing down so there isn't any opportunity to "regen" during a normal flight.
Yea that makes sense. Can't be hitting stall speed. Although maybe there's an argument for less load / wear on the turbine motor if the electric motors could be involved during landing ?
Or maybe you keep your downsized turbine motor running closer to peak efficiency even during landing and increase the mechanical energy harvesting to your APU or alternator thing or whatever to control speed and use that to charge a small supercapacitor. In the event of an aborted landing the supercapacitor could provide a boost of thrust to your electric motors. This would keep wear on the battery lower and also allow it to dump energy at its peak efficiency
I'm not sure how that would help. If you're descending it's probably to land, and then you can recharge the batteries with electricity from the ground. That would be more efficient (and cheaper) than burning fuel to charge them.
1300 HP electric engine power, now that's an achievement.
I do wonder if prop engines can act as "windmills" (similar to turbine engines, which often in accidents still have been found to provide a bit of hydraulic power), which means regenerative braking could be used instead of speedbrakes.
There are times where the intent is to rapidly shed energy and systems/procedures designed for that (spoilers, speed breaks, slipping the aircraft). I suspect though that implementing any kind of efficient regenerative capability during these times will be something that is developed later/as an afterthought initially. That being said dismissing the idea out of hand is probably wrong. If this system is used during takeoff and climb and would be needed for landing/go around then I imagine they would want minimum energy levels in the batteries so there is probably going to be some system to tap the generator to give them an ability to have minimum energy for landing. Why tap the generator if you can scavenge along the way or just build in robbing power from the prop as the connection to get that power. This system probably naturally beefs up the generator substantially on these aircraft to give them essentially this exact capability. Hmmm.. as I type, the more I think it may happen very quickly honestly. But I'm not an aerospace engineer so this is all a guess!
There would never be a reason to use windmilling props in flight for electrical generation. Even when descending, airliners like the Dash 8 need at least some forward thrust in order to maintain control and stay on the glide slope. They only use reverse thrust for a few seconds during the landing roll.
I've seen references to this capability in reporting elsewhere. It's less useful in an aircraft than a car because you can gradually descend, reducing power proportionally as you're preparing to land. But I'm guessing this is part of where the claimed 30% improvement in efficiency comes from.
After they land, they usually sit around for a while. Not really super important to charge the battery on decent I wouldn't think. Would be interesting to get rid of the battery entirely (or reduce the size) and have a beefed up magsafe plug that was attached during lift off, and came unplugged once it was at cruising altitude.
At that point seems like you might as well use a launch system like aircraft carriers use
anything that gets rid of leaded fuel on prop aircraft is a win even if 0% efficiency improvement
This is about replacing a turboprop with a slightly smaller turboprop and electric motor/generator+battery. The turboprop burns Jet-A, not leaded aviation gasoline.
If you care about leaded fuel, the culprit now is general aviation - small airports, Cessnas, not commercial flights. If you go work on it, let me know, I'll help you!
Dont we have approved fuel replacement for leaded fuel now finally ?
Also I swear there was a company replacing a push pull type plane with a hybrid ?
Nothing on recharging the battery in the landing phase, so room for improvement.
Not really much room. Unlike a car going down a hill, an airplane descending still wants thrust from the engines, just not as much as in cruise.
Not an aerospace engineer - but that sounds like a lot of extra cost/complexity/weight, for pretty minimal benefit.