Electric planes

[swaltner]

Two different types of thrust reversers:

https://upload.wikimedia.org/wikipedia/commons/4/4c/Airbus_A340-313E,_Finnair_JP7679383.jpg

https://upload.wikimedia.org/wikipedia/commons/3/3c/Klm_f100_ph-kle_arp.jpg

In both cases, the air flowing around the outside of the core of the turbine engine is redirected forward instead of going straight out the back of the engine. This gives thrust that is at least partially directed forward, and slows down the aircraft. The (Space) Shuttle Training Aircraft actually put the thrust reversers on in flight to be able to replicate the rapid descents the Space Shuttle did in glider mode. "Folks, we'll now begin our descent from 30,000'. We'll be on the ground in 75 seconds. Please buckle your seatbelts."

Regarding the electric airplanes and being able to regen power from cruise altitude, I don't see that as being a viable option. If possible (not exceeding Vne, not exceeding the red-line on the rpm of a piston engine, not exceeding the max airspeed when below 10,000' AGL (above ground), not exceeding the airspeed limit if you're not in smooth air, a pilot will keep the power in as much as possible. You're cruising along at 200 mph at 10,000' and it's time to descend, so the power stays in and you descend at 225 mph or whatever it takes to have you reach the pattern altitude a couple miles before the airport. You will have to adjust the power a little on the descent, but as someone else mentioned, you don't bring the engine to an idle (or even close to it), so the real chance to recoup energy via regen isn't there.

I think the electric system like what Pipistrel offers in the Alpha Electro really shines in the pilot training market. This flight profile is typically traffic pattern work right around the airport. While doing touch and go's, the electric motor could regenerate power. During a descent in the traffic pattern at the airport, you add flaps. This increases both lift as well as drag that is generated by the wing. The added drag is helpful because it allows steeper descents without increasing your speed. When using flaps, this energy is just wasted. If instead of flaps to increase drag, you generated electricity from the electric motor, that is doing something useful, which is recharging the battery for your next takeoff. The AOPA video referenced below says for every 6 trips around the pattern, you are able to recoup enough energy for an extra trip around the pattern.

One other benefit of using regen instead of flaps is in the event of a go-around, or aborted landing due to an unexpected event. While this shouldn't be an issue, the chain of events leading many aircraft accidents starts with a poorly executed go-around. The pilot is in a low power, low energy, high drag state and needs to build up speed before they can safely start a climb. With flaps, the plane goes to full power and then you have to SLOWLY raise the flaps. Raise them too quickly, and you lose too much lift and settle to the ground (crash). Don't raise them fast enough and the added drag of the flaps will keep you from being able to climb quick enough (crash). With a propeller in regen and no flaps, you can simply set the electric motor to full power and instantly get rid of all the drag you wanted for your descent but not lose the precious lift that the wings is generating.

This is very similar to how hybrids and EVs regen power. Instead of wasting energy by putting it into the brakes as heat, they put it into the battery for use to get the car back up to speed. In the case of the electric airplane, instead of deploying flaps to waste energy by moving more air, they recharge the batteries for having a little boost on the next takeoff. In a car, it's more efficient to coast to a stop than use regen. In a plane, it would be more efficient to do a long descent without going into regen. regen is not 100% efficient, so if you can avoid it, you're better off. Having said that, while it's not 100% efficient, it is better than the alternative of not trying to recover any energy.

AVweb posted a video where they took the Pipistrel Alpha Electro for a flight. Pipistrel refers to the propeller as a propmill (propeller / windmill - efficient in both thrust and drag modes). It's definitely a different shape than standard propellers. AOPA also posted a "review" of the Alpha Electro. The AOPA video has a lot more info on the components in the Alpha Electro.

 

[daniel]

Yup - Thrust reversal - Wikipedia

Two different types of thrust reversers:
<...snip...>

Very cool. Thanks for all that!

 

[Etna]

You will have to adjust the power a little on the descent, but as someone else mentioned, you don't bring the engine to an idle (or even close to it),

You bring an interesting point. Private airplanes do not fly descents the same way as airliners. As a private pilot I also descend as fast as possible, so that's with adding some power as you mention. Airliners (and private jets) on the other hand always try to fly the entire descent with engines in flight idle. I was referring to airliners because we were discussing turbofans and didn't think about private airplanes.

As for traffic patterns, using regen instead of flaps could work to some extent but I am skeptical about the 6 trip claim. Moreover, this would train a pilot for an approach/landing technique that has a practical application only in a training environment, which is self defeating.

 

[Laserbrain]

A new electric VTOL (vertical take-off and landing) jet:

 

[daniel]

^ They begin with the caption "We believe in a world in which everyone can fly anywhere anytime." Does anybody really believe it would be remotely safe to allow everyone to fly a plane? VTOL certainly increases the opportunities for air transportation for those wealthy enough to afford it. But a world in which everyone can afford to have a jet pick them up at home is not going to happen any time soon. The VTOL jet has some distinct advantages over a helicopter, but I doubt it will significantly reduce the cost of the kind of trips where helicopters are commonly used.

Great idea. Extremely over-hyped.

 

[RubberToe]

No person would be flying these, they would be completely automated. That is a given.

What is happening in the "flying car" space is the convergence of 3 things:
1. Battery energy density increasing
2. Rapid transition in control technology from piloted flight to autonomous flight
3. Massive investment in vehicles capable of providing short range (30-50 mile) flights

There are VERY many use cases that could take advantage of these kinds of capabilities. The technology is going to be available very soon to be able to move a couple passengers from point A to point B very rapidly, with the JFK to Manhattan shuttle being only a single example.

I think the biggest hurdle will be when they try getting the regulatory framework in place for operating these kinds of systems. The FAA will take probably 1-2 years before even allowing limited test flights in remote areas. Then another 1-2+ years to iron out the rules for letting these vehicles fly in controlled airspace (i.e. from JFK to Manhattan). Countries with less stringent air space control may see this happen before we do in the U.S. Then there is the requirement for what needs to essentially be "hack proof" command and control, so that the bad guys can't take control of the systems and do a large scale 9-11 type event.

I would be willing to bet that within 5 years it will not be an uncommon sight to see these flying 100 feet above a crowded freeway. I don't think that the average person (even on this forum) realizes what a game changing technology this is going to be. That is the reason why there are so many companies working on these prototypes.

The EV analogy from 5 years back being that there were no Tesla Model S's on the road in early 2012. They are now making thousands per week.

RT

 

[Etna]

Agreed. Speaking of the Lilium Jet, IMHO they do not have the right concept. All these little fans are terribly inefficient and noisy - knowing that the most efficient VTOL aircraft is a single fan (I think these are called helicopters). You want the minimum number possible for redundancy without the need for autorotation, and that's probably just four fans like the CityAirbus or the Pop.up concepts.

 

[JRP3]

I would be willing to bet that within 5 years it will not be an uncommon sight to see these flying 100 feet above a crowded freeway.

Seems like a bad idea since it would almost guarantee any crash would land on the most number of vehicles. I also we'll see a large number of these, if any, in 5 years time.

 

[daniel]

I think it will be more then 5 years before these are capable of completely automated flight. The time will come, I just think it's further off than that. As for their claim that everyone can fly, well, they are going to be extremely expensive, even as a taxi service. Flying 100 feet over freeways? No way! Safety will prohibit that. Finally, air space congestion will be a big issue that will limit how many there can be in any given region, thereby keeping the price up. Note also that if they're automated, there needs to be a protocol for air traffic control to communicate with the computer. This will take time to develop. And if you use these to get to the airport, well, the air space around an airport is already very congested.

For at least 50 years these will be a luxury for the very rich. Not something for "everybody."

 

[Etna]

Sure let's forget the freeways. There is no point to constrain the trajectory to a freeway.
Airspace congestion is an entirely different matter when you reserve low altitude (<1000ft) airspaces to autonomous air vehicles. The autonomy allows fairly dense traffic schemes, not as dense as cars on a highway but certainly a lot more dense than current aviation traffic around airports.
Now I completely agree this will take years (10-15?) to develop but maybe in five years some places like Dubai will have limited experiments ongoing.
Airbus is thinking $250/hour operating cost for this type of transportation pod - a 10 minutes ride would cost something like $25. That's about the same ballpark as a taxi ride. I don't see this as a luxury product at all.

 

[JRP3]

I also doubt we'll see a large number of these, if any, in 5 years time.

*Too late to edit the original

 

[ggr]

Agreed. Speaking of the Lilium Jet, IMHO they do not have the right concept. All these little fans are terribly inefficient and noisy - knowing that the most efficient VTOL aircraft is a single fan (I think these are called helicopters). You want the minimum number possible for redundancy without the need for autorotation, and that's probably just four fans like the CityAirbus or the Pop.up concepts.

Really? Then why is NASA building the X-57?

NASA’s aeronautical innovators hope to validate the idea that distributing electric power across a number of motors integrated with an aircraft in this way will result in a five-time reduction in the energy required for a private plane to cruise at 175 mph.

 

[daniel]

Sure let's forget the freeways. There is no point to constrain the trajectory to a freeway.
Airspace congestion is an entirely different matter when you reserve low altitude (<1000ft) airspaces to autonomous air vehicles. The autonomy allows fairly dense traffic schemes, not as dense as cars on a highway but certainly a lot more dense than current aviation traffic around airports.
Now I completely agree this will take years (10-15?) to develop but maybe in five years some places like Dubai will have limited experiments ongoing.
Airbus is thinking $250/hour operating cost for this type of transportation pod - a 10 minutes ride would cost something like $25. That's about the same ballpark as a taxi ride. I don't see this as a luxury product at all.

I can imagine a time when the on-board computer can track every other aircraft within, say, ten miles, and shake hands with every one to avoid collisions. Note that without well-defined lanes (as on roads) it's not enough to see the vehicles around you, but you need to communicate and decide which way to turn to avoid collision. And with many vehicles, all must communicate with every other one, in real time, and agree not just one-on-one, but as a collective.

This just isn't going to happen sooner than 20 or 30 years, and my guess is more like 50 years. You need computing power, software, protocols, and hardware. The real flaw in the flying car idea is the notion that the air space can accommodate a thousand times more vehicles than we have at present. Because we do have airplanes and helicopters now. They are expensive to buy and maintain, and require extensive training to operate.

I believe it costs around $1,500 per hour to charter a helicopter. I don't believe these "flying cars" will be available for $250/hour. Airbus may be saying that, but I call b.s., especially since they're taking about a machine that does not yet exist.

There's an annoying thing that happens in the media: They'll report on some fascinating new discovery or invention, with real potential, and then speculate wildly on how it will cure cancer or solve all the world's problems. The small VTOL jet is fascinating, but it ain't gonna be cheap transportation for the common man. When I was young, nuclear energy was going to be so cheap we would not have electric meters. Just a monthly fee for the maintenance of the wires to our houses. Look where that went! Predictions of cheap miracles to come never pan out.

 

[ecarfan]

I think some of you are being overly pessimistic about the ability of technology and software to solve the personal air transport challenge. The Lilium is a remarkable design created by people with passion and vision. I am cheering them on. I don't know if they will achieve their goal, but I admire their spirt. Lilium

 

[Etna]

Really? Then why is NASA building the X-57?

Really. The X-57 is not a VTOL aircraft. Its purpose is to validate the concept of Distributed Electric Propulsion (DEP) by careful placement of the propellers relative to the wing - something the Lilium Jet is not trying to do.

The general idea of DEP is to allow much higher lift coefficients (on the order of 5) for takeoff and landing, so you can have a wing that's basically half the area. That means you need to cruise at a much higher lift coefficient and thus higher drag, which is a "really bad idea"™. I have read a few technical papers on DEP and how they claim a higher lift to drag ratio and thus lower overall power requirement for a given speed, but I am sceptical. Certainly their claim on the page you linked of a five-time reduction in energy requirement is a claim that raises a red flag.

 

[daniel]

I think some of you are being overly pessimistic about the ability of technology and software to solve the personal air transport challenge. The Lilium is a remarkable design created by people with passion and vision. I am cheering them on. I don't know if they will achieve their goal, but I admire their spirt. Lilium

I agree. I just think the control issue is vastly more complicated in a 3D environment, and that it will take longer to solve than the optimistic estimates. I also think flying cars will never be economically feasible as daily transportation for the common folks.

Here's a thought experiment: Imagine there were no roads, but whole cities were flat open spaces, and people drove a straight line from point to point. No stop signs, no lanes. In fact, no brakes allowing cars to stop. You just had to swerve to avoid hitting each other. Now square that difficulty to account for the whole thing being in the air. This is not a trivial computation-communication problem for a system of autonomous vehicles in an extremely congested environment. These things are being tested today in empty air space just to see if they can take off, control themselves, and land. As soon as you have two in the same air space, it becomes much more difficult. Now imagine a city where everyone wants to fly rather than drive and you have a computational problem today's computers are entirely inadequate to cope with. This will change. But not overnight.

I, too, applaud Lilium. I only criticize those who see this as a harbinger of cheap autonomous flying cars for the common man in five years.

 

[Sancho]

Calling the Lilium jet a "flying car" both overstates and understates what it really is. It's light aircraft, a mature business category. There are just two main innovations that need to be fully worked out, which together are game changers:

1) Electric powered. Everyone on this forum knows the huge advantages this brings in terms of operating, maintenance and construction costs, noise reduction, environment, etc

2) Vertical liftoff and landing. Not only eliminates need for airports, but also makes it much easier to learn to fly. Couple this with more sophisticated autopilot advances, and anyone could operate it, or it could even get someplace on its own.

The other major Lilium innovation is its proposed business model of a taxi service that is 10 times faster than a taxi at a lower fare. That is the kind of disruption that reinvents an industry.

Tesla already has connections to Lilium (it's new hiring director was formerly one of Tesla's first key employees) and there are obvious synergies between the two companies. I wouldn't be surprised to see a merger or joint venture before this jet accepts its first paying customer.

 

[daniel]

I'm skeptical of the statement that VTOL makes it easier to fly. Helicopters are much harder to fly than conventional fixed-wing aircraft. Computer control changes everything, of course, but then we're no longer talking about learning to fly. Now we're just talking about learning to type a destination into a keyboard. And you still have the problem of scaling up to a congested environment.

I think the term "flying car" is overused, and I doubt there'll be much of a market at a realistic price point for a machine that drives on the road and flies. VTOL eliminates the need for driving on the road.

 

[tga]

I think the term "flying car" is overused, and I doubt there'll be much of a market at a realistic price point for a machine that drives on the road and flies. VTOL eliminates the need for driving on the road.

I'm not sure how we got on "flying cars" - Lilium doesn't make that claim, and this isn't a road-going vehicle.

There are some eyebrow-raising claims on their site, like "energy efficiency will be better than or comparable to an electric car," "90% less energy than drone-style aircraft," and "in flight, the Jet's power consumption per km will be comparable to an electric car."

Really? At 300 km/hr? I guess the laws of physics don't apply in Lilium-land. At those speeds, your S would draw 9x the power than at 100km/hr.

Maybe they're comparing consumption per km to a 300km/hr electric car.

 

[WarpedOne]

I'm skeptical of the statement that VTOL makes it easier to fly. Helicopters are much harder to fly than conventional fixed-wing aircraft.

The missing detail is helicopter uses one giant prop to generate all the lift. It thus suffers big rotational force that must at all times be exactly matched with tail rotor or else...

These VTOL concepts all use multi-prop setups where rotational force is canceled out between pairs of props.