Electric airplanes

Factual Questions
141

It’s more like the blown flaps of an F-104. The small thin wings were designed for high speeds but the plane was a complete menace on landing. Because of this they added a system that directed bleed air from the engine onto the flaps. It decreased the landing speed by 30 knots.

This is basically what they’ve done to the X-57. When you consider they added the weight and complexity of 12 additional motors along with 12 folding props along with two cruise motors it doesn’t really make much sense. they could have simply put 2 stacked electric motors in the nose of the plane and eliminated all the drag they tried to save with a thinner wing over the original twin engine design.

142

This seems a huge complication: scatter expensive batteries all over the countryside (must be avoided by other aircraft as they come to earth) then undergo the expense of gathering then up and transporting them to where they’re needed.

143

An update on the Eviation Alice… Apparently, the prototype burned up in a battery fire before it could fly. This was convenient, since the design appears to have completely changed anyway.

Gone are the wingtip engines and the tail engine and the taildragger configuration, and the fuselage design, and… The new airplane is a more more investor-friendl… er, conventional twin tail mounted engined, T-tail aircraft (if the speculation is correct in the flight global article).

They are now saying first flight will be in 2021, and certification in 2023, which is still crazy. And the plane shown has wings that are very long and thin for such a plane, and they look pretty close-coupled to the tail. If that thing is airworthy, it’ll need a large runway and will have some unique handling issues. But that wing looks inadequate to me. They probably need to model it like that to make the numbers work for efficiency.

I just noticed that the original design was more of a tandem-wing design looking at how far forward the wing was and how large the V-tail was, I suspect they were trying to make them both lifting surfaces for efficiency like a canard. but the new one has a standard tail, and the wing is significantly further back on the fuselage, to fhe point where is looks like the airplane will be short-coupled.

So over a year since we talked about it, nothing exists except a render, yet they prokise to have it not uust finished but certified in two years.

Keep hanging on to your money. The Moller Skycar has been ‘six months from untethered flight’ for about 30:years now.

144

Update to the Electro-Beaver. MSN
Even they admit an electric jetliner is decades away, but with battery technologies developing at such a rapid rate the new batteries in the EB will be beneficial. Baby steps.
FWIW, one of the biggest stumbling blocks for any new airplane design is getting certification and liability coverage. Both are so risk averse that novel designs are very difficult and expensive to get into production, which is why so many designs start with an existing airframe.

145

So I got around to doing some more reading on the X-57 Maxwell. It’s a very interesting experimental program, and I have no reason to doubt NASA’s preliminary estimates for efficiency, etc.

Unfortunately, they indicate just how far we have to go to make electric aviation possible for mass use.

Here’s the original aircraft specs from the plane they started with, a Tecnam P2006T:

Passengers: 4
Payload: 618 lbs
Empty Weight: 1765 lbs
Max Speed: 178 mph
Cruise: 155 mph
Stall speed: 54 mph
Range: 853 miles
Endurance: 4.25 hours
Wing Loading: 16 lbs/sq ft

Now, the X-57 Maxwell plans to take that plane and ultimately put a new wing on it with 2.6 times the wing loading. By my rough guesstimate, that puts the stall speed up around 75-80 mph, way outside certification limits for a small plane. So NASA uses the 12 little electric motors on the wings to blow air past them to bring the stall back down to FAR 23 limits.

Once at cruise, or maybe a cruise climb of 120 mph or so, the inboard engines are shut down and the plane flies on the two tip engines. This is the cruise configuration that could see up to 500% improvements in efficiency. Some of that comes from electric propulsion, some comes from mitigation of wingtip vortices, but most comes from essentially putting a jet wing on a prop plane. You could get the same efficiency without the 12 little motors if you were willing to have a 175 mph plane that approaches at 120, lands at 100, and stalls at 80, and takes forever to climb to altitude. So the 12 electric motors are there essentially to improve low speed performance to the point where it can match other general aviation planes.

So, the specs of the Maxwell, if all goes according to simulation:

Capacity: 2 people
Range: 100 miles
Endurance: 1 hour
Battery Weight: 860 lbs
battery Usable Capacity: 47 kWh
Stall Speed: 63 mph (small motors running)

So basically what they’ve done is said, “We know wings are a compromise between low speed and high speed performance. Let’s just design a wing optimized for cruise, and use electric motors to augment it for takeoff and landing.” That’s a smart idea, and if it works it might really improve performance of all aircraft. Heck, imagine a normal gas-powered plane with electric-augmented wings. You could either make a normal plane a STOL plane, or you could put cruise wings on, say a Piper Cherokee and without hurting takeoff and landing performance make cruise more efficient and more stable against turbulence. A hybrid plane like this might make a lot of sense.

However… I sense many roadblocks here, especially around certification. Consider that any time you are below 80 mph, you are at the mercy of those little motors. If the motors shut off, instant stall. On takeoff, until you are above 80 (and at a reasonable angle of attack), if the motors quit you are simply going to crash. Even if only one quits just after rotation, it might be enough to stall a wing and crash the plane. The FAA hates flight modes like that.

Also, there will need to be extensive testing of what happens if one, or two, or three of those little motors fail. You’re going to get weird flows over the wing. If an outboard lift engine fails at a slow enough speed, you could get a stall-spin situation. So that entire system will not only have to be super reliable and multiply redundant, but every failure condition will have to be tested extensively. I suspect the motor system will have to be computer controlled, so if one motor fails or develops low power the system would instantly compensate by increasing power to others. That sort of thing. But that’s also a certification nightmare.

After all of this, though, and even if the numbers work out, you wind up with an aircraft that is barely usable. You’re going from an airplane that can carry four people to one that can carry two, and a range that drops from 853 miles to 100 miles. An endurance of 1 hour is only 15 minutes with VFR reserves. So this becomes a very marginal airplane. Also not clear is just how much 14 aviation-certified electric motors and all the control hardware will cost. Or how those 12 spinning or folded propellers will cause additional drag at cruise. Finally, extensive trials will be required for what happens when a tip motor fails at cruise. That should be exciting.

Great technology, and I can see electrically-augmented flight in our future if it works out. But I still don’t think we can do away with fossil fuels. This research could turn into a 10-20% improvement in light aircraft efficiency overall. Maybe more.

Those little motors are 14.4 KW, or 172 KW for all of them. If you only had to run them until you got to a cruise climb of 120 mph, You might only need them for 5 mins at the start and end of each flight. That’s about 28 kWh. We might be able to package that size of battery for a weigh of 400 lbs or so in the future. You’d claw back some of that for having a lighter wing, and you might not need as large a primary gas engine. Knock 50 lbs off the engine, and 200 lbs off the wing, and for the cost of a single passenger you might be able to get an airplane that cruises at half the energy as a traditional one but maintains the same takeoff and landing performance. For planes that travel long distances, this could actually work. They’d still use avGas, just a lot less of it.

146

Here’s an article looking at the current state of three likely E-planes: the Eviation Alice, Lillium Jet, and Heart Aerospace ES-19.

147

The Jetson One has had their official launch. More like a scooter that flies than a plane though and, as I understand it, only has 20 minutes of battery. Still, it is a flying electrically powered craft that can carry a person.

148

A couple of weeks ago, I saw a report on CBS Sunday Morning about the Opener BlackFly, a single-seat electric airplane with a range of about 25 miles.

149

Carrying a person with electric power is not hard, and hasn’t been for a long time. The trick is doing it safely, with reasonable range. The ‘Jetson 1’ looks like a death trap. 20 minutes of battery isn’t enough to do much of anything except maybe fly around in the desert so a drone can take pretty pictures. It has four single points of failure, any one of which will kill you.

There are a number of these craft out there, that basically had the big idea of scaling up a quadcopter and putting a person in it. Great for fund raising on Kickstarter, not great for an actual aircraft. They make much of things like two propellers on each arm as ‘redundancy’, but any FOD picked up and blown though one is likely going through the other as well. And then you get to die.

150

Not read the entire thread. But I wonder if props could be feathered on decent to help recharge batteries.

Sort of like a hybrid car uses for braking.

151

You could do that, but the plane would descend at a much higher rate than is usual for airliners.

152

Yeah, I thought of that. A much stepper decent. Just thinking out of the box.

153

You wouldn’t want to feather them. The purpose of feathering a prop is to stop it windmilling and if you are trying to harness energy, you want them windmilling.

154

Ture. And I doubt the vehicle would recover enough energy in the batteries to do much of anything but…maybe…they’d get enough juice to engage the motors for the last (say) 30 seconds of flight so they could land. Just enough for a controlled landing.

I dunno though. Just speculating.

155

Yeah I used the wrong term. I guess I meant that you may need to adjust the angle of the prop.

Was in an amphibious plane once and the pilot did that and backed the plane into shore.

157

This thread was mostly inspired by NASA’s X-57 Maxwell experimental e-plane. Word now is that the program will be concluded without the X-57 actually taking to the air. Apparently there’s some safety issues with the prototype that they can’t fix.