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  #101  
Old 12-11-2019, 10:45 PM
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Originally Posted by Dr. Strangelove View Post
Another thing that comes to mind, while I'm thinking about it: eliminate the vertical stabilizer.

Of course, craft without a tail or rudder already exist, but carry some disadvantages relating to how they achieve stability. Commercial craft haven't yet gone this route.

Electrics offer another method of achieving stability. Due to the ease of having lots of engines, as well as their fast response time, you can use differential thrust to control yaw.

Because you can have lots of motors, you don't have to worry about what happens when a handful of motors fail. You can even retain control in case of complete loss of battery power by using the props on one side as generators and routing power to the other side. You can balance torque by balancing thrust/drag between more inboard vs. outboard ones.
the vertical stabilizer is not something that can be replaced with differential thrust. In fact, one of the primary functions is to maintain 3 axis control in the event of differential thrust due to an engine failure.

As for sticking a prop at the end of the wing, that just exacerbates stall problems if one of the engines fail. To compensate for it requires a larger tail.

Last edited by Magiver; 12-11-2019 at 10:46 PM.
  #102  
Old 12-11-2019, 11:17 PM
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the vertical stabilizer is not something that can be replaced with differential thrust. In fact, one of the primary functions is to maintain 3 axis control in the event of differential thrust due to an engine failure.
And yet the B-2 Spirit eliminates the vertical stabilizer with, in part, differential thrust. A many-motor e-plane has much more control authority than a pair of turbojets, so it should work even better.

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As for sticking a prop at the end of the wing, that just exacerbates stall problems if one of the engines fail. To compensate for it requires a larger tail.
One of the engines? The X-57 has 14 motors. The loss of just one will reduce the thrust a bit, and probably the range, but not nearly enough to cause stability problems. It won't need all the motors for sustained flight.
  #103  
Old 12-11-2019, 11:55 PM
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Originally Posted by Sam Stone View Post
In any event, we're talking about a few percentage points of efficiency, maybe.
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A smaller, highly loaded wing pushes around with high power is not really an efficient way to go.
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What you're gaining in wing efficiency (maybe a few percent) generally isn't worth the problems with handling, spin recovery, engine failure procedures, and the need for a third engine because you can't fly with one engine out if the other is at the tip of the wing.
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Sure. But you should also expect that tradeoff change to make a relatively small difference in the specification you are trying to improve.
Not to put too fine a point on it, but you're directly contradicting NASA here. NASA is hoping for a 5x efficiency increase (not a few points), and the multi-motor concept, short wing, wingtip motors, and so on are all fundamental to it.

That's not to say that you're definitely wrong--this is still a research program after all--but between you and them, I know who I would put my money on.
  #104  
Old 12-12-2019, 12:21 AM
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And yet the B-2 Spirit eliminates the vertical stabilizer with, in part, differential thrust. A many-motor e-plane has much more control authority than a pair of turbojets, so it should work even better.
the B2 uses elevons and split rudders for yaw, not differential thrust.

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One of the engines? The X-57 has 14 motors. The loss of just one will reduce the thrust a bit, and probably the range, but not nearly enough to cause stability problems. It won't need all the motors for sustained flight.
The little motors are to add lift to the wing for takeoff and climb and are shut down in cruise flight.
  #105  
Old 12-12-2019, 12:39 AM
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the B2 uses elevons and split rudders for yaw, not differential thrust.
http://www.airvectors.net/avb2.html
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The decelerons have to be opened about five degrees before they are effective, and in normal cruising flight they are left slightly open. However, this undermines stealth, so when the bomber is in hostile airspace, it uses differential engine thrust for yaw control.
And anyway, this is just obvious basic physics. Of course you can use differential thrust for yaw. It's rare but not impossible. Highly responsive and redundant motors make it more practical.
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The little motors are to add lift to the wing for takeoff and climb and are shut down in cruise flight.
Yeah, and under normal circumstances the wingtip motors would be enough to provide yaw control. Under a failure situation, you can spin up the small motors for extra control. You just need to land safely in that circumstance.
  #106  
Old 12-12-2019, 01:18 AM
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Not to put too fine a point on it, but you're directly contradicting NASA here. NASA is hoping for a 5x efficiency increase (not a few points), and the multi-motor concept, short wing, wingtip motors, and so on are all fundamental to it.
If you read their cite is says 3.3X lower energy use savings at high speed over the original P2006T and then 5X over that for a total of 16.5. I don't know what they're talking about but it's not the thrust needed at cruise. No way.

It's more likely they're talking about the lifting capacity of the wing at takeoff using the little motors because the small wing won't generate enough lift with the 2 large motors.

I'm not even sure what the claim means. If it takes 150kw to fly it now and they're going to use 120 kw of cruise motors what is a 16.5X lower energy savings at high speed? it's a 20% savings in power and that is at the expense of the power used by all the little motors getting it to cruise altitude. 12 little motors at 10.5 kw is 126 kw which means 246 kw on climb-out. The original plane would use 39% less power on climb-out.

Last edited by Magiver; 12-12-2019 at 01:20 AM.
  #107  
Old 12-12-2019, 01:47 AM
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If you read their cite is says 3.3X lower energy use savings at high speed over the original P2006T and then 5X over that for a total of 16.5. I don't know what they're talking about but it's not the thrust needed at cruise. No way.
See this page:
https://www.nasa.gov/centers/armstro...ts/FS-109.html

The 3.3x is from the original to "Mod II" improvements, and then an additional 1.5x from the Mod II to Mod III improvements, for a combined 5x.

It also says:
Quote:
This design driver includes a 500 percent increase in high-speed cruise efficiency
That seems reasonably clear to me, though I suppose it depends exactly on their starting point.

In any case, I'm not here to quibble about the exact improvement factor. The point is just that they're shooting for far more than a few percentage points.

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12 little motors at 10.5 kw is 126 kw which means 246 kw on climb-out. The original plane would use 39% less power on climb-out.
Ok, but maybe it climbs faster. If they can get more power with little weight penalty, that means they can spend less time in a slow, inefficient flight regime. Low speed means high induced drag.
  #108  
Old 12-12-2019, 01:50 AM
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http://www.airvectors.net/avb2.html

And anyway, this is just obvious basic physics. Of course you can use differential thrust for yaw. It's rare but not impossible. Highly responsive and redundant motors make it more practical.
I'm not sure if I should give you partial credit for that. Maybe 25%. Certainly an interesting tid bit. But you can't use differential thrust for yaw control.

My Cite: The elevons and rudders also control the plane's yaw (rotation along the vertical axis).

What you're describing sounds more like rudder trim than outright yaw control. Rudder is 3rd axis control used to compensate for loss of engine and crosswind component.


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Yeah, and under normal circumstances the wingtip motors would be enough to provide yaw control. Under a failure situation, you can spin up the small motors for extra control. You just need to land safely in that circumstance.
It is my understanding that the little motors are there for wing lift. If you drop a motor on one side and try to compensate with the little motors you're going to lift the wing while the opposing propulsion motor pushes the plane around. You'd have to shut the other engine down and start up all the little motors. So you're trading 126 kw of power used mainly for lift to act as propulsion motors. While they will produce propulsion they will also produce drag with 12 propellers spinning. As far as I can tell their main purpose is to add lift to the wing.

Or to put it simply, the plane has to be certified to fly on just the little motors. Power-wise they should be able to keep it in the air. They represent 79% power which should be more than enough if it was the original wing. However, it's a much smaller with wing loading increased from 17 lbs/sq ft to 45 lbs.
  #109  
Old 12-12-2019, 01:56 AM
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See this page:
https://www.nasa.gov/centers/armstro...ts/FS-109.html

The 3.3x is from the original to "Mod II" improvements, and then an additional 1.5x from the Mod II to Mod III improvements, for a combined 5x.

.
I stand corrected. Blurry vision just saw the 5X.
  #110  
Old 12-12-2019, 02:12 AM
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I stand corrected. Blurry vision just saw the 5X.
Understandable, since their material isn't as clear as it could be.

I did run into this video, which clears up a few things. It contrasts the original craft with the new designs. In particular, it gives these figures for the original:
13 nmi/gal avgas
0.7 km/kWh

And these for the Mod III:
51 nmi/gal
2.9 km/kWh

Now, 13 nmi/gal converts to about 0.7 km/kWh if you do a direct thermal energy conversion: approximately 32 kWh/gal for avgas. The Mod III indeed does several times that figure on a pure electric drive.

So a good portion of the initial 3.3x improvement is just due to the horrible thermal efficiency of piston engines. That's fine, but we expected that just by virtue of it being an EV.

Some of the remainder of that 3.3x will be from aero improvements, and all of the 1.5x will be aero. So, not 5x, but perhaps 2x combined, depending on how you do the math. Regardless, it's significantly better than a handful of percentage points.
  #111  
Old 12-12-2019, 02:26 AM
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Not to put too fine a point on it, but you're directly contradicting NASA here.
No, heís not. Rather, youíve misunderstood whatís going on.
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Notably, the majority of the efficiency increase comes from the switch from gasoline piston engines in the baseline aircraft to high-efficiency electric motors, but a substantial increase still occurs due to the improvement in aerodynamic-propulsive efficiency.
Cite: https://ntrs.nasa.gov/archive/nasa/c...0170001218.pdf

So the 3.3x-5x ďefficiencyĒ goal involves a much, much smaller drag reduction than you understood. Because piston engines are only about 30% efficientóthey waste 70% of their fuel energy as heatóthis isnít an apples-to-apples comparison. (Gas turbines are about 30% more efficient than piston engines in aircraft applications).

In the real world, Iíd be thrilled for the X-57 team if the total aircraft drag at 150 KIAS was reduced by 25-50%. A 100% drag reduction would be extraordinary. 15-20% seems realistic to me.
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That's not to say that you're definitely wrong--this is still a research program after all--but between you and them, I know who I would put my money on.
Oh, are we betting?!? OK!

Several pilots and at least one aerospace engineer have cautioned you that the 500% number isnít realóand weíve explained why. Weíve further fleshed out why the real improvement is likely to be 5-10% of the press-release number. And yet youíre sticking to the press release.

Like the poker aphorism says: if youíve been at the table half an hour and you donít know who the sucker is, itís you.
  #112  
Old 12-12-2019, 02:32 AM
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Understandable, since their material isn't as clear as it could be.

I did run into this video, which clears up a few things. It contrasts the original craft with the new designs. In particular, it gives these figures for the original:
13 nmi/gal avgas
0.7 km/kWh

And these for the Mod III:
51 nmi/gal
2.9 km/kWh

Now, 13 nmi/gal converts to about 0.7 km/kWh if you do a direct thermal energy conversion: approximately 32 kWh/gal for avgas. The Mod III indeed does several times that figure on a pure electric drive.

So a good portion of the initial 3.3x improvement is just due to the horrible thermal efficiency of piston engines. That's fine, but we expected that just by virtue of it being an EV.

Some of the remainder of that 3.3x will be from aero improvements, and all of the 1.5x will be aero. So, not 5x, but perhaps 2x combined, depending on how you do the math. Regardless, it's significantly better than a handful of percentage points.
I was going to bring up the likelihood they were going to use thermal efficiency as a way of making some numbers look good. The implication was that they were creating some new efficiency in the structural design.

It's an end run around the thermal efficiency per lb that petrol has over Lithium Ion batteries and if I remember correctly that was something like 40 to 1. As has been mentioned before, weight isn't a problem with cars nearly as much as it is with airplanes.
  #113  
Old 12-12-2019, 02:32 AM
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Weíve further fleshed out why the real improvement is likely to be 5-10% of the press-release number. And yet youíre sticking to the press release.
See my post above. At the very least, the (hoped for) Mod II -> Mod III improvement of 1.5x is fully due to aero.

Note that I am only disputing the incremental "few percentage points" figure. Or 5-10%, as you're putting it. Whether the real aero improvement is 1.5x, 2x, or more is irrelevant to my point here.

And to be clear, Sam Stone was the one to try taking the hydrocarbon chemical energy figure and go from there. So with respect to that, the 5x figure is the correct one.
  #114  
Old 12-12-2019, 02:36 AM
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No, heís not. Rather, youíve misunderstood whatís going on.
Also, you failed to read all of the claims that I responded to. For example:
Quote:
A smaller, highly loaded wing pushes around with high power is not really an efficient way to go.
That's a highly specific claim that's exactly contradicted by the NASA work, and has nothing to do with the degree of improvement they ultimately make.
  #115  
Old 12-12-2019, 02:36 AM
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As has been mentioned before, weight isn't a problem with cars nearly as much as it is with airplanes.
Yes indeed. This is the fundamental barrier to battery-powered airplanes right now.
  #116  
Old 12-12-2019, 02:37 AM
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Richard Pearse, I'm curious about something.

It looks like the busiest domestic corridor in Australia would be the Melbourne to Sydney run* - a distance of 713km, according to this site.
What sort of available range would you feel comfortable with, if you were flying that route?

*Geez, close to 9 milion passengers a couple of years ago.
I have flown that route, many times, but mostly late at night when ATC holding is less of a problem.

On a nice day, about an hour and twenty minutes of extra endurance would be good. This is enough to have the typical 20 minutes of traffic holding, plus a couple of approaches, and still land with 30 minutes remaining endurance.

On a day with bad weather, enough range / endurance to get to Sydney, fly two approaches, then divert to an alternate (e.g., Canberra), fly an approach and land with 30 minutes remaining. In a passenger jet that equates to around two hours additional endurance.

On a nice night, after curfew, when there isn't much traffic, I'd be happy with an hour.
  #117  
Old 12-12-2019, 02:41 AM
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Also, you failed to read all of the claims that I responded to. For example:
Nah. Your misunderstandings are fundamental. Iím not going to respond to all of them; Iím not your TA.
  #118  
Old 12-12-2019, 02:46 AM
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Nah. Your misunderstandings are fundamental. Iím not going to respond to all of them; Iím not your TA.
Do you agree, or not, that the expected aero improvement is on the order of 1.5x?

Last edited by Dr. Strangelove; 12-12-2019 at 02:49 AM.
  #119  
Old 12-12-2019, 02:49 AM
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In all sincerity, I don’t understand what you’re asking. If you clarify your question, I’ll happily answer it.
  #120  
Old 12-12-2019, 02:52 AM
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Look, Sam Stone stated this:
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In any event, we're talking about a few percentage points of efficiency, maybe.
1.5x, or 50%, is not "a few". 25% is not "a few" either. So is it "a few," or not?
  #121  
Old 12-12-2019, 02:53 AM
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I have flown that route, many times, but mostly late at night when ATC holding is less of a problem.

On a nice day, about an hour and twenty minutes of extra endurance would be good. This is enough to have the typical 20 minutes of traffic holding, plus a couple of approaches, and still land with 30 minutes remaining endurance.

On a day with bad weather, enough range / endurance to get to Sydney, fly two approaches, then divert to an alternate (e.g., Canberra), fly an approach and land with 30 minutes remaining. In a passenger jet that equates to around two hours additional endurance.

On a nice night, after curfew, when there isn't much traffic, I'd be happy with an hour.
H'm. So,worst case day, you'd want about 3.5 hours endurance?
Fuel load in an electric plane is not an issue, obviously. A full charge weighs the same as depleted. So, no matter what, you could always 'fuel' for worst case...unless charging time then becomes the main factor, now that I think of it. Hmmmmmm.
....anyway. I'm wondering, as an interim step, they might want to try a "hybrid" airplane. One with both burner engines - jet or turboprop - and electric engines. Use all engines for takeoff until you reach cruising altitude and speed, and then throttling back the burners and cruising on the electric alone.
I've never seen anything like this suggested or in development, so there's probably something wrong with the idea.
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Old 12-12-2019, 03:02 AM
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I think youíre conflating me with Sam Stone. I never said I thought the drag reduction from the blown wing would be ďa few percent.Ē

I said I thought that a realistic drag reduction would be not 500% but rather 5-10% of that number. That means I expect a net drag reduction for the whole plane of about 25-50% compared to the piston-powered original.

I certainly donít think the X-57 project is a waste of time, if thatís what youíre thinking. But you accepted the press-release claims at face value, arguing that electric planes were so fundamentally different from internal-combustion planes that 500% seemed a realistic number to you. Itís not realistic to me, and I said so. Thatís as far as it goes.
  #123  
Old 12-12-2019, 03:03 AM
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In all sincerity, I donít understand what youíre asking. If you clarify your question, Iíll happily answer it.
Also, in fairness, I misread your post above. You said "5-10% of the press-release number" (not 5-10% total--my mistake), which would be a 25% to 50% improvement. I agree with that, and noted in my post before yours that only the 1.5x from the Mod II to Mod III is clearly only from aero. If they can only achieve a 1.25x improvement, I wouldn't be utterly shocked.

Nevertheless, even the lower end of your range counts as far more than "a few percentage points".
  #124  
Old 12-12-2019, 03:04 AM
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Duplicate postóNM

Last edited by EdelweissPirate; 12-12-2019 at 03:06 AM.
  #125  
Old 12-12-2019, 03:16 AM
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I think youíre conflating me with Sam Stone. I never said I thought the drag reduction from the blown wing would be ďa few percent.Ē
You responded to a post which was entirely directed at Sam Stone, who made several statements which contradict what NASA is saying. They would still contradict what NASA is saying whether the improvement is 25%, 50%, or 500%. Or 10%, for that matter. I probably should not have cited the 5x, but the actual figure isn't crucial to the response.

We are in agreement that the 5x does not reflect the real aero improvement--that is more like 1.5x, as I noted before your post. I'd even agree that this is probably optimistic. Nevertheless, it is still not an incremental improvement.

BTW, thanks for the link above; I'd been looking for something more substantial than press releases and videos.
  #126  
Old 12-12-2019, 03:38 AM
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H'm. So,worst case day, you'd want about 3.5 hours endurance?
Well... On a worst case day, Canberra isn't suitable as an alternate, and so you'd need more endurance to reach a further alternate. The more endurance your aircraft has, the less likely it is to be grounded by the weather forecast for the destination and close alternates.


Quote:
Fuel load in an electric plane is not an issue, obviously. A full charge weighs the same as depleted. So, no matter what, you could always 'fuel' for worst case...unless charging time then becomes the main factor, now that I think of it. Hmmmmmm.
....anyway. I'm wondering, as an interim step, they might want to try a "hybrid" airplane. One with both burner engines - jet or turboprop - and electric engines. Use all engines for takeoff until you reach cruising altitude and speed, and then throttling back the burners and cruising on the electric alone.
I've never seen anything like this suggested or in development, so there's probably something wrong with the idea.
For electric to go big, as in 100+ seat passenger aircraft, you'd need to have a way to vary the battery load and weight, you'd also need to cater for a 30 minute turn around. Having slot out / slot in (SOSI) batteries would allow for an appropriate number of fully charged batteries to be installed for a specific flight.

As for a hybrid craft, I can see it working for a turboprop but I'm not sure if an electric ducted fan would be compatible in profile with jet engines. You are not the first to think of it. https://www.nbcnews.com/mach/science...net-ncna862001
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Old 12-12-2019, 04:17 AM
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I was thinking more along the lines of, say, the Convair B-36, in a way. Picture one of those, way smaller, with the pusher props being connected to electric motors, with a couple of turbofans using basic jet fuel slung underneath the wing, say. It's sort of an interim step to full electrically powered aircraft.
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Old 12-12-2019, 04:32 AM
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I'm not sure if I should give you partial credit for that. Maybe 25%. Certainly an interesting tid bit. But you can't use differential thrust for yaw control.
But they do. Yaw control is entirely by thrust differential when they're going for low radar cross-section. Otherwise it's done with the split flaps, which are basically just like controllable air-brakes. But because they stick out they increase the radar cross-section.

Now, when using the thrust differential, they probably don't have a lot of control authority. The turbojets are slow to respond and being close to the center, probably have some limit to how much torque they can actually apply. But this wouldn't be true for a whole row of motors.

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It is my understanding that the little motors are there for wing lift. If you drop a motor on one side and try to compensate with the little motors you're going to lift the wing while the opposing propulsion motor pushes the plane around.
That's true enough, but the plane still has ailerons which could compensate for that. Obviously you aren't going to control all this with hydraulic or mechanical controls; it would have to be all fly-by-wire. It may not even be a good idea in the end, but given the popularity of flying wings in concept designs (and very occasionally in real life), there's obviously a lot of interest in getting rid of control surfaces.

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You'd have to shut the other engine down and start up all the little motors. So you're trading 126 kw of power used mainly for lift to act as propulsion motors.
You would have to reduce it, but not necessarily shut it down completely. I'm not sure even an ordinary rudder could compensate for a full motor-out scenario without doing something with the motors on the other side. It would definitely be a less efficient regime, but again this is a failure scenario.
  #129  
Old 12-12-2019, 07:12 AM
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Canada's Greg McDougall hopes to fly a Magnix retrofitted electric Beaver this Wednesday.

https://crosscut.com/2019/12/worlds-...could-take-pnw
It flew on Tuesday ... for 15 minutes.

A 62-year old plane electrified. Nope, not a contrast at all.
  #130  
Old 12-12-2019, 08:52 AM
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For electric to go big, as in 100+ seat passenger aircraft, you'd need to have a way to vary the battery load and weight, you'd also need to cater for a 30 minute turn around. Having slot out / slot in (SOSI) batteries would allow for an appropriate number of fully charged batteries to be installed for a specific flight.
People keep coming back to battery weight in this thread. Again, everyone I know of working on storage and propulsion for larger planes is not even considering batteries.
  #131  
Old 12-12-2019, 11:32 AM
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https://jalopnik.com/electric-aviati...ght-1840374924

Electric flight. On Tuesday, commercial flight began.

Rats, beaten to it.

Last edited by smithsb; 12-12-2019 at 11:33 AM.
  #132  
Old 12-12-2019, 11:37 AM
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Originally Posted by Dr. Strangelove View Post
You responded to a post which was entirely directed at Sam Stone, who made several statements which contradict what NASA is saying. They would still contradict what NASA is saying whether the improvement is 25%, 50%, or 500%. Or 10%, for that matter. I probably should not have cited the 5x, but the actual figure isn't crucial to the response.

We are in agreement that the 5x does not reflect the real aero improvement--that is more like 1.5x, as I noted before your post. I'd even agree that this is probably optimistic. Nevertheless, it is still not an incremental improvement.

BTW, thanks for the link above; I'd been looking for something more substantial than press releases and videos.
We appear to be talking past each other. I was not talking about the NASA plane at all.
  #133  
Old 12-12-2019, 04:30 PM
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We appear to be talking past each other. I was not talking about the NASA plane at all.
Fair enough, and for the record I'm more or less in agreement about Eviation. Though the entirety of aviation is filled with fly-by-night (heh) startups, so I'm not sure about the overall relevance.

Nevertheless, you made a few specific, general claims, like that a highly-loaded wing is not the way to go for efficiency. But that's very explicitly a goal of the NASA project and they expect significant improvements--not a few points, but up to 50% (combined with the other factors).

Probably Eviation will fail, but NASA isn't limited by the poor track record of aviation startups; they're just doing basic research. If their low-area wing pans out, we'll see the data.
  #134  
Old 12-12-2019, 05:14 PM
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On Tuesday, commercial flight began.
Note that this was a test flight of a new propulsion system. It's hoped this will eventually pass certification and conduct commercial flights with paying passengers.
  #135  
Old 12-12-2019, 05:49 PM
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Originally Posted by Dr. Strangelove View Post
Fair enough, and for the record I'm more or less in agreement about Eviation. Though the entirety of aviation is filled with fly-by-night (heh) startups, so I'm not sure about the overall relevance.
It's a pet peeve of mine, as I've been involved in aviation for 30 years, including the homebuilt movement, and I'm getting sick of hucksters taking money off of people for fanciful planes with $40 'info-kits', or funding pleas for planes that only exist as sketches or empty mockups. The kickstarter era has turned this type of scam into an art form.

This stuff is made worse by the incredibly poor tech reporting from silicon-valley 'tech' journals like The Verge, who seem to think that every idiotic 'disruptive' kickstarter project or fanciful idea existing only as CGI renderings is the next great thing to be breathlessly reported.

Eviation just has the feel of another company that is mainly engaged in rent-seeking and separating investors from their money. I'd love to be wrong, as their machine would be cool if it works. But then again, so would a Moller Skycar.

The electric Beaver looks like a much better bet, because they started with a robust certified airframe and built out the electrics to solve a very specific problem that fits within the limitations of electric propulsion. The STC process is still going to be a bitch, and they need to double the range to make it work, but at least it's in the ballpark.

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Nevertheless, you made a few specific, general claims, like that a highly-loaded wing is not the way to go for efficiency. But that's very explicitly a goal of the NASA project and they expect significant improvements--not a few points, but up to 50% (combined with the other factors).
I don't believe I said a high wing loading is inefficient. I said that a high wing loading is a tradeoff - poorer takeoff, landing, and climb performance in exchange for higher cruise speeds. It sounds like NASA is trying to use the small electric motors to overcome the takeoff and climb issues so fhat they can use a higher wing loading. I will have to read up on that plane some more,

[/quote]Probably Eviation will fail, but NASA isn't limited by the poor track record of aviation startups; they're just doing basic research. If their low-area wing pans out, we'll see the data.[/QUOTE]

Yes, but NASA does LOTS of research on things that never pan out. Hell, NASA is studying the EM drive, and it's not even physically possible. So the mere existence of a NASA program means next to nothing when it comes to near-future products. I still support the research, and we will learn some important stuff from it that will make electric aircraft more feasible, but NASA doesn't have to worry about little things like certifiability, comfort, cost, manufacturing difficulties, maintenance, etc.
  #136  
Old 12-12-2019, 05:55 PM
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Note that this was a test flight of a new propulsion system. It's hoped this will eventually pass certification and conduct commercial flights with paying passengers.
We talked about this plane at length earlier in this thread. The current incarnation has only a 15 minute range with a 25 minute reserve. That's not nearly enough. But they think they can repackage the batteries and bump the range up to 30 minutes with a 30 min reserve - which is the bare minimum they need for their use case of short island-hopping flights.

This plane would not be usable for much more than that. A 30 minute endurance in a plane like that would be enough for taxi, takeoff, landing, and about 15 minutes of cruise, or a range of maybe 50 miles. That's fine for flying between two of the many islands around Vancouver, but that's about it. Works for them, but will never be a reasonable general purpose electric airplane solution.
  #137  
Old 12-12-2019, 06:07 PM
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Originally Posted by Sam Stone View Post
I don't believe I said a high wing loading is inefficient. I said that a high wing loading is a tradeoff - poorer takeoff, landing, and climb performance in exchange for higher cruise speeds. It sounds like NASA is trying to use the small electric motors to overcome the takeoff and climb issues so fhat they can use a higher wing loading. I will have to read up on that plane some more
You did use the word "efficient" but I won't harp on that point. Perhaps you were mentally including all the extra stuff you need to make a high-load wing have reasonable performance, such as more powerful engines and lift augmentation devices.

The distributed nature of the electric motors enables what they call a "blown wing" configuration. Any pilot already knows that you get significant lift from prop wash; the small motors enables that over the entire wing (with, they say, minimal extra drag at cruise).
  #138  
Old 12-12-2019, 06:44 PM
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Originally Posted by Dr. Strangelove View Post
You did use the word "efficient" but I won't harp on that point. Perhaps you were mentally including all the extra stuff you need to make a high-load wing have reasonable performance, such as more powerful engines and lift augmentation devices.

The distributed nature of the electric motors enables what they call a "blown wing" configuration. Any pilot already knows that you get significant lift from prop wash; the small motors enables that over the entire wing (with, they say, minimal extra drag at cruise).
Yeah, I imagine they function somewhat like leading edge slots - diverting more air over the wing to prevent boundary layer separation at high angles of attack. Is that correct? If so, it could easily be more efficient, as you aren't diverting high pressure air flow from under the wing.

I'll have to read more about that plane. Sounds interesting,
  #139  
Old 12-13-2019, 12:28 AM
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Yeah, I imagine they function somewhat like leading edge slots - diverting more air over the wing to prevent boundary layer separation at high angles of attack. Is that correct?
I think it's really just pushing a lot more air over the wing--it's not a "trick" to get a higher angle of attack without stalling. Whatever extra speed the props can give the air, that directly corresponds to a lower takeoff speed. Like a self-generated headwind.
  #140  
Old 12-13-2019, 01:20 PM
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For electric to go big, as in 100+ seat passenger aircraft, you'd need to have a way to vary the battery load and weight, you'd also need to cater for a 30 minute turn around. Having slot out / slot in (SOSI) batteries would allow for an appropriate number of fully charged batteries to be installed for a specific flight.
Random thought from a nontechnical person: the batteries don't weigh [measurably] less when they discharge...but they're no longer helping, while they do add significant weight. In World War II we extended the range of aircraft with drop tanks, disposable fuel tanks whose weight and drag were eliminated by dropping them from the aircraft. Would droppable SOSI batteries be a good idea? Obviously with a recovery system, like a chute and beacon, or even drone flight back to a charging station. Fly partway out, discharge some battery packs, dump them, keep going without their weight.

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Originally Posted by Ruken View Post
People keep coming back to battery weight in this thread. Again, everyone I know of working on storage and propulsion for larger planes is not even considering batteries.
Battery technology is showing a lot of promise now that interest in renewable energy has stimulated research money. Expect large improvements in the near future.
  #141  
Old 12-13-2019, 06:56 PM
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Yeah, I imagine they function somewhat like leading edge slots - diverting more air over the wing to prevent boundary layer separation at high angles of attack.
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  
Old 12-13-2019, 09:29 PM
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... with a recovery system, like a chute and beacon, or even drone flight back to a charging station. Fly partway out, discharge some battery packs, dump them, keep going without their weight.
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.
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