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.
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.
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.
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.
the B2 uses elevons and split rudders for yaw, not differential thrust.
The little motors are to add lift to the wing for takeoff and climb and are shut down in cruise flight.
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.
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.
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.
See this page:
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:
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.
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.
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.
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.
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.
No, he’s not. Rather, you’ve misunderstood what’s going on.
So the 3.3x-5x “efficiency” goal involves a much, much smaller drag reduction than you understood. Because piston engines are only about 30% efficient—they waste 70% of their fuel energy as heat—this isn’t an apples-to-apples comparison. (Gas turbines are about 30% more efficient than piston engines in aircraft applications).
In the real world, I’d 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.
Oh, are we betting?!? OK!
Several pilots and at least one aerospace engineer have cautioned you that the 500% number isn’t real—and we’ve explained why. We’ve further fleshed out why the real improvement is likely to be 5-10% of the press-release number. And yet you’re sticking to the press release.
Like the poker aphorism says: if you’ve been at the table half an hour and you don’t know who the sucker is, it’s you.
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.
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.
Also, you failed to read all of the claims that I responded to. For example:
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.
Yes indeed. This is the fundamental barrier to battery-powered airplanes right now.
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.
Nah. Your misunderstandings are fundamental. I’m not going to respond to all of them; I’m not your TA.
Do you agree, or not, that the expected aero improvement is on the order of 1.5x?
In all sincerity, I don’t understand what you’re asking. If you clarify your question, I’ll happily answer it.
Look, Sam Stone stated this:
1.5x, or 50%, is not “a few”. 25% is not “a few” either. So is it “a few,” or not?