5 ms·

How do you cancel out the total system mass? I'm not completely following your derivation. Keep in mind with your math that you have to account for the VTOL co

How do you cancel out the total system mass? I'm not completely following your derivation. Keep in mind with your math that you have to account for the VTOL component of flight too. Also, the ducted fans they're using are not efficient at all in your stated flight regime. There are other considerations at work here as well. Having a 70l/d ratio means huge wings on something that will be this heavy. Gliders have big wings for their size and they have a very very low gross weight (ASK 21 2 seater at 900kg and a 17m wingspan). On top of just being absolutely massive, having all that wing area can cause problems in other than optimal weather situations. An ultra low wing loading will mean that the craft responds like a kite to any air movement so it would be very uncomfortable for the passengers who just want to get where they're going. I agree that the chances of power loss are remote, especially compared to the current tech in use, but especially with new designs, you need a way of handling it. The issue isn't so much any individual motor but the batteries and support systems. They'll need a cooling system most likely and any failure will turn the whole thing into a Note 7. Also, sorry about using LiPo as a generic term. LiS batteries will definitely help the situation somewhat.

jacquesm 10y ago
And intra-pack screening of batteries to avoid cascading failures.
Robotbeat 10y ago
It's all in that first 0.5 ratio. Basically, that ratio says that if I have a 10 ton aircraft, 5 tons is for the payload, airframe, engines, miscellaneous structure, and takeoff/landing batteries while 5 tons is for the cruise batteries. That 0.5 ratio is probably very optimistic for a real-world vehicle, but not crazily so. Burt Rutan's Voyager and the Virgin Globalflyer got much more ambitious figures, like 0.75 and 0.83, respectively (in this case referring to fuel mass instead of battery mass). You refer to L/D ratio and lack of wing loading as if it's the same thing but it isn't. High performance gliders are generally ballasted (sometimes about half the weight is ballast) in order to increase the wing loading and increase flight speed. Our aircraft would have a 300km/h cruise speed, and so will similarly have a fairly high wing loading, even if it does need long, high aspect-ratio wings. This is helped by the fact that the sort of high performance lithium-ion batteries that you'd be likely to use (i.e. like Tesla uses) would be a good twice as dense as fuel. This increases your effective wing loading for the same outer mold line, allowing higher speed efficient cruise at the same altitude. As far as battery and support system being a central failure point: One interesting thing with electric propulsion is it's fairly easy to split the batteries and subsystems up /as well/ as the motors. So each motor pod could have its own small battery nearby for (possibly emergency) take-off and landing (this could also help reduce cabling mass, especially for the high currents you're likely to need for the vertical takeoff and landing portions). The cruise batteries could remain centralized, since you'll need a lot more of them and mass efficiency will be critical (and you could rely on your very good glide ratio).

jacquesm 10y ago

And intra-pack screening of batteries to avoid cascading failures.

Robotbeat 10y ago

It's all in that first 0.5 ratio. Basically, that ratio says that if I have a 10 ton aircraft, 5 tons is for the payload, airframe, engines, miscellaneous structure, and takeoff/landing batteries while 5 tons is for the cruise batteries. That 0.5 ratio is probably very optimistic for a real-world vehicle, but not crazily so. Burt Rutan's Voyager and the Virgin Globalflyer got much more ambitious figures, like 0.75 and 0.83, respectively (in this case referring to fuel mass instead of battery mass). You refer to L/D ratio and lack of wing loading as if it's the same thing but it isn't. High performance gliders are generally ballasted (sometimes about half the weight is ballast) in order to increase the wing loading and increase flight speed. Our aircraft would have a 300km/h cruise speed, and so will similarly have a fairly high wing loading, even if it does need long, high aspect-ratio wings. This is helped by the fact that the sort of high performance lithium-ion batteries that you'd be likely to use (i.e. like Tesla uses) would be a good twice as dense as fuel. This increases your effective wing loading for the same outer mold line, allowing higher speed efficient cruise at the same altitude. As far as battery and support system being a central failure point: One interesting thing with electric propulsion is it's fairly easy to split the batteries and subsystems up /as well/ as the motors. So each motor pod could have its own small battery nearby for (possibly emergency) take-off and landing (this could also help reduce cabling mass, especially for the high currents you're likely to need for the vertical takeoff and landing portions). The cruise batteries could remain centralized, since you'll need a lot more of them and mass efficiency will be critical (and you could rely on your very good glide ratio).