Science Forums 2

[CraigD]

Originally Posted by Roadam

I think the future of air travel is in flying wings. No matter what kind of engine you use to propel yourself, its the aerodynamics that limits you to certain speeds and power requirements. Making a plane with less drag compared to what it carries it the way to go.

There’s an aerodynamic conundrum around flying wing vs. “tube and wing” and mixed “blended wing-body” designs. The total surface area (wetted area) is reduced in and minimized in blended wing-bodies and flying wings, reducing parasitic drag (drag not related to producing lift) but in most cases, the wingspan (and resulting aspect ratio) must be reduced, increasing induced drag (drag related to producing lift). The result of this tradeoff is often that blended wing-bodies and FWs often have about the same efficiency as equivalent conventional TAWs.

Another issue with FWs – one of which model airplane hobbyists like myself, and early full-scale designers like the legendary Jack Northropare/were acutely aware – is stability, in particular pitch (nose up/down) stability.

With a TAW aircraft, either of the conventional horizontal stabilizer wing in the rear or in the front (canard) configuration, static (no control input required) pitch stability is due to differences in the lift of front and rear wing at different pitches. With a single FW, however, this difference must occur in different sections of the wing, and, unless the wind is extraordinarily long, such as with a delta wing, the effective arm of the correcting force much shorter than the TAW. It’s hard to build an efficient flying wing that’s statically stable, so most of even the oldest flying wings have used some sort of active system, either mechanical or computerized.

Practically, however, the stability issues is, I think, a non-issue, as most large modern aircraft are currently critically dependent on computer control systems. The old-fashioned design goal of making a plane that can be flown in the event of a complete power outage is, except for the small niche of lower-quality control “sport/experimental” planes, and “brush planes” and others designed for lower-tech support environments, of little relevance.

Originally Posted by Roadam

And another thing. Flying wings have more or less flat top. So it would be easier to fir PVs onto it.

A very good point, IMHO.

Originally Posted by Roadam

But I think that Any heavier than air aircraft that would be useful cannot rely solely on solar cells.

I agree. This is almost a certainty, as for a typical solar power output, the solar power flux at Earth is at theoretical max about 680 W/m^2. A large aircraft like a 747 has a total top cross section area of about 1000 m^2, so a 100% efficient direct solar system on a clear summer day would have about 700000 W, but needs about 200 times that much to have the same performance.

Pure solar airplane are possible, but capable only of low speeds - though very high altitudes. MacCready’s Gossamer Penguin, Solar Challenger (4500 W prop shaft output with the highest speed, about 50 km/h) and the unmanned Helios (21000 W, highest sustained altitude of any winged aircraft of 29523.8 m) are examples.

In cloudy weather and at night, pure solar planes get hardly any power, so can’t sustain altitude.

Practically, then, electric planes must have energy storage, such as batteries of fuel cells. The electric-powered Sonex Waiex sport plane is an example.

I’ve been doing a bit of figuring since I stated

Originally Posted by CraigD

There don’t appear to me to be any major show-stopper barriers to, in the next few years, building a “plug-in solar” electric airliner in the 30-seat, 500 km/h, 1500 km range “regional” class (eg: the BAe Jetstream 41)

The numbers are daunting.

Converting the Jetstream 41 to run on conventional LiPo batteries would give it about 9 minutes flight time, vs. its usual 2.5 hours – about enough take off, run a single pattern, and land. Assuming the best batteries available by about 2012, the flight time increases to about 27 minutes. Hydrogen fuel cells give about the same result.

Clearly, a 30-seat battery electric airliner would need to be designed from scratch, minimizing airframe mass and maximizing battery mass. By rough scaling estimate, the total mass would need to be about 15 times that of an equivalent kerosene turboprop, or about half the mass of a 400 passenger 747.

These number cause me to seriously rethink the feasibility of battery or fuel cell electric airliners, and lead me to wonder what options remain. Nuclear?