From news coverage, it seems that fleet fuel efficiency is the single defining aspect that determines an airline’s profitability in the current high fuel cost environment.
So, I was curious, just how fuel efficient could we make future airliners?
Obviously, even at $40/barrel, fuel costs were significant and fuel efficiency was a primary design concern for any airliner, but on NPR I heard an industry expert declare, “no airliner, not even the 787 Dreamliner, was designed to operate in a $120/barrel environment.” Starting from scratch, maximizing aerodynamics and the use of low weight materials, even allowing planes that flew 20-30% slower, how much more fuel efficiency could we extract from fairly, “conventional,” airliner technologies (that is, no solar-powered planes or lighter than air craft)?
Are there any aircraft companies that could engineer and deliver a significantly more fuel efficient plane than the upcoming, “state of the art,” aircraft such as the 787 or A380 within a decade?
Finally, just how fuel-efficient are current aircraft in terms of passenger mile per gallon or some other standard measure? Obviously this depends greatly on mission profile, etc., but I would be curious to know that in measurements I’m familiar with. Is there an unbiased resource somewhere that lists the, “mileage,” for modern aircraft?
You mean commercial aircraft companies other than Boeing or Airbus? No. Unless, of course you want to add in any military aircraft companies, but the answer to the question as asked is still no.
The power available to the most advanced solar-powered aircraft possible with state-of-the art technology is enormously smaller than what would be required for practical air transport. Indeed, if it were possible to make solar cells that were 100% efficient (all radiation is converted to usable electricity), it would still not be possible to propel an aircraft at jet-type speeds.
And it seems to be a common misconception that lighter-than-air craft are efficient. They aren’t, except when the need is for very low speeds. At anything above about 60mph, the work of moving a huge hull through the air guarantees poor efficiency, especially when measured in the way that’s most important: passenger-miles per gallon (or for freight, ton-miles per gallon).
Here are rough numbers for a B-747, based on technology of about 20 years ago:
Cruise speed: 550 knots (= 633 mph)
Fuel consumption at cruise: 3400 gph (pretty close to a gallon per second)
Full load of passengers : 440 (but aircraft configurations vary a lot)
This yields around 82 passenger-miles per gallon.
My understanding is that with current-technology engines, fuel consumption would be something like 20% lower. And modern aircraft like the 787 and A380 should do meaningfully better.
I wish I had more specific knowledge about this and hopefully someone more knowledgeable will come along and add to this.
One possible improvement is in use and should be coming more widespread in the commercial airliners is relaxed dynamic stability. If the center of gravity is moved backwards closer to the center of lift, the drag is reduced. Traditionally, the CG was kept forward so the aircraft was dynamically stable, which was necessary as long as a human being was in the flight control feeback loop.
With computers controlling the feedback on the flight control system, the stability can be relaxed and drag is reduced. A computer can make the minute adjustments needed to keep the pointy end pointed forward in an unstable aircraft much faster than a human. A less draggy airplane uses less fuel.
The next step in engines is most likely to come from gearing down the fans instead of direct-driving them, allowing them (and their turbines) to operate at their most efficient speeds instead of some compromise. That will also substantially lower their noise levels. Other projects for large aircraft involve large turboprops, something tried in the Eighties but without commercial success.
Efficiencies of airframes are improving more than it might appear, both from increased use of composites for large structures (like in the 787) and in the maturation of computational fluid dynamics (CFD) codes to replace wind-tunnel work. We’re talking the tens of percents already vs. 1980’s designs.
But yes, those are evolutionary, not revolutionary, developments. We’re doing the same things better, not different things, and it’s hard to see what else might be available. Longer-term, some change away from fossil fuels is as imperative for aviation as it is for everyone else, but there are much more constraints on aircraft (mainly from weight) than for ground vehicles. Meanwhile, there are ways to make existing fossil fuels burn with less greenhouse-gas production, and that’s under way.
No doubt. But would an aircraft with solar panels all over be able to supplement its fuel requirements to any useful degree? After all, during day flights, planes are in an ideal position to take advantage of the sun’s rays for most of the flying time.
Although I confess I can’t imagine how a hybrid turbine/electric engine scheme would work.
Let’s assume that a special 747 has wings completely covered with 100% efficient solar panels, and a propulsion system that can somehow use the power thus generated. Solar power is roughly one kilowatt per square meter; the 747’s wing area is 542 square meters. So with the sun directly overhead, 542 kilowatts are available.
At cruise speed, a 747’s engines are putting out around 60,000 kw. The solar cell scheme would thus reduce fuel burn by a bit less than 1%.
Thanks, your post provided a good jumping off point for a lot of topics. One thing I’ve also been unable to pin down is the relative fuel effecincy of turboprops vs. regional jets, although I was suprised to learn that apprently the turboprops are more effecient in many cases. Certainly that qualifies for conventional technologies that could be implemented in the short term, although the geared engines were also very interesting.
Again, I wish there was a website that listed passenger mile per gallon effeciencies or even a calculator where you could enter a mission profile and see the results for a number of planes, but that’s some interesting stuff. I hadn’t realized quite how much more fuel effecient newer designs were compared to ones from the 1980’s, I had assumed that the differences were extremely small because of their outwardly similar appearance.
Short-haul aircraft spend much of their time low and relatively slow, where the propulsive efficiency of a propeller is higher than a fan. That parameter is just the ratio of the airspeed to the average discharge velocity - any “leftover” exhaust velocity is just wasted. Jets are popular in RJ’s because of customer acceptance (they’re quieter and look like “real” airliners).
The manufacturers’ sales and marketing departments have their own, generally proprietary codes that do just that.