Reddit Expertise on Aerospace Engineering

There are SO MANY things one could talk about.

The Solar Impulse (1, 2, 3, 4) is flying all the way around the world on solar power. Some solar UAVs have flown for 3 days - and even broken the Blackbird's altitude record - but the couple of solar-powered manned airplanes out there have only flown for a couple hours. Of course, the main problem here is that there is only so much solar power shining on each square foot of airplane, so you can only go so fast before the drag on each square foot catches up with the solar power from each square foot. (Add to that the fact that the airplane is not 100% covered with solar cells, the fact that solar cells are not 100% efficient, and the fact that they ideally should not use up all the solar power and instead should save some of that energy in batteries for flying at night, and… bottom line, they literally have to fly at like 25-30mph).

Electric airplanes are becoming increasingly practical. Of course, their main problem is that batteries have much lower energy density than fuel: Replace the fuel tank with a battery of the same weight, and when you take off, you’ll only have about 5% as much energy to fly as you did with fuel. (Add in a lot more batteries, and you might be up to 15 or 20%). So these airplanes must either fly with very little power, i.e. slowly (i.e. be similar to ultraights or gliders), or they can be flown like a normal airplane but for only 15-30 minutes (e.g. 1, 2). Either way, you’re not going very far. But for flight-training, where what matters are the hours in the air rather than “getting places”, this is starting to become a viable technology. There’s a lot of hype around the E-Fan, but I believe that Randall Fishman’s electric airplanes are even more impressive. See also the Elektra–One and the Pipistrel WattsUp.

Boeing recently flew the first manned fuel-cell-powered airplane. A PEM fuel cell is basically a hydrogen engine that generates electricity. The exhaust product is water, and it can be turned back to hydrogen through electrolysis, i.e. you can “recharge” your fuel tank by plugging it in, just like a battery. So it has the “best of both worlds”: It’s electric, low noise, zero emission, rechargeable, and has the high energy density of hydrogen fuel. Fuel-cell-powered UAVs have set endurance records. There are a few out there.

Bombardier recently flew the first airliner with a geared turbofan engine. Airbus flew theirs shortly thereafter. Mitsubishi will fly theirs any day now, and Boeing is not far behind. These are engines where, due to a geared-down “transmission” (gearbox), the big fan out front spins more slowly than the turbine inside the engine that is powering the fan, like a car in low gear. So the fan can be bigger (and the bigger the fan, the more efficient it is, because it pushes on more air per second and gives less extra speed to each bit of air than if the engine were narrower) without the tips going supersonic (which would require much more power, losing efficiency), and the inside of the engine can spin faster (which is also more optimal, i.e. more fuel-efficient, allowing the turbine to be smaller and lighter while still generating the required horsepower).

The X-48B / X-48C explored Blended-Wing-Bodies, which have less drag than “tube and wing” airplanes. Until the X-48, the stall/spin characteristics of flying wings/BWBs were not well understood: Each one stalled/spun a little differently, i.e. they were all a little surprising (in a bad way), some could not be recovered from this condition and killed their pilots (e.g. the Edwards after whom the base is named). So before we can safely have non-military pilots flying airplanes with this very slick shape, we need to figure out its stall/spin behavior, so that BWB airplanes can be designed to be able to manage and recover from stalls/spins. The X-48 provided some good data on that front.

The X-56 is currently testing flutter-suppression technologies that should allow faster airplanes to fly with thinner, more glider-like, lower-drag wings. The wings on the X-56 are modular, so as soon as they gather data from one set, they can design and build an improved set of wings and “pop it in”.

The X-43 and X-51 showed that sustained scramjet flight is possible. Sure, so far they can only fly for a few minutes at a time, but that actually gets you pretty far when you’re going at TEN TIMES the speed of sound.

Agusta is certifying a civilian tiltrotor. I want one!

And yes, there was an accident, but work is progressing towards allowing regular people to buy seats on a passenger craft that goes to space. Also, with reusable rockets almost becoming a reality, the cost of access to space is about to plummet, because each flight will only pay for the fuel, rather than having to pay for the one-time-use rocket.

Military stuff? The X-37 is a little unmanned space shuttle, flown into orbit by the USAF where it has stayed for more than a year doing who-knows-what. (I bet the ion thrusters are not the most interesting thing it’s testing). The X-47B has taken off and landed on carriers autonomously, and refueled in flight. The A160/YMQ-18 has flown unrefueled for over 18 hours, which is unparalleled for a helicopter.

X-45s have flown in groups, and made decisions (based on the fuel and armament on each X-45 in the group) about which airplane should divert to attack a pop-up target and which should continue on to the original target. When they got to their targets, they calculated and flew their bombing runs to maximize the chance of hitting the target while avoiding enemy attack. Scary AI stuff.

The F-35 is a VTOL, stealthy, supersonic airplane, powered by the most powerful engine ever put in a fighter.The F-22 can break the sound barrier in a vertical climb (and is also stealthy, and more maneuverable than anything else out there).

Companies are trying to develop flying cars; not just this one but also this one, this one, this one, this one…

And then there’s the idea of airplanes that use lots of little electric motors, e.g. here and here and here…

How could I forget the EcoDemonstrators? 737 (1, 2), 757, and 787, each of which tested a variety of new aerodynamic, propulsion, controls, materials, and electrical technologies, all aimed towards reducing fuel burn: Variable-geometry nozzles, adjustable mini trailing edges on ailerons, fuel cells, active laminar flow control, electronic flight bags, ceramic-matrix composites... and a mysterious winglet that no one seems to want to talk about. Speaking of laminar flow control, after many decades of research, the 787-9 is finally implementing it, after some prior testing. Details here, here, and here.

The largest airplane in the world is currently being built. About the size of two 747s, it will carry a rocket large enough to put stuff into orbit. (So, in a way, that accomplishes the "reusable first stage" goal that SpaceX has been having difficulty with.

Also... Supersonic business jets may be on the horizon, after some interesting research into reducing sonic booms: See the shaped demonstrator (here's an enormous but informative PDF) and the Quiet Spike (1, 2, 3).

Lots of neat stuff going on at the moment. It’s a fun time to be in aerospace :)