Robot landing pad for planes without landing gear
Here's an idea that seems a bit wild and scary at first, but it's doable today and has broad benefits: Small aircraft that don't have landing gear, but instead land and take off from robotic "can't miss" platforms pulled by cables on short airfields.
For every small aircraft purchaser, a big decision is whether to get retractable landing gear. They are very expensive, and create a risk of failure, but your plane will fly a lot faster and be more fuel efficient if you get them. What if we could leave the landing gear on the ground?
Imagine a wheeled platform on the runway with robotic control and a variety of systems to perfectly track an approaching aircraft. Pulled by cables, it can accelerate at several "g"s forward and back and left and right. As the aircraft approaches it tracks it and the cockpit display indicates positive lock. If the plane veers left, it veers left. If the plane speeds up it speeds up. Pretty much no matter what the pilot or winds do (other than missing the runway entirely) the plane can't miss landing on it. It's spring loaded so even if the landing is a bit hard the shock is cushioned. Done right, it's just like having fancy shock absorbing landing gear. The platform could also have a means to grab the plane, such as a strong electromagnet (with corresponding use of magnetic metal in the base of the aircraft) or even robotic hooks which grab a bar on the bottom of the aircraft and lock. This would allow the plane to then be slowed very quickly if desired, more quickly than an ordinary plane can do.
The same platform would do take-off, though that's much simpler, at least until the plane lifts off. The guidance system could still track the plane in case it did some odd landing abort. If there are hooks or an electromagnet, the platform could accelerate the plane much more quickly than its own engines, allowing take-off on a much, much shorter runway. You need either 100% assurance of release (which is why an electromagnet has advantages) or enough room to return to a stop if for some reason the aircraft can't be released, but again you can do that in much less room than an independent aircraft. You don't strictly need any connection at all for an ordinary takeoff, though. The plane would be accelerating itself and the platform would just be tracking it, seeing the plane get lighter. You only need a connection for a short-runway takeoff.
There is a huge downside of course. You can only use runways with such a system, and even if such a system were planned, at first there would be few runways equipped with it, leaving a hard chicken-and-egg problem. Ordinary aircraft designs need landing gear as their nose propellers can strike the ground if they land without gear, destroying the propeller. There are aircraft designs where the engine sits above and to the rear, or other places the propeller won't hit the ground in a belly landing. Such layouts could offer very short (partially embedded) wheels on which a landing could be performed in a pinch. Otherwise in an emergency landing propeller damage would be very likely. In the Zeppelin era, they built planes with no landing gear that took off and "landed" via a hook from the Zeppelin.
Or you could even have retractable gear, the very thing whose cost we wanted to eliminate. This makes little sense, unless the grappling system comes into play. With the grappling system, it becomes possible in theory to make workable airstrips for light aircraft that are just 100 meters long! (at 1.5g it takes under 50m to get to a suitable takeoff speed, and then you need another 50m for abort space.) I can't predict what length we would actually settle upon with safety margins, but suddenly it becomes possible to have airstrips in all sorts of places they can't fit today. It's also even possible to make a square or circular airstrip so that landings and takeoffs are always aligned with the wind. With no crosswind landings, things become much simpler for pilots. Of course, the area around the airstrip still has to be free of tall obstructions, as you need room to climb once you leave the platform.
In the future world of electric aircraft, no battery power would be needed for the initial takeoff acceleration, increasing range. The quieter motors of electric aircraft might also allow airports to exist in areas they can't be today.
What does the landing platform system look like? For existing runways, you could build it as a set of 4 robotic trucks in a cross configuration with intersecting cable sets where the platform is. The trucks are heavier than the platform and aircraft, so with rubber tires they can still accelerate the platform very fast to match all movements of the aircraft. For safety, the rear truck would be very short, shorter than the platform. In an emergency, all trucks could release cables and veer away. At a dedicated airport, the cables would be integrated and there need not be motors in the platforms. The main cable could be buried like a cable car to avoid all risk of interaction with wayward aircraft.
It could also be on a single truck, indeed a truck which can drive to any location, including anywhere with enough clear roadway, allowing emergency landings and landings in remote locations. A truck can't accelerate at 1.5gs though it could pay out a cable with a platform that could accelerate that fast.
There would also need to be a system (using small wheels embedded in the bellies of the aircraft) to transfer aircraft off of the platform onto other truck platforms which taxi them to the terminal or hangar.
If we go a bit further into the future, we can also imagine the planes being robotic and even electric. The ability to put airports anywhere would mean that at least for the rich, short hop plane trips around cities become possible. Imagine a twin-engine electric aircraft that flies itself. (Aircraft fly themselves all the time these days, that's nothing new.) Because electric engines are not much more efficient as they get larger the way other engines are, it's more cost effective to make twin engine electrics than twin engine liquid fuel aircraft. With two engines and two battery systems, a twin engine electric can have full redundancy for safety.
Now imagine that these aircraft can take off and land from short tiny airstrips, and fly themselves at 200mph to another airstrip 20 to 50 miles away. That's a commute or a local business visit, not what today we would do by plane. This sort of short range is within the range of batteries today where long trips are not, though may are hoping for new battery technology to change that.
Now merge this with the robocar world, so you get in a robocar, it takes you on a short hop to a nearby short airstrip. You quickly step out and into the aircraft waiting next to you and strap in. Moments later, you are pulled into position onto a takeoff platform. A few seconds of 1.5gs and you're up in the air. 20 minutes later you are 50 miles away and land on another landing platform, where another robocar is waiting next to where your plane stops to take you the last few miles.
This could be a realizable form of the flying car dream that never materialized. (Though I should note that some people feel that because of the higher torque of electric motors, multi-engine electric aircraft may be able to do vertical take-off cost effectively, which could also bring about that vision if battery technology gets 2-4x better.)