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.

Flying cars

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.)

weight of reinforcement

Not knowing enough about planes I would question whether the need to have a distributed, large reinforced structural plating on the underside of the plane would not result in the total fuel cost exceeding the cost of the the small-contact-point structure for the landing gear. This also has poor characteristics for non-standard emergency landings...

Large plating

I am not sure you need that much extra structure. The plane of course already has the structure to put all its weight on the existing gear which come from the wings and under the engine (or sometimes the tail) or in above-wing aircraft the gear are all on the fuselage so the support is already there. Steel is not usually used but it could be easily substituted in a few key places with only a modest amount of weight. Or possibly some rare earth magnets too.

I don’t think this extra weight would be more than the weight of the gear you are losing. Note that retractable gear offer 10-15% more speed or fuel economy, and the gearless craft will do better as it doesn’t have the weight of the gear.

As for how to grab the aircraft, that is a problem to be researched. The magnet is one idea which both is less likely to not let go on takeoff and simple for grabbing on landing. Of course on aircraft carriers they drag a tailhook. I could imagine a retractable bar or hook on the aircraft. If, for some reason it does not catch and you were hoping for a quick stop you would have to do a go-around. You would want to avoid a system that could partly catch. A tailhook makes sense for landing. For takeoff you can manually attach a clamp which releases when the plane gets lift. But this is in the “not quite sure what the best way will be” category, or some may wonder if it can be cost effectively solved. Mainly my goal is to wonder, if we can solve it, what might change in aviation with super short runways and cheaper, more efficient aircraft.

a step further

Half of the energy used in a 200 mile flight is spent during take-off and ascent. Cruising energy is directly proportional to the craft's coefficient of friction. Descent and landing energy is miniscule as long as a landing retry is not necessary.

How about an aircraft tug system that assists takeoff and ascent, then detaches and returns to the airport of origin? The tug could be remotely piloted or automated. The tug expends the energy for take-off and ascent, leaving the aircraft to be lighter (it won't have to carry as much fuel, have as large a battery pack or have as large of an engine/motor because it won't need to achieve the steep ascent slope required during takeoff, the tug does that).

This tug system might work nicely in combination with your automated landing platform. This results in an aircraft without landing gear, an energy storage system (fuel tank or battery) whose mass is cut in half (or kept the same with the vehicle's range doubled) and a smaller quieter engine (more room for people/cargo).

Tug system

This is interesting, particularly if you can get a robotic tow. It is of course quite common for people who do soaring/gliding to get a tow for liftoff, and the tools are well established. Of course, gliders don’t have spinning props up front so this would require an aircraft without a nose propeller. (I doubt people would want to consider starting their engines after releasing the tow cable, except perhaps with electric engines.)

It would be interesting to see if an electric tow aircraft could be made that could pull up a small aircraft very quickly to allow super short takeoff. A cable system on the ground can do any acceleration the airframe and passengers can tolerate; perhaps an electric tow could approach this?

Without that there still is the fuel advantage you point out, even with a long runway. And it also offers the ability for robotic takeoff on aircraft that are not otherwise automated. The tow-drone need not be an autonomous robot entirely, it could still be flown by remote pilots just as many drones are today, but it would have all the advantages of drones — no need to carry the weight and volume of humans and ability to go autonomous for boring things like return to airfield.

My recollect is that you don’t have to pay too much for a glider tow because pilots are working on their skills. Otherwise it would make sense to experiment with remote controlled drone tows at a soaring airport. In that case, the “remote controller” could be the pilot of the sailplane, but it would then need to land itself since that pilot will be busy after.

Aircraft Carriers

If this idea were feasible, I'd expect the navy would be using something like it on aircraft carriers. No chicken-and-egg problem there...

On carriers

The navy isn’t interested in flying lightweight propeller planes from carriers, nor that interested in saving money and fuel. And they do use the tailhook to grab the jets to slow them down. For takeoff they use super-powered jets and a runway high above the sea. And of course they use jumpjets like the Harrier and JSF, which are hugely expensive to operate.

Carriers do use catapults to get the planes in the air, which is what I’m talking about here. The planes still have landing gear though. Without the catapults and catchers they could not land on the short runway. Done as it is, it’s dangerous technology. I’m suggesting that robotics could make it safe enough for everyday use, and so good you don’t even need to keep the gear on the plane.

USS Macon

The US Navy's "trapeze" aircraft flown from airships had (fixed) landing gear -- see photos at: USS Macon. Problem is, though, are you going to put a "catcher" vehicle on every random airstrip where you might have to make an emergency landing?

This idea might improve endurance for drone aircraft, though, and you might be able to put the catcher equipment on a regular truck.

The trapeze aircraft

No Don, while they did fly some aircraft with landing gear, the Macon’s sparrowhawks (and other aircraft for this purpose) were later modified to not have landing gear. Instead the gear were replaced with more fuel capacity.

As I discuss, an emergency landing in such an aircraft is a belly landing, so you want a design where the prop is not destroyed in a belly landing.

As long as emergency landings are rare — and they are — it’s cheaper to do the repairs from the rare belly landings than pay the 10% speed and larger fuel penalty of fixed gear. Or so I am guessing, as I don’t know how many aircraft have been designed to make belly landings more tolerable.

You could certainly make the catcher on a robotic truck that can drive to any location you wish to land. While a truck can’t veer left and right as quickly as something on cables can, it can probably do the job. The maximum acceleration of an aircraft in rough winds on landing is probably not multiple gs.

I think there are some

I think there are some significant problems with this:
- Private planes spend a lot of time at small, rural airports. These airports are more convenient places to refuel during long journeys, or are nice uncrowded places to practice landings, and are usually more fun destinations for pancake or cheese burger meet ups. These airports are sometimes little more than a long piece of concrete and a fuel pump, and would be unlikely to have a tug.
- No-gear landings are very expensive. Props start at about $10k, and engines require a complete rebuilt after a prop strike ($6-10k). And it can take months to get a plane back into flying shape after this. This article has a detailed cost estimate:
http://www.suttonjames.com/uploadedFiles/Resources/News/AviationConsumer...
- Landing gear is not all that heavy or that much of a drag. For planes that are available in both retractable and fixed versions, the retractable is only a few knots faster. No gear is of course faster than retractable, since the wings won't have to generate lift for the weight of the gear, but I do not think this would be significant.

My technology wish list for aviation:
- ADS-B (the new GPS position beacon system for aircraft) to be non-stupid, for it to add to safety.
- Add non-mandatory VHF text messaging between the air traffic control and aircraft while keeping the voice frequency as an option. Most of the ATC traffic is the same thing over and over again, and could easily be compressed into a dozen bytes for each aircraft (assigned heading, altitude, lane, etc). In advanced planes this could even automatically update the altitude and heading reminder 'bugs' on the instruments, or even update the 'sky highway' in modern cockpits (like the G1000). I think a LOT of fuel is wasted over most cities by the restricted bandwidth of the current voice only system we have. If you've ever listened to how fast people talk near busy airports, you'll hear pilots waiting their turn for a few seconds on the voice channel to request something (usually to request some small route optimization, like cutting a corner between two navigation points). Planes need to land and take off with a certain spacing, to be clear of each other's wake turbulence, but I also feel that sometimes the landing and take-offs are slower than optimal because people can't communicate effectively when there are so many planes talking to the same controller. It's like a big game of tetris where you have to control the pieces over a radio-- I feel that if the controller could make a plan for the planes directly on his terminal, which would then automatically text the planes (and get confirmations back from the pilots), that much more optimal stacking could be done.

Gear savings vs. short takeoff

There are two benefits to this approach. On is the savings of the gear. Fixed gear can cost 15 knots on the faster aircraft (about 10%) and retractable gear cost lots of money. In addition, as you say, there is the weight of the gear. The second benefit is the potential for super-STOL at airports that are just 100 to 150m long, and thus can be put just about anywhere.

As you say, at first, you want to land at these remote airports. So the chances are you would have gear (retractable or otherwise) as well as the tailhook or other modifications at first. You would do it to access these tiny airports.

Only after there were lots of airports ready to do such a landing would you dare make a plane that had no gear. (Or had mini-gear for emergencies like the wheels inside floats.)

As to the cost… Certainly if making a new airport, the landing platform would be vastly, vastly cheaper than the extra land and paving required to make a regular length runway.

On an existing airport, of course any new cost is a new cost, not taken without a good reason. In theory, if a landing platform truck (which has a much easier job on a long runway) cost under $100K it would not take too many pilots wanting to base there in cheaper gearless aircraft to justify it, but that would take time.

I probably put the benefits in the wrong order above. The first thing I thought about was saving the gear, but then I realized that allowing a safe airport as short as an aircraft carrier was the bigger win.

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