Urban Air Mobility — Make Every Landing Into a Forced Landing
Uber envisioned people zipping around the Bay Area in electric air taxis. However, the road from concept to FAA certification is proving to be long. This reality sunk in and Uber sold its Elevate venture to Joby, a leader in electric aircrafts based in California.
Missing from the futuristic designs proposed are plausible ways to land safely in case of major malfunctions. Most have very small wings, some none at all. The drone-inspired designs rely on fancy electronics to save the day. All are shown taking off vertically to altitude, something even helicopters don’t dare do.
Small airplanes and helicopters can land fine without engine power, albeit a little rough. Ideally, urban aircrafts should be designed to perform routine landings essentially the same way as forced landings. This way pilots will be highly proficient in handling emergencies over cities.
The viability of urban aviation hinges on the capability to land unpowered safely on busy city streets
Let’s explore what exists today and what needs inventing for urban aircrafts to have this capability.
Wings Save The Day
Small aircrafts like the venerable Cessna 172 are simple machines. They’re fine gliders when the engine quits. In fact, landings are routinely done without much engine power at all. Given some grassy field, chances are you’ll make it down safely with a dead engine.
The Cessna 172 has a glide ratio of 8:1. That means it can glide 8,000 feet horizontally for every 1,000 feet of altitude lost. That’s almost 2 miles!
Moreover, the flight controls are operated by steel cables and pilot muscles. These keep working even when everything else fails.
Newer aircrafts like the Cirrus SR22 have a whole-aircraft parachute system. But it’s used only as a last-resort since the parachute cannot be steered or controlled.
So wings can save the day. However, the shallow approach, high speed and long ground distance make city landings dicey propositions.
Still, recent advancements in short takeoff and landing (STOL) are quite impressive. The current landing distance record is 9 feet! Of course, it helps to have strong offshore headwinds.
Are bush airplanes the most viable design today for city operations? You might say that cities and the great outdoors have in common a lot of rough terrain.
Dead Man’s Curve
Helicopters can glide too when the engine quits, provided they have enough horizontal speed. The windmilling rotor blades then act as a wing in autorotation. That’s why helicopter pilots always seem in a hurry to gain forward speed on takeoff.
They’re careful in avoiding Dead Man’s Curve. That’s the nickname given to a chart describing speeds and altitudes where an unpowered forced landing is not survivable. Typically that’s too low and too slow.
Having a glide ratio around 4:1, an unpowered helicopter can glide almost one mile in any direction from an altitude of 1,000 feet. The steeper angle of descent is actually a good thing over a city. Unpowered helicopters remain highly maneuverable too, perfect for tight urban canyons. Best of all, the landing footprint is very small.
This Bell UH-1 Huey above is practicing an unpowered landing with autorotation. The engine is kept running for safety, but it’s not giving any power to the rotor. An impressive job there!
So helicopters have decidedly many of the capabilities required for urban operations. But they are noisy. And those long spinning blades are dangerous near the ground. They can get snagged in wires, strike pedestrians or even fly off.
These are similar to the drone you might have gotten for Christmas. They rely on a high-speed flight computer to control the motors at all times. No human pilot is able to fly a multicopter unassisted.
But if the computer dies, there are no good options. Multicopters usually don’t have wings, so no gliding. Autorotation like helicopters is not possible either because the propellers are just too small.
Therefore, their Dead Man’s Curve probably starts around 20 feet. And there’s no safe altitude above that. Even with a parachute, you’ll need enough height for it to open. It will be a wild ride down, dangerous for you and everyone below.
More promising are multicopters with tilting wings or motors. They fly like airplanes then transition into vertical mode. But this mode is still inescapably subject to Dead Man’s Curve. So real life takeoffs and landings will probably be a lot more horizontal that depicted.
However, multicopters have perfected the use of their small motors for exceptional maneuverability. Urban aircrafts could benefit from this idea to supplement control surfaces at very low speed.
Fly Like A Bird 2.0
Mother Nature has already produced some of the finest aeronautical designs for operation in cities. Birds boast advanced technologies like wing morphing enabling them to routinely perform tight city maneuvers.
This eagle can fold its wings to swoop down with precision. The V-shaped dihedral reduces lift, resulting in a steep angle of descent. But it also provides excellent stability and maneuverability.
When close to the ground, the eagle extends its legs to enable maximum shock absorption. It will then rotate its body into a full stall and extend fully its wings and tail to trap air ahead into a cushion. The eagle will finally flap wings forward to use their lift and weight as levers to push itself back as is lands.
On the takeoff many birds are capable of climbing vertically. But this requires a lot of effort and comes with a Dead Man’s Curve of its own. Birds are well aware of this. That’s why they prefer taking off like helicopters with a forward motion.
Inventing A Safe Urban Aircraft
Having explored the state of the art in avian and aircraft designs, we’ll explore in Part 2 the must-haves for a viable urban aircraft. It will borrow from birds, helicopters, airplanes and multicopters. But our aircraft might look very different from what we’ve seen so far.
About The Author
Pierre is a deep learning engineer at a major cloud provider. His workday duty is to ensure customers are highly successful in using and deploying artificial intelligence and machine learning technologies. Pierre is an FAA-licensed airplane and helicopter pilot. He also muses about audio streaming for 5G and how interactivity and dynamic personalization will change advertising, podcasting and music forever.