Fuel-Less VTOL Plane
Many new VTOL (vertical take-off and landing) planes are being designed and created. I am a big fan of the Zuri eVTOL project of my friend. eVTOL aircraft typically use many electric engines for the initial lift for take-off as they allow for accurate plane control. However, the batteries are still too heavy for a reasonable travel distance using just an electric propulsion.
I have an idea of how to solve it and create a practically fuel-free plane. But first, let’s outline some basic concepts of designing an aircraft. There are only a few options for how to extend the flight distance.
1. The first way is to increase the density of energy of the fuel. So the overall weight of the plane can be lower. Of course, we could wait for much better batteries. Or we can use a hybrid power system. Zuri eVTOL project uses an extra genset turbine to power the plane while moving horizontally. It can use traditional petrol or e-fuel, with 30x higher energy density than the best batteries. The turbine is an extra weight, but it allows to recharge the batteries during the flight providing the energy needed for the vertical landing. So the plane can use just half of the batteries. The obvious disadvantage is that the turbine is an extra weight during the take-off and landing. And batteries are dead weight during the horizontal flight.
2. Reduce the energy needed for the flight by more efficient plane design. The reduction of the drag significantly extends the travel distance. The most efficient experimental passenger plane is the Celera 500 (or the new variant Celera 800 ), which claims to achieve a 59% improvement in fuel burn over similarly sized aircraft. It has a single-push propeller, very minimalistic wings, and an odd cucumber shape of the aircraft body. That combination allows them to achieve almost the same energy efficiency as a family car. The induced drag, generated by the airflow vortexes, is reduced to a minimum.
3. Flying high and limited hovering saves energy. The atmosphere is thinner, which allows higher speed while keeping the drag smaller. The potential energy of reaching high altitudes is recovered later during the descent phase of the flight. On the other hand, the plane cabin needs to be pressurized, which increases the cost for small commercial aircraft. VTOL also needs to reduce the time required to hover because hovering is 10x more energy-demanding than cruise flight. The Lilium VTOL project uses 36 (!) small ducted fans to power the aircraft. Ducted fans are more efficient for horizontal flight. On the other hand, this design demands a lot of energy to hover maneuver. That does leave just a small reserve (1 minute!) for an error (e.g. somebody accidentally blocking the landing zone).
4. Make the plane lighter. The weight is directly impacting the fuel/energy consumption. That is also one of the advantages of planes powered by traditional fuel. The aircraft weight is getting lower as much as 40% as the fuel is consumed during the flight. Unfortunately, the batteries weigh the same all the time. Most planes are made of aluminum alloys or carbon fiber composite material to reduce their weight. However, what if we use another method of defying gravity, which was prototyped about 100 years ago?
It is called an airship — a cigar shape balloon. They use helium or hydrogen and in large volumes, the airship can get buoyancy in the air. No energy is wasted on keeping the aircraft flying. Companies like LTA Research or HybridAirVehicles believe that airships could be reintroduced as the means of climate-friendly air travel.
The airships are a nice concept but were abandoned for several reasons.
a) The first was safety. The maximum lifting power is provided by hydrogen, which is highly flammable. The tragic end of the Hindenburg airship unveils the danger. Helium, the inert gas used today, is much more expensive and doesn’t provide as strong lift as lighter hydrogen. On the other hand, we are learning how to treat hydrogen as its “green flavor” (made from electric power generated by renewable sources) might be the e-fuel of the future.
b) Secondly, the airship travel speed was low. Even the modern AIRLANDER 10, which generates its lift partially by aerodynamics, has a top speed of 130 km/h. That is at least 6x lower than a commercial passenger aircraft. Hence the the airships are rather considered as competition to ships or trucks.
c) Airships are huge with fixed shape and volume. So they need a lot of space and nice weather conditions to operate safely. They need to be attached to the ground all the time. Any weight change immediately impacts the buoyancy and the aircraft could shoot upwards rapidly.
So could we create an aircraft using the advantages of an energy-free balloon like VTOL and the efficiency and speed of a plane? What if we create a kind of convertible?
Imagine that we will inflate a two pair of balloons and lift the plane. Once we reach high altitude, we will compress the gas back to the storage and glide to our destination. The higher the ascent during the initial phase, the longer the plane could glide. The only energy we will need is to power the compressor. Let's do the calculation, how much is this feasible?
What size of the plane are we talking about? The concept can’t be used for the smallest planes, because we need some volumes in wings to store the lifting gas. I made some calculations for an aircraft for 20 people. So let's assume that the plane chassis will be 5.000 kg. That is about 5x the weight of the most common Cessna plane accommodating 4 passengers. We can use carbon composite to shape tanks as an integral part of the wings. With 20 people on board, we will need a passenger allowance of about 2.000 kg. In addition, there is the weight of the balloons, hydrogen polymer liner and the hydrogen gas. To be safe, let's say we will need to lift about 10.000 kg assuming that the wings will act as the storage tanks.
The amount of mass that can be lifted by hydrogen in air at sea level, equal to the density difference between hydrogen and air, is (1.292–0.090) kg/m3 = 1.202 kg/m. Hence we will need to fill in the volume 10.000 m3 at the ground. That is about four spheres with 8 meters radius. Not small balloons, but not something unimaginable. We will need about 840 kgs of hydrogen for that. The balloons will be made of flexible latex to expand to spheres with 10 m radius equalizing the lower pressure at higher elevations. It is about half the pressure and air density at 8000 meters above sea level.
After about 20 minutes of ascent, we will need to compress the hydrogen as quickly as possible using as little energy as possible to convert the aircraft into the glider. We could leave balloons partially inflated, but for simplicity we will now consider storing all the hydrogen in the tanks inside the plane. The larger the volume we can have in the wings, the faster we can do it as we don’t need to achieve so high pressure. A blended wing-shaped aircraft will be the best choice. It combines the wing and the main hull into one body. This shape provides smooth integration between the wings and the plane body with about 20% extra efficiency validated by the Airbus MAVERIC blended wing demonstrator.
We can achieve at least 150m3 of volume in each wing (12m length of the plane, 2m high, and 12m span of each wing of a triangle shape). That means the hydrogen gas will be pressurized to approximately 30 atmospheres. This will require about 2% of the energy content of the stored hydrogen using highly efficient centrifugal compressor technology. We could recover about half of the energy during the balloon inflation using the compressors as generators and a small onboard battery.
The glide ratio defines how far a plane can glide. The record is 1:70, so the aircraft can glide a distance of 70km while losing 1km of elevation. The typical plane has a glide ratio of 1:20. If we could convert some of the hydrogen to electric power using efficient fuel cells, we could power a pair of propels to extend the reach of our aircraft further. They will also provide additional lift during the transition phase from ascending to the gliding phase till the optimal cruising speed is achieved. With a gliding ratio of 1:50 and 8km above sea level starting point, we could travel almost energy-free up to 400 kms. Then we can inflate the balloons and repeat the ascent maneuver or execute a VTOL landing.
How much it will cost? The hydrogen is currently 1.5 EUR/kg (made of methane) up to 5 EUR/kg for green hydrogen made by use of electric power from renewal resources. So we will need energy of about 5kg hydrogen depending on the efficiency of the fuel cell and the compressor regeneration (50%) for a single flight of 400 km flight with 20 people. That is 7.5–20 EUR per flight for hydrogen used for the compression. Less than 1 EUR per passenger.
So we could refill our hydrogen tank on the ground or use some light solar photovoltaic powerfoil on our large wings. With more than 250 m2 of area, we could generate up to 50kW of power either to use the propellers during the flight or to generate some hydrogen using a small onboard electrolyzer. The plane will refuel itself during the sunny days in few hours.
There is no need for better battery chemistry, superlight materials or to accept the risk of limited hoovering capabilities. You only need a smart plane design, four efficient compressors, and latex balloons to create a practically fuel-less plane, which will be the most efficient type of travel.
