Gas Turbine versus Battery Power
Batteries are a sensible alternative to gasoline for powering many cars, but for aircraft they are problematic to say the least. Mainly, they are too heavy for the power they hold – some 50 times less that the same weight of the synthetic kerosene mentioned above, but then of course you have to factor in the thermal efficiency of the internal combustion engine (say 30%). Even then, however, there is still a serious reduction in the cruise speed of the aircraft and/or the distance it can fly. As an example, the gas turbine powered Autocopter has a range of 1,000 miles at a max cruise speed of 190kt, whereas Its wingless battery-powered variant can do 150 miles at 120kt. Possibly the best answer for the battery-powered VTOL machine is then to design an aircraft with an efficient wing, and for example the Joby multi-rotor aircraft does this very well, so raising its cruise speed to perhaps 150+kt. The Autocopter can also do this, but by already using a highly efficient coaxial main rotor it doesn’t benefit from this quite so much.
Weight, however, is not the only problem with batteries in aircraft. Four of the most serious ones are: loss of power if the battery temperature falls below 0 deg. C.; the battery constitutes a permanent deadweight; Li-ion batteries pose a fire risk if overcharged; the battery’s life is a function of the number of times it is recharged, so for a short-range air taxi this could mean several times a day, leading to a battery life of not much more than a year.
Battery powered helicopters may look an acceptable solution for an aircraft that travels from a city-centre hub to a major airport and back. But from a productivity standpoint they cannot compete with a gas turbine machine due to their lower speed and time to recharge. Sometimes, the difference can be as much as 5:1 in the gas turbine aircraft’s favour.
There is also the issue of charging infrastructure. City-centres hubs and major airports will definitely provide this, but anywhere else may present a problem. Then, although electrical power is available everywhere, the power required by a helicopter is huge so the recharge time can be significant.
For the gas turbine Autocopter it will be a question of access to fuel. The Autocopter has a unique type of combustion system that will allow it to fly on diesel, gasoline, synthetic kerosene, jet fuel, Sustainable Aviation Fuel (SAF) and, at pinch, hydrogen. Diesel can, however, only be used in-extremis and only to get the aircraft to a site where it can be refuelled by one of the jet fuels. The absolute ideals are synthetic kerosene and SAF.
People sometimes look at battery powered cars able to do 400 miles on one charge (if lucky) and think why a helicopter can’t do the same. The reason is simple. Automobiles rarely use more than 25% of their max useable power because of speed limits and traffic congestion. A helicopter is completely different: it is able to fly in a straight line at max power, and it almost always does, so range in a battery-powered helicopters is perhaps 25% of that in a car.
For the Autocopter it therefore comes down to range. Horizon believes that a range of 150 miles is too short for the markets it wishes to sell its aircraft into. Horizon then offers a choice between battery power and gas turbine power, so letting the market decide. 1,000 miles was the gas turbine target range settled on. As such, a gas turbine powered Autocopter used as an air taxi could fly all day, making many stops, on just one refuelling.
There are indications that the power density of batteries will in the future increase substantially, but what Horizon ideally needs is one that’s fifteen times as dense. Researchers are talking about this, but it will probably take the automobile industry to fund it before aircraft see it. We could be looking at 10yrs. If and when it happens, it will be a quick adaptation for the Autocopter since it already has a fully electric drive system.