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| Fig. 1: Helicopter Energy Usage vs Distance for each type. [1-3] (Source: A.Tidd) |
The future is electrifying. Americans have been moving toward electric appliances for the past couple of decades and they just started moving toward electric cars in the past couple of years. This push for electrification has many reasons, but outside of the economic ones, they have been electrifying for the increased energy efficiency and reduction of greenhouse gas emissions associated with going electric. The transportation industry accounts for about 1/4 of US CO2 emissions each year, so it will be impossible to achieve net zero without addressing this sector. With Tesla and others bringing electric vehicles (EVs) to market, access to personal ground transport is expanding rapidly. The space that needs more attention is aviation, more specifically, the planes and helicopters that dominate the space. This begs the question: is it possible to take the advancements in EVs and apply them directly to aviation? This study takes an EV approach to the aviation industry by examining how much energy it takes to fly a helicopter and how likely it is for the same batteries being used in EVs to be adapted for use in helicopters.
Before beginning the analysis, it is important to understand how and why helicopters use energy. This study examines helicopters with a single main rotor and tail rotor. These types of helicopters fly because the main rotor produces more thrust than the weight of the helicopter. The spinning rotor pushes air downwards, and because of Newton's Third Law, that results in the helicopter moving up. Once off the ground, the helicopter can adjust the pitch of the rotors to move forward, backward, or sideways. This analysis determines the energy used by three types of helicopters (light utility, multi-mission, and medium utility) by multiplying the average power, which is a fixed percentage of each helicopters rated power, with the time traveled given a certain distance. The light utility helicopter has a relatively low engine power to design gross weight (DGW) ratio given its reciprocating engine design and is commonly used for the transportation of people and compact cargo with a combined weight of less than 500 kg. The helicopter used in this study that is representative of the light utility helicopter type is the Schweizer 300C. [1] The multi-mission helicopter uses a gas engine with approximately four times the power as the reciprocating engine and a DGW that is about double the light utility. It is more versatile than the light utility and is commonly used for complex search and rescue missions, law enforcement, EMS, and personal travel. The multi-mission helicopter representation used is the Bell 206L4. [2] The medium utility helicopter has twin turbine gas engines, and it is typically much heavier than the multi-mission helicopter. It is used for missions with ranges surpassing 500-700 km or with cargo exceeding 1000 kg. Furthermore, it is commonly used in the oil and gas industry and for executive travel. The Airbus EC175 represents the medium utility helicopter type in this study, and some of the specifications for all three of the helicopter types can be seen in Table 1. [3]
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| Table 1: Specifications of each helicopter type [1-3]. |
To start the analysis, the average power used by each type of helicopter over the course of a mission was determined by taking 75% of the rated power of each helicopter. This factor was chosen as the estimate given typical operating conditions of helicopter motors while carrying payloads proportionate to their power-to-DGW ratios. This estimate is also based on the expected hover-to-cruise time ratio. The power used to hover and climb is more than that needed to cruise, so more time spent hovering/climbing will bring the average power closer to the rated power, and the more time spent cruising will bring the average power further below the rated power. This is due to induced drag. As the rotors spin, they push the air down through them. This results in an area of lower pressure above the wings than below, thus creating lift. While hovering, however, the air that is situated above the rotors rushes in to fill the low-pressure area. This gives the air above the rotors a downward (induced) velocity. The helicopter is essentially sitting in a tunnel of downward-flowing air, so to stay in the same position, the rotor needs a higher speed or pitch resulting in more power. This problem of induced drag does not occur during cruise because the helicopters horizontal velocity moves it into air that has not yet had a chance to gain an induced velocity.
After determining these power measurements, the amount of energy used by each type of helicopter was calculated using Energy = Power × time, where the time for each mission was determined using the cruise velocities in Table 1. This analysis yielded Fig. 1, which shows the linear relationship between energy use and distance traveled for each helicopter type.
Fig. 1 shows that the amount of energy needed to travel just 150 km (which is 4-6 times less than the helicopters standard ranges) is 100, 300, and 1070 kWh for the light, multi-mission, and medium utility helicopters respectively. The most energy-dense EV batteries on the market today are in the Tesla Model 3 and Audi e-Tron which have a specific energy of about 260 Wh/kg. [4] This means that each helicopter would need a 384, 1154, and 4115 kg EV battery respectively to fly only 150 km. This would push almost every helicopter over their DGW making them unflyable. This way of analysis also provides batteries with the benefit of the doubt. As helicopters get heavier, the power needed to fly them increases. This would increase the energy requirements further, resulting in an exponential graph of energy use vs distance traveled instead of linear. The same problem exists with traditional fueling methods, however, because the specific energy of oil is over 10 kWh/kg the problem is not as apparent. Given the energy intensity of helicopter flight, without significant improvements in battery density, the future for electric helicopters is bleak. There is theoretical backing for lithium-ion batteries to more than double their energy density to 580 Wh/kg by incorporating nickel into the lithium manganese oxide. [4] But even with that improvement, the total ranges of the electric helicopters would still be at most half of the traditional ones. This means that it is not possible to electrify the current aviation sector using the most prominent tools of the EV industry.
It is impossible to achieve net zero without decarbonizing aviation. As it stands today it is unlikely that there will ever be a Schweizer 300C, Bell 206L4, or Airbus EC175 that is battery-powered and can perform better than their fossil-fuel-fueled counterparts. Fortunately, we do not need to electrify current helicopter designs. We need to find ways to transport cargo through the air quickly and sustainably. There is a plethora of research going into biofuels, which have similar energy densities and are relatively simple to make, but they are currently more expensive and not as plentiful as traditional fuels. Furthermore, scientists are looking into hybrid helicopters with batteries for times when high power is needed and hydrogen fuel cells for times when low power is needed. There are also engineers working to redesign what it means for something to be a plane or a helicopter; they are combining the best aspects of both and creating a new market of electric vertical take-off and landing aircraft. So, although we are not going to solve the problem of how to electrify modern helicopters, with some more innovation, we may still find a way to have sustainable air travel.
© Andrew Tidd. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Schweizer 300C Piston Helicopter," Schweizer Aircraft, February 2007.
[2] "Bell 206L4," Bell Helicopter, 2015.
[3] "EC175," Airbus Helicopters, 2014.
[4] M. S.E. Houacheet al., "On the Current and Future Outlook of Battery Chemistries for Electric Vehicles - Mini Review," Batteries 8, 70 (2022).