Sometimes the laws of physics seem to be casually ignored by a new invention which greatly benefits mankind. At Reden, we refuse to have our heads in the cloud and like to apply the numbers to see if the amazing claims can be correct.
The Celera 500L, according to www.ottoaviation.com, flies 6 to 12 times farther on a gallon of fuel than a ‘comparable jet’, and has twice the flying range. How does it do this? The webpage explains that the superior performance is achieved by having extremely low drag and a highly efficient propulsion system. Do other designs overlook these areas? Surely, the search for low drag and efficient propulsion has been on since Orville Wright flew 120 feet at 10.9 km/h in 1903, while his brother jogged alongside?
Flying machines which are not lighter than air use the flow past wings or winglike body parts to generate lift. In stable flight, the lift (L) is exactly equal to the weight of the airplane. The flow also causes drag (D), but this is much less than the lift. The ratio L/D is an important measure for the efficiency of the airplane. The other is quite obviously the weight of the construction including passengers, luggage and fuel. If the weight is less, then so is the drag, and thus the energy required to move a given distance. The third important factor is the propulsion, which has to be both light and efficient. It is interesting to note that speed does not affect the efficiency directly. An airplane which flies twice as fast with wings a quarter the size generates the same lift and drag. It will arrive earlier and consume the same amount of fuel. (There is a ‘but’ which will play an important part later!)
Remember for now that FD = L/D * weight.
The values of L/D achieved so far are 20:1 for the Airbus A380, and the Albatross (according to Wikipedia), and 37:1 for the Virgin Atlantic Global Flyer. The Celera 500L achieves 22:1, according to its inventors.
If we accept the very good value of 22:1, the Celera 500L would require a forward force of 4.5% of its weight to keep it in the air. The work done by this force is force times distance.
The mass of the Celera is not disclosed. We can make an estimate based on the number of passengers. The site states that there can be six passengers. Presumably, there is also a pilot. If we assume, therefore, that the passenger, pilot and some luggage weigh a total of 700 kg, and multiply this by 4 to get the empty weight of the craft (the A380 needs 5 times the passenger weight), and add fuel for the promised range of 4500 NM, (5200 statute miles, at 18 to 25 mpg, so roughly 210 gallons or 795 litre, roughly 650 kg), the start weight becomes 4150 kg.
To keep this airborne, a thrust of 4150*9.8/22 = 1849 N is needed. Per kilometer, this works out at 1849 kJ= 1,9 MJ. However, just as when you row, the propellors are not pushing against something solid, but against something that moves, so the work done is much more. If we take 2 for this slip factor, the value becomes 3.8MJ/km. A litre of diesel fuel ‘contains’ 45 MJ, which a very efficient ‘small’ engine converts with 40% efficiency to rotation, of which the propellor loses another 25% (remainder: 13.5 MJ/l). This results in a fuel efficiency of the plane of 13.5/(3.8) = 3.55 km/l or 8.4 mpg. Otto Aviation claims 18-25 mpg, which therefore seems optimistic, but still in the same order of magnitude. 8mpg is better than what, according to Otto Aviation, ‘a comparable jet aircraft’ achieves: 2 – 3 mpg.
(Even if we leave out the losses of the propellor and slip, the consumption is still 9 km/l= 21 mpg, lower than the claimed performance)
Back to the ‘important but’. The lift is normally provided by the wings, and the fuselage has a shape which produces mainly drag. This drag does increase with speed, so that the faster plane becomes less efficient. The shape of the Celera 500L, however, appears to be capable of a useful L/D of its own. This may explain the very good overall L/D for the plane. However, this is a trick that cannot be scaled easily; for a larger airplane to have the same advantage from this shape, the flying speed would have to be reduced, in order to keep the Reynolds number within the limits for this design.
The Celera 500L’s claims are amazing, but not impossible if it is, in fact, much lighter than we have assumed. It may prove successful in competition with ‘comparable jet aircraft’, but for larger passenger numbers, the ‘normal’ airplanes will prevail.
A note on the impact on the environment of flying: compared to cars, efficient airplanes are not that bad per passenger and per kilometer. However, it is easier to fly large distances in the same time, which leads to more people covering larger distances. Besides, the smell near airports demonstrates the fact that jet engines don’t have catalytic converters, which makes them more polluting per litre of fuel than modern cars.