“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Monday, December 28, 2020

The Celera 500L

Image credit: Otto Aviation

As anyone who's spent any time reading my publications knows, I'm a pilot and have been involved (non-commercially) in aviation for over 40 years. As such, I keep track of developments in the field. Thus, I was fascinated by the news of the Celera 500L by Otto Aviation. The performance claims made for the airplane are spectacular, to say the least.

Otto claims that the aircraft has a range of 4,500 miles (statue rather than nautical as far as I can tell) at a speed of 460 m.p.h. (again, statute m.p.h., not knots, as far as I can tell). It's stated that the Celera 500L does so while burning "8 times lower fuel consumption" and "5-7 times reduction in operating cost." For those who don't follow general aviation (that is, all aviation other than air carriers and military), the claimed speed is well above the speeds of high end turboprop business aircraft and not far below those of business jets. For example, the King Air B360ER turboprop achieves 349 m.p.h. in high-speed cruise, the Cessna Latitude business jet has a maximum speed of 512 m.p.h. But the range of the King Air is 3,092 miles and that of the Longitude is 3,105 miles. Otto claims that the Celera 500L achieves 18 - 25 miles per gallon fuel economy. What might be considered a comparable small jet, the Embraer Phenom 300E will get, perhaps, 5 miles per gallon. 

Otto states that the Celera 500L achieves these spectacular specifications due to a design for laminar flow over both the wings and the fuselage. Laminar flow is a fluid flow state wherein there is minimal mixing between layers and the flow is "smooth" rather than turbulent. It is a flow regime that minimizes drag, and all (well, most) aircraft designers seek to maximize laminar flow. It is a well-known phenomenon and no aeronautical engineer looks at Celera's web site or the many web sites and YouTube channels featuring the Celera 500L and slaps him or herself on the forehead and says "laminar flow, why didn't I think of that?"
Image credit: RED Aircraft

 The engine for the Celera 500L is the "RED A03" by RED Aircraft, GmbH. This is a 
compression ignition (i.e., diesel) engine. The engine is stated to be a V12 configuration with each six cylinder side operating independently. It's also stated to be all-aluminum in construction. Per RED's website, the engine is approved by both the FAA and EASA (the European Union Aviation Safety Agency). While diesel engines are typically very efficient due to the high compression ratio required for combustion of the fuel-air mixture, they are also typically heavy as a consequence of the strength required due to that high compression ratio. I'm not aware of any other all-aluminum compression engines.

As can be seen in the photo above, the aircraft is a "pusher" configuration, the propeller is behind the aircraft and pushes the airplane rather than the standard configuration wherein the propeller(s) pulls the airplane. This configuration has the advantage of letting the wings "see" a flow undisturbed by prop wash. Of course, the propeller is composed of airfoils as well, and they now see air disturbed by the wings and the fuselage, though not nearly so much as the propeller causes, especially given the laminar flow claim. It's also the case that the propeller is more subject to damage from ice shed from the wings in icing conditions and, at the altitudes stated in Otto Aviation's material, icing is certainly possible.

With all of that said, what is the likelihood that Otto Aviation can live up to their claims for the Celera 500L? It's difficult to do a thorough analysis given that nearly 100% of the numbers given consist only of those claims. The only hard data I could find is the takeoff power of the engine. They do claim that drag has been reduced by approximately 59% in comparison to similar sized aircraft. What can we infer from this?

With thanks to "Simplex11" at aviation stack exchange for the approach, we'll use the "59% lower drag" claim along with figures for my airplane, a Cessa 441. My airplane cruises at about 345 m.p.h., using 450 horsepower per side in cruise, or a total of 900 horsepower. The Celera 500L cruises at 460 m.p.h. and a drag of 0.41 ("59% less") times that of a typical aircraft.

We then have that ~D1=\frac{1}{2}C_{D1}\rho S_{D1}V_{D1}^{2}~ and ~D2=\frac{1}{2}C_{D2}\rho S_{D2}V_{D2}^{2}~, where ~D1~ is the total drag on the Celera 500L and D2 is the total drag on the C441. The Cs, Ss, and Vs are the drag coefficients, reference areas, and velocities of the Celera 500L and the C441 respectively, and ~\rho~ is air density. We can then combine these to get ~\frac{D1}{D2}=0.41(\frac{V1}{V2})^{2}~. Then, we know that power is speed times force, so we have ~P1=D1V1~ and ~P2=D1V2~ and so ~\frac{P1}{P2}=0.41(\frac{V1}{V2})^{3}~. Plugging in numbers, we can calculate that the Celera 500L needs something like 875 horsepower* to achieve the claimed speed. These numbers are, of course, approximate, but note that the maximum continuous power of the RED A03 is 460. And, as a reminder, this assumes that the "59% reduction in drag" is true.

What about fuel economy? Otto claims "18 - 25 m.p.g." (again, as above, I assume statute miles). The fuel economy can be calculated from the speed, power, and energy density of Jet-A fuel. Should the hordes demand that I show my work, I'll show the calculations but, having already produced an equation dense post, I'll just give results. And, as above, these are approximations based on sparse information. If the power requirement calculated above is correct, the Celera 500L would achieve something like 11.5 m.p.g. If, on the other hand, the speed of 460 m.p.h. can be achieved with the 460 maximum continuous power of the RED A03 engine then a figure of about 21.8 m.p.g. could be achieved. But, as I indicate above, it does not seem plausible to travel at 460 m.p.h. with less than 875 horsepower. And, yet again, all of this is contingent on the Celera 500L achieving the claimed 59% drag reduction. Time will likely tell.

Further questions are raised by Otto's claim that "The Celera 500L has a glide ratio of 22:1 (typical GA aircraft of similar size have a glide ratio of < 9:1)." While I can't question the 22:1, the "<9:1" claim is absolutely false. My airplane has a glide ratio of 14.8:1. Many business aircraft do better. Possibly such airplanes as the Cessna 172 Skyhawk (a four seat, fixed gear airplane with wing struts) may have glide ratios in the range mentioned by Otto, but no light jets or turboprops do. When falsehoods are stated as facts on web sites, I have to question all of the information to be found there.

And finally, in the immortal words of Carl Sagan, "extraordinary claims require extraordinary evidence." Otto Aviation's claims for the performance specifications of the Celera 500L are, without a doubt, extraordinary and I've seen no evidence, let alone extraordinary evidence.

Otto Aviation has completed their A round of financing. They anticipate B round financing in 2021 and 2022, during which they plan to begin FAA certification. In 2023 to 2025, their web site calls for C round financing and the beginning of manufacturing and first commercial deliveries. Based on my strong skepticism, I'm not a participant in Otto's financing!

*It should be noted that Simplex11's calculations are slightly different and yield a larger power requirement for the Celera 500L.