For the past three years in the aviation press, occasional spy photos have appeared, showing an unusual blimplike airplane parked at the former George Air Force Base, now Southern California Logistics Airport, near Victorville, California. This oddity has now made its formal debut. Its name is Celera 500L, and it is a product of Otto Aviation.
Otto makes impressive claims for the Celera, notably that it will cruise at 400 knots at 65,000 feet and have a range of at least 3,900 nautical miles. Powered by a diesel engine and burning jet fuel, it’s said to be eight to 10 times more efficient than comparable jets, to achieve a carlike 25 or so miles per
gallon and a 22-to-1 glide ratio.
As I write this, very few specifics about the physical characteristics of the airplane have been published—other than that it accommodates 6 passengers in a spacious cabin with a stand-up center aisle. Its circular fuselage is about 78 inches in diameter and 35 feet long and has the profile of a laminar-flow airfoil section (something similar to a NACA 67-018). The aft-mounted engine drives a multibladed propeller whose diameter seems to be about the same as that of the fuselage. It appears that the elliptical wing spans about 55 feet. If the airplane weighs 7,000 pounds—just a guess—it would require at least 210 square feet of wing area to achieve the FAA-mandated 61-knot stalling speed for single-engine airplanes. That implies an aspect ratio of about 14.
The German-manufactured 374-cubic-inch, 12-cylinder Red A03 engine is rated at 500 hp, using jet fuel. The dry weight of the engine is 800 pounds; radiators and accessories would add a hundred pounds. Its specific fuel consumption is 0.35 pounds per horsepower-hour; this represents a 17 percent improvement over the 0.42 of comparable spark-ignition engines, as well as a 47 percent improvement over a Pratt & Whitney PT6. According to Red Aircraft’s website, a version with two stages of turbocharging can operate at up to 50,000 feet, but its critical altitude—the highest altitude at which it develops rated power—is 25,000 feet.
The principal claim made for the Celera is that it supports an unusual amount of laminar flow and therefore has unprecedentedly low drag. I ran the sketchy information I have through a performance-calculation program in order to see if I could guess the assumptions on which the performance projections were based.
The basic parameters that frame the performance of an airplane are the span and area of its wing, the efficiencies of its engine and propeller, and a fictional number—the “equivalent flat-plate area,” which is a way of expressing parasite drag—represented by the letter F. Parasite drag is drag as a result of skin friction and flow separation, as opposed to induced drag, which is an inevitable byproduct of lift. A third category of drag, attributable to engine cooling, is included in F and, by rule of thumb, is of the order of 0.2 sq. in. per horsepower in cruise. Turbines, whose engines do not require cooling, largely escape this penalty, as do sailplanes.
The efficiency of the engine is expressed by its specific fuel consumption, which in this case we know to be 0.35. (Parenthetically, this is an entirely plausible number, not a wild claim.) For this simulation, I used a propeller efficiency of 85 percent, meaning that 15 percent of the power supplied by the engine is lost in the process of being converted to thrust.
The only knob left to twiddle is F. Luckily, when wingspan is known, it is possible to get an approximation of F by back-calculating from the glide ratio. The answer is 3.5 sq. ft.
Strangely, even using the full 500 hp, I could not get the computer to yield a top speed of more than 300 knots at 30,000 feet, and it gave a more trucklike than carlike 10.8 miles per gallon. (Incidentally, jet fuel is about 10 percent denser than avgas, and so there is more energy in a gallon.) Shaking the monitor didn’t help.
Read More from Peter Garrison: Technicalities
Well, I thought, the magic number must be 65,000 feet. Sure enough, here, 500 hp yields 430 ktas—at 120 kias—and 17 nmpg. A more realistic cruising power of 400 hp yields, well, nothing, because the minimum power required to sustain flight at this altitude, at 325 ktas, is 425 hp.
We seem to be in the ballpark, but the Celera may encounter several difficulties on its way to FL 650.
Operating a diesel engine at rated power at that altitude, where the density of air is about a sixth of what it is at sea level, will require prodigious levels of both turbocharging and cooling. I doubt that it has ever been done. The fact that Red Aircraft reports a critical altitude of only 25,000 feet with two stages of turbocharging hints at the difficulties involved. During World War II, however, the Germans operated diesel-powered recon airplanes at 40,000 feet, so we at least know that’s possible; but the airplanes were slow, and I doubt the engines were putting out anything near their rated cruising power.
Another problem is the propeller. With the airplane traveling at Mach 0.7 and the prop tips going Mach 0.75 circumferentially, the outer portions of the prop blades are getting into the transonic drag rise. Furthermore, the propeller is operating in the disturbed wake of the airframe. The initial prop-efficiency estimate of 85 percent may not be borne out in reality.
But that’s all right. Nobody insists on cruising at FL 650 anyway, unless maybe supersonically. So, let’s drop down to a more hospitable FL 450—where blood will not boil if the cabin decompresses. Here, we find that we can do a comfortable 325 ktas at 80 percent power and 16 nmpg. Can the engine develop 80 percent of power here? Well, no, but human ingenuity knows no bounds, so perhaps it can be made to do so.
The final question, then, is whether 3.5 square feet is a realistic value for F.
Cooling drag is the big unknown. I imagine it might account for about half a square foot of F, which leaves 2.7 for the rest of the airframe. The total surface, or “wetted,” area of the airplane is probably around 1,100 square feet. A wetted-area drag coefficient of 0.0025 (2.7 divided by 1,100, roughly) would be eye-opening. The lowest value, net of cooling drag, that I have heard of is 0.0034.
The designers of the Celera will point to the expectation of unusually extensive laminar flow. Laminar flow has about half the skin-friction drag per square foot that turbulent flow does. The wings and tail surfaces are already laminar on any airplane that wants them to be, however, and so what is special about the Celera is its laminar-profile fuselage. The fuselage accounts for about half the wetted area, and ideally, about half of that would be laminar (because laminar flow ceases just past the biggest diameter).
Personally, I doubt that it will be possible to maintain laminar flow over more than about a quarter of the fuselage, and probably not even that much. A body of revolution does not produce such favorable conditions for laminar flow as a wing does. The inevitable seams at windows, the nosewheel and boarding doors are very likely to trigger an early transition to turbulent flow, as would any deviation of a few degrees from perfect alignment with the direction of flight.
The Celera 500L is not so revolutionary as it claims to be. Other airplanes—the BD-5, the Viken VK-30, the Lear Fan—have been conceptually similar. What sets the Celera apart is its huge passenger cabin and efficient diesel engine. I doubt it will achieve the announced performance—I’m happy to be proved wrong—but if it cruises at 260 ktas at 30,000 feet burning 90 pph and has a toilet inside, that will be good enough.
This story appeared in the December 2020 issue of Flying Magazine