Celera 500L fiction or fact?
For the previous three years at the aviation media, occasional spy photographs have appeared, showing an odd blimplike airplane parked in 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, also it is a product of Otto Aviation.
Otto makes impressive claims for the Celera, notably it will cruise at 400 knots at 65,000 ft and have a range of 3,900 nautical miles. Poweredby a petrol engine and burning jet fuel, it is said to be eight to 10 times more effective than comparable jets, to achieve a carlike 25 or so miles per gallon plus a 22-to-1 slide ratio.
As I write this, very few specifics about the physical features of the airplane have been printed –besides that it accommodates 6 passengers in a spacious cabin with a stand-up center aisle. Its circular fuselage is all about 78 inches in diameter and 35 feet long and has the profile of a laminar-flow airfoil section (something very similar to a NACA 67-018). It looks like the barbell crosses about 55 feet. In case the plane weighs 7,000 pounds–just a guess–it would need at least 210 square feet of wing area to achieve the FAA-mandated 61-knot stalling rate for single-engine planes. That implies an aspect ratio of approximately 14. The dry weight of this engine is 800 pounds; radiators and accessories would add a hundred pounds.
0.42 of similar spark-ignition engines, in addition to a 47 percent increase over a Pratt & Whitney PT6. In accordance with Red Aircraft’s website, a version with two phases of turbocharging can operate up to 50,000 feet, but its crucial altitude– the maximum altitude where it develops rated electricity –is 25,000 ft.
The principal claim made for its Celera is that it supports an odd quantity of laminar flow and consequently has unprecedentedly low haul. I ran the sketchy information I have via a performance-calculation program so as to see if I could guess the assumptions on which the operation projections were established.
The basic parameters that frame the performance of an airplane would be the length and region of its wing, the efficiencies of its own engine and propeller, and a literary number– the”equal f lat-plate region,” that is a way of expressing parasite drag–represented by the letter F. Parasite drag is drag as a result of skin friction and flow disturbance, instead of induced drag, which can be an inevitable byproduct of lift. Per horsepower in cruise. Turbines, whose motors do not require cooling, mainly escape this punishment, as do sailplanes.
The efficiency of the motor is due to its particular fuel consumption, which in this case we all know to be 0.35. With this simulation, I used a propeller efficiency of 85 percent, which means that 15 percent of the energy supplied by the motor is lost in the process of being converted to push. The only knob left to twiddle is F. Luckily, when wingspan is famous, it is likely to receive an approximation of F by back-calculating in the glide ratio. The answer is 3.5 sq. ft.
Strangely, even using the entire 500 hp, I could not get the computer to yield a top speed of over 300 knots at 30,000 ft, and it even gave a more trucklike than carlike 10.8 miles per gallon. (Incidentally, jet gas is about 10 percent denser than avgas, so there is more energy in a gallon.) Shaking the monitor didn’t help. I thought, the magic number has to be 65,000 ft. A more realistic cruising power of 400 hp yields, well, nothing, since the minimum power needed to sustain flight at this altitude, at 325 ktas, is 425 hp.
Operating a diesel engine at rated power at that elevation, in which the density of air is about a sixth of what it’s at sea level, will require prodigious heights of turbocharging and cooling system. I doubt that it’s ever been done. How Red Aircraft reports a critical altitude of only 25,000 ft with two stages of turbocharging hints at the difficulties involved. Throughout World War II, but the Germans operated diesel-powered recon airplanes at 40,000 feet, so we know that’s possible; however the planes were slow, and that I doubt the motors were putting anything out close to their rated cruising power. Another problem is the propeller.
Together with the airplane traveling at Mach 0.7 and the prop tips going Mach 0.75 circumferentially, the outer parts of the prop blades are getting involved in the transonic drag rise. Furthermore, the propeller is operating from the disturbedwakeoftheairframe. The initial prop-efficiency estimate of 85 percent might not be carried out in fact.
But that’s all perfect. Nobody insists about cruising at FL 650 anyhow, unless possibly supersonically. Thus, let us drop down to some more hospitable FL 450– where blood will not boil if the cabin decompresses. Here, we discover that we can perform a comfy 325 ktas in 80 percent power and 16 nmpg. Can the motor develop 80 percent of power here? Well, no, but human creativity knows no boundaries, so maybe it can be forced to do so.
The final issue, then, is if 3.5 square feet is a sensible value for F. Cooling haul is the big unknown.
I imagine it may accounts for about half a square foot of F, which leaves 2.7 for the rest of the airframe. The total surface, or”wetted,” region of the plane is most likely approximately 1,100 square feet. A wetted-area drag coefficient of 0.0025 (2.7 split by 1,100, approximately ) will be eye-opening. The lowest value, net of cooling , I have heard of is 0.0034.
The designers of the Celera will point to the anticipation of remarkably extensive laminar flow. Laminar flow has roughly half of the skin-friction haul per square foot that tumultuous flow does. The wings and tail surfaces have been already laminar on any plane that wants them to be, nevertheless, and so what’s unique about the Celera is its laminar-profile fuselage. The fuselage accounts for approximately half of the wetted area, and ideally, about half of that would be laminar (because laminar flow stops just past the largest diameter).
PersonallyI doubt that it will be possible to keep laminar flow more than just a quarter of the fuselage, and likely not even that far. A body of revolution does not create such positive conditions for laminar flow as a wing does. The inevitable pits at windows, the nosewheel and boarding doors are extremely likely to trigger a historical transition to tumultuous f low, as would any deviation of a couple degrees from perfect alignment with the direction of flight. Other airplanes–the BD-5, the Viken VK-30, the Lear Fan–happen to be conceptually similar. What sets the Celera apart is its own massive passenger cabin and efficient diesel engine. I doubt it will achieve the announced performance–I’m pleased to be proved wrong–however if it cruises at 260 ktas at 30,000 ft burning 90 pph and contains a toilet inside, which will be fine enough.