Nice video about the project. Mentions that the engine is already use in an updated Yak and a drone.
This project is attracting a lot of interest lately. So I thought some math and cold facts confrontation would help… (I stole some of the cons from this thread, thank you).
First the engine: according to the manufacturer, they now have a twin stage turbo version which delivers 550 HP, maintains full take off power to 25’000 ft and has a service ceiling of 50’000 ft.
In a standard atmosphere, the pressure at 25’000 ft is approximately 0.37 sea level.
at 50’000 ft, pressure is down to 0.11 sea level.
So if the turbocharged engine’s critical altitude is 25’000ft, at 50’000 ft it will have (roughly) 30% of rated power left. That’s 165 HP.
Now the prop. The typical efficiency of a well matched prop is around 85% in cruise. I doubt that a prop operating in the wake of the tail surfaces would achieve this. It is therefore reasonable to expect a decline in efficiency. This will negate some of the drag decrease on the fuselage due to the suction in the tail.
Next challenge is cabin pressurization: to maintain a 8000 ft cabin at 50’000 ft, one must boost outside air pressure by a factor of 6 or so .
The temperature will increase by several hundred degrees K in the process, so it will then take a lot of very sophisticated cooling to get it to a usable T.
This is not just a technical challenge, it comes at a horsepower cost and with massive cooling drag. Most probably, huge and heavy AC compressors will also be required.
They’ll need to be fault tolerant please, say the guinea pigs in the oven… (and the FAA?)
Now wing area. These small thin wings and low rotation angle allegedly give the aircraft “the take-off performance of a midsize jet” – read: very very long take-off roll.
I am not sure what the stall speed of this thin wing airfoil will be at FL500. Will the aircraft with 165 HP come even close ?
The pressurized fuselage will need to be built for 13 psi of differential pressure, over 2 times more than say a PC-12. A lot of structure and airliner style windows will be needed.
So no matter how aerodynamically clean the design is, it will be hampered by relatively high weight (causing substantial induced drag at altitude) and the cooling drag associated with the enormous compression ratios of both engine and cabin air.
And finally, I’ll propose the following comparison. The Extra 400 is my “home turf” in terms of real world performance.
A good one will do 215 KTAS at 25’000 ft on 260 HP (75%).
Let us assume that the Celera’s much larger and heavier body and engine will miraculously achieve the same total drag.
And that the engine will, in a future iteration, deliver the same 260 HP at FL500.
So same power, same total drag. If the Celera flies higher, its TAS will increase by roughly 20 knots per 1’000 ft. Am additional 25’000 ft buys 50 knots.
And here you have it: cruise TAS of tadaaaaaa:
215+50 =
265 knots…
Does engine power reduce linearly with pressure or density as you go from 25k to 50k ft?
Cooling of pressurized air could be done via the fuselage itself? Presumably it is quite cold due to high altitude?
Does power needed to overcome induced drag scale with KIAS or with KTAS? Linearly or exponentially?
Flyingfish wrote:
Next challenge is cabin pressurization: to maintain a 8000 ft cabin at 50’000 ft, one must boost outside air pressure by a factor of 6 or so .
The temperature will increase by several hundred degrees K in the process, so it will then take a lot of very sophisticated cooling to get it to a usable T.
Flyingfish wrote:
The pressurized fuselage will need to be built for 13 psi of differential pressure, over 2 times more than say a PC-12. A lot of structure and airliner style windows will be needed.
Some math must be off, because my plane does 8000ft cabin at FL510 with 9.2 diff, not 13.
loco wrote:
Some math must be off, because my plane does 8000ft cabin at FL510 with 9.2 diff, not 13.
9.2 would be right. (Actually, when I calculate I get 9.4.)
Does power needed to overcome induced drag scale with KIAS or with KTAS? Linearly or exponentially?
Thrust vector as measured in Newton vs total drag: parasite drag in KIAS*KIAS + induced drag in 1/(KIAS-VS)
HBadger wrote:
Does engine power reduce linearly with pressure or density as you go from 25k to 50k ft?
Cooling of pressurized air could be done via the fuselage itself? Presumably it is quite cold due to high altitude?
Does power needed to overcome induced drag scale with KIAS or with KTAS? Linearly or exponentially?
These are back of the envelope calculations, based upon observing the power decay on the Thielert diesels’s data sheets – roughly linearly from the critical altitude.
The power needed to overcome induced drag is related to the angle of attack which must be increased in lower density air. We know that air density decays in a non-linear way, reducing lift, but the required increase in AOA is airfoil specific. To make things simpler, while induced drag increases, parasite drag does the opposite.
At the end of the day, at least in my aircraft (FL250 ceiling) the combined effect boils down to: 2 additional knots TAS per 1000 ft of altitude.
The reason I highlighted the induced drag, cooling drag and prop efficiency issues is to show how theses elements will cap the benefit from the theroretically “zero drag” fuselage/propeller architecture.
Sorry about the overestimated cabin pressure differential, @loco, I stand corrected.
The main point is the beefy structure that is required to cope with the roughly doubled pressure differential. This structure would have to be much stronger than the pressure vessel of the aircraft I used for the final comparison.
This correction will soften the problem of cooling the pressurized air, but it remains a massive challenge. I once read a great paper explaining how the different cooling problems are affected by altitude and air-to-air inter cooling was by far the biggest problem.
Maybe loco can explain how bleed air is cooled in business jets?
And while we are at it, can someone elaborate on the challenge of keeping the Celera above its stall speed at FL500?
The engine is liquid cooled, so you can use that liquid to temper cabin air, too. So it’s not air-to-air cooling, which wouldn’t work in FL500 or higher.
Flyingfish wrote:
Maybe loco can explain how bleed air is cooled in business jets?
It is cooled by passing through heat exchangers and expanding in turbines.
https://en.wikipedia.org/wiki/Air_cycle_machine
