AF wrote:
This is a revolutionary design that uses the body shape for impulse propulsion.
What is “impulse propulsion” in this context. Googling only give me various spaceflight pages, including info on the Star Trek Enterprise’s impulse engines.
Jujupilote wrote:
Some info in french here :
http://www.aerovfr.com/2020/08/letonnant-celera-500l/550hp, water-cooled V12 diesel. Zero forward visibility, zero side visibility.
The visibility issue I could imagine being mitigated up to a point by a camera in the nose. What strikes me most – as a layman – is the comparatively skinny wing which could suggest a need for long runways to get up to take-off speed, and the rather small propeller which is probably not the most efficient one. Weight & balance might also be interesting for such a tail-heavy plane.
Airborne_Again wrote:
What is “impulse propulsion” in this context. Googling only give me various spaceflight pages, including info on the Star Trek Enterprise’s impulse engines.
Do they perchance mean a lifting body? The pusher layout would make it a bit easier to use the fuselage more efficiently with less disturbed airflow over it.
Since the plane already flies, they must have gotten the basic aerodynamics right; the actual performance numbers will be interesting.
Sebastian_H wrote:
What strikes me most – as a layman – is the comparatively skinny wing which could suggest a need for long runways to get up to take-off speed,
Takeoff distance is quoted as 3,300 ft, which is pretty terrible for something that is trying to compete with a light jet.
Sebastian_H wrote:
Do they perchance mean a lifting body?
I don’t the Celera fuselage fits the definition of a lifting body, just one that is super low drag. When I think of lifting bodies I think of things like the SR71 and the Space Shuttle. with bodies that are curved on top and fairing into the wings.
Airborne_Again wrote:
What is “impulse propulsion” in this context.
Right, I’ve hashed the terminology again.
What I’m referring to is a combination of pressure thrust and wake thrust.
wake thrust
pressure thrust
more here
another example
Sebastian_H wrote:
“Assuming a perfectly spherical cow on a frictionless surface …”
Yeah, it all gets to be something like that unless someone builds the thing and it flies in the real world like it did on paper
Hmm, is all I can say. On the speed, if it’s 40% as draggy and has 50% more power, let’s compare to an SR22. It should go at cube-root(2.5 × 1.5) times faster, which my calculator says is 280 KIAS at the equivalent altitude. FL650 is not a great idea – there’s an excellent reason Concorde was limited to FL600, which has to do with people not exploding in the event of a depressurisation. (I heard an extremely messy and nasty story of this happening for real in a U2). Doing the IAS/TAS conversion, 450 KTAS at FL600 does seem possible.
Like all not-quite-paper aircraft, a lot can be done before it has to be proven. Just look at what the DA42 was going to do.
A google for e.g.
does a person explode in a vacuum
digs out a lot of stuff and shows that the above is a myth. Certainly the difference between FL600 and FL650 won’t make any difference as to what happens to a human if the pressurisation is lost (one dies fairly fast due to lack of oxygen). I do wonder what the Concorde certification arguments were… a dive to a lower level for sure but you have only a few minutes at most.
I think this project is a classic investor / VC milking exercise. I am sure a number of people will make a nice living and put their kids though private schooling on it 
I’ve lost 100% on some such projects.
johnh wrote:
there’s an excellent reason Concorde was limited to FL600, which has to do with people not exploding in the event of a depressurisation. (I heard an extremely messy and nasty story of this happening for real in a U2)
Certainly being exposed to (near) vaccuum is very bad for you, but it’s a myth that people explode. The skin is quite strong and will easily hold a 1 atm. pressure differential.
The real problem is that air will be forced out of your lungs and that even at 100% O2, oxygen partial pressure is not high enough.
johnh wrote:
Hmm, is all I can say
Indeed
It’s a bit interesting though, and not at all uncommon:
The positive effect of “wake-filling” on propulsive power requirements has long been known from the field of marine propulsion. Ship propellers are typically located at the aft-body of the vessel and operated within the boundary layer flow close to the ship’s body surface. The physical principle ultilised in this configuration is known as Boundary Layer Ingestion (BLI) or wake-filling propulsion integration. It is also applicable to airborne systems.
Of course, the location of a ship propeller is a practical consideration first and foremost. It was placed where it is placed long before anyone thought about “impulse propulsion”. Besides, car ferries along the Norwegian coast have propellers aft and fwd, or fwd and aft, they have no fwd and aft, goes equally well in both directions. A practical solution to make the ferrying as efficient as possible (getting cars on and off).
A water jet use a somewhat similar principle. It draws water from the boundary layer to get the inlet velocity as low as possible for best efficiency, not by using a fwd facing scoop as would be the case for an aero jet engine.
A small diameter, aft mounted propeller combined with the right shape of the fuselage can, at least in theory, have benefits for efficient propulsion. Will it work in practice, and will such a configuration be practical at all?
AF wrote:
What I’m referring to is a combination of pressure thrust and wake thrust.
Hmm, I can only access the abstract of the first cited article, and it seems that they talk about venting jet exhausts via the trailing edges; it seems to be about efficiency gains of jet aircraft.
The second cited PDF is about rocketry, and the closest I could find to the unclear terms are a mentioning of “pressure drag” on p. 6 & 14, and classical rocket propulsion on p. 9/10.
Third cited web page of an EU research seems to point to something:
In a fuselage wake-filling propulsion arrangement, the airflow around the fuselage is ingested by fans or propellers located at the aft-fuselage and accelerated just so that the aerodynamic drag of the fuselage is compensated. Compared to a locally separated compensation of the fuselage drag through jet excess momentum produced by classically installed (podded) engines, wake-filling immediately allows to reduce jet excess velocities, hence, the kinetic energy losses in the aircraft wake […] The basis of this is a single boundary layer ingesting fan, the so-called fuselage fan, that encircles the aft-section of the fuselage.
OK, so as far as I understand it, the ingestion and acceleration of the boundary layer by a pusher prop as in the case of the Celera would provide an advantage. I assume that somehow this might be achieved ideally having a laminar boundary layer at the aft end of the fuselage, because otherwise I can’t see how that would not be achieved similarly by the Rutan pushers.
The fourth cited document raises immediately my eyebrows:
Synergy Aircraft is the only company to fully understand why that occurred, and to commit 100% of our resources to using the required technologies correctly.
The track record of GA inefficiency makes it obvious that something deeply hidden must be wrong in standard aeronautical practice.
Reading further:
The assumption of “closed” thermodynamic calculations (the ones always used to figure up a new aircraft) is that the drag when gliding unpowered is always equal to the drag at the same speed even when under power, which isn’t necessarily true when one is using that power to reduce drag. Synergy uses power first to dramatically reduce drag, then to make thrust.
Aha! Active boundary layer control. Wasn’t that used already e.g. on the F-4 Phantom II? With more or less successful outcome?
And now the “pressure thrust”:
A small amount of engine power can be used to reduce the boundary layer thickness of the aft fuselage. This “powered pressure recovery” can be used to create a larger area of high pressure in the back of a body than in front of it, creating a slight forward push called pressure thrust.
However, attempts to use the phenomenon for propulsion are misguided and inefficient. Done properly, pressure thrust is actually just major drag reduction for the price of a little suction.
Zero or even negative drag has been easily reached in experiment, but it quickly becomes inefficient to go beyond the goal of drag cancellation.
And of course they neglect to provide a citation for their claims of “zero or even negative drag has been easily reached in experiment”.
Since the Celera already flies, it is far better than the vapourware Synergy in that respect; it will be quite interesting to see if they achieve their efficiency goal, and if the whole drag reduction is more than what would be expected due to a very clean streamlined fuselage.
@Sebastian_H
I’m definitely not an expert on this, I just read up on it because I was interested in investing in these developments (don’t worry @Peter, I didn’t)
The rocketry paper describes the pressure differential between the front of the rocket and aft end. If the shape is specifically designed for a specific range of velocities through a known medium, like air, you can utilize the pressure differential to recover thrust rather than induce drag.
In layman’s terms (for those like myself), if the length and shape are the right balance for a given velocity, the recoil of the air from being compressed at the nose is utilized as it expands toward the tail.
Sebastian_H wrote:
“zero or even negative drag has been easily reached in experiment”.
I’m also not sure about this. In theory it does work, but in practice, the airfoil must be absolutely defect free, which is extremely difficult to achieve.
There are some elements to the Synergy design that are indeed pretty novel and make use of pressure differentials that other aircraft designs haven’t really.
But it is still vaporware until it’s built at full scale…
