Otto Aviation Group LLC Celera 500L

since ozone is a greenhouse gas in the troposphere but the opposite in the stratospere, it would be useful to fly as high as the Celera. But I highly doubt that NOx or Ozone will remain in any athosphere for 100 years, there is way to much mixing going on.

NOx and Ozone at the ground have a very short life expactation of several hours to a couple of days, no way that any of them will remain in the athmosphere for 100 years!
 
there is no doubt that CO2 can rise up from the ground level to stratosphere otherwise it wouldn’t affect the Klima. Also, FCKW and other refrigerants can rise up from the ground level to the stratosphere, but Ozone, NO, NO2 and N2O will always stay in the same height for decades??? This is ridiculos!
 
No, that's not correct. The combustion process of a Diesel is much more complete thant that of a gazoline engine due to the fast burn rate (high compression ratio) and surplus Air. Otto engines have higher combustion temperatures due to the (in most cases) almost stiochiometric combustion. There is a NOx peak at Lambda 1,1 (10 % exess air) which is about the ratio gazoline aircraft engines are operated for max. efficiency (leaning). Diesel engines are operated at much leaner Lambda in cruise flight which reduces NOx emissions.

In a car, the conventional catalyst will emininate almost all NOx as long as Lambda is </= 1, with lean burn, it will be much more effort (by using Urea, of NOx trap with periodical regeneration).

I don't know the situation in the US, but in Europe the suphur content of Diesel fuel is equal to Otto fuel, it depends on the regulation, not on the engine parameters.

Do you have any quatation about NOx affecting the Ozone layer?

Similar in Canada where most diesel trucks are placarded: "Low-sulphur diesel fuel only." This is not an issue at filling stations because Canadian stations are only allowed to stock low-sulphur or no-sulphur diesel fuel.
 
The problem with Europe is that the devil has already done his deed.
It's 20 years ago that Europe should have banned diesel engines for personal transportation and controlled diesel quality. Instead they ramped it up full speed (some countries with weak automotive prospects leading the charge) just to shield their industry from foreign imports. *

Now, it's too late... And we have passed over our murderous fleet of dusty vehicles to Africa and they thanks us through global air mass circulation. Something that would stand for the next couple of decades, probably.

*France is still allowing heavy duty fuel from the cheapest grade into the shipping industry, a major contributor of Nox emissions through that usage of low grade fuel. Very few countries still allow that.
 
The air in Europe is as clean as never before! Since about 1990 the NOx, Soot, HC and Co emissions went down continously and are just a fraction of everything meassured before. Of course, we never meassured the pollution of the 19 th century or the medial age, but it was shurly worse than in the late 20 th century with all its fires (backeries, blacksmith, cooking etc. in every house.
 
some (too rare...) news about the Celera 500L

At least flying tests with Boundary Layer Data System during June-September 2021.


And, according to a press article (shared from the website of Otto Aviation) :

“The volumetric capacity of the fuselage and specific configuration choices allows our design to adopt either hydrogen or battery electric propulsion. We’re eager to build upon the initial version of the 500L and deliver a zero emissions version of the aircraft.” Otto projects that a viable zero emission aircraft will be available by 2027.


To be continued...
 
The batteries will be empty when reaching 45.000 ft and a fuel cell will loose a lot of efficiency in the thin air (high energy consumption of the charging system). Both doesn't work for planes which should fly very high and far.
 
It looks the business and the case for it seems to be elf explanatory. I hope it goes well for them.
 
The batteries will be empty when reaching 45.000 ft and a fuel cell will loose a lot of efficiency in the thin air (high energy consumption of the charging system). Both doesn't work for planes which should fly very high and far.
We can imagine synthetic fuel too. I think that they have a good basis that they can develop.
 
Yorba Linda, Calif. – Nov. 17, 2021 – Otto Aviation has officially concluded Phase One testing of its
Celera 500L aircraft with 55 total successful test flights and roughly 51 hours of flight time. The Celera
500L is the most fuel-efficient, commercially viable business aircraft in the world.

The final Phase One test flight was flown using Sustainable Aviation Fuel, further validating the Celera
500L’s potential to revolutionize sustainable air travel in an aircraft that already has 80% lower fuel
consumption than comparable aircraft. All test flights have validated the aircraft’s operating
performance goals.

The Otto Aviation team used industry standard flight test performance methods that aided in refining
the statistical certainty of the prototype performance. The most recent flights took place from July to
November. Several flights reached airspeeds of over 250 mph at altitudes up to 15,000 feet which
projects to an airspeed of 460 mph at 50,000 feet.
 
Some news/advertisings from the Otto Aviation webpage :


-Passenger Travel: Significantly reduced operating costs will open private aviation to large segments of travelers who previously relied on commercial aviation.

-The Celera 500L enables the air taxi model: Lower fuel and maintenance costs drive a lower per-passenger ticket price, increase utilization, and ultimately increase profitability of the air taxi model. The Celera 500L makes the short haul air-taxi model viable overnight.

-Cargo operations: Capable of carrying D-sized cargo containers, the Celera 500L can carry freight directly from a product’s point of manufacture to point of sale, skipping traditional cargo hubs. Low costs per lb of payload on both short and long-range missions makes the Celera 500L effective at carrying small volumes of high value cargo. Superior range capabilities enable new missions that were previously only possible by narrowbody-sized aircraft, opening up a new potential market niche.

-Military Applications: Superior cruise speed, range, and loiter time opens the Celera 500L to a wide range of military missions including personnel travel, ISR and cargo capabilities. The unique long-range capability of the Celera 500L allows for efficient transportation to remote bases and communities previously only served by larger and more costly aircraft.

-Market opportunities: Massively increases the overall size of the private air travel market and opens the air-taxi market. Ability to compete with both private air services and premium-class commercial airfares. Premium-class airfare revenues estimated to be ~$196 billion.

-Electric/Hybrid-variants: As battery technology becomes viable for aircraft, hybrid and electric propulsion systems can further improve the efficiency of the Celera 500L airframe. The Celera 500L design lends itself well to future adaptations of improved propulsion technology.

-Celera 1000L: The Celera 500L can be scaled up 20% while maintaining its extensive laminar flow. This increase results in a cabin volume nearly double the size of the current configuration. This means more passengers. It also means a much larger amount of freight, including the ability to fit five D-sized shipping containers, a standard in larger scale air cargo.

-Drone: The Celera 500L has significant potential as a drone because its combination of range, speed, service ceiling and cost are unattainable by existing unmanned aerial vehicles (UAVs).
 

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Some news :


Otto Aviation has confirmed plans for a 19-seat, hydrogen-powered version of its Celera aircraft that could be operated on flights of up to 1,000 nm. The Celera 750L is a stretched version of the 500L (...) ZeroAvia will integrate its ZA600 hydrogen-electric powertrain as an alternative propulsion system to the Red A03 diesel engine being used for the 500L.
(...)
In December 2020, Otto announced plans to develop a zero-emissions version of the 500L and have it available by 2027. That is two years after the first Celera model is due to enter service.

According to ZeroAvia, the Celera 750L will provide even more competitive operating costs due to reduced maintenance expenses for the propulsion system and the lower price for hydrogen versus jet-A. Within the next few weeks, ZeroAvia expects to start flying a 19-seat Dornier 228 testbed aircraft fitted with the 600-kW ZA600 powertrain. It has a pair of the turboprop twins that it will work on at its facilities in California and the UK.

Hopefully, unlike the failure of many such projects, this one will become a reality...

To be continued.
 
Lower price for hydrogene than for Jet-A???? Sounds a bit far fetched...

Fuel Cells are loosing efficiency in thin air, since a large amount of work has to be invested in compressing the air. I believe a hydrogene combustion engine would be a better choise for this application.

Maybe the Redair company is in trouble because of their Russian background and they have to change their focus...
 
I wonder how "stretching" the fuselage works given the constant curvature to give laminar flow. Maybe this is actually a new fuselage but scaled up? Or similar airfoil with lower thickness/chord?
 
they showed a version with a bigger fuselage which should also be able to provide laminar flow, could be this one. I would prefere to see one version getting closer to production instead of seeing new even more ambitios plans....
 
No, but they have a flying prototype which is waitung for the multi stage turbocharging for the maximum flight hight. They are a start up and years away from a product.
 
The articles says :

In December 2020, Otto announced plans to develop a zero-emissions version of the 500L and have it available by 2027. That is two years after the first Celera model is due to enter service.

Then, nothing is foreseen to be sold before 2025. But even 2025 could be difficult for such a project...

See you in 2025, to confirm... or not...
 
I like their design and their approach, but to me it isnt a good sign, if big changes or a new concept are introduced instead of focusing to enter the marked.
 
A few observations on this aircraft:

The high laminar flow model isn't practical in real life on a passenger aircraft certified for IFR and known icing plus dealing with typical insect impacts. How will they de-ice this aircraft? Where will the shed ice end up? Think that pusher prop...

The 3 stage turbo system required to fly a diesel at 50,000 feet isn't practical. The intercooler drag and volume would be a show stopper here.

Relighting a diesel at 50,000 feet will be a rather large certification challenge.

Pressurization differential will have to be impressive and will the FAA certify a single engine piston aircraft for 50,000 feet? I'm doubtful.

Few people will spring $4M+ for a piston powered aircraft. Turbines rule here. Folks with this kind of money don't care about fuel economy.

I have some concern about C of G range with the layout. Challenging when operating with pilot only vs. passengers all loaded well forward of the wing.

The latest iteration with hydrogen can't be for actual practical use since no infrastructure exists for the fuel. Maybe as a technology demonstrator.

I don't see this aircraft being accepted into the market in any numbers and believe it won't meet the lofty initial performance projections.
 
A laminar flow wing is a wing which is optimized to have a large proportion of laminar flow. I guess, all modern wings try to do so, even if other constrains (stability) or manufacturing (rivets, de-icing etc.) might limit the gains. The Lance Air planes are equipped with a laminar flow wing and I’m sure, these are not the only planes in GA. Electrical heating the leading edges or weeping wings should enable a smooth leading edge without disturbing the laminar flow.

The Grob Strato 2c also had a three-stage turbo charging system and despite its large engine nacelles it produced net thrust by cooling (Meredith effect). In the Patent file of Otto you can clearly see, that they are aware of the importance of a proper cooling system and they are planning to use something like a jet cooler with additional draught by the exhaust gases. The Strato 2C was even flying higher than the Otto Celera (24. Km with 18 km being the most effective flight height, if I remember correctly).

Restarting a Diesel (or a turbine) in great height is not trivial, but it should not be impossible. For Diesel engines, I know a quite simple trick which works, but I can’t describe it here.

I don’t know if any of us is a marketing specialist, but I have my doubt, that most people really care what device is driving the prop, the Celera offers unmatched range, interior space, speed and low operation cost. It will also help to reduce the CO2 emissions significantly and running on expansive synthetic fuel will be feasible, unlike in a thirsty small jet.

The problem with fuel cells is, that they need a higher boost pressure even than Diesel engines and they produce very cold exhaust gases (80°C) which contain to little energy to drive a turbocharger without electrical support. Therefore, fuel cells are losing efficiency with increasing height. The cooling system also has lower temperatures which makes it more difficult to create net thrust. In a Diesel engine, only 15-20 % of the waste heat is in the cooling system, the majority is in the exhaust (plus charge cooler), in a fuel cell, the waste heat is mainly in the cooling system, so that the coolers of fuel cells have to be much bigger and heavier than those of combustion engines.
 
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The problem with fuel cells is, that they need a higher boost pressure even than Diesel engines and they produce very cold exhaust gases (80°C) which contain to little energy to drive a turbocharger without electrical support. Therefore, fuel cells are losing efficiency with increasing height. The cooling system also has lower temperatures which makes it more difficult to create net thrust. In a Diesel engine, only 15-20 % of the waste heat is in the cooling system, the majority is in the exhaust (plus charge cooler), in a fuel cell, the waste heat is mainly in the cooling system, so that the coolers of fuel cells have to be much bigger and heavier than those of combustion engines.
What sort of fuel cells are you talking about?
 
PEM, these are the only feasable for any mobile applications. High temperature fuel cells need to run continously (not more than three starts per year permited).
 
Why the need for a supercharger in a fuel cell powered vehicle? What needs to be supercharged?
The membrane in a PEM fuel cell needs to operate in a narrow temperature/pressure band. Anything to do with that?
 
The power output of a fuel cell is about linear to the gas density inside (temperature plays also a big role, but the max temp is limited to about 80°C for enough life expactation), so you need to increase the gas pressure to get a decent amount of power. The ordinary peak pressure in PEM fuel cells is about 3 bar (absolute). The pressure of the H2 side and the air side must be well balanced to not destroy the membrane inside.
 
Have you looked at SOFC for your comparison ?
Solid oxide fuel cells are a class of fuel cells characterized by the use of a solid oxide material as the electrolyte. SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the hydrogen, carbon monoxide or other organic intermediates by oxygen ions thus occurs on the anode side. More recently, proton-conducting SOFCs (PC-SOFC) are being developed which transport protons instead of oxygen ions through the electrolyte with the advantage of being able to be run at lower temperatures than traditional SOFCs.

They operate at very high temperatures, typically between 500 and 1,000 °C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.

Solid oxide fuel cells have a wide variety of applications, from use as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. In 2009, Australian company, Ceramic Fuel Cells successfully achieved an efficiency of an SOFC device up to the previously theoretical mark of 60%.[2][3] The higher operating temperature make SOFCs suitable candidates for application with heat engine energy recovery devices or combined heat and power, which further increases overall fuel efficiency.

Because of these high temperatures, light hydrocarbon fuels, such as methane, propane, and butane can be internally reformed within the anode. SOFCs can also be fueled by externally reforming heavier hydrocarbons, such as gasoline, diesel, jet fuel (JP-8) or biofuels. Such reformates are mixtures of hydrogen, carbon monoxide, carbon dioxide, steam and methane, formed by reacting the hydrocarbon fuels with air or steam in a device upstream of the SOFC anode. SOFC power systems can increase efficiency by using the heat given off by the exothermic electrochemical oxidation within the fuel cell for endothermic steam reforming process. Additionally, solid fuels such as coal and biomass may be gasified to form syngas which is suitable for fueling SOFCs in integrated gasification fuel cell power cycles.

Source: Wiki
 
A laminar flow wing is a wing which is optimized to have a large proportion of laminar flow. I guess, all modern wings try to do so, even if other constrains (stability) or manufacturing (rivets, de-icing etc.) might limit the gains. The Lance Air planes are equipped with a laminar flow wing and I’m sure, these are not the only planes in GA. Electrical heating the leading edges or weeping wings should enable a smooth leading edge without disturbing the laminar flow.

The Grob Strato 2c also had a three-stage turbo charging system and despite its large engine nacelles it produced net thrust by cooling (Meredith effect). In the Patent file of Otto you can clearly see, that they are aware of the importance of a proper cooling system and they are planning to use something like a jet cooler with additional draught by the exhaust gases. The Strato 2C was even flying higher than the Otto Celera (24. Km with 18 km being the most effective flight height, if I remember correctly).

Restarting a Diesel (or a turbine) in great height is not trivial, but it should not be impossible. For Diesel engines, I know a quite simple trick which works, but I can’t describe it here.

I don’t know if any of us is a marketing specialist, but I have my doubt, that most people really care what device is driving the prop, the Celera offers unmatched range, interior space, speed and low operation cost. It will also help to reduce the CO2 emissions significantly and running on expansive synthetic fuel will be feasible, unlike in a thirsty small jet.

The problem with fuel cells is, that they need a higher boost pressure even than Diesel engines and they produce very cold exhaust gases (80°C) which contain to little energy to drive a turbocharger without electrical support. Therefore, fuel cells are losing efficiency with increasing height. The cooling system also has lower temperatures which makes it more difficult to create net thrust. In a Diesel engine, only 15-20 % of the waste heat is in the cooling system, the majority is in the exhaust (plus charge cooler), in a fuel cell, the waste heat is mainly in the cooling system, so that the coolers of fuel cells have to be much bigger and heavier than those of combustion engines.

Lancairs don't have a laminar flow fuselage which is where Otto claims to have cut a large percentage of the drag. This would require deicing on the fuselage as well, otherwise a huge hit in overall drag. Simply not practical to deice the whole airframe (fin and stab too) to maintain laminar flow.

The Lancair Evolution has boots so no laminar flow happening there. Other Lancairs are not flown in known icing. Weeping wings wouldn't allow laminar flow and isn't very suitable for such a long range aircraft due to the limited supply of fluid. Electrically heated wings are challenging on a composite structure due to low thermal conductivity. Foil on the external surface is likely to trip the boundary layer again. The question still remains if you can do it, will that prop "process" the shed ice without damage?

The Grob and other ultra high altitude aircraft had much lower wing loadings plus higher aspect ratios and huge propellers. Very different from Celera. The sheer volumes of turbomachinery and HXs required for a 500hp CI engine at 50,000 feet won't fit in the fuselage. The 1st stage compressor housing for each bank would have to be on the order of something like 14+ inches in diameter. You are talking about 6 turbos here and multiple HXs for intercooling. CI engines are not practical at these altitudes due to the high pressure ratios required to make useful hp.

Exhaust augmented cooling would be very difficult to incorporate in this design due to the number and placement of HXs required. There is no free ride here either with a transfer of energy from one system to another.

The Grob used a turbine engine to feed high pressure air to the turbocharged Contis, not a 3 stage turbo systems to my knowledge. Can you cite a reference for net cooling thrust on the Grob? From the photos, I don't see the required duct shape there to maintain or increase exit velocity. Perseus B used a 3 stage turboed Rotax engine but this weighed 2.6X more than the base engine (turbos, plumbing and HXs).

In this price range you'll find no piston aircraft. Folks here want the cache and reliability of turbine power. They also don't care about CO2.

I don't know anything about fuel cells but I wonder where this aircraft will operate to fuel up with hydrogen?

Much as I am impressed that this aircraft was designed "outside the box" and was actually built and flown, they don't seem to understand high altitude turbocharging, the proposed market, certification requirements of an IFR capable (that means de-iced) single CI engined, pressurized aircraft flying at 50,000 feet.

I'll go out on a limb and predict that this aircraft will never meet the original projections nor will it ever be certified and go into production in the form we see here. Maybe has a chance with a turbine engine.
 
The Lancair evolution has a glide ratio of 1:24, I proved the claims of Otto with that glide ratio, and the targets can be archieved with it (in short: 3500 kg*9.81 m/s²*205 m/s *1/24 = 293 kW, without additional trust by cooling or exhaust). Keep in mind, the Celera is intended to fly with a constant, optimal angle of attack with the best glide ratio. So, the aerodynamic qualities of the Otto Celera don’t have to be superior to the Lanceair to reach their targets. I don’t see a reason, why a fuselage can’t be optimized for laminar flow, this was already the case for the Piaggio Avanti.

There are electric heated leading edges on the LX-7 available and I guess also for the Lanceair evolution. As far as I know, these are working fine and they a commercially available. Despite that, in 50.000 ft you don’t have icing anymore, so you would need a large amount of deicing fluid in a weeping wing. Icing occurs only on the leading edges of something, surly also on the nose of the Celera, so there might be a need for a little deicing the nose but not on the whole fuselage. Pusher aircrafts in great heights are nothing new (B-36) and icing is never mentioned as a big problem here.

The Grob definitely had a three-stage turbo system (I have a book in which it is described in detail), they used a low pressure turbocharger which was built out of a PT6 (You can find all the information in “Flugmotoren und Strahltriebwerke; Kyrill et.al.). Unlike in gas turbines for propulsions, turbochargers in piston engines have higher temperature spread between turbine and compressor and a lower pressure ration (turbocharger vs. turbine), so that there is much more reserve for keeping the power output constant by excessive turbocharging. The Grob engines could keep the maximum charge pressure up to 24 km (78700 ft). About the additional trust, in this book they mentioned 12 % additional trust by the gas expansion behind the low pressure turbocharger, on a website (still searching to re find it….) it was the same amount for the combined cooling air/exhaust gas outlet. There is a paper about the aerodynamic and propulsion of this plane, unfortunately not for free: https://www.sciencedirect.com/science/article/abs/pii/S1270963801011270

Take a look:
View: https://www.youtube.com/watch?v=QEfYBAxDJgM&ab_channel=EdutainmentEdutainment


Keep in mind, that the air density in 24 km (maximum flight height of the Grob Strato 2C) is more than four times lower than at 15 km (maximum flight height of the Otto Celera) with the temperatures being nearly equal (https://www.digitaldutch.com/atmoscalc/). Cooling and turbocharging for the Otto Celera is more than 4 times easier than for the Grob Strato 2C, the faster speed also helps to keep the cooler small.

The engine position in the rear and the large diameter of the fuselage of the Otto Celera will give a lot of internal space for the cooling system. The engine bay has approximately 60 % of volume of the passenger cabin, take a look in the interior and imagine an engine in the middle with all the coolers/diffusors around.

In this price range, you can’t find any aircraft which offers this range, interior space and speed, there is simply no match. Furthermore, there is a high chance, that fossil Jet-A will be fully or partially replaced by something CO2 neutral in near future. This replacement could be biofuel or synthetic fuel or a blend from both, but in any case, it will be something which is much more expansive than today’s Jet-A. Having a fuel-efficient airplane will not be only a huge advantage in term of range, but also in cost. If people charter an airplane, they will care for travel time and comfort but not for the propulsion system. Some might prefer a Jet over an “old fashioned” propeller, but most want even recognize if there is a Diesel or Turbine driving the prop.

The Otto Celera simply couldn’t achieve the targets with a turbine, because turbines are losing to much power in extreme altitudes. There are good reasons, why Grob used piston engines (with a PT6 derived turbocharger), piston engines can use almost all the compressed air and keep the power output constant, whereas turbines simply can’t do it, because they operate with much surplus air.

Another big advantage of piston engines is the much more efficient operation for short distances, were turbines burn a big part of the fuel during taxing and stress the turbine with many load changes.

Relighting a Diesel is not that difficult if you’re using water cooling, water cooled charge coolers (for the last stage) and some chemical help. I’m not the FAA, but I don’t see the need to restart a Diesel engine at a flight height which is much higher than the Mount Everest, you could simply wait with the starting procedure until crashing into a mountain becomes an issue.

About fuel cells: I see it as a long term solution and not suitable for aircrafts with long range and great flight height. You might have noticed, that I’m not to optimistic about the success of a fuel cell variant of this plane.
 
Icing occurs only on the leading edges of something, surly also on the nose of the Celera, so there might be a need for a little deicing the nose but not on the whole fuselage. Pusher aircrafts in great heights are nothing new (B-36) and icing is never mentioned as a big problem here.

Keep in mind, that the air density in 24 km (maximum flight height of the Grob Strato 2C) is more than four times lower than at 15 km (maximum flight height of the Otto Celera) with the temperatures being nearly equal (https://www.digitaldutch.com/atmoscalc/). Cooling and turbocharging for the Otto Celera is more than 4 times easier than for the Grob Strato 2C, the faster speed also helps to keep the cooler small.

The engine position in the rear and the large diameter of the fuselage of the Otto Celera will give a lot of internal space for the cooling system. The engine bay has approximately 60 % of volume of the passenger cabin, take a look in the interior and imagine an engine in the middle with all the coolers/diffusors around.

The Otto Celera simply couldn’t achieve the targets with a turbine, because turbines are losing to much power in extreme altitudes. There are good reasons, why Grob used piston engines (with a PT6 derived turbocharger), piston engines can use almost all the compressed air and keep the power output constant, whereas turbines simply can’t do it, because they operate with much surplus air.

Another big advantage of piston engines is the much more efficient operation for short distances, were turbines burn a big part of the fuel during taxing and stress the turbine with many load changes.

Relighting a Diesel is not that difficult if you’re using water cooling, water cooled charge coolers (for the last stage) and some chemical help. I’m not the FAA, but I don’t see the need to restart a Diesel engine at a flight height which is much higher than the Mount Everest, you could simply wait with the starting procedure until crashing into a mountain becomes an issue.
I'm not sure you've ever been in heavy icing. I can assure you, ice can form anywhere. My point was mainly that any icing on the fuselage or wing immediately trips the boundary layer and you'd be back to turbulent flow and suffer a 30+% increase in drag (if we believe Otto's laminar numbers and the drag reduction quoted). Speed and range would suffer accordingly as long as the ice remains. You may have to climb and descend through a thick icing layer to get to 50,000 feet. If some ice remains in places on a turbulent flow airframe, it's not of so much concern as it has far less aerodynamic impact.

I'm guessing Celera will use a composite propeller. The FAA will require that that remains viable in ice shedding tests.

Turbocharging SI and CI engines is quite different since CI engines require much higher pressure ratios to make the same power. I estimate the RED engine would be running a PR of around 19 to 1 at high cruise power.

The 3 stage turbo system used on Perseus dwarfed the engine both in volume and weight. It would be no different here. It would be a plumbing and maintenance nightmare to fit all the ducting and HXs required in an efficient manner to keep the cooling drag under control. Look at the size of the components on a P47 with only 2 stages and flying at 30,000 feet for a comparison. The Rotax 3 stage system used a 20 inch diameter 1st stage compressor housing on only an 80hp engine. Make that two even larger ones in the case of the Red engine. You begin to see the packaging problems here. It's completely impractical given that some fuel must also be carried in the fuselage on Celera.

Turbines are simply flat rated to achieve good altitude performance. The MQ-9 operates to over 50,000 feet using conventional Garrett turboprop engines. Some modern PWC turboprops offer BSFC figures at altitude comparable to existing aero CI engines. Turbines are proven very reliable and durable. I don't think most worry about turbine stress with load changes.

I think you're forgetting the time of useful consciousness at 50,000 feet is less than 10 seconds and less than 1 minute with 100% O2 and no pressure mask. The FAA certainly will be concerned about that fact in certifying this aircraft for flight at these levels in the event of a flame out of the single engine. No passenger carrying single engined piston aircraft has every been certified by the FAA. Count on that being a big hurdle.

Otto hasn't proven any of their lofty projections as they lack the turbo system fitted to get up to these levels. No useful flight test data has been released and they have masked internet flight data. Let's wait and see how this pans out but it sounds like the diesel may already be dead as they consider fuel cells. Perhaps someone knowledgeable in turbocharging has finally educated them on the folly of their original plan.

The RED engine is certified but has virtually no flight history to speak of compared to legacy aero engines. They have a lot to prove as well at these altitudes where it's never been operated.
 
So, I believe you’re right icing might occur and not being totally washed of by deicing fluid, but this is not a safety concern and will only happen occasionally. Once the plane has reached its cruise height, icing want be a problem anymore. In case you will fly a long distance over water, you need to be sure, that no icing occurred during the starting phase or you need to get rid of it by choosing the right atmospheric conditions. I guess, in more than 90 % of the flights, icing is no problem an the Otto Celera can make full use of the laminar flow optimization.

The Dassault Falcon flies as high as the Celera is intended to do, so it is possible to certificate planes for that height. Maybe this is not possible for airliners, but for smaller business jets (and surly as well for "business props").

BMW had a production Diesel engine with six cylinders and 5 turbos and three stages (2 x high pressure, 2 medium pressure, one low pressure turbo). Multi stage supercharging is nothing new, but built for millions of cars and almost all of the trucks.

For a flight height of 15.000 m (Otto Celera) even a two stage charging system would work:

.The air density at 15.000 m is about 0,193 kg/m³, the air speed is 205 m/s, so we can add some dynamic pressure to this:

With 80 % pressure recovery we can add ps=(0,193/2)*205²= 4055 Pa. The atmospheric pressure is 12044 Pa so the total pressure will be 16100 Pa. The pressure increase of about 33% will not fully correspond in a density increase due to warming an due to losses in the pressure recovery. The temperature will rise about 8.5 % so that the density with 100 % pressure recovery is roh = 0.193 * (16100/12044)*(1/((16100/12044)^0.285) = 0.2375 kg/m³. Let’s estimate some pressure losses (80 % pressure recovery) so we have a p0 of 0.229 kg/m³ and a p0 of 14820 Pa or 0,148 bar.

The engine will propably need about 3.2 bar absolute, so the required pressure ratio is about 22.5 (taking account of some flow losses), this is about 4.75 per stage. This is quite high under normal circumstances, but the maximum pressure ratio of a turbocharger is usually limited by the compressor outlet temperature, because the max. tolerable temperature of the aluminum compressor wheel is limited to around 250 °C. Since the incoming air is extremely cold (216 K or -57 °C) a pressure ratio of 5 or even more should be totally OK for the low pressure turbo. So a two stage charging system with two pressure ratios of 5 (low pressure) and 4.5 (high pressure) would be totally feasible.

I’m not doing the turbomatching for Redair for free (they can do it themselves, I’m sure…), but I took a look at Garret. For the matching, we need to be aware, that the density of the low pressure turbo is only about 1/5 th of the sea level air density and so we can only assume 20 % of the norm mass flow in a compressor map (the rest stays almost the same). With around 370 kw engine power, an AFR of 18 and SCF of 220 g/kwh we will have an air mass flow of about 0.4 kg/s. Because of the low density, this corresponds to 2 kg/s in a compressor map or 1 kg/s if we are using two low pressure turbochargers, which makes sense in a V-engine. So here is one (probably not the optimum) turbo that could do the job for one half of the engine:

https://www.garrettmotion.com/wp-content/uploads/2018/05/Comp-Map-GTX5533-88mm-2-scaled.jpg

If you take a look on the outer dimensions (https://www.garrettmotion.com/wp-content/uploads/2020/02/GAM-Flange-Diagram-G57.pdf) this is nothing extreme cumbersome and could be easily applied to the Redair (or even better) engines. As said, multi stage turbocharging is a standard solution for Diesel engines even for road vehicles. Keep in mind, that the high-pressure turbo can be much smaller because the density will be about 5 times higher.

The weight this turbocharger is around 14 kg (https://www.turbosbytm.com/de/g57-3000-super-core), so a second stage (2x) would add about 30 kg to the engine.

The load cycles for turboprops are indeed a factor and a main reason why twin piston engine planes are sometimes kept in service a long time for island hopping. Turboprops are absolutely unsuited for that, because of excessive fuel consumption and fast aging. Turbines are also much more expansive even than aero Diesel engines which are built in much smaller numbers. With the much better part load efficiency, the best angle of attack can not only be achieved with altering the height but also by altering the speed. In a turbine, you would loose so much engine efficiency, that there is no gain by flying slowr.

Flat rating of a turbine means, you take an oversized turbine and don’t make use of the power potential.

I know an engine design which is much better than the Redair engine (lighter, more efficient, longer lasting with lower rpm) but the Redair engine is a good starting point. The Continental Aero Diesels are much more reliable than their gasoline counterparts, despite being much more complex. The FADEC system is the cause of the increased reliability. Operating spark, mixture, speed manual by a pilot is extremely anachronistic and the main reason for engine failures (after fuel issues). Diesel engines in trucks are extremely long lasting and reliable, Aero engines could be the same. One really nice feature about the Redair Diesel is, that both sides of the engine run independently, with two separated engine controls, cooling systems, charging systems, injection systems, so that in almost all failure scenarios you can still limp home with one side of the engine. This approach is even safer than a twin engine, since a failure of one side of the engine can be handled much easier by the pilot than an unsymmetrical thrust.
 
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Have you looked at SOFC for your comparison ?
Solid oxide fuel cells are a class of fuel cells characterized by the use of a solid oxide material as the electrolyte. SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the hydrogen, carbon monoxide or other organic intermediates by oxygen ions thus occurs on the anode side. More recently, proton-conducting SOFCs (PC-SOFC) are being developed which transport protons instead of oxygen ions through the electrolyte with the advantage of being able to be run at lower temperatures than traditional SOFCs.

They operate at very high temperatures, typically between 500 and 1,000 °C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.

Solid oxide fuel cells have a wide variety of applications, from use as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. In 2009, Australian company, Ceramic Fuel Cells successfully achieved an efficiency of an SOFC device up to the previously theoretical mark of 60%.[2][3] The higher operating temperature make SOFCs suitable candidates for application with heat engine energy recovery devices or combined heat and power, which further increases overall fuel efficiency.

Because of these high temperatures, light hydrocarbon fuels, such as methane, propane, and butane can be internally reformed within the anode. SOFCs can also be fueled by externally reforming heavier hydrocarbons, such as gasoline, diesel, jet fuel (JP-8) or biofuels. Such reformates are mixtures of hydrogen, carbon monoxide, carbon dioxide, steam and methane, formed by reacting the hydrocarbon fuels with air or steam in a device upstream of the SOFC anode. SOFC power systems can increase efficiency by using the heat given off by the exothermic electrochemical oxidation within the fuel cell for endothermic steam reforming process. Additionally, solid fuels such as coal and biomass may be gasified to form syngas which is suitable for fueling SOFCs in integrated gasification fuel cell power cycles.

Source: Wiki
As said, these are not feasible for any mobile application (maybe only as auxiliary power unit in large ships), since they need to run continuously. Every temperature cycle will age them and that s why in CHP applications you are not allowed to switch them off more than three times a year. Another issue is the extreme long start up time, this could be accepted for large passenger liners which are about 3/4 of their lifetime in the air, but not for business planes which operate unregularly and must be ready for the next flight within a short time span. They also want exept fast load changes which is a serios risk in aircraft propulsion.

Another drawback is the extreme price of these fuel cells, even in serial application for home heating they cost about 30.000 € with an electric power of only 1.5 kW. Larger fuel cells might be somewhat cheaper, but aircraft applications will make everything 10 times more expansive than ground applications. T
 
They are indeed good for APU. Back in the early 2010's there was a successful joint program b/w Airbus and Boeing to study that application. Last time I checked, it was in a good path.

Their advantages is the use of onboard fuel instead of hydrazene and simplicity in refueling, including replacing the Fuel cells material.

I am surprised that you are pointing toward a susceptibility to load variation when Fuel cells have to be integrated with a propulsion strategy and doesn't make much senses as a stand-alone solution.

ZeroAvia recent crash report (modified Piper-Malibu) illustrates how a credible strategy could be pretty well adapted to aviation and self-reliant (I mean without the complexity of external systems such as is Hydrogen fuelled a/c inducing a price increase ill-suited for the GA industry.

Notice that you can do SOFC with H2, Methane or general aviation fuel. This flexibility is an outstanding bonus that should not be sidelined when it comes to make cost projection.

Methane can be for example harvested in agricultural fields and then used to propel agricultural planes. That plane can then fly on its own to the next airport, refuel with JP-7 and then cruise at the other end of the state to execute another agricultural contract. All this with the same propulsion unit.

We can see how the green energy revolution has adapted to a variety of energy production means. It would be foolish today to look at the future as if there was only one way of doing thing and start to replicate the Oil/gas industry but with a new fuel.
This is probably not how the outcome will look like.

The GA industry must understand that nobody is gonna field the massive costs inherent with stocking and distributing hydrogen in small airfields before H2 is on one form or another massively fielded in the civilian infrastructure (cars mainly, something that is not absolutely certain to happen as seen from today).
In other words, we would be in total denial if we do not see GA ending orphaned by those massive projects led mainly by states incentives. There is realistically no room for a popular aviation in them.
 
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In a APU of a large airliner the fuel cells could run almost continously, this might be a more usefull application than propulsion for small aircrafts. I m pretty sure, the ZeroAvia is not using a SFOC but a PEM fuell cell. As fas as I know, the accident happend during the switch from the Batterie to the fuel cell, when the prop was free spinning in induced some voltage at the wrong moment for the inverter.
 
about future propulsion, synthetic fuels to replace Jet-A is the only realistic short term solution, Ammonia (better than Hydrogene) for the mid and not so lang anymore distances would be the next step. Battery electric planes will find their niche for flight shools, island hoppings and the good old urban mobility.

if you concider the low price proclaimed by the US Navy for synthetic jet fuel produced on board of an nuclear powered carrier, it is vital to believe, that GA will simply switch to synthetic fuels. Any efficiency improvment by hydrogene or ammonia will not be large enough to make a real price difference for planes which are used 300 h p.a. or less.
 
The Dassault Falcon flies as high as the Celera is intended to do, so it is possible to certificate planes for that height. Maybe this is not possible for airliners, but for smaller business jets (and surly as well for "business props").

BMW had a production Diesel engine with six cylinders and 5 turbos and three stages (2 x high pressure, 2 medium pressure, one low pressure turbo). Multi stage supercharging is nothing new, but built for millions of cars and almost all of the trucks.

For a flight height of 15.000 m (Otto Celera) even a two stage charging system would work:

The engine will propably need about 3.2 bar absolute, so the required pressure ratio is about 22.5 (taking account of some flow losses), this is about 4.75 per stage. This is quite high under normal circumstances, but the maximum pressure ratio of a turbocharger is usually limited by the compressor outlet temperature, because the max. tolerable temperature of the aluminum compressor wheel is limited to around 250 °C. Since the incoming air is extremely cold (216 K or -57 °C) a pressure ratio of 5 or even more should be totally OK for the low pressure turbo. So a two stage charging system with two pressure ratios of 5 (low pressure) and 4.5 (high pressure) would be totally feasible.

The load cycles for turboprops are indeed a factor and a main reason why twin piston engine planes are sometimes kept in service a long time for island hopping. Turboprops are absolutely unsuited for that, because of excessive fuel consumption and fast aging. Turbines are also much more expansive even than aero Diesel engines which are built in much smaller numbers. With the much better part load efficiency, the best angle of attack can not only be achieved with altering the height but also by altering the speed. In a turbine, you would loose so much engine efficiency, that there is no gain by flying slowr.

Flat rating of a turbine means, you take an oversized turbine and don’t make use of the power potential.

I know an engine design which is much better than the Redair engine (lighter, more efficient, longer lasting with lower rpm) but the Redair engine is a good starting point. The Continental Aero Diesels are much more reliable than their gasoline counterparts, despite being much more complex. The FADEC system is the cause of the increased reliability. Operating spark, mixture, speed manual by a pilot is extremely anachronistic and the main reason for engine failures (after fuel issues). Diesel engines in trucks are extremely long lasting and reliable, Aero engines could be the same. One really nice feature about the Redair Diesel is, that both sides of the engine run independently, with two separated engine controls, cooling systems, charging systems, injection systems, so that in almost all failure scenarios you can still limp home with one side of the engine.

The Falcon and other aircraft certified for flight at around FL500 are mostly twin turbofan business jets. The chances of a double engine failure on these and subsequent cabin pressure loss is many times lower than than an unproven, single engine diesel. I'd be very surprised if the FAA will certify this configuration.

Compound diesels have been around for decades in terrestrial applications however they make less sense for high altitude aviation applications. The BMW B57S engine was discontinued in 2020, being too complex and expensive to build and update.

We rarely run turbos at the top of the compressor map in aircraft as efficiency falls off, life is reduced and surge/ choke margins are more narrow up there making matching more difficult over a range of power settings. Also, the 2nd stage compressor, given its size won't be developing a PR of 4.75. This will necessitate 3 stages of compression IMO. My background is 45 years of turbocharging experience, 20 years of that in aviation (I fly a turbocharged Experimental) and a decade in assisting Reno race teams (3 times winning) with FADEC equipment and turbocharging advice.

I think your weight estimates on the turbo system are way low. Here is a breakdown on an actual 3 stage system used for high altitude testing some years back:

rot.jpg

Note this is for an 80hp engine. The components will be MUCH larger and heavier on a 500hp class CI engine (many times more air mass flow required). The 3 stage powerplant with the required HXs will be immense and complicated to inspect and service. The turbine is ultimate simplicity by comparison.

Island hopping aircraft generally fly low where turbines are not competitive on fuel flows. That's a different story at FL500 where Celera will fly long legs. Turbines make more sense here.

Most turbines (turboprops) are flat rated today to give good altitude performance and they work just fine up there. I believe the engineers working in this field know what they are doing. Turbines are light enough that using a larger core, doesn't impact weight very much.

Continental doesn't make an engine anywhere near large enough to use on Celera so that doesn't apply much to this thread. The RED engine is a little weak for Celera. It will be running hard to produce even middling climb performance compared to existing single turboprops like the TBM960 and PC12, especially in icing conditions where the excess power is welcome. Celera won't be able to match the balanced field lengths of these aircraft either due to the lower power, higher wing loading and lack of effective high lift devices. This will limit the fields where they can operate from, reducing utility.

I believe in the likely $5M range Celera will cost, most buyers will want the proven reliability, durability and low maintenance costs of turbine power. Many of the CI aero engines so far offered require quite a few expensive parts replaced during their life- turbos, injectors, pumps, gearboxes etc. Their overall cost per hour over the TBO/TBR period, after their higher initial costs, make them a wash with current piston SI engines at US avgas prices. That will be different in other parts of the world where JET and Avgas costs are much further apart. I'd maintain that folks buying $5M aircraft don't care much about fuel costs though so I see a small market for Celera with diesel power in the US at least.

At this stage, Celera hasn't proven flight at 50,000 feet so only time will tell what it can really do. I don't put much faith in new startup aviation companies with no track record in producing certified production aircraft. They have a lot to learn the hard way. It always costs more and takes longer than anticipated, especially when your design is so far away from the norm. Part 23 certification by 2025 is certainly a pipe dream.
 
About the pressurization: The Failing of the engine doesn’t imply a sudden loss off pressure, in a sealed cabin only the venting system must be closed. The large internal volume of the Celera contains enough air for breathing a long time before the occupants would run out of air. The plane could surly glide down to earth before breathing become critical. But even long before touching the earth, breathing with the oxygen system and venting the internal volume down to atmospheric pressure could be done long before. An engine failure combined with a fuselage failure would be a very rare event…

Compound Diesel make a lot of sense for high power applications. The two-stage charging system for aero engines can be much simpler than the standard two stage charging system for cars. In cars, the systems are optimized for max. torque over a wide rpm range. Usually, at low rpm, only the small turbo is used, at medium rpm ranges, a two staged configuration is used and at high rpm only the large turbo is doing the job. In an aero engine the system only needs to switch from singe stage (small turbo at low height) to 2 stage operation in great height. All you need for that are two turbos in a row with each of them having a waste gate, this is quite a simple system without lot of valves and bypasses as in cars.

We did simulation of turbo systems for an aero engine, the good thing about it is, you always run along the line of highest efficiency (if done right). With increasing the flight high, and constant power output, you stay right on the line and the compressor map doesn’t have to be wide. Even part load doesn’t change a lot about it, as turbine friends might know, the torque over rpm characteristic of a prop matches very well the behavior of a turbine or turbo system. You don’t run into surging during normal operation until you are doing fast load drops (very untypical for planes). In any case, there are many ways to prevent surging and anti-surging valves can be found on many cars. This engine will have a FADEC system and it relies on electronic anyway, so the application and control of surging is no big deal.

With compressing cold air, the safety margin of the compressor becomes much higher since they are in fact not really simply limited by the compressor speed but by the combination of compressor outlet temperature and speed. I estimated a boost pressure of 3.2 bar (absolute) for the Redair engine but this was much too high. I took my calculator and calculated the pme, it is “only” about 18 bar pme so something about 2.5 bar would be totally sufficient. The total pressure ratio needed will not be higher than 20, this can very well be done with a two stage system. Turbochargers of marine or stationary engines can operate with a pressure ratio of 5.5 bar continously.

BTW: I’m developing combustion engines since about 25 years, mainly doing the design (I was involved in an aero Diesel project once long time ago and also now), but I also did a lot of thermodynamics and charging layout (e.g., for truck engines and a race Diesel engine).

Thanks for the interesting example of the Rotax engine! This configuration is for 85.000 ft, the air density is only 18 % of that of the flight height of the Celera. Without doing to much calculation, it should be clear that you need one stage more, if the total pressure ratio is about 5.5 times higher. As said for the Strato 2C, the cooling systems weight and volume will also be 5.5 times higher at 85.000 ft than at 50.000 ft.

The specific weight of turbocharger gets lower with increasing engines size. The Garret turbocharger which I choosed as an example weights only 14 kg and is recommended for (ground) engines with 6-12 L displacement. An ordinary car turbocharger for a 1.5 L engine doesn’t weight much less (let’s say 10 kg), so the small Rotax engine will suffer much more from a three-stage charging system than the way bigger Redair engine.

I’m convinced, the Otto Celera will make use of elaborated lifting devices, just as the TBM 700 does. BTW the wings of the TBM 700 were originally intended to be used on a very capable piston powered plane by (Mooney M301 only one prototype). The propulsion system of the Otto Celera has a unique feature, between the engine and the prop, a torque converter is used which acts as a CVT for variable prop speed. Props are very suitable to produce a lot of trust at low speeds, so starting it from ordinary runway for jets, shouldn’t be an unsolvable problem.

Excluding Continental from this track, as you proposed, doesn’t make sense at all, especially when you included the much smaller (and not even a diesel) Rotax engine. Continental prooved, that FADEC controlled CR injected, water cooled Diesel engines can be more reliable than traditional aero engines. The Redair approach is very similar to the Continental Diesels, both are using FADEC, CR injection, small cylinders, high rpm and water cooling. Both have a layout which is typical for car engines, so they are absolutely comparable (the cylinder numbers are the biggest difference).

I don’t agree to that what potential buyers or customers really want, but it doesn’t make a lot of sense to discuss it any further. Keep in mind, that Otto will make such flight affordable for a much higher number of people so that chartering a plane becomes an alternative to booking business class. Their aim, is a new and a much bigger market than a few multi-Millionaires This cannot be done with current jet-propelled planes.

The Otto Celera will fligh long distances, but they also promote there advantages for short distances over turbine powered planes. Charter planes or personal planes will be operated very flexible and sort distances cannot be excluded.

More important, the world is changing and the cheap AVGAS prices might be gone in just a few years. Synthetic fuels are the only viable option for long range business planes when fossil fuels are ruled out. The cost of these fuels will be much higher than todays AVGAS prices. Fuel efficiency will be much more important in future, even for business jets. Remember, the Gas turbine powered locomotives in the US as well as the gas turbine powered ships were all ruled out and replaced by Diesel engines when the fuel price increased.
 
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I found an interesting link about Turkish publication about turboprops for high altitude. I’m pretty sure, they used propulsion systems which were already flat rated otherwise it would have made any sense.

https://dergipark.org.tr/en/download/article-file/990400

As we can see, a turboprop which has about 1200 kW at sea level is down to 404 kw at 12190 m altitude. This es about equal to the max. power of the Redair Diesel which can (if they are smart enough to design a proper two stage charging system) be held constant up to 15000 m altitude. The difference in air density between 12190 m and 15000 m is bigger as one might think, it’s a factor of 1.55 meaning, that these turboprops would have only about 260 kW left in 15000m altitude. This shows, that a Diesel engine like the Redair with is equivalent in great height to turbine with 1860 kW ground performance. The PT6 couldn’t do the job, it would take something like the PW100 to get enough power. The PW 100 weight around 400 kg, the Redair Diesel 363 kg (with single stage charging and without cooling system). The power to weight ratio of turbines and Diesels is almost equal for height altitude propulsion with the Diesel having a better fuel economy, more flexibility (efficiency is less affected from different engine loads) and by far lower cost.
 

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