Celera 500l Progress

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henryk

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major prop manufacturers
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V<750 km/h !
 

trimtab

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@trimtab

Can you explain this section "The power required for a given IAS rises with altitude (proportional to 1/sqrt of density differences) "
I think it is related to something I have read multiple times years ago over on BT. That for piston planes, for a constant L/D ratio (e.g. constant IAS), the MPG stays the same regardless of altitude. Are these related, and does it make sense (and do I recall it correctly).

Tim
Lift is proportional to the density times v squared. So is drag. So the increase in velocity is proportional to the square root of the ratio at the densities.

Because power is force x velocity, if the velocity increases for the same drag force more power is involved proportional to the increase in velocity, given by the relationship above.

If you apply this reasoning to, for example, a P210, you will find the numbers for the brake horsepower available at 23000 feet when used for the 2,000 ft performance correspond closely to this approximation. In fact, the calculated airspeed will be several knots above the actual demonstrated indicated airspeed, again because the propeller loses efficiency as one gains altitude. You can use this to calculate the achievable true airspeed and you will arrive at the figures that look a lot like what are in the poh at 23000 feet. if you simply count on constant IAS the p210 will be a rocket ship compared to its actual performance.

Applying the same reasoning to the Otto airplane is pretty straightforward once a few assumptions are made about what was going on at fifteen thousand feet during the tests. if they were producing maximum horsepower at 15,000 ft to achieve roughly 250 knots, then it is clear that they would be expected to achieve somewhere between 350 and 400 knots TAS at 50,000 ft. The only way to project a 450 to 500 knot speed would be if they were only producing 50 or 60% of full rated horsepower during those tests to achieve those 250 knots.

But the bottom line is that as air density drops, the power required to achieve the same IAS rises. This is also the reasoning behind why Vx and Vy converge with altitude.

The only plausible explanation for the Celera 500 is that, during the testing, the engine power output was greatly reduced in achieving 250 knots, which is certainly possible, and would be an astounding achievement to be able to move that much airplane with only about 250 horsepower. But that makes more trouble for me for a couple of reasons. Firstly, I don't understand how laminar flow could provide anything like that performance increase, and secondly because the adjacent calculation of using the purported 22:1 lift drag ratio and a reasonable weight for the aircraft based on its stall speed and some other broad assumptions could yield that kind of ability with 250 hp.

And so, again, I remain interested in the kinds of calculations that make this possible, especially if they are obvious and I am simply missing it.
 
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trimtab

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So I think I just figured out a large part of the puzzle. my assumptions assumed that the Reynolds number really didn't change significantly. That works for aircraft that I've usually looked at. However, this aircraft is flying at 50,000 ft and the Reynolds number changes radically. Running some quick calculations shows that it is entirely plausible for the laminar flow regime to encompass the entire fuselage. This was the missing piece for me. It also explains why they were using an infrared camera to establish where the turbulent transition zone occurred at 15,000 ft. This analysis could be used to establish the aft progression of the transition point at 15,000 ft. That information could be used to project whether the transition point would encompass the entire fuselage essentially.

Two things happen if that were to occur, and they do appear to be at least theoretically possible. First, a large flat plate drag reduction could occur. 25% to 35% is entirely within the realm of possibility. This alone could theoretically make speeds approaching 450 mph possible based on the increase in excess horsepower alone. The other effect of having the fuselage completely involved in laminar flow is to reduce the roughly 7% to 10% losses from the boundary layer in the propeller disc in the pusher configuration. If you cut this in half you would gain more horsepower on top of the excess horsepower already gained.

So at long last this project may actually be pretty darned interesting for reasons related to the simple lack of good assumptions and evaluating it initially.

In short, if laminar flow is indeed able to envelope the entire aircraft, and if the drag reduction can approach forty or more percent, it seems very likely if 500 horsepower can be maintained that the aircraft should be able to achieve 450 mph or even more based on the reported conditions at 15,000 ft. The issues related to the ability of the compression-ignition engine to recover from a unstart and the propeller efficiency issue based on Reynolds number are still pretty existential to the success of a project like this. But at least the numbers actually look plausuble in the napkin math finally.

This is also interesting in the context of the company's claim to be able to make a larger aircraft. the company claims it would have to go to 60,000 feet, and this was puzzling except in this context. Scale is very important in all of this.

So, in short, a slower climb in a draggy turbulent condition during the climb, followed by the ability to accelerate as the aircraft is more deeply involved in laminar flow at high altitude. all of this is possible due to the fact that Reynolds number is falling off of a cliff to offset the increased horsepower required for a given ias.
 

rv6ejguy

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I think they were saying 250 mph so 217 knots at 15,000 feet. I can't see that small prop being very efficient at 50,000 feet and 450 mph. Available thrust is a tiny fraction of static thrust at that speed.

In the real world, a few dozen bug splats will negate much of the laminar flow.

I'm certainly impressed with what they have done so far and it's really thinking outside the box. Wishing them the best of luck here as they head towards certification and production. Even if they fall short of their projections somewhat, it will still be impressive and interesting to see how they do all this technically.
 
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trimtab

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It looks like there are a number of academic exercises to get through with the project first before things can get refined into a product. But at least I'm taking the effort out of the same column as Theranos, Enron, Lily, and WorldCom.
 

PMD

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Given Otto's background, I am inclined to give him the benefit of the doubt, but most of what has been raised here are some very good questions. Can't wait to see how the actual hardware plays out. As I believe I already said: even if this thing only comes close to its stated ability, it will still be a game changer - at the very least from a technological frame of reference.
 

rv6ejguy

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Unless things go very smoothly with the powerplant and airframe testing, it will be a while before they get this sorted to work well at FL500. Will the FAA even certify a single engined diesel for flight up there? Never been done before and that usually adds years to the process. I'm thinking pressurization failure would be hard to deal with. Non-pressure O2 masks don't give you much useful consciousness up there. They may be limited to more like 31,000 like a TBM.

Then for IFR, they'll have to certify for icing. How will that be accomplished? EM pulse maybe? Can't use boots as that will ruin the laminar flow. The diesel can't generate enough heat to do the cabin, de-ice and keep the fuel from gelling up there. I see a lot of technical hurdles to get over first.
 
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Victor Bravo

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Small Thorium reactor onboard for running electric or steam de-ice, electrical power, and cabin equipment. Chances are the nuke would be easier to certify than the diesel 😬
 

qchen98

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That's
Unless things go very smoothly with the powerplant and airframe testing, it will be a while before they get this sorted to work well at FL500. Will the FAA even certify a single engined diesel for flight up there? Never been done before and that usually adds years to the process. I'm thinking pressurization failure would be hard to deal with. Non-pressure O2 masks don't give you much useful consciousness up there. They may be limited to more like 31,000 like a TBM.

Then for IFR, they'll have to certify for icing. How will that be accomplished? EM pulse maybe? Can't use boots as that will ruin the laminar flow. The diesel can't generate enough heat to do the cabin, de-ice and keep the fuel from gelling up there. I see a lot of technical hurdles to get over first.
Would a wing radiator do the job? That will also eliminate the intake at the rear.
 

rv6ejguy

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Looking at the Drive article and claimed fuel economy, making some estimates on speed and using a BSFC figure of .35, I come up with around 303 hp for 400 mph and 481 hp for 460 mph. These fit a straight cubed power/ speed function calc, assuming prop efficiency stays the same at high speed, which it won't. Can this do 400 mph on 300 hp though? Only flight testing will prove or disprove that.

A TBM 930 requires around 660 hp to cruise at 370 mph by comparison with a smaller cabin at 31,000 feet.
 

danmoser

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It certainly is an interesting concept, but suspicion is always going to be aroused by a lack of published test results and other specifics. Wind tunnel tests of similar laminar fuselages with Boundary Layer Ingestion have indicated that it is a very promising approach. However, I am skeptical of their claims about Red V12 diesel operating costs and BLI prop performance.

Seems like the concept is more suited to a 19 seat regional carrier with electric propulsion .. Maybe 300 mph at 25,000 ft. with a 500 mile range and proven low operating costs seems more reasonably attainable and commercially viable.
But hey, it's their airplane!.. I'm interested to see what they can do with it. :)
 

trimtab

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Another article says the altitude was 17,000' @ 251 mph. It also inferred the engine was operating below max hp, but I think it was being confused with the planned improvement to raise the critical altitude to 40k' or 50k'.
 

rv6ejguy

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Another article says the altitude was 17,000' @ 251 mph. It also inferred the engine was operating below max hp, but I think it was being confused with the planned improvement to raise the critical altitude to 40k' or 50k'.
It will be interesting to see how they pack 6 turbos on this engine to make close to SL rated hp at 50,000 feet.

xA202.jpg
This is the 2 stage engine but with a max practical pressure ratio of around 4 to 1 from each stage, that puts us at 16 to1. Ambient pressure up there is around 3.3 inches, 3.3 X 16= 53 inches, likely not enough to make 500ish hp, so would need to add one more turbo per bank which is becoming a bit ridiculous.
Most CI aero engines are using 75-85 inches. It would need intercooling after each stage- a plumbing nightmare to fit the piping and HXs into an efficient duct. The first stage compressor becomes very large physically with a 3 stage setup. With a required pressure ratio (PR) of something in excess of 20 to 1 to make MCP, the heat load from the charge cooling is immense and with the low air density up there, you need big HXs to pull the heat away. HX effectiveness drops with decreasing air density and increasing PR. More space, more drag. Turboprops start looking really good once you consider these problems. CI engines are simply a poor choice for high altitude flight because of the high MAP required to make power.

Looks like it turns 4000 rpm for takeoff-500hp, 460hp at 3750rpm (MCP) and 400hp at 3500 rpm. Dry weight will be in excess of 800 pounds. It's a beast.

Given the 6.1L displacement and these rpms I'd conjecture that they are running a bit less MAP that something like the Austro AE and Conti-CD series engines. I'd put TO MAP at 70 inches. I estimate the cruise MAP to make 400hp at around 65 inches.

RED claims sea level power only to 25,000 feet with the 2 stage model. That seems very low since they would have the potential for 176 inches MAP (11 X 16) at that altitude using the figures above. Not sure what gives but they have probably never flown one of these up there yet so have a bunch to learn.

It would seem that a turbine engine as described here: https://www.aiaa.org/docs/default-source/uploadedfiles/education-and-careers/university-students/design-competitions/1st-place-undergraduate-team-the-university-of-kansas.pdf?sfvrsn=bcf213fc_0 would offer close to the same BSFC as the CI engine at far less weight, complexity and cooling drag.
 
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Voidhawk9

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It will be interesting to see how they pack 6 turbos on this engine to make close to SL rated hp at 50,000 feet.
...
At least they should have plenty of space to work with. They could arrange things in similar manner to the Republic P-47, which used aft fuselage volume to fit lots of turbo and rads and ducting.
p47_turbo.jpeg
 

rv6ejguy

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At least they should have plenty of space to work with. They could arrange things in similar manner to the Republic P-47, which used aft fuselage volume to fit lots of turbo and rads and ducting.
View attachment 118433
This is what is required to be efficient at 30,000 feet with 2 stages of compression and 2000hp. Note the size of the HXs. Scale this down to 500hp but with 3 stages and see the problems of fitting it all in there- with the coolant HXs. They don't have the space IMO. The RED is a big engine in relation to the airframe with 6 turbos and lots of ducting and HXs tacked on.
 

Voidhawk9

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The real answer is probably one of the simpler ones: Don't fly higher than 2 turbos can reasonably manage. High altitudes are cool in theory...
 
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