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10/23 Raptor Video

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BBerson

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I will dig out my turbocharger book today. Much to review. The YouTube commenters throw the word "turbo" this and "turbo" that and I don't know if they are referring to the "radial inflow power turbine" or the "radial outflow turbine air compressor" when they use the slang word "turbo".
First, I want to know if the power turbines are piped in parallel or in series and what is pressurizing the cabin?
 
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rv6ejguy

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These are piped in series.

The term turbo (short for turbocharger) describes a centrifugal compressor driven by a turbine on a common shaft, both of which components (along with the turbine housing A/R) need to be matched for the application. These are separate components which must be referred to separately in terms of matching.

Pressurization air could be tapped off either stage of compression, not really important at this stage, since Raptor can't be pressurized at this time due to a cracked window and some other fuselage mods.
 

BBerson

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Ok, I didn't find anything about dual turbos in the airplane chapter of the Hugh MacInnes book. But I did find a "tandem" turbocharger schematic in the tractor pulling chapter. Hugh does not recommend Homebuilt aircraft turbochargers for safety reasons.
 

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Jay Kempf

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Not sure what tandem means. Seems like a series installation to me. One pump feeds the inlet of the next, once removed from ambient. Same as compressor stages.
 

pictsidhe

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Ok, I didn't find anything about dual turbos in the airplane chapter of the Hugh MacInnes book. But I did find a "tandem" turbocharger schematic in the tractor pulling chapter. Hugh does not recommend Homebuilt aircraft turbochargers for safety reasons.
Yes, like that. But Peter only has an aftercooler, no intercooler. He is calling his aftercooler an intercooler. Having both would give better efficiency, but is more complex.
 

BBerson

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I don't understand this YouTube post. I would think that mass flow would be the same all through a series system, volume would change?

rv6ejguy
@michael Onines The 1st stage compressor only flows about half the mass flow needed to feed the 2nd stage compressor. The turbos are not matched correctly as has been stated many times for months now. At low altitude for testing, a single turbo will do the job more efficiently and with less weight
 

rv6ejguy

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In order to match staged turbos, you normally start with the 2nd stage first, then calculate what is required from the first stage to supply the second stage. You need to account for the pressure change and temperature change from compression and intercooler effectiveness plus pressure drop- in other words density ratio. Flow on compressors is described as INLET flow, prior to compression. Lots of people miss this very important point. You need to know the pressure ratio, compressor efficiency, intercooler effectiveness and intercooler pressure drop in order to calculate density ratio which is really what's important.

Normally, you try to keep the pressure ratios between stages about the same.

In a simplified example, assuming no temperature changes, no pressure drop and a pressure ratio of 2 between first and 2nd stages, you'd need about twice the first stage inlet mass flow (before compression) to feed the 2nd stage at that required inlet flow (now at double the pressure).

When both compressors are about the same size as in Raptor, the 1st stage will certainly be in the choke region trying to supply the 2nd stage compressor. Operation near choke results in low compressor efficiency and high compressor discharge temps- exactly what you don't want where you're demanding the already high pressure ratios that diesels require to make any sort of power at high altitude.

Calcs can be done using volume (CFM in old terms) or Mass (lbs./min. usually in old terms). I use mass since that's how modern Garrett compressor maps are labeled.

An easy to visualize example of the physics here, is to look at a cutaway of an axial flow gas turbine engine. The first compressor stages are larger than the downstream stages and on the turbines, the first stages are smaller than the downstream stages. The same physics applies to staged turbochargers.
 
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Victor Bravo

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An easy to visualize example of the physics here is to look at a cutaway of an axial flow gas turbine engine. The first compressor stages are larger than the downstream stages and on the turbines, the first stages are smaller than the downstream stages. The same physics applies to staged turbochargers.
That's the easiest and most useful explanation that I have heard on the subject of how/why the Raptor staged turbochargers are not matched. Thank you for bringing the concept down to a level I could grasp more easily.
 

wsimpso1

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I'd imagine that spring rate makes quite a difference to the handling. Any articles on that?
Without the spring, the airplane already has stick force gradients in force per g and force per speed change. A trim spring adds force per stick travel. Various groups, including USAF, have weighed in on what the think the gradients should be.
 
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BBerson

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Ok, got it. Peters 1st stage compressor is only flowing about half of what is needed. (misread the sentence as a recommendation)
Does the turbo company offer installation help? Or do they avoid aviation sales?
 

rbarnes

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Ok, got it. Peters 1st stage compressor is only flowing about half of what is needed. (misread the sentence as a recommendation)
Does the turbo company offer installation help? Or do they avoid aviation sales?
Probably could have reached out to one of the dozen companies that specializes in this and gotten a properly sized system with a short phone call and email.

 

rv6ejguy

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While staged turbo setups are way more common now than they were even 5 years ago, few people have much experience with high altitude matching but at least they would have told Peter you never use compressors with the same flow rate.
 

BBerson

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Probably could have reached out to one of the dozen companies that specializes in this and gotten a properly sized system with a short phone call and email.

Easier to understand when they call it a big turbo and small turbo instead of 1st and 2nd. The intake air goes into the big one first. The exhaust into the small one first. About all you need to remember.
He may have started 5 years ago when less common.
 

rbarnes

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Easier to understand when they call it a big turbo and small turbo instead of 1st and 2nd. The intake air goes into the big one first. The exhaust into the small one first. About all you need to remember.
He may have started 5 years ago when less common.
That video is from 2016. Diesel hot rod guys have been doing this for years and years. Another key take away from the video is notice that the compressor outlet on the large turbo is the same size as the inlet on the small turbo and the exhaust outlet on the small turbo is the same size as the inlet of the exhaust on the big turbo. This is hugely important to pairing the two turbos and keeping the flow between them smooth.
 

BBerson

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With 1500 deposits why are turbo sales engineers not involved?
It can't be that difficult to exchange for the correct matched twin turbos even if they require a bump on the cowl.
 

BBerson

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At the factory, we put a metal bubble over the hole in the fiberglass cowl from the exhaust burning through. I have also repaired an exhaust burn thru on a C-206. Stainless might be best in the turbo area. MacInnes said the installation should be Fail-Safe to avoid loss of airplane.
 

rbarnes

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With 1500 deposits why are turbo sales engineers not involved?
It can't be that difficult to exchange for the correct matched twin turbos even if they require a bump on the cowl.
It would require a complete rework of all hot and cold piping on the engine. This is the current problem as far as I can make out. No resources in current location to make needed changes beyond taping stuff on to the plane.
 

flywheel1935

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Here's another example of an aircraft that will 'change the face of GA' Can you see the lose similarity ,
£1 million, spent, 2 aircraft built, then.................bust, and this was a team of aircraft specialists
Not quite a 'white elephant' more a 'white baby seal' , just a shame as here in the UK we have all but lost GA manufacturing.
 
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