WI: NACA Modified P-38

[Draconis]

Here's another interesting tidbit from the first document (NACA-TN-1602, the one with the steady flow compound engine compared to a turbosupercharged one): they also tested both setups (stead flow compound and turbo-S/C) without the engine-driven S/C stage. Here's the results for the turbo engine (note, this engine DOES NOT use any compounding, but is strictly a turbo-S/C as found on the P-38):

Of course, in order for a P-38 (or P-81) to use the exhaust-jet thrust we will have to redesign the exhaust outlet and wastegate; but, I think it should be possible. What this means is that even without turbo-compounding we can gain a mean of 16% thrust and a 10% nsfc reduction in our existing airplane. Assuming they realize this and give it a try ITTL prior to war's-end.

That is an interesting approach. Gets rid of the horsepower consuming mechanical supercharger. I haven't read the article yet. I'm curious as to how well the single stage turbine driven supercharger would cope with the different altitudes and engine power settings. Its dynamic range you could call it.

And whatever exhaust energy is not used to drive the supercharger is basically used as a jet exhaust blown out the rear of the turbine? The installation of this type of turbo-supercharger will require the turbine exhaust placed as close to the airplanes' thrust line to avoid large pitch changes with engine power changes. Where to fit this type of turbo S/C in a P-38?

 

[EverKing]

Not to be a nitpicker, our last name is Pasteris...thanks, keep it up!

Jeff

My goodnes... What a thing to screw up.
I will fix it at my earliest convenience. So very sorry about that.

 

[EverKing]

Where to fit this type of turbo S/C in a P-38?

Yeah, that's the real problem with this. I mean if you could figure out how to make everything fit, you could exhaust through a cutout on the inboard side of the boom, in pitch line with the main line of thrust but slightly inboard. This thrust could then help improve airflow over parts of the elevator too. Trouble is, with the main wheel taking up all the space immediately aft of the turbo you really can't do this. So, I don't think this set up is a good option for the P-38.

 

[Draconis]

Yeah, that's the real problem with this. I mean if you could figure out how to make everything fit, you could exhaust through a cutout on the inboard side of the boom, in pitch line with the main line of thrust but slightly inboard. This thrust could then help improve airflow over parts of the elevator too. Trouble is, with the main wheel taking up all the space immediately aft of the turbo you really can't do this. So, I don't think this set up is a good option for the P-38.

I think we might have to move that turbocharger forward a little ways to allow room for a short and slightly angled jet exhaust tube. As you describe it must be level with the pitch line. The two exhausts will balance each other out in the yaw and should have little to no effect on roll. Single engine characteristics might be a little more difficult.

I'm thinking just how drastic an angle can be put on the exhaust tube without ruining the turbine performance with too much back pressure or melting the tube. Or a split exhaust tube Y-ing out?
I'm winging it here.

 

[sonofpegasus]

Might an off Line exhaust thrust be useful in countering single engine yaw?

 

[EverKing]

I'm thinking just how drastic an angle can be put on the exhaust tube without ruining the turbine performance with too much back pressure or melting the tube. Or a split exhaust tube Y-ing out?
I'm winging it here.

This is the crux of the issue in getting exhaust thrust after the turbo in the P-38. Ultimately I think this is a dead end. In a more traditionally designed twin-engine--i.e. sans booms--you can exhaust directly out the rear of the nacelle.

Might an off Line exhaust thrust be useful in countering single engine yaw?

It could help a little but likely only at high speed and altitude where the exhaust pressure and ambiant pressure are dramatically different--conditions which you should generally avoid when running on a single engine. Also, it will not help with the torque which is usually the bigger problem when running single on a P-38.

 

[tomo pauk]

...
It could help a little but likely only at high speed and altitude where the exhaust pressure and ambiant pressure are dramatically different--conditions which you should generally avoid when running on a single engine. Also, it will not help with the torque which is usually the bigger problem when running single on a P-38.

(my bold)
IMO - just one engine running on the big & heavy P-38 woud've induce less torque problems than a normal power-on operation on a much lighter & smaller P-39/40/51. Despite not having an inclined rudder, there are two vertical tails that can get enough of trim to counter the adverse torque.

 

[wizz33]

i read somewhere that the designer of the harrier engine said that hot turbine gas does not lose trust/energy in turns

 

[Draconis]

This is the crux of the issue in getting exhaust thrust after the turbo in the P-38. Ultimately I think this is a dead end. In a more traditionally designed twin-engine--i.e. sans booms--you can exhaust directly out the rear of the nacelle.

The same problem will exist when trying to fit the turbo-compound engines into your P-81. But seeing as how the P-81 is a little bigger than the P-38 perhaps there are some possible arrangements.



I finished reading the paper you quoted that among other things describe an engine configuration using a modified turbo-supercharger to carburetors than to an aftercooler. The engine driven supercharger is dispensed with to remove that horsepower drain. As you quoted in your comment it produce good results.

What if a similar system was fitted to a P-38? What kind of turbine could be used that doesn't produce enough thrust to require a changed location and orientation but can still drive a somewhat larger supercharger? From reading the paper I wasn't able to determine the feasibility of this idea.

As I understand it the NACA engine experiments where using a turbine designed to produce thrust in a turbo-compound type system that was then adapted to the modified turbo-supercharger system. What if the turbine designed for a turbo-supercharger was used in a system without the engine driven supercharger? Would we see a similar amount of improvements? I don't know. But if there was some gain we would also not have the problem of how to duct out the turbine exhaust thrust.

 

[EverKing]

just one engine running on the big & heavy P-38 woud've induce less torque problems than a normal power-on operation on a much lighter & smaller P-39/40/51

I suppose you are correct that the torque by itself wouldn't be as much of an issue but the other dynamics of the P-38 tend to exacerbate any rolling tendency due to flying on a single engine. As we've discussed before, the outward turning (at the top of the arc) propellers cause the airplane to want to roll into the "dead" side which, when combined with the decrease of lift under the dead side center wing and the added drag of the dead engine, produces a substantial rolling effect. This was really what I was thinking about.

The same problem will exist when trying to fit the turbo-compound engines into your P-81.

Yes. Even the P-81 would have to have the booms substantially re-designed to fit a steady-flow jet exhaust turbine of this type. To the point where it may be easier to just re-design it to a more standard single-fuselage + long nacelle arrangement.

What if a similar system was fitted to a P-38? What kind of turbine could be used that doesn't produce enough thrust to require a changed location and orientation but can still drive a somewhat larger supercharger? From reading the paper I wasn't able to determine the feasibility of this idea.

Yes, the paper(s) is (are) somewhat vague on some of the details of the test engine setups. Still, a GE C-series turbine may fit the bill. They are larger than the B-series used in the P-38 and may be able to make use of increased exhaust pressure to the point of reducing the exit pressure to near atmospheric pressure, thereby using more exhaust energy and turning the larger compressor, etc.

What if the turbine designed for a turbo-supercharger was used in a system without the engine driven supercharger?

I don't think you'd get the same type of gains unless you dramatically upgrade the turbine and compressor. The installed Turbo-Supercharger is already matched to the airplane to produce x boost at maximum turbo-RPM, which is then further compressed by the engine-stage S/C. Without the "primary" engine-driven supercharger performing this additional compression, the induction air will hit the Turbo boost limits and suffer a reduction in Max. Manifold Pressure and Critical Altitude. At least, that's how I see it.

 

[Draconis]

Yes, the paper(s) is (are) somewhat vague on some of the details of the test engine setups. Still, a GE C-series turbine may fit the bill. They are larger than the B-series used in the P-38 and may be able to make use of increased exhaust pressure to the point of reducing the exit pressure to near atmospheric pressure, thereby using more exhaust energy and turning the larger compressor, etc.

I don't think you'd get the same type of gains unless you dramatically upgrade the turbine and compressor. The installed Turbo-Supercharger is already matched to the airplane to produce x boost at maximum turbo-RPM, which is then further compressed by the engine-stage S/C. Without the "primary" engine-driven supercharger performing this additional compression, the induction air will hit the Turbo boost limits and suffer a reduction in Max. Manifold Pressure and Critical Altitude. At least, that's how I see it.

Are you saying yes and no for a P-38 using the modified turbo-charger. I like the idea your describing in the first paragraph here about using a larger C-series turbine.

I like the idea of being able to get rid of the engine driven supercharger for the increase in engine efficiency. Not sure if using an aftercooler brings any additional benefit by replacing the before the carb intercooler. But if there was a practical way to effectively power the superchargers, whether single or two stage only with the energy contained in the engines' hot exhaust gasses that would make for more available power to the propellor.

 

[Rath]

Outboard thrust would help with engine-out, while inboard thrust would provide extra airflow over the elevator.

I have an idea for a typhoon-style chin affixed to the bottom of the boom, with the turbine blowing down through it. You could even go full motorjet by adding an intake in front of the turbine chin and running the driveshaft to the compressor.

 

[EverKing]

Outboard thrust would help with engine-out, while inboard thrust would provide extra airflow over the elevator.

I have an idea for a typhoon-style chin affixed to the bottom of the boom, with the turbine blowing down through it. You could even go full motorjet by adding an intake in front of the turbine chin and running the driveshaft to the compressor.

The trouble with that set up (pushing thrust through the underside of the boom) is that the main gear will be directly in the thrust jet during take off. I think we have to just accept that the Twin-Boom design of the P-38 (or anything directly derived from it) is not well suited for any sort of "easy" add of a turbine of this type.

Speaking of designs derived from the P-38, I have been working on the engine design and layout for the XP-81. I have had to re-contour some of the nacelle and move some panel lines to get everything to fit properly and for access panels/doors to line up. So, which of these three Nacelle designs do you like more?

1) Similar to the original and will use an automatic inlet shutter similar to our ATL P-38 but with simpler operation (simple "lift-to-open" type)
2) Small opening, long intake duct (may not be able to offer as efficient cooling)
3) Large opening, short intake duct (may be offer TOO efficient cooling)

 

[Rath]

I like the idea of the first one for lower drag when it is closed in WEP, while the last one looks coolest. It reminds me of the LaGG series almost for some reason.

 

[Draconis]

It looks like #1 offers the best streamlining and more precise control of the airflow into the intercooler. Probably closed completely when flying economical cruise in the ETO Stratosphere and wide open near sea level in the Tropics at WEP and varying enough to cover everything in between. That design would have worked with the P-38 too.

 

[thorr97]

I've always been partial to the P-38s with the chin mounted radiators. Those just look "right" to me more than the earlier models that had the flush inlets immediately below and behind the props.

So, from an aesthetic perspective, I'd go with #2. Then #3.

 

[EverKing]

I like the idea of the first one for lower drag when it is closed in WEP, while the last one looks coolest.

Closed when in cruise. Open in WEP when the turbo is pushing more boost and the air needs to be cooled more. I agree that that last one has potential for looking "tough." I even have an alternate which broadens it a little and gets rid of the top mounted inlet (just aft of the prop spinner) by placing everything down in it. Almost looks like the XP-72 but with an almost F-8 or A-7 style inlet. I think it will be overkill for the cooling requirements and will produce too much drag for this A/C.

It looks like #1 offers the best streamlining and more precise control of the airflow into the intercooler.

Absolutely. But it also adds complexity and a little weight (machinery to open/close the shutter, etc.). Withe the design as it, I think this is the one that makes the most sense though.

So, from an aesthetic perspective, I'd go with #2. Then #3.

I agree, I really like the look of #2 and #3 has potential with a few adjustments.

 

[phx1138]

As pretty as it is...it is no Alfa Romeo Tipo 33 Stradale. I am obsessed with that car. Even more so considering it was producing about 230 hp out of a 2.0L Naturally Aspirated V8 in 1968! That's 120hp/litre, numbers we barely see even today except in Forced Induction engines.

Impressive performance. It'd be around 6th on my list, behind the '58 Vette, & Porsche 904 & 911/912 (& maybe the '72 'vette).

Well, one could argue that we are currently in the TC era of F1. When they went down to the Turbo-V6s a few years back they include a Heat Motor Generator Unit (MGU-H) which can most easily be described as a turbo-compound turning an electrical generator which powers a battery store and can be used together with the Kinetic Motor Generator Unit (MGU-K) to power the Kinetic Energy Recovery System (ERS-K, formerlly KERS) which is a hybrid-type electric motor providing additional power to the drivetrain.

I'd argue that means "hybrid era", but wouldn't argue strongly.

But, yes, a TRUE TC era prior to the 1980's Turbo insanity, could be epic. Even more likely than an early TC era in F1, however, would be a TC era in LeMans where endurance and power are pushed to the limits. I could definitely see this happening during heyday of the 1960's and after a few early years of regular powerplant failures they work the worst of the bugs and make it useful and reliable for road/racing use...and then everything changes.

An F1 TC before the '80s really frightens me, given the OTL deaths. LeMans, or GTP/Group B/Group C, could see the 917s a lot sooner--& as insane as Group C got, it might be enough to produce a ban. OTOH, underpowered cars might be less abundant; if the postwar MBs & Ferraris can't put power on the track, & the Coopers (with more juice than OTL) can... (Or maybe it's OSCA or Porsche?)

Something else I overlooked: this could easily overpower the bias-ply tires in NASCAR, like the OTL Daytonas did, & lead to more than a few deaths there, too. And to restrictor plate racing a lot sooner ...but also to better tire tech, & maybe *HANS sooner. (Could easily overpower the tires in F1 & GTP/Group C, too.)

I have actually been thinking about this off-and-on for the past few years. I love the idea of TC and think well developed TC kits could extend the life of popular old V8s. I think the best market to hit first would be the Chevy Small Blocks just because everyone seems to modify them. A TC kit would probably require a custom bell housing to incorporate the gearbox connecting the blowdown turbine to the flywheel before the clutch and installation may be a problem in vehicles where the engine sits far back against the firewall. I do think it is possible, though. Of course, an easier "bolt on" application could use a chain or even a belt to transfer power to the front of the crank.

That's about what I thought. Though it occurs to me the SBC might not be so popular as OTL. It might be replaced by a TC 225/250/292 Jimmy 6. Especially in police cruisers (with the smaller 6, or maybe a big 4, in taxis): good power & good mileage; they don't need the *440 Chrysler (which legend says "could pass anything on the highway but a gas station"), because they have cop radios... So maybe the 265, 283, & 327 are as big as it gets?

Here's the diagram from the report:
View attachment 386260

I was thinking of a photo of the actual use. Thx for the diagram, tho.

as much as I want you to get back to the "shoot 'em up" parts of the story, this is fascinating!!!!

what would something like this do to a design like the B-50? seems to me you could set up an underwing pod for the engines and generate enough horsepower where mounting additional jet engines wouldn't be necessary...

OR, what would this do to a Mosquito???

I'm not seeing "podded" B-50 engines, but redesigned nacelles: more an XB-38 appearance?

I'm less sure it benefits the Mossie, given how long adapting this to the Merlin might take. Unless we posit a V1710TC'd Mossie.

There is plenty, plenty of power more or less wasted though the exhausts. Per this diagram, for every 3 HP provided to the prop, there was a potential of extra 5 HP that just exited through exhausts - piston engines of the era were lousy in 'HP per fuel used' category. Granted, some of this power was used to provide exhaust thrust

That's fairly staggering.

It sounds like you mean this would be comparatively simple supercharger add-on engineering, rather than complete redesign. It that's so, I'll withdraw my V1710 Mossie, & say this: it's going to dominate drag racing almost from the get-go. It won't be long before civilian truck makers postwar are copying this, & pirating turbines is dead easy. It won't be long after that before somebody starts building dedicated TC kits...& it won't be TF diggers with 6-71 blowers on the cover of Hot Rod (& page one of National Dragster), it'll be ones with TC 392s. Which also (probably) means the trend to poking holes in hoods & giant snorkel scoops never happens, either. (Sleepers, anybody?)

Yeah, that's the real problem with this. I mean if you could figure out how to make everything fit, you could exhaust through a cutout on the inboard side of the boom, in pitch line with the main line of thrust but slightly inboard. This thrust could then help improve airflow over parts of the elevator too. Trouble is, with the main wheel taking up all the space immediately aft of the turbo you really can't do this. So, I don't think this set up is a good option for the P-38.

I wonder if you could do it by changing two things: sweep up the rear of the boom, in a "scorpion tail", & turn the gear wheel so it lies flat in the well.

That said, probably this variant wouldn't appear before war's end...and the XP-81, as a clean sheet design, wouldn't need that kind of fix.

As for the gear being in line with jet thrust on takeoff, what about fitting "thrust flaps" to redirect the jet until the gear retracts? Fit the control system to the retract mechanism. Or set the jet exhaust so it only exits above a given boost pressure? (Tho it occurs to me you'll be at high boost on takeoff, so maybe not practical...)

I also like the #3 inlet design esthetically. Except I wonder about the FOD hazard, with stones kicked up by the nosewheel, & maybe the gas ingestion problem from the guns. (Maybe even, just, sucking empty cases, too.)

 

[EverKing]

As much fun as I'm having designing a proper V-1710 TC for the XP-81, I am leaving the math and performance calculations for later. The next narrative chapter will bring us back to the PTO for a check-in on how things are going there and I expect it to be up sometime next week. After that I move over to Allison so you can all learn how their engine development is going, or I have a couple options for Europe (9th AF tactical attack mission will come up some time and I have a very specific mission planned for August '44 with the 8th AF). Also, sometime in fall of '44 we will see the final Block of P-38s rolling off the line with the last couple improvements the airplane is still waiting for--from then until the end of production all of the A/C will be the same and there will be no more room to grow. I would also like to pay Herr Limberg a visit so we can see what the LW is doing to compete with the Lightning and Mustang (plus maybe a drop a line to Hilgert to learn what he's been up to since we last saw him).

We will really start to see some major divergences from OTL missions and war progress.

 

[Draconis]

I also like the #3 inlet design esthetically. Except I wonder about the FOD hazard, with stones kicked up by the nosewheel, & maybe the gas ingestion problem from the guns. (Maybe even, just, sucking empty cases, too.)

Could stones from the nosewheel reach the engine nacelles? Seems like an extreme angle for the nosewheel, especially if taxiing or rolling at high speed. Anyhow I think the intercooler is pretty robust and a dent or two in the cooling fins from a stone or shell casing isn't a show stopper. And I would truly like to see what kind of aerobatic maneuver could cause gun exhaust or shell casings from the nose guns to be ingested by one or both of the intercooler intakes. The lomcevak is not a recommended combat maneuver.