A thought about WWII turbo chargers longevity

[onetrack]

It is true, as johnm says, that the Nazis did not have access to the exotic metals - nor the pool of skilled metallurgists that the U.S. had - that were needed to produce the higher quality metal alloys that could resist the extremely high temperatures inside jet engines. Not only a shortage of nickel, but also a shortage of titanium, vanadium, molybdenum, tungsten, and chromium.

Added to that, they had forced labour in their factories and a level of sabotage by those forced to work, that the Allies didn't have to contend with.

Both sides had to contend with human error in the manufacturing process, but trying to watch for sabotage as well, would have been another burden that simply exacerbated normal human errors.

[facthunter]

I don't think turbo chargers were used on the Majority of applications. The other types of Kompressor don't need refractory metals in their construction.   Nev

[onetrack]

Nev is right, mechanically-driven  superchargers were far more common than turbochargers on WW2 piston engines, because they did not need operate at the high temperatures that turbochargers operate at.

 

However, turbochargers were being used on aircraft engines early in WW2, but they were initially referred to as "turbosuperchargers" by the Americans. Early on, they were used in conjunction with geared superchargers to gain maximum benefit of forced induction, especially at high altitudes. However, the U.S. engineers well understood the over-pressurisation problems caused by geared superchargers, so the two-speed and two-stage superchargers were then introduced.

 

Below is a link to a 1943 article written by GE engineers fully explaining all aspects of WW2 turbosuperchargers. Interestingly, they used ball and roller bearings for the turbine shafts, and also had their own independent oil supply and tank.

There is also a description in the article of how these turbosuperchargers were balanced and repaired in that era.

 

http://www.rwebs.net/avhistory/opsman/geturbo/geturbo.htm

 

Edited December 7, 2023 by onetrack

[facthunter]

Geared superchargers operate best at full throttle height. That will vary with RPM chosen and consequently the HP achieved.   Nev

[LoonyBob]

8 hours ago, johnm said:

On the subject of P47’s and WW2

 

Reading my last book ……… the Luftwaffe did not have much access to metals such as nickel so their max boost (super or turbo ?) for the ME 109 and FW 190 was say 18000 ft where as the P47 & P51 was something like + 24000 ft

 

The height at which the B17 flying fortress flew at - + 24000 ft

 

A distinct advantage to the allies and their bombing campaign in Europe. A distinct disadvantage to Luftwaffe pilots who had to get up to the B17 height

 

Not something well considered by many ? – these crude technical comments are made for discussion !

The DB601 & 605 used a fluid coupling for their superchargers, and varied the compressor speed by adjusting the fluid level on the fly... the max boost was limited only by the design & "trim" of the compressors. As Nev says, superchargers care not about heat.

 

To my mind, the JUMO 004 using hollow turbine blades welded from arcs of tubular steel was a more significant indicator of the lack of superalloys...

 

[facthunter]

Hollow turbine blades are the order of the day now in the big stuff.. Nev

[johnm]

so Loonybob / and others - are the facts about Luftwaffe and say USA aircraft performance correct ? - via my crude technical ability

 

Did Luftwaffe craft perform not as well at the higher altitude and if that is correct what was the thing that determined that ?

 

ta

[skippydiesel]

I must have read this somewhere a long time ago: The Germans had automatic boost/altitude compensating  superchargers while the Poms required the pilot to manually change boost pressures - true/false???

[facthunter]

Dunno . I was always of the view  based on period information, the Germans could go higher. A different view is new to me.  Nev

[LoonyBob]

On 08/12/2023 at 6:24 AM, johnm said:

so Loonybob / and others - are the facts about Luftwaffe and say USA aircraft performance correct ? - via my crude technical ability

 

Did Luftwaffe craft perform not as well at the higher altitude and if that is correct what was the thing that determined that ?

 

ta

First, the engines:

 

The fixed-speed geared superchargers the UK started the war with, could not be used on takeoff or the donk would destroy itself; they had one optimum altitude, and fell off above it. The two-speed superchargers did the same, but twice...

The Bf & FW supercharged engines could run optimum boost at any altitude up to ~22,000ft (later they used larger compressors, to go higher); but the supercharger was not at its own best efficiency most of the time.

The US toyed with Turbochargers, which have built-in altitude compensation up to some ceiling, though not peak efficiency all the way up.

 

But airframes!:

 

The thick wing on the P-38 and Typhoon limited them to below ~14,000ft iff'n they wanted to run with the pack. Anything using thinner turbulent (non-laminar) airfoils - Bf109, Spitfire below Mk.21, Grumman fighters, P-47s, Mossies - had a higher rate of climb and smaller turning radius (ok, except for the P-47 & Mossie!) than the P-51D, and some were faster; but above ~25,000ft, the Mustang was faster and outclimbed anything I've listed so far.

 

I have not seen the outcomes of any flt-off between a P-51D, a Yak-3, and a Hawker Tempest/Fury (granted, the Hawkers were a generation ahead of the Stang).

 

The Me163 and 262 were pretty good at high altitude, but the later 109's were weighed down with anti-bomber ordinance...

 

My summary understanding was that the Luftwaffe had the edge at very high altitudes (in fighter terms), until the 'stang came along. The P-47 was not great from compressibility, but had the power and tankage to be up there with the bomber swarm, and the 8 .50 cals made an impression.

 

From memory, Clousterman wrote of an ocassion when he was stooging around Germany in a Tempest at ~4,000ft, looking for landing 262's to molest, when "for some reason i glanced in my mirror... three FW190s were following me, the Nitrous Oxide turning their exhausts white-hot" (the Gremans had fitted FW190s with NOx to try and catch the Tempests); Clousterman applied WEP and simply flew away from them... not high altitude of course, but it thinne dout the 262s.

[danny_galaga]

On 07/12/2023 at 8:58 AM, facthunter said:

Not sure that's an accurate portrayal.  Recall "beware of the Hun in the Sun". They frequently attacked from above.    Nickel is added to many steel high strength alloys as well as Inconel a turbine blade alloy which has been superseded by better  in later years. Many superchargers are direct gear drive with 2 ratios. It's only turbochargers that need the special turbine blades to take extreme temps.  Nev

And this I guess is at the crux of my question - machinery that needs special alloys because of high velocity and temperature don't seem to me to be able last for a very long time, especially since they would be glowing red hot during normal operation. During the war it didn't matter but for 80 year old restored aircraft spare turbo chargers must be like hens teeth. And they're frikken HUGE! Hence my musing that heavy earth moving machinery might supply something adaptable. But even then, when you look at the plumbing of a P47, there doesn't seem to be much wiggle room 🤔

 

[facthunter]

Having all that plumbing doesn't help efficiency. A turbo charger uses exhaust heat for it's energy.  Nev

[danny_galaga]

I thought it was more about pressure? In any case, the distance would help the air going back to the engine cool, on top of the intercooler. You see this with the P38 as well. Just doesn't seem to be easy to get it all up close the front. Unlike a car with its relatively wide front end.

[facthunter]

Back pressure increase takes power from the engine. It's probably a combination of both but the advantage was that it was more efficient than direct drive where you knew exactly how much power was used in driving the  BLOWER.. It was CLAIMED that turbo gave it to you almost  free and the complex drive problems were avoided as well. Engines like the Wright Double cyclone Use 3 Power recovery Turbines coupled to the Crankshaft and that engine is the most fuel efficient of all aero engines of the period.. Nev 

[johnm]

thanks for all that peoples  - I'll add I think for the turbo - the intercooler needs to be in there as well 

 

cooling down of intake gas and fuel ? before it goes in the engine 

 

I'm going back to that book where I mislead it or I misunderstood it 

 

(I still think ? the Luftwaffe climbed to have the height advantage for speed / gravity - to get some performance)

[facthunter]

You need the power to get the height till you eventually get into U2 territory  where you do everything for height and lose high speed.  Nev

[danny_galaga]

More on what's happening with current WW2 aircraft. This page tells me there are only 3 P47s flying with a working turbo. 

 

https://www.worldwariiaviation.org/aircraft/republic-p47-thunderbolt

 

First I thought that supported what I was taking about. But then elsewhere I read there are only 4 P47s left flying!  So then I thought, well 75% isn't a bad rate.

 

But I'm back to where I started- that it is getting problematic. If you've spent millions restoring a war bird, it seems to me you wouldn't stop at the turbo, unless you just can't replace it. 

[LoonyBob]

On 10/12/2023 at 7:30 AM, danny_galaga said:

And this I guess is at the crux of my question - machinery that needs special alloys because of high velocity and temperature don't seem to me to be able last for a very long time, especially since they would be glowing red hot during normal operation. During the war it didn't matter but for 80 year old restored aircraft spare turbo chargers must be like hens teeth. And they're frikken HUGE! Hence my musing that heavy earth moving machinery might supply something adaptable. But even then, when you look at the plumbing of a P47, there doesn't seem to be much wiggle room 🤔

 

 

The GE Turbo-SUpercharger used in the P-47, used an axial-flow turbine; so turbine blade growth would have been the life-limiting factor. The TIT - turbine inlet temperature - is crucial to controlling turbine aging*; the lower, the better. The long pipework would very much help limit the maximum TIT.

 

Just to confuse things, superalloy growth rates under constant stress at constant temperature are not linear, particularly in the first 20~200 hours. At its best (jet engine) operating temperature, NiMoNic 75(?) blades actually shrink for about 80 hours, then take another ~~70hrs to grow back to its original length. NiMoNic 60 doesn't shrink to speak of, but holds constant for a few tens of hours. Both are stretching like taffy at 600~750hrs... in a turbocharger, which does have lower TIT than a jet, they should last thousands of hours, subject to usage, manufacture, and salt exposure. I have NO idea what superalloy GE used at the time.

 

In engineering terms, heat is energy and energy is heat. A radial inflow turbine - as used in small turbos for cars etc - gets its energy from the gas velocity, the gas inertia, and a slight bonus from the exhaust pulses. Back pressure is created only by the flow losses through the tortuous guts of the thing; but as the drive comes mainly from inertia, not dynamic pressure (Pelton wheel style), the secondary losses are quite large. An axial turbine gets its energy from the gas velocity, like a propeller in reverse, and the only losses to speak of are frictional.

Perhaps the most accurate enginge heat breakdown in the public domain was for the Merlin, of which 25% of the heat in the fuel went out the crankshaft and twirled things; ~50% went out the exhaust pipe... modern cars do about the same under acceleration, so there's plenty of free heat - one just adds an amount of boost equal to any turbine-induced back pressure, and everything else the turbo gives is free.

 

A centrifugal compressor cannot achieve much better than 70~ adiabatic efficiency, because the air entering the eye is not rotating, but the "channels" between the vanes are. That means that ~30% of the drive energy (or more when outside the optimum mass flow/pressure ratio "island") is heating the air; which is great at causing detonation destruction and mechanical death for spark ignition engines. Thus, "pre-cooling" or "intercooling".

Interestingly, adding a supercharger and lowering the compression ratio will give a higher (maximum) power to weight than just bumping up the compression ratio, with a greater margin of safety from detonation to boot. One may then disregard intercooling for low boosts, as seen on many/most inter-war aero engines.

An axial-flow compressor achieves >96% adiabatic efficiency, thus heating the air charge very little, as seen on DB601s and peers.

 

<>

 

The Hun in the Sun was a sound practice in WW1, but the "attack from above" has more complex reasons. Imagine two fighters in a level(ish) turning duel; the faster the following fighter goes, the larger the turn radius, the prey gets away - but slow down or bleed energy, and the prey gets away! However, if the attacker rolls level whilst maintaining the same G, a 1/4 of a turn sees the vector near-vertically up and curving in towards the far side of the circle of pursuit, ahead of the target (deflection, woo hoo!). Also, in the arc over the top, the wings are unloaded, so induced drag almost vanishes - an energy advantage! Shoot down target, rinse and repeat.

This tactic - which dates from Boelke - was the reason the Bfs and Fws were designed to have a better initial climb gradient / "zoom' climb than the Allied fighters of the time.

 

 

[LoonyBob]

6 minutes ago, danny_galaga said:

More on what's happening with current WW2 aircraft. This page tells me there are only 3 P47s flying with a working turbo. 

 

https://www.worldwariiaviation.org/aircraft/republic-p47-thunderbolt

 

First I thought that supported what I was taking about. But then elsewhere I read there are only 4 P47s left flying!  So then I thought, well 75% isn't a bad rate.

 

But I'm back to where I started- that it is getting problematic. If you've spent millions restoring a war bird, it seems to me you wouldn't stop at the turbo, unless you just can't replace it. 

One can buy WW2 superalloys off the shelf, at not vast expence, and the blades were investment cast, like almost all of the zillions of car turbos made each year. The most problematic part would be disassembling an original and measuring it to within an inch of its life.

[facthunter]

You can't get the high octane fuel so you have to derate most of this stuff. Fighters will be a lot lighter than they were when operational

[facthunter]

Some GE turbine blades are cast as a single crystal of metal and are hollow for cooling purposes. This is really high tech. stuff. An Engine shut down at high power removes metal from the cases and causes Blade tip leakage and a power loss till the blades grow a bit again.  Nev

[danny_galaga]

1 hour ago, LoonyBob said:

One can buy WW2 superalloys off the shelf, at not vast expence, and the blades were investment cast, like almost all of the zillions of car turbos made each year. The most problematic part would be disassembling an original and measuring it to within an inch of its life.

I suspect people are just going to fly them sans turbo from the sounds of it. The up shot I suppose is you might be able to gain a few hp by simplifying the exhaust AND inlet by permanently removing the turbo. A bit of extra horse at low level would be just what the doctor ordered for an aircraft mostly flown at low altitudes at air shows 😎

[facthunter]

 Reducing the Manifold pressure by using less throttle is the easiest way. You set to limiting MP anyhow for all take offs. Full throttle height is just an efficiency thing. Desired but not essential.   IF you continue climbing  above full throttle height you will reduce the Power anyhow just as you do with a non boosted engine from  ground level.  Nev

Edited December 11, 2023 by facthunter

[LoonyBob]

8 hours ago, danny_galaga said:

I suspect people are just going to fly them sans turbo from the sounds of it. The up shot I suppose is you might be able to gain a few hp by simplifying the exhaust AND inlet by permanently removing the turbo. A bit of extra horse at low level would be just what the doctor ordered for an aircraft mostly flown at low altitudes at air shows 😎

Personally, I suspect an unarmed P-47 has sufficient power without boost...

[danny_galaga]

51 minutes ago, LoonyBob said:

Personally, I suspect an unarmed P-47 has sufficient power without boost...

Well, like I say, most of its flying now would be low, where you wouldn't normally use the turbo. My original concern was basically that it might be hard to have a 100% working warbird if you couldn't maintain the turbo. As you see in the pics I posted, it's a not insignificant part of the aircraft. I guess even more so for the P38 since it is a visible part of the airframe. 

 

It seems the answer to my question really is that if the turbo isn't working, they just don't use it.