enginehistory.org

[jjuutinen]

So, what is the truth: is pure turbocharging more efficient speed wise than mech blower plus exhaust thrust? The latter theory is usually proposed by makers like RR who failed to develop workable turbochargers. My understanding is that maximizing power available to drive the propeller (i.e. turbocharging) is more efficient than having a mech blower and exhaust thrust as the velocity of the escaping exhaust if very high compared to the speed of the aircraft, therefore inefficient.

[jrussell]

A very interesting question I have long pondered! As I am a Docent at a museum, I get to look very closely at a P-38 L, a p-51D, and a mk16 Spitfire. The one thing that seems very interesting is the difference in exhaust stacks on the p-51 and the Spitfire. The Spit exhaust stacks flare towards the end promenently, compared to the p-51. I have wondered if this was an example of R-R trying to lower exhaust velocity to improve thrust effiency?I have been told that the Brits gained a considerable speed increase working with exhaust stacks to improve thrust, but I'm not sure that the stacks I'm looking at, are the ones that were a result of this work. I can send comparative photos if you would like them. ( The beauty of Warbirds - the ones we get to look at nowadays are of questionable accuracy as far as these details are concerned ).Anyway, I have always wondered why there was seemingly NO attempt to gain thrust from the turbo on the p-38. I have read that the GE turbo's were very sensitive to backpressure, but this doesn't seem to make sense to me, as I find it hard to believe that using exhaust thrust would create that much backpressure ( Anyone care to weigh in on that one?).To me it has always seemed that the optimum approach, would be to use a turbo AND it's associated exhaust thrust. It always seemed strange to me that R-R didn't go the turbo route as they had a very good handle on the thermodynamics of centrifugal compressor design, as well as high temperature materials, as evidenced by their jet designs!

[rinkol]

Pratt and Whitney tried to get thrust from a turbocharger arrangement in the R-4360 witht he Variable Discharge Turbine (VDT) concept, as shown at http://www.enginehistory.org/P&W/R-4360/Image06.jpg.

This engine never reached production, despite a significant R&D effort. Presumably the practical problems proved intractable. Also, the immense size of the overall engine may have been a problem.

With a conventional aircraft engne turbocharger, the boost is regulated by varrying the amount of exhaust gas flowing through the turbine. This would have complicated the problem of obtaining jet thrust. Also, there may have been issues in cooling the turbines.

With respect to the design choices made by RR, it might be noted that they did succeed in getting a serviceable engine with good high altitude performance into service relatively quickly. Many of the early turbocharger designs were plagued with technical problems, particularly relating to the control and production issues. Although, Bell experimented with a turbocharger in the first P-39, they never reintroduced it, even in the later P-63. Also, AFAIK, nobody was ever able to come up with a successful turbo isntallation in single engined fighter aircraft with inline engines. With the conventional front mounted engine, there was a problem finding space for the turbocharger and ducting that did not involve moving the pilot close to the tail (XP-37) or increasing the fuselage cross section to accommodate ducting for a rear mounted turbocharger (FW 190C).

Robert

[szielinski]

Just to muddy the waters, JJ's choice may be changed if the engine
is two-stroke, as that type of engine is unfussed by back-pressure.

[mjohnston]

I did some studies on this a few years back when I was doing some PD work on a new design unlimited racer. I had developed a simplified performance model of the Merlin engine and looked at the trade off of mechanical supercharging vs turbocharging. This was done by removing the HP losses associated with the supercharger and substituting the HP losses due to the turbo back pressure. I had assumed that the work extraction through the turbo would make the exhaust gas pressure low enough that there would be little available to turn into thrust.

What I found out was that at lower airspeeds (from memory 300 -350 mph range), the turbos had a slight advantage. The HP loss due to the increase back pressure was quite a bit less than the mechanical power losses for the supercharger. The available exhaust thrust at these lower speeds was not enough for the supercharged cycle to make up the difference.

At higher speeds (450 - 500 mph - Reno race conditions) the situation changed. The exhaust thrust turned out to be a considerably higher and the supercharged engine had a decided power advantage. This is mainly due to the relationship HP= Thrust (lb) * Velocity (ft/s) / 550, so the increased speeds gave a favorable nod to the supercharged cycle.

Reviewing some of the NACA and ARC reports of the day regarding engines and installations, there was a lot of studies on what they termed dispersed installations where the radiator, intercooler and turbocharger were placed in different locations on the aircraft. The theory being that this would allow a reduction in aircraft drag by distributing these components to more optimum locations. Taking this into consideration, a lower drag configuration might be obtained using a distributed approach using a turbocharger that a supercharger. The lower drag could offset the higher power potential of a supercharger. Some of the last generation piston aircraft at or after WWII were just starting to explore these concepts, but it was never fully explored since turbines were coming on the scene.

Hope this helps

Mark Johnston

[gryan]

Mark

You mention there was research done into dispersed installations. Can you post a list of references or papers? I would like to obtain them and have a read!

Thanks

Gerald

[mjohnston]

Unfortunately I am on business travel until mid-Jul, and my references are at home. However, I did find a bit of info lurking in my laptop:

Hartshorn, A.S. and Nicholson, L.F, “ The Aerodynamics of the Cooling of Aircraft Reciprocating Engines, “ Aeronautical Research Council, Report and Memoranda No. 2498, 1947.

This was a fairly decent report issued by the equivalent of NACA in the UK. You may be able to get this off the web.

Here are a couple of sites where you can download old NACA and ARC reports. A lot of info can be found here with the proper choice of search terms.

NACA technical report server:
http://naca.larc.nasa.gov/

Cranfield University archive of old Aeronautical Research Council:
http://aerade.cranfield.ac.uk/reports.html

The Cranfield site also acts a mirror for the NACA report server and most of the online reports can be found through here.

Hope this helps and happy hunting.

Mark

[hfriedman]

One of the more notorious (not necessarily truthful) scandals of World War II aviation is the tale of the aborted re-engining of P-38s with Merlins. The story is that after the spectacular success of replacing Allisons with Merlins in the P-51, a Lightning was lent to Rolls-Royce or to a British testing facility for a test replacement. Then American politics, perhaps sparked by General Motors’ influence in high places, killed the project with a demand that the airplane be returned immediately. Thus was America cheated of a world beating airplane; another victory by malafactors of great wealth. So goes the story.

In contrast to the early Mustangs, the P-38 had turbo superchargers and the high altitude performance that they produced. The change to mechanical superchargers and, certainly, any attempt to adapt the Merlin to the turbos, would seem to have involved much greater engineering problems than the P-51 conversion.

Does anyone have any more on this?

[jrussell]

What I have read on the subject, is that the main reason that the Merlin was not installed was that the engineering required to change the cowling and mounts would require a production interuption, and was therefore declined. When you consider that the two stage Allison engines were expected to come on line sooner than they did, the decision seems more reasonable - but it is hard to imagine that SOME political pressure wasn't involved! Another element in the long line of P-38 what ifs that leaves P-38 lovers wondering about all the deceisions that seemed to hold the Lightning back.
_________________
It runs best just before it blows!

[jjuutinen]

Well, I think the quickest way to boost P-38´s power would have been by fitting it with the R-2800. Though definitely a heavier engine, how much heavier would the installation have been when the radiators and coolant of the 1710 could have been deleted? Probably not much.

Option is water injection. Can´t really comprehend why ADI was never fitted with series produced Lightnings.

[jjuutinen]

Of course, the ultimate Lightning would have been a version with turbocharged Napier Sabres with ADI:) That is 6000+ hp available for the pilot...Climbing capability would have been simply outrageus.

[szielinski]

Not as outrageous as the fuel load....
(says the same guy who mentioned two-strokes)

[gryan]

Mark

I am having trouble locating the paper "Hartshorn, A.S. and Nicholson, L.F, “ The Aerodynamics of the Cooling of Aircraft Reciprocating Engines, “ Aeronautical Research Council, Report and Memoranda No. 2498, 1947." Was it on the Cranfield University server? The search comes back that it is unknown.

Would I be correct in surmising that most of the research work on this topic occurred from about 1942 until 1950?

Any other recommended papers?

Thanks

Gerald

[sithomas]

Only 19 years late, but for those in Canada (and likely many other countries except the US) the report can be found here:
https://hdl.handle.net/2027/mdp.39015024198973

A VPN may prove beneficial.

UK crown documents that were commercially available have a copyright life of 50 years.