On big superchargers and small aircraft

[tomo pauk]

Snaching idea from the title of a controversial book of the 1st Croatian president, I'll try to steer the discussion about the fiddly bits that make difference in aircraft engines' altitude performance, and aircraft that were supposed to use the newly-earned capability.

It is all very well and good to talk about installing two stage superchargers on P-39s. p-40s and early P-51s...........except...........
they don't fit.
In order to work they need intercoolers. Otherwise the intake mixture gets to too hot and the engine goes into detonation.

The 2-stage engines worked without intercoolers (V-1710, DB-603L versions, Jumo 213F, DB 605L), and they also were installed in fighters as small as Bf 109 or Yak-9 (that one was wih intercooler). Granted, some of those engines used a bit too big compression ratio as a self-inflicted wound.
Early P-51 was outfitted with 2-stage engine and went above 440 mph after some tweaking.

This was the trouble the P-38 had from about the 'D' model until the "J". It was the problem the P-63s had.

Finding another 1 ft or less behind the engine for the larger supercharger housing wasn't the problem (until Allison put the 1st stage in a separate housing and drove it with a seperate drive system). it was finding room for the intercoolers and ducting.
Please note that a 2 stage Allison in the P-63A could make 1800hp at sea level using 75in and water injection. WIthout water injection it made about 1500hp at 60in but by the time you got to to 21500ft it was down to 1180hp, whether you used water injection or not.
Please note a Merlin 46 could make 1100hp at 22,000ft with a single stage single speed supercharger and make 1440hp at 14,500ft. No intercooler either.

The P-38 even with 'bad' intercoolers was eventually able to make 1425 HP at 25000 ft, and 1600 HP at 15000 ft. The V-1710 with worst 2-stage S/C gained ~7000 ft in rated altitude vs. the best 1-stage version.

Everybody wants the performance of the two stage supercharger, nobody wants to pay the cost.
early P-51Bs were slower than Allison power Mustangs at low altitudes because of the higher drag of the larger fuselage, larger radiator/intercooler and heavier weight of the two stage engines. Once you got high enough for the two stage engine to start making significantly more power than the single stage the Merlin Mustangs really went into a world of their own.

People want/wanted to pay the cost.
Probems with V-1710 in 2-stage flavor were:
- it was running late vs. other competitors
- USAAF, in it's wisdom, didn't see fit to push for P-51 nor for P-40 to have it installed ASAP
- after all, it got installed in the redundant aircraft (P-63)
P-51B was either as fast or faster than P-51 or P-51A in a choosen engine setting between SL and 20000 ft.

A two stage Allison in a P-63 was around 150lbs heavier than the engine in a P-39N, it was longer and expecting to cool an engine making several hundred hp more needs a slightly bigger radiator (about 20-25lbs more for the P-63) the P-63 used a propeller about 60 lbs heavier.

Granted, bigger radiators were needed. The P-39 will not get along with long V-1710, unfortunately people at Allison didn't thought that engine length matter at A/C that have engine amidship.

You can do it, it takes a lot more changes than some people seem to think.
The Merlin system worked but it was also a bit on the crude side (at first look) and many engineers were trying to go one better. The Merlin ALWAYS both impellers spinning at the same time and spinning at the same speed. Effective but not very flexible with a two speed drive. P & W's two stage (first flown in 1939 so two stage superchargers aren't unknown in the US) had a single speed supercharger on the engine and a two speed drive with neutral on the auxiliary supercharger drive given three possible combinations. Allison stuck a hydraulic coupling in the drive the auxiliary supercharger giving an infinite number of combinations between a high and low limit. More elegant from an engineering point of view but you had to pay for it somehow.

RR's 2-stage supercharger was excellent in two crucial categories: power at (but not just) high altitude, and ability to be retrofitted into existing A/C due to being compact in execution. From practical point of view, it was as elegant as possible.

 

[BiffF15]

Gents,

In layman's terms would you explain the difference between speeds and stages of superchargers? Also explain variable speed ones if you would. Analogy seems similar to transmissions but not sure that's encompassing enough.

Cheers,
Biff

 

[Shortround6]

The 2-stage engines worked without intercoolers (V-1710, DB-603L versions, Jumo 213F, DB 605L), and they also were installed in fighters as small as Bf 109 or Yak-9 (that one was wih intercooler). Granted, some of those engines used a bit too big compression ratio as a self-inflicted wound.
Early P-51 was outfitted with 2-stage engine and went above 440 mph after some tweaking.

It depends on what kind of boost you are looking for. A Merlin 61 was making 15lbs boost at 23,500 (60in) which is about 2.0 Ata and the supercharger is compressing the ambient air over 5 times. The DB-603L with intercooler (granted the German gas needed help) was rated at 1.4Ata? The 605L was rated at 1.75Ata? Granted at higher altitudes but how many production aircraft were flown with the 3 German engines?

The P-38 even with 'bad' intercoolers was eventually able to make 1425 HP at 25000 ft, and 1600 HP at 15000 ft. The V-1710 with worst 2-stage S/C gained ~7000 ft in rated altitude vs. the best 1-stage version.

I believe parts of the solution to getting the P-38 engines to work at higher altitudes was bigger radiator housings that stuck further out into the air stream and a modified oil cooler system, Automatic cooler flaps or something? Better engine cooling can help but those modified radiator housings on the P-38 (even if the cores were the same) cost drag. I don't know about the oil cooler set up.

People want/wanted to pay the cost.
Probems with V-1710 in 2-stage flavor were:
- it was running late vs. other competitors
- USAAF, in it's wisdom, didn't see fit to push for P-51 nor for P-40 to have it installed ASAP
- after all, it got installed in the redundant aircraft (P-63)
P-51B was either as fast or faster than P-51 or P-51A in a choosen engine setting between SL and 20000 ft.

A P-51 B could make more power but at low altitude the Allison planes could be as fast or faster.
British got one to do 360mph at 1000ft using 56in on an F3 engine. they got 392mph out of it at 7,900ft.
Using an F21 engine (9.60 gears) they got 366mph at 1000ft and 377 at 4000ft.
Americans got 363mph out of a P-51B at 5000ft using 60.5in map.
Yes the Merlin was soon rated at 67in and then 75in with 100/150 fuel but that doesn't change the fact that the Allison powered versions were just about as fast on less power.
American test had a P-51 with -39 engine doing 388mph at 12,500ft on 1200hp. P-51B using 60.5in in low gear at 10,000ft was making 394mph on 1485hp.
That is the cost. Too many people in these "what ifs" want to take the extra power and turn it all into better performance and not pay the cost of weight and drag.

RR's 2-stage supercharger was excellent in two crucial categories: power at (but not just) high altitude, and ability to be retrofitted into existing A/C due to being compact in execution. From practical point of view, it was as elegant as possible.

Unfortunately some people just couldn't avoid violating the KISS principle.

 

[wuzak]

Gents,

In layman's terms would you explain the difference between speeds and stages of superchargers? Also explain variable speed ones if you would. Analogy seems similar to transmissions but not sure that's encompassing enough.

Cheers,
Biff

Speeds are like gearing in car.

Stages is the number of superchargers, basically, in the supercharger system.

A single stage supercharger has one compressor.
A two stage supercharger has two compressors.

The advantage of the 2 stage supercharger is that for a given overall pressure ratio (ie absolute pressure at outlet/absolute pressure at inlet) each stage has to do less work.

a PR of 4:1 is tough for a single stage supercharger, but a 2 stage supercharger has each operating at PR of 2:1.

 

[Shortround6]

Wuzak has it pretty well. The two stage superchargers had compressors in series regardless of how they were driven. Output of 1st stage blew into the inlet of the 2nd stage.
Variable speed superchargers used a fluid coupling much like the torque converter in a car. There was a definite range over which they operated, like form a low of 7 times the input speed to the speed of the impeller to a high of just over 10 to 1. for the German DB engines.

Problems were that even with the increased efficiency of the two stages over a single stage the temperature of the air in the intake manifold could still be several hundred degrees even if the outside air was below freezing. This lead to detonation (a bit like pinging in a car climbing a hill but much, much more sever).

 

[tomo pauk]

It depends on what kind of boost you are looking for. A Merlin 61 was making 15lbs boost at 23,500 (60in) which is about 2.0 Ata and the supercharger is compressing the ambient air over 5 times. The DB-603L with intercooler (granted the German gas needed help) was rated at 1.4Ata? The 605L was rated at 1.75Ata? Granted at higher altitudes but how many production aircraft were flown with the 3 German engines?

I'm looking to improve altitude power of a choosen engine. The DB 605L was not an indeal engine, but it provided extra 480 PS over the DB 605A at 9 km (29530 ft), or more than 50% more. The DB 603L was rated at 1.75 ata, and 2 ata with MW 50.
Number of German (or other people's) aircraft powered by 2-stage engines has next to nothing to do with 2-stage engines working without intercoolers.

...
That is the cost. Too many people in these "what ifs" want to take the extra power and turn it all into better performance and not pay the cost of weight and drag.

I am not one of these people
Use the darned 2-stage engine in area where it excels - high altitude power, the altitude band being well suited to escort the US bombers. Allies don't need yet another fighter that is fast down low. This is a reason why I trumpet the installation of 2-stage V-1710 on the P-51, while killing off the XP-63 project already in 1942. Yes, it will lag vs. the Merlin Mustangs by 10-20 mph, but that is still enough to plow through Fw 190s and lesser fighters.

Unfortunately some people just couldn't avoid violating the KISS principle.

Today it is doubly so.