high/low blower super charger gears

[Charge]

http://naca.central.cranfield.ac.uk/reports/1941/naca-tn-795.pdf

Just a link. Enjoy.

-C+

[EagleDNY]

Interesting link. 

God but there were MEN in those days - imagine figuring all that out with the old slide rule folks....

[gripen]

Generally nothing special will happen assuming that throttle valve is able to prevent the overpressure. The efficiency and power is just lower due to unnecessary workload for supercharger and higher temperature of the incoming air.

[Captain Virgil Hilts]

CVH , Obviously if you over boost an engine serious things will happen very soon, but the question we are talking about does leaving a supercharger in high alt gear over boost the engine. With an automatic butterfly valve or other device are you sure it would over boost?

HiTech

If there is an automatic throttle valve that controls boost by referencing manifold pressure, provided it has the operating range to close the throttle enough, it will prevent excessive boost. In other words, if whatever automatic control can close the throttle ahead of the blower enough, yes, it can prevent excessive boost.


If there is no automatic throttle control to prevent excessive boost, it will over boost if you leave the throttle wide open. Example: Let's say that if you drive the blower at 1:1 (crankshaft speed) at or below 18K, and at War Emergency Power, it produces 80" of boost, which is all the engine will stand without detonating, provided it is cool enough. To produce the same 80" of boost at say, 27K critical altitude, let's say it requires you to drive the blower at 1.30:1 crankshaft speed (30% overdrive). So if you dive from 24K to 16K, leave the blower in overdrive, operating at WEP, then the blower is going to over boost the engine in just a few seconds, pretty much the instant it gets dense enough air, figuring 90% efficiency, you're going to get about 20-25% more boost, so now you have close to 100" of boost. On an engine already operating on the ragged edge, you have maybe a few seconds before it starts killing itself in a rather spectacular manner.

I'm not sure an automatic throttle control is present, I'd have to look at some diagrams to see if it is. Remember, these piston engine propeller driven fighter aircraft have a system that uses propeller pitch to control RPM, not throttle position. Throttle position is what governs boost on these systems. The throttle is controlled by the pilot, manually. So, assuming that the pilot left the throttle in the War Emergency Power position, unless the throttle itself, in the wide open position, presents enough restriction, if the blower drive system does not shift down to low speed, the boost will exceed the maximum rating.

To give you an example of how large a difference blower drive ratio makes, with a typical 8-71 blower on a 450 cubic inch engine (a properly sized blower for good performance and efficiency), drive ratio change from 1:1 to 1.2:1, or 20% over drive, a very common change, will take you from 8-10 pounds of boost (we don't use "inches of mercury or water to measure boost in racing) to 12-14 pounds of boost on a standard blower. On an engine that is operating as close to the edge as a World War II fighter engine, if it is designed to operate at 8-10 pounds of boost and you run 12-14 pounds of boost, the engine can be destroyed (catastrophic failure) in one ten second pass. Back when we were running the full 1/4 mile, or 1320 feet, and doing it in 4.4 seconds, we could make a small blower pulley change and predict almost exactly (within 20 feet) where the engine would reach an over-boost condition and blow up like a hand grenade.

[Captain Virgil Hilts]

Don't they use pop-off valves on Roots type blowers?

Yes, we use preset pop off valves with springs, the area is around 2 to 4 square inches, depending on the blower and the intake. That's an emergency valve, and is not intended be held in the unseated position for any real length of time, it's to prevent severe damage in case the engine were to backfire through the intake tract. The venting from the pop off valve opening releases fuel and air under pressure, in a relatively closed area, say, inside the engine cowling of an airplane, odds are leaking voltage from the ignition system would ignite the fuel and air, causing at least a small explosion.

On the Top Fuel car I crew part time, we also have a burst panel on the intake manifold, that, combined with the pop off valve, is intended to keep the blower from being blown off the intake in the event of an intake valve hanging open.

On turbochargers, if they are mounted in front of the throttle, we use a "sneeze valve", it's just there to keep the turbocharger bearings from being damaged if the throttle is slammed shut during high boost conditions.

[Charge]

"and higher temperature of the incoming air."

And that is what I'd be worried about.

-C+

[Captain Virgil Hilts]

"and higher temperature of the incoming air."

And that is what I'd be worried about.

-C+

Well, if you increase the boost, you increase the air temperature, unless you have an intercooler (found only on turbocharger equipped planes, best I can tell). Add 20" of boost and 50 degrees of air temperature and you'll be detonating in seconds.

[MiloMorai]

Merlin engines from the 60 series on had intercoolers.

[Stoney]

Saw a blown Merlin off American Beauty at Reno a couple of years ago.  A connecting rod snapped from being overboosted and it blew the whole bottom of the oil pan off.  There was a hole big enough to stick your foot through.  Obviously they're using ADI and running 120" or so, but that's the result.

[Edit]Also, good description of the P-51D automatic boost control in the POH in the AH Wiki.  May be worth looking into a F4F manual or something older that had manual boost controls to see what the procedures were...

[Guppy35]

All I remember is reading many comments by the Spit drivers talking about the 'thump' of the high blower kicking in as they hit a certain altitude.  it never sounded like something they set on their own but that it was set for certain heights to kick in.

[Charge]

Well initially they were manually controlled but it does not take much to impelent a device which monitors air density and boost pressure and if they both fall to certain level the second gear is clutched automatically. They even installed a cuckoo clock in FW190 and it did a fine job on engine management. 

"Add 20" of boost and 50 degrees of air temperature and you'll be detonating in seconds."

AFAIK this is also what happens if you rotate the impeller too fast in too dense air and as the impeller starts to stall it causes the charged air to heat up which eventually leads to uncontrolled detonation.

-C+

[Captain Virgil Hilts]

Saw a blown Merlin off American Beauty at Reno a couple of years ago.  A connecting rod snapped from being overboosted and it blew the whole bottom of the oil pan off.  There was a hole big enough to stick your foot through.  Obviously they're using ADI and running 120" or so, but that's the result.

It also almost certainly had either aftermarket rods or Allison rods, as a Merlin never won Reno until around 1969 (not the exact year) or so when someone figured out how to put Allison rods in it. Several companies, such as Oliver, now make 4340 forged billet rods for the Merlin and the Allison. The oiling system on the Merlin is weak, and the big end of the stock rods will not stay round.

Of course, they're spinning around 4500 RPM as opposed to the original red line of around 3000. That 1500 RPM does not seem like a lot, until you consider that it is more than 40% above the original design rating, and until you hold a rod and piston in your hand, the stuff in them is massive. Boost does not break the rods, what it does to the rods is to force the big end out of shape, that pinches the bearing, which then sticks to the crank journal and spins, burning the rod until it becomes weak and breaks.

Tweedling methanol/water into the engine doesn't make any more power, in fact, they only put in what they have to in order to keep the combustion temperature down, any extra kills too much power.

[Captain Virgil Hilts]

Well initially they were manually controlled but it does not take much to impelent a device which monitors air density and boost pressure and if they both fall to certain level the second gear is clutched automatically. They even installed a cuckoo clock in FW190 and it did a fine job on engine management. 

"Add 20" of boost and 50 degrees of air temperature and you'll be detonating in seconds."

AFAIK this is also what happens if you rotate the impeller too fast in too dense air and as the impeller starts to stall it causes the charged air to heat up which eventually leads to uncontrolled detonation.

-C+

Yeah, like I said, I'm pretty sure the blower drive speed was controlled by a aneroid bellows that shifted at a preset altitude. An automatic throttle control to use the throttle to control boost could be made much the same way a "throttle stop" is used in drag racing today (I hate them), the driver, or in this case, the pilot, would open the throttle all the way, but an automatic device would be able to close the throttle at a pre-determined boost level, with a telescoping/sliding segment in the throttle cable, or a second set of throttle plates.

Yes, compressor stall will heat the air, but it also causes pressure surges, and then you have serious inefficiency that kills performance, not to mention you can damage the supercharger.

[Strip]

FYI ...

The water/alcohol injection on a Merlin (or any engine) does not appreciably affect combustion
temperatures. It does however affect the intake air temperatures and helps prevent detonation
or pre-ignition. Detonation can cause cylinder pressures to spike well over normal limits and shocks
the piston, rod, ring package, and crankshaft. In fact spark knock (as its sometimes called) is the
audible sound of two wave fronts colliding. In a over boosted condition this would be the most likely
cause of damage to an engine.

Strip

[Mike Williams]

All I remember is reading many comments by the Spit drivers talking about the 'thump' of the high blower kicking in as they hit a certain altitude.  it never sounded like something they set on their own but that it was set for certain heights to kick in.

I’m reminded of Bobby Oxspring's experience flying Spitfire IXs with 72 squadron in North Africa during early 1943:

"The second stage supercharger of the Merlin 61 had an automatic barometric control gauged to cut in at 19,000 feet.  Since barometric instruments are notoriously imprecise, the effect on a squadron climbing for altitude at high power settings meant that twelve superchargers cut in at slightly different times.  The effect of the formation with these widely fluctuating power settings was like shuffling a pack of cards and trying to keep the same suit together.  To overcome this discrepancy, we decided to climb well above the automatic setting utilizing the manual override.  On a radio order from the leader we flipped the 'auto' switch and cut the superchargers in together.  We were then able to contend with the foe at high altitudes, but it was prudent to keep the knowledge of our capability from him as long as possible.  Bearing in mind the efficiency of the enemy’s 'Y' service, we devised a code for 2nd supercharger engagement.  The formation leader's instruction for simultaneous action by other pilots was 'Up your pipe'; Up your' being the warning, and 'Pipe'’ being the executive command to cut in.  The stratagem had the desired effect and 'Pipe' sent the squadron soaring aloft like a pack of homesick angles."