Spit Mk1 Boost?

[Al Schlageter]

Originally Posted by Crumpp

Only after significant modification to handle such pressure, modifications that did not exist in 1940.

So it only took RR maybe 1-2 months to make the significant modification and test the engine at +16lb boost and obtain certification for that boost level. Not bad at all.

[Crumpp]

We're saying the same thing btw, no need to argue.

There is no argument at all. I was trying to help you guys find answers to your questions about these machines.

RR maybe 1-2 months

I don't know as I would have to see the actual timeline. I highly doubt it though.

Changes to aircraft do not generally happen overnight. First the change has to be thoroughly tested because the safety margins are by their very nature very tight in aviation.

Once approved, it can't happen overnight either. The changes have to be disseminated to the folks who will implement them. They need the knowledge to enact the change.

Just as important, the people making those changes need the resources to enact it. That means the manufacturer has to retool or reset the production lines, train employees, and get the new parts made. Making enough for the aircraft in service is just the tip of the iceberg too. You have to have enough replacement parts sitting on the shelves to keep the airplanes in service flying. The standard rule of thumb is one part on the airplane, three replacements on the shelf.

So if you have 25 airplanes, you need 100 parts in total.

Of course once the part is made, it still has to reach the flight line too.

In short, it has to be approved, disseminated, manufactured, and distributed before anyone picks up a screw driver to loosen up the first screw to make the change. On average, that process took ~6months in WWII for most designs. The more technical the change, the longer and vice versa.

Just because a memo says it was approved does not mean it was in operational use from that day forward.

[41Sqn_Banks]

Maybe the +16 lbs were allowed for Sea Hurricane IA that operated from CAM ships (catapult armed merchantman) without the possibility to land - engine life is not a problem in this case.

[Al Schlageter]

Since Sea Hurricanes were in service in Feb 1941, then the +16lb boost Merlins were available in 1940 since it took, according to you, ~6 months.

[Viper2000]

Originally Posted by Crumpp

Only after significant modification to handle such pressure, modifications that did not exist in 1940. Your document clearly separates it from the earlier Merlin II and III engines.

The Sea Hurricane was a special case; it was used on CAM ships and could not be recovered. The pilot would either parachute out or ditch when he ran out of fuel, as CAM ships had no way of accepting the aeroplane back for landing.

As such, both engine and airframe life were expected to be no more than one sortie, and it was therefore perfectly reasonable to give them a Viking funeral.

[Crumpp]

As such, both engine and airframe life were expected to be no more than one sortie

You know I read that part about one sortie life span. Thanks for pointing that out. Makes perfect sense to put your obsolete motors and so extremely over-boost them in a disposable airplane. Even then I would like to see what the engine was approved for at the boost. I would think it was just to get off the CAT.

Even a disposable fighter is worthless if it cannot fly to a target and fight.

[Viper2000]

Originally Posted by Crumpp

You know I read that part about one sortie life span. Thanks for pointing that out. Makes perfect sense to put your obsolete motors and so extremely over-boost them in a disposable airplane. Even then I would like to see what the engine was approved for at the boost. I would think it was just to get off the CAT.

Even a disposable fighter is worthless if it cannot fly to a target and fight.

I don't think they needed the power for takeoff because the "catapult" in this context was a whacking great rocket.

Given the stability & control difficulties associated with using really high powers at low speeds in these aeroplanes, and the obvious risk of engine failure at low speed & altitude off the bow of a ship which probably can't stop or avoid the aeroplane if it ends up in the drink, I would have thought it more likely that the launch was conducted at normal takeoff power.

I think that the extra combat power was primarily intended to be used for a rapid climb to cloudbase in order to intercept/scare off the bomber that was threatening their convoy; in the absence of RADAR, the CAM's fighter was only launched when the enemy was within visual range, and so warning time was strictly limited.

The climb would be unlikely to go higher than cloudbase because the bomber would just cloud-hop home (and probably couldn't hit a ship from any higher anyway, so either way it would be a mission kill); the fighter would be unlikely to give chase because of the obvious navigational challenge of finding their convoy again afterwards in an aeroplane not really equipped for naval navigation (at this time, most Fleet Air Arm aircraft, even the fighters, had a navigator).

For this reason, I suspect that they'd be unlikely to use the extra power for more than a couple of minutes in the sortie; I'm guessing that an early Hurricane with +16 would probably exceed 3000 fpm climb rate, and the cloud base would usually be less than 6000 feet...

[Blackdog_kt]

Originally Posted by SEE

Thanks guys, just one qestion. At alt 17K and above, the manifold pressure in Clods Spit Mk1 drops significantly (less than 2 Ibs per Sq.in.) - full throttle - straight and level - The boost cut out has no effect as pointed out earlier in the thread. So, based on the above - this appears to be incorrect for that altitude?

You got me curious so i set forth to investigate this by converting some units and making a couple of simple comparisons

My personal conclusion is that while i can't be sure if it's accurate because i don't have original performance specs, +2 boost is nothing to scoff at in the wider sense of things.

First of all boost is measured in relative units, it's PSI above the normal sea level ambient pressure or in other words, zero boost means full throttle at sea level in a non-supercharged engine.

To put this in perspective, +0 boost would be equivalent to about 1 Ata (for Luftwaffe aircraft) or 29.92 inches of mercury (for USAF aircraft)

According to conversion tables, 1 psi = 2.036 inHG = 0.068046 atmospheres.

I don't know if Ata units correspond exactly to standard atmospheres, but here's how it looks thus far with relative boost measuring:
1 atmosphere = 29.92 inches Hg = +0 psi boost

If we measured absolute psi we'd get:
1 atmosphere = 29.92 inches Hg = 14.696 psi (absolute)

If this gets confusing, just keep in mind that boost is like setting the scale arbitrarily: +0 psi boost = 14.696 psi absolute


Following this train of thought we have:
+2 psi boost = 16.696 psi absolute (14.696 absolute + 2 from the boost) = 1.13609 atmospheres = 33.993 inHg

If we keep in mind that variants of late war fighters with turbo-charged engines like the P-47 were limited to about 42 inches Hg, getting the equivalent of 33.99inHg from a supercharged Merlin (no turbos involved) is not shabby at all.

Like i said i don't know how accurate it is, it's just that it doesn't seem implausible when making comparisons to later designs. I know this doesn't answer your question in a satisfactory manner, i was just in the mood for some investigating until someone with more accurate data can pop in

[SEE]

Thanks for taking the time to do the maths Blackdog. I don't use the 'boost' because I have never really noticed any gains that are worth the risk of engine damage if you forget to disable it.

But I do look at the manifold pressure guage to check I haven't blown a gasket......

I have read Spit pilot accounts where they describe (in passing) enabling 'boost' or 'throttle through the gate' to get out of trouble and head for base.

If I fly straight and level - full throttle, with the boost cut out positioned for that extra bit of throttle movement - I don't see any increase in RPM, IAS, etc. That left me wondering what should I expect to read on my gauge at 18000ft for example and what performance gains I should expect to see or get.

[Ze-Jamz]

Originally Posted by SEE

Thanks for taking the time to do the maths Blackdog. I don't use the 'boost' because I have never really noticed any gains that are worth the risk of engine damage if you forget to disable it.

But I do look at the manifold pressure guage to check I haven't blown a gasket......

I have read Spit pilot accounts where they describe (in passing) enabling 'boost' or 'throttle through the gate' to get out of trouble and head for base.

If I fly straight and level - full throttle, with the boost cut out positioned for that extra bit of throttle movement - I don't see any increase in RPM, IAS, etc. That left me wondering what should I expect to read on my gauge at 18000ft for example and what performance gains I should expect to see or get.

Id like to know this..

TC, help please