Spit Mk1 Boost?

[41Sqn_Banks]

Yes considerably ...

http://www.spitfireperformance.com/spit1-12lbs.jpg

full throttle would give about 17 lbs. sq.in.

If however I well understood, the boost cut out was put because this high pressure was damaging to the engine in the long run and therefore its use was limited to real emergency cases.

Could the fact that it is limited right now be linked to that perhaps?

[Osprey]

TomcatVIP, would you please stop posting if you aren't going to add anything other than hearsay to this. From reading the thread =XIII=Wedge has made some very interesting points - I for one am trying to understand the detail of this but clearly I'm a bit more stupid than other pilots. Understandably I don't really appreciate you throwing in undermining comments based on your own personal knowledge and trying to deride other posters arguments on the spurious suggestion that the length of registration on a forum somehow dictates your knowledge on the topic. Should Jeffrey Quill have registered today would you be telling him the same thing?

Frankly, if you are one of those people that doesn't believe that the RAF had the 100 octane in use on fighters thus achieving 12lbs boost then I'm afraid I don't think I'll be able to take anything you say seriously, ever. I'm confident that the development team aren't so unreasonable when it comes to looking at evidence.

=XIII=Wedge, please carry on.

[=XIII=Wedge]

Function of the Automatic Boost Control
My understanding was that the original reason for the throttle being design with an automatic boost control was to reduce the workload on the pilot and prevent over-boosting the engine.

It allowed them to set the desired boost pressure with the throttle lever and the ABC would ensure that that desired pressure setting was maintained up to the rated altitude.

Without it the pilot would constantly have to keep adjusting the throttle to maintain the same boost as the plane changed altitude.

Using the boost cutout disables this function.

@TomcatViP - It is possible, but it is more likely that pressure to release the product meant that development was scaled back.

There is clear evidence of this, due to having two variants of spitfire MK 1A that differ only by propellor. (DH-2S and Rotol-CS)

No problem with this it happens all the time, features are dropped to get a release to the market place.

However if you take these pressures out of the equation, a more realistic flight model is still a desirable attribute and is infact a major selling point.

Red vs Blue arguments don't really come into it.

Multiplayer is an important aspect of the game, but with out greater realism we might as well be playing Tom Clancy's H.A.W.X.

[41Sqn_Banks]

Originally Posted by =XIII=Wedge

Function of the Automatic Boost Control
My understanding was that the original reason for the throttle being design with an automatic boost control was to reduce the workload on the pilot.

It allowed them to set the desired boost pressure with the throttle lever and the ABC would ensure that that desired pressure setting was maintained up to the rated altitude.

Without it the pilot would constantly have to keep adjusting the throttle to maintain the same boost as the plane changed altitude.

Using the boost cutout disables this function.

This is correct. This are exactly the points mentioned in the Merlin II/III engine manual.

Some notes:
You discribe a "variable datum type" automatic boost control. Early production Merlin II engines had a "fixed datum type" automatic boost control (however even the manual mentions that these are replaced by the "variable datum type"). As you already wrote with a "variable datum type" boost control each throttle setting gives you always the same boost (up to rated altitude). The "fixed datum type" had the follwing drawback:

AP 1590B Merlin II and III Aero-Engines

240. (...) With the fixed datum type of automatic unit for which the control has been set, maximum permissible boost is obtained soon after the throttle is opened past the slow-running position. In consequence of this, below rated height and particularly near the ground level, it is only possible to open the throttle a small amount with the result that a considerable degree of lost motion in the cockpit throttle operating lever occurs.

What the "modification" to the boost control does is simply limiting the boost pressure to rise above +12lbs even if the boost control was disabled. However my understanding is that you don't have the benefits of the "variable datum type" boost control anymore (as long as boost control cut out is enabled). In later Merlin marks this was changed and the "boost control cut out" didn't really disable the boost control but it did set a higher boost for each throttle setting.

[=XIII=Wedge]

So given that we currently have an earlier Spitfire using only 87 octane fuel:-


With the ABC enabled (normal situation)

• The maximum boost obtainable will be +6.25 lbs.
• Once set the selected boost (datum) will be maintained regardless of changes in altitude up to the rated altitude.
• Above rated altitude the pressure will drop off, but the datum point will remain the same.
• Pilot workload is reduced and there is no risk of overboosting the engine.

With the ABC disabled (done by operating the cutout)

• The maximum boost obtainable would be +17 lbs @ sea level.
• Any rise above + 6.25 would have disasterous impact on the engine due to pre-ignition of the fuel.
• Provided that the pilot retarded the throttle they would have manual control of the boost, but would have to be careful not to overboost the engine.
• The indicated boost pressure would change as the altitude changed.

Assuming that this is a correct description then from a game design point of view their are a few options.

1. The operation of the cutout would only be of value if the game simulated failures to the ABC unit either through the damage model or the physical weathering setting.
2. If this was not the case then the cutout option could be removed (Only the sadistic would take pleasure from using the cut-out to intentionally overboost a Merlin)

If any of my assumptions are incorrect then please let me know.

Does this seem like a reasonable analysis of the issue?

[Osprey]

I feel more clever already

[SEE]

Originally Posted by =XIII=Wedge

So given that we currently have an earlier Spitfire using only 87 octane fuel:-


With the ABC enabled (normal situation)

• The maximum boost obtainable will be +6.25 lbs.
• Once set the selected boost (datum) will be maintained regardless of changes in altitude up to the rated altitude.
• Above rated altitude the pressure will drop off, but the datum point will remain the same.
• Pilot workload is reduced and there is no risk of overboosting the engine.

With the ABC disabled (done by operating the cutout)

• The maximum boost obtainable would be +17 lbs @ sea level.
• Any rise above + 6.25 would have disasterous impact on the engine due to pre-ignition of the fuel.
• Provided that the pilot retarded the throttle they would have manual control of the boost, but would have to be careful not to overboost the engine.
• The indicated boost pressure would change as the altitude changed.

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?

[Viper2000]

Originally Posted by 41Sqn_Stormcrow

If however I well understood, the boost cut out was put because this high pressure was damaging to the engine in the long run and therefore its use was limited to real emergency cases.

ABC was introduced in order to prevent pilots from breaking engines by overboosting them below FTH.

I suspect that the cutout was originally provided in case the ABC failed, so that the pilot would still have access to direct throttle control.

The cutout modification was basically a hack which allowed the cutout to function as a combat power detent without extensive modification to the rest of the cockpit.

Later Spitfires didn't have a boost control cutout, but were instead fitted with a gated throttle which allowed the independent setting of the various engine ratings, including the combat rating. They also had double-acting ABC instead of single acting ABC, which meant that the throttle behaviour was a bit different. This is explained in the relevant AP from the period (I forget the number now, but it's been linked to in quite extensively in this forum).