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The Staff of SOH
3 Spitfire Mk IX Specials
- Thread starter ndicki
- Start date
Led Zeppelin
Charter Member
Beautiful, many thanks one again!
MajorMagee
SOH-CM-2022
Two things to note:
Spitfire FR.IXc MK915 shows up as Spitfire FR.IXc X in the selection list
Spitfire FR.IXc NH270 shows up as Spitfire FR.IXc NH970 in the selection list
Spitfire FR.IXc MK915 shows up as Spitfire FR.IXc X in the selection list
Spitfire FR.IXc NH270 shows up as Spitfire FR.IXc NH970 in the selection list
greycap.raf
Charter Member
As this marks the first appearance of the +25lbs boost Merlin 66 in the series I'll say a couple of words.
The normal "full throttle" boost of the Merlin 66 was +12lbs and the emergency boost was set at +18lbs. When the 150 grade fuel became available in summer 1944 the emergency rating was increased to +25lbs but to my knowledge the maximum continuously allowed boost was still the same +12lbs as always. However, I've chosen to set the full throttle boost at +18lbs and hitting the emergency boost gives the full power, admittedly the gauge won't read +25lbs as CFS3 doesn't correctly model the efficiency losses at higher boost pressures so the correct power figure is achieved with slightly less boost than in real life.
The main reason behind this choice was that it would feel quite silly to have an engine that runs at either 1315 or 2050 bhp with no options in between, in real life the pilot could use any power setting but our only options are full throttle and emergency boost so I decided to give the chance of using the normal +12lbs (90% throttle if I remember correctly), +18lbs (full throttle) and +25lbs (emergency power) to have a bit more flexibility in the engine management.
As the extra boost in real life was only available below the supercharger critical altitude - the supercharger can only move so much air no matter what the fuel - it tapers off as the altitude increases in such a way that when the aircraft reaches 22.000 feet, the critical altitude for the +18lbs boost, there's no more extra to be had and it performs like a normal Merlin 66 from there on. Admittedly with the exception of having unlimited +18lbs boost but with CFS3 being built the way it is we can't have everything.
It's not a perfect system but it was the best I could come up with. The other option would have been to have the entire boost range under the normal throttle travel but I didn't want to do that as it would have allowed running at full emergency boost all the time and it didn't sound like a good idea from a simulation point of view, especially when the real aircraft required a dedicated move of the throttle lever to engage the emergency boost.
The normal "full throttle" boost of the Merlin 66 was +12lbs and the emergency boost was set at +18lbs. When the 150 grade fuel became available in summer 1944 the emergency rating was increased to +25lbs but to my knowledge the maximum continuously allowed boost was still the same +12lbs as always. However, I've chosen to set the full throttle boost at +18lbs and hitting the emergency boost gives the full power, admittedly the gauge won't read +25lbs as CFS3 doesn't correctly model the efficiency losses at higher boost pressures so the correct power figure is achieved with slightly less boost than in real life.
The main reason behind this choice was that it would feel quite silly to have an engine that runs at either 1315 or 2050 bhp with no options in between, in real life the pilot could use any power setting but our only options are full throttle and emergency boost so I decided to give the chance of using the normal +12lbs (90% throttle if I remember correctly), +18lbs (full throttle) and +25lbs (emergency power) to have a bit more flexibility in the engine management.
As the extra boost in real life was only available below the supercharger critical altitude - the supercharger can only move so much air no matter what the fuel - it tapers off as the altitude increases in such a way that when the aircraft reaches 22.000 feet, the critical altitude for the +18lbs boost, there's no more extra to be had and it performs like a normal Merlin 66 from there on. Admittedly with the exception of having unlimited +18lbs boost but with CFS3 being built the way it is we can't have everything.
It's not a perfect system but it was the best I could come up with. The other option would have been to have the entire boost range under the normal throttle travel but I didn't want to do that as it would have allowed running at full emergency boost all the time and it didn't sound like a good idea from a simulation point of view, especially when the real aircraft required a dedicated move of the throttle lever to engage the emergency boost.
gecko
Charter Member
Great skins Nigel, and nice explanation Rene! I appreciate the work that goes into both! How did you go about determining the correct boost setting for the sim to account for the efficiency loss at high boost? I'm running into similar issues on a different project. My current solution is to use the propeller tables in the air file to produce the correct airspeeds at different power settings using the full boost values found in the manuals, but it's looking like this may not work out like I hoped.
Also, what is your understanding of the Spitfire's boost system? When I did the Mk.VIII FM I allowed +18lbs at full throttle without using CFS3's emergency boost function, believing this to be consistent with how the system operated (though with the drawback of being able to run +18lbs continuously, though the main issue at the time seems to have been managing radiator temps, since the engine was proven to be able to handle the pressure for fairly long periods). The earlier single-speed Merlins had a boost control cutout that had to be activated separately, though I've not been able to determine if full emergency boost was activated all at once when it was activated by moving the throttle past it, or gradually by means of extended throttle travel. What's your take?
Also, what is your understanding of the Spitfire's boost system? When I did the Mk.VIII FM I allowed +18lbs at full throttle without using CFS3's emergency boost function, believing this to be consistent with how the system operated (though with the drawback of being able to run +18lbs continuously, though the main issue at the time seems to have been managing radiator temps, since the engine was proven to be able to handle the pressure for fairly long periods). The earlier single-speed Merlins had a boost control cutout that had to be activated separately, though I've not been able to determine if full emergency boost was activated all at once when it was activated by moving the throttle past it, or gradually by means of extended throttle travel. What's your take?
greycap.raf
Charter Member
I usually set the correct "full throttle" boost, then adjust the engine to produce the correct power, and then use as much boost as it takes to produce the correct emergency power. I think my Merlin 63 and 66 are somewhere around +16½lbs when full emergency power is engaged while they should be at +18lbs but since I'm getting the correct power, so be it. The very noticable increase in charge temperature causes quite an efficiency loss but CFS3 doesn't know to handle that, it only understands losses as a function of rpm which remains the same. Back in the days of the Bf 109G/K series I used correct boost pressures for both full throttle and emergency power, as a result the engines were slightly down on power at full throttle. It's a matter of less than 50 bhp anyway so perhaps even falling into the production tolerances of wartime engines.
The throttle lever on the Spitfire had a "gate", probably some kind of a friction stop, to indicate the transition from "normal" boost to overboost. It wasn't all or nothing, if the pilot wanted for example +16lbs on an engine rated at +12/+18lbs it was possible, the throttle was simply pushed through the gate but not firewalled. There were actual restrictive devices to prevent the full travel (and thus full boost) from being used when engines modified for +25lbs were, for one reason or another, running on the normal 100/130 grade fuel instead of the 100/150 grade rocket stuff.
The throttle lever on the Spitfire had a "gate", probably some kind of a friction stop, to indicate the transition from "normal" boost to overboost. It wasn't all or nothing, if the pilot wanted for example +16lbs on an engine rated at +12/+18lbs it was possible, the throttle was simply pushed through the gate but not firewalled. There were actual restrictive devices to prevent the full travel (and thus full boost) from being used when engines modified for +25lbs were, for one reason or another, running on the normal 100/130 grade fuel instead of the 100/150 grade rocket stuff.
gecko
Charter Member
How are you measuring/calculating bhp? I've never attempted to, just plugged in the published numbers and limits for the engine in question and adjusted the prop to match. But maybe dialing that in would help.
I'm not sure what you mean by this: "but CFS3 doesn't know to handle that, it only understands losses as a function of rpm which remains the same."
With a constant speed prop a loss in power needs to be quite significant to cause an RPM drop while in in the air in real life (static ground runs are another matter) because the prop governor will adjust the prop blades to a finer pitch to compensate and maintain the same RPM. A drop will only occur after the blade reaches the fine pitch stop, which will only happen at very low airspeeds. With a constant speed prop, the increase in speed or climb rate that comes with increasing manifold pressure is ultimately due to the propeller blades being turned to a coarser pitch by the governor, which causes them to create more thrust. So I guess it goes back to my question about calculating bhp, I guess you're saying the increase from +12 to +18 or +25 produces too much bhp. So how are you determining that?
I'm not sure what you mean by this: "but CFS3 doesn't know to handle that, it only understands losses as a function of rpm which remains the same."
With a constant speed prop a loss in power needs to be quite significant to cause an RPM drop while in in the air in real life (static ground runs are another matter) because the prop governor will adjust the prop blades to a finer pitch to compensate and maintain the same RPM. A drop will only occur after the blade reaches the fine pitch stop, which will only happen at very low airspeeds. With a constant speed prop, the increase in speed or climb rate that comes with increasing manifold pressure is ultimately due to the propeller blades being turned to a coarser pitch by the governor, which causes them to create more thrust. So I guess it goes back to my question about calculating bhp, I guess you're saying the increase from +12 to +18 or +25 produces too much bhp. So how are you determining that?
ndicki
Charter Member 2016
It's meant to... As with the other Spitfire I've released, if it carries a squadron code and/or an individual letter, I use that to identify it. I only use the s/n if it does not have an individual letter.
Fair cop... I think we can all do the modification.
ndicki
Charter Member 2016
It isn't. Shouldn't be there...
