EDIT: I saw my mistake... I was at way too low altitude for the RPM make an effect. But at altitude, Accusim reproduces everything perfectly. Sorry for the confusion. Can't wait having these birds in MSFS! We need this kind of quality over there.
Sorry for the confusion.
Hi guys, I have a question on the behavior of the direct drive coupled supercharger in the Spitfire I/II.
I noticed that when I set MAP on the throttle governor, the MAP indicated is not affected by the rpm of the engine. My understanding is that the faster the engine shaft spins, the faster the blower turns, hence I would expect a rise in MAP. Also I should not be able to reach high MAP boost on low rpm.
When you read here about Rolls-Royce Merlin Automatic Boost Regulator:
https://www.enginehistory.org/Piston/Ro ... nABC.shtml
Then for instance a 2.3:1 compression requires the blower to spin at 24'000 rpm, which in turn requires the engine shaft to spin at 2600 rpm. At lower rpm it will (exponentially so) I would then expect less comression/boost as the blower will spin proportionally slower. Hence I undertand that the maximum boost obtainable will be lower the slower the engine spins. You can see also a rpm requirement in obtaining specific boost at altitude in Fig.9 of the report linked above.
Why do I not see a reduction of indicated boost by reducing rpm? Why is boost about constant anywhere between 1800 and 3000 rpm of the engine shaft? Is this compensated by the aneroid that stabilizes boost (vs ambient air pressure, I assumed) above a throttle setting of 0 boost? But how could it do so? When I am flying down low at +8 boost, going down to 1800 rpm will not produce +8 boost when it is geared to do +12. So maybe it could maybe 4 or so (I'm guessing). And there is no way opening the throttle plate to get positive boost that I can think of?
Is this like it should be, then why? Or can it be a sim limitation?
P.S.: I am aware the in non aspirated engines, MAP will remain constant when playing with the prop pitch governor. At the price that 0 boost is the most I can get.
Question on superchargers in the Spitfire - SOLVED
-
Killratio
- A2A Spitfire Crew Chief
- Posts: 5785
- Joined: 29 Jul 2008, 23:41
- Location: The South West of the large island off the north coast of Tasmania
- Contact:
Re: Question on superchargers in the Spitfire - SOLVED
Hi FHS,
I see you have marked as “solved” but here is my twopenethworth anyhow
You were operating on the assumption that the power produced by the supercharger is proportionate to the RPM because it is direct drive. More rpm = more compression = more boost.
That is not how the system works. Whilst more rpm = more compression (and THAT is proportionate), it does not have that extra connection to the “boost” part of the equation in that way.
The reason for this is faulty assumption number two… that the supercharger is GEARED to produce +12 boost at 2600rpm. It isn’t. It is geared to produce the amount of compression required to produce +12 boost at FTH at 2600rpm. In fact at many regimes of flight both above AND below FTH the super charger is happily spinning away contributing absolutely nothing to boost. If the current boost setting is below that which the engine can deliver without the supercharger helping, the aneroid is “cutting out” any extra power the supercharger would otherwise be delivering. If you increase RPM and the supercharger supplies greater pressures, they are being wasted by the throttle plate being closed more. If you reduce RPM and the supercharger delivers less power, the aneroid is opening the plate and the engine is generating extra boost. RPM changes and the boost stays the same. This will work at any time the supercharger is adding less power than it is capable of at the current throttle setting.
The argument was often seen with the CLoDs crowd (I LOVE calling them CLoDs
) who took car Turbos/superchargers as a reference point. The difference being that an aircraft supercharger is primarily designed to maintain engine power as altitude increases (absent takeoff power considerations) not to generate huge amounts of power at ground level. That is why you see two stage superchargers in aircraft, to optimize even more for differing altitudes.
Regards
Darryl
I see you have marked as “solved” but here is my twopenethworth anyhow
You were operating on the assumption that the power produced by the supercharger is proportionate to the RPM because it is direct drive. More rpm = more compression = more boost.
That is not how the system works. Whilst more rpm = more compression (and THAT is proportionate), it does not have that extra connection to the “boost” part of the equation in that way.
The reason for this is faulty assumption number two… that the supercharger is GEARED to produce +12 boost at 2600rpm. It isn’t. It is geared to produce the amount of compression required to produce +12 boost at FTH at 2600rpm. In fact at many regimes of flight both above AND below FTH the super charger is happily spinning away contributing absolutely nothing to boost. If the current boost setting is below that which the engine can deliver without the supercharger helping, the aneroid is “cutting out” any extra power the supercharger would otherwise be delivering. If you increase RPM and the supercharger supplies greater pressures, they are being wasted by the throttle plate being closed more. If you reduce RPM and the supercharger delivers less power, the aneroid is opening the plate and the engine is generating extra boost. RPM changes and the boost stays the same. This will work at any time the supercharger is adding less power than it is capable of at the current throttle setting.
The argument was often seen with the CLoDs crowd (I LOVE calling them CLoDs
) who took car Turbos/superchargers as a reference point. The difference being that an aircraft supercharger is primarily designed to maintain engine power as altitude increases (absent takeoff power considerations) not to generate huge amounts of power at ground level. That is why you see two stage superchargers in aircraft, to optimize even more for differing altitudes. Regards
Darryl
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>
Re: Question on superchargers in the Spitfire - SOLVED
Hi Darryl
I marked my topic as "solved" as I realized that I far underestimated the altitude required for the Spit II to make MAP drop by reducing RPM. When I repeated it higher up, it became evident that things are as they must be with the Spit and that I made an error in my initial testing.
I do however appreciate your comment, as you highlighted that I indeed oversimplified things in a way that made me do a faulty experiment (not enough altitude) to check for the drop in MAP when reducing revs.
You are spot on by stating that there's more to a blower producing *pressure" than its rpm. This way the Spitfire Mk.V LF (often referred to as Spitfire Mk.VD) can achive higher boost pressure down low, but due to the smaller diameter of the rotor, the blower capacity drops faster with decreasing ambient pressure compared to the standard Spitfire Mk.Vc. (And crews hated that aircaft for that.)
Where one can see the effect is above critical altitude of the blower, as there, the wastegate is always closed and all the boost you see on the MAP indicator is what the supercharger can provide. This Accusim Spitfire shows that nicely. Once I went up high enough.
Having said that, I would like to point out where I'm coming from when doing such a test. And that is that beautiful (one and only so far) quad prop plane we have now for MSFS. (I don't really use P3D anymore, less because of looks, but performance. Other than that I also use DCS, and BoX more or less regularly and own most of their offerings.) It's a gorgeous plane, great fun as well.
But in that one, when you climb high above critical altitude of the blower, you find that when I am high enough (FL225, blower first gear) that full trottle gives me only 30 inches MAP (it does about 60 on ground). Then up there, anywhere between 1500 rpm and 2300 rpm I get 30 inches MAP. Funny enough, when I rev up past 2500, then power output drops and at 2800 rpm (max.) engine power output essentially collapses. (Probably the power requirement for 2800 rpm exceeds available power at a constant 30 inches.) Below 1500 rpm, the engine for obvious reasons doesn't want to run properly anymore. And I think this is due to a limitation of how MSFS currently can handle superchargers. They basically act as increased ambient air pressure with regards to the engine that then does as if it was a non-aspirated engine. It also makes the engines generally produce more thrust at lower rpm as soon as you get to altitude.
What I want to do if I go really high, I want to rev up in order to get as much air in my engine as possible and I will not hurt my engine that much with doing so. And in that MSFS aircraft I mentioned, high up you cannot use revving up the engine to get the last bit of air or maximize your engine performance to your needs above critical height. You even reduce your power output by doing so. Hence, I wanted to backcheck with how A2A do superchargers.
But please, don't see that as me shaming a competing developpers product. I am certain they know exactly how things really work with that aircraft and I think what we see is a limitation of MSFS (or one of those) that keeps Scott from releasing aircraft for MSFS.
Anyway, I apreciated your input Darryl! Thanks!
Cheers,
Florian
I marked my topic as "solved" as I realized that I far underestimated the altitude required for the Spit II to make MAP drop by reducing RPM. When I repeated it higher up, it became evident that things are as they must be with the Spit and that I made an error in my initial testing.
I do however appreciate your comment, as you highlighted that I indeed oversimplified things in a way that made me do a faulty experiment (not enough altitude) to check for the drop in MAP when reducing revs.
You are spot on by stating that there's more to a blower producing *pressure" than its rpm. This way the Spitfire Mk.V LF (often referred to as Spitfire Mk.VD) can achive higher boost pressure down low, but due to the smaller diameter of the rotor, the blower capacity drops faster with decreasing ambient pressure compared to the standard Spitfire Mk.Vc. (And crews hated that aircaft for that.)
Where one can see the effect is above critical altitude of the blower, as there, the wastegate is always closed and all the boost you see on the MAP indicator is what the supercharger can provide. This Accusim Spitfire shows that nicely. Once I went up high enough.
Having said that, I would like to point out where I'm coming from when doing such a test. And that is that beautiful (one and only so far) quad prop plane we have now for MSFS. (I don't really use P3D anymore, less because of looks, but performance. Other than that I also use DCS, and BoX more or less regularly and own most of their offerings.) It's a gorgeous plane, great fun as well.
But in that one, when you climb high above critical altitude of the blower, you find that when I am high enough (FL225, blower first gear) that full trottle gives me only 30 inches MAP (it does about 60 on ground). Then up there, anywhere between 1500 rpm and 2300 rpm I get 30 inches MAP. Funny enough, when I rev up past 2500, then power output drops and at 2800 rpm (max.) engine power output essentially collapses. (Probably the power requirement for 2800 rpm exceeds available power at a constant 30 inches.) Below 1500 rpm, the engine for obvious reasons doesn't want to run properly anymore. And I think this is due to a limitation of how MSFS currently can handle superchargers. They basically act as increased ambient air pressure with regards to the engine that then does as if it was a non-aspirated engine. It also makes the engines generally produce more thrust at lower rpm as soon as you get to altitude.
What I want to do if I go really high, I want to rev up in order to get as much air in my engine as possible and I will not hurt my engine that much with doing so. And in that MSFS aircraft I mentioned, high up you cannot use revving up the engine to get the last bit of air or maximize your engine performance to your needs above critical height. You even reduce your power output by doing so. Hence, I wanted to backcheck with how A2A do superchargers.
But please, don't see that as me shaming a competing developpers product. I am certain they know exactly how things really work with that aircraft and I think what we see is a limitation of MSFS (or one of those) that keeps Scott from releasing aircraft for MSFS.
Anyway, I apreciated your input Darryl! Thanks!
Cheers,
Florian
-
Killratio
- A2A Spitfire Crew Chief
- Posts: 5785
- Joined: 29 Jul 2008, 23:41
- Location: The South West of the large island off the north coast of Tasmania
- Contact:
Re: Question on superchargers in the Spitfire - SOLVED
No problem at all Florian,
It is a fascinating subject and all the more interesting for the aneroid being inserted into the process.
The mixture, I have to add, is also automatic (albeit with a choice of "starting point" in the Spitfire I/II/V.)
which further complicates the practical workings of the system (or at least the OUTPUT) of the system.
If you also try your experiment with the fixed pitch airscrew, you will get a slightly different set of effects
from memory and this is this is to do with propeller efficiency. The effect is not on boost but rather on
proportionate differences in speed. I do seem to recall but we are talking over ten years ago now in testing!
Yes, pilots loved the "clipped" hated the "cropped'. I've even heard it referred to as "Clipped, Cropped and Crapped" rather than the more polite "Clipped Cropped and Clapped":)
regards
Darryl
It is a fascinating subject and all the more interesting for the aneroid being inserted into the process.
The mixture, I have to add, is also automatic (albeit with a choice of "starting point" in the Spitfire I/II/V.)
which further complicates the practical workings of the system (or at least the OUTPUT) of the system.
If you also try your experiment with the fixed pitch airscrew, you will get a slightly different set of effects
from memory and this is this is to do with propeller efficiency. The effect is not on boost but rather on
proportionate differences in speed. I do seem to recall but we are talking over ten years ago now in testing!
Yes, pilots loved the "clipped" hated the "cropped'. I've even heard it referred to as "Clipped, Cropped and Crapped" rather than the more polite "Clipped Cropped and Clapped":)
regards
Darryl
<Sent from my 1988 Sony Walkman with Dolby Noise Reduction and 24" earphone cord extension>
Who is online
Users browsing this forum: Google [Bot] and 9 guests