Spitfire engine control management

[Finkeren]

So the modeling is correct then. I thought so, but LukeFF made me uncertain.

[JtD]

No, modelling is wrong. Just tried it in game.

 

With abc cut out boost stays at 16lb fixed independent of altitude at any setting above 30% throttle. Wrong.

With abc working, at high altitude (25k in my test), changing throttle from 75-100% has no impact at boost. Wrong.

The two errors, however, are consistent. It's not how I think the abc and the cut off worked, though.

 

It's fundamental to understand that the pilot with the throttle lever gives a certain throttle valve position from which the abc can only reduce the throttle valve opening. It cannot open the throttle any further. As such, it is wrong that having the throttle at 100% at high altitude gives the same boost as having the throttle at 75%. The game is assuming the abc opens the throttle to 100% when the lever is set to 75%, which it doesn't.

The manual is pretty clear that only with the throttle set to maximum (i.e. rated boost) can the regulator open the throttle valve fully.

 

Cutting out the abc completely runs the abc control mechanism against the high end stop and leaves throttle valve position solely dependent on the throttle lever position.

[Farky]

Imho, what we should have when using the abc-cut off, is P-40E behaviour limited to 16lbs boost - so the upper range of the throttle would be useless at low altitudes when 16lbs are reached, but throttle should directly control boost with lower settings or at higher altitudes.

...

The manual is pretty clear that only with the throttle set to maximum (i.e. rated boost) can the regulator open the throttle valve fully.

 

 

In early models of Merlin (around Battle of Britain time frame), using Automatic Boost Control Cut-Out indeed resulted in "P-40E behavior limited to maximum emergency boost". In case of Merlin 45 and 46, it was different. Here are some pages from Merlin 45/46/55 manual. Already posted to testers and hopefully also developers, because behavior in game is not correct.

 

ABC Cut-Out should fully opens throttle valve with throttle lever at or beyond the gate (49% throttle in game), not at 30% of throttle like is it now. Throttle in Spitfire V was set to maximum (rated boost) at the gate, not at fully forward position.

 

Throttle lever at the gate (49% throttle in game) from S.L. up to Full Throttle Height should always result in +9 lbs boost, in case when ABC Cut-Out is not activated of course.

 

 

[JtD]

How would the gate be at 49%? You're speaking of the graphics in game, right? That's around 0 lb in game. The gate should be, as you say, at 9lb. It should also be somewhere at around 80% travel, I think, given that there was a lot more lever travel before than there was past the gate. The game doesn't make sense here. Thanks for having me take a look at that.

 

I don't know where exactly we disagree, if we disagree, but at all throttle setting below the gate, the Merlin with abc cut off operates like the P-40E we already have, as the pilot directly opens or closes the throttle. The cut off will still leak boosted pressure into the suction side and thereby cause higher boosts, but won't reach 16lb any more. Due to the different throttle lever position the position of the piston valve (7) will reach the neutral position at lower boosts (balance of spring loaded throttle input against boost pressure in 5 working on the aneroid (6)) than it would with the throttle at the gate.

 

I don't see why the cut off would not work at settings below the gate (even if lb depended on throttle lever position and limited to not fully open throttle valve), and I agree that above the gate it's a fixed 16lb boost with a potentially fully open throttle valve.

 

Edit: Ah, sorry. I think I got your point now. I'm quite tired today so if my above statements don't make sense (I know some don't), just ignore them. You're saying that even with abc cut off, the Merlin 45 & 46 as we have in game still have some altitude correction/constant boost function, as opposed to the Allison in the P-40E, which has none.

Edited July 8, 2017 by JtD

[Farky]

 You're saying that even with abc cut off, the Merlin 45 & 46 as we have in game still have some altitude correction/constant boost function, as opposed to the Allison in the P-40E, which has none.

 

Yes, exactly.

And I agree 100% with your points, for example gate should be imho at higher % of throttle.

[ZachariasX]

How would the gate be at 49%? You're speaking of the graphics in game, right? That's around 0 lb in game. The gate should be, as you say, at 9lb. It should also be somewhere at around 80% travel, I think, given that there was a lot more lever travel before than there was past the gate. The game doesn't make sense here. Thanks for having me take a look at that.I don't know where exactly we disagree, if we disagree, but at all throttle setting below the gate, the Merlin with abc cut off operates like the P-40E we already have, as the pilot directly opens or closes the throttle. The cut off will still leak boosted pressure into the suction side and thereby cause higher boosts, but won't reach 16lb any more. Due to the different throttle lever position the position of the piston valve (7) will reach the neutral position at lower boosts (balance of spring loaded throttle input against boost pressure in 5 working on the aneroid (6)) than it would with the throttle at the gate.I don't see why the cut off would not work at settings below the gate (even if lb depended on throttle lever position and limited to not fully open throttle valve), and I agree that above the gate it's a fixed 16lb boost with a potentially fully open throttle valve.Edit: Ah, sorry. I think I got your point now. I'm quite tired today so if my above statements don't make sense (I know some don't), just ignore them. You're saying that even with abc cut off, the Merlin 45 & 46 as we have in game still have some altitude correction/constant boost function, as opposed to the Allison in the P-40E, which has none.

The Cut-Out switch just opens a bypass valve for the regulator, allowing higher boost. See here:

http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml

 

The throttle is correctly modeled with the Cut-Out as we have it.

Edited July 9, 2017 by ZachariasX

[ZachariasX]

 

 

ABC Cut-Out should fully opens throttle valve with throttle lever at or beyond the gate (49% throttle in game), not at 30% of throttle like is it now. Throttle in Spitfire V was set to maximum (rated boost) at the gate, not at fully forward position.

 

Why should the Cut-Out only be active, when the throttle lever is at the gate? As I understand it, it is a simple bypass valve, telling the aneroid to make the engine pump more air (at given position of the pilots control lever of the governor, the "throttle") by opening the throttle valve a tad more, such that it is able to reach the higher max. boost. when the lever is at the gate. If you have the throttle farther back than the gate, max. boost is not reached, proportionately dropping boost to you prgressively throttling more. This is what we have in game.

 

Thus, I would say the Cut-Out switch doesn't disengage anything, it just sets boost pressure limits differently.

[ZachariasX]

 

 

the Merlin 45 & 46 as we have in game still have some altitude correction/constant boost function, as opposed to the Allison in the P-40E, which has none.

 

The Merlin just has a correrction for for you not exceeding max. boost. What it absolutely doesn't do is keeping boost constant when climbing (or descending) unless you are at max. boost. Then you have constant (max.) boost until critical altitude is reached, just because the regulator will not allow you to exceed max boost below critical altitude of the blower, and not because you're "corrected for altitude".

[JtD]

There are many errors with the throttle and the ABC as it is now:

 

49% is the wrong throttle position at which the gate is bypassed. It should be around 80%+.

0lb is the wrong boost at which the gate is bypassed. It should be 9lb, i.e. rated boost.

Boosts below rated boost should not be maintained until full throttle altitude by the ABC, it should be necessary to manually adjust boost after a certain altitude is reached. ABC cannot open the throttle fully at throttle lever positions lower than the gate.

Therefore, at high altitudes above full throttle altitude, reducing throttle from full to lower settings should reduce boost.

At throttle positions at the gate and above, operating the ABC cut out will result in boosts of 16lb, maintained over altitude until full throttle altitude.

At throttle positions below the gate, operating the ABC cut out will result in boosts of less than 16lb, maintained over altitude, but not up to full throttle altitude.

 

Really, throttle valve position equals throttle lever position MINUS ABC correction. If the ABC does not reduce boost, the relay (13) is at the full stop to the left in the diagram. It cannot go further left and give extra boost beyond the setting of the throttle lever position. So at this left stop position, throttle valve position equals throttle lever position, and throttle lever at the gate means fully open throttle valve. Lower lever settings mean less opened valve, i.e. lower boost at low altitude and lower full throttle altitudes (since it's not full throttle).

The piston (13) goes right and closes the throttle valve at low altitudes, where boost exceeds pre-set values, which depend on the throttle lever position (input via cam (21)). The pre-set come from the balance of spring forces (20,23) working against boost (5) and standard pressure (19) effecting an aneroid (6). Any off balance situation moves the control valve (7), admitting boosted pressure to either side (via 11 or 12) of the piston (13) so that the piston (13) is moved, which in turn moves the throttle valve (15), so that a constant boost pressure is being maintained up until the piston is in the extreme left position at high altitude.

By operating the cut out you leak boosted pressure from the aneroid boost chamber (5) - you throw off the balance which is pre-set to achieve a manifold pressure of 9lb at the gate. Depending on the size of the leak hole (31), the new balance will be reached with the piston (13) having moved left some way, opening up the throttle valve (15) in the process, to achieve a higher boost. In our case the leak hole has a size so that things settle at a new balance of 16lb if boost is at 9lb originally, and the throttle lever in the gate position.

It should be obvious that with a lower throttle lever position and a resulting balance in the boost regulator at a lower boost pressure of say 6lb, the cut off valve leaking (less) boosted pressure through the same sized leak hole, will not cause the piston to move further and the throttle to be opened further than it was at 9lb.

 

Edit: WRT keeping boost constant over altitude, read the paragraph from the manual already posted, reposted.

Edited July 9, 2017 by JtD

[HagarTheHorrible]

What it absolutely doesn't do is keeping boost constant when climbing (or descending) unless you are at max. boost.

 

I thought that is exactly what point 53 said it did do ?  "determined by the cockpit throttle lever"

Edited July 9, 2017 by HagarTheHorrible

[ZachariasX]

 

 

Edit: WRT keeping boost constant over altitude, read the paragraph from the manual already posted, reposted.

 

 

I thought that is exactly what point 53 said it did do ? "determined by the cockpit throttle lever"

 

Point taken. All the better. I'll have to go though the mechanics again, but thnx for your detailed explanation of your point JtD.

[Farky]

Why should the Cut-Out only be active, when the throttle lever is at the gate?

 

 

Misunderstanding. I wasn't talking about activation, but about the point where +16lbs boost is possible. ABC Cut-Out can be of course activated at any time, regardless position of throttle lever.

 

 

If you have the throttle farther back than the gate, max. boost is not reached, proportionately dropping boost to you prgressively throttling more. This is what we have in game.

 

 

We have this in game, kinda, but not quite correct. The throttle lever in game begins to influence boost at 29% of throttle or less, gate is visually at 49% of throttle, functionally at some 90% of throttle (throttle lever position for +9 lbs boost).

 

 

The Merlin just has a correrction for for you not exceeding max. boost. What it absolutely doesn't do is keeping boost constant when climbing (or descending) unless you are at max. boost. Then you have constant (max.) boost until critical altitude is reached, just because the regulator will not allow you to exceed max boost below critical altitude of the blower, and not because you're "corrected for altitude".

 

ABC is keeping boost constant when climbing, well, to some extent -

 

To appreciate the limitations of control of lesser boost pressures, consider the example of take-off and climb at +4 lb/sq.in boost. With engine at slow running, pilot's lever is moved to position on quadrant giving +4 psi boost, this being only part of the possible range on quadrant, throttles will only partially open and cam "Y" will only partly depress the sensitive aneroid. Regulator will again control, but owing . to throttles partial opening by pilot's lever, relay movement available to correct is reduced. On climb, boost pressure will be maintained by return of relay piston to forward position which continues until piston contacts forward stop. Boost pressure would then fall with increase of altitude, although throttles are not wide open; to continue the climb at +4 lb boost is only possible by manual operation of the pilot's lever. When this is at full forward position on quadrant, throttles are wide open and at higher altitudes the boost pressure will necessarily fall.

 

From here - http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml

 

 

There are many errors with the throttle and the ABC as it is now:

49% is the wrong throttle position at which the gate is bypassed. It should be around 80%+.

0lb is the wrong boost at which the gate is bypassed. It should be 9lb, i.e. rated boost.

Boosts below rated boost should not be maintained until full throttle altitude by the ABC, it should be necessary to manually adjust boost after a certain altitude is reached. ABC cannot open the throttle fully at throttle lever positions lower than the gate.

Therefore, at high altitudes above full throttle altitude, reducing throttle from full to lower settings should reduce boost.

At throttle positions at the gate and above, operating the ABC cut out will result in boosts of 16lb, maintained over altitude until full throttle altitude.

At throttle positions below the gate, operating the ABC cut out will result in boosts of less than 16lb, maintained over altitude, but not up to full throttle altitude.

 

I agree with all points. One more thing -

At throttle position beyond the gate, in situation when ABC Cut-Out is not activated, boost should be +12 lbs.

[ZachariasX]

ABC is keeping boost constant when climbing, well, to some extent - To appreciate the limitations of control of lesser boost pressures, consider the example of take-off and climb at +4 lb/sq.in boost. With engine at slow running, pilot's lever is moved to position on quadrant giving +4 psi boost, this being only part of the possible range on quadrant, throttles will only partially open and cam "Y" will only partly depress the sensitive aneroid. Regulator will again control, but owing . to throttles partial opening by pilot's lever, relay movement available to correct is reduced. On climb, boost pressure will be maintained by return of relay piston to forward position which continues until piston contacts forward stop. Boost pressure would then fall with increase of altitude, although throttles are not wide open; to continue the climb at +4 lb boost is only possible by manual operation of the pilot's lever. When this is at full forward position on quadrant, throttles are wide open and at higher altitudes the boost pressure will necessarily fall. From here - http://www.enginehis...erlinABC.shtml

Reading it again, (it was early before on a sunday ) I see that point. It is however still a regulation of "lesser boost pressures" and not a device that makes you giving a set value and the goveror will always hold that. It can only do so in a smaller range. (Can I call it a "Diet Kommandogerät"?) It works within a narrower window (although a useful one) and most importantly it does keep you from blowing your engine in power on dives. With increasing boost however, the range where the ABC can control your set MAP increases as well, making it "less diet".

 

So I wasn't really thinking of it as a device to hold MAP constant, but as a safety device that takes care of your engine. (One can learn things even on a sunday morning in forums, should one choose to do so )

[unreasonable]

 (One can learn things even on a sunday morning in forums, should one choose to do so )

 

If only not to drink quite so much on a Saturday night....

[ZachariasX]

If only not to drink quite so much on a Saturday night....

^^

[Gavrick]

Thanks to everyone for the technical details found. Some materials I have not met before. As time will be, I will study and take into account.

[JtD]

Looking forward to it.

 

Personally I think it's great we can have discussions about this level of detail. 'Complex' used to be 0-100% on a slider and a W for WEP.

[BlitzPig_EL]

This is a great example of how all technical threads should work.

 

Well done gents.

[ShamrockOneFive]

I can't pretend to say that I fully understand all of the mechanics involved here but I wanted to say kudos for this really interesting and informative thread. Great details, primary source materials and a real academic discussion that may even be valuable towards further enhancing the accuracy of the Spitfire.

 

 

This is a great example of how all technical threads should work.

 

Well done gents.

 

Completely agree!

Wonderful.

[unreasonable]

I agree, (not only about the not fully understanding part   ) - and also +++ Gavrick for being so prompt to see the discussion and taking an open minded view.  

Edited July 10, 2017 by unreasonable

[ZachariasX]

Fantastic Gavrick! That's cool!

 

 

Well then, for the lesser technicians like me, I’d like to recapitulate then what it should do. I have gone through your list of details, JtD.

 

So I’d like to sum up the workings of the mechanics as I understand it. Feel free to correct me (so I or someone else can learn).

 

With the Merlin, we have a set of what are at first glance contradictory set of engine controls. We have:

• A boost control system (abc)
• Throttle gate (above setting for rated power)
• The ominous Boost Cut-Out Override Switch

 

Now why would have one both a cut-out switch and a throttle gate? Both seemingly installations made for keeping you from blowing your engine.

 

Looking how the came in this world, it becomes evident why we have them as such:

 

First, there was a direct linkage between throttle lever (it was really that, like in our P-40), where you set the carburetor opening, resulting in a manifold pressure (MAP) that can be read on the respective gauge.

 

Taking the example of a  Merlin with compression ratio of 2.3:1, you can get about 20 psi positive boost at see level by moving the throttle all the way forward. If your engine starts pre-detonating beyond 9 psi positive boost (about 24 psi total pressure, assuming 15 psi outside pressure), it means you can, assuming linear (witch it isn’t) MAP increase, move the throttle lever forward 2/3 of the travel and you have max. permissible MAP.

 

This is your take-off power. When you take off and climb, you can say, for the allowed 3 minutes I continue to move the lever a bit forward so that the MAP will remain at +9 psi boost. You have to use both hands to remain at full power, one on the stick, one on the throttle (apart form occasional re-trimming).

To keep the pilot from working delicately on the red line of the engine limits and making him less work the throttle to keep climbing power, the automatic boost control (abc) was introduced. It is a cylinder, the aneroid, that senses the pressure produced by the blower. This cylinder is linked to the throttle control, or say the pilots governor to adjust MAP. In between both max. and min. position of the cylinder, it can further open or further close the carburetor, thus modulating the pilots input in both ways. (Contrary to what I said above.)

 

How does that work: Taking the example of a plane sitting on the ground for take off, assuming again 15 psi outside pressure and a rated power of +9 psi boost, a total of 24 psi MAP. If I moved the throttle all the way forward with nothing keeping me from doing that, I would reach 34 psi boost and blowing the engine instead of taking off. Now this cylinder, in the full “back” position with 15 psi ambient pressure, counteracts the throttle opening via linkage and lets it only open such that it produces 9 psi boost (24 psi MAP). Thus, the aneroid cylinder can decrease MAP over a range of 10 psi (34 psi – 24 psi = 10 psi). This way, I can happily shove the throttle all the way forward to get +9 psi rated boost and take off.

While I’m climbing ambient pressure is dropping and the cylinder moves gradually forward from the “back” position until it reaches the full “forward” position. As I am still (naughty me) climbing with the throttle lever all the way forward, at some point I will have reached critical altitude and boost (MAP) will start to drop.

 

In this example with a gearing for 2.3:1 compression,  this will be at:

 

24 / 3.3 = 10.25 [psi],  meaning about 3 km altitude or so.

 

This also means that if I set for +4.5 instead of +9 psi boost after take off, the piston of the aneroid is less “back” and consequently it will reach “forward” position sooner. This means, I will hit critical altitude before I hit it when setting throttle for +9 psi. Thus we can say:

 

The less throttle I give (less boost), the smaller is the range that the system can help me and walk the throttle for me during climbing (vice versa when sinking), maintaining set boost/MAP. In this sense, yes it does keep boost constant, but within a certain window of boost and atmospheric pressure.

 

Now, we the abc. But there is also the “gate”. Why having a gate when you have the automatic system?

 

Now, taking off and landing are (besides getting hot at) the most dangerous parts of the flight, as they are closest to the ground. Keep in mind, plane crashes always are a result of insufficient space between the aircraft and the ground! Making that process of using this little extra power that you can only use for a short time more accessible (easier to safely use) to the pilot, the gate system comes to help. What does the gate do?

Lets assume the boffins at Rolls-Royce say, “Ok, for just take off, you can use +12 psi boost. But just for that, 3 minutes, else you pay engine maintenance yourself!”

To make the correct use easy, the came up with “the gate”: It is a position forward of the range that the abc can control. In this example, the abc will act that it can reduce the boost for 10 psi, resulting in +9 psi with the throttle lever fully forward at sea level. They added a notch allowing the pilot to move the throttle lever even more forward, beyond what the abc’s limits, allowing 3 psi more boost.

 

Now, when taking off, I can move the throttle lever all the way forward, reaching 9 psi (where it is capped by the abc), then pushing it through the gate, opening the throttle valve in the carburetor a tad more, and getting another 3 psi for a total of +12 psi boost. With this, I can take off safely and after 3 minutes of permitted operating time at 12 psi, when climbing, I will have lost the 3 psi that I was getting anyway very quickly and withing allowed time for +12 psi and I will have reached +9 psi boost where the abc can keep it until I’ve reached critical altitude. Again, I can just shove the lever all teh way forward and I'm not blowing my engine, but this time I even get a little extra power as well.

 

 

Now, with having controlled boost and the gate, why the need for the boost cut-out override switch?

 

First, it was a fail safe device. When they installed the aneroid first, they didn’t trust it enough to not give the pilot an option to shut it off, basically giving the pilot full direct control over the MAP should he want that. Same as in an early P-40 (as we have it in the game).

It is like this in the MkI and early MkII Spitfires. The little lever is connected to a cable that pulls a valve open for complete bypass of the abc. In the Hurricane AFAIK it is a red knob that can be pulled (“pulling the dyke by the tits”) and it got some popularity amongst the pilots as it allowed them to use boost pressured beyond the allowed margins up to where they themselves heard bad noises coming from the engine. Thus, a fail safe system was abused for getting more power. No wonder.

 

How exactly the abused fail safe device turned into a system for power increase is not documented in detail (I didn’t find specific documents), but what happened is that peeps seemingly got very happy pulling the “tit” and it must have dawned to the boffins that there should be something done about that. And they did when the 100 octane fuel was introduced, allowing for higher boost.

What they did is the following: they changed the system such that pulling the “tit” didn’t completely bypass the abc anymore, but instead opened a valve that just increased the airflow in the system such that the aneroid cylinder is re-calibrated to higher boost values, e.g. +16 psi rated boost instead of  previously +9 psi rated boost, depending on the fuel you’re using. So, while the little lever kept his name, it did something it didn’t do before. With this, the pilot has another tool to use maximum power when he really needs it without being at risk of accidentally abusing the engine in normal flight modes.

 

In all, it’s a really cool system I think. But it is a throttle governor and NOT a Kommandogerät.

 

[edit: typos]

Edited July 10, 2017 by ZachariasX

[unreasonable]

 

In all, it’s a really cool system I think. But it this throttle governor is NOT a Kommandogrerät.

 

I think I almost understood that, it is a bit like school teachers telling schoolboys that they have to have short back and sides haircuts, then letting them grow their hair just over the collar.  The boys feel a sense of rebellious freedom, essential to their task of scoring girls, while the masters prevent the boys from growing their locks down to the shoulders, shooting heroin, reading Satre etc.

[YoYo]

Thank you.

I my game these are the default controls for the Bf 109/110 water radiator says nothing about spitfire.

Why can't we have one set of controls for engine management

+1.

 

I use also lever for water radiator. Please to connect it with the others.

[Farky]

Nice job ZachariasX, just few nitpicks -

 

 

How does that work: Taking the example of a plane sitting on the ground for take off, assuming again 15 psi outside pressure and a rated power of +9 psi boost, a total of 24 psi MAP. If I moved the throttle all the way forward with nothing keeping me from doing that, I would reach 34 psi boost and blowing the engine instead of taking off. Now this cylinder, in the full “back” position with 15 psi ambient pressure, counteracts the throttle opening via linkage and lets it only open such that it produces 9 psi boost (24 psi MAP). Thus, the aneroid cylinder can decrease MAP over a range of 10 psi (34 psi – 24 psi = 10 psi). This way, I can happily shove the throttle all the way forward to get +9 psi rated boost and take off.

 

 

It should be 35 psi (15 + 20), thus range of 11 psi ? And why even bother with MAP ?

Let's say that outside pressure is conveniently 14.7 psi and we want rated power of +9 psi boost. 14.7 psi is 0 boost (in standard atmosphere, is it equivalent of 29.92 inHg), our engine will produce +20 psi boost on full throttle, goal is +9 psi. 20 - 9 = 11. Therefore, the aneroid cylinder can decrease boost over a range of 11 psi.

 

 

First, it was a fail safe device. When they installed the aneroid first, they didn’t trust it enough to not give the pilot an option to shut it off, basically giving the pilot full direct control over the MAP should he want that. Same as in an early P-40 (as we have it in the game).

It is like this in the MkI and early MkII Spitfires. The little lever is connected to a cable that pulls a valve open for complete bypass of the abc.

... but what happened is that peeps seemingly got very happy pulling the “tit” and it must have dawned to the boffins that there should be something done about that. And they did when the 100 octane fuel was introduced, allowing for higher boost.

 

 

Complete full direct control over the MAP like in P-40E was the very first variant of ABC Cut-Out. When the 100 octane fuel was introduced, they modified Automatic Boost Control units so that the maximum possible boost was +12 lbs. They just drilled hole through the cut-out valve. After some time they change the ABC Cut-Out to system we know from Spitfire V..

 

 

[ZachariasX]

It should be 35 psi (15 + 20), thus range of 11 psi ? And why even bother with MAP ?

 

Thnx Farky. Well, that all has to do with my bias toward our english friends with their way of being charmingly impractical. And due to England not being known to me as an area of high pressure. So I rated the recognition value of the integer '14' in '14.7' higher than the correctly rounded integer, coming up with a more practical "atmosphere". I can make up units too!

 

Their units themselves, impractical as they are, are further enriched with this made up pressure unit called "boost". I mean, seriously. I can understand that the pint is such a great unit, that when you're dealing with that, you can easily skip lunch. And dinner. And breakfast. I can only imagine that in such a state a mentally sane person would come up with "boost", since you already made your life miserable by using units like pounds per square inch. It's like using "slaves per tug line" as a unit for force when building pyramids. You may also say, it is useful in that case too. (On the other hand they even built the pyramids like that, while the ones going metric can't even build an airport...) Hence my tendency to use MAP. Because that is what matters to the engine.

[Tag777]

Well gents, very informative thread indeed. And polite, that is not less important, seeing the "ammo caliber" of the discussions in another threads.

Now, I have perhaps a silly question for many of you, but anyways: how do you achieve for example 540 km/h (336 mph) at 7620 m (25,000 feet)?

I was testing the Merlin 46 last night, expert mode, 2850 rpm, +9 lb boost to climb, as indicated, but at that altitude the plane barely achieve 200 km/h (124 mph approximately). Even with the WEP activated and at 3000 rpm there was nearly none difference. Radiator was at 60%. I suppose I am doing something wrong, but what.

[unreasonable]

Tag777 do not forget to convert from IAS to TAS.

Edited July 11, 2017 by unreasonable

[Tag777]

Aha, ok, I did not consider that. Thanks.

[Tag777]

Ok, the value for TAS after the conversion is 186 mph.

[ZachariasX]

Ok, the value for TAS after the conversion is 186 mph.

At 25k feet? No way. That would hardly keep you flying.

[Tag777]

At 25k feet? No way. That would hardly keep you flying.

Yep, it was hard. I had to descend to regain some speed. So, I suppose that I managed the engine tweaks incorrectly, but I can not find what.

[ZachariasX]

just checked. I get about 240 mph on the dial at 25k feet which makes about 400 mph TAS. All well.

[unreasonable]

Just tested myself, I am getting about 250 on the speedo at 26,000ft.   Tag777 - you do know that the dial in the cockpit is mph, not kph? Sorry to ask but your result is so bizarre .... (is your gear still down )

 

Z what IAS/TAS converter are you using? The ones I have seen all suggest about 2% per 1000ft ie your 240 IAS ~ 360 TAS

Edited July 11, 2017 by unreasonable

[ZachariasX]

Just tested myself, I am getting about 250 on the speedo at 26,000ft.   Tag777 - you do know that the dial in the cockpit is mph, not kph? Sorry to ask but your result is so bizarre .... (is your gear still down )

 

Z what IAS/TAS converter are you using? The ones I have seen all suggest about 2% per 1000ft ie your 240 IAS ~ 360 TAS

One that lies to me on the internet, but that accepts temperature as well. Your rule of the thumb for sure is applicable. I didn't even try to get an exact reading, I just wanted to double check that it is quick up there, and it is.

[Tag777]

Just tested myself, I am getting about 250 on the speedo at 26,000ft.   Tag777 - you do know that the dial in the cockpit is mph, not kph? Sorry to ask but your result is so bizarre .... (is your gear still down )

 

Z what IAS/TAS converter are you using? The ones I have seen all suggest about 2% per 1000ft ie your 240 IAS ~ 360 TAS

Yes of course I know the Spit dial is in mph, not km/h. And yes, my gear was up 

I will try again this night. Some aspects that I did not consider were propeller pitch and mixture. I will check and comment my results.

 

Thank you all for the help 

Edited July 11, 2017 by Tag777

[Gambit21]

What are you guys doing with the mixture lever?

Leaving it alone until you reach a certain altitude, then moving it to the forward position?

[ZachariasX]

What are you guys doing with the mixture lever?

Leaving it alone until you reach a certain altitude, then moving it to the forward position?

Use "rich" for all flight regimes except cuising at low power. "Lean" will give you best economy at about 60% (or so) power (power, not rpm!) output.

 

One thing to remember is that both mixture settings will enrich > 8% best power mixture with increased power output of the engine. Operating with the lever set on "lean" will produce higher engine temps than "rich" at higher power. While the mixture will be still > 8% mixture, in a "rich rich" state, it will depend on your abilities to keep engine temps down to make the slight extra shaft power worth the while. Operning the rads more to cool the engine in "lean" mode more probably hurts your intentions more than leaving it to "rich". You are also putting more wear on the engine.

 

For your purposes, use "rich" and leave it there. The last thing of concern is economy on small maps like these.

[Gambit21]

Thank you.

[BlitzPig_EL]

I was cruising on the Moscow summer map yesterday at 25,000ft. 60% power, 60% prop pitch and the mix set to auto lean, and was indicating 200mph as I recall.

[Gambit21]

I was playing around last night, did some sight seeing, and also a few furballs so that I can start to get my gunnery skills back to where they used to be.

 

I know what the Specs page says, and was using as a guideline...however I'd be interested if any of you have come to any conclusions about in-between settings or methodologies for general combat/flying around.

Not only while you're engaged, but pretty much anything other than long transit flights.

 

I cant' manage this aircraft quite like the Yak because of the boost, so I was trying to find the closes thing to a "set it and forget it" manifold pressure setting, but came to the conclusion that this isn't practical

with this aircraft.  I was probably futzing around more than need be with both the throttle and the RPM in actual combat.

 

The other part to this is that I'm rusty across the board and haven't flown anything in quite a long time.

Just working the kinks out, and doing so with the Spit.

 

Haven't blown up an engine...but trying to be as efficient as possible.

 

Fired up the A-5 for a few minutes too.

That was to simple...so back to the Spit.