Manifold Pressure

[nickpike 0]

Hi all, using the variable manifold pressure in XML. As the engine revs increase, so does this variable. My experience of this pressure is that is reduces with revs, that is, a lower pressure exists because more air is being pulled through the manifold. Should this variable be subtracted from a possible maximum? I also thought of subtracting this from atmospheric pressure, as it must vary according to changes in atmos pressure, but the values can then become negative. Can someone point me to a solution for showing a realistic manifild pressure. This is obviously for a piston engine. I have searched the archives here but not fould a solution.cheers,nick

[taguilo 33]

Nick,I know nothing about manifold pressure. However, I do know a bit of XML :-) So, if you could bring a practical example of what values do you need to relate and how, maybe I could think of something.Tom

[Guest _FSAviator_]

Greetings Nick,The answer has two parts. How this stuff works in FS9 and how it works in real life. You probably understand the first part, but I will explain for the benefit of any readers who do not, since it is often misunderstood. Whether in FS9 or in real life such gauges only display a value. They do not calculate it, or its relationship to any other value. If the FS9 gauges are displaying the current values in memory the gauges are working perfectly, whether or not the values are realistic. In FS9, just like every MDL, output gauges just sit there and wait for a flight dynamics author to tell them what to do. Their code holds no relationships and no consequential behaviour. If this were not the case default gauges could not exist. They do exist and they can display thousands of different relationships for thousands of different engine types and marks. All the gauge author has to do is retrieve the current value calculated within FS9, to the order of the flight dynamics author, and then make the needle point at that value. A non default MAP or RPM gauge does not require code that is absent from a default gauge. There is nothing missing from default MAP and rpm gauges. No new code needs to be developed. A different bitmap may be desirable and since e.g. 25 inches may be at a different position on that bitmap new code is needed to point the needle at 25 inches where it is on that new bitmap, if 25 inches is what the flight dynamics author encoded for the circumstance.The relationships, the equations that drive them, and the aircraft behaviour that ensues, are encoded elsewhere. Partly in the hard coded flight model (FM) provided by Microsoft, and partly within the individual third party engine and aircraft specific flight dynamics (FDE), provided by the flight dynamics author. Moving to real life.If the engine lacks a constant speed airscrew, and also lacks a supercompressor, with everything else held constant, the manifold pressure will always tend to rise whenever engine rpm rise because the throttle cable opens the butterfly valve in the choke tube to promote that result. When a more complex piston engine has an rpm lever as well as a throttle, even if the engine also has a supercompressor, there is no reason why MAP should never rise when rpm are increased and equally no reason why MAP should never fall when rpm are increased. It just depends.Those real life dependencies are precisely what flight dynamics authors encode, on an engine type, variant, and mark basis, air file by air file.

[Au MaV 0]

Hi NickThis Prob total wrong but hey I like getting shot down in flames its how I learn no pain no gain:) My understanding of Manifold pressure is as the revs raise so will the manifold pressure,more air is forced into the engine means more pressure.As alt climbs air thins meaning less pressure,then the supercharger is engaged(make it SO number one :D ) to stuff more of the thin air into the engine cheersWozza

[nickpike 0]

Many thanks guys, so using RECIP ENG1 MANIFOLD PRESSURE without any modifying factors should give a realistic value. Maybe of interest, this particular gauge has the range 10 to 35 inHg for the needle. The numeric value has a minimum of about 5 inHg. cheers,nick

[pve 0]

Nick The MAP gauge with engine off normally gives the ambient pressure, Cross check with the altimeter at airfield altitude. 35in is good for normally aspirated but say a Turbo Senneca will give around 39 to 41 on takeoff ground roll at near sea level with the boost lights flashing and climbout close to high 30.

[adr179 0]

I was also looking for a kind of a workaround for the manifold pressure gauge behaviour, since it definitely is NOT REALISTIC, not even in the default FS9 DC3. After some 4000 hours of flying "the real thing"I feel competent to say that. The main point is that this statement by Aviator: "A non default MAP or RPM gauge does not require code that is absent from a default gauge. There is nothing missing from default MAP and rpm gauges. No new code needs to be developed." is misleading, since I have a feeling it's almost impossible to tweak the variables that are sent to the gauges, we do need a different code for the MP gauge to make it at least fake correct behaviour that is:In a variable pitch propeller and a non-turbocharged engine combination the manifold pressure is decreasing(without touching the throttle levers, that is)at an average rate of 1in.Hg per 2000feet in altitude change and this is modelled in the simulation. The situation that is absolutely not modelled in the simulation is that at a constant altitude when the propeller levers are moved, thus changing theRPM there is a noticeable change in the MP, lower RPM, higher MP and vice versa.In a turbocharged engine MP is kept the same with the altitude changes by means of a special vastegate up to a certain altitude, reffered to as "critical", even there is nothing critical in the situation, and then at higher altitudes MP falls of at a standard rate, and this behaviour is NOT MODELED in the simulation at all, and the same situation is with the RPM changes, where the MP raise and drop is also not modeled. So what we need are actually two MP gauges, one for the normaly aspirated, variable pitch propeller/engine combination and another for a turbocharged/variable pitch propeller combo.I'm ready to provide help in form of information of realistic behavior and values to the XML guru's who might want to give it a try, since the gauge will become well to complicated (editing two additional values internaly from three variables provided by the sim) for my modest knowledge of XML.Those interested, please send a PM, since I really don't want to readposts cluttered with statements of how everything is actually perfectly done already. If that was the case, many topics like this one wouldn't have been written.

[Guest Douglas K]

Here

[n4gix 4,947]

>In brief: the airfile author can change the flight>dynamics code (airfile, aircraft.cfg file) to allow the engine>gauge variables to present the correct information. Then, if>necessary, and in the gauge code, you can change that code as>required to tailor the final indication displayed by the>instrument to suit your needs.That is the precise sequence I follow:1) code the gauges to report the requisite sim variables, calibrating to any non-linear scales...2) send project to FDE person...3) make any needed scalar tweaks to the gauge's variables as needed to reflect reality as closely as possible.The major point being made though is simple. The flight dynamics must be properly set prior to making "gauge tweaks..." ;)

[taguilo 33]

""The situation that is absolutely not modelled in the simulation is that at a constant altitude when the propeller levers are moved, thus changing theRPM there is a noticeable change in the MP, lower RPM, higher MP and vice versa.In a turbocharged engine MP is kept the same with the altitude changes by means of a special vastegate up to a certain altitude, reffered to as "critical", even there is nothing critical in the situation, and then at higher altitudes MP falls of at a standard rate, and this behaviour is NOT MODELED in the simulation at all, and the same situation is with the RPM changes, where the MP raise and drop is also not modeled. ""***********************************I believe that FS unmodelled behavior you stated can be perfectly simulated using XLM code. However, you'll need to know which are the real numbers for the relationship between RPM increase/decrease and MP decreas/increase. Basically two things to consider before coding startup:-If there should be a significant difference in engine performance as MP raise/drop in response to RPM changes, you would have to tune RPM and throttle events by code, faking its respective lever bitmaps'positions (ie "throttle lever position", etc)- On the opposite, if there is not such a difference noticeable in some kind of flight characteristics alteration, should be enough to fake the gauge's readings using polinomials extracted from real tables.Tom

[adr179 0]

Thank's for Your reply Tom.In real life there is a "significant difference in engine performance", even dramatical, it quits after a serious abuse by means of setting tolow an RPM against to high an MP setting. However, by applying just normal, recommended factory settings for combinations of RPM/MP that produce certain recommended power settings for 55, 65 and 75% power reffered to as long range, economy and fast cruise by some manuals, there is no obious penalty anywhere, not in engine performance or in fuel flow, since it actually ends up equal once all the settings to the RPM an MP are done.The main purpose of all this is to produce a realistically behaving gauge and by doing this also increase the cockpit workload to a realistic level. Or is it? Since I flew the kind of hardware for over 4000 hours I'm maybe even more disturbed by lack of a correct reaction from the gauges, since I'm used to do the correct actions when changing power, so the "easy way" is actually a harder way for someone that knows what handling of this kind of engines is all about. What needs to be done is to add a dependency to the output manifold pressure values that are displayed by the gauge in a way that the MP will rise if the RPM is decreased (RPM lever position)and at the same time to compare the RPM handle position against the throttle lever position and increase or decrease the MP pressure only if the position of RPM handle against throttle(MP) handle is relatively changing. A realistic value of change to start with is some 10% of change in MP with 10% of change in RPM, so if the RPM is decreased from 2500 to 2250(10%) the manifold will raise from 23 to 25.5 and this increase and decrease of MP can be considered linear thru the normal operating ranges. It's not that I'm lazy or something, but this kind of programming in XML is simply well above my current capabilites.At the end I need to appologise for a mess in my first post, I overlooked that the aircraft I was testing has some entries in the engine section missing, thus producing strange, but actually correct behavior of the MP gauge, that is otherwise correctly programed- reffering to critical altitude, fixed vastegate part.

[taguilo 33]

""What needs to be done is to add a dependency to the output manifold pressure values that are displayed by the gauge in a way that the MP will rise if the RPM is decreased (RPM lever position)and at the same time to compare the RPM handle position against the throttle lever position and increase or decrease the MP pressure only if the position of RPM handle against throttle(MP) handle is relatively changing. A realistic value of change to start with is some 10% of change in MP with 10% of change in RPM, so if the RPM is decreased from 2500 to 2250(10%) the manifold will raise from 23 to 25.5 and this increase and decrease of MP can be considered linear thru the normal operating ranges. It's not that I'm lazy or something, but this kind of programming in XML is simply well above my current capabilites.""*************All this seems feasible to code in XML...Ball goes to Nick's field now! :-)Tom

[nickpike 0]

Thanks Tom, I've caught the ball but don't know what game I'm playing :)From what I have read above, this is not a simple parameter to model.I think the only way to model this is to obtain performance characteristic graphs from the actual engine itself. Not easily obtainable on the net. The engine, FYI, is a Teledyne-Continental Model IO-550-B. Or, unless I have missed some points, can someone state, maybe in tabular format, pressure characteristics.cheers,nick

[adr179 0]

Hi Nick!Actually You don't need any kind of performance data for both the gauge to act realisticaly and the flight model to perform exactly the way it performed before the mods, provided that You stay within the parameters I described in my previous post. A slight discrepancy will only be happening only when the RPM handle is moving against the throttle handle being at a fixed position. Once the power settings are done, one will again end up with the same relationship of MP&RPM and the gauge influence to engine power output will be exactly the same as before, so I don't see why You need any kind of performance graphs just to tweak the gauge behavior internally.The value of 10% that I stated in my previous post maybe looks like a rough guess, since it's a round figure, but it's correct and will actually produce a realisticaly behaving gauge.

[Guest _FSAviator_]

I will respond to specific matters arising and then provide some insight into the real problem that lies at the heart of all the follow up posts about