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DCS: P-51D Mustang

DCS P-51D Mustang alpha version

Manifold Pressure

Given that the Manifold Pressure (MP) indicator will quickly become one of the primary cockpit instruments used when flying the Mustang, a discussion of some of the principles behind its indication is worthwhile. Before we begin, remember that manifold pressure is measured in inches of Mercury (in.Hg).

First, let’s review the general airflow through the induction system of the P-51D Merlin engine, equipped with a carburetor and a two-stage, two-speed supercharger. Initially air is ingested through the air intake(s) (of a couple of possible types, which we’ll discuss in another note) and flows past a throttle valve that controls airflow volume into the carburetor. Here, fuel is added to create a fuel-air mixture of a specific ratio. The mixture is then passed through the supercharger, where it is highly compressed, becoming significantly hotter in the process. To prevent the compressed and very hot mixture from causing detonation, as well as to allow more of it to be “packed” into the cylinders, it is cooled twice – by the intercooler between the first and second supercharger stages and by the aftercooler just prior to entering the manifold. Finally, the mixture is passed into the manifold for induction into the cylinders. The manifold itself is a very strong structure surrounded by about 8 mm of aluminium alloy – a necessity given that pressures attained here may be as high as two atmospheres.

Cooling of the fuel-air mixture is performed by the aftercooling system, which is completely separate from the engine cooling system and circulates as much as 36 gallons of coolant per minute under peak performance conditions. The radiator of the aftercooling system is installed as a single unit with the engine coolant radiator in the aft section of the air scoop underneath the fuselage, although they are functionally independent from each other. To protect the manifold from backfires, it is equipped with flame traps – essentially metal filters designed to prevent flames from expanding throughout the entire manifold.

If we get rid of everything, except the throttle valve, carburetor and the manifold, we are left with a conventional, naturally aspirated engine. Let’s consider what happens with pressure in the manifold as we open and close the throttle valve while maintaining a constant engine speed (RPM). With the throttle completely open, air flows freely and manifold pressure equals ambient atmospheric pressure. As the throttle valve is closed, the cylinder pistons begin to “suck” air through a limited opening, creating a partial vacuum in the manifold and a corresponding drop in manifold pressure. 

Similarly, when the throttle valve is partly open while engine RPM is increased, manifold pressure drops, because with increased RPM the cylinder pistons must “suck” more air into the manifold through the same narrow throttle opening. The same effect can be witnessed when bumping the throttle up from idle power. Initially the RPM are kept down by low engine power output, but as power output increases when the throttle is moved forward, an initial boost in manifold pressure takes a dip as RPM begin to catch up.

Let’s now return everything we removed earlier and take another look at how RPM affect manifold pressure. Pressure increase (boost) levels in the supercharger have a very non-linear relationship with engine RPM. Thus, under relatively low RPM (60-75%) and throttle settings, typically manifold pressure will drop as RPM is increased, similarly to the situation described above. Under high RPM settings, however, supercharger boost levels significantly outweigh the pressure drop immediately past the throttle valve, resulting in increased manifold pressure.

In the Merlin engine, things are even more interesting, thanks to an automatic manifold pressure regulator installed to help ease the pilot’s workload. For any given throttle setting, manifold pressure can change dramatically as flight conditions change (in particular as air density changes with altitude). The automatic regulator tries to maintain the manifold pressure set by the pilot's throttle lever, minimizing any additional throttle “jockeying” required to hold this setting in flight. The automatic regulator does not work throughout the entire performance envelope of the engine. In the V-1650-7 model engine featured in DCS Mustang, it begins to function at 40 in.Hg. Below this value, manifold pressure is controlled exclusively using the throttle handle and all of the effects described above can be witnessed. At 40 inches and up, however, the throttle handle sets the desired pressure value and the automatic regulator attempts to maintain it by adjusting the throttle valve opening as necessary.

Operation of the automatic regulator consists of the following primary elements. An aneroid sensor coupled to a piston valve moves vertically in reaction to pressure changes, closing and opening vent lines leading to a relay piston. The relay piston moves horizontally in response to pressure differentials created by the aneroid piston valve to maintain equal pressure to either side inside a cylinder. As the relay piston moves forward or back, it opens or closes the throttle valve until pressure equilibrium is re-established, returning the aneroid piston valve to a neutral position and stabilizing the relay piston in place, which may be forward or back from its original position. The relay piston is connected to the throttle valve via a differential linkage system with the throttle handle in the cockpit. Within the operating range of the automatic regulator, the sum movements of the throttle handle and the relay piston determine the actual position of the throttle valve at any given time.

Let’s consider an example. We’ll assume the engine is driven to 3,000 RPM on the ground and the throttle is advanced fully forward. Under these conditions, the supercharger is capable of producing much higher pressure in the manifold than the maximum permissible pressure of 61 in.Hg. The regulator’s purpose is to limit pressure to 61 inches and maintain it there as long as the throttle handle is in the full forward position. As soon as engine RPM reaches levels at which pressure climbs above 61 inches, the aneroid becomes unbalanced, shifting the relay piston to close the throttle valve. The regulator operates in the same fashion throughout the manifold pressure range of 40 – 61 in.Hg.

In practical terms, what this means is that the pilot uses the throttle handle to set his desired manifold pressure and the regulator operates the relay piston to open or close the throttle valve to maintain this setting. As altitude increases and air density decreases, resulting in lower pressure, the regulator opens the throttle valve to maintain manifold pressure. Conversely, as altitude decreases and air density increases, the regulator closes the throttle to maintain manifold pressure. 

In the above example of 61 inches of MP, when critical altitude for maintaining this pressure is reached, the relay piston and the throttle handle are both fully advanced, and the throttle valve is fully open. When manifold pressure is set substantially lower than maximum, for example the Maximum Continuous setting of 46 inches at 2,700 RPM, the regulator will attempt to maintain pressure as altitude increases, but will eventually hit the fully open position of the relay piston, even though the throttle valve is only partly open, because the throttle handle in the cockpit is not fully advanced. In this case, it will become necessary to move the throttle handle up to further open the throttle valve in order to maintain manifold pressure, because the automatic regulator will have no further authority due to having reached the relay piston’s limit of range of motion. As critical altitude for this pressure setting is reached, the throttle handle will have to be all the way forward to maintain it. Here, we have to remember that the supercharger is a two-speed system and switches into high blower mode somewhere around 19,000 feet. When this happens, manifold pressure increases dramatically and the throttle handle has to be moved back, otherwise resulting in a climb at 61 in.Hg at 2,700 RPM. Not deadly, especially using quality gasoline, but not recommended, either.

As you may have deduced, 61 inches at 3,000 RPM is full Military, or Takeoff power, nominally limited to 15 minutes. Let’s take a brief look at War Emergency Power (WEP) mode, nominally limited to 5 minutes of operation. WEP can be mechanically implemented in a number of ways. The first option is to artificially lower the pressure acting on the aneroid by opening an escape line, resulting in an opening of the throttle valve by the regulator so as to “maintain” pressure – while in fact boosting it beyond the value set by the throttle handle. This method was used on early Mustangs, which featured a special control handle in the cockpit to engage WEP. Another option is to design the throttle linkage assembly such that the relay piston is in the fully closed position when the throttle handle is set to full military power. The pilot would then push the throttle handle past this setting into the WEP position, further opening the throttle valve and the relay piston would be unable to act upon it to close it. And the final option is to design the linkage system such that the throttle handle position past full military power would produce manifold pressure up to 67 or even 75 in.Hg.

Given the limitations of most HOTAS controllers used by virtual pilots, DCS Mustang will model the first method. This allows us to avoid having to rely on throttle detents or limit their range of movement in the pre-WEP range. As such, we will have a dedicated input command to engage WEP as a simulation of a cockpit control handle.

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Lead Flight Model Programmer
Dmitry "Yo-Yo" Moskalenko

Eagle Dynamics The Fighter Collection
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