Our lead flight model programmer has put together a few developer note articles that discuss some of the more interesting features of both the real and the virtual DCS P-51D. We will be releasing these over the course of the the next couple of weeks.
The first and probably most lengthy article overviews some of the principles behind the Manifold Pressure indicator in the cockpit.
Enjoy!
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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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Our next subject - the propeller governor:
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Propeller Governor
First, some fundamentals. A propeller is essentially a set of little wings, which produce lift and are subject to drag much like a normal wing. Also like a normal wing, a propeller moves through the air with a so called angle of attack. The greater the angle of attack, the greater the lift (or thrust in the case of propellers) at the cost of increased drag, making it harder to move through the air. And again like a wing, the angle of attack is affected not only by the structural position of the propeller, but also by the speed and direction of the airflow passing over it. Finally, and once again like a wing, a propeller produces a downwash, or an induced velocity. Because the linear velocity of any propeller blade section is a function of the propeller radius (the outer blade edges of a spinning propeller move through the air faster than the inner edges), the blades are shaped such that the blade angle is progressively reduced toward the outer edges.
Early propellers were constructed with fixed blades, where the blade pitch angles cannot be changed. In a fixed-pitch propeller, as the aircraft’s speed increases, the angle of attack is reduced, which in turn reduces the thrust produced by the propeller. If airspeed continues to increase, the propeller eventually turns into a kind of airbrake, producing reverse thrust, but still demanding power from the engine to turn it. Ultimately the propeller can start to windmill, where it itself begins to turn the engine instead of the other way around.
The problem with fixed-pitch propellers is that they only work well in a narrow range of airspeeds. The pitch can be optimized for low speed, useful for maximum takeoff thrust, but then efficiency begins to drop as airspeed climbs. Conversely, the pitch can be designed for best climbing speed or higher speeds in general, but at the cost of low speed efficiency, which reduces takeoff performance.
Variable-pitch propellers were introduced to remedy this problem. The pilot could now manually control the propeller pitch angle. This was, no doubt, an exciting time for aircraft engineers. For pilots however - fighter pilots in particular - it was another headache in flight. It became much easier to break the aircraft by overstressing or overspeeding the engine as a result of mismanaging propeller pitch control. So in a spur of innovation, designers began to work on mechanisms around the 1930s that would automatically adjust propeller pitch to maintain a constant engine RPM – propeller governors. All the pilot would have to do is set a desired engine RPM and the governor would load or unload the propeller by adjusting the pitch angle to maintain this setting.
Let’s now take a look at the Hamilton Standard propeller governor system used on the P-51. Inside the prop spinner is a propeller dome, which houses a horizontal piston cylinder. The piston is surrounded by oil to either side – low pressure engine oil in the forward side and propeller governor oil, pressure-boosted by a pressure pump, in the rear side. Relative pressure of the oils to either side of the piston determines its position. As the piston moves in reaction to pressure differentials, a special mechanism translates this motion to the propeller blades to adjust their pitch.
Oil flow to and from the cylinder is controlled by a vertical pilot valve in the governor assembly. The pilot valve’s neutral position is maintained by a balance of forces between a tension spring that pushes it down and special flyweights that pull it up under the centrifugal force of spinning action when the engine is running. This balance is maintained and the propeller pitch remains constant as long as the engine RPM is stable. When RPM changes, the flyweights and the tension spring become unbalanced, moving the pilot valve to open oil lines to and from the piston cylinder. Oil moves into one side of the cylinder and out of the other, the piston moves in response to a pressure change, and the propeller pitch is adjusted until equilibrium is restored. Tension of the spring is controlled by the pilot’s RPM lever. As such, moving the RPM lever in the cockpit unbalances the pilot valve and again moves the piston to adjust the propeller pitch until equilibrium between the tension spring and the flyweights is restored at the set RPM value.
For example, if RPM increases, the flyweights move outward under increased centrifugal force, overcoming the tension of the spring and pulling the pilot valve up. The pilot valve opens oil lines to push high pressure governor oil into the rear side of the cylinder and engine oil out of the forward side of the cylinder. The piston moves forward and propeller pitch is increased. As propeller pitch increases, the higher drag increases the load on the engine and RPM is returned to its original value. The flyweights return to a neutral position and equilibrium is restored over the pilot valve, closing the oil lines. Conversely, if RPM is reduced, the tension spring overcomes the flyweights, moving the pilot valve down and pushing engine oil into the forward side of the cylinder and governor oil out of the rear side. The piston moves back, decreasing the propeller pitch and unloading the engine to increase RPM until equilibrium is restored.
There is a third element affecting propeller pitch angle – the centrifugal force of the propeller itself, which moves the blades toward lower pitch. It’s important to point out that in the absence of oil pressure inside the piston cylinder, the propeller will set to low pitch.
So, what does it all mean in practice?
Running at maximum RPM is very stressful for the engine, even if manifold pressure is kept down. It’s generally best to maintain the lowest RPM possible for any desired flight condition. A number of manifold pressure and RPM combinations are recommended for various parts of the flight envelope. These are provided in the manuals and graphs, but can be determined independently given a sufficient understanding of the principles involved.
A special case worth considering is an engine failure. In autorotation, the prop effectively acts as an airbrake, so assuming the governor remains functional, the RPM should be immediately set to full decrease. In this case, the aircraft might attain a glide ratio of 9-10:1. If RPM is left high, this ratio will drop by as much as a third. Worst of all is a situation where the propeller stops altogether due to a jammed or poorly turning engine. In this case, the prop’s surface area of nearly 1 square meter will reduce the glide ratio approximately by half. Luckily, getting the propeller to stop mid-flight, even with the engine turned off, is practically impossible. Although, if the oil is frozen and airspeed is low, it does become likely. As long as the engine is warm – this could only happen in a spin or maybe a complete loss of airspeed, such as at the top of a stalling vertical maneuver. Once stopped, spinning up the engine is impossible, regardless of airspeed.
Here are a couple of new screenshots where you can see the visual difference between a low and high pitch setting of the propeller:[/quote]
You can see the screenshots here:
http://forums.eagle.ru/showpost.php?p=1441...mp;postcount=92Prop wash effect(can you do this on any other sim)
http://www.youtube.com/watch?feature=playe...p;v=pUgDqhPnekwNot everyone is happy with this news though:
http://www.youtube.com/watch?feature=playe...p;v=iAoZP6EuaDk
Posted: Sat Jun 30, 2012 12:41 pm
