DCS Mustang Developer Notes

[sobek]

There is no "gradually opening" with WEP, as far as I understand the mechanics of the engine, so a mere keypress would do the job pretty well. But as sobek knows way more than I do...

 

Are you sure? The way that i understood the mechanics, there is simply a sealed stop on the throttle that you break for WEP, which gives the throttle additional travel forward. All this is talking about RL, of course.

[159th_Viper]

Goodness Gracious.........of all the things to get Pernickety about :megalol:

 

Just keep in mind that in some Mustangs, WEP was not incorporated into the throttle: WEP was engaged by pulling a Boost Control lever.

[PeterP]

:D exactly!

By answering one question six new will rise...

It's like the head of the Hydra!

 

 

Just keep in mind that in some Mustangs, WEP was not incorporated into the throttle: WEP was engaged by pulling a Boost Control lever.

So...:huh:? What does this tell us...;)

Sometimes I think you enjoy to confuse us!

<<< Yes , have a look - and you will understand why I think this.

 

 

______________

However, as with the throttle levers of the Shark, I'd consider this less serious/important than negligible

My analogy to the throttle levers of the BlackShark was of course only meant as example how WEP can be implemented into the sim. ;)

Edited April 24, 2012 by PeterP

[159th_Viper]

Sometimes I think you enjoy to confuse us!

 

:D

 

Nothing untoward - just means that the keypress method of enabling WEP currently In-SIM can be, if one chooses to, be equated with the Boost Control lever.

[Luigi Gorgonzola]

Are you sure? The way that i understood the mechanics, there is simply a sealed stop on the throttle that you break for WEP, which gives the throttle additional travel forward. All this is talking about RL, of course.

As far as I understood the actions triggered by breaking the seal with the throttle lever, yes. But as I cannot acces my notes and links atm, I have to admit I might be wrong.

 

However, my comment was meant to refer to the "additional axis" thingy. The additional way you'll have to move the throttle lever to get WEP does probably not allow for any kind of fine adjustments (especially when in a fight for which WEP was meant?). IMHO the seal and gate are just meant to require an intentional although short movement of the lever by the pilot with the seal providing some protection against unintentional WEP activation (and an additional visual cue for the ground crew, maybe).

 

Briefly said, a kepress will do the job just fine although it might not match the visual experience (GUI). Keypress and the earlier WEP lever might match better ;)

[Luigi Gorgonzola]

Goodness Gracious.........of all the things to get Pernickety about :megalol:

 

Oh weren't we meant to do all this just for your entertainment, were we? ;) :megalol: :music_whistling:

[effte]

Goodness Gracious.........of all the things to get Pernickety about :megalol:

 

Just keep in mind that in some Mustangs, WEP was not incorporated into the throttle: WEP was engaged by pulling a Boost Control lever.

 

Aaaah.... but we're getting a D model, and as per the developers notes,

 

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.

 

(My boldface.)

 

;)

[159th_Viper]

Aaaah.... but we're getting a D model.....

 

Yes, exactly. AAF Manual 51-127-5, 15 Aug 1945.

 

;)

[Tailgate]

And remember, the Mustang doesn't have a hotas, so lock your joystick between your knees and get two mice to simulate your hands. ;)

[effte]

Yes, exactly. AAF Manual 51-127-5, 15 Aug 1945.

 

;)

 

Hence, no separate control handle for WEP but rather a wired gate. We're not getting the "some Mustangs" with separate boost control levers, and PeterP is indeed right in his suspicions. :D

[Graill]

I don't get it. Can someone explain it for a simpleton why the manifold pressure stuff is important? I can't slam the throttle forward and aft at will is that what this is all about? I though that if there's no leak in the system then you don't have to worry about manifold pressure.

 

And the mustangs being developed late in the war did not use fuel injection but the BF109s which was developed since the start of the war did?

 

 

In the simplest terms you are going to use that guage to monitor at a glance and ensure you do not grenade your engine due to detonation or overpressure, adjusted for altitude.

 

Seasoned pilots can do this by sound and rote action in the real deal so it becomes like breathing.

[effte]

Which engine version are we getting? Unreliable boost control below 41" would, to the best of my knowledge, mean a -3 or early -7?

 

Is the automatic boost regulator the same as for the Spitfire installation of Merlin 66s, described in the documents linked in post #2? I've been scouring the web for images, and the external shots of the P-51 engine with accessories show me a regulator which I do not recognize from the drawings in the Merlin 66 manual. The developer notes describe the Merlin 66 installation spot-on, but while the Spitfire/Merlin 66 POHs mention adjusting the boost as you climb at lower boost settings, the Mustang POHs I've been able to find do not mention this.

 

Which documents for the V-1650/P-51D confirm the same boost regulator in the Packards? Are those documents available anywhere? If not, I'll probably be able to acquire them if only I get a reference. Packard redesigned other parts of the induction system, so I'm not prepared to make the assumption that Rolls Merlins and Packard Merlins would have used the same boost control.

 

Before anyone gets all upset, let me be very clear: I'm not saying that I think a mistake has been made. I'm just saying that I have not seen authoritative sources which tell me that this is in fact the way the P-51 Packard engine works, and I would very much like to read those documents.

 

Oh, and I must back up a bit on the separate boost control lever. They do appear to have been installed in early D models, meaning it is still a possibility in our simulated steed.

 

Edit:

What manifold pressure does, and why we need to control it: The amount of power produced by an engine depends largely on the amount of fuel-air mixture it sucks in on each induction stroke. More fuel-air, more power. The swept volume of the cylinders (displacement) is fixed, so you have two ways of increasing the fuel-air amount: Increase RPM, or force the fuel-air mixture in at a higher pressure. With fixed propellers, you adjust your throttle setting until you have the desired RPM which you know corresponds to the power you want. With constant-speed propellers, the propeller blade angles automatically adjust to maintain a constant RPM regardless of the amount of power applied to the prop, meaning we have to find a different means to monitor and regulate the engine power setting. As we already monitor and control the prop RPM, what we need is a gauge telling us the pressure at which we are letting air into the cylinders - the manifold pressure.

 

I would be concerned if people were setting their MaP by ear rather than by the gauge...

Edited April 26, 2012 by effte

[Nate--IRL--]

Oh, and I must back up a bit on the separate boost control lever. They do appear to have been installed in early D models, meaning it is still a possibility in our simulated steed.

 

No Boost lever - just a wire gate on the throttle.

 

Nate

[effte]

No Boost lever - just a wire gate on the throttle.

 

Thanks!

 

Viper: :P

[Yo-Yo]

Which engine version are we getting? Unreliable boost control below 41" would, to the best of my knowledge, mean a -3 or early -7?

 

Is the automatic boost regulator the same as for the Spitfire installation of Merlin 66s, described in the documents linked in post #2? I've been scouring the web for images, and the external shots of the P-51 engine with accessories show me a regulator which I do not recognize from the drawings in the Merlin 66 manual. The developer notes describe the Merlin 66 installation spot-on, but while the Spitfire/Merlin 66 POHs mention adjusting the boost as you climb at lower boost settings, the Mustang POHs I've been able to find do not mention this.

 

Which documents for the V-1650/P-51D confirm the same boost regulator in the Packards? Are those documents available anywhere? If not, I'll probably be able to acquire them if only I get a reference. Packard redesigned other parts of the induction system, so I'm not prepared to make the assumption that Rolls Merlins and Packard Merlins would have used the same boost control.

 

 

The manual for V1650-7 gives full information regarding the boost regulator. THe original Packard Boost regulator was similar to Merlin but was not exactly the same. As far as I remember even the servo piston travel direction was reversed. But the main scheme was the same.

 

We model earlier device INTENTIONALLY because we wanted to show our engine modelling features. Full range automatic boost regulator does not allow to show all effects of MP due to altitude, rpm, etc.

Frankly, had we decided to make full range boost regulator the current engine model would not have been made. Current AI model can do the same EXCEPT producing true MP gauge reading for non-automatic range.

 

In fact there is no difference for the sim user how exactly WEP is done. For the majority of users that have not long-travel throttles with WEP detent the way WEP is engaged is under a veil.

Let's presumed that in non-WEP mode the 100% of throttle is 61" then there is no difference if our 100% input transforms it into 67" as boost regulator setting or 67" MP is transformed into 61".

Maybe I was wrong unveiling our internal realisation and creating misunderstanding... :)

 

.

[159th_Viper]

 

Viper: :P

 

Yeah yeah yeah :)

 

As I said before, we get the appearance of the later D model with the functionality of the earlier D model.........No need for any scripting witchcraft or the like.

 

Pernickety :megalol:

[EvilBivol-1]

Our next subject - the propeller governor:

 

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:

 

 

Edited April 29, 2012 by EvilBivol-1

[EvilBivol-1]

For those looking for some drawings of the governor system, you will find some good illustrations here:

http://napoleon130.tripod.com/id697.html

 

Keep in mind, the P-51 propeller was a different version that did not allow feathering.

[Grimes]

Slightly OT, but the first screenshot posted is making me wonder if there will be multiple pilot models to choose from.

[Daze]

Nice screenshots. Must be almost ready for release now? :D

[joey45]

This is way over my head.

[element1108]

I'm a visual learner...although I'm happy with the screen shots (I'll never complain about screen shots)...I think to better grasp the written component a video demonstration of the prop system might serve me better ;)

 

(worth a try).

 

Thanks for the info

[cichlidfan]

I learn by doing!! *hint* :D

[element1108]

I learn by doing!! *hint* :D

 

 

What he said:joystick:

[Agg]

Slightly OT, but the first screenshot posted is making me wonder if there will be multiple pilot models to choose from.

Probably one of the Tuskegee airmen ;)