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Are the hydraulic failures simulated on the jet? I tried forcing them on in the mission editor, even all the systems at once (1A,1B,2A,2B), but it doesn't seem to make any difference on the plane. I tried forcing other different failures, and all they work. But I cannot simulate any hydraulic failure.

TL;DR first: The flaps for the DCS P-51D get blown back up at high speed, and do not stay fixed in place like they should. There is an unusual though inconsistent exception however, when airspeed is lower than necessary with the engine idled. THE EXPECTATIONS: First let's establish what is expected for flap behavior in the P-51. I'll start with the mechanical linkages first: The P-51 actually has a rather unique flap mechanism for the fighters of the time, and are rather automatic in nature. Aircraft that used hydraulic power to move their flaps typically required the pilot to manually move their flap control lever in the cockpit up or down to actively make the flap strut actuate the flaps up or down. The pilot would then return his flap control handle to the neutral position to stop the flap movement. This is the case for the flap control in the P-47, Mosquito, and ( believe) the P-38. If you don't have or know these aircraft, you can see how these flap handles operate here, at 11:09: Now you may be wondering why the P-51's flap controls work so automatically, without requiring the pilot to determine the time when he needs to stop the flap movement. It's all in the linkages: When you push down on the flap handle to lower the flaps, this pulls the Follow Up Control Rod forwards, and thus hinge/point #1 forwards. However, hinge #3 is somewhat fixed in its position. As the Follow Up Control Arm (not the rod) is pulled forwards at hinge #1, hinge #3 acts as the fulcrum around which the Control Arm will rotate. Note that the Follow Up Control Arm is a solid linkage that extends from point #1 to point #3, and does not bend at point #2. Connected to the center of the solid Follow Up Control Arm is the Control Valve Rod Assembly, at hingepoint #2. As the Follow Up Control Arm is pulled forwards at point #1, a part of this movement is transferred to the Control Valve Rod Assembly, which is connected directly to the Wing Flap Control Valve's lever. This directs the valve to allow fluid to pass to the strut, which will push on the Torque Tube Arm, turning the Torque Tube. But how does the flap know when to stop actuating automatically? Well, as the torque tube rotates, it will pull hinge/point #3 aft via the Follow Up Control Linkage. This brings hinge/point #2 back to its previous neutral position, and thus the Flap Valve's lever back to its neutral position. This kind of automatic functionality allows the pilot to predetermine a desired setting for the flaps, and the torque tube (and flaps) will neutralize its own controls once it's found the new equilibrium between points #1 and #3, in order to bring #2 back to its neutral position. Of interesting note, the Flap Valve's lever acts essentially exactly as the flap control handle in the P-47, Mossie, and P-38, as they are usually directly linked 1:1 to each other. In case the official drawing above is too cluttered to follow, I also made these simplified drawings to try to show how this automatic function of the flaps works. Now we can start on the hydraulics: Note that I won't cover the entire layout of the hydraulics, just the parts pertaining to flaps operation. The engine driven pump will continuously supply pressured fluid to the Unloading and Relief Valve at 1500 PSI. However, the Unloading and Relief Valve will maintain the hydraulic system's pressure at a lower 1050 PSI (specifically the manual says 1000-1100 PSI). When the system pressure reaches 1050 PSI, the unloading valve will open, creating an idling circuit just between the resevoir, pump, and Unloading and Relief Valve, isolating the hydraulic system from the pump until there is a demand on the system that consumes pressure. The wing flap selector valve does not look like this as pictured, but this is merely a visual demonstration to show roughly how the flow of fluid works. Take note of the check valves on the pressure and return feed ends on the flap valve. This is how the Wing Flap Selector valve operates the flaps circuit: And of course, I've made another drawing that simplifies this, pertaining to just the flap operations: Now, with the the hydraulic and mechanical systems in mind, what should occur if the flaps are mismanaged and oversped beyond their limits? With no relief and bracketed with check valves, the hydraulics will not yield. The weak point is the mechanical linkages. If you can imagine an immense pressure exerted on to the flaps, attempting to push them up, this will apply direct pressure on to the hinges of the torque tube arms. The weakest point taking that pressure are the bolts within the hinges, which could shear. These hinges were noted to be the weakest link in the flaps' strength. As such it was designed so that in the event of overpressure on the mechanical system, the first hinge (bolt) to shear would be that connecting the torque tube to the hydraulic strut. The two hinge bolts used to connect the torque tube to the two flaps were larger in diameter than the one hinge bolt connecting the strut to the torque tube. It was critical that any failure was to occur here, as this ensured that both flaps would simultaneously fail in sync. This helped to prevent assymetric drag and lift throughout the aircraft's flight. If the flaps are oversped and the hinge fails (bolt shears), then both flaps will weathervane with the direction of wind. So, if they were previously down when being overspeed, they will both snap back upwards at most speeds. When coming in for landing, or generally very slow however, the flaps will drop on their own, as the wind force on them decreases. THE DCS BUGS: In the DCS P-51's flap behavior, there are a couple behaviors that conflict with the mechanics and hydraulics of the real aircraft. 1) Flaps blown up when they're oversped, while strut is currently actuating flaps down. What would prevent this in reality would be the two check valves that are immediately upstream and downstream of the Wing Flap Selector Valve, to prevent flow reversal. These would be what would stop the flaps from getting blown back up when oversped, while the strut is currently attempting to actuate the flaps down. This results in the flaps not moving until airspeed decreases, or the flaps snapping lose when the strut's hinge bolt shears. 2) Flaps blown up when they're oversped, after strut is finished actuating flaps down. There are now two measures in place to prevent fluid from flow reversal. Now not only are the check valves upstream and downstream (pressure and return) of the Wing Flap Selector Valve in place, but the Wing Flap Selector Valve itself will have all its poppet valves closed. This will isolate the wing flap circuit from the main circuit, holding the fluid in place. The flaps will not move, unless the flaps are oversped and snap lose when the strut's hinge bolt shears. 3) The DCS exception... However in DCS currently, if A) your engine is idled; and B) your airspeed is >100MPH, the flaps will have a chancec of finally behave correctly, and could remain fixed in place when you dive with them. Why is it odd? Because no matter the other external factors (aside from the pilot moving the flap handle in the cockpit), so long as the flaps achieve the angle they were determined to match, they will "lock" (the flap selector valve will close). This, in testing, appears to be possible with full flaps as fast as 150MPH IAS. Airspeed (and the force against the flap surfaces) is what will affect if the hydraulic system has the pressure required to lower the flaps. What affects if the flaps will "lock" or not, is if the flaps reach their predetermined setting or not. Not only is 100MPH IAS too slow, but also it's only an indirect reason for determining if the flaps should "lock." Here's a video highlighting the issues: TURN ON SUBTITLES Timestamps of interest: 0:30 2:40 7:15 P-51 Flap Blowup test.trk CORRECTIONS: 1) The flaps should NOT get blown back up uncommanded under ANY circumstances, short of a component failure. The check valves and flap selector control valve ensure that fluid will not give or reverse flow to permit flaps to blow up against excessive force on the flap surfaces. 2) Add a component failure. As flaps would be oversped while deployed beyond limits, the most likely point of component failure will be the torque tube arm hinge between the strut and the torque tube. If the bolt in this hinge were to shear, this would cause both of the flaps to suddenly weathervane into the wind, and essentially flap freely. This would create some interesting problems when it would come to landing. Slapbladder if you have read this far, message me in Discord with the codephrase: Blue sky