Specific sub-systems - The Winding System
In addition to the cone valves that you see in figure five, there are several other types of valves that various organ builders use. One of the most common is the curtain valve. The curtain valve consists of a rubberized cloth curtain which rolls or unrolls over a grid which is usually on one side of the inlet valve box. As the top of the regulator ascends, the curtain covers more and more of the grid. The curtain is always on the high pressure side, so that the air pressure and flow keep the curtain tight against the grid at all times. When there is a demand for air, the top of the regulator descends, and this makes a little more of the curtain wind up on its roller and uncover more area of the grid to let more air through to the regulated side of the system.
The advantage of the curtain valve is that there is no force on the curtain due to unequal pressures that might influence its performance. The disadvantage of the curtain valve is that the top of the regulator must descend through a greater distance between zero and full organ, although this defect can be somewhat reduced by operating the curtain through a system of levers or differential-diameter pulleys so that the curtain motion is greater than that of the regulator top.
In actual practice, however, the unbalanced air pressure that exists on the two sides of a cone valve disc decreases as the top of the regulator descends, and as long as the cone valve disc is not too large, the effect of this unbalance generally has no noticeable effect. In my experience, if the area of a cone valve is 1/9 or less the area of the regulator top, you will encounter no problems from the pressure differential between the blower side and the regulated side.
A third type of valve, which some builders have used consists of a bellows type regulator whose movable top is connected, usually by a cable or a chain, to a butterfly valve in the supply pipe from the blower. These generally give poor results for several reasons. First, the characteristics of a butterfly valve are such the the top of the regulator must descend a considerable distance (in some cases up to 6" to open the valve for full organ) and as we now know, excessive regulator top travel can result in very noticeable pressure sags. In spite of what logic might suggest to us that the force on a butterfly valve is always equal on both halves of the valve disc and therefore should represent a net force of zero, actual tests on butterfly valves show that when the butterfly is at some intermediate position other than fully open or fully shut, if there is a flow through the butterfly valve, the forces on the two halves of the valve disc are very unequal and result in considerable torque. (Think lifting force on airplane wings vs flow of air over and under the wings) Therefore tight control of air pressure via butterfly valves becomes somewhat difficult, and in my experience, in general gives poor results. This fact is exemplified by the relatively small number of pipe organs that you encounter that use butterfly valves for pressure control.