Hey Guys, John Snider here. Harry e-mailed me and let me know that there was considerable interest in my anti-servo kits here on the forum. I do still have kits available and the price is $275 plus shipping. The kit is very complete with all hardware, rivets, cable, etc. included and as Harry mentioned, the installation manual is very detailed so you can't hardly mess it up.
Below is an explaination of the tab but I was unable to get pictures attached. If someone is interested please e-mail me directly and I can send you pictures. Be sure to e-mail me directly as I rarely get a chance to visit this forum but will try to check in occasionally.
John Snider
[email protected]
812-890-8257
When a RAF 2000 or any aircraft for that matter is yawed to one side or the other, the relative wind strikes the cabin from the side. This not only results in a direct force being applied to the upwind side of the cabin, but also a low pressure on the downwind side. The cross sectional area of the cabin of a RAF 2000 is greater than the cross sectional area of the vertical stabilizer and rudder combined (vertical tail) when the doors are on. With the mast representing the yaw or vertical axis, (not exactly but very close) there is more area in front of the yaw axis than behind. This is somewhat analogous to an arrow with the feathers at the front. It wants to swap ends and fly backwards. The only reason that is does not is because the pilot senses the yaw and reacts by applying rudder which has enough authority that it can overcome the forces being applied on the cabin. The pilot is essentially acting as a yaw damper. Unfortunately, if the pilot removes his feet from the rudder pedals or worse yet, a rudder cable breaks, the RAF will swap ends in flight. When the doors are off the situation is greatly improved as the air can simply pass through the cabin in yawed flight but there is still some tendency for the aircraft to diverge from coordinated flight once a certain angle of yaw is reached.
The above can be easily illustrated by standing twenty or so feet to one side or the other of the RAF, directly abeam the mast and facing the gyro. I will use the left side for this discussion. With the rudder perfectly aligned with the vertical stabilizer, it can be clearly seen that the cross section of the cabin is larger that the cross section of the vertical tail and that the mast or pivot point is very nearly half way between the two. Now imagine that the wind is your eyes and it sees what you see if the aircraft were in a 90 degree yaw. In the absence of rudder control, the wind would blow the rudder to the right, not only reducing the cross section of the vertical tail even more, but also effectively applying a nose right rudder command which is exactly the opposite of what is needed in this situation.
An anti-servo tab is a form of trim tab that is hinged and thus moveable. It effectively acts as an aerodynamic centering device by moving in the same direction as the rudder when the rudder moves. This movement tends to return the rudder to its neutral position when deflected. The more deflection there is the greater the force trying to return the rudder to neutral.
The anti-servo tab replaces the existing fixed trim tab on the rudder. All but about one half inch of the existing trim tab is cut off leaving only enough to rivet a piece of piano hinge to. The anti-servo tab is nearly the same shape and size of the portion of trim tab that is cut off and has the other half of the piano hinge riveted to it allowing this new tab to be hinged and thus movable on the rudder.
This new movable tab has control horns on it similar to the control horns on the rudder itself. Two cables link these control horns to a bracket attached to the keel beam. The cables are routed inside the rudder and are crossed so that when the rudder moves to the right (nose right) the tab also moves right. The opposite is true for left movement. The movement of the tab is caused by the movement of the rudder and requires no input from the pilot.
The installation of the anti-servo tab reduces the undesirable yaw characteristics of a RAF 2000 to a much more acceptable level without the huge cost, weight, hassle of preparing and painting, and detriment to the stock look of the RAF associated with adding more vertical tail.
The results of installing the anti-servo tab on my RAF 2000 are as follows:
Doors Off: The gyro flies in perfect coordinated flight with my feet off of the rudder pedals at cruise power settings. (The tab can be ground adjusted to provide coordinated flight for the power setting you use most often. Most likely cruise.) When power is higher, some right rudder pressure is required. When power is lower, some left rudder pressure is required.
Doors On: The gyro is substantially easier to control in the yaw axis and requires much less effort to maintain coordinated flight. If I remove my feet from the rudder pedals, the gyro may fly for some time before eventually yawing to the left or right about 10-15 degrees. It no longer will continue to diverge and swap ends in flight. If I intentionally apply sufficient rudder in either direction to create a yaw of well over 25-30 degrees, the gyro will return immediately to the 10-15 degree yawed condition allowing for continued flight and landing in the event of a rudder cable failure.
Power Off: When power is removed or dropped below about 3500 RPM, the anti-servo tab does not have sufficient airflow across it to do its job effectively. Consequently, at low or idle power settings and engine inoperative, the gyro behaves almost exactly as it did without the anti-servo tab installed. However, anytime one is flying with reduced or no power as in approach and landing or engine out landing, they would or should have their feet on the rudder pedals anyway. If you have an engine failure and rudder cable failure all in the same flight then you’re just having a really bad day.
I am not advocating flying around with feet not on the rudder pedals but this tab will allow the pilot to remove his feet from the pedals to relieve cramps and readjust for comfort without having the gyro try to swap ends. More importantly, it makes the gyro much less likely to become uncontrollable in the event of rudder cable failure.
John Snider
[email protected]
812-890-8257
John Snider