Chucklehead,
The horizontal tail is mainly there to stop the aircraft from doing a loop at high speed. It fights the rotor's very strong tendency to pitch and keep going when disturbed (because the rotor reacts to an angle of attack change by increasing its moment in the same direction, a function of how the advancing blade sees the angle of attack increase, and flaps in the direction of the disk angle).
For example, if the nose is pitched up by a disturbence, the advancing blade sees the angle increase and gains lift, so it starts up, and gets to peak flapping over the nose, which in effect makes the nose up disturbence worse.
The horizontal tail sees the pitch up attatude change as a change to its angle of attack that increases its up load. That way, the up force from the tail fights the main rotor and re-balances the helo. If your horizontal tail suddenly disappeared in high speed cruise flight, your helo would almost immediately do a loop, either nose up or down depending on what the first disturbence was.
The horizontal tail is sized to exactly fight the main rotor at the highest speed and the worst CG (usually aft CG). The tail size must be increased if the speed is increased, or if the CG is moved aft, or if the gross weight is increased.
You can tell if the tail is too small during pullup maneuvers at high speed. If the nose wants to go further up, and it takes forward stick to stop it, the tail isn't strong enough to balance the main rotor, and you have to help it. We call that "negative maneuvering stability" and usually make the designers fix it.
That is the dominant force the horizontal tail creates, and its reason to exist. Old helicopters (H-13, H-55, H-34) have tiny tails because the have low Vne, so the instability of the main rotor is milder.
John, I am pretty sure the Apache has the hardware but not the electronics and software, but I will check. I know that the Army got tired of Hughes messing around with the Apache problems in pitch and just made them buy the Black Hawk's tail and be done with it. In fact, they also tried to get Sikorsky's chief developer (our friend and mentor, Bob Zincone) to get assigned to Hughes to fix the Apache's early woes. Bob simply refused!