I have a question, Why is it necessary to reset the gyro direction indicator periodically due to precession but it is not required to reset the attitude indicator?
That's an excellent question, and one that really needs a diagram to properly explain. Mike Busch does a good job in the following article:
The Gyro with an Attitude
The attitude indicator has a different function than the heading indicator. it's job isn't just to be stable in one plane, like the heading indicator. The only job of the gyo on the heading indicator is to maintain a stable platform to give a reference better than the compass...one that doesn't lead and lag and have acceleration and dip errors. The drawback is that it needs to be reset occasionaly, and the amount is owing to it's condition, the condition of the bearings, it's speed, the amount of maneuvering and precessive forces applied to the gyro, etc.
The attitude indicator must remain erect or upright, all the time. It's sole purpose is to give a true picture of the world outside, around the airplane. It's got to have a way to find up...not such an easy thing to do, if you think about it. It's got to have a way to stand that vertical gyro up, or "erect" itself, without any pilot input. the pilot can always refer to the wiskey compass to set up the heading gyro (and in more advanced aircraft, flux gate compasses do this automatically).
Some attitude gyros incorporate a "fast errect" mechanism using electrical input, others use a mechanical pull knob to "cage" or fix the gyro in an erect position...useful when doing aerobatics, and also for quickly erecting the gyro. Once it's up to speed and upright, it needs to stay that way. The gyro uses a weighted gyro gimbal, as well as "pendulous vanes" which direct the flow of air through the gyro case, to force it upright.
A pendulous vane is just what it sounds like. it's an exhuast port through the gyro assembly, which is covered by a pendulum. The pendulum hangs down, blocking the exit. If the gyro is tileted in a manner that causes the pendulum to swing free of the exhaust port, the airflow out the exhaust port creates a force, a "jet" if you will that helps upright the instrument internally.
This can only protect against so much tipping...if the instrument is tipped too far, it may "tumble" and then have to erect itself again.
Attitude indicators do precess, however. As mentioned before, acceleration errors cause the attitude indicator to show a slight climb when increasing forward speed, and dip a little to show a slight descent when rapidly slowing. Other fairly insignificant precession errrors also occur with the attitude indicator.
The short answer to your question is simply that the heading indicator and the attitude indicator are designed for differrent purposes, and act differently becase of their internal mechanical construction.
The following link contains a number of different instrument explainations, which may be useful:
attitude indicator (artificial horizon)
The following link contains some good diagrams of pendulous vanes at work:
4-4
Another powerpoint presentation on instruments. Gyro information and diagrams begin on frame #21:
freegroundschool.com/Free%20Ground%20School/Files/Aircraft%20Instruments.ppt