I always have wondered about that small surface in the trailing edge of the elevator, at the beginning, I thought it's for stabilizer trimming, but later on, it dawned on me that it was made for another purpose, and it's called balance tabs.
Balance tabs was made to assist in moving the elevator, so the loads on the control column (Yoke) are significantly reduced, which makes the airplane easier to fly.
When the control is given an input, like when the pilot pull or push the control column, the trailing edge of the balance tab is deflected in the opposite direction of the trailing edge of the elevator. Here is an animated photo illustrating the deflection :
This action creates a small aerodynamic force, which acts in the opposite direction to the main aerodynamic force created by the elevator, and this results in a reduction in control effectiveness, as the tab force is opposite to the control force. So now maybe you're thinking why somebody wants to reduce the control forces required to move the elevator, in fact, it's very effective in critical situations such as hydraulic systems failure, because it will help the pilot for better manual controlling, since it will reduce the loads applied on the control column.
OK that's it, that's why Boeing use the balance tabs on their 737 airplane, I hope this post was helpful, so if you found it interesting please share it with your friends via your Facebook, twitter or google+ to help the others figure out the role of balance tabs. if you have any question, don't hesitate, just leave a comment bellow, I will be glad to help you :)
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