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I was just enjoying a movie of one Svetlana Kapanina flying her EA300. One camera looks out of the cockpit over the left wing. In a couple of scenes, the aileron of the wing can be seen quite clearly. The picture (see 1) shows it deflected downward, except for the red "end" deflecting upward. Other scenes show that when the aileron deflects upward, the red "end" turns downwards.

What is the reason to have this red "end" tilting in opposite direction to the aileron?

Wing with aileron deflected downward

NB, you can enjoy the whole movie here:

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That is an enclosure for a counterweight, either made from steel or lead to balance the aileron and/or reduce control flutter in various flight regimes. The extension provides an additional lever arm to alter the moment of inertia about the aileron’s axis of rotation. These are usually added or refined during flight testing to remove undesirable flutter or other qualities inherent in the design. Counterweights are usually installed at the factory with the airplane but custom made counterweights can be used on a specific aircraft to provide a specific feel to the controls to meet customer requirements. Many high performance experimental category acro aircraft do this on aircraft custom built for a master aerobatic flyer.

The comment above about this providing an aerodynamic counterbalance is incorrect. Such devices for this purpose do exist but in the form or one or more ‘shovels’ or ‘spades’ on the underside of the control surface.

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    $\begingroup$ If you would like to add a source: POH Section 7.3 $\endgroup$
    – Fiddlesticks
    Mar 5, 2018 at 3:47
  • $\begingroup$ On first OP's picture, we can see (close to the root) that the whole aileron's leading edge overlaps full span above extrados, do you know the reason for this? $\endgroup$
    – jkztd
    Mar 5, 2018 at 10:51
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    $\begingroup$ @qq jkztd: the POH says "To reduce pilot's hand forces the hinge line of the ailerons is positioned 25% of the aileron depth." In other words, the entire aileron is aerodynamically balanced to reduce control forces. $\endgroup$
    – Fiddlesticks
    Mar 5, 2018 at 12:50
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The small red tab is an aerodynamic balance, used the reduce the control loads required from the pilot when power assistance is not available.

Normally, the aerodynamic load on a deflected aileron will tend to return it to a neutral position. For slow and light aircraft the pilot can easily handle the required load to maintain the required deflection. For high performance aircraft these loads can be come significant.

By adding a small part of the aileron ahed of the pivot (a horn), the aerodynamic load on that portion will tend to deflect the aileron further into the flow. The tab is still small enough that the main part of the aileron will return the surface to neutral, but the force from the horn effectively lightens the load that the pilot needs to handle.

The EA300 is a high performance aerobatic aircraft. The pilot needs light control loads in order to perform the extreme maneuvres the aircraft is capable of. Using aerodynamic balances achieves that without the need for a heavy hydraulic system.

You may also find aerodynamic balances on the elevators and rudder of some aircraft, for the same reason.

The geometric shape trailing from the wing is a sighting aid to help the pilot judge the aircraft attitude with respect to the horizon - nothing to do with the ailerons at all.

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