Why don't I see split ailerons that work like spoilers going up and like normal ailerons going down? Isn't that a good way to get rid of adverse yaw?
Adverse yaw is caused by difference in drag for aileron up and down positions, aileron down has more drag than aileron up; in this configuration when aileron is up additional drag is created because of split. –
It is possible to produce a split aileron, which would indeed be(much) less susceptible to adverse yaw. But there are downsides as well:
Above pic shows how adverse yaw is eliminated without adding complexity: the frisé aileron has a lower corner that pokes out when deflected upwards, thereby increasing drag.
As stated in @Mark’s answer, the later versions B-52 had replaced its small ailerons with spoilerons. Not for reducing adverse yaw, but for eliminating aileron reversion. From the wiki:
Because of the B-52's mission parameters, only modest maneuvers would be required with no need for spin recovery.
B-52s prior to the G models had very small ailerons with a short span that was approximately equal to their chord. These "feeler ailerons" were used to provide feedback forces to the pilot's control yoke and to fine tune the roll axes during delicate maneuvers such as aerial refueling.[102] Due to twisting of the thin main wing, conventional outboard flap type ailerons would lose authority and therefore could not be used. In other words, aileron activation would cause the wing to twist, undermining roll control. Six spoilerons on each wing are responsible for the majority of roll control. The late B-52G models eliminated the ailerons altogether and added an extra spoileron to each wing.[102] Partly because of the lack of ailerons, the B-52G and H models were more susceptible to Dutch roll.
Present day high subsonic passenger jets mostly have two sets of ailerons, inboard and outboard, plus spoilerons. Inboard ailerons to be used at high speeds, also to eliminate aileron reversion.
in this configuration when aileron is up additional drag is created because of split.
Looking at the following picture taken from this NACA report, it can be seen that, the lift coefficient being the same, a split flap/aileron generates less drag than a plain flap/aileron, especially at high AoA:
So, with a split flap/aileron the opposite of what you'd like to achieve actually happens.
Also for the pitching moment similar results are obtained, with the split flap/aileron modifying the moment less than a plain flap/aileron, the $C_l$ being the same.
Both plain and split flaps/ailerons were superseded after WW2 by the more efficient slotted flaps/ailerons.
Bonus material
Spoilers are normally used to... well spoil the airflow on the upper surface of a wing, basically triggering a controlled stall.
Even if they can be termed spoileron and used as primary control surfaces (typical example is the B-52 and many, if not all, jet fighters), they shouldn't be confused with ailerons as such:
The B52 has "spoilerons". Roll control is possible by raising spoilers differentially. The advantage is that adverse yaw is not created.
On the B-52, this greatly relieved stresses on its very tall vertical stabilizer of early models, which was needed for stability at the 50,000+ feet and high subsonic airspeeds the swept wing bomber was designed to fly at after design modifications in the late 1940s, by eliminating the need for rudder inputs to counter-act adverse yaw.
It works so well, later models have no ailerons at all. Spoilerons also help reduce wing twisting forces that ailerons can produce at higher airspeeds.
First, we can define split flaps from the reference and note: there is not a picture of a split flap in the question.
What is pictured in the top image is more akin to a spoileron.
The middle image shows an airfoil with no control surface deflection, and the bottom one shows a downward flap deflection.
It is the downward deflection that causes adverse yaw. Differential spoilerons will cause a roll into the turn, rather than away, eliminating the need for rudder to counter-act adverse yaw.
