I saw a boomerang looking trim tab on the bottom of the ailerons of the Yak .
1 What is it called ?
2. What is it for ?
3. How does it work ?
4. Is it for any plane ? ( Like my U Can Do ? )

Stupid question . But I do not know.

It's called an aileron spade. It boosts the aileron to make it easier for the pilot to give full control.

A spade.  Thanks.<div>
</div><div> So in a R C world , is it needed ? </div>

Not really. Just use big servos.

Properly installed and used, boost tabs are a big help. Much lower current drain (smaller batteries and lighter weight) and more positive control. There is also less force on the control rods (less blow back or deformation). Improperly used or to large and then big problems can occur. You want them to be not greater than 10% of the movable surface and absolutely no slop in the hinges.

As Bax explained to me when I asked the same question - it does not apply so well in RC as we usually control our ailerons independently (when the plane is big enough to matter). For the spade to help, you need the whole system working to a single input.

Bedford

The spade is rigidly connected to the aileron. They make one single unit. The aileron and spade will be worked by a "single input" in our usual servo-to-aileron setups.

The two most popular ways to help reduce the control load on surfaces are the spades and boost tabs. Boost tabs look like smaller "ailerons" set into the ailerons (or elevators or rudders). They move opposite to the surface they boost. They can be powered to move independently to their subject surface. You'd probably consider them as controlled by a different input than the surface and therefore not working from a single input with the surface. Most I've seen on models are connected to a horn and simply respond to the movement of the surface versus that rigidly mounted horn. (Horn is usually fixed on the wing/stab/fin.) That setup would be one where the boost tab and surface were controlled by a single input.

So the single input idea only applies to a system that is one of a couple of systems.

BTW, boost tabs look very much like trim tabs. In the case of boost tabs, they are driven opposite the direction you wish to move the surface. Their aerodynamic influence then drives the surface, or helps it move in the direction you want it to move. The spades actually cause a force that directly contradicts the resistance caused by the surface's movement. They create drag force opposite the drag the surface is causing that has been discovered to be "too much" for the pilot.

Spades can be a pita, as they restrict the surface's movement in one direction. And they can often create much more force in one direction than they create in the other. And who needs imbalance........... They appear to be a "duct tape and baling wire" solution to inappropriate design.

In full-size aircraft, aileron spades reduce control forces to move the ailerons quickly. That's because the ailerons are inter-connected to the pilot via the stick. When an aileron is deflected up, the spade catches more air and helps the aileron move upward, reducing the stick force needed to move the aileron. When it moves down, the spade is more in line with the airflow, so does not oppose the aileron movement.

On full-size aircraft that use aileron spades, they spades are adjusted in size and angle until they work such that the pilot is comfortable with the feel of the ailerons when the airplane's in flight. You cannot just put them on and expect them to work. They need to be adjusted by 'feel', and the servo(s) in the model just won't give you that kind of information.

Also, the ailerons in the mode are not interconnected, that is, if you removed the aileron servo(s), deflecting one aileron won't cause the other one to deflect...unless you have an older model where the aileron servo moves a wire that activates a bellcrank in each wing panel in front of each aileron. If you have that kind of installation, you might be able to use aileron spades if you could determine the proper size and angle to allow the servo to work less. However, it would be all "hit and miss" unless you use complex instrumentation to tune the ailerons and servo to the actual forces involved.

Is the spade parallel to the wing cord line?

Kurt

No, parallel to the FSD (Free Stream Direction) at that point of the wing chord. Simply, in S&L flight there is no (or at least equal) force transmitted back through the control system.
Evan, WB #12.

Hi- there are no 'stupid' questions, just stupid answers! The Boeing B-9, of which I am building a model, has a boost tab on its huge rudder. As stated in other posts, the boost tab is a little different than the spade in its application and function.

And in fact there are at least three configurations/things that people think of when talking about reducing the load on a surface using aerodynamics. Problem us people have when thinking about something is that we often mix up our labels.

There are also horn balancers. The rudders that have a projecting section on their tops, forward of the hingeline sometimes have those sections called boost tabs by some people. But they're probably more correctly called horn balancers.

We also have surfaces with their hingelines aft the LE of the surface. Thankfully, there aren't too many who call that arrangement boost tabs.

Good thing about spades is they're scarce as hen's teeth, and the name is really, really distinctive.

Bad thing about boost tabs is they're scarce AND look so much like trim tabs nobody notices them.