Can someone phrase the explanation to this in simple words as you would explain to a student?
This is real easy, no explanation required:
Tell your student to stick their hand out the window of the car driving 25mph in the city and rotate it like an aileron.
Then have them do it on the freeway doing 60mph.
Feel the difference? Did smaller movements produce larger effects at higher speeds?
Practical examples are typically more effective at driving a point home than a lot of words.
Very simply, because the dynamic pressure drops as you slow down, making all aerodynamic controls more sluggish.
Aileron works by locally increasing the lift of one outboard wing and decreasing the lift of the other, thereby creating a torque or rolling moment. For simplicity, let's think that for every degree of aileron deflection, there is an associated increase and decrease in the local lift coefficient for TE down aileron and TE up aileron, respectively1.
You may remember that the total lift differential is proportional to the lift coefficient, but also to the dynamic pressure. You may also remember that dynamic pressure is proportional to the square of the true airspeed. Therefore, as you slow down, even though the lift coefficient differential remains more or less constant for the same aileron action, the total lift differential decreases as a square of the speed, and the same for the rolling moment.
1 The change in local lift coefficient is not constant across flight regime. As the AOA increases toward stall, significant nonlinear effect arises.
To make the answer as simple as possible, an aerofoil will increase lift in two ways, either by going faster or increasing the angle of attack. When the aileron moves up or down it’s changing the shape of the wing and either increasing or decreasing the angle of attack. So, if you fly fast just a small aileron deflection will produce the necessary lift to raise the wing. Conversely, if the aerofoil is going slow the aileron will require a greater deflection to create a larger angle of attack for the same amount of wing movement. Hence at high speeds the aircraft will feel very responsive and at low speeds will feel unresponsive particularly as it approaches stall speeds. You should understand this is a very simple overview and there are many other aspects to consider.
The lift that is generated by an aileron at a fixed deflection (e.g. 5 degrees down) is proportional to the volume of air that is deflected downward per unit of time (e.g. per second). As speed is reduced the volume of air being deflected downward per unit time is reduced so the lift created by the aileron is also reduced and the aileron become less effective.
In the flight envelope you describe, the aileron response is affected by two factors:
- the change in lift from the movement is related to the speed of flight. As the speed reduces, the force diminishes, and,
- when the wing stalls, airflow over the aileron is substantially turbulent and no usable effect results from aileron movement.
This applies equally to other aircraft, not just light trainers. However, sophisticated fly-by-wire controls are often designed to not allow stalling and prevent the effect from being noticed under normal flight conditions.
why does aileron response decrease ... from slow flight up to stall.
Yes, controls definitely get more sluggish slowing down from cruise speed to slow flight, due to the reduction of aerodynamic force on their area. From slow flight to stall, with ailerons, one must be aware of additional effects beyond reduction in airspeed.
Ailerons work by moving one up and one down. All well and good until one goes to higher angles of attack. There is a loss of effectiveness for the upturned aileron due to flow separation approaching stall. This is compensated by increased angle of attack of the downtrend aileron, but it will stall first. This is the dreaded "aileron reversal" that can lead to a spin. Note that in the PARE spin recovery method, ailerons are nuetral.
From slow flight to stall, use your rudder and turn coordinated to stay safe.
Easy - it's no different than the WHOLE WING exhibiting diminished response at low speeds! When you pull back on the yoke, that IS because you are getting diminished response from the wing. The ailerons likewise require more displacement for the same effect at low speeds compared to high speeds. If the student then can't grasp why the whole wing doesn't work at slow speeds, then you at least have uncovered the level of the student's intuition and understanding!