Q. What are flaps and what is the trim tab? A. They are secondary flight controls.
The primary flight controls are Ailerons, elevator and rudder. Trim Tab and Flaps are secondary flight controls.
A trim tab is a small control surface that is hinged at the trailing edge of a primary flight control that when deflected into the wind, uses airflow to apply a force on a primary flight control in the opposite direction. It also serves to neutralize or reduce the pressure required on the primary flight control.
Changes in Airspeed and Power changes the pressure needed on flight controls.
Trimming off the control forces relieves the pilot from maintaining continuous pressure on the control wheel which reduces pilot fatigue as well as sloppy control.
Trim tabs can be fixed in which they are set at the factory and not moveable or they can be electronically or manually moved from the cockpit.
When you move the trim tab, it moves the opposite direction of the desired control surface movement.
If you want to maintain your desired pitch attitude and keep the nose from dropping, you need to hold back the elevator control pressure by using the nose-up trim to relieve the back pressure in which you move the trim control in the “Nose Up” direction. This moves the trim tab down which puts an up force on the elevator which puts a down force on the tail and moves the nose up.
If the nose tends to rise above the desired altitude when the forward elevator control pressure is relaxed, use the nose up trim to relieve the forward pressure which moves the trim tab up. This puts down force on the elevator which moves the tail up and the nose down.
Before takeoff, trim tabs should be set to the takeoff position.
Wing flaps are attached to the trailing edge (rear edge) of the wings between the fuselage and the ailerons. These flaps change the shape of the wing to increase lift, increase drag, and decrease stall speed.
Using flaps allows you to takeoff and land at slower speeds and in shorter distance as well as to descend at a steeper rate and angle of descent without an increase in airspeed. In the air, the flaps help you to get down and to slow down.
How Flaps Work
Flaps change the shape of the wing by changing the curvature of the wing or changing the camber of the wing as well as increasing the surface area of the wing.
Since the shape and size of the wing changes with the flaps, so does the speed at which the wing stalls.
For a Cessna 172SP Skyhawk, the stalls speed without flaps is 48 knots and with full flaps, the stall speed is 40 knots indicated air speed.
Using Flaps During Takeoff
Many aircraft manufacturers recommend using flaps for takeoff and the Cessna 172SP Skyhawk models call for 0 to 10 degrees of flaps for takeoff. Takeoff roll is reduced and the total distance to clear an obstacle is reduced by 10% by using 10 degrees of flaps.
If using flaps during takeoff, it is important to remember to maintain the flap setting until the airplane is a safe distance from the ground and you are maintaining a position rate of climb.
Reducing the flaps setting, you may feel the aircraft “sink” a little which can be offset by adding additional back pressure.
The climb checklist at 1000 feet5 will help you remember to raise the flaps back to 0 degrees. Forgetting to raise the flaps will slow your climb due to the excess drag and you may risk structural damage to the aircraft if you fly faster than the recommended speed for extended flaps.
Using Flaps to Land
In most normal situations, you will want to use full flaps when landing, however, during gusty or turbulent conditions, you will; want to land with a reduced flap setting to increase the ability to control the aircraft.
If you cannot land or reject the landing and want to go around to try again, you will need to :
- apply full power
- pitch to reach a climb attitude.
- raise the flaps to a partial setting
- 20 degrees in the Cessna 172 Skyhawk.
- (If you raise the flaps up to the full position without accelerating, you will be in greater risk of coming closer to a stall spee and it will allow the aircraft to sink down which delays you’re climb.)
- accelerate to a safe climb airspeed
- around 60 knots in the Cessna 172 Skyhawk
- retract the flaps to 10 degrees until all obstacles are cleared
- return the flaps to 0 degrees once you have reached a safe altitude