A Fowler flap is a movable panel at the trailing edge of an aircraft wing. It travels rearward and deflects downward, extending the wing section and changing its camber. Tracks and supporting mechanisms guide that movement. The animation below shows a single panel and its guide rollers in side view; the calculator compares its dimensions and position.
Fowler Flap Mechanism Interactive Calculator
Choose a wing chord, flap size, aft travel and deflection. The results describe the fully extended position; the animation cycles between retracted and extended. This is a geometry example, not a model of a particular aircraft.
Equation Used
C is reference wing chord, f is flap chord as a percentage of C, a is aft travel as a percentage of C, and δ is downward deflection. Δx is the trailing edge’s horizontal displacement from its retracted position. d is its vertical drop relative to the flap’s leading edge. The illustrative leading-edge path is horizontal. These dimensions do not predict lift, stall speed, loads or slot airflow.
Rearward travel followed by downward deflection
The fixed wing supports the flap through its guides and drive. Unlike a panel that only turns around a fixed hinge, a Fowler flap also moves aft. Its position therefore depends on both translation and angle.
In this illustration, the first half of deployment translates the panel while keeping it level. The second half continues the translation and adds the selected downward angle. The panel is rigid: its chord and the spacing of its rollers remain constant throughout the movement. Reversing the motion retracts it.
The blue and gold rollers follow separate fixed guide paths. Their guides are shown in side projection and are assumed to occupy separate spanwise planes. Their precise shape is chosen for this example’s motion. The drive, support structure, seals and three-dimensional clearances are not shown.
What changes when the flap extends
Fowler flaps are high-lift devices. Aft travel exposes more of the flap, and downward deflection changes the wing section’s curvature. A slot can direct air from beneath the wing toward the flap’s upper surface. Actual lift and drag depend on the complete wing, flap and slot geometry and on the airflow conditions.
The single panel here illustrates the movement without representing a particular aircraft’s number of flap elements or operating settings. Aircraft-specific configurations and limits come from that aircraft’s documentation.
Calculate position from chord, travel and angle
Let C be the reference wing chord and f the flap chord percentage. The flap chord is c = Cf/100. If the aft-travel percentage is a, the leading edge moves by s = Ca/100.
A panel tilted downward through angle δ has horizontal chord projection c cos δ and vertical projection c sin δ. Its trailing edge therefore moves horizontally by Δx = s + c cos δ − c, and drops by d = c sin δ relative to its leading edge.
These are planar distances. A negative Δx means the trailing edge ends forward of its original horizontal position, even though the leading edge has moved aft. It does not mean that the flap has a negative area. At zero aft travel, the model illustrates rotation without the extension characteristic of a Fowler flap.
Example: a 0.5 m flap with 0.4 m travel
With a reference chord of 2 m and a flap chord of 25%, the panel chord is 0.5 m. Aft travel of 20% gives s = 0.4 m. At 30° downward deflection, its horizontal projection is 0.433 m.
The trailing edge extends by 0.4 + 0.433 − 0.5 = 0.333 m and drops by 0.5 sin 30° = 0.250 m. With the same translation and no deflection, extension would instead be 0.4 m and drop would be zero.
Geometry does not determine flight performance
The lift equation is L = ½ρV²SCL, where ρ is air density, V is airspeed, S is the chosen reference area and CL is the corresponding lift coefficient. A flap’s chord and angle alone do not supply that coefficient. Suitable aerodynamic data are needed for the actual configuration.
The calculator consequently reports distances rather than assigning a lift increase or stall speed. It also omits actuator force, bending, bearing loads and interference between structural parts. The animation is useful for understanding travel and rotation, not for selecting flight settings or designing a flight-critical assembly.
Frequently asked questions
What distinguishes a Fowler flap from a plain flap?
A Fowler flap extends rearward as well as deflecting downward. A plain flap mainly rotates about a fixed hinge.
Why show two guide rollers?
Two constrained points can guide a rigid panel’s position and angle. The illustration uses separate guide paths in offset spanwise planes; real support arrangements vary.
Does changing wing chord change the drawing’s apparent size?
The drawing fits the available screen width, so it remains normalized to wing chord. Changing chord scales all reported distances; the percentage and angle controls change the visible proportions and motion.
Do the numerical results change during autoplay?
No. They describe the selected fully extended position. Autoplay cycles through deployment and retraction; Pause and the Animation control let you inspect intermediate positions.
Why can trailing-edge extension be negative?
Rotation shortens the horizontal projection of the flap chord. If that reduction exceeds the aft translation, the trailing edge ends forward of its retracted horizontal position.
Can this calculator give stall or approach speed?
No. It has no aircraft weight or aerodynamic coefficient data. Its outputs describe panel geometry only.
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