Down and Dirty - Modeling Wing Lift Augmentation Part 1
Trailing Edge Flaps

by Michael Benolkin

Introduction

Most aircraft kits are designed so the wings are molded with the flaps and ailerons integral with the wing to keep the cost of engineering and production down. As modelers, we sometimes want to pose the aircraft ‘down and dirty’ – flaps extended – to add some detail and dimension into our projects. The simplest thing to do is simply cut out the flaps from the wing halves, glue them together, fill in the gaps with styrene strips, and glue the result back onto the assembled wing at an appropriate angle. Simple, right? Perhaps, but you may also be shooting yourself in the foot if you’re trying to present an accurate model.

To help you with your next projects, here is a quick tutorial on wing lift devices, what to look for in reference photos, and how to replicate those details in scale. This tutorial will look at flaps as used on most flap-equipped aircraft in the 1930s and 1940s. We will cover slats and leading edge flaps in Part 2.

Background

In these days of modern aircraft and airports, it is easy to lose sight of why flaps were needed in the early days of aviation. In the years between World War I and World War II, aviation was becoming more commonplace, but airfields were still little more than large grass clearings that were kept relatively clear of holes and other unpleasant obstacles to permit safe flight operations. Most of these fields were only a few thousand feet long in any direction and usually were surrounded by trees, power lines, fences, etc. around their perimeters.

To safely land an aircraft on one of these fields, you’d need to fly as slow as safely possible and with a descent rate that can be arrested safely prior to touchdown. With these relatively short runways, you needed to touch down as close to the approach end of the field as possible to allow sufficient length for a safe rollout before encountering the obstacles on the other end of the field. To safely clear an obstacle at the approach end of the field meant that your approach would cause you to touchdown further down the field and thereby shortening your available roll-out distance. The alternative was to approach with a steeper descent angle, but an aircraft would accelerate in that angle and thereby use more runway to get stopped. The solution to allow steeper landing approaches without accelerating was landing flaps.

Figure 1 - Flaps-up versus flaps-down Landing

Quick Tutorial on Aerodynamics

Without diving into a bunch of theory, fixed-wing aircraft are affected by four basic forces: lift, drag, thrust, and gravity:

• Lift is provided by the wings as they move through the air. Lift offsets the force of gravity and wing area will determine how much weight the aircraft can lift given the available thrust.
• Drag is the result of airflow resistance across the airframe. All aircraft create drag as a result of trying to push the airframe through the air. The creation of lift also creates drag.
• Thrust is provided by the exhaust pressures of a turbine engine or through the translation of engine torque into thrust through the rotations of a propeller. To keep an aircraft aloft, thrust must offset the drag to sustain airspeed and altitude.
• Gravity is the force that returns an aircraft (or any other object) to the ground without sufficient lift to offset the force of gravity.

Figure 2 - Basic Aerodynamic Forces

Some additional definitions before we move forward:

• Angle of incidence – angle at which the wing is attached to the airframe
• Angle of attack – angle of the wing relative to the direction of flight
• Direction of flight – not only refers to the compass bearing direction but also to the vertical direction
• Ground roll – the distance an aircraft travels down a runway for take-off or landing
• Longitudinal axis – the line from the nose to the tail of the fuselage that defines the aerodynamic centerline and roll axis of the aircraft
• Stall – condition where a portion (or all) of the wing ceases to create lift
• Wing Chord – the line that extends from the leading edge of the wing to the trailing edge

Figure 3 - Angles in Cruise Flight

The illustration above shows the relationship of the various angles in cruise flight. The nose is pointing only slightly above the direction of flight thanks to the angle of incidence maintaining most of the required angle of attack in level cruise flight.

Figure 4 - Angles in Slow Flight

In contrast, at an airspeed used to to land, it takes more angle of attack to maintain level flight. The high angle of attack creates additional drag, which in turn requires more engine power to maintain airspeed.

Figure 5 - Angles in Descent - No Flaps

Pushing the nose over and throttling back, the aircraft will descend toward the runway, but you can see that visibility over the nose is poor to see where you're going and a steeper descent angle will only increase your airspeed again.

Figure 6 - Angles in Descent - Full Flaps

With the flaps extended, you must push the nose over to maintain airspeed as the flaps have created more drag as well as altered the aerodynamic chord so you can see over the nose again without speeding up in the descent. This allows for steeper approaches into airfields though I hope this chap remembers to lower the landing gear...

Figure 7 - Changes of Aerodynamic Chord with Flaps Down

Compare the diagram above with the first one showing the wing with its flaps up and you can see the difference in the purple line representing the aerodynamic chord.

Okay, we've covered WHY flaps are used and probably in more detail than the average modeler really cares. Now will look at a number of the more common types of flaps used on aircraft.

Different Types of Flaps

Split Flaps

Split flaps were the early answer to creating sufficient drag to allow for steeper landing approaches into airfields. Some examples of split flap-equipped aircraft include the F4F Wildcat, Hawker Hurricane and Supermarine Spitfire, but these were widely used in aircraft leading into World War II.

Figure 8 - Split Flaps

Figure 9 - Split Flap Interior of Fairey Battle

Figure 10 - Split Flaps on de Havilland Vampire

Split flap designs are recognizable as the upper surface of the wing remains unchanged when the flaps are lowered. These flaps are essentially a portion of the underside of the wing that would hinge down to increase drag. When looking up into the flaps, you can see structural details on the inside of the flap as well as up inside the exposed underside of the wing.

Conventional Flaps

Later conventional flaps simply rotated a portion of the wing trailing edge downward which also increased drag as with the split flaps, and was also more effective in reducing stall speed which further shortened landing distance. A good example of this type of flap is on the P-51 Mustang.

Figure 11 - Conventional Flaps

Figure 12 - Conventional Flaps on P-51 Mustang

Fowler Flaps

Fowler flaps are similar to conventional flaps as they also rotate down into the airstream, but unlike conventional flaps, they slide out from under the wing on tracks to increase wing area for additional lift at low flap settings before moving down to full flaps. The flaps are extended and retracted using either pushrods or jackscrews. The A-10 Thunderbolt II is one example that uses the Fowler flap system.

Figure 13 - Fowler Flaps

Figure 14 - Fowler Flaps on A-10 Thunderbolt II

Figure 15 - Underside of Fowler Flaps on A380

P-47 Fowler Flaps

The Republic engineers implemented a variation of the Fowler flaps, but instead of sliding the flaps out on tracks with a pushrod or jackscrew to extend and retract the flaps, Republic used an interesting set of hinges to create the Fowler effect.

Slotted Flaps

Most modern airliners and even some combat aircraft have slotted flaps. These are Fowler flaps that break up one huge flap section into two or more thinner airfoils that allow air moving under the wing to pass through the flaps and provide augmented lift like blown flaps, but without the bleed-air plumbing.

Figure 16 - Slotted Flaps on 747

Flaperons

Flaperons are outboard flap sections that behave like ailerons in normal flight, but will extend down with the inboard flaps for additional lift/drag. These flaperons will still move about while extended to provide roll control.

Blown Flaps

You'll see references to blown flaps out there and these were usually conventional flaps that had engine bleed air blown over the top of the flaps to further reduce the airspeed at which the boundary layer would depart the surface of the wing and create a stall. To the normal observer, these blown flaps look like conventional flaps.

Other Types

Junkers had a completely different approach to flaps and low-speed flight which had advantages in utility aircraft but created too much drag in high-speed applications. We won’t get cover the Junkers flaps in this article.

No Flaps

You might think that once flaps were in common use, every aircraft would be equipped with flaps. The North American F-100A and F-100C Super Sabres, Convair F-102 Delta Dagger, Convair F-106 Delta Dart, and Lockheed SR-71 are all examples of more modern aircraft that did not have flaps.

Do Your Homework

You can find out what type of flaps the subject you're modeling was equipped with either in your favorite reference materials or by going online and looking at good photos. Just be aware that sometimes a given aircraft type can change the type of flaps it uses during production, such as the example above where the early F-100 Super Sabres had no flaps, while the F-100D and F-100F were equipped with conventional flaps. Another example is the MiG-21 - early MiG-21s were equipped with Fowler flaps, but starting with the MiG-21PFM(SPS) Fishbed F, all subsequent MiG-21s were changed to blown flaps (which look like conventional flaps).

Each aircraft has different maximum and usually intermediate flap settings. Get a look at the angles that these flaps are set to replicate this correctly on your model. For instance, Hurricanes and Vampires have split flaps that are down almost 90 degrees when fully extended.

Modeling Flaps

Now that we've covered the visible differences of these flaps and what they do, let's look at how to model flaps:

Split Flaps

Depending on how detailed you wish to be, you can simply remove the molded-on flap section under the rear of the wing, sand the inside of the removed flap section flat and then to where the trailing edge is relatively sharp. The underside of the upper wing half mating surface also needs to be sanded to remove any ridge and to sharpen the trailing edge. You may need to add styrene strip to block off the inside of the flap well from the hollow interior of the styrene wing, then re-attach the flap at the desired angle.

More skilled modelers can add strip styrene inside the flap structure as well as inside the flap well, or you can get a photo-etched flap detail set like those produced by Eduard and simply origami the photo-etched parts into an intricate structure complete with structural lightening holes.

Conventional Flaps

Many modelers simply cut out the upper and lower flap sections from the wings, glue them together, then reattach at the desired angle. This might work, but you might want to do a few other steps to enhance the appearance of your work.

Sand the trailing edge of the wing where you removed the flaps and create a bevel of about 45 degrees like the area in green above. Glue the flap halves together, then add some square or rounded styrene stock to the leading edge of the flap and sand into shape to create a wing-like leading edge on each flap. Don't forget to gap-fill the sides of the flaps! Re-install the new flaps into the bevels and you'll have a solid join to glue together.

Flowler Flaps

Like conventional flaps, you'll be removing the upper and lower flap sections from the wing. Unlike conventional flaps, the flap section visible on the upper surface of the wing is much narrower than the lower surface.

This means you'll need to add more styrene to build-up the airfoil shape of the flap as shown in green. The upper half of the wing will need to be thinned as shown in red to create the flap well and a bulkhead (also shown in green) to close off the inside of that hollow wing. The fowler flap equipped wings use tracks to move the flap out and then down, and these are generally housed in fairings like the A380 image earlier. You'll need to alter the kit fairings to hold your flap in its new position and you're all set.

Conclusion

With these simple techniques, you can replicate these and other types of flaps in scale using little more that strip styrene and your original flap sections. Care must of course be taken not to damage the wing while you're doing this surgery. If the flaps get damaged, you were going to add filler strips anyway...

If you don't want to scratchbuild the flaps themselves, these tips will be useful for getting the flaps off the wings so you can use some of the nice aftermarket resin flaps that are out there for quite a few different aircraft and scales.

As with all projects, check your references to see what your project is supposed to look like when completed, but these tips will help you understand what you're looking at with these different approaches to flaps. In Part 2, we'll flip the wing around and look at leading edge flaps and slats.

Good hunting and happy modeling!