Steve Kerry writes, regarding the Fowler flaps for his He 177 model:

The model designs I've seen and/or read about usually have the flaps moving back and down in a constant arc. But all my research (and I've been digging up quite a bit of it) shows the He 177 flaps moving back with hardly any droop at all. This would give a large increase in wing area across the entire span (including the ailerons!) with only a slight camber change, presumably for takeoff. Then, at the end of their travel, the flaps (excluding ailerons) can droop some 30-40 degrees for landing with virtually no rearward movement at all.

From : Don Stackhouse

There are several common ways of hinging Fowler flaps. The simplest is to pivot the flaps at a point well below the wing, so the flaps swing aft and down in an arc, as you describe. This method is mechanically the simplest, but the travel geometry may not be aerodynamically optimum, and it usually means having large brackets for the flap hinges hanging out in the airflow under the wing. The DC-9, DC10, a number of regional airliners like the ATR series, and the Beech Sundowner are examples of aircraft that use this system.

Another pupular method involves cam rollers on the flaps that ride in slots on brackets extending aft from the fixed portion of the wing. The geometry of the slots and the location of the rollers determines the geometry of the flap travel. Generally there are two rollers at each bracket, and either one or two slots in each bracket. The Boeing airliners and the venerable Cessna 150 and 172 all use this system.

Other systems are also possible. For example, the Me 262 uses a swing arm at the nose of the flap and a roller-and-slot at about 30% of the flap chord.

An excellent source for drawings of a variety of flap linkage systems are the beautiful cut-away views of various aircraft published in "Flight International" magazine. These often include a little cross-section view showing the flap travel geometry and the linkages that create it.

...in flight, which way are Fowler flaps being pulled? If all the connections were to suddenly shear off, leaving the flaps free to run up and down their tracks/rails, where would they go? Gravity would call them out and down, but I suspect the airflow would push them back up at least some of the way. What I mean is, what sort of loads can I expect and which direction are they going to be going?

The loads at full deflection are generally huge, and normally in the direction that tends to retract the flaps, at least to the point of nearly zero angular deflection. If you had an initial section of their travel that was straight back, before they began to deflect downward, they might tend to float near the aft end of that travel segment.

You can get a feel for just how huge the forces are by flying some full-scale light aircraft that have manually-operated flaps. The Beech Sundowner, the Piper Cherokee series, and the DHC-1 Chipmunk are examples of this. The flap handle is generally enormous, and it still takes a fairly muscular pull to get the flaps extended to the last detent in flight.

Another indication is flap extention redline speeds. For example, on the DC-3 (split flaps, but the numbers are similar to equivalent aircraft with Fowlers), the aircraft's maximum indicated airspeed ("clean") is 286 mph. You can extend the wheels at 150 mph, and you can extend 1/4 flap at 152, but for 1/2 flap or full flap you must be below only 112 mph. Normally you get that slow in flight only during the last stages of the landing approach.

Flaps make a lot of lift, but they put a lot of load on their structural attachments and actuators in the process of doing so. This should not be a surprise, when you consider that a good flap system can approximately double the total lift of the wing!

There was a rather graphic example at Chicago O'Hare airport a number of years ago of what happens when flaps become unrestrained. The DC-10 uses two shear pins to attach each wing engine, designed to fail and let the engine fall safely free of the aircraft if it locks up. Without this, the tremendous rotational inertia of the engine rotors could rip the engine out of the airplane, perhaps taking the pylon and parts of the wing spar with it. These forward pins, which normally carry most of the weight of the engine, have required inspections. There is a special (and somewhat time-consuming to install) fixture used to support the engine while the forward pin is pulled for inspection. Some of the mechanics were taking a shortcut, using a forklift and a strap instead of the fixture to support the engine. They didn't realize that in the process they were overstressing the rear pin, setting it up for a premature failure.

Just as the subject aircraft (with a full load of passengers) rotated on takeoff, the rear pin failed. Still producing takeoff thrust, the engine pivoted forward and upward around the forward shear pin, going up and over the top of the wing and taking all of the hydraulic lines that ran inside the leading edge of the wing with it. These lines supplied the flap actuators. Suddenly without hydraulic pressure to keep them open, all the flaps on that side of the airplane immediately RETRACTED.

With takeoff flaps still deployed on one wing, and zero flaps on the other, the ailerons were hopelessly overwhelmed by the massive difference in lift between the two wings. The airplane immediately and inexorably rolled toward the flapless wing, carving a long, deep furrow in the ground with its wingtip before finally impacting nose-down and nearly inverted. There were no survivors. The missing engine, with its failed shear pins and some fragments of hydraulic lines, was found lying on the runway not far from the point of liftoff.

They grounded the entire DC-10 fleet while they figured out the cause, and for a while the DC-10 was the safest airplane in the airlines' inventory - it didn't carry passengers, or leave the ground!

Think about that when you're designing your flap linkages, and try to make it as unlikely as possible for the flaps to be deployed unequally. If that unfortunate situation does occur, it is extremely unlikely that your model will survive.

Don Stackhouse
DJ Aerotech