2007 Scale & Non-Scale Events

Are Flaps Necessary?

It' s always amazing that with the wealth of information constantly flowing from model aviation magazines and how-to books apparently so little is known about model aerodynamics.

Take aircraft flaps, for example. Unlike cyber-space and high speed modems, aircraft flaps have been with us for eons, yet a mystery persistently lingers about their true purpose, aerodynamic function and the pros and cons of their virtues regarding scale R/C models. The simple truth is that flaps and what they do, is the easiest type of airfoil to comprehend. (Click on Pictures to Enlarge)

Though flaps were first experimented with in the 1920s it was not until the advent of the monoplane and its higher wing loading that they came into widespread use in the rnid-1930s. Since monoplanes characteristically had higher stall speeds than most biplanes they subsequently landed at higher air speeds and required more runway length to become airborne. To counter this increase in landing and takeoff speeds the installation of flaps, or additional lift/drag capability, was the logical answer. Aerodynamically, the flap is a barn door- a lift/drag-creating device which permits slower landing speed and, therefore, decreases the required landing distance. 

Second, because they permit a comparatively steep angle of descent without an increase in speed they offer safe obstacle clearance on approaches to small airfields. An added benefit is that they may be deployed to shorten takeoff distances and provide a steeper climb path to altitude. An example of their effectiveness is seen in the WWII-era 8-25 Mitchell, landing at a gross weight of 30,000 Ibs. With flaps up the venerable Mitchell came "over the fence" at 120 MPH, whereas with full 45-degree flap setting this speed was reduced to only 90 MPH with a resulting decrease in rollout of only 2500 ft of runway, as opposed 4000-ft with flaps up (and a lot less wear on brakes and tires). Since flaps allow steeper angles of descent with the nose pitched lower, they also give pilots a better view of the landing area.

Second style is the split flap which lowers from the bottom of the wing as primarily a drag inducing surface. The split flap does not offer an appreciable increase in lift and was successfully used on such aircraft as the Douglas DC-3, British Spitfire, Curtis P-40, and North American T -6, among countless others. The third type was the patented Fowler Flap which deployed on tracks from beneath the wing in an expanding manner that when fully deployed, also increased the total wing area, thereby decreasing wing loading while still providing additional lift and substantial drag.

Fowler Flap installation showing (1) up-stop inspection slots, (2) wing Splice, (3) wing T.E., (4) aileron, (5) flap-track support, (6) flap track, (7) flap carriage, (8) flap extending cable attachments, (9) flap, (10) wheel well fairing, (11) engine nacelle fairing.

When Cessna revised the airfoil in its 1977 models for better stall characteristics the pilot's handbook then recommended the use of 20-degree flap settings for ALL takeoffs. Large high performance aircraft like the B-25 Mitchell require 20-degree flap settings for take-off, yet oddly enough one of Jimmy Doolittle's B-25 TOKYO RAIDERS mistakenly took off from the deck of the USS Hornet without lowering any flap on the launch of their historic 1942 bombing mission. Named the "Ruptured Duck," and piloted by the late Lt. Ted Lawson, the heavily-loaded bomber dipped precariously close to the sea but did manage to fly after a deck run of 700 feet with maximum fuel and bomb load.

That's how flaps perform on big birds but are they truly necessary on scale RC models? The answer boils down to a matter of scale size. The larger the scale the more the model emulates the flying characteristics of its full size counter part. This is because air density does not vary while airfoil lift factors do. Mathematically, Reynolds numbers which deal with theoretical chordwise airfoil lift factors com into play. Without getting to far into the math books, lets just say that large scale models of high performance aircraft, like most WWII-era fighters and bombers, greatly benefit from operating flaps. Contrarily, they are of dubious value on large scale (over 5-foot wingspan) light planes such as the Piper Cub, Taylorcraft BC-12D and Aeronca 7 AC Champ varieties which inherently boast light wing loadings and have slow landing speeds.

Remember too, that improperly used or deployed too prematurely flaps can be hazardous to flight performance since they drastically alter approach speeds to landing. The matter of model size is critical in regard to approach airspeeds since flaps on a small 36-inch Piper Tri-Pacer will also serve to lower its landing speed, just as they would on a small scale Spitfire, or P-5l Mustang. The difference here is the problem of additional mechanism weight; the complications of additional servos, push rods, bell-cranks, etc. In other words, the question becomes a trade-off in handling effectiveness versus model flying efficiency; which is more valuable? The smaller the model the more critical weight becomes. Is this then a viable trade-off in a light model that has reasonably slow approach speeds? Personally, I doubt it is.

To summarize the question of flaps we have only to look at the great air racers of the 1930s -the speedy Gee Bees, Laird Solution, Mr. Mulligan and the others of their era that broke world records at the Thompson and Cleveland Air Races. Few of these speedsters were equipped with flaps because the weight of this additional mechanism would denigrate their quest for pace-setting speed. For the most part these were finely-tuned machines primarily built for a single purpose, to win trophies and break records. They were flown by the hottest pilots in the world from large airfields that could handily accommodate today's Boeing 747. The answer then "to flap or not to flap" lies strictly in the type and size of scale model a R/C pilot is skilled enough to handle. The choice is yours alone...

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