There are several types of flaps and a fair amount of information required to obtain optimum performance from flaps. First, there are several types of flaps commonly used on stunt ships.
One of the most common configurations is built-up with either straight or geodesic ribs. This type of construction has both advantages and disadvantages. The biggest advantage is that they tend to build with both flaps having equal stiffness. They are less dependent on obtaining near identical wood for each of the two flaps. With near equal stiffness, they usually perform satisfactorily without obvious faults. If they flex under load, at least, they flex equally and usually do the job of providing an expected amount of lift. This twisting under load is referred to as "washing out". Personally, I have never seen really stiff built up flaps. I gave them a pretty good try when I built my Hornet. Even with a couple of layers of glass and epoxy, they weren't stiff enough to be depended upon to provide the lift the Hornet was going to need. Lacking really effective flaps, I faced the probability that the airplane would lack the really tight corner required for top competition. I stopped work on the airplane and hung it up, unfinished. Maybe finishing the flaps with carbon and epoxy would have given it the strength necessary for a really competitive airplane, but I doubt it. In any case, light carbon cloth wasn't generally available in 1978 when I built the Hornet.
Another common flap construction is to simply use the sheet flap, cut to outline, hinged and left untapered. This flap has a high degree of sensitivity around neutral, builds easily, and is easy to rig accurately with its parallel flat surfaces. It isn't the most efficient use of material from the weight and strength standpoint. Still, it is an easy type of construction and also very satisfactory in use if care is taken in wood selection to obtain equal stiffness of the two flaps. Even slab flaps will "wash out" if the wood is too light or a finish used which doesn't significantly strengthen the surface of the wood.
The next type of flap is the tapered flap of sheet construction. This is also a very satisfactory type of construction which has the aerodynamic effect of being less sensitive around neutral. This too is a characteristic which can be exploited if one understands the aerodynamic implications of tapered flaps. The thinner trailing edge of tapered flaps place an even greater demand on wood selection and finishing to make flaps of adequate and equal stiffness. Tapered flaps tend to be weaker and more prone to "wash out" but have a more finished appearance, particularly on semi-scale airplanes.
The last type flap is the "balls to the wall" all out effort to build the stiffest possible flaps where the absolute maximum possible lift is needed from the flaps. I use this construction. To build these, a carbon torque tube leading edge is used with tapered balsa behind and covered with epoxy and glass cloth. This carbon tube transmits torque from the flap horn all the way to the wing tip. The carbon tube is rigid and has an exceptional ability to resist twisting. It will bend, but this isn't a problem as it is attached to the wing trailing edge with a number of hinges and the wing itself provides the strength to resist bending. If a twisting force is applied the the torque tube flap, there is very little "wash out" as the distance from the flap horn increases. If the same twisting force is applied to flaps with no torque tube, those flaps will twist and "wash out" somewhat, depending on the strength, thickness, construction and grain of the wood. "Wash out" in most flaps increase as the distance from the flap horn increases. Flaps glued to the torque tube have very little "wash out" toward the tips.
My original molded Mustang, "E2-S", had 3/8" quarter grain, 6 lb wood. They weighed about 2 1/2 oz for the pair. They were covered with two layers of glass cloth and epoxy glue. Unfortunately, these flaps were not made from matching sheets of wood. This resulted in the outboard flap being much less stiff in torsion than the inboard flap. It "washed out" under load much more than the stiffer inboard flap. The caused the inboard flap to roll the airplane in maneuvers. This rolling was controlled by reducing tipweight. I flew the 1974 NATs, with no tipweight, to 3rd place. The absence of tipweight limited the airplane's ability to fly well in wind. Fortunately, the winds were calm throughout the contest. After the NATs, I discovered the problem and glued extra area onto the trailing edge of the outboard flap. This increased that flaps ability to generate as much lift as the stiffer inboard flap. This improved the airplane dramatically. With a balanced and stiffer flap set, I was able to get a bit more lift for tighter corners. I was also able to return to normal tipweight for a big improvement in line tension and windy weather capability. It was a good lesson.
When I built the next Mustang, "Snaggletooth", and the Mustang "G4-G", intended for the World Championship, I used matching 7 lb quarter grain 3/8" flaps with the two flap sheets cut side by side from the same block of wood. This is what I call "matched" wood. These flaps were covered with five layers of glass cloth and epoxy. This made very stiff flaps. There were no difficulties in trimming these airplanes.
"Snaggletooth 2" was destined to be a heavier airplane because of the use of a heavier engine and heavier glue in the assembly. I could probably have saved several ounces by using water thinned Tightbond. Water thinned Tightbond is adequately strong and very light when dry. I wanted to offset the greater weight of the airplane by obtaining more lift from the flaps instead of increasing the wing area. Howard Rush had posted a photo of an airplane with built up flaps based on a torque tubes for torsion. It was an intriguing idea, likely to work, if a satisfactory solution could be found to fabricate them. I had a jig constructed for routing the leading edges, and another jig machined for drilling the carbon tubes. The flap wood was glued to the carbon tubes and covered with one layer of glass cloth and epoxy glue. The stiffness of this set of flaps is amazing. They are the stiffest flaps I ever built by far. In flight, these extra stiff flaps overpowered the elevators. This made the airplane turn slowly in corners. I had to tape the elevators to get more leverage from the tail. This worked too, and I wound up with both the lift I was seeking in design, and the all the corner I could use.
Today, I am building a new set of carbon torque tube flaps for my Critical Mass to obtain more lift to compensate for installing a Saito 91 where the design originally called for the lighter Saito 72. The modifications to the Critical Mass will also include larger elevators, with tightened hinge gap, to compensate for the increased flap lift which would otherwise overpower the elevators unless taped. I'd rather add elevator area than tape the hinge line. This modification to my design parameters will be incorporated into all of my new designs having carbon torque tube flaps.
I have a new, unfinished "Millennium 2" designed for a PA or Ro-Jett 65. Needless to say it will also get carbon torque tube flaps, larger elevators and a larger heavier engine, in the form a Ro-Jett .77.
I do not recommend carbon torque tube flaps for typical stunt ships. They are not needed. Realistic, semi-scale airplanes, however, tend to build overweight. In fact, they are not overweight, they are "underwinged". It amounts to the same thing except for the appearance and performance of these small wing, relatively powerful, airplanes.
For typical stunt ships, in my opinion, matched flaps of reasonably stiff. quarter grain 7 lb wood would be optimum.
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Once again, when using nontapered slab flaps, there are both structural and aerodynamic considerations. Structurally, nontapered flaps are unnecessarily strong and heavy, for their purpose, as the distance from the hinge line increases. Maximum lift available from flaps is dependent on the area, deflection and stiffness of the flaps. Very stiff tapered flaps may have as much or more lift than less stiff slab flaps.
Aerodynamically, slab flaps will have increased sensitivity. Some airplanes, the Challenger 604, for example, have blunt trailing edges on the ailerons. This improves aileron effectiveness on that airplane. Is it necessarily desirable to have very sensitive flaps with small deflections? Slab flaps will have more lift with small flap deflections than a tapered flaps. With small control imputs, elevator response will be the same but tapered flaps will lift less than slab flaps. This reduced flap sensitivity around neutral emphasis the elevators for flight path changes.
By now, you have probably noticed that the same logic applies to the elevators. Less elevator sensitivity around neutral was the reason some builders intentionally added "slop" to their elevator control. Small control imputs would give relatively little reaction from the elevators in pitch allowing the airplane to climb or descend slightly from changing the lift of the sensitive flaps. On the L-1011, we called this effect of changing lift without pitch "direct lift". Direct lift made for smoother glide slope corrections. We could correct deviations from the glide slope, up or down, with Little or no pitch change giving a smoother ride to the passengers.
I design a bit of direct lift into my airplanes as well without using slab flaps. I use thicker stabilizers than elevators to create a "dead spot" around neutral. I think this makes for smoother level flight corrections and nicer rounds.
Al
