Section 2 : Design Philosophy
Before getting into the construction details of your RV, let's take a look at the design philosophy and goals that are the basis for this airplane. The goal was to achieve the maximum overall performance, flying enjoyment, ease of construction, building and flying economy, ease of maintenance, and pleasing appearance possible for a two-place airplane. Understanding how this was achieved might help you better appreciate many of the RV's features as you encounter them during construction.
The formula for achieving total performance is amazingly simple: Maximize thrust, minimize drag; maximize lift, minimize weight. The implementation of this formula is a bit more complex, however.
Thrust, for a given HP engine, has been maximized through use of a good propeller, streamlining of the engine cowl (through the use of a crankshaft extension), and directing the engine outlet rearward. Drag was minimized by keeping the aircraft frontal area to a minimum, using smooth surfaces, and shaping all airframe components to reduce aerodynamic drag. Lift was maximized through use of a wing with adequate area, good airfoil, and smooth surface finish. Weight is minimized by careful structural design, by using the best airframe materials, and by installation of only essential instrumentation and equipment.
Most of the literally hundreds of features which comprise the overall RV package have been determined in the design stage and involve no choices for the builder unless he chooses to make major modifications. There is little that a builder is likely to do which will have much effect on either thrust or lift of his RV. However, construction techniques and choices of installed equipment can have noticeable effects on both drag and weight; the archenemies of performance.
The RV's "traditional" configuration; tractor engine, monoplane, stabilizer in the rear, is an exercise in logic, not simply a concession to convention. There are many good reasons why light planes have been built this way for decades, other than the often heard arguments of "entrenched design mentality" from those seeking "technological breakthroughs". The bottom line is that this configuration has proven to offer the best compromise resulting in the best all around airplane. A discussion of every factor involved in analyzing and choosing each design feature is too involved for this presentation. However, let's examine one choice; tractor vs. pusher engine/prop installations.
Pusher engine/props have always intrigued designers. They offer the possibility of drag reduction, by eliminating the disturbed airflow over the fuselage and inboard wing surfaces, and contribute to better cockpit visibility. So far so good, but with the propeller aft of the main wheels, ground clearance becomes a problem as rotation is required for take-off and landing. If the pusher engine and prop are located near the trailing edge of the wing, this problem is minimized, but others are encountered; i.e.: the blunt shape of the aft fuselage and the need to build a more complex twin boom arrangement. The end result is of questionable value from the drag reduction standpoint. Locating the prop at the extreme rear of the airplane, behind the tail surfaces, optimizes streamlining but requires a complex, heavy, and expensive drive shaft system between the engine and prop. Either way, engine cooling is impaired.
One other seldom discussed drawback of a pusher is that, for any given take-off and landing requirement, it will require more wing area (and thus additional weight and drag) than a tractor.
Ironically, the reason for this is the same as given as the benefit of the pusher configuration; the fuselage, inboard wings, and tail surfaces are out of the prop stream. Without the accelerated airflow from the tractor propeller over these surfaces, some lift and controllability is lost, thus requiring a higher landing speed for any given wing loading. The point is, some otherwise good ideas don't always work well in the real world.
The constant chord wing plan form chosen for the RV series offers the ultimate in construction ease,stability, and lifting ability. The possible drag and aesthetic penalties for the rectangular wing are negligible in light of its advantages. The airfoil used is a modified NACA 23013.5, an old wing section often maligned In " airfoil selection" articles and texts. However, this basic airfoil section has been used on some of the world's most successful airplanes ranging from the Taylorcraft and Helio Courier on one end of the scale, to the Turbo Commander and even the Cessna Citation on the other end. Others using it include the DC-3, all tapered wing Beechcrafts, and many of the Cessna twins. The RVs are in good solid company.
In addition to a good stable wing plan form, the RVs incorporate a relatively long fuselage along withlarge tail surfaces for plenty of control authority.
Seating arrangements vary between the RV designs, depending on the primary mission envisioned.
Side-by-side seating was chosen for the RV-6/6A because this arrangement is generally preferred for its primary mission: cross-country flying. Specific advantages of the side by side configuration includeequal visibility for both occupants, more easily achieved dual control capability, lots of instrument panel space, minimized CG travel for various loading conditions, and a full cowling with room for engine accessories and plumbing.
Tandem seating was chosen for the RV-4 because this arrangement offers lower drag, better pilot visibility, and better aesthetics with more appeal to the " sport" pilot. Because the visibility is better, the airplane is flown solo from the front seat. Handling and center of gravity would be better with rear seat solo, but visibility is of sufficient importance to override these considerations. The superb front seat visibility of the RV-4 adds much to its ease of landing and the feeling of confidence experienced by the pilot.
The RV-8/8A retains all the advantages of tandem seating, but the roomier cockpit and second baggage compartment make cross-country travel more comfortable and practical. The full width cowling was designed to handle engines up to 200 horsepower.
Designers often use the term "Mission Profile" which simply refers to the function an airplane is designed, to perform. The RV's mission profile is rather broad- they were intended to fill nearly all sport flying needs - speed, STOL, limited sport aerobatic. Meeting all these needs required a design "balancing act". Favoring one need often adversely affects others. An example would be emphasizing cross country cruise performance by installing extra radio, instruments, and upholstery. The weight added would adversely affect all other performance parameters. This is not a "maybe" , it is a certainty.
Whether the trade-off is worthwhile is a decision that can only be made by the builder.
We feel that an RV in its basic form with fixed pitch prop, modest instrumentation and radio, and a carbureted engine, represents the best compromise.
Cruise speed of an RV can best be improved by reducing drag; through attention to finish and fitting details. Constant speed props do not necessarily improve cruise speed, but they do offer the opportunity for RPM control, which improves cruise fuel consumption. Additions of instrumentation, radio, and interior appointments do not help cruise speed, but do add to the ease and comfort of X-C flying.
STOL performance, or at least take-off and climb, can be improved with a constant speed prop.
However, under most conditions, landing performance dictates field length requirements, so the constant-speed would offer unneeded performance at the expense of dollars and weight.
Obviously, we could go on and on, covering every design decision, compromise, or concession.
However, it should be obvious by now that every feature of the RVs, whether major or minor, was the end product of much deliberation. In almost all instances, these features are so inter-related that altering one will affect several others; meaning that a builder should not consider making changes unless he is willing and capable of analyzing the overall impact of the change.