SECTION 2
DESIGN PHILOSOPHY
RV AIRCRAFT
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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 planform chosen for the RV-3/4/6/7/8 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 RV-9 series uses a Roncz airfoil, designed specifically for the job the RV-9 is intended to do.
In addition to a good stable wing planform, the RVs incorporate a relatively long fuselage along with large 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-7/9 because this arrangement is generally preferred for its primary
mission: cross-country flying. Specific advantages of the side by side configuration include equal 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.
RV AIRCRAFT
SECTION 2
DESIGN PHILOSOPHY
section 2r2.doc 3/05/02
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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 carburated
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.
