Little Plank (oz5163)

About this Plan

Little Plank. Radio control flying wing sailplane.

Quote: "Editors Preface: After receiving the construction article for the Little Plank from the authors, Chuck Clemans and Dave Jones, I was totally intrigued by the design of the aircraft and by its performance potential. The RCM prototype which I constructed from the authors original plans weighed 25 ounces and had a wing loading of approximately 5 ounces per square foot. The amount of 'rake-up' of the elevons proved to be less than shown on the plans and no engine of any type was used, our Little Plank prototype being set up for Hi-Start and electric winch usage.

The first few flights required some sorting out of the aircraft to obtain the proper trim for thermal flying. The model proved to be one of the most remarkable thermal soaring machines we have flown to date and competed favorably with many high performance thermal machines in the area. In fact, the full thermal potential of this ship was never fully realized even with over 100 flights to its credit.

It goes up on an electric winch, or Hi-Start, like it was on rails and, while its flight speed is quite fast compared to the conventional sailplane, it virtually acts as a 'thermal sensor' in the air. Without any dihedral whatsoever, and its extremely low wing loading, the model reacts to even the smallest thermal by raising a wing tip sharply. Turns can be made so tight that it is almost like a pylon racer and staying in a thermal is virtually a unique experience. While the Little Plank can be thermaled in a circular flight path similar to any conventional sailplane, by applying full up trim on our particular prototype, the model would shoot straight up the center of the thermal.

Once properly trimmed, and once the flyer has become used to this unusual machine, we found that it equalled the best thermal times of the high performance ships that we pitted against it.

The Little Plank is definitely not a beginners machine by any stretch of the imagination. With sensitive elevons, no dihedral, and a fast forward flying speed coupled with excellent penetration, the Little Plank requires your constant attention to fly it. You cannot take your eyes off this ship and expect it to be in the same attitude as you left it! This ship is more analogous to flying a full house competition multi insofar as it has to be flown all of the time and you can easily become disoriented due to its configuration.

We were extremely impressed with the performance characteristics of this model and its ability to perform with the best of them in thermal competition. If you are a soaring pilot who has passed the novice stage, we recommend this design to you for a complete change of pace in soaring as well as a challenge to your thermal piloting skill. It is an excellent design based upon several years of research by the authors and has a performance potential that has yet to be reached to its fullest extent.

The Little Plank is a design for slope soaring, thermal soaring, and sport flying. It makes use of the plank configuration to provide ease of construction and maximum performance with !ow power. The principle design features are:

Configuration: Flying wing with zero sweep back and constant chord.
Fuselage: No internal bulkheads.
Rudder: Centrally located, non-moving. permanently attached to wing.
Power: .049 to .10 in pusher configuration, prototype used TD .051 with Cox tank mount and TD .09 with Tatone mount and internal tank.
Controls: 2 channel with elevons for pitch and roll functions. Control Linkage: Uses sliding servo for roll and fixed servo for pitch. Both servos are mounted on a plywood rack to allow removal as a unit. Breakaway linkage to prevent servo damage due to crash.
Wing Section: S-1 with no under-camber. A 9i/i% section with mild reflex. A reflexed Eppler 385 section has been developed and will be tested in the near future.
Dihedral: None, tip shape provides small dihedral effect.
Construction: All balsa using standard wood sizes. Plywood used at stress points.
Weight: Less than 2 lbs., using PCS radio and KPS-10 servos and TD .051-2.5 lbs. with TD .09. 25 ounces as thermal soarer without power.
Performance: Surprisingly fast with TD .051. Capable of 10' diameter loops, rolls, and inverted flight. Six foot diameter turns possible on the slope. Will fly hands off when adjusted for circular flight. Tows well on electric winch or Hi-Start.

The Little Plank is a sport design for the Sunday flyer or others desiring a change of pace. While the plank configuration sacrifices efficiency due to the reflexed airfoil, it offers advantages in compactness, ease of construction, maneuverability and versatility. What other design can be thrown off a slope, winched, fly under power, and will loop, roil, fly inverted, alarm spectators and frighten birds?

Historical: The plank, or ironing board configuration, is not new. Our first one was a Dutch design called the 'Ironing Board'. This was in the late 1940's. Later the British magazine Aeromodeler published three views for a towline glider and the coordinates for a so-called self-stabilizing airfoil. This airfoil, a 9.5% reflexed, under-cambered section was the basis for a design series which included a 300 sq in towline glider and a 1200 sq in radio controlled slope glider in addition to the Little Plank. The Little Plank utilizes a flat bottom version of this wing section for improved penetration and simplified covering.

Design: The current design is the joint effort of Chuck Clemans and Dave Jones. Dave is the proprietor of Western Plan Service, 5621 Michelle Drive, Torrance, California 90503. A request to the above address will produce a list which includes a variety of designs for slope and thermal, with such unusual items as a semi-scale Spitfire for slope and a 2500 sq in plank for FAI record attempts. Dave is responsible for the basic aerodynamics and configuration while Chuck, who resides in the Seattle area, can be given credit for power modifications, construction and flight test.

The Little Plank is an attempt to produce an aircraft which one can carry assembled to the field and is at home either at the slope or in a thermal. It is maneuverable and requires a minimum of power. The pusher configuration was used because of efficiency advantages and to reduce the post-flight cleanup problem.

The size of the Little Plank allows the use of a TD .051 for sport flying or an .09 for the more adventurous. Power could be increased still further but is not recommended. The Little Plank is stong enough to accept a .19 but two problems must be resolved. The flat bottom airfoil with reflex will cause large trim variations between high and low throttle due to center of pressure movement. Further, the center of gravity must be between 17% and 20% for reasonable stability, which may be difficult to achieve with the weight of a larger engine in the tail.

The design of the wing tips provides a small amount of effective dihedral which, while improving stability for circling in thermals, does not cause problems in maneuvers such as roll, Immelman, etc. Elevons for control in pitch and roll simplify the installation since separate pushrods and bellcranks are not required for the ailerons. Aileron differential is not used on the original and does not appear to be needed.

Flight Tests Due to the unusual configuration and the variety of flight modes intended for this design, an extensive series of flight tests was undertaken. These tests were conducted over a period of several months in widely diverse weather conditions and resulted in several improvements to the original design. Slope tests were conducted with engine and propeller in place in conditions ranging from mild shear to straight-in at 35 mph. While the Little Plank can not be considered an exceptional dead-air machine, it is outstanding in high winds. Extended inverted flight (the reflexed airfoil requires about 1/4 down stick), rolls, and loops were accomplished with ease.

Powered sport and thermal flying presented nothing unusual but did suggest some changes. The engine run with the TD 051 using the Cox tank mount was considered a bit short for sport flying. A 1 oz. internal tank was mounted for use with the TD 051 and later tests with the TD 09. One thing to remember is that pushers tend to richen up in the nose-up attitude due to tank position which is the opposite of tractor configurations. This means that the engine should be adjusted with the aircraft in a slightly nose-up attitude prior to launch.

A TD 09 was installed to evaluate sport flying characteristics with additional power and to provide a faster climb rate to thermal country. The TD 09, while improving the sport flying characteristics, sacrifices soaring ability due to the extra 1/2 lb. which must be carried in the form of engine, tank, and ballast.

During the tests with the TD 09, it was noted that the Little Plank would sometimes protest audibly when the speed got above a certain point. A series of dive tests was undertaken to investigate this problem..."

Direct submission to Outerzone.

Update 15/03/2017: Have rescaled this one up to fullsize at just slightly over 56in, thanks to anon.

Supplementary file notes

Article pages, text and pics.

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