Building the MachNone

The booming ultralight movement has been recognized by many as the beginning of a new era of low cost fun flying. The flurry of activity among builders and the joy of flying these simple machines is a long-awaited breath of fresh air to both the established aviation community and, more importantly, to those still on the outside looking in. The "blue sky" innovation possibilities make the design of an ultralight aircraft among the most interesting of challenges.

The Blue Light Special is a one-of-a-kind ultralight designed and built by a small group of Cessna Aircraft engineers which included Don Berry, Lyle Bitker, Mike Lovelace, Larry Weaver and Steven K. Wood. The airplane is admittedly and unabashedly conventional in appearance and was designed with the philosophy that good adherence to well-known principles can produce reasonable performance on relatively low power. The present trend of improving ultralight performance by the simple addition of bigger, more powerful (and subsequently heavier) engines is alarming and directionless. This trend must be arrested if we in the sport intend to remain self-policing and continue to show those watching us grow (read FAA) that we can exercise enough restraint from "bigness" to maintain the safety record inherent in our low kinetic energy flying machines.

The hang glider heritage of ultralights is still very much present in the current crop of production models. This is understandable. If it were not for the humble beginnings of the hang gliding movement a decade ago, ultralights would never have gotten the "foot-in-the-door" advantage over their licensed counterparts that they have enjoyed. To attempt to achieve optimum performance commensurate with the current foot-launching requirement is only natural. Yet, most designers have avoided conventional layouts and structures because they appear heavy and hard to build. It should be kept in mind, however, that most airplanes are configured the way they are for some very good structural and aerodynamic reasons. This does not mean that what has already been done in the field of ultralight design is bad. It simply means that we can achieve good results by putting to use well known design principles.

The design of the Blue Light Special involved reaching an acceptable compromise of the following basic objectives:

These criteria are so familiar and simple that they hardly bear repeating. However, a clear definition of the desired objectives is essential to decision making when compromises must be made.

As noted, the foremost design requirement of the Blue Light Special was foot launchability. All other goals had to yield to the stall speed and empty weight limitations of this requirement. Thus, as with all airplane projects, the wing was designed first. The wing features the relatively thick NACA 43018 airfoil section, which gives good low speed performance and allows the use of a single, deep (and thus lightweight), cantilever spar. A rectangular platform was selected to minimize building time and insure gentle root-first stall characteristics without the added complexity of geometric twist. The wing was designed in two panels which are joined together and to the fuselage by four bolts. This feature allows moderate transportability and a short assembly time.

Since the wings do not mount ailerons, the torsional strength requirements, and thus the panel weights, are low. This also simplifies the control system and reduces building time significantly. Six degrees of dihedral has proven more than adequate for good roll response with the two-control system.

After considering every imaginable combination of aluminum, foam, wood and fiberglass for the structure, we chose a composite of Dow blue Styrofoam and wood. Low cost, availability and workability were key considerations here. Throughout the airframe, foam is used as shear web material and wood is used to carry bending loads. Needless to say, this method cannot be used on bigger, heavier aircraft because they lack adequate cross-sectional area (in the wing, primarily) to carry the shear loads. The concept, however, could be utilized by substituting a more dense foam with a higher shear strength. A positive ultimate load factor of 6 was used during the design analysis.

The empennage members are cantilever and are constructed similar to the wing. The stabilizers are two-spar in structure while the rudder and elevators each utilize a single spar/torque member and wood trailing edge. Control surface hinges are of cloth (after model airplane fashion) and, surprisingly, are vastly over-strength. This feature reduces the hardware requirement and building time significantly.

The fuselage is basically a foam box with wood longerons at the corners of the cross-section. The turtle-back is for streamlining and is not structural. Aircraft plywood was used to distribute concentrated loads at the landing gear and wing attachments. Since the straight-tube axle serves as the forward edge of the seat, the load path for pilot inertia loads during landing is very direct and superfluous structure is eliminated.

The entire airframe is covered with a lightweight material called sheath lining. It is available in any fabric store and is commonly used as a liner for suit jackets and coats. The material was attached to the airframe with latex contact cement so that no stitching is required. The sheath lining is heat-shrinkable and comes in a variety of colors. Two coats of polyurethane varnish were applied to seal the cloth and provide a somewhat glossy finish.

The main landing gear is comprised of a cantilever aluminum tube to which moto-cross bicycle wheels are mounted. A tail-skid made of sheet metal is mounted to the stern post. Recently, the wheels were fabric-covered for streamlining and aesthetics.

The Blue Light Special is powered by a McCulloch MC-101B with a 3 to 1 multiple v-belt reduction unit. This combination is fitted with a 48" x 22" Ritz propeller which seems fairly well matched to the application. A flat-wrap cowling of .020 aluminum streamlines the engine and reduction unit. Thus far, the engine has been operated with its factory-equipped muffler box with no effort toward tuning. The noise level is fairly intense, and ear plugs are a must for long flights.

Construction was begun in early May of 1980 after about two months of design work. The project progressed at a steady pace and the bird was ready to fly by the end of August. Total cost of the project stood at $974.62 when the craft was completed. The materials selected proved easy to work, and no major difficulties were encountered. The single most time consuming stage of construction was the fabrication of the reduction unit, components of which were made on various lathes in the Wichita area.

After taxi-testing and the addition of drag reducing fairings, the Blue Light Special was test flown on September 4, 1980. The bird was "crow-hopped" the length of Graham Field at Wichita, Kansas several times before the first circuit was made. These short flights allowed time to evaluate the effectiveness of the two-control system and the engine performance. The test flying went without any difficulty whatsoever and the aircraft soon endeared itself to all who flew it.

The Blue Light Special possesses that rare and wonderful flying quality of being docile yet spry. Thus, its handling qualities have been most pleasing to "old timers" and beginners alike. The two-control system seems to work especially well with new fliers as the craft was successfully soloed by Lyle Bitker (whose previous flying experience was nil) after a few supervised low-level hops.

Stability about all axes is excellent without impairing maneuverability. Naturally, control forces are light, yet over-controlling has not been a problem. Everyone who has flown the airplane has commented their surprise at the effectiveness of rudder-only lateral/directional control. Of course, the aircraft yaws briefly during initiation and recovery from banked turns. Once a turn is established, however, the aircraft will maintain coordination on its own.

Stalls are mild with plenty of advance warning and no tendency to "break" unless purposely precipitated from extremely nose-high pullups. Recovery is uneventful, with altitude loss all but indiscernible.

Directional control on the ground is straight forward and will surprise pilots accustomed to heavier "taildraggers" since the combination of low inertia and relatively large aerodynamic restoring forces makes the aircraft impossible to ground loop.

With the rope starter removed for weight saving, the engine is hand-propped for starting. A cockpit-controlled compression release takes the "snap" out of the operation. The McCulloch engine has proved its reliability by providing trouble-free power for a present total of about 35 flying hours.

If the performance data in the accompanying table seem less than dazzling in comparison to the advertised performance of other ultralights, it is because no attempt has been made to exaggerate the truth. It has been our observation that many promoters of ultralights are given to overstatement of cruising speed, glide ratio and rate of climb figures and to understatement of stalling speed and empty weight figures. Often, glide ratio and rate of climb information is stretched by a factor of two or three from the aircraft's true performance. This sad fact creates a "first-liar-doesn't-stand-a-chance" syndrome among advertisers and destroys credibility in the sport. It would almost seem that only those with nothing to sell can afford to "tell it like it is".

No commercial endeavors are planned for the Blue Light Special since the project was undertaken primarily for fun. The educational benefits and the pleasure of flying the aircraft have been ample reward for the effort invested. It is the hope of its builders that some features of the Blue Light Special will prove their merit by being included in future ultralight designs.