Background

Contemporary aerobatic aircraft have evolved directly from, and are conceptually very similar to, conventional light aircraft. They have an engine at the nose, tail surfaces at the tail, with the pilot somewhere in between, and monoplane or biplane flying surfaces. What distinguishes today’s best aerobatic aircraft from other light aircraft is primarily:

• Structural strength.
• Power-to-weight ratio.
• Inverted capability (airfoils, fuel and oil systems, etc.).
• Control authority.
• Stall/snap/spin behavior.

If one looks objectively at this conventional light-aircraft configuration, however, one sees serious compromises for aerobatic flight. The heaviest component, the engine, is placed at the forward extremity of the airplane where, along with the mass and gyroscopic inertias of the propeller, it resists attitude changes in pitch and yaw. The pilot’s visibility (vital for precision maneuvering) is inevitably restricted by the engine, fuselage, and wing. Engine torque, propeller gyroscopic effects, and a rotating propeller wash cause the airplane to behave asymmetrically, requiring pilots to learn many maneuvers twice—left and right. High yaw inertias and a high-set rudder degrade spin safety. Little consideration is given to knife-edge (side) flight or to aerodynamic control at low and zero airspeeds. The 4-cycle engines used in most Western aerobatic aircraft were originally designed for light cruising aircraft; they are heavy, bulky, and expensive, and the punishment subjected upon them by unlimited aerobatics leads to frequent (expensive) rebuilds.

The Evolution of the Aerobatic Airplane

Early sport aerobatic aircraft were adapted from conventional military trainers and sport aircraft. They were generally oversized and under-powered, leading to sluggish maneuvering rates and dismal vertical performance. Aircraft would often have to climb to altitude between maneuvers, and they did not enjoy negative-G flight.

A significant breakthrough occurred with the arrival of one of the first purpose-built aerobatic machines, the Pitts Special, during the 1950’s and 60’s. The Pitts achieved superior maneuverability through its small size and high power-to-weight ratio. The Pitts S1S and S1T (basically high-powered Pitts Specials with symmetrical wings) dominated aerobatic competition until the arrival of a new breed of monoplanes based on the Stephens Acro. The monoplanes eclipsed the Pitts in one major area: vertical performance, achieved through lower parasite drag. The lower drag permitted higher airspeeds, which correspond to higher energies, which could be translated into altitude. Thus, even though the early monoplanes were often heavier than a similarly powered Pitts, their vertical performance was considerably better.

The early aerobatic monoplanes were soon eclipsed by the Russian Sukhoi SU26 and the German Extra 300. These aircraft represented an incremental evolution with refined aerodynamics and control systems, modern composite structures, more powerful engines, and larger propellers. The prohibitive cost of these “superships” has since led to several custom-built variants, such as the Edge 540 and Staudacher 300, while at the same time there has been a push towards lighter, cheaper variants using the Lycoming 0-320 and 0-360. The Rihn DR-107 “OneDesign” and DR-109 and the Giles G200 and G202 represent valiant attempts to lower the cost of unlimited aerobatic competition, while giving up little performance. Nevertheless, the quest for additional performance is ongoing, as is demonstrated by the development of carbon wings for the OneDesign and two high-powered variants of the G200: a Lycoming 0-540 powered machine for a member of the US aerobatic team, and a Pratt and Whitney PT-6 turbine powered machine for a well-known airshow pilot. These latter two aircraft represent probably the state-of-the-art with respect to aerobatic performance. Yet conceptually they are identical to previous aircraft, with the identical in-built limitations.

What is Aerobatic Flight?

In 1961 José Luis de Aresti published his dictionary of all possible aerobatic maneuvers—possible, that is, for the Bücker Jungmeister. This dictionary has since grown from around 3,000 maneuvers to some 15,000 as pilots have learned to exploit the capabilities of modern aircraft. History has shown that aerobatics evolves as aerobatic aircraft evolve; as aircraft have become capable of new maneuvers, pilots have always been quick to exploit those capabilities.

Consequently we may ask: What exactly is aerobatics? Removed from all technical limitations, aerobatic flight could be defined as precise maneuvering in 3-dimensional space. Such maneuvering has three aspects:

1. Position (in 3 dimensions)
2. Velocity (in 3 dimensions)
3. Attitude (on 3 axes)

“Ideal” aerobatic flight would involve complete precision on all three points. That is, an idealized aerobatic aircraft would be capable of attaining any combination of position, velocity, and attitude, and could move to any other combination quickly and precisely.

Obviously, perfection on all these points will never be attained by a flying machine which relies upon dynamic interaction with the atmosphere. This definition does give us a measuring stick, however, with which to judge our existing aircraft and, more importantly, to help us develop something better.

The State of the Art

Today’s best aerobatic aircraft are well-executed machines which perform well within their conceptual limitations. Due to flaws built into the standard configuration, however, they remain relatively limited on the vertical dimension (both when ascending and descending) and they cannot fly efficiently at unusual lateral attitudes. A far-forward propeller location degrades longitudinal stability and limits the propeller’s use as a drag-producing device. Despite claims of “knife-edge spins” and “knife-edge climbs,” no existing aircraft has addressed the lateral aerodynamics required to fly efficiently on its side. Nor has any aircraft adequately addressed engine torque. A conventional aircraft behaves differently right and left. It may have sufficient thrust to hang on its propeller for a moment—until engine torque takes over and rotates the airplane. The maneuver then becomes essentially an uncontrolled rolling tail-slide, and the aircraft has to flick around and regain airspeed before control is regained. Whenever a pilot is forced to relinquish control of his aircraft in order to exit a maneuver, we have a serious design flaw.

It has been claimed that the new high-powered Giles machines will have sufficient thrust to hover while controlling torque with full-span ailerons. It remains to be seen just how much controllability will remain in this rather precarious flight condition.

Traditionally, the quest for vertical performance has led to serious compromises in other areas. Larger engines and propellers have demanded larger, heavier airframes. Combined with the need to restrict size to maintain maneuvering rates, increasing power has led to higher wing-loadings. Higher wing-loadings equate to higher speeds through all maneuvers and higher G forces for a given maneuver. Consequently, aerobatic sequences cover a larger area of sky and are limited by the size of the competition box. Airshow routines become less interesting since the aircraft is often far away.

Speed is not necessarily a valuable attribute for an aerobatic aircraft. It has been sought after mainly because it can be traded for altitude.

A Question of Scale

All else being equal, a small aircraft will maneuver more quickly than a larger one. This is because of the lower mass inertias (of both the aircraft and the surrounding air mass) resisting attitude changes. These inertias increase exponentially with the size of the aircraft, so the effect is very noticeable.

Larger, heavier, more powerful machines have maintained high maneuvering rates through higher wing loadings, higher speeds, and advanced control-surface technology—but, we ask, at what cost? Aerobatic sequences must be performed at high speeds through large areas of sky while requiring high G forces. A smaller, lighter aircraft, with lower wing-loading, can complete an aerobatic sequence with lower G forces, or with the same G forces while tightening up the sequence into a smaller space. The pilot has an easier job staying in the competition box and can position himself to his advantage in relation to judges and spectators. This is an obvious advantage for both airshow and competition aerobatics. Moreover, a smaller, closer aircraft will appear to be flying faster and more nimbly than a larger aircraft which is further away.

The Hummingbird Solution

Based on the foregoing discussion, the mechanical problem of aerobatic flight has been reconsidered and a new configuration has been devised. The solution presented here promises to enhance performance on the vertical dimension through higher thrust-to-weight ratios (thus reducing the need for high speeds), while retaining a small, light, agile aircraft. The configuration also offers new capabilities on the lateral axis and in slow flight, resulting in an aircraft with the potential to change the way aerobatics is flown.