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The Optimum Configuration for Aerobatics?

Lifting System

In this section I present my argument that Hummingbird represents the optimum configuration for Aerobatics—given known technology, of course. Notice the “aero” in aerobatics, meaning relying on interaction with the atmosphere. We are not talking about anti-gravity-batics or ufo-batics, or any such “unknown” technology!

To evaluate an aircraft configuration we look at how it measures up to a specific mission. Having defined our Idealized Aerobatic Mission we will proceed to investigate how various possible configurations measure up, eliminating many possibilities along the way. To simplify this procedure we will look according to the following criteria:

  • Lifting System
  • Propulsion
  • Structures
  • Controllability
  • Ergonomics

Lifting System: Lift/Drag Ratio. Upright, Inverted, and Side-Flight.

An aerobatic aircraft (like any other aircraft) creates lift by imparting momentum to the atmosphere. The device used to impart the bulk of this momentum is generally called a “wing”, though here we will generalize the term by calling it a “lifting system”. This is because we tend to think of a wing as a plane (thus the source of the term “aero-plane”), while other geometric forms will work just as well, and sometimes better. A wing which is not basically straight and flat is generally called a “non-planar” wing.

A lifting system can (and often does) incorporate more than one lifting surface (wing). Of note are the biplane, canard, tandem, and three-surface configurations. Of these, the biplane (or triplane, or quadraplane) separate the lifting surfaces vertically, while the canard, tandem, and three-surface configurations separate the surfaces longitudinally:

Multiple surfaces, longitudinal separation

Multi Surface (sm)

None of the longitudinally-distributed lifting systems are suitable for our aerobatic mission, for two significant reasons:

  • Inverted capability. Longitudinally-distributed lifting systems require varying lift coefficients between the lifting surfaces to achieve pitch stability and stall/spin safety. Generally the forward surfaces must operate at higher lift coefficients. Go inverted and (without major geometry changes) the opposite will be the case: the forward surfaces will now be operating at a lower lift coefficient. Pitch trim, pitch stability, and stall behavior are drastically altered. In short, it would be difficult to render such a configuration aerodynamically symmetrical as is required for efficient aerobatics.

  • Stall characteristics. Our aerobatic mission requires the airplane to stall asymmetrically with yaw (just one side of the lifting system stalls, creating the snap roll), while maintaining pitch stability and control. Any configuration which distributes lifting surfaces longitudinally must stall each surface at precisely the same AoA to avoid drastic pitch-trim changes. This would be next to impossible to accomplish under every flight condition.

Multiple surfaces, vertical separation.

Vertically separating the lifting surfaces brings two major benefits:

  1. Opportunities to efficiently brace the structure, leading to structural weight savings
  2. Distributing the lifting forces vertically, thus reducing energy losses and therefore induced drag, which equates to higher span efficiency.

An aerobatic airplane is faced with a peculiar dilemma. It cannot fly too fast or it will be unable to stay in the competition box (or in sight of the crowd). Nor can it drastically increase span, or structural weight and maneuverability on the roll axis suffer. Yet it must pull high G’s (lots of lift, therefore induced drag) while limiting energy losses. What to do? Non-planar wings! This is why aerobatics is the last bastion of the biplane.

But is the biplane the best non-planar lifting system for aerobatics? If it was, the planar systems (monoplanes) would not be taking over. Hummingbird offers an alternative non-planar lifting system that offers induced drag and structural benefits similar to the biplane, while reducing parasite drag and integrating propulsive and side-flight requirements.

It is no accident that knife-edge (side-flight) loops have never been flown. A true knife-edge loop demands extraordinary aerodynamic efficiency (lift/drag ratio) on the lateral axis, and the required lift/drag ratios must be attainable at speeds low enough to complete the bottom of the loop without exceeding flutter speeds, over-stressing the airplane, or passing out from “side-g” (however that might feel).

Lifting Systems Compared (sm)

A vertical planar wing of sufficient area and L/D to side-loop an Extra-class airplane would be at least 12 feet span—clearly unmanageable on the ground and ungainly in the air. This explains why several knife-edge maneuvers were recently removed from the Aresti manual: By conventional solutions, true knife-edge aerobatics is simply impractical.

So we have a problem: how to create sufficient lift and L/D in a vertical wing that cannot be taller than about 8 feet. This demands a light weight aircraft along with very high span-efficiency. The solution is the annular wing, which produces just half the induced drag of a planar wing of the same span producing the same lift. Moreover, this annular wing works equally well at every bank angle.

It is said that an efficient design makes each component do more than one job. Hummingbird utilizes an annular wing in a completely novel manner, having it perform 5 key functions:

  1. Produces efficient aerodynamic lift at all lateral attitudes.
  2. Increases static thrust over free propellers of the same diameter.
  3. Aerodynamically couples the propulsion system and lifting system, creating a unique "powered lift" effect.
  4. Efficiently braces the wing in bending and torsion (with help from the ventral fins and stators).
  5. Structurally connects aft fuselage to forward fuselage.

Lifting System (sm)

Hummingbird’s primary lifting system consists of three components: wings, duct, and ventral fins. The stators are not considered “lifting” because they never see a significant angle of attack; their primary aerodynamic function is to help stabilize the airplane when the propellers are impeding airflow through the duct, by stabilizing the disturbed airflow before it reaches the tail surfaces. The ventral fins are not crucial aerodynamically but are structurally required; their aerodynamic effect is to increase the maximum available side-lift and move the lateral center of pressure forward, while sacrificing some vertical span-efficiency over the pure annular wing.

I rest my case that Hummingbird’s Lifting System is optimum for aerobatics.


Hummingbird’s counter-rotating, ducted propulsion system, using 2-cycle or Wankel engines, offers the following advantages over any existing system:

  • Higher power to weight ratio (without using large engines or turbines).
  • Higher thrust to weight ratio (without using large propellers).
  • Full, torque-free power available on demand.
  • Propulsion system can be used to generate drag.
  • Unique aerodynamic coupling of propulsion and lifting systems creates a low cost “powered lift” effect.
  • Propulsive symmetry (aircraft behaves identically left and right).
  • Lower costs (for both purchase and overhaul).


Hummingbird’s novel structural configuration offers the following advantages:

  • The duct (in combination with wings, fins, and stators) constitutes an offset octagonal truss which efficiently braces the wings against bending and torsion and limits distortion of the duct.
  • No additional struts or wires are required to complete this structural arrangement. While the ventral fins and stators contribute to drag, they have aerodynamic roles also, and the stators have a third function of supporting the aft fuselage and engine.
  • Engine loads are distributed, reducing stress concentrations.
  • Propellers run on a large-diameter tubular axle. There are no cantilevered propeller shafts!
  • While special attention will be required to bring fuselage loads through the fins and stators, nowhere in the airplane will the structures see anything like the loads in a monoplane wing.
  • Aircraft can be broken down into sections no larger than about 10 feet in length and about 200 lb in weight.


Controllability is enhanced over other configurations by the following features:

  • The aircraft’s mass inertias are reduced by positioning heavy components near the CG.
  • Gyroscopic forces acting on the airplane are minimized by employing counter-rotating propellers at low rpm, and placing them near the CG.
  • Propeller location just aft of the CG avoids stability degradation due to propellers and permits greater flexibility in using propellers for drag control.
  • Statorons and tail surfaces are located immediately behind the propellers, providing three-axis control with power, even at zero or negative airspeeds.


  • Forward pilot location allows unrestricted visibility.
  • G forces correctly anticipate aircraft acceleration (cockpit is forward of CG).
  • Reclining seat does not degrade visibility (as in conventional configurations).
  • Forward sighting device.
  • VERY quiet cockpit, since props turn slowly inside a duct, while exhausts exit behind the propellers.

I rest my case that Hummingbird represents the optimum configuration for aerobatics.

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© Copyright 1992-2009 Philip Carter