The Optimum Configuration for Aerobatics?
In this section I present my argument that Hummingbird represents the optimum configuration for Aerobaticsgiven 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: 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
None of the longitudinally-distributed lifting systems are suitable for our aerobatic mission, for two significant reasons:
Multiple surfaces, vertical separation.
Vertically separating the lifting surfaces brings two major benefits:
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 Gs (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).
A vertical planar wing of sufficient area and L/D to side-loop an Extra-class airplane would be at least 12 feet spanclearly 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:
Hummingbirds 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 Hummingbirds Lifting System is optimum for aerobatics.
Hummingbirds counter-rotating, ducted propulsion system, using 2-cycle or Wankel engines, offers the following advantages over any existing system:
Hummingbirds novel structural configuration offers the following advantages:
Controllability is enhanced over other configurations by the following features:
I rest my case that Hummingbird represents the optimum configuration for aerobatics.
© Copyright 1992-2009 Philip Carter