ZIMMERMAN'S FLYING FLAPJACKS: DESIGNS AHEAD OF THEIR TIME

Flight Journal, Apr 2005 by Wainfan, Barnaby

In an age in which we think we've seen everything and designers constantly produce flying machines that push the boundaries, we tend to forget that our history is littered with brave experiments that, for one reason or another, stopped barely short of answering the question that they were designed to answer. Because of this, some worthwhile concepts have been ignored for decades. These definitely include Vought's "Flying Flapjacks," the V-173 test airplane and the heavier and more powerful XF5U-1, which were the result of the work done by NACA engineer Charles Zimmerman.

Zimmerman was the first researcher to investigate the characteristics of very low aspect ratio wings in a scientific way. Other experimenters had tested low aspect ratio airplanes and models with various degrees of success, but Zimmerman did proper wind-tunnel tests at the NACA Langley Research Center. He discovered that his models behaved very differently from conventional wings.

Low aspect ratio wings have certain interesting aerodynamic characteristics at high angles of attack (AoA). Aspect ratio is a measure of the wing's "slenderness." A high aspect ratio wing has a long span and a narrow chord. A low aspect ratio wing has a wide chord and a short span. If the aspect ratio is low enough, the tip vortices begin to dominate the flow as the angle of attack approaches the point at which the airfoil would normally stall. The post-separation airflow forms a pair of stable, rolled-up vortices that come up and over the tops of the wingtips. These vortices sweep over enough of the wing surface to actually delay a stall. The flow in the core of the vortex is at much lower pressure than the ambient air. This low pressure acts on the upper surface of the wing and generates lift. The development of a strong, organized, vortex flow allows the low aspect ratio wing to go to very high AoA without stalling in the conventional sense. It also causes the lift loss at angles of attack beyond maximum lift to be gradual. This benign behavior at high AoA is one of the features that attracts designers to low aspect ratio designs as a solution to the problem of stall/spin accidents.

Zimmerman wind-tunnel-tested low aspect ratio wings with rectangular, circular and elliptical planforms, and he published two reports: "NACA Report no. 431: Characteristics of Clark Y Airfoils of Small Aspect Ratios" (1932) and (NACA Technical Note no. 539: Aerodynamic Characteristics of Several Airfoils of Low Aspect Ratio" (1935).

Among his conclusions were:

1. "There is a range of aspect ratios extending approximately from 0.75 to 1.50 wherein end flow causes a marked delay in the breakdown of the longitudinal flow as the angle of attack of the airfoil is increased."

Aspect ratio is technically defined as the wingspan squared divided by the wing area: AR=B^sup 2^/S, where B is the span and S is the area. For rectangular wings, aspect ratio is the ratio of the span to the chord, so an aspect ratio 1 "rectangular" wing would be square.

2. "The value of the maximum lift coefficient and the angle of attack at which it occurs is greatly affected by the shape of the extreme tip of the airfoil."

The low aspect ratio elliptical planforms he tested for TN 539 became the basis for the V-173 and XF5U-1.

Like other experimenters in the mid-1930s, Zimmerman initially set out to use the stall resistance of low aspect ratio wings to create a safe, light plane for personal flying. With the advent of WW II, he and the rest of American industry turned to more serious matters.

The configuration

The goal of the XFSU project was to produce an airplane that performed well as a fighter, would take off and land at extremely low speeds and, ultimately, would be able to hover. The idea was to combine the lightness and high AoA capability of the low aspect ratio wing with very large-diameter propellers to create a near-VTOL fighter that could be operated off merchant ships to defend convoys from German bombers.

The XF5U-1 and the V173 test airplanes were essentially flying wings. Most of the configuration was the wing, which had an aspect ratio of 1.275 and a planform that had a half-ellipse leading edge and a half-ellipse trailing edge joined at a straight quarter-chord line. The cockpits of both airplanes were partly buried in the wings' leading edges; small nacelles protruded ahead of the wing, and there were canopies on top of the wings' leading edges. The engines were buried in the wings.

High angle of attack

Zimmerman's experiments in the wind tunnel and, later, with electric-powered models, showed that a low aspect ratio machine with a semi-elliptical planform similar to his later wind-tunnel models could fly under control at very high AoA without stalling. His electric model, which was flown on a tether like later control-line models, demonstrated that it was possible to transition from level flight to vertical and back under control. Being able to go to high AoA made it possible to rotate the airplane and aim the thrust up to oppose gravity and carry some of the airplane's weight.