Visual Illusions and Downwind Turns

VISUAL ILLUSIONS AND DOWNWIND TURNS

Wayne Hershberger

Introduction

RC pilots who talk about downwind stalls are sometimes chided for harboring naive superstitions about the wind. Aerodynamicist Ralph Gross listed seven such fallacies in his paper "Airplanes and the Wind." Don Lowe excerpted Gross's list in his "Flying Lowe" column (Radio Control Modeler, May 1994). Gross' list reads:

1. As you turn upwind, you will gain altitude and/or airspeed.
2. As you turn downwind, you will lose altitude and/or airspeed.
3. As you are turning from upwind to crosswind, the wind can get under a wing and cause the airplane to roll into a steeper bank.
4. The airplane will tend to yaw, so that it is heading into the wind (weathervane).
5. The airplane can gain altitude faster when headed into the wind, or it cannot gain altitude as fast when heading downwind.
6. To hold a heading when flying crosswind, you will have to hold rudder.
7. When flying downwind, the airplane is more likely to stall; or when flying upwind, the airplane is less likely to stall.

According to Gross, none of these seven statements is true — but he is speaking only about the direct aerodynamic effects of the wind. He discounts the indirect aerodynamic effects of the wind that are mediated by the pilot. When one considers these pilot-mediated effects, it appears likely that most, if not all, of the statements are true for many RC pilots.

Direct versus indirect effects

Aerodynamicists explain that wind is the movement of a mass of air relative to the earth's surface, whereas aerodynamics concerns movement of air relative to airfoils. Therefore, unless an airfoil is attached to the earth (e.g., in a wind tunnel), wind should be irrelevant: an airborne airfoil doesn't even "see" the wind. The wind has no direct aerodynamic effect upon the performance of a flying airplane.

That said, this does not imply the wind has no indirect aerodynamic effect on airplanes flown in the wind. Pilots may be affected by what they "see" about the wind and the airplane's motion; whatever affects a pilot is likely to affect the airplane's aerodynamic performance because a pilot controls the flight path only by altering the airplane's aerodynamic characteristics. A pilot who changes the aircraft's performance to offset illusory visual effects of the wind is inadvertently mediating an indirect aerodynamic effect of the wind. These indirect (pilot-mediated) effects are real aerodynamic effects, even though they are not direct effects of the airflow on the airfoil.

A pilot-mediated downwind stall is still a stall, occasioned by the wind insofar as the wind induces the visual illusion that leads the pilot to make compensatory control inputs.

Why pilots perceive climbs and descents downwind vs. upwind

If downwind stalls result from pilots' false impressions of losing altitude, why do RC pilots think they are losing altitude going downwind and gaining altitude going upwind? The source is visual geometry and perceptual error, not an aerodynamic miracle.

An RC airplane flying at a constant altitude (above the pilot's eye level) will appear to climb or descend in the pilot's field of view depending on viewing distance. The angular elevation increases as viewing distance decreases (maximum directly overhead) and decreases as viewing distance increases (minimum at the horizon). Any change in angular elevation that is not caused by a change in viewing distance is attributable to a change of altitude, and vice versa.

Our brains use a perceptual principle (the size–distance invariance hypothesis) in judging an object's altitude (or size) and viewing distance. Provided the object is registered by the pilot's brain, any change of the model's angular elevation will be interpreted as a change in altitude or distance of regard (or both). Errors in one estimate imply errors in the other. RC pilots routinely misjudge distance and altitude (a tree or fence seeming to "grab" a model), and these errors are exacerbated when wind affects the rate at which viewing distance changes over time.

Four simple heading cases

For simplicity, consider a model flying at constant altitude in four basic heading/wind relationships: directly with the wind toward the pilot, directly with the wind away from the pilot, directly into the wind toward the pilot, and directly into the wind away from the pilot.

• Flying with the wind toward the pilot: The model will ascend rapidly in the pilot's field of view, giving the appearance of climbing and accelerating. A cautious pilot may throttle back to "slow down." After the model passes overhead and the wind is at the pilot's back, the model will rapidly descend in the field of view, appearing to dive. The pilot may then feed up elevator to maintain altitude. Throttling back and holding up elevator is a recipe for a stall — the classic pilot-mediated downwind stall.

• Flying with the wind away from the pilot: The model will descend rapidly in the field of view after passing overhead, again creating misleading impressions that may prompt inappropriate control inputs.

• Flying into the wind toward the pilot: The model will ascend in the field of view relatively slowly; it may seem to be trading airspeed for altitude. A pilot wanting to speed up or maintain altitude may feed down elevator, inducing a shallow dive.

• Flying into the wind away from the pilot: The model will descend in the field of view relatively slowly, again giving the false impression that it is climbing; pilots may react by feeding down elevator and diving slightly.

In short, the illusory climbs when flying downwind (and illusory climbs when flying upwind in some viewing geometries) lead pilots to add up elevator downwind and down elevator upwind. Those inputs change airspeed and control effectiveness and thereby create the very phenomena (airspeed and stall differences) that Gross labeled fallacies.

Turning, ground path, and the "weathervane" impression

An airplane flying a perfect circle at constant altitude in a moving air mass will trace an asymmetric ground path. The ground path's radius of curvature varies continuously with the airplane's heading relative to the wind.

• When turning from upwind to crosswind, the ground-path radius gradually lengthens; the turn slows and the airplane seems more steeply banked at the beginning of the turn. Because pilots often feed in down elevator upwind and up elevator downwind, the increased airspeed upwind makes the airplane roll into a steeper bank when turning from upwind to crosswind (supporting Statement 3 in practice).

• When turning from crosswind to upwind, the ground-path radius shortens; the turn quickens and the airplane seems to "weathervane," bringing its nose smartly into the wind (Statement 4 as a pilot-mediated effect).

These visual effects arise because the pilot is earthbound; a pilot seated in the airplane or flying from a moving deck with the same groundspeed would not perceive the same distortions.

The wind both distorts the apparent flight path (the ground path) and inclines the apparent flight plane in complex ways. Visually, it is as if the wind tips or bends the sky: level flight planes can seem tipped, and tipped flight planes can seem level. RC pilots, wanting to maintain level flight, may try to hold the model in what appears to be a level plane but is actually a tipped visual plane — producing inappropriate control inputs.

Practical implications

• The airspeed will tend to increase going upwind and decrease going downwind if pilots adopt nose-down attitude upwind and nose-up attitude downwind. Control surfaces will be more effective upwind, and the airplane will be more responsive upwind than downwind.
• Many RC flight phenomena commonly attributed to direct aerodynamic effects of the wind are actually the result of pilots compensating for visual illusions. Although these are not direct aerodynamic effects of the wind, they are real aerodynamic effects that are mediated by the pilot's reactions to what he or she sees.

Because these adverse aerodynamic effects are the result of visual illusions, pilots can minimize them by learning to recognize and resist those illusions. Developing the habit of flying on reference (airspeed and attitude) rather than purely on visual angular motion will reduce pilot-mediated stalls and other wind-related mishaps.

Conclusion

The truth or falsity of Gross's seven statements depends on the pilot. For an ideal pilot who does not mediate undesirable indirect effects, Gross is correct: the statements are false. For most RC pilots, however, who do respond to visual illusions produced by the wind, the statements often hold true in practice. Learning to recognize these visual illusions and resist compensatory reflexes is the best defense against pilot-mediated downwind stalls and related phenomena.

Wayne Hershberger 436 Gayle Ave. DeKalb, IL 60115

Transcribed from original scans by AI. Minor OCR errors may remain.