Experiments page (older content) for Aeroxperiments website

EXPERIMENTS

Steve Seibel
www.aeroexperiments.org
steve at aeroexperiments.org

Material from April 2003
Preface and brief revisions August 10 2004

 

Preface: This "Experiments" section describes some of the questions I've been interested in and some of the methods I've used to address these questions. The experimental results, and the underlying theory, are addressed only briefly here.

This material was prepared for an earlier version of this website. Some of this content is still rather rough-cut but it will provide the reader with some idea of the questions I've been interested in. Look for this content to be revised, completed, and incorporated into the main site map at a later date.

This material was created in April 2003 after I carried out a series of experiments with rudders and wing-tip drag devices to look at coupling between yaw and roll on flex-wing hang gliders, but before I had the opportunity to repeat these experiments on a glider with a VG, so this material does not address the relationship between the VG setting and the amount of effective aerodynamic anhedral contained in the wing. Nonetheless, all the content should be valid, since the later experiments with the Airborne Blade confirmed and added to the earlier findings.

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EXPERIMENTS:

In this section of this website you can read about some interesting experiments I’ve conducted in hang gliders, airplanes, sailplanes, and R.C. gliders. I’ll also touch on theory in this section, in just enough detail to give some motivation for the experiments. For much more on theory, turn to the "theory" section.

Index to experiments section:

PART 1: AIRCRAFT USED FOR EXPERIMENTS, AND ILLUSTRATED LIST OF TOOLS

Aircraft used for experiments

Illustrated list of tools

PART 2: OVERVIEW OF BASIC AREAS OF EXPERIMENTATION, WITH BRIEF NOTES ON UNDERLYING THEORY AND EXPERIMENTAL RESULTS:

Area A: basic turn dynamics: Sideslip, pitch control inputs, and the interplay between angle-of-attack, pitch attitude, flight path, airspeed, and G-loading.

Area B: Coupling between yaw and roll in flying-wing aircraft, especially hang gliders.

PART 3: MORE DETAILED NOTES ON EXPERIMENTAL TOOLS AND STRATEGIES

PART 4: DETAILED QUESTIONS ADDRESSED VIA IN-FLIGHT EXPERIMENTATION

Area A: basic turn dynamics: Sideslip, pitch control inputs, and the interplay between angle-of-attack, pitch attitude, flight path, airspeed, and G-loading.

Area B: Coupling between yaw and roll in flying-wing aircraft, especially hang gliders.

 

PART 1: AIRCRAFT USED FOR EXPERIMENTS, AND ILLUSTRATED LIST OF TOOLS

 

Aircraft used for experiments:

  1. Wills Wing Spectrum hang glider; Wills Wing Raven hang glider; Wills Wing Skyhawk hang glider. (A glider with a V.G. will be the next test subject.)
  2. Cessna 152
  3. Slingsby Swallow sailplane; Schweizer 2-33 sailplane; Schweizer 2-22 sailplane; Grob Acro 2-place sailplane for aerobatic experiments.
  4. Superfloater ultralight sailplane (3-axis aerodynamic controls); Swift motorized ultralight sailplane (2-axis elevon controls with no moving rudder surface).
  5. Rogallo-style RC model trike (i.e. powered hang glider); Zagi RC model with increased sweep to resemble a hang glider wing planform, and with modified wing spar system for ground-adjustable dihedral or anhedral.

 

Illustrated list of tools:

1. Visual reference guide for accurately measuring bank angle in a hang glider

2. Yaw string for measuring sideslip in a hang glider

  1. "Bowsprit" and keel-mounted yaw strings for measuring sideslip angles at the extreme nose and rear keel of a hang glider, for looking at "airflow curvature" effects.
  2. Helmet-mounted mirror for viewing yaw string mounted on rear of keel of hang glider.
  3. Slip-skid bubbles (similar to an aircraft slip-skid ball) for looking at aerodynamic sideforce during a sideslip in a hang glider.
  4. Homemade G-meter for use in hang gliders and airplanes.
  5. System of bubble levels to look at pitch attitude at various airspeeds in a hang glider.
  6. Keel-mounted controllable rudder for hang gliders; also could be locked to serve as a fixed vertical fin.
  7. Wing-tip-deployed drag chutes to study asymmetrical yaw effects in hang gliders.
  8. Mid-span-deployed drag chutes to study asymmetrical yaw effects in hang gliders.
  9. Tape recorder for recording data in hang gliders, airplanes, and sailplanes.
  10. Variometer with barograph capable of recording airspeed and sink rate in 1-second intervals.
  11. Sliding-disk-type and rotor-type of airspeed sensors.
  12. Tool for holding aircraft control yoke in a fixed position in the fore-and-aft sense, while allowing normal roll control.
  13. Lightweight, fixed-deflection rudder for Rogallo-style RC model trike, adjustable to put the rudder area either above or below the main line of the keel of the glider. Also a wingtip-mounted drag ribbon as an alternative way to yaw this aircraft in flight.
  14. Several controllable, removable rudders for modified Zagi RC rigid-wing model, variously configured in several sizes and with centers of area above, below, or in line with the CG of the Zagi.
  15. Emergency cloud-flying probe for a hang glider, consisting of forward-mounted GPS, compass, and yaw string.

 

PART 2: OVERVIEW OF BASIC AREAS OF EXPERIMENTATION, WITH BRIEF NOTES ON UNDERLYING THEORY AND EXPERIMENTAL RESULTS:

 

Area A: basic turn dynamics: Sideslip, pitch control inputs, and the interplay between angle-of-attack, pitch attitude, flight path, airspeed, and G-loading.

I’ve been interested in these topics since I sat in on a meeting of a group of hang glider pilots at the University of Wisconsin, Madison over 10 years ago. They were discussing how they were learning to "coordinate" their turns and it all seemed a bit odd to me, especially in relation to my own experiences (At the time I had just gotten signed off on my private rating, for sailplanes, flying at America’s most amazingly affordable soaring club, located near Wichita Kansas.) Much later, when I began taking hang gliding lessons, I realized that that the established view of turns and coordination and sideslips in the world of hang gliding was indeed quite different from the prevailing understanding in the rest of aviation. (Not that there aren’t any parallels, for example Langewiesche’s classic "Stick and Rudder" contains some ideas that are very much in line with the thinking of the hang gliding community. Langewiesche writes that pitch inputs, as well as rudder inputs, have a strong effect on whether the aircraft will slip or skid during a turn. I’ll also mention quite a few other similar sources elsewhere on this website. The more mainstream view of the larger aviation community is that rudder inputs control slip and skid and pitch inputs do not--but even with this more correct understanding of how the controls work, the "explanations" given in pilot training manuals of slips and skids are sadly lacking in any real understanding of the fundamental forces at work, almost without exception). At any rate I began daydreaming about ways to test some of these differing ideas experimentally on a hang glider and on other aircraft, long before I was cut loose on my first altitude flight or even my first flight from the top of the training dune. I also immediately began driving my hang gliding instructor crazy with questions related to these topics, right from day 1 on the training hill!

I believe the results of the experiments described in this section can be used to simplify and streamline our instructional materials, and will allow for a better general understanding of hang glider aerodynamics, particularly useful for modeling and simulation efforts. Some other specialized, practical applications include the formulation of an optimum strategy to descend quickly in the presence of widespread lift, and the improved analysis of the complex and poorly understood phenomenon of lockout during towing.

In short, I’ve found that hang gliders "coordinate" exactly like other fixed-wing aircraft: yaw inputs (if available via the addition of a rudder) control slip and skid and serve to precisely align the nose with the actual direction of travel of the aircraft through the airmass, in the yaw axis, in spite of adverse yaw effects and yaw rotational inertia effects. Pitch inputs control angle-of-attack and thus play a central role in the interplay between bank angle, G-loading, airspeed, flight path, and pitch attitude, but have little to do with preventing or creating slips or skids. When a hang glider pilot "coordinates" a turn by easing the control bar forward as he rolls the glider into the turn, he increases the angle-of-attack and G-loading, which prevents the nose from dropping due to an abrupt downward curvature in the flight path and a rise in airspeed. This has little or nothing to with preventing a "slip"—allowing the nose to drop and the airspeed to rise as the glider enters the turn will not make the hang glider "slip" sideways through the air. (Much more on the detailed physics of this in the "theory" section.) The sensations that many hang gliders believe to be evidence of a sideways "slip", are actually the sensations of an abnormally low G-loading in comparison to the bank angle, along with a strong downward curvature in the flight path and a rapid increase in airspeed. These are the natural consequences of rolling the glider into a turn without moving the control bar forward to increase the angle-of-attack and "hold the nose up", or of actually pulling the control bar aft at the same time that the glider is rolled from wings-level into a steep turn. "Slips" and "skids" are an entirely different matter—the glider tends to yaw toward the outside or high side of the turn whenever the pilot increases the bank angle, especially if the roll rate is high, regardless if the pilot’s pitch-control input. This yawing motion is in fact a slip. So hang gliders tend to slip as they enter a turn, and the reverse is also true—when rolling back to wings-level, adverse yaw tends to swing the nose toward the inside or low side of the turn, which is a skid. Without a rudder there is little that can be done to prevent these slips and skids, and they are not of a lot of concern in practical flying, except that they tend to make the higher performance gliders, with their increased yaw and roll inertia and reduced sweep and reduced yaw stability, a little bit squirrelly to handle. Pitch control inputs really can’t do anything to reduce or prevent these slips and skids; on the whole there is very little relationship between a pilot’s pitch "coordination" inputs, and the amount of slip that will occur as the glider rolls into a turn. However, a swept-wing hang glider with anhedral tends to have a much higher roll rate at low angles-of-attack, when the bar is well pulled-in, than at high angles-of-attack with the bar well forward. This higher roll rate at high airspeeds might also be accompanied by a noticeable increase in adverse yaw.

A hang glider gives very little physical evidence to the pilot when it does slip or skid, because the flying-wing nature of the design generates very little in the way of an aerodynamic sideforce component, for a given sideslip angle through the airmass as evidenced by a yaw string or telltale. Also the freely-hanging nature of the pilot’s connection to the aircraft means that the pilot is not going to directly "feel", as a tangible sideways "push" on his body, any aerodynamic sideforce components that may be present. Instead he will simply find himself tending to hang slightly toward the high side or low side of the glider centerline.

As hang glider pilot’s we’d do well to drop the idea of "coordinating a turn prevent a slip" from our hang gliding training materials, and replace it with the idea of "coordinating a turn to control the airspeed and to prevent a dive". (It would no doubt be a loosing battle to propose that we entirely stop using the words "coordinate" and "coordination" to describe the way that we ease the control bar forward to give a nose-up pitch input as we enter a turn, though these words are used primarily—but not exclusively—in the rest of aviation to refer specifically to the use of the rudder to prevent slips and skids). We could also do without the various diagrams, explanations, and analogies (bobsleds, cars on curved tracks, etc) that purport to show how a "slip" is caused by inadequate lift, inadequate vertical component of lift, excess gravity, inadequate pitch control, inadequate curvature in the flight path, inadequate horizontal component of lift, inadequate centripetal force, inadequate centrifugal force, etc.. Slips and skids are caused primarily by adverse yaw and other related effects which yaw the nose toward the high side or outside of the turn, in relation to the glider’s actual direction of travel through the airmass at any instant. Besides the simple streamlining of ideas and the greater accuracy that we could obtain by dropping most of our current ideas about "slips", we would also place hang glider pilots in a better position to understand the control inputs used in sailplane and airplane flying, should they ever find themselves interested in such things, and finally we would make the world of hang gliding more natural and intuitive and interesting to knowledgeable sailplane and airplane pilots.

 

Area B: Coupling between yaw and roll in flying-wing aircraft, especially hang gliders.

How do yaw and roll couple in hang gliders and other flying-wing aircraft? What is the role of sweep, and what is the role of anhedral or dihedral, in yaw-roll coupling? How does one even begin to quantify the amount of dihedral or anhedral which is present in a swept wing with a great deal of washout? Since very few hang gliders have rudders, very little is known about these topics in the general hang gliding community, and even many designers of hang gliders are not able to answer these questions definitively. Not that they would really need to—again, very few hang gliders have rudders—but this information does form some of the basic foundation of a comprehensive understanding of how our wings work. Originally I assumed that the swept wing on my Wills Wing Spectrum, which appeared to have a very modest amount of anhedral, would surely yield positive coupling between yaw and roll. In other words, if the nose of the glider were forced to yaw to the left in relation to the actual direction of travel through the airflow, or if the glider were suddenly struck by a crosswind gust from the right, the aerodynamics of the swept wing would make the glider roll toward the left, leading to a left turn. These assumptions even made it into a couple of articles I pubished on some of the topics mentioned in Area A, and though I received quite a lot of feedback on the articles, no one had much to say about the topic of yaw-roll coupling—though I now know that my original assumptions were in fact in error. I’m now convinced that most modern hang gliders exhibit a negative coupling between yaw and roll, with the possible exception of flight at very low airspeeds such as min. sink or below, where yaw-roll coupling may be neutral. At faster speeds, yaw-roll coupling is negative, meaning that if the nose is forced to yaw to the left of the glider’s actual direction of travel through the airmass, or the glider is struck by a sudden crosswind gust from the right, then the glider will tend to roll toward the right, which leads to a right turn. The anhedral designed into the wing is apparently dominating over the sweep, in terms of yaw-roll coupling. Many hang gliders appear to have very little anhedral until the observer takes into consideration the effects of washout. By twisting the average chord line of the wing, washout changes the geometry of a swept wing in a way that actual can create a fair amount of effective anhedral, even if the leading edges of the glider are totally in the same plane as the keel or the wing-root chord line when the glider is viewed head-on, either on the ground or in flight. We’re not talking about a flex effect here, instead we’re just looking at the way that changing the mean chord line changes the overall 3-dimensional geometry of a swept wing, including the amount of anhedral or dihedral that is present. This is rather counterintuitive and will be discussed in much more detail in the theory section. At any rate, the experiments outlined in this section involve flying a hang glider that was equipped with a controllable rudder. Some experiments used wing-mounted drag chutes as an alternative means of yawing the glider. Also some experiments were conducted with a Zagi RC model, modified to include a rudder and a ground-adjustable wing that could be set at various dihedral or anhedral angles.

Obviously, no suggestion is intended that a rudder or asymmetrical drag chute would be a practical addition to a hang glider for everyday flying!

 

PART 3: MORE DETAILED NOTES ON EXPERIMENTAL TOOLS AND STRATEGIES

Many of the experiments described in this website don’t involve any aerodynamic modifications to the aircraft, just some simple procedures and some simple gadgets for carefully measuring bank angle, G-loading, slip angle, aerodynamic sideforce from slips and skids, etc. Interested persons are highly encouraged to replicate these experiments, while exercising due caution. Feel free to contact the author for more information on these experiments.

On the other hand some of these experiments involve adding a controllable rudder or a small, wing-mounted drag chute to generate a yaw torque on a hang glider. Do not try these experiments! If you do try any of these experiments use extreme caution, and contact the author for more information. At least one person has been killed while flying a trike (powered hang glider) with a homemade rudder. At least one person has been killed when a (tandem) hang glider encountered a small parachute that was intentionally dropped earlier in the flight. (These incidents happened several years ago and had no connection to the author’s experiments.) Some key principles for safety: ground testing—in my experiments, before the rudder was flown on a hang glider, it was mounted on top of a car and tested at speeds up to 70 mph. Keep It Simple Stupid—the rudder used in these experiments only deflected in one direction (to the left) and then back to neutral because this made the control lines much simpler to design and install. This system was more than adequate for collecting detailed, carefully measured experimental data on the reaction of the glider to rudder inputs. The control lines were simply two thin ropes (one for deflection, one for centering) that could be tensioned and then put into jam cleats mounted on a down tube. This allowed the rudder to be held at a set deflection while accurate data were collected on the behavior of the hang glider in turns, wings-level flight, etc.. During these experiments the rudder was locked at a certain angle of deflection but the glider was controlled entirely in the conventional manner, i.e. by weight-shift roll and pitch inputs. For launch and landing the rudder was always locked into the neutral position. An alternative and much more dangerous approach would have been to install some kind of sliders on the base bar so that the normal pilot weight-shift roll control inputs also activated the rudder. Attempting to mix roll and yaw control inputs in this manner, before the glider’s reactions to rudder inputs were well understood, would not have yielded much in the way of meaningful, measured experimental data, and could well have been disastrous, since it was discovered that left rudder actually made the glider roll and turn to the right! Imagine what would happen if a pilot rigged up a hang glider in this manner, with base-bar sliders controlling both yaw and roll, and then moved his hands down onto the base-bar sliders immediately after launch! In contrast, in my case I didn’t even fully realize that the rudder had turned the glider in the "wrong" direction, until I was back with my feet on the ground listening to the cassette of recorded observations that I had made in flight. Basic control of the glider, using the normal weight-shift pitch and roll inputs, was never an issue. Another key principle—gradualism. I started with small deflections of the rudder and didn’t increase the deflection until I knew that I could control the aircraft at all airspeeds and bank angles, using the normal weight-shift pitch and roll controls, with the rudder locked at the smaller deflection angle. Likewise when I was experimenting with wing-mounted drag chutes, I started with very small chutes and increased the size only gradually. (In truth with the largest size chutes I tested, control at high airspeeds was marginal enough that pulling the bar all the way in, for maximum airspeed, was totally out of the question). Since the reaction of the glider to a deflected rudder or a wing-mounted drag chute is extremely airspeed-dependent, with the most severe reaction occurring at high airspeed, a pilot’s normal reaction to pull in for extra airspeed in case of difficulty will work against him. I planned for failure—I assumed that the rudder could jam into place at any time, and also that a drag chute could become jammed and unable to be jettisoned. I didn’t go to full rudder deflection, or deploy the largest –sized drag chutes, until I was convinced, from earlier experiments, that I could safely land the glider using the controls in the normal manner, even if the experimental surface became jammed in place in the fully deployed position. (The landing would have been modified to use a lower-than-normal airspeed on final approach—again, flight at the highest airspeeds with full rudder deflection or the larger sized drag chutes was definitely a marginal proposition.) Of course, I made every attempt to plan for all possible failure modes. If the simple jam-cleat system used for the rudder became utterly jammed, one quick slice with a knife would have freed the control line and allowed the rudder to float back to neutral, unless the line had become looped or snagged back at the rear of the keel. Similarly one slice of a knife immediately cut away the wing-mounted drag chute, or if the knife was lost—and the backup knife was also lost--then a pull on a piece of tape released the standing end of the drag chute line, which was simply wrapped three or four times around a down tube. However there was some remote possibility of the line snagging as if paid out through the eyelet mounted at midspan or out at the tip. When the chute was flown from the midspan position there was an alternative way to easily jettison the whole setup even in the case of such a tangle, but this wasn’t possible when the chute was flown from a wingtip-mounted eyelet. Again, the experiments were done in a gradual manner, over many flights, and the glider was never allowed to be configured—even momentarily—in any way in which a safe, controlled landing would not have been possible, even if the various aerodynamic devices had become jammed in a fully deployed position.

This brief account is by no means a full description of the various techniques and safety provisions that were developed over many months of experimentation. Again, do not try the experiments involving the rudder or the wing-mounted drag chutes. Or at the very least, contact the author for more information.

CAUTIONARY TALE #1—At least one person has been killed while flying a trike (powered hang glider) with a newly-installed homemade rudder. I have few details. The accident happened sometime in the 1990’s, in the general vicinity of North Plains, Oregon.

CAUTIONARY TALE #2—A few years back a tandem hang glider crashed after accidentally snagging around a flying or landing wire, a small parachute with a toy bear attached, which had been dropped earlier in the flight. One person died and one was severely injured. The crash involved an uncontrollable turn or spin.

CAUTIONARY TALE #3—This account was taken from the internet. I have communicated with the pilot involved. It is the exact opposite of the "gradualistic" approach—an attempt was made to drastically modify the control system of a hang glider, without first collecting any data on the aerodynamics or structural considerations involved (insert aileron story here).

 

PART 4: DETAILED QUESTIONS ADDRESSED VIA IN-FLIGHT EXPERIMENTATION

(Note: this section is still under construction. All of these experiments have already been carried out--I just haven't posted the detailed methods and results yet.)

Area A: basic turn dynamics: Sideslip, pitch control inputs, and the interplay between angle-of-attack, pitch attitude, flight path, airspeed, and G-loading.

A1. When a hang glider rolls from wings-level into a turn, does it sideslip during the turn entry? Does it sideslip during the subsequent steady turn after the bank angle is stabilized? How much sideslip is involved in these various phases of flight?

A2. Imagine a hang glider slipping sideways at some given angle: for example, perhaps the nose is yawed 5 degrees to one side of the direction of the airflow (relative wind) and flight path, as indicated by the deflection of a yaw string. How much aerodynamic side force is created as the aircraft moves sideways through the airmass? How much aerodynamic side force would be created by the same slip in an airplane or sailplane? How much aerodynamic side force is created when a rigid-wing flying wing aircraft such as the Swift (or a Zagi RC glider) is sideslipped through the airmass?

A3. Is it really true that a hang glider will "slip" sideways through the air if the pilot fails to make the proper nose-up pitch input while entering a turn, as described in nearly every hang gliding training manual in existence? Similarly, is it really true that a hang glider will "slip" sideways through the air if the pilot sharply pulls in the control bar while flying in a steep bank?

A4. Is it really true that an airplane or sailplane will "slip" sideways through the air if the pilot fails to make the proper nose-up pitch input while entering a turn, as described Wolfgang Langewiesche’s classic physics-for-pilots book "Stick and Rudder", and in the USAF flight training manual "Modern Airmanship 3rd edition" (Major Van Sickle, ed.), and other sources? Similarly, is it really true that an airplane or sailplane will "slip" sideways through the air if the pilot sharply pushes forward on the control stick or yoke while flying in a steep bank?

A5. When a hang glider enters a turn without the usual bar-forward, nose-up pitch input, what kind of trend is seen over time in the G-loading? How does this relate to the change in G-loading over time in a more normal turn entry where the pilot gives a bar-forward, nose-up pitch input to hold the airspeed constant and prevent the nose from dropping and the flight path from diving downward? In particular what do we see if we hold the bar in a constant position, in the fore-and-aft sense, and let the aircraft go through a whole series of pitch oscillations, all triggered by the initial roll-in from wings-level to a banked attitude? How do the airspeed, G-loading, and flight path change over time? Do these changes have any relation to the sensation of "slipping" as described by many hang glider pilots when they enter a turn without making the usual bar-forward, nose-up, pitch "coordination" input? Does this sensation of "slipping" have any relationship to actual sideslip as demonstrated by the deflection of a yaw string or telltale?

A6. This question deals with how to sustain a high sink rate in a hang glider, for example to maintain a positive descent rate to escape widespread strong lift. One author advocates a series of reversing turns, each entered in an "un-coordinated" manner—meaning that the pilot doesn’t make the usual bar-forward, nose-up, pitch input as he enters each turn. Instead the pilot makes a bar-aft, nose-down pitch input as he enters each turn. The author reports that this makes the hang glider "slip" sideways through the air, at least for a short time, after which the slip must be renewed by rolling back to wings-level and making another "uncoordinated" turn entry, hence the series of reversing turns. The end of each temporary "slip" is said to be signaled by an increase in the G-loading as the wing starts to fly in a "coordinated" manner. Is there anything to this? Specifically, is the average descent rate yielded by this method any higher than would be maintained in a steeply banked, fully-pulled-in, high-speed turn? Or in more detail: if we compare these two methods with the common parameter of some given maximum bank angle, which method yields the higher sink rate? If we compare these two methods with the common parameter of some given maximum allowable G-loading, which method yields the higher sink rate?

A7. If we investigate the questions of #A5 in an airplane, do we see dynamics that are similar to what we would see in a hang glider? What if we make a special tool to hold the control yoke in an absolutely fixed position in the fore-and-aft sense, so that only roll inputs are possible? (This would be rather difficult to accomplish in a hang glider but is quite easy in an airplane with a control yoke). If we roll quickly from wings-level to a steep bank, what kind of changes over time do we see in pitch attitude and flight path, airspeed, G-loading, and slideslip? How do these different parameters relate to each other? If we remove the tool and make roll quickly from wings-level to a steep bank, while simultaneously applying aft pressure on the control yoke to hold the nose up and prevent the airspeed from rising, how does this change the response that we see in the flight path, airspeed, and G-loading? And how does this change the response that we see in terms of sideslip?

A8. Let’s use the tool described in #A7 to continue to investigate the dynamics of an aircraft with limited pitch control. One of the major differences between a hang glider and an airplane or sailplane is that the airplane or sailplane has much more pitch control. In an airplane or sailplane the pilot can easily reduce the wing’s angle-of-attack to a zero-lift or negative-lift position, just by shoving the control stick forward. In an airplane or sailplane, sustainable top speed in a glide—either wings-level or banked—is limited only by the structural strength of the aircraft, or in the cases of a few extremely strong aircraft, by whatever terminal velocity can be developed when the pilot unloads the wing to the zero-lift angle-of-attack and allows the aircraft to follow a vertically straight-down flight path. In many hang gliders, the sustainable top speed in a glide—either wings-level or banked at some given bank angle-- is limited by the length of the pilot’s arms. By shifting his weight forward, the hang glider pilot reduces the wing’s angle-of-attack, which in turn reduces the lift coefficient, yielding a higher airspeed, but there is no possibility of holding the wing in a zero-lift or negative-lift angle-of-attack. Therefore there is no possibility of sustaining a straight-down dive—at least for reasonable bank angles—even if structural strength were not an issue. This limited pitch control is the real source of the difficulty of maintaining a high sink rate in a hang glider, as noted in question #A6. Let’s continue to explore similar dynamics in an airplane. We’ll use our special tool to hold the control yoke of an airplane in a constant position, in a fore-and-aft sense, and experiment with different ways of maintaining a high average sink rate. For a given maximum bank angle, is it better to hold the aircraft constantly at that bank angle to maximize the sink rate, or will a higher sink rate be developed by a series of reversing turns as proposed in question #A6? (Remember that we are not making any nose-up pitch "coordination" inputs as we roll into each turn?) What about for a given maximum G-loading?

A9. In practical terms how can a pilot keep an airplane under control if the pitch control system (elevator) is jammed? How does this relate to techniques used by hang glider pilots to control pitch attitude and airspeed during aerobatics or flight in very turbulent air? How does this relate to the interplay between pitch attitude, flight path, airspeed, and G-loading as explored in question #A4?

A10. Can we detect the "airflow curvature" effect by observing yaw strings or telltales positioned at various locations on the keel of a hang glider?

A11. To hold a given angle-of-attack, how do we have to change our pitch control inputs as we increase bank angle? In an airplane or sailplane, does the control yoke or stick have to be moved further forward to create a stall when the aircraft is steeply banked, than when the aircraft is wings-level? In a hang glider, does the bar have to be pushed out further forward to create a stall when the glider is steeply banked, then when the glider is wings-level?

A12. What sort of error does a Hall-type airspeed indicator show in a steep turn? Can the Hall-type airspeed indicator be used as an angle-of-attack indicator, regardless of bank angle?

 

Area B: Coupling between yaw and roll in flying-wing aircraft, especially hang gliders.

A few terms: by "coupling between yaw and roll", or "yaw-roll coupling", we are getting at the answer to the following question: "Which way does the glider tend to roll, if it is forced to fly through the airmass in a yawed (slipping or skidding) manner?" By "positive coupling between yaw and roll", or "positive yaw—roll coupling", we mean that if the nose is yawed to left of the airflow (relative wind), i.e. to the left of the direction that the glider is travelling through the airmass, then the glider tends to roll left. The same would apply if a glider with "positive yaw-roll coupling" were struck by a sudden crosswind from the left. By "negative coupling between yaw and roll", we mean that if the nose is yawed to the left of the airflow (relative wind) and to the left of the direction that the glider is travelling through the airmass, the glider tends to roll to the right. "Neutral coupling between yaw and roll" means that the glider shows no tendency to roll when the nose is yawed to the left or right of the direction that the glider is actually travelling through the airmass.

B1. Does a modern flex-wing hang glider such as the Wills Wing Spectrum show positive, negative, or neutral coupling between yaw and roll?

B2. Does an older Rogallo-style hang glider show positive, negative, or neutral coupling between yaw and roll? Is it essential that a hang glider have anhedral, and negative yaw-roll coupling, in order to be controllable via weight-shift?

B3. Does a rigid-wing aircraft such as the Millenium show positive, negative, or neutral coupling between yaw and roll?

B4. This question explores some flexible-geometry effects that relate to the action of the rudder and the tip-dragger devices. Some of the roll response to these devices may be due to changes in the geometry in the airframe, rather than due to the simple fact that the glider is being yawed relative to the airflow. If we apply tension to a line that runs between rear of the keel and a wingtip, how does the glider respond in the various axes, particularly in roll? Will the glider tend to roll toward, or away from, that wingtip? How strong will the roll response be? What if we tension a line that runs between a corner of the control frame, and the corresponding wingtip? In all these cases, how will the response vary with airspeed?

Questions B5 through B6 relate to some experiments performed on a Zagi RC flying wing with adjustable dihedral/anhedral, and a rudder.

B5. How much anhedral is needed to make the Zagi respond like the Spectrum as described in B1? How does this relate to actual amount of anhedral, if any, that is contained in the wing of the Spectrum? Is the yaw-roll coupling that we see in the Spectrum highly analogous to the yaw-roll coupling that we would see in a rigid-wing aircraft of similar geometry, or is there evidence that flexible-wing effects are also playing an important role in the yaw-roll coupling that we observe in the Spectrum?

B6. In terms of stability and control, including roll rate, what are the advantages and disadvantages of the following arrangements: a swept, dihedral wing with no vertical fin; a swept, dihedral wing with a vertical fin; a swept, flat wing with no vertical fin, a swept, flat wing with a vertical fin; a swept, anhedral wing with no vertical fin, and a swept, anhedral wing with a vertical fin? What are the advantages and disadvantages of a vertical fin, in relation to these various wing planforms? Can we see any evidence of the handling penalties created by a vertical fin, as reported by some hang glider pilots?

 

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