The recent resurgence of board wings is exciting. The performance increment they offer is opening up new vistas for economically endowed pilots. Transitioning to rigid wings from the realm of flex wings appears to be easy enough for most pilots, however, it is quite possible to miss some important concepts along the way. Ignorance of these concepts can threaten your life. We are speaking of rigid wing spin behavior and VNE limitations.
Because flex wing hang gliders are very resistant to spins and have a wide margin of structural integrity, most hang glider pilots have not been trained to avoid spins, and may not even be aware of the dangers. It is in order to remedy this situation that we offer this discussion.
Hang Glider Spins
A spin occurs when only one wing of an aircraft stalls. This usually happens when a pilot slows down too much in a turn. The stalled wing creates a great increase in drag and a subsequent loss of lift. The result is that the stalled wing retards while the other wing rotates rapidly around it. The spin will normally continue unless the pilot does something appropriate to stop it.
A spin can take place with the wings fairly level (a flat spin), like a maple seed whirling to the ground, or the bank can be steep with the aircraft in a very nose-down attitude (a tail spin). In the first case the descent rate is fairly low, and many pilots have survived flat-spin crash landings. In the latter case, crash survivability depends on the influence of your guardian angel. Which type of spin you experience depends on your aircraft's configuration and how you entered the spin.
A flex wing hang glider is very difficult to spin. The reason is that all the washout (twist) in the wing keeps the outboard portion of the wing at a low angle of attack. The tips don't stall readily. (Typical hang glider twist is 12" to 15", while it's about 3' in a sailplane or airplane.) Most hang glider rigid wings have twist similar to that of sailplanes. Why not give them more twist and be done with it? The reason is that the wing's rigidity would make them very poor performers at higher speeds, because, unlike flexible wing tips, these wings would not reduce their twist as the glider's angle of attack is lowered (that is, at higher speeds). Thus, rigid wings spin more readily than flex wings.
The problem is that, unlike pilots of airplanes and sailplanes in general, we often fly hang gliders close to the terrain, frequently scratching to get up after launch. Conditions in these situations are normally turbulent as the air rolls up the slope, punctuated with thermals. Turbulence is a culprit in accidental spins because it can cause one wing to stall by increasing its angle of attack with an upward gust.
There is a long history of rigid wing use in hang gliding. What can we learn from this? All the best-known pilots who flew the Mitchell Wing as a hang glider spun it inadvertently. It is still being flown as a powered ultralight and its spin behavior has been thoroughly explored. It likes to enter a spin with little warning and spin steeply no matter what the control configuration, according to pilots I recently interviewed. On the other hand, the Icarus V, the Easy Riser and the Fledgling were fairly spin resistant, although they would spin nicely when provoked.
Why this difference? There are three additional design factors besides twist that affect spin characteristics. They are sweep, taper and airfoil distribution. Sweep (angling back) in a wing promotes spins because upwash form the inboard part of the wing increases the angle of attack on the trailing outboard part. (You can visualize this phenomenon by imagining a wing in front of two smaller wings behind and outboard. The upward-flowing part of the leading wing's tip vortex would meet the trailing wings and increase their angle of attack.) While the physical twist of a flex wing may be 15", the virtual aerodynamic twist is much less due to the sweep effect.
Taper promotes spins because it increases the local loading out at the tips. In effect, if the chord of a wing gets smaller at the tips, the airfoil is working harder. For this reason, the airfoil can also make a difference. A thicker airfoil in relation to the chord length at the tips helps reduce spin susceptibility. It is interesting to note that we could easily spin standard gliders in the early days due to all their sweep and taper, despite their huge helping of twist (20" or more). One design with a tighter sail and long, pointed tips was called the Windlord, but it quickly earned the nickname "Spinlord."
With all this in mind, why do they sweep or taper our wings? Sweep is important for pitch stability in a flying wing. (Flying wings with little or no sweep must rely on other dodges such as lots of reflex as in the Marske Monarch and Backstrom wings, or negative elevons as in the Mitchell wing.) Taper is useful for performance. The most efficient planform is an ellipse, and a tapered wing simulates an ellipse without the fancy curves which are hard build into a leading edge D-tube (note that the latest sailplanes have multiple straight lines to simulate this curve). A non-tapered wing (affectionately known as a Hershey bar, such as the Exxtacy's) is spin resistant, but involves a weight, loading and possibly a handling penalty. It is interesting to note that the E7, essentially an Exxtacy clone with a tapered wing, has at least as much performance, reportedly quicker roll response, and weighs nearly 15 pounds less.
In view of the above we can comment on the spin characteristics of the current crop of rigid wings. The Exxtacy and its exact copies are fairly spin resistant, although you can make them do it. I have heard of no inadvertent spins occurring, although I'm sure they may. The E7 with its tapered wing should be a bit more susceptible to spins. The Swift has been spun unintentionally and the Millennium may do so as well. Both of these craft are somewhat swept and tapered with low twist. The Atos isn't too tapered but will spin like any sailplane if aggravated.
Spin Dangers, Defenses, and Recovery
When you are in a spin you are out of control and can go nowhere unless you stop the spin and resume normal flight. The main danger of spinning is, of course, hitting the ground. Spins still account for a large percentage of fatal accidents in both sailplanes and airplanes. It is important for those of us flying rigid wings to know enough about spins and spin recovery to prevent the same statistics in our sport.
One of the most dramatic things that happens in a spin is disorientation or vertigo. When I first started doing spins on my Sensor years ago, I noted how quickly the effects of vertigo build up. The reason for this is that a spin tends to rotate you around more quickly than even a very tight 360. Dr. Fred De Lacerda writes a column in Sport Aerobatics magazine illuminating this and other subjects concerning human physiology in dynamic flight situations. Much of this material is summarized in our book Performance Flying. I expect that a good number of spins continue to the life-threatening stage due to complete disorientation of the pilot. I have witnessed two spins of hang gliders that went to the ground. Both pilots rang their bells but dusted themselves off and said essentially the same thing: "I didn't know which way was up." Neither pilot changed his control position of holding the bar out, so profound was the disorientation.
The FAA no longer requires spin practice for acquiring a pilot's license. The reason is that too many fatalities were occurring, despite the fact that the spins were expected, pilots initially had ample ground clearance, and often instructors were on board. The United States Ultralight Association requests spin entry recognition and recovery, but not spins for this reason. Within the last decade or so the gurus of aerobatics (airplanes) have revised the spin recovery techniques recommended for all pilots. The old standard, "stick forward, opposite rudder, level wings" was resulting in excessive recovery speeds or "over the top" spins too frequently. Now the technique consists of stopping or slowing the rotation with opposite rudder control before applying forward stick.
Besides vertigo, what else can go wrong in a spin? One thing is overbanking. At least one flex wing has gone upside down, and I watched a Fledgling go well past vertical in a low spin. The cause was the pilot initiating too vigorous a spin so the inside wing was severely stalled while the outside wing whipped around at a high speed and high angle of attack. But the biggest spin problem is not enough altitude to recover, as is the case when scratching low or, for sailplanes and airplanes, during landing setup. When a spin happens at low altitude the tendency is to try to pull the nose up soon, since it tends to drop radically in most spins. Of course, any nose-up control only deepens the spin.
The best way to avoid spins is to recognize the situations in which they occur and how they feel. Spins occur when you are flying slowly in turbulent conditions or you are in a shallow-banked turn (30° or less) and slow down too much. Spin prevention consists of maintaining a bit of extra insurance speed when soaring low, and when thermaling in a turbulent thermal. In addition, you should know the glider's signals of an incipient spin very well. These signals may include a "sticky" wing feeling (the inside wing wants to retard), a harder push-back of the basetube or pull-forward of the stick, and buffeting of control surfaces. Some craft may give no overt signs.
Spin recovery consists of lowering the nose in most rigid wings, since we often have no direct control over yaw, except with tip rudders. However, a word of warning here: Do not lower the nose too much or you introduce the possibility of structural damage. I know of two rigid wings that have suffered such a calamity and doubtless there have been more. Generally, returning the control bar or stick to the neutral trim position is all that is required to effectively stop a spin in hang gliders and airplanes.
It seems reasonable to me that rigid wing manufacturers should publish the spin characteristics and recovery methods for their wings. Also, it seems reasonable for rigid wing pilots to explore the spin characteristics of their wings, but not on a casual basis such as under the urging of another pilot who is not an instructor with extensive spin training. This article is certainly not sufficient to train you to do spins. Perhaps the safest route is to take spin training in an aerobatic airplane with a competent practitioner. You will learn how disorienting spins can be and how the entry and recovery feels.
Here are two safety addenda: First, we should note that Tom Knauff, one of the sailplane pundits, teaches pilots to use 45° bank angles for landing approach turns in order to prevent spins. In a 45° turn there is a smaller speed differential between the inside and outside wings so an inadvertent stall is less asymmetrical. In addition, it takes much more control force to stall in such a bank so an unintended stall is less likely. On a flex-wing hang glider, at least, you will find it impossible to spin in a very steep bank.
Second, we should take a lesson form the paragliding world. These wings will spin readily (a bit of taper and very little washout). I have seen three of them spin in a gaggle during competition. One went to the ground and was fatal. The others ended up under a bloom of Nylon. All of them were unrecoverable. One I didn't see ended in a midair with a double fatality. It seems to me that if spins occur with any regularity in rigid wings, then they should be separated from flex wings in competition. I enjoy flying a flex wing with rigid wings, but not in the death-threat gaggles at start gates. The two types of hang gliders are different enough that thermaling circles don't exactly coincide.
Many rigid wing pilots do not bank as steeply as flex-wing pilots because rigid wing tips do not wash out so they can't be slowed as much in steep banks. We have already had one midair between a rigid wing and a flex wing. To prevent more I suggest that meet directors run separate start gates for the two classes, just as they would if paragliders and hang gliders were competing together.
Spins are here to stay, because in our quest for better performance one clear design path is rigid wings with aerodynamic controls. Such configurations allow performance-enhancing aspect ratios and cleaner wings. So we had better start educating ourselves about the causes and prevention of, and recovery from spins just as pilots in the sailplane and airplane communities do. This article is one small step in that direction.
Next month we'll look at VNE speeds, in Rigid Wings Part II - Spins, Speeds, and Safety