Last month we investigated the possibility of spins in rigid wings. We discovered a bit about their cause and prevention, as well as recovering from them. We also mentioned that overspeeding and structural damage are real possibilities if spin recovery is not properly executed. In this article we will consider rigid wing structural issues.
Part of the problem we are trying to address is that many new rigid-wing pilots are former flex-wing hang glider fliers. Just as these pilots have had the luxury of ignoring the spin problem that plagues general aviation, they have also generally had to worry less about aircraft structural integrity than pilots in any other forms of aviation -- except when they whack seismically, of course.
The reason for this state of affairs is that a flexible wing bends under increased wing loading. The leading edges bow in, the sail washes out and the wing sheds its burden of lift, especially outboard. The result is an automatic limit to the maximum G-load the glider will allow. We used to think that flexible hang gliders were unbreakable under positive loads, no matter how they were handled, until a pilot flying a heavily loaded HP blew it up with a strong push-out in a screaming dive. Despite that incident, we generally consider that our wings are essentially unbreakable as long as they remain upright.
On the other hand, rigid wings aren't so worry free. The reason for this is their rigidity, of course. More rigidity doesn't necessarily mean weaker, but since the wings don't bow appreciably they don't limit the load that is built up in maneuvers or turbulence. To better understand the limits, let's look at G-loading and V speeds.
Loading A Wing
When we engage in curving flight (turns or pull-up from a dive) or encounter turbulence we increase the loading on our wings. You are quite familiar with this phenomenon because you feel it all the time in your car in curves or dips. You also experience the turbulence effect when you drive over bumps. Increased speed around the curves, through the dips and over the bumps increases the forces you feel. We call these forces G-loading. One G is simply the force of gravity -- your weight for example. Two G's would be twice your weight, three G's three times your weight, etc.
Typical sailplanes are stressed for 5.3 G's positive and 2.7 G's negative, while airplanes come in at 3.8 positive for normal category and 4.4 positive for utility category, and half those figures in the negative sense. Rigid-wing hang gliders appear to be designed to the sailplane standard. Flex-wing hang gliders are not tested for structural strength in the same way as airplanes or sailplanes (loading them up with weight or "sandbagging") because they are small enough to fit on a vehicle, and their flexibility makes it difficult to load them with weights. Typically, they are driven down the road on a test platform with the appropriate angle of attack at a minimum airspeed. The weight of the truck provides the load force. If the gliders don't break or bend, they pass the load test. What is this minimum airspeed?
To find out, we must learn a bit about forces on our wings. When we fly in a curving flight path the apparent centrifugal force adds to our weight according to how fast we enter the curving path and how tight we make the curve. Again, you are familiar with this principle from your car or bicycle experience. In a similar sense, the faster we fly through turbulence the more severe the loads on our wings due to gusts.
So airspeed appears to be one of the main factors in structural loading. We can get a feel for this by noting the fact that the greatest potential lift on a wing occurs just before stall (the point of maximum coefficient of lift). When a wing does stall, the smooth airflow is disrupted and the forces on the wings are reduced. Thus, we can find a relationship between our flying speed and stall speed to determine how much load we can potentially place on our wings.
There is a well-known equation that gives the possible G-loading resulting from abrupt maneuvers relative to the airspeed you are flying. It is:
Immediately we can see some simple results. Let's assume that most hang gliders stall at 20 mph (32 km/h). If we are flying at 40 mph our formula gives
as the maximum possible G-loading in an abrupt maneuver (we'll discuss turbulence below). If we are flying at 60 mph the math becomes
See how quickly the potential G forces rise as we add speed?
The HGMA testing generally considers flex-wing hang gliders to be in the utility category. That means they should withstand 4.4 G's of loading if they are to comply with FAA requirements. This means that a glider which stalls at 20 mph could be flown at 42 mph and suffer no structural damage no matter how hard you maneuvered. If you look at the certification placard on your glider you'll find similar numbers. (Any discrepancy is due to the fact that the test airspeed indicators are calibrated and generally read a higher stall speed than we read under our wings.)
From the above calculation we get the glider's maneuvering speed. This is the maximum speed that the glider can be flown without fear of structural damage. This speed is called Va in aircraft usage and is referred to as maneuvering speed on the placard.
The Va speed is also good for turbulence. Turbulence gusts can greatly load up a wing. Fortunately for us, our light weight (and flexible frame for flex wings) limits the gust loading. The formulas for determining gust loading are beyond the scope of this article (see references below), but we can show you the results. For an average topless hang glider encountering a normal updraft of 1,500 fpm, the additional G-loading is .2 G's at 30 mph and .3 G's at 48 mph. At double the updraft strength (considered by the FAA and OSTIV, the sailplane technical authority, to be realistic in short shots in desert areas), these numbers double.
This doesn't seem like much -- .6 G's at the most -- but a couple of factors can make it worse. If you are maneuvering (pulling out or turning) when the gust hits, the effects will be added. More importantly, when strong up or down currents hit concurrently (remember those slack wires!) the G-loading can climb considerably. All in all, Va speed or below is your safety umbrella in turbulence.
VNE Limitations
Certainly most of us have flown faster than our Va speed. If you look you'll notice another speed, Vne, on your placard. The Vne speed is the "Velocity Never to Exceed." It is the velocity at which your glider has been tested to withstand the load test and not permanently deform. Typically, for the HGMA testing program, Vne is 1.15 times Va. Common Va and Vne speeds are 46 and 53 mph respectively.
The importance of Vne in aircraft usage cannot be overstated. If you fly faster than Vne there is no guarantee that permanent distortion or control flutter will not occur. These effects are just from the speed, not from G-forces, since we are assuming linear flight. The point to make perfectly clear is that you can fly at Vne in safety, but you cannot expect to make sudden maneuvers at this speed or fly in turbulence without possible damage.
Okay, many of us dive to goal in a bar-stuffing, Vne-pressing hurry through thermal turbulence. What's going on here? Basically it's the law of the pioneer: The first guy across the desert didn't die, so it must be safe for the rest of us. We have indeed proven by experiment that our (flex-wing) gliders can take it. Remember that merciful off-loading flexibility.
But this is precisely the potential problem with rigid wings, especially the new crop. There haven't been enough pioneers to test the limits of the wings. The factory test pilots shouldn't be expected to fly beyond the design Vne. If they are smart enough to limit their airspeed, shouldn't you? Rigid wings do not flex and off-load nearly as much as rag wings or even sailplanes. In addition, they are very capable of picking up speed very quickly. There is no doubt that every one of them will easily exceed their design Vne unless a system such as lots of fixed twist is built in.
The purpose of all this discussion is to make us all aware of the distinct opportunity we now have of breaking our gliders in the air. If you choose to fly a rigid wing you no longer have the luxury of ignoring speed limitations. At least one rigid wing has folded up due to overspeeding and maneuvering. We can prevent further similar incidents through knowledge and training.
Rigid wings are coming to their glory as we enter the third millennium. They offer a wider range of flying options, including more thermals to choose from, more routes to follow and more worthy flying days. Their only penalty appears to be weight and cost -- matters that have always been overcome by eager pilots. We embrace the many facets of our flying sports, but let us proceed with awareness so we don't have to lose friends to learn the limits.
References
- Performance Flying, by Dennis Pagen, Sport Aviation Publications, 1993. This book has a section on flying physiology which will help you understand how the brain reacts to turns, spins, spirals, etc.
- Laminar Aircraft Structures, by Alex Strojnek, published in 1983 by EAA. This books is the best book for the layman interested in aircraft structures, loading, etc. (the laminar in the title is just a catch word).
- Laminar Aircraft Design, by Alex Strojnek, published in 1984 by EAA. This book is one of the best for learning aerodynamics.
- Human Factors, by Dr. Fred G. DeLacerda, published in Sport Aerobatics. Dr. DeLacerda writes a column that illuminates many mental effects in flying. Sport Aerobatics magazine is published by a chapter of the EAA. You can get copies of the articles by Dr. DeLacerda by calling the EAA.