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Nap-of-the-Earth Flying & The Save
by Gary Osoba

Many questions beg the attention of a pilot about to embark on a cross country adventure. He wonders "Is it too early to launch? Can I get away easily with this wind? How high will they go today? Did I bring enough water?" The questions tug away at a pilot's mind as he readies everything for flight. However, the question which urges itself upon you so insistently, so frequently, is.... "Will I have to land out unexpectedly?"

Yes, the landing. Where will it be? How will it go? Many a cross country flight has been abandoned, even in the planning stages, by concerns over an unknown landing. Many a cross country trek, beautifully flown, has been marred in its final moments by a landing that resulted in harm to the glider or pilot. Obviously, anything which can reduce the risks of landing out will do much to enhance the frequency and enjoyment of cross country flight. Traditional soaring literature is replete with many fine suggestions in this regard. A wise pilot will not only carefully consider these, but will implement them.

However, among the advantages of a newly emerging group of soaring craft is their ability to significantly limit landing out in an unsafe manner. Their design strengths, by nature, make premature landings rare. And they make the well-timed landing an easy one. As such, the growing field of ultralight and entry-level (or light) sailplanes will do much to encourage cross country soaring.

To illustrate the point, I can't remember the last time I worried about landing out when preparing to embark on a cross country flight in the prototype Carbon Dragon. In fact, after logging the first 100 hours of cross country time, there was not a single unplanned out-landing. I was well into the second 100 hours before the first and only one finally occurred. What happened then?

I was flying a quick100 km triangle in prefrontal conditions. Frontal passage was not predicted until some 8 to 12 hours after launch. Nevertheless, things developed early. And quickly. During the second leg of the triangle, a very strong cross wind began to develop. Shortly thereafter, the sky, which had been spotted by small and infrequent cu's, began to develop a threatening darkness to the west. I aborted the triangle, and turning into a strongly building headwind, headed toward the home gliderport (Figure 1). The darkness was approaching quickly, the wind kept building, and within a minute or two, overdevelopment turned the sun off like the flick of a light switch. The entire return course now being shaded, I landed out in a wheat field some 8 miles from the airport. Helped by my friend Bob Drennon, we quickly trailered the glider, snapped a picture of the massive cumulus mammatus behind it (Figure 2), and raced back to the hanger before the storm hit.

But let's get back to all the unplanned landings which could have occurred...and never did. Why is it that pilots in gliders like the Carbon Dragon will worry very little about this common soaring predicament?

To begin with, these gliders are designed to maximize soarability. Racing around with high speed efficiency, although respected, is not the top design priority. They stay up when nothing else can. They launch way early, sometimes hours before conventional sailplanes are soaring. And they land way late, after using every little bit of lift there is to find. The result is dramatically longer average flight times. And consequently, a significantly reduced number of takeoffs, tows and landings per unit of soaring time.

When it does come time to land, the number of suitable landing sites is much greater than that for the conventional sailplane. Not only can these gliders utilize microlift, but they can perform what we may term microlandings. The contributing factors are obvious. With landing speeds approaching sometimes one half that of a conventional sailplane, many sites which would otherwise be passed up are now usable. Combined with lower gross weights, the low speeds result in dramatically short roll outs. For example, on one flight last year I flew a little over 200 miles from southern Kansas up into Nebraska, then turned and flew back another 10 or 20 miles to land closer to my chase crew. Setting up for a landing near sundown, I selected the corner of a soybean field with short crops and widely spaced rows (Figure 3). This put me right next to a paved highway with a farm road by the field. After landing (and attending to another duty or two which tend to develop on a 6 hour flight) I stepped off my landing roll at 21 feet.... in negligible wind! Although the short roll out was not needed in this field, it will come in handy in others. In an emergency situation, consider the difference in inertial mass between a glider touching down at 20-25 knots with a gross weight of 300-500 lbs. and one weighing 800 or 1000 lbs. which is landing at 40-50 knots... over unimproved terrain!

Also helpful are the shorter spans and good maneuverability possessed by these designs, allowing them to squeeze down into smaller fields surrounded by trees or other obstacles. And to use areas with somewhat undulating grades which are otherwise unlandable. Of course, the excellent soarability of this class of gliders can sometimes work against you. Last year we took the Carbon Dragon with us on a trip to visit my wife's family in Wisconsin. I located a site with a farm road about 20 minutes away where the owner allowed local hang glider pilots to conduct tow operations. I had brought my static tow system and was able to enjoy a nice flight after taking a tow from my wife, Mary. When it came time to land, I had selected a small field several miles away which was bordered on the downwind side by a row of trees some 60 feet high. The plan was to fly 180's over the trees until descending to an altitude just above them, then turn final and drop into the field for a landing. The problem was that the wind was blowing 10-15 knots and with the excellent sink rate of the glider, I wasn't descending at all through the lift formed by the line of trees. So, I just made passes for a while, soaring the "ridge", and then resorted to my spoiler in order to effect the planned landing (Figure 4).

Micropatterns also affect average flight times and the frequency of landings. How so? Well, consider the rationale behind a typical, 1000' landing pattern. It's interesting to note that not only is this altitude applicable to conventional sailplanes, but many experienced hang glider pilots use it as well. The primary purpose of flying a pattern is to provide time for accurate perception... perception of current sink rate, perception of resultant glide, perception of field layout, any obstacles or other dangers, and other aircraft. A correctly flown pattern gains the pilot a grasp of perspective. Time is what's required. Even though a hang glider pilot typically flies his approach at half the speed of a sailplane and can land in some incredibly small areas, his sink rate is double that of a good sailplane. And so, the 1000' pattern is flown to provide the time necessary to size up all the variables. The sailplane has a good sink rate, but with the higher speeds, needs much more area to land in. In this case, the time provided by a 1000' pattern gives him the ability to fly a sufficiently large pattern, thoroughly scoping out his landing.

On the other hand, with gliders like the Carbon Dragon, 1000' patterns are just not necessary. A pilot entering the pattern at that altitude might as well set his alarm 5 minutes into the future and take a nap! With the sink rate of a high performance sailplane and the ability to land in areas nearly as small as a hang glider can, 500' is certainly adequate. I like to contrast it this way: Why enter a landing pattern at an altitude higher than I climbed away from at the beginning of the flight? Would the pilot of a 15 meter racer think of entering a landing pattern at 3000' after a soaring flight initiated from a 2000' aerotow? Hardly. Likewise, here's how it usually works for me: I take a 600' to 800' auto tow by static line. If I contact lift above 200' during the tow, I release early and fly away (Figure 5). If I take the full tow to 800' or so, it usually takes a few hundred feet to find a small thermal and then begin the afternoon's trek in that fashion. During the flying season, I get away almost every time.

Entering a landing pattern at 1000' is therefore not only unnecessary, but... well, wasteful. I don't know any other way to state it simply. On one flight last summer which was about to end, I had committed from base leg and was turning final at somewhere between 150' and 175'. I generally won't try below 200', and please don't think I'm recommending it to others, but in this instance I contacted smooth lift in light winds. So... I did it. Another unwanted landing prevented. Another flight significantly prolonged. Keep in mind that the Carbon Dragon uses about 20'-25' of vertical altitude in a coordinated 360 degree turn, enjoys a full stall recovery in about the same and a spin recovery in about 60' or 70' (if you can entice it to even enter one in the first place). It's really most genteel, without a dissonant note in it's entire repertoire.

So what kind of net effect can be expected from using 500' micropatterns instead of the standard 1000 footer? The sum, in this case, is dramatically greater than the parts. Very dramatically so. Its not as though the extra 500' on a day with 5000' thermal tops gives you 10% more time to contact another thermal. And that consequently, on the average, you'll avoid 10% of the unwanted landings. Getting 10% more air time. No, the dynamics of micrometeorology enter the picture and the whole formula begins to change. For here, within 500' or so of the surface, the magic of microlift phenomena is truly alive. It's vibrant, and can give birth to a microsave when you absolutely need one!

For the sailplane pilot who feels that nothing useful can be negotiated at these low altitudes, think of all the times you scratched, and hunted, and struggled to stay up... only to commit to a premature landing. And sure enough, well into final, there's the lift...too low to do anything with. But high enough to play havoc with your final glide path. It's not that you missed it earlier...flew around it... it's just that you weren't low enough yet. That's right, low enough.

I've spoken with many experienced hang glider pilots who know what I'm talking about. From time to time, they've benefited from the phenomenon. They just don't yet possess the performance levels to reliably exploit this near- earth soaring environment.

Raptors certainly recognize the reality of nap of the earth microlift. In this narrow altitude band where they're most frequently found flying, their technique is truly inspiring. What soaring pilot, possessed of a rudimentary knowledge of a hawk's performance capabilities and yet observing the same in action, has not scratched his head in wonderment? And remarked that surely the laws of mathematics must have been temporarily suspended in this location.

Do the hawks know something we don't? I believe so. But maybe we can get to know it, too. My experiences in the Carbon Dragon have led me to construct an increasingly clear mental picture of what's going on down here, within 500' or so of the surface. The numbers and relationships which are presented now may vary somewhat with location, topography, etc., but the essentials should hold true. The conclusion I've drawn is that on any given day where convection is working as a result of solar heating, every likely thermal producing source can provide you with a save, whether it's "cycling" or not. Basically, every time. I say this because it's been my experience in the Carbon Dragon over dozens and dozens of instances. Now if it's too early in the day, its not going to work. If it's at the end of the day, it may not work. If the sun has shut down, as in the instance related at the beginning of this discussion, it won't be reliable. Otherwise, it's there for you. Absolutely. In fact, the referenced instance was the only time it hasn't worked for me in many hours of cross country soaring.

It amazes me that at this point in soaring history there is still a fundamental debate among some about what a thermal really looks like. There are those who will say that all thermals essentially resemble a chimney in structure, providing a constant source of lift over a thermal producing source. For any who would argue against it, its pretty hard to convince someone who has witnessed a massive dust devil towering skyward from the same field all day long that their eyes were just playing tricks on them. On the other hand, there are those who argue that all thermals are essentially big bubbles, which having reached a temperature sufficiently greater than the air surrounding them, break away as a discreet air mass, floating upward. The ring vortex model fits into this latter category. For those arguing against this approach, its pretty hard to convince pilots who have entered a thermal right below another glider, only to find the lift gone! Or those who have been spat out of a thermal who then re-enter the column to note mixed air or sink, but no more lift... no matter how long they circle.

We should be able to agree that all of these concepts of thermals, and many variations in between, exist at various times. In the case of the "chimney" thermal, it would seem that consistently strong conditions, under direct, strong sunlight, with light winds, would favor their formation. In these instances, the powerful energy of the sun just continues to pour into a ground source, such as a freshly plowed black field. Enough of a temperature differential between the source and its surrounding terrain exists so that the energy going in essentially equals the energy going out... and up. The air mass doesn't really need to pause to build up enough heat, it's more or less a constant process. The sun's radiation in... the earth's convection out. Needless to say, these kind of conditions pose no particular problem for any of the types of soaring craft mentioned in this discussion. As a matter of fact, they are truly sought after, albeit quite rare in my part of the country! The guaranteed save will be there at 500'. It will be there at 1000'. And it will probably be there at 5000'.

What about the bubble (and ring vortex) model? In this scenario cycling takes place. At times, the heated parcel of air in the near-earth environment will be hot enough to break away, or is triggered into doing so, possibly even sustaining for a while in chimney-like fashion. Then, the cycle will shut down to start the heat building process all over again. Mild and indirect solar radiation will inhibit the strength and frequency of the cycles, as will stronger winds which tend to trigger the cycles early and redistribute the heat horizontally through the atmosphere. In these conditions, smaller, weaker thermals or even incipient ones are favored, depending upon what's going on with the upper air masses. The variations in low level wind gradient will also exert an influence one way or another, for obvious reasons.

Once again , the problem is not when the cycle is switched on... but off. Which seems to be the case most of the time. What's really going on then? Is there still something there that we can use? Recall our searching sailplane pilot who was simply not low enough to utilize lift. The lift turns out to be there, essentially all the time, but at micro-rates and micro-altitudes. Think about it this way: What's happening at the top of the bubble while it's waiting to build enough total energy to break away from the surface? Is there some sort of firm barrier that prevents the warm air "in" the bubble from mixing or moving into the air above the bubble? Not that I'm aware of. In fact, as the temperature of the bubble mass builds, it's still subject to the laws of thermal dynamics and will therefore seek equilibrium with surrounding air. It's migrating, leaking off if you will, into the upper air. We might call the result leak-off microlift. Consider the example of a hot air balloon and it's definitive fabric barrier which is designed to contain a "thermal". In spite of the existence of the barrier, considerable leak off still takes place, necessitating frequent blasts of the burner to keep matters in check. In fact, the leak off is of a high enough order to allow soaring birds to sustain flight above the balloon's envelope.

But back to our bubble's cycle. In the very early stages of the process, the temperature differential may only be a few degrees, which is why the perimeter of the bubble may be encountered some 300' to 500' above the surface. Progressing a few degrees warmer now, the leak off may result in weak, disorganized lift which moves upward another few hundred feet. It might only amount to 75, or 100, or 125 fpm, but it's there. A little while longer, and the bubble's heated mass has accelerated rapidly from the surface contact and the whole mass begins to break away for another up cycle.

What I have found repeatedly is that if I will park myself over a good source when conditions are working, I'll eventually get my save. I can count on it. I might descend for a while through really weak leak-off to 300' or 400', then just barely sustain on top of it for a time until it starts to break away. Or, I might find it starting to leak off for a slow climb rate to 700' to 900', waiting then at that altitude until it organizes and roars upward in a cycle. But it always seems to be there for me.

When barely sustaining and playing the waiting game, I have to be careful to fly as efficiently as possible and to utilize shallow bank angles. It would appear that the performance capabilities of the Carbon Dragon (circa 100 fpm minimum sink) in combination with the low speeds and small turning radii are just barely inside the parameters necessary to utilize leak- off microlift. Conversely, to circle too tightly introduces just enough degradation in sink rate to render the overall technique ineffective. Of course once the bubble breaks away, the structure seems to concentrate into a smaller column and then tighter coring is definitely in order.

A variation along this theme occurs in higher winds. Instead of the bubble building in time over a singular location, the surface winds regularly detach the weak leak-off bubbles early from their source and they begin drifting with the wind. Then another forms pretty soon over the original source, the wind tears it away, and on we go. What microlift technique can be utilized in this instance? Park yourself over the source, continuing to descend to the 500' level or lower if necessary, in the hopes that a big enough one will break free to send you back to the upper levels. If not, take the next one that leaves and commit to drift with it. What happens in these conditions is that the weak lift may take you a few hundred feet higher, but no more. Stay with it. You've made your decision. Sustain in the bubble, not over the ground source. Do not exit and try to find a stronger one. Not only is it unlikely that you will find a stronger one in a random search of these conditions, but you certainly don't have the altitude or time to explore for very long.

What happens in these situations is that the bubble you are with will eventually contact another good ground source, combine with it's heated potential, and nearly always provide you the energy to go back to the upper levels. In this instance, you'll find yourself working what we may refer to as cumulative thermals. It can actually be quite predictable... drifting along, barely sustaining over green fields... and spotting a big plowed one coming up in about a mile or so. Sure enough, when you get there, it all comes together and you're gone!

Without a doubt, there's usable lift to be found down here, in close proximity to the earth. Capturing it's potential requires a combination of the right equipment and the right techniques. Of course, nothing presented herein should be construed as a contradiction of the old soaring adage "Get high and stay high!". But when everything else has failed you, and you're just not yet resigned to landing, nap-of the-earth microlift may prove to be your answer.

Hopefully there's something here which will prove useful to you whether you fly a hang glider, a standard class sailplane, or a Nimbus IV. And, I hope it gives impetus to those interested in exploring the emerging field of ultralight and light sailplanes. Whether it's the excellent soarability, the increased number of usable landing fields, the efficiency of micropatterns, or the reliability of a nap-of-the-earth save, this class has much to offer. In case you haven't already guessed it, I'm thoroughly enjoying myself in this regime!

Best Regards & Air Time To All, Gary Osoba


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