Hang gliders only ascend when the surrounding air is ascending. Wind deflected by hills. Pockets of warm air rising. Hang gliders go up for the same reason kites fly. And kites don't need complex airfoils. They're just sails.
Powered aircraft ascend by using their engines to move fast enough to provide their wings with wind. Wind flowing over wings produces lift.
So why do gliders have airfoils- wings with top surfaces longer than than their bottom surfaces- if they're not what makes them rise? Because gliders need to be able to remain in the air as long as possible. To loiter and maneuver to locate the rising air. Just as powered aircraft use airfoils to convert forward motion into altitude, gliders use theirs to convert altitude into forward motion.
What if you put an engine on a hang glider? It's been done. Many ultralights use hang glider wings. Such aircraft land on wheels. Some people employ a simpler approach, attaching a small 15hp engine directly to their flying harness. These are launched and landed on foot.
Powered hang glider require special skill on take-off and landing. And they're not as appropriate for mountain launches. When flying under full power, the pilot is pushed forward through the control bar. This results in a control attitude that is equivalent to a power dive. It's not a very strong position to be in. The pilot is also managing an extra source of aerodynamic directional control because she is directly attached to the source of thrust. This added dimension of control partially obscures the "natural" control motions normally required to fly a hang glider. Also, the propeller provides a substantial amount of drag when the engine is off. Usually this is addressed by building in features that allow the prop to feather or fold back.
Powered hang gliders are chimeras. They are neither gliders nor aircraft, but represent a compromise between two incompatible ideals. On one hand is pure soaring flight to which the added noise, cost, weight, and drag of engines is anathema. Hang gliders are designed to be gliders. They're designed to be controlled via weight shift.
On the other hand is powered, three-dimensional, acrobatic flight. Or, if you prefer: high-speed / long-distance passenger service. Aircraft design varies accordingly. Powered aircraft rarely look anything like hang gliders.
If you're a pilot that's interested in soaring, organic control, and the elegance of physically carrying your wing until it carries you, then you're attracted to hang gliding. Otherwise, you'll probably head elsewhere.
However, I'm thinking that there probably isn't a HG pilot that wouldn't appreciate an engine occasionally. Sometimes you want to take off from flat ground and fly flatland thermal- without going to the multiplied trouble of being towed by another aircraft. Sometimes you'd want to boost yourself back into ridge lift instead of drifting down to a faraway valley at the loss of hours of flying. And then there are times when circumstances have left you no option but to crashland in unsuitable terrain- forests, rocks, cacti. In addition to an emergency chute, a bit of thrust would make for an excellent safety feature.
But if you look at the options already available, having an engine for these special occasions means managing a significant amount of awkwardness and drag on a constant basis. So,
What I'm proposing is that the hang glider be equipped with an odd number of small electric ducted fans attached directly to the glider's wing- not to the harness. The fans already exist. They're made for radio controlled scaled-down jet aircraft. And batteries have never been more advanced.
Ducted fans are an efficient way to produce low-speed thrust- in the regime of 0-100mph. Ducts drastically reduce blade tip losses- vortexes of turbulence that don't contribute to thrust. Ducts add complexity and weight, however, which may outweigh the efficiency gains at higher speeds. Also, high speeds entail greater amounts of induced drag caused by the ducts themselves. But DFs, with their shorter diameters, can also operate at higher RPMs than similarly powered open props. Electric motors are a perfect choice for taking advantage of this.
When not under power, the drag a propeller's generates is proportional to the area of the circle of the propeller's sweep. That doesn't mean that the induced drag is exactly equivalent to what would be caused by a flat solid disk of the same diameter. It's significantly less than that. It only means that the wider the propeller, the more drag it creates when not under power. A helicopter, for instance, with its massive prop-diameter, produces enough drag to actually land safely even if the engine quits (see autorotation). Propeller = parachute.
Ducted fans allow for comparatively tiny cross sections.
By using several of them, let's say five, you take advantage of a miniature expression of the multi-engine advantage. Why do large aircraft- WWII bombers, for instance- employ multiple engines? It's not because a single engines couldn't be built that could provide enough power. Bigger engines almost always provides a better power-to-weight ratio (this doesn't apply as much to electric motors). Instead, it's because propellers would have to get very large and be driven very fast- and the blade tips would be exceeding the speed of sound which would generate all kinds of horrible turbulence. Multiple engines allows for smaller prop diameters to generate the same amount of thrust.
That's one side of the argument in favor of multiple engines. The other side is that smaller prop diameters entail far less drag. Assuming you have efficient small engines, and assuming those engines aren't always running, multiple small engines entail far less drag.
Let's do some math, using two small DFs as examples.
First, tuck this away: a Mosquito powered hang glider uses a prop with a 1.35m diameter. That translates to a prop-circle area of 1.42 m^2. The engine / prop combination the Mosquito uses produces about 130 lbs of thrust. That's about 96 lbs / m^2. Sorry about the mixed units. I'm an American.
Let's take a look at some electric DFs: this produces 18+ lbs of thrust with a 120mm diameter or 0.0113m^2. That's about 1592lbs / m^2.
Meanwhile, this one produces 28+ lbs of thrust with a 156mm diameter (0.0191m^2). That equals 1464 lbs / m^2.
The drag induced by a 30% greater diameter cancels out the proportional advantage of 55% greater thrust. Let me put it this way.
The least common multiple of 18 and 28 is 252.
14 x 18 = 252 lbs
9 x 28 = 252 lbs
To get 252 lbs of thrust, you could use 14 of the 18lb ducted fans or
9 of the 28lb ducted fans.
14 x 0.0113m^2 = 0.1582m^2
9 x 0.0191m^2 = 0.1719m^2
If you needed to produce 252lbs of thrust, and if all you cared about was the amount of drag the ducted fan would produce when not in use, 14 slightly smaller DF's' would entail around 8% less drag than 9 of the larger, more powerful ones.
That's how ducted fans roll. They make smaller diameters more stream-lined for similar amounts of power.
Let's assume that each DF is designed to produce some amount of static thrust at some particular operating speed. Either slower or faster is less efficient. Let's call this peak efficiency, or PE.
Let's also assume that each DF can produce significantly more power- albeit at a lower efficiency / longevity. Let's call this peak power, or PP.
Let's also assume that you can also operate your DFs to produce just enough thrust to overcome the drag they produce just sitting there. We'll call this peak longevity (I'd say "endurance," but I already used the letter "E"), or PL.
Why use an odd number of DFs?
If you had five DFs in a row (and I'm not saying it would have to be five. Likely you'd end up wanting between 7 and 11)- two on each wing and one in the center- you could turn them off in the follow progression:
5- all on
4- all but the center one on
3- one on each wing plus the center
2- one on each wing
1- just the center
Let's run some scenarios.
A. Let's say you find yourself in a momentary emergency situation. You need to produce maximum lift in the least amount of time to avoid hitting an imminent obstacle. You turn all five DFs to PP and switch to PE as soon as you're out of the woods.
B. You've dipped into a mountain valley and need to fight your way back into the ridge lift. You don't dare fly too close to the sheer mountain walls, where turbulence is unpredictable. You have open airspace to operate in, and- if necessary- an emergency place to land down at the valley floor. So you switch all five fans to PE and work your way back up until you've above the ridgeline again.
C. You're flying cross country and you haven't even needed your DFs at all. Your batteries are full. So you operate all five fans at PL to give yourself the best possible flight characteristics.
D. You're at 8000ft AGL, flying cross country, and a wide long lake is in your path. You believe that you have enough altitude to glide across with up-to a mile of glide to spare. However, you'd be at such a low altitude on the other side that you'd have precious little chance to find another thermal and make your way back up to cloudbase. Your flight would probably be over for the day. You ask yourself: am I a soaring purist, or am I a pilot? You decide you'd rather fly that break a record, so you set two of the five fans to operate at PE, providing yourself just enough thrust to maintain your present altitude. Halfway across the lake, you encounter sinking air. Instead of wasting time maneuvering in an attempt to find more favorable air you turn on another fan- also operating at PE. After five minutes, you cut to two. Once your shadow hits the shoreline you turn all but one of them off. You're still at 6500 ft when you encounter your first hint of a thermal. Ten minutes later and your high-altitude swim is all but forgotten.
E. You've been flying cross country for several hours and you could really use a break. It's the middle of the day. You spy a truckstop with huge, mostly vacant parking lots so you meander down, flaring out in the grass at the edge of the lot. You tie your glider down, jog to the bathroom, catch some lunch, and then, with crowd of bemused onlookers wondering what you have in mind, you strap back into your harness, face into the wind, run, turn on all five DFs at PP, lift off, and make a lazy wide circle over the hot parking lot. After you've gained fifty feet of altitude, you switch to PE. Another fifty, and you're getting some positive help from the inevitable black-top thermal. You're back at altitude and on your way.
D. You're contemplating a mountain launch, but the ideal launch direction the site provides is facing 45 degrees away from the direction of wind. What's worse, the wind is averaging 5 mph less than what'd you'd like. So you angle yourself straight into the wind. Two volunteers hold your wings as you ready yourself. You turn on all five fans at PE and start your run. Your HG tugs against your ground handlers. You shout, "clear," and instead of five steps, you're off in three. Instead of dipping down fifty feet before leveling off, you lose nothing. You're alerady climbing. Thirty seconds later, you cut out all five DFs and emerge into a high pocket of ridge lift. For the rest of your flight, your DFs are running at PL.
E. You've just driven 10 hours to fly at a particular mountain in the foothills of Idaho. It's early afternoon and when you arrive, the windsocks are laying limp. You wait an hour, and other than an occasional 8mph gust, it's a still day. You're not looking for an epic journey, you just want to get off the ground. So you power up, setting all five DFs at PE, and up you go. You let all five DFs run at PE until the battery is about dead, stealing altitude from an uncooperative sky. Then you switch to PL and you quietly glide back to earth. What would have been a five minute flight has been stretched into almost an hour.
F. The unimaginable has transpired. One of your under wing guy lines has broken its shackle in a violent bout of vertical turbulence and now your wing is slightly skewed and threatening to deteriorate further. You were flying over steep, tree-covered terrain when it happened. A quarter mile away is an inviting mountain meadow. You have a choice- throw your parachute immediately and land in the trees- or gently nudge your wounded wing into more inviting terrain before hitting the silk. Only problem is, the winds just aren't cooperating with your plans. So your first reflex is to give yourself some emergency power. You point your nose to safe terrain. Simultaneously, one hand has gone to your chute, pulling it from its pocket on your chest. You mentally rehearse your throw, ready to move the moment things get any worse. A subjective hour later, you're over the meadow. You have a few hundred more feet of altitude than you would have had you not used the DFs. Parachute in hand, you take a few extra moments to survey the terrain. Time slows. Satisfied, you make your throw. Your forward progress is abruptly halted and you begin to descend at an angle, turning slowly as you go. You see that you're headed for one of the few small pines that occupy the otherwise open meadow so you goose the throttle on your DFs, producing moments of PP whenever you're facing away from the tree. You radio back that you're okay, fold up, and hike out. You realize that if you'd deployed your chute immediately, you very well might have spent the remainder of the day, scratched and bleeding, a hundred feet up in a tree.
G. You've decided to fly across the United States. You're going from west to east, following the prevailing winds. You keep within sight of roads and highways as you go. Each day, you fly as far as the weather and wind will permit, using your DFs to boost your altitude whenever expedience requires. On cooperative days, the sky gives up hundreds hundreds of miles. Some days you fly from morning to afternoon. Others, you stop several times at towns, or rural gas stations, and beg electricity in exchange for telling your story. You're often invited to supper. Most of the time, people offer you a place to sleep. After you cross the Mississippi, you encounter a patch of rainy weather that lasts for most of a week. You fold up and take advantage of a friendly stranger's offer to store your glider in his garage. When the weather clears, you head east again. After two months, you swing out over the Atlantic and then come to rest on a beach in North Carolina. You're greeted by a crowd of well-wishers who've been following your progress online.
I could write more about the technical details. The types of batteries you'd use (at least 40lbs of compact rechargeable lithium ion batteries designed for small electric cars). I could talk about how, with five DFs, you'd have two on each wing and one on the centerline and that by varying the thrust bilaterally, one could steer the wing. I could talk about how you could use either a system of switches to turn DFs on and off and a single throttle, similar to a motorcycle's throttle control, that varys the power of everything that's turned on. The pilot could have a second throttle-like control, or lateral sliding control, that moves power from side to side to assist in steering. Finally, the pilot could have presets that would keep the DFs running at PP, PE, or PL in whatever configuration she desires.
The DFs could be mounted in a number of different ways. I expect the most practical arrangement would be to mount them to the crossbar under the wing. The center DF could be mounted to the keel. It's important that the DFs be mounted in a way that doesn't cause them to crushed or filled with grass during a noseplant.
I used an example above with 252 lbs of thrust. That's well more than necessary. Somewhere around 170lbs of PP thrust should be sufficient. More than that, and the pilot would be tempted to stray into acrobatic flight.
Total 5 x PE flight time might be around 30 minutes. Dividing the number of DFs multiplies the endurance.
In summary, what makes this idea potentially superior to powered hang gliders that already exist is that it hews closer to the ideal of pure soaring flight. It allows the hang glider to handle like a hang glider again. Instead of turning a hang glider into a small ultralight, it provides occasional assistance, extra flexibility, and emergency power. It allows for take-off from flat ground /and/ hill launches without reconfiguration. It enhances cross-country endurance by providing a bridge between hotspots.
And, if you were to install lightweight photovoltaic cells- like the thin copper indium gallium diselenide films promised by Nanosolar, you might have yourself a solar soarer capable of indefinitely extending its daytime endurance or, in a pinch, of getting off the ground again after a wilderness landing.
Q. Isn't having a powered option kinda like cheating?
A. Yes... but, you should ask yourself whether the added versatility, longevity, would make you a better pilot, or worse. If the answer is "worse," then this concept isn't for you. On the other hand, if you think of this as a safety device, then no, it isn't cheating.
Q. How much will they cost?
A. About $12k fully assembled and tested.
Q. What if I'm building one myself?
A. Between $7k and $10k.
Q. What would you need to make this happen?
A. An investor- initial prototyping should be achievable for less than $50k / 6 mos. A hang glider engineer- someone with a lot of experience repairing, rebuilding, and modifying hang gliders. An "RC Engineer"- someone with experience working with the propulsion equipment. And a test pilot- an advanced hang glider pilot with powered experience. A spokesman (probably one of the aforementioned persons)- someone that can navigate the news media to generate free advertising.
Q. What will this do to the sport of hang gliding?
A. This has the potential for attracting new interest to the sport of hang gliding, as well as reviving the interest of some percentage of existing pilots. People who live in flat areas, or are repelled by their impression of the inherent risk and uncertainty of unpowered flight, may use see this as an excuse for giving the sport a second chance. DFs will also add to the cool-tech factor, expanding the potential customer base accordingly.
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