Kite Strings

HIGHER EDUCATION
WELCOME TO FIN LAND

by Drs. Lisa Colletti and Tracy Tillman

Fins to the Left, Fins to the Right...Welcome to Fin Land!
(Jimmy Buffett, refs 1,2).

TRACY: It's no secret that we are serious, probably obnoxious, parrot-heads. In fact, you're a famous parrot-head. Your picture made the Tampa Bay Online news when we were there for a concert last year (ref 3).

LISA: Yeah, we really enjoy going to Jimmy Buffett concerts, and we've held way-fun annual "MargariTOWville" fly-ins here at Cloud 9 for years. Jimmy's current concert tour is called "Welcome to Fin Land." We figured that would be a good title for this month's article, since our topic is about the use of fins on hang gliders.

TRACY: "Welcome to Fin Land" is perfect, because our flying site is located in the frozen tundra of the north, and because nearly everyone who aerotows with us here at Cloud 9 flies with a fin on his/her glider.

LISA: That's true. Even though we don't require the use of fins, almost everyone in our club flies with one. It's a main factor contributing to our record and reputation for safety. Last year, our club received the National Aeronautic Association's (NAA) Safety Award for hang gliding. The use of fins on gliders is a huge part of why our club has been able to maintain such an outstanding safety record over the years.

TRACY: It might be good for us to explain why most pilots here at Cloud 9 fly with a fin on their gliders.

LISA: There are several reasons: (#1) it makes towing safer, easier, and more likely to get to release altitude without a problem; (#2) the right-sized fin enhances handling and performance, (#3) our club has a culture of safety; and (#4) our best and most experienced pilots fly with fins, which make them good role models for new pilots.

TRACY: OK, let's look at reason #1. Why does flying with a fin make aerotowing safer, easier, and more likely for the pilot to get to release altitude?

LISA: A fin adds directional stability. Think of what it looks like when someone rides a unicycle compared to a bicycle. Because hang gliders are flying wings, they have a certain lack of directional stability as compared to airplanes or sailplanes, which utilize vertical stabilizers for directional stability. Pilots towing hang gliders without fins don't have much directional stability, so they often have to make relatively severe and quick movements to keep the glider near the center of the cone of safety while on tow. People on a unicycle sort of look like that, too. Pilots in free flight don't need to maintain such a high level of directional control and precision as is required when on tow, so the lack of directional control is not as noticeable in free flight as it is on tow. The pilot can let the glider wander all over the place in free flight, and they won't really notice. But that's not case when towing, especially aerotowing. From the view of the tug pilot, some hang glider pilots towing without a fin often look like a Raggedy-Ann doll on the end of a rubber band, bouncing all over the place while trying to maintain control on tow.

Whereas, when pilots use a fin to enhance the directional stability of their glider, they tend to be much more smooth and rock-solid on the centerline during tow, and they are not moving and bouncing around during tow nearly as much as a pilot who is not using a fin. It puts less stress and wear on the weak link, and it is more likely that the pilot will get a successful tow to release altitude without a weak link break or lockout--which makes it more likely for them to get a great soaring flight. This is a positive reinforcement system, as it contributes to building the confidence of the pilot, making him/her more likely to fly more often--which results in building more skill, more confidence, and more flying. That's what a safe, fun flying experience does. Scary, dangerous flying experiences drive people out of the sport. Pilots who get scared or hurt have a negative experience. They're not having fun, they lose confidence, and they are less likely to continue flying.

TRACY: And worse things can happen. Pilots who lockout near the ground, crash, and die have a more significant negative experience and don't continue flying, either. OK, what about reason #2--why does the right-sized fin enhance handling and performance?

LISA: The key is having the right-sized fin for a particular pilot/glider combination. Single-surface gliders usually have more directional stability and don't need fins. However, some--especially newer--pilots flying a single-surface glider may have trouble with pilot-induced oscillation (PIO) and will benefit from use of a fin. There has been a recent trend for some manufacturers to reduce directional stability in their newer single-surface glider designs, in an effort to enhance light handling for experienced pilots who may prefer to fly a single-surface glider. However, this just makes the glider harder to fly for new pilots, especially by tow. Because of the reduced directional stability in some newer single-surface gliders, it makes sense to put a fin on them for first solo flights.

Then there is the issue of pilots moving up to a more advanced-level glider. There is a tendency for pilots to PIO when moving up to a higher-performance glider, because they have gotten used to the stability, roll/yaw coupling behavior, lighter weight, and quicker response of their lower-performance glider. Higher-performance gliders are heavier and have more aspect ratio, meaning the wings are relatively longer in span and thinner in chord, and thus tend to be stiffer--meaning harder and slower to turn. Designers usually have to make them less roll stable to try to make them easier to turn. The result is that pilots new to a high-performance glider may over-control and oscillate. PIO is even more likely to occur when towing a hang glider as compared to free flight, because of the higher speed of towing and the physics of the pull of the tow line on the glider and pilot. Pilots should use a fin when they are transitioning to a higher-performance glider, especially if they are going to tow it. Using a fin makes the transition safer and builds confidence in the pilot who is new to a glider.

TRACY: First impressions are important. If a pilot gets scared or hurt flying a new glider, he/she may never like it. He may get frustrated, sell her new glider, and perhaps leave the sport. Whereas, if a pilot has had a good experience with a new glider and feels safe in it, he/she is likely to be more confident and fly more often, maintain currency and skill, and stay in the sport. A good experience in a new glider also helps to boost the reputation and sales of that glider design for the manufacturer.

LISA: When an instructor/dealer orders a new double-surface glider for a student or customer, he should ask the manufacturer to prep it for a fin. It would be good for the customer to also order a fin with his/her new glider. But even if he doesn't, the instructor/dealer can mount a rental fin on the glider for the pilot's initial flights in it, since the keel has already been prepped at the factory for mounting a fin. Wills Wing makes fins (ref 4) and can prep the keels on their gliders to accept fins at the factory. This has probably helped to increase sales of Wills Wing gliders, relative to some other glider manufacturers who do not produce fins or prep their gliders for fins. Having a glider prepped to accept a fin at the factory eliminates the need to use an additional keel extension/adapter later on for mounting a fin, and having a glider prepped to accept a fin enhances resale of the glider--both of which are good reasons why it is smart to get the glider prepped for a fin at the factory in the first place. It is important to remind the factory to pin or bolt the stinger to the rest of the keel, so it won't freely rotate when a fin is mounted on it.

If the manufacturer does not offer prepping of their keels for fins, then the instructor or dealer should prep it when the new glider is delivered, or at least have a clamp-on fin available to put on the glider for the customer's initial flights in it.

TRACY: What about the size of the fin?

LISA: Just as gliders come in various sizes, it makes sense for fins to come in various sizes, too. Wills Wing makes just one size fin, a fairly large one. It is an excellent size for adding a significant amount of directional stability, which is good not only for pilots transitioning to a more advanced glider, but also for some pilots all of the time. However, some pilots may not like or need that much directional stability, particularly if their glider is already fairly stable and/or stiff in roll. We have found that a fin that is about 30% smaller is a great size for use on stiffer comp-level gliders and some smaller gliders. It significantly enhances tracking and smoothness while on tow, but does not adversely affect handling or performance off tow.

TRACY: Why does a fin not adversely affect performance in free flight?

LISA: Think about it. The keel of a hang glider is not like an arrow flying straight through the air. The keel is dropping nose-high through the air as the glider flies forward--it drops through the air at some particular glide angle, because the wing's airfoil must have some angle of attack to produce lift. That is why some pilots will try to fly head low--to keep their body as streamlined as possible with the upward relative airflow. Because the keel is dropping through the air--not flying straight through it--the fin that is mounted on top of the keel is flying in the resultant wake above and behind the keel. The fin is not really going to add drag and perhaps may even help to streamline the swirling air above and behind the keel and thus reduce drag. Your university students' fin design is somewhat long and not very tall; it is shaped like a fairing at the base and provides a streamlined shape down a long portion of the top (wake-side) of the keel.

TRACY: In reality, the effect of streamlining the top of the keel probably doesn't reduce drag very much, but it doesn't hurt, either. The bottom line is that a good fin design does not hurt performance.

LISA: Also, some gliders have a tendency to start oscillating on their own at higher speeds, such as on fast glide or while on tow. Flying with a fin can help to reduce or eliminate these problems, both on and off tow. If a glider is not flying smooth and straight on a high speed glide, its glide performance is not going to be optimal. If a pilot has to constantly move his/her body around to try to keep the glider flying straight and smooth on a fast glide, his/her body will produce more drag in the airstream and thus reduce glide performance. In addition, when a pilot is constantly moving his body around, battling his glider for control, he/she will get tired quicker and flying performance, judgment, and ability to land well may become impaired.

TRACY: But what about pilots who claim that a fin makes their glider stiffer and more work to fly?

LISA: First, a lot of pilots who make that claim have probably never actually put a fin on their glider--they don't know what they are talking about, especially in a towing environment. However, if they have tried using a fin on their glider and found it to be stiffer and more work to fly, they simply used a fin that was too big for the glider. Wills Wing makes just one size of fin. It's a big one and can be too big for some gliders. If a right-sized fin is used, it will not make the glider stiffer and harder to turn--rather, it will make the glider easier to tow and fly at high speed, and, thus, less work to fly.

TRACY: Actually, a fin can make a glider easier to thermal and climb faster. You and I and other pilots here at Cloud 9 know and understand that.

LISA: That's right. Most pilots everywhere know that high-performance gliders usually need to be high-sided while holding turns in thermals. Because high-performance gliders have long, thin, heavy, high aspect ratio wings, designers of high-performance gliders must decrease the dihedral or add anhedral to make them less roll stable and easier to turn. Because these gliders tend to have negative roll stability (and thus reduced directional stability), pilots must high-side the glider to maintain a certain bank angle and turn radius while thermaling--otherwise, the glider will continue to roll into its bank more and more, and turn tighter and tighter on its own. Usually, more high-siding is required as more VG is added. This forces many pilots to circle in thermals with less VG, and get lower climb performance--or try to hold themselves farther out on the high-side while trying to fly the glider with more VG to get a more efficient wing and better climb performance. Trying to do that is a lot more work and makes the pilot get tired faster. In many cases, it takes too much work to try to circle in thermals using much if any VG.

Most pilots don't realize that a fin can allow them to circle in thermals with more VG and less high-siding and get better climb performance with less work. It is obvious, both in theory and in practice, that a fin does the same thing as high-siding, as it helps to keep the glider from tightening the turn while circling in a thermal. A right-sized fin will not impair handling--rather, it will help handling in thermals. With a right-sized fin, a pilot can use more VG in thermals, get better climb performance, be more relaxed, and get less tired on longer flights.

TRACY: I'm sure a lot of pilots will not want to believe that a fin can help them fly better in thermals. The key is having the right-sized fin. Based on our early experiments of flying with different-sized fins on various sizes and models of gliders, as well as years of additional experience verifying the results, the advantages of flying with a fin when thermaling are obvious to us. But, the fin must be the right size.

Are there any other flying qualities that are enhanced by use of a fin?

LISA: Yes, it makes the glider behave better when hit by a cross wind on take-off and when landing. If the glider gets hit by a cross wind gust when taking off from a launch cart, the fin will make the glider momentarily yaw and weather-vane a bit to help it more correctly maintain a crab angle into the relative wind behind the tug. A fin helps to yaw the nose of the glider into the wind gust, which makes the upwind wing slow down a little and the downwind wing speed up a little. This helps to keep the upwind wing from lifting and will help to prevent the glider from entering a downwind lockout near the ground. This momentary yaw can help the glider stay on the centerline of tow, or help keep it a bit on the upwind side of the centerline of tow. Staying on centerline is best. But if a glider gets off the centerline of tow when launching, there is more room for error and more time to recover if the glider is on the upwind side of the centerline, rather than the downwind side of the centerline. A lockout can happen very quickly if an upwind wing lifts at take-off and the glider gets on the downwind side of the centerline of tow. In addition to the downwind roll, the crosswind will even more quickly push the glider away from the centerline of tow, toward a lockout. If the glider is on the upwind side of the centerline, the crosswind will push the glider back toward the centerline, away from a lockout.

TRACY: And then there is the additional effect of wind gradient. If the upwind wing gets lifted through the gradient into a higher speed of crosswind, the high wing will be lifted up even more, be harder to keep down, and more likely to progress into a lockout very quickly. Whereas, if the downwind wing lifts through the gradient into the greater crosswind above, it will tend to be pushed back down to level and help to avoid a lockout.

LISA: A fin helps in a similar way if you get hit by a crosswind gust on landing. The fin will help the gliders momentary yaw into the crosswind gust, which will help keep your upwind wing down as you are leveling off in your round-out and it will help point the nose of your glider more directly into the wind so you can land with minimal ground speed and so the wind doesn't get under a wing and lift it when you flare.

TRACY: What about fins on rigid wings?

LISA: Rigid-wing hang gliders have more dihedral than high-performance flex wings, which adds roll stability and aids directional stability. But, in spite of having more dihedral, a rigid-wing hang glider is easy to roll because it has aerodynamic control surfaces. Rigid wings are usually much more stable and controllable on tow than high-performance flex wings and don't need a fin. But, instead of needing a fin to enhance directional stability, many rigid wings need to have a horizontal stabilizer mounted on the keel for pitch stability--which can prevent the mounting of a fin.

TRACY: Are there any high-performance flex wings that don't fly better with a fin?

LISA: Based on our experience, a right-sized fin will enhance the flying qualities of any high-performance flex wing and make them significantly easier to tow.

TRACY: As we've said, most of the pilots who fly in the club here at Cloud 9 have a fin on their glider. It is not so common at other flying sites. Can you explain to our readers why that is?

LISA: First, for many years we have been consistently teaching our students and new club members about the benefits of flying with a fin. Second, it helps when you and I provide a positive example by flying with fins on our own gliders. Third, we are able to provide right-sized fins for the range of gliders our pilots fly. Fourth, you and I have tried to create a culture of safety here (a) by example, (b) by teaching, and (c) by financially rewarding pilots who meet our "Safety Mentor" criteria--part of which includes using a fin if flying a double-surface flex-wing glider. And fifth, it helps when most other pilots who fly here also have fins on their gliders and prove the value of having a fin by towing smoothly, having great flights, and making good landings. Here, it is "cool" if you fly with a fin and "not cool" if you don't.

TRACY: Peer-pressure and leading by example make a big difference. New pilots will emulate experienced pilots and those who get the best flights. It is relatively easy for a club, school, or flying site to create a culture of safety if the best and most experienced pilots demonstrate safe flying practice. We can't eliminate all risk in flying--but with a culture of safety, it is less likely that something bad will happen. If instructors and advanced pilots hot dog and do dumb stuff to show off, and otherwise lead by bad example, it will result in a culture at that flying site of other pilots following that example and doing dumb stuff, too. That kind of culture makes it more likely that something bad will happen there, sooner rather than later.

LISA: As pilots, we accept the baseline risk of flying because flying adds quality to our lives. However, in addition to quality of life, it is good to have quantity. There is no quality of life if you "aren't." There are many risks in flying that can be prevented and/or minimized to reduce our overall risk (ref 5). It takes the right attitude, good training, good equipment, and risk-minimization behavior. Take it to heart. This is not an understatement.

TRACY: No, it's not. We're all heard the phrase that the sea is a cruel and unforgiving mistress" (ref 5). Well, the air can be a lot more fun than the sea, but also less forgiving. If you screw up, the air can be something more like an unforgiving ex-wife than a mistress--something along the lines of "evil, sadistic bitch." That's not an understatement, either.

LISA: Right, you don't want to go there.

TRACY: NOT ME!

Lisa is the Associate Dean and Professor of Surgery at the University of Michigan Medical School, and is past chair of the USHPA Towing committee. Tracy is a retired university professor, current chair of the Towing committee, and regional director for USHPA Regions 7 & 13. He is also a FAAST Team Safety Counselor for the FAA Detroit FSDO area. Lisa and Tracy are both very active multi-engine commercial airplane and glider pilots, tug pilots, and tandem hang gliding instructors for the Dragon Fly Soaring Club at Cloud 9 Field (46MI), Michigan. Please feel free to contact them about towing-related issues at cloud9sa@aol.com.

References

1. http://www.last.fm/music/Jimmy+Buffett/_/Fins
2. http://www.buffettworld.com/tourdates/
3. http://www2.tbo.com/photoalbum/2010/apr/24/jimmy-buffett-2010/19/
4. http://willswing.com/Accessories/stabilizer.asp
5. http://www.planeandpilotmag.com/pilot-talk/grassroots/the-character-of-risk

Tracy

"Welcome to Fin Land" is perfect, because our flying site is located in the frozen tundra of the north, and because nearly everyone who aerotows with us here at Cloud 9 flies with a fin on his/her glider.

Lisa

That's true. Even though we don't require the use of fins, almost everyone in our club flies with one. It's a main factor contributing to our record and reputation for safety.

Last year, our club received the National Aeronautic Association's (NAA) Safety Award for hang gliding.

The use of fins on gliders is a huge part of why our club has been able to maintain such an outstanding safety record over the years.

Tracy

It might be good for us to explain why most pilots here at Cloud 9 fly with a fin on their gliders.

Lauren Tjaden - 2008/03/23 22:20:15 UTC

When Jim got me locked out to the right, I couldn't keep the pitch of the glider with one hand for more than a second (the pressure was a zillion pounds, more or less), but the F'ing release slid around when I tried to hit it. The barrel release wouldn't work because we had too much pressure on it.

Anyhow, the tandem can indeed perform big wingovers, as I demonstrated when I finally got separated from the tug.

Davis Straub - 2011/08/26 14:04:52 UTC

We had six weaklink breaks in a row at Zapata this year.

Lisa

There are several reasons: (#1) it makes towing safer, easier, and more likely to get to release altitude without a problem;

(#2) the right-sized fin enhances handling...

...and performance, (#3) our club has a culture of safety;

...and (#4) our best and most experienced pilots fly with fins, which make them good role models for new pilots.

Tracy

OK, let's look at reason #1. Why does flying with a fin make aerotowing safer, easier, and more likely for the pilot to get to release altitude?

Lisa

A fin adds directional stability.

Think of what it looks like when someone rides a unicycle compared to a bicycle.

Because hang gliders are flying wings, they have a certain lack of directional stability as compared to airplanes or sailplanes, which utilize vertical stabilizers for directional stability.

Pilots towing hang gliders without fins don't have much directional stability, so they often have to make relatively severe and quick movements to keep the glider near the center of the...

...cone of safety while on tow.

People on a unicycle sort of look like that, too.

Pilots in free flight don't need to maintain such a high level of directional control and precision as is required when on tow, so the lack of directional control is not as noticeable in free flight as it is on tow. The pilot can let the glider wander all over the place in free flight, and they won't really notice.

But that's not case when towing, especially aerotowing.

From the view of the tug pilot, some hang glider pilots towing without a fin often look like a Raggedy-Ann doll on the end of a rubber band...

Gregg Ludwig - 2006/01/22 18:15:15 UTC

I just can not understand why operators continue to use poly towlines (for aerotow ops) when spectra towlines are clearly superior. Poly is less expensive... but when considering the cost of a tow plane and HG and the advantages of spectra, a few dollars of savings is foolish.

Gregg Ludwig - 2006/01/23 16:32:52 UTC

Towline elasticity produces a rubber band effect that results in ever changing towline forces that can also produce significant airspeed changes as well. Since Spectra does not offer elasticity (or very little) this effect does not occur and a safer tow results. Trikes normally tow with longer lines of 200-250 feet so the advantages of Spectra are even greater.

...bouncing all over the place while trying to maintain control on tow.

Whereas, when pilots use a fin to enhance the directional stability of their glider, they tend to be much more smooth and rock-solid on the centerline during tow....

...and they are not moving and bouncing around during tow nearly as much as a pilot who is not using a fin.

It puts less stress and wear on the weak link...

Towing Aloft - 1998/01

A weak link is the focal point of a safe towing system.

...and it is more likely that the pilot will get a successful tow to release altitude without a weak link break or lockout--

Adam Parer - 2009/11/25

Due to the rough conditions weak links were breaking just about every other tow and the two tugs worked hard to eventually get everyone off the ground successfully.

...which makes it more likely for them to get a great soaring flight.

Jim Rooney - 2011/08/28 19:39:17 UTC

The frustration of a weaklink break is just that, frustration.
And it can be very frustrating for sure. Especially on a good day, which they tend to be. It seems to be a Murphy favourite. You'll be in a long tug line on a stellar day just itching to fly. The stars are all lining up when *bam*, out of nowhere your trip to happy XC land goes up in a flash. Now you've got to hike it all the way back to the back of the line and wait as the "perfect" window drifts on by.

I get it.
It can be a pisser.

This is a positive reinforcement system, as it contributes to building the confidence of the pilot, making him/her more likely to fly more often--which results in building more skill, more confidence, and more flying.

That's what a safe, fun flying experience does.

Scary, dangerous flying experiences drive people out of the sport.

Lauren Tjaden - 2008/03/23 22:20:15 UTC

When Jim got me locked out to the right, I couldn't keep the pitch of the glider with one hand for more than a second (the pressure was a zillion pounds, more or less), but the F'ing release slid around when I tried to hit it. The barrel release wouldn't work because we had too much pressure on it.

Anyhow, the tandem can indeed perform big wingovers, as I demonstrated when I finally got separated from the tug.

Pilots who get scared or hurt have a negative experience. They're not having fun, they lose confidence, and they are less likely to continue flying.

Jim Rooney - 2011/06/12 13:57:58 UTC

Most common HG injury... spiral fracture of the humerus.

NMERider - 2012/06/02 08:17:03 UTC

I've met a few who broke the same arm a year or so later and then they quit. I think more pilots may quit after their friends get hurt. That may be a good reason to be vigilante with fiends.

Keith Skiles - 2011/06/02 19:50:13 UTC

I witnessed the one at Lookout. It was pretty ugly. Low angle of attack, too much speed and flew off the cart like a rocket until the weak link broke, she stalled and it turned back towards the ground.

Tracy

And worse things can happen. Pilots who lockout near the ground, crash, and die have a more significant negative experience and don't continue flying, either.

OK, what about reason #2--why does the right-sized fin enhance handling and performance?

Lisa

The key is having the right-sized fin for a particular pilot/glider combination. Single-surface gliders usually have more directional stability and don't need fins.

However, some--especially newer--pilots flying a single-surface glider may have trouble with pilot-induced oscillation (PIO) and will benefit from use of a fin.

There has been a recent trend for some manufacturers to reduce directional stability in their newer single-surface glider designs, in an effort to enhance light handling for experienced pilots who may prefer to fly a single-surface glider.

However, this just makes the glider harder to fly for new pilots, especially by tow.

Because of the reduced directional stability in some newer single-surface gliders, it makes sense to put a fin on them for first solo flights.

A minimum USHGA Novice (II) level of pilot proficiency is required to fly the Falcon safely, unless under the direct supervision of a qualified instructor.

Then there is the issue of pilots moving up to a more advanced-level glider. There is a tendency for pilots to PIO when moving up to a higher-performance glider, because they have gotten used to the stability, roll/yaw coupling behavior, lighter weight, and quicker response of their lower-performance glider.

Higher-performance gliders are heavier and have more aspect ratio, meaning the wings are relatively longer in span and thinner in chord, and thus tend to be stiffer--meaning harder and slower to turn. Designers usually have to make them less roll stable to try to make them easier to turn. The result is that pilots new to a high-performance glider may over-control and oscillate.

PIO is even more likely to occur when towing a hang glider as compared to free flight, because of the higher speed of towing...

...and the physics of the pull of the tow line on the glider and pilot.

Tracy Tillman - 2012/06

Dr. Lionel D. Hewitt, professor of physics and developer of the 2-to-1 center-of-mass Skyting bridle for surface towing, is well respected for his knowledge of towing, bridles, and weak links.

Pilots should use a fin when they are transitioning to a higher-performance glider, especially if they are going to tow it. Using a fin makes the transition safer and builds confidence in the pilot who is new to a glider.

Tracy

First impressions are important. If a pilot gets scared or hurt flying a new glider, he/she may never like it. He may get frustrated, sell her new glider, and perhaps leave the sport.

Whereas, if a pilot has had a good experience with a new glider and feels safe in it, he/she is likely to be more confident and fly more often, maintain currency and skill, and stay in the sport.

A good experience in a new glider also helps to boost the reputation and sales of that glider design for the manufacturer.

miguel - 2012/05/31 22:57:42 UTC

The glider had a bicycle handle release. When it came time to release, it would not release and in fighting with it, it came loose on the downtube, rotating around it. Fighting with it put the glider in the beginnings of lockout so I flew the glider. I noticed that the truck had stopped but the reel was free wheeling. I had done step towing before so I flew large circles around the towline and landed into the wind. I kissed the ground and swore never to tow a hang glider again.

Lisa

When an instructor/dealer orders a new double-surface glider for a student or customer, he should ask the manufacturer to prep it for a fin. It would be good for the customer to also order a fin with his/her new glider.

But even if he doesn't, the instructor/dealer can mount a rental fin on the glider for the pilot's initial flights in it, since the keel has already been prepped at the factory for mounting a fin.

Wills Wing makes fins and can prep the keels on their gliders to accept fins at the factory. This has probably helped to increase sales of Wills Wing gliders, relative to some other glider manufacturers who do not produce fins or prep their gliders for fins.

Having a glider prepped to accept a fin at the factory eliminates the need to use an additional keel extension/adapter later on for mounting a fin, and having a glider prepped to accept a fin enhances resale of the glider--both of which are good reasons why it is smart to get the glider prepped for a fin at the factory in the first place.

It is important to remind the factory to pin or bolt the stinger to the rest of the keel, so it won't freely rotate when a fin is mounted on it.

T.B. - 2007/01/17 03:08:33 UTC

I reach to hit the release and accidentally push the release on the outside of the downtube...

Lauren Tjaden - 2008/03/23 22:20:15 UTC

When Jim got me locked out to the right, I couldn't keep the pitch of the glider with one hand for more than a second (the pressure was a zillion pounds, more or less), but the F'ing release slid around when I tried to hit it.

Janni Papakrivos - 2008/06/30 15:35:44 UTC

With a bunch of tows to boot I can say that it happened once that I tried to release but missed the brake lever, instead I just pushed it around the downtube and had a much harder time reaching and actuating it.

Rick Maddy - 2009/04/04 20:17:05 UTC

I was overcontrolling like crazy. I actually went for the release just above tree level and missed. I hit the top of the release and knocked it sideways a little.

miguel - 2012/05/31 22:57:42 UTC

The glider had a bicycle handle release. When it came time to release, it would not release and in fighting with it, it came loose on the downtube, rotating around it. Fighting with it put the glider in the beginnings of lockout so I flew the glider.

If the manufacturer does not offer prepping of their keels for fins, then the instructor or dealer should prep it when the new glider is delivered, or at least have a clamp-on fin available to put on the glider for the customer's initial flights in it.

Tracy

What about the size of the fin?

Lisa

Just as gliders come in various sizes, it makes sense for fins to come in various sizes, too.

Tracy Tillman - 2012/06

Based on several decades of experience and hundreds of thousands of tows conducted by numerous aerotow operators across the county, the de facto standard has become use of a 260 lb. weak link made as a loop of 130 lb. green spot IGFA Dacron braided fishing line attached to one end of the pilot's V-bridle.

Jim Rooney - 2007/07/22 22:30:28 UTC

I've heard it a million times before from comp pilots insisting on towing with even doubled up weaklinks (some want no weaklink). I tell them the same thing I'm telling you... suck it up.

Wills Wing makes just one size fin, a fairly large one. It is an excellent size for adding a significant amount of directional stability, which is good not only for pilots transitioning to a more advanced glider, but also for some pilots all of the time. However, some pilots may not like or need that much directional stability, particularly if their glider is already fairly stable and/or stiff in roll.

We have found that a fin that is about 30% smaller is a great size for use on stiffer comp-level gliders and some smaller gliders.

It significantly enhances tracking and smoothness while on tow, but does not adversely affect handling or performance off tow.

Whereas, when pilots use a fin to enhance the directional stability of their glider, they tend to be much more smooth and rock-solid on the centerline during tow....

Tracy

Why does a fin not adversely affect performance in free flight?

Lisa

Think about it. The keel of a hang glider is not like an arrow flying straight through the air. The keel is dropping nose-high through the air as the glider flies forward--it drops through the air at some particular glide angle, because the wing's airfoil must have some angle of attack to produce lift. That is why some pilots will try to fly head low--to keep their body as streamlined as possible with the upward relative airflow. Because the keel is dropping through the air--not flying straight through it--the fin that is mounted on top of the keel is flying in the resultant wake above and behind the keel. The fin is not really going to add drag and perhaps may even help to streamline the swirling air above and behind the keel and thus reduce drag. Your university students' fin design is somewhat long and not very tall; it is shaped like a fairing at the base and provides a streamlined shape down a long portion of the top (wake-side) of the keel.

Tracy Tillman - 2012/06

The worst performance comes from a relatively short Spectra or thin pro-tow bridle, especially when the weak link is attached directly onto the thin curved pin of the secondary release...

Tracy

In reality, the effect of streamlining the top of the keel probably doesn't reduce drag very much, but it doesn't hurt, either. The bottom line is that a good fin design does not hurt performance.

Lisa

Also, some gliders have a tendency to start oscillating on their own at higher speeds, such as on fast glide or while on tow.

Flying with a fin can help to reduce or eliminate these problems, both on and off tow.

If a glider is not flying smooth and straight on a high speed glide, its glide performance is not going to be optimal. If a pilot has to constantly move his/her body around to try to keep the glider flying straight and smooth on a fast glide, his/her body will produce more drag in the airstream and thus reduce glide performance.

In addition, when a pilot is constantly moving his body around, battling his glider for control, he/she will get tired quicker and flying performance, judgment, and ability to land well may become impaired.

Higher-performance gliders are heavier and have more aspect ratio, meaning the wings are relatively longer in span and thinner in chord, and thus tend to be stiffer--meaning harder and slower to turn. Designers usually have to make them less roll stable to try to make them easier to turn.

Adam Parer - 2009/11/25

Due to the rough conditions weak links were breaking just about every other tow and the two tugs worked hard to eventually get everyone off the ground successfully.

Davis Straub - 2011/08/26 14:04:52 UTC

We had six weaklink breaks in a row at Zapata this year.

Tracy

But what about pilots who claim that a fin makes their glider stiffer and more work to fly?

Doug Hildreth - 1991/06

Good launch, but at about fifty feet the glider nosed up, stalled, and the pilot released by letting go of the basetube with right hand. Glider did a wingover to the left and crashed into a field next to the tow road.

Lisa

First, a lot of pilots who make that claim have probably never actually put a fin on their glider--they don't know what they are talking about, especially in a towing environment.

Tracy Tillman - 2012/06

We could get into details of lab testing weak links and bridles, but this article is already getting long. That would be a good topic for an article in the future. Besides, with our backgrounds in formal research, you and I both know that lab tests may produce results with good internal validity, but are often weak in regard to external validity--meaning lab conditions cannot completely include all the factors and variability that exists in the big, real world.

However, if they have tried using a fin on their glider and found it to be stiffer and more work to fly, they simply used a fin that was too big for the glider. Wills Wing makes just one size of fin. It's a big one and can be too big for some gliders. If a right-sized fin is used, it will not make the glider stiffer and harder to turn--rather, it will make the glider easier to tow and fly at high speed, and, thus, less work to fly.

Tracy

Actually, a fin can make a glider easier to thermal and climb faster. You and I and other pilots here at Cloud 9 know and understand that.

Lisa

That's right. Most pilots everywhere know that high-performance gliders usually need to be high-sided while holding turns in thermals. Because high-performance gliders have long, thin, heavy, high aspect ratio wings, designers of high-performance gliders must decrease the dihedral or add anhedral to make them less roll stable and easier to turn.

Because these gliders tend to have negative roll stability (and thus reduced directional stability), pilots must high-side the glider to maintain a certain bank angle and turn radius while thermaling--otherwise, the glider will continue to roll into its bank more and more, and turn tighter and tighter on its own. Usually, more high-siding is required as more VG is added. This forces many pilots to circle in thermals with less VG, and get lower climb performance--or try to hold themselves farther out on the high-side while trying to fly the glider with more VG to get a more efficient wing and better climb performance. Trying to do that is a lot more work and makes the pilot get tired faster. In many cases, it takes too much work to try to circle in thermals using much if any VG.

Most pilots don't realize that a fin can allow them to circle in thermals with more VG and less high-siding and get better climb performance with less work. It is obvious, both in theory and in practice, that a fin does the same thing as high-siding, as it helps to keep the glider from tightening the turn while circling in a thermal. A right-sized fin will not impair handling--rather, it will help handling in thermals.

With a right-sized fin, a pilot can use more VG in thermals, get better climb performance, be more relaxed, and get less tired on longer flights.

Tracy

I'm sure a lot of pilots will not want to believe that a fin can help them fly better in thermals. The key is having the right-sized fin. Based on our early experiments of flying with different-sized fins on various sizes and models of gliders, as well as years of additional experience verifying the results, the advantages of flying with a fin when thermaling are obvious to us. But, the fin must be the right size.

Are there any other flying qualities that are enhanced by use of a fin?

Lisa

Yes, it makes the glider behave better when hit by a cross wind on take-off and when landing. If the glider gets hit by a cross wind gust when taking off from a launch cart, the fin will make the glider momentarily yaw and weather-vane a bit to help it more correctly maintain a crab angle into the relative wind behind the tug. A fin helps to yaw the nose of the glider into the wind gust, which makes the upwind wing slow down a little and the downwind wing speed up a little.

This helps to keep the upwind wing from lifting and will help to prevent the glider from entering a downwind lockout near the ground.

This momentary yaw can help the glider stay on the centerline of tow, or help keep it a bit on the upwind side of the centerline of tow.

Staying on centerline is best.

But if a glider gets off the centerline of tow when launching, there is more room for error and more time to recover if the glider is on the upwind side of the centerline, rather than the downwind side of the centerline. A lockout can happen very quickly if an upwind wing lifts at take-off and the glider gets on the downwind side of the centerline of tow. In addition to the downwind roll, the crosswind will even more quickly push the glider away from the centerline of tow, toward a lockout. If the glider is on the upwind side of the centerline, the crosswind will push the glider back toward the centerline, away from a lockout.

Tracy

And then there is the additional effect of wind gradient. If the upwind wing gets lifted through the gradient into a higher speed of crosswind, the high wing will be lifted up even more, be harder to keep down, and more likely to progress into a lockout very quickly. Whereas, if the downwind wing lifts through the gradient into the greater crosswind above, it will tend to be pushed back down to level and help to avoid a lockout.

Lisa

A fin helps in a similar way if you get hit by a crosswind gust on landing. The fin will help the gliders momentary yaw into the crosswind gust, which will help keep your upwind wing down as you are leveling off in your round-out and it will help point the nose of your glider more directly into the wind so you can land with minimal ground speed and so the wind doesn't get under a wing and lift it when you flare.

Tracy

What about fins on rigid wings?

Lisa

Rigid-wing hang gliders have more dihedral than high-performance flex wings, which adds roll stability and aids directional stability. But, in spite of having more dihedral, a rigid-wing hang glider is easy to roll because it has aerodynamic control surfaces. Rigid wings are usually much more stable and controllable on tow than high-performance flex wings and don't need a fin. But, instead of needing a fin to enhance directional stability, many rigid wings need to have a horizontal stabilizer mounted on the keel for pitch stability--which can prevent the mounting of a fin.

Tracy

Are there any high-performance flex wings that don't fly better with a fin?

Lisa

Based on our experience, a right-sized fin will enhance the flying qualities of any high-performance flex wing and make them significantly easier to tow.

Tracy

As we've said, most of the pilots who fly in the club here at Cloud 9 have a fin on their glider. It is not so common at other flying sites. Can you explain to our readers why that is?

Lisa

First, for many years we have been consistently teaching our students and new club members about the benefits of flying with a fin.

Second, it helps when you and I provide a positive example by flying with fins on our own gliders. Third, we are able to provide right-sized fins for the range of gliders our pilots fly. Fourth, you and I have tried to create a culture of safety here (a) by example, (b) by teaching, and (c) by financially rewarding pilots who meet our "Safety Mentor" criteria--part of which includes using a fin if flying a double-surface flex-wing glider.

Davis Straub - 2008/04/22 14:47:00 UTC

Here is the requirement from the 2007 Worlds local rules (which I wrote) for weaklinks:

Pilots must use weaklinks provided by the meet organizers and in a manner approved by the meet organizers. All weaklinks will be checked and use of inappropriate weaklinks will require the pilot to go to the end of the launch line to change the weaklink.

Weaklinks will consist of a single loop of Cortland 130 lb Greenspot braided Dacron Tolling line and should be placed at one end of a shoulder bridle.

Jim Rooney - 2008/11/24 05:18:15 UTC

I've personally refused to tow a flight park owner over this very issue. I didn't want to clash, but I wasn't towing him. Yup, he wanted to tow with a doubled up weaklink. He eventually towed (behind me) with a single and sorry to disappoint any drama mongers, we're still friends. And lone gun crazy Rooney? Ten other tow pilots turned him down that day for the same reason.

And fifth, it helps when most other pilots who fly here also have fins on their gliders and prove the value of having a fin by towing smoothly, having great flights, and making good landings. Here, it is "cool" if you fly with a fin and "not cool" if you don't.

Christian Thoreson - 2004/10

Thus wheel landings, the safest and easiest way to consistently land a hang glider...

Tracy

Peer-pressure and leading by example make a big difference. New pilots will emulate experienced pilots and those who get the best flights.

It is relatively easy for a club, school, or flying site to create a culture of safety if the best and most experienced pilots demonstrate safe flying practice. We can't eliminate all risk in flying--but with a culture of safety, it is less likely that something bad will happen. If instructors and advanced pilots hot dog and do dumb stuff to show off, and otherwise lead by bad example, it will result in a culture at that flying site of other pilots following that example and doing dumb stuff, too. That kind of culture makes it more likely that something bad will happen there, sooner rather than later.

Lauren Tjaden - 2008/12/31 04:29:12 UTC

came in with no wind after an hour and had right wing drop. instead of wrestling gilder straight i tried to flare while desperately trying to straighten.

Lisa

As pilots, we accept the baseline risk of flying because flying adds quality to our lives.

However, in addition to quality of life, it is good to have quantity. There is no quality of life if you "aren't."

There are many risks in flying that can be prevented and/or minimized to reduce our overall risk.

It takes the right attitude, good training, good equipment, and risk-minimization behavior.

Take it to heart. This is not an understatement.

However, some--especially newer--pilots flying a single-surface glider may have trouble with pilot-induced oscillation (PIO) and will benefit from use of a fin.

However, this just makes the glider harder to fly for new pilots, especially by tow.

Because of the reduced directional stability in some newer single-surface gliders, it makes sense to put a fin on them for first solo flights.

Then there is the issue of pilots moving up to a more advanced-level glider.

There is a tendency for pilots to PIO when moving up to a higher-performance glider, because they have gotten used to the stability, roll/yaw coupling behavior, lighter weight, and quicker response of their lower-performance glider.

The result is that pilots new to a high-performance glider may over-control and oscillate.

Pilots should use a fin when they are transitioning to a higher-performance glider, especially if they are going to tow it.

Using a fin makes the transition safer and builds confidence in the pilot who is new to a glider.

If a pilot gets scared or hurt flying a new glider, he/she may never like it.

He may get frustrated, sell her new glider, and perhaps leave the sport.

Whereas, if a pilot has had a good experience with a new glider and feels safe in it, he/she is likely to be more confident and fly more often, maintain currency and skill, and stay in the sport.

A good experience in a new glider also helps to boost the reputation and sales of that glider design for the manufacturer.

When an instructor/dealer orders a new double-surface glider for a student or customer, he should ask the manufacturer to prep it for a fin.

It would be good for the customer to also order a fin with his/her new glider.

But even if he doesn't, the instructor/dealer can mount a rental fin on the glider for the pilot's initial flights in it, since the keel has already been prepped at the factory for mounting a fin.

If the manufacturer does not offer prepping of their keels for fins, then the instructor or dealer should prep it when the new glider is delivered, or at least have a clamp-on fin available to put on the glider for the customer's initial flights in it.

First, for many years we have been consistently teaching our students and new club members about the benefits of flying with a fin.

Wills Wing Vertical Stabilizer

Wills Wing has developed a Vertical Stabilizer attachment for the purpose of helping hang glider pilots minimize or eliminate problems with roll / yaw oscillations. The tendency towards oscillations of this type is present to some degree in all high aspect ratio gliders, particularly when flying at faster speeds in turbulence, and when towing (especially aero-towing). Pilots with sufficient skill and experience can usually control or prevent such oscillations. Pilots who are making the transition from entry level gliders to high performance gliders, and pilots learning new skills such as aero-towing, are often unable to adequately prevent or recover from these oscillations. In the case of aero-towing, this can be enough of a problem to be a barrier to learning this new skill. The Stabilizer is a great accessory for the advancing Beginner to Intermediate pilot.

A fin helps to yaw the nose of the glider into the wind gust, which makes the upwind wing slow down a little and the downwind wing speed up a little.

What we have covered in this article is practical information and knowledge gleaned from the real world of aerotowing, developed over decades and hundreds of thousands of tows by experts in the field. This information has practical external validity. Hopefully, someone will develop methods and technology that work better than what we are using as standard practice today. Like the methods and technology used today, it is unlikely that the new technology will be dictated onto us as a de jure standard. Rather, to become a de facto standard, that new technology will need to be made available in the marketplace, proven in the real world, and then embraced by our sport.

Based on several decades of experience and hundreds of thousands of tows conducted by numerous aerotow operators across the county, the de facto standard has become use of a 260 lb. weak link made as a loop of 130 lb. green spot IGFA [ref 12] Dacron braided fishing line attached to one end of the pilot's V-bridle.

The worst performance comes from a relatively short Spectra or thin pro-tow bridle, especially when the weak link is attached directly onto the thin curved pin of the secondary release, which imparts high loads over a very small area of the weak link.