Flexies

This page was inspired by the lack of centralized information on Flexwing (Rogallo wing) boost glider designs. Almost all of this info was derived/gleaned from RMR postings and posters. Credit to original authors is given when possible. Unless cited by author, this information has been extensively edited and may not reflect the original meaning. My comments and/or edits are entered between square brackets, [like this]. Cavaet Emptor.

Basics

A flexwing glider uses a flexible material (plastic, mylar, fabric, etc.) to form the wing surfaces of the glider. A common example of a flexwing is the older, triangular shaped hang glider. A flexwing glider is traditionally constructed of a center, or keel, spar and two wing spars covered with light-weight plastic film. The spars are usually made of small diameter wooden dowels. All the spars are joined by a pivot mechanism at the forward end and with wing spars extended, the flexie has a triangular shape. The wing spars pivot at the apex and may be folded against the keel spar so that the glider can be loaded into the rocket booster. Upon ejection, a torsion mechanism (spring, rubber band, etc.) unfolds the wing spars which stretch open the wings and the glider (hopefully) glides back to Earth.

Compared to fixed wing gliders, a flexwing of the same size is extremely light and exhibits very low wing loading. They are capable of very long glide durations and a primary problem with a good flexwing glider is simply getting them back; just trying to improve the odds of recovery has led to several flexwing design innovations.

Flexwings are notoriously hard to trim into a stable glide configuration and the flexible wing material makes glider trimming more of an art than a science. While a flexie is an aerodynamic vehicle, traditional aerodynamic characteristics are generated by non-traditional means and the rigorous analysis of a flexie design is not typically possible.

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Information, References, and Links

1. A good source of information is the USENET newsgroup;   rec.models.rockets.

     For WWW access, try Google Groups: rec.models.rockets
     The Google Groups site allows you to search the message database for information.

2. The RMR rocketry FAQ has info on flexwings.

     Check out Bob Kaplow's updated Section 8 Glider FAQ.

3. The ASTRE club newsletter has a couple of articles on flexwing construction in PDF format.

4. A NARAM R&D report on flexwing glider trimming is available in PDF format.

5. The NAR Technical Services (NARTS) has several publications for sale which
    encompass flexwing gliders.

     Publication NIRA4 includes a flexwing construction articles.

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Kits

There are few flexwing glider kits on the market.

1. Qualified Competition Rockets (QCR) has a couple kits
    and a publication on flexwing glider design.

2. Chris Taylor has an article on his Maxy Flex glider on Essence's Model Rocket Reviews.

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Basic theory

An ideal glider in still air converts potential energy (altitude) into motion (horizontal glide). The conversion efficiency of a glider can be expressed as a glide ratio; the amount of horizontal distance traveled per unit distance in altitude lost. For instance, if a glider moves ten feet horizontally and loses one foot of altitude in the process, it's glide ratio is 10:1. Several factors affect glide ratios, but a primary factor is the wing loading of the glider. Wing loading is the the weight of the glider divided by the surface area of the wing. Flexwing gliders are very light as compared to traditional fixed-wing gliders and exhibit low (small) wing loading and, therefore, large glide ratios. It is this efficiency which is exploited by flexwing gliders and it affects flexwing glider design and design tradeoffs.

Flexwing gliders are subject to the same aerodynamic principles and forces as are traditional gliders, but the lack of traditional control surfaces requires a flexwing to generate stabilizing forces through global, rather than discrete, airframe/wing changes. Whereas a traditional glider may be stabilized by making discrete changes to elevators or rudder assemblies, a flexwing does not incorporate those assemblies and stability changes usually require modifications affecting the shape or weight of the entire glider or wing surface, which, in turn, may or may not affect other flight characteristics. The interaction of global shape/weight changes and flight characteristics make flexwing gliders notoriously difficult to trim for desired glide profiles and stability.

A glider maintains a constant glide path when the net forces acting upon the glider are balanced, except for a slight net thrust in the direction of flight. The forces acting upon a glider affect it's motion (acceleration) and orientation in a three dimensional space. The net motion of a glider is determined by the net forces of weight, lift, thrust, and drag. The orientation of a glider is affected by the distribution of the individual components of those four forces across the airframe by affecting the attitude of the glider along it's roll (rotation), pitch (up and down), and yaw (side-to-side) axes with respect to the center of gravity (mass) of the glider. If the individual vertical components of lift, for instance, are longitudinally (fore to aft) unbalanced across the center of gravity, the glider will either pitch up (attempt to climb) or pitch down (dive). Unbalanced horizontal components along the longitudinal axis will result in yaw (turning) changes. Lateral (side-to-side) unbalance can result in both roll and yaw changes. Considering this, it can be seen that symmetry is an important requirement for flexwing (or any aircraft) design and, in fact, introducing aerodynamic asymmetry is how the glider's flight profile and stabilty are controlled.

Stability refers to the ability of an airframe to maintain, or return to, it's desired flight attitude in the presence of impulse (temporary) forces. Stability is accomplished by the generation of restoring forces as the glider moves away from it's desired flight attitude. A stable rocket, for instance, generates a restoring force because the angle-of-attack of the fins changes slightly as the rocket pivots causing the fins to generate a lift force in opposition to the attitude change. The resulting force imbalance tends to rotate the rocket's longitudinal axis back parallel to the relative wind. In an unstable rocket, the restoring fin lift aids the original force imbalance and in an extreme case the rocket will fly in loops as it rotates about it's center of gravity (mass). Stability requires a glider to generate restoring forces for pitch, roll, and yaw inducing imbalances. For instance, if a glider dives too steeply, it should generate a restoring force to pitch the nose up. Conversely, too shallow a glide angle may cause the glider to stall and the restoring force should pitch the nose down. To keep the dive and stall restoring forces from "fighting" each other and causing an oscillating flight profile, the restoring forces need to be as small as practical or the response of the airframe to these forces must be damped, or both. In addition, a desired flight profile may require the glider to turn and circle the launch area or exhibit other, non-constant flight profile characteristics. Since this also requires a force imbalance, the generation of restoring forces and desired control forces should be decoupled as much as possible to prevent them from opposing each other and decreasing overall stability.

In a practical glider, most of these concerns are automatically addressed by the basic design, else it would not be a viable glider design in the first place. But when modifying a design, or trimming one for flight, these interactions must be kept in mind. Trimming a glider to slowly circle, for instance, may result in an unstable design that simply spirals down in an uncontrolled manner if the wrong mechanism is used to impart the turning force, ie, it intereferes with yaw stability.

For an ideal, three spar flexwing glider, attention should be paid to symmetry during construction; equal distributions of weight, lengths, and materials. Once a glider with a reasonable chance of performing has been constructed, small asymmetries can be added, such as weights or changes in local wing tension, to tailor the glide characteristics. Maintaining stability may require experimentation with more than one method for implementing a desired flight profile. For instance, if adding weight to impart a turn seems to impact stability, a change in wing tension may effect the desired result without impacting stability.

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Basic flexie construction

Materials

1. Wing material. Most builders use 1/4mil plastic sheet. Good wing candidate material includes dry cleaner bags, plastic dropcloth, plastic bags, etc. Mylar is used, but tends to wrinkle. Depending on the material, tape, CA glue, or contact cement can be used to attach the wing material to the spars. Stitching the fabric to the spars is also a viable technique.

2. Spars. Spruce dowels seem to be common. Balsa dowels and stock can be used, but strength becomes an issue.

3. Wing extension. Most flexies are boost gliders; the flexie is folded up and placed within a rocket airframe. The glider is released (usually expelled) from the rocket by the motor ejection charge. The glider's wings must unfold when it is ejected and this is usually accomplished by springs applying a force to the pivoting wing spars. The most common method uses coiled and formed music (piano) wire springs, though there is no universal extension method. Flat springs, rubber bands, and any number of other devices have been used to extend flexie wings.

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Design

1. Most flexies utilize three equal length spars; left wing, right wing, and center, or keel, spars.

2. The unmounted wing material is shaped either as a semicircle, with the spar lengths defining the radius of the circle, or as a quadrilateral whose long edges equal the wing spar length and the trailing edges are straight. The material is affixed to all three spars. There should be enough wing material to allow it to drape over the spars and not be stretched tightly across the entire wing surface. Indeed, the looseness, or billow, of the wing determines the majority of the aerodynamic properties of the glider.

3. The angle formed by the extended wing spars (how far they open) is typically from 90 to 120 degrees, although gliders with more, and less, wing spar angle have been built. The best angle to use for a given glider is still a topic of debate. [Flexible wing hang gliders have been built with spar angles approaching 180 degrees and truncated tip wings with high aspect ratio, ie, almost "normal" looking wing planforms.]

4. Flexies seem to glide best with a small amount of dihedral in the design. Dihedral is just a slight up angle of the wings. In a flexie, this means the aft ends of the wing spars are above the center spar. When viewed from the back, the spar ends form a shallow "V". With twelve-inch spars, a good dihedral is about three-quarters to one inch. Dihedral is usually added by the wing extension spring; the wing spars are unfolded in a slight up direction (the wing planes intersect the plane containing the keel spar). Alternatively, the keel spar can be mounted at an angle, but the diameter of the folded flexie must fit within the booster rocket airframe.

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Hints and Tips

A flexie should have a taut wing surface towards the nose, but the aft section of wing needs to be loose and billowy to form a Rogallo wing shape and control pitch stability. A wing spar which gently curves in towards the keel along it's length can give the appropriate wing tension, but the curves are hard to reproduce between models and spar material. George Gassaway has a different method to accomplish this:
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Author: George Gassaway [RMR post]

There is a trick to making flex-wings have good pitch stability without having to resort to other tricks such as tails, canards, and weights.

About 20-25% back from the nose, use some very sticky tape such as a strip cut from a band-aid (I use some good old sticky adhesive mylar but that's not easy to find). Use the tape strip to pull the plastic taut into the [left] spar and another piece to pull the plastic taut into the right spar. The remainder behind should be allowed to drape loosely so it billows with air when it glides.

Since you will need to experiment, do not attach the tape down permanently until you get the tautness just right for a good glide trim ( I usually only have a small poriton lightly stuck to the plastic, with the rest of the tape strip peeled up into the air). After all, if you use the kind of thin plastic as I use (1/4 mil dropcloth) once the tape is down good you will rip the plastic before the tape will come up if you later want to try to adjust the tautness and billow.

When the tautness and billowing are right, the taut front end acts as a built-in canard, sort of like having a flying wing that has elevators on the inboard leading edges instead of on the trailing edges (not that I can think of any real flying wings that were like that). I won't get into it any deeper since it is mostly just a matter of trying it and adjusting it until it glides stably. Once you get in the ballpark, you can tell, though if stalling persists you might need to add a little noseweight too (clay falls off too easily, I often glue a scrap piece of spruce or something else, even a piece of solder, to the nose center spar to move the CG a bit forward). But you can expriement with clay at first to make sure it is a CG problem before glueing anything.

Sometimes to get a good stable glide I've allowed one side to billow a bit more than the other, making the flex-wing glide in a turn of about 10 feet in diameter. But too much billow on one side can make it tend to spiral down. Again, trial and error in learning will teach far more than I could type about it.

Now, canards can do the job but they add extra complexity and difficulty. Takes a lot less time to find out how to trim by this nose tautness and rear billow method than do any actual design changes....which you would also still have to learn how to trim out anyway.

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Author: Brett Buck [RMR post]

Third magic trick - use fins about twice as big as you think you need on the booster. I don't think anyone has calcuated Cnf for a nose that consists of 3 sticks with a dropcloth between them! Boost instability is a common issue.
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