S4 – Boost Glider
- Flown in three standard rounds and, if needed, one or two flyoff rounds
- Flown in the 2.5 N-sec (A) power class for both Seniors and Juniors
- Maximum time per standard round is 180 seconds
- Score is the sum of the duration of the three flights
- If tie for 1st, 2nd, or 3rd place after three standard rounds, 1st flyoff round is flown with max of 300 seconds
- If still any ties after 1st flyoff round, 2nd flyoff round is flown with unlimited time
- Maximum of two models may be entered for the three standard rounds
- One additional model may be entered for the flyoff rounds
- S4A model requirements: no jettisoned parts (everything that goes up must come back gliding); maximum mass = 60 grams
- Reliable model
- High performance: good boost, good transition, excellent circling glide
- Finding thermals/lift
- Recovery of models
The S4 event (free flight boost glider) has undergone a transition. At one time, the preferred S4A design was the classic pop pod glider. Europeans developed the very successful scissors/flop design (see the Venus Rocketry “Gizzard” model for an example). The UK developed the Jacomb design that uses a folding/pivoting wing such that the model looks like a rocket for boost but then deploys for gliding recovery. A fourth option was internally carried gliders (typically flying wings) that are boosted in 40mm (or larger) boosters and unfold at ejection. However, some flying wing gliders were made from foam and had such low wing loading that it was difficult to tell if the model was gliding or just tumbling along in the breeze.
Starting in 2013, the FAI rules for S4 have been changed to require that “everything that goes up must come back gliding.” This eliminates pop pod designs and internal carry gliders. Therefore, S4A is now similar to the NAR Rocket/Glider (R/G) event.
To be competitive in S4A at a WSMC, you will need a reliable, high performance model. If any of your three flights fails or has mediocre times, you will not be competitive. Individual medal winners typically max (180 second flight) at least two of their flights, and there is often one round of flyoff due to multiple fliers having three maxes. Winning a team medal normally requires an average score of at least 400 seconds for each of the three team members, which can only be achieved with zero DQ’s or flights less than 60 seconds and probably at least one max.
Boost reliability is very important. Pop pod designs, such as the popular Hummingbird design (see the files below), are no longer legal. Some people have tried variants of the Hummingbird with a sliding wing and fixed pod. This design may have potential but needs further development.
The scissors/flop design usually has good boost reliability. However, the design has somewhat high drag in the boost configuration. Boost altitude might be improved using other designs. The Jacomb design may have lower drag during boost. However, the Jacomb design is mechanically complex, which may impact reliability. American-style sliding wing R/G designs have good boost reliability. However, the weight of the sliding and deployment mechanism will affect boost altitude and glide performance.
An S4 model must have reliable boost-to-glide transition. This can be achieved by having 2-3 degree incidence between the wing and horizontal stabilizer. Sufficient dihedral (~30 degree inclusive angle) is good for stable glide. Dihedral also improves visibility for model recovery. Glider stability can be calculated using the XFLR5 program.
Glide performance is primarily dominated by wing loading and drag. Scissors/flop designs are somewhat challenged here since they have a lot of exposed components (rubber bands, hooks, wing stops, pivot, and pod). Efforts to “clean-up” a scissors/flop design by using torsional springs instead of hooks and rubber bands may provide benefits.
Finding lift is very important. It is difficult for an S4A model to get a maximum time in dead air. Methods for finding lift are documented on the Construction & Techniques page. At a WSMC, the U.S. team will generally have several Kestrel weather stations deployed to help identify wind/temperature patterns that indicate thermal formation. Watching the descent performance of models recently launched can also help understand lift situations.
Recovery of models is very important. Only two models may be used for the three standard rounds. Therefore, at least one of the models from the first two flights must be recovered in order to make the third flight. One additional model may be entered for the flyoff rounds. If that is the only model available, then that model must be recovered in order to fly in the 2nd flyoff round. Fortunately, at a WSMC, the U.S. team members who are not flying that day will be deployed for short, mid, and deep recovery. Exploits and achievements of the U.S. “deep recovery team” are legendary!
Lightweight construction methods are important. Lightweight balsa can sometimes be difficult to find at local hobby stores. Hand-selected lightweight balsa is available (at a premium price) from Sig. Graphite tubes (available from CST, ACP, and others) are light, stiff, and strong, and are often used for fuselages. Carbon tubes made by the DPP process (available from A2Z and CST) are somewhat lighter than standard carbon tubes. Some work has been done using vacuum bag methods to create foam/fiberglass wings for S4 models. It is challenging to use these methods (foam cutting, vacuum bagging) at the small sizes needed for S4 components.
Most U.S. competitors fly using European motors with 2N average thrust and 3-second delays. These motors will provide higher boost altitudes than US motors, and this may increase duration and the probability of finding lift. However, European motors have different thrust-vs-time characteristics than U.S. motors. Models that boost well using U.S. motors may not work properly when flown with European motors. Some pre-WSMC experience is recommended to make sure that the motor characteristics are understood.
Some competitors use piston launchers to increase launch altitude. Others fly without piston launchers in order to eliminate “one more thing that might go wrong.” For practice flight in the US, most flyers use A3-4T or A10-3T motors. The A3-4T motor has a more suitable thrust-time history. However, the four second delay may be too long for some models. The delay time of the A10-3T motor seems to be about right. However, the high initial thrust spike of the A10 may stress the model more than an A3 or a European motor.
Dethermalizers have been used to help with model recovery. The traditional approach has been to use dethermalizer fuse/wick. However, this method has disadvantages due to the weight and drag of the wick and snuffer tube. In case of launch misfires, the wick must be removed and the model re-prepped. There is also the danger of setting a fire if the fuse is not extinguished by the snuffer tube.
Some efforts have been made to use electronic dethermalizers. However, a small electronic system (such as the Pico timer) with battery would likely weigh 4-5 grams. This is a significant penalty for a small model.
This is an area that still needs more development.
|Bottom view of Russian swing-wing S4A||Other||May 28, 2014, 3:13 am||705 KB|
|GibletS4A Instructions||May 28, 2014, 3:13 am||6 MB|
|JACOMB SPECIAL S4A Instructions (UK, Oct 10)||May 28, 2014, 3:13 am||630 KB|
|Russian FAI Flop Pivot S4A||Other||May 28, 2014, 3:13 am||59 KB|
|Russian S4A - Pivot Flop 2||May 28, 2014, 3:13 am||42 KB|
|S4A - UZB Design||May 28, 2014, 3:13 am||68 KB|
|Scissors/flop design by Chris Flanigan||October 12, 2014, 6:17 pm||14 KB|