S9—Gyrocopter Duration

Event Highlights

• Flown in three standard rounds and, if needed, a flyoff round
• Flown in the 2.5 N-sec (A) power class for both Seniors and Juniors
• Maximum flight duration 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
• Maximum of two models may be entered for the three standard rounds
• One additional model may be entered for the flyoff round
• S9A model requirements: minimum length = 500 mm, minimum diameter = 40mm for at least 50% of length, maximum mass = 60 grams

Major Challenges

• Reliable model (good high boosts, good ejection that doesn't damage the model)
• Rapid deployment of blades at apogee with immediate spin-up
• Light construction and high blade rotation velocity
• Finding thermals/lift

Discussion

This video by experienced US Team member (and 2104 Bronze medalist in S9) Trip Barber discusses designs for S9:  https://youtu.be/GaGxLxJmNho

To be competitive for a medal at a WSMC (Junior or Senior), you will need to get an average duration of at least 160 seconds on each of your three flights in the standard rounds, and probably multiple 180-second maxes.  Unless there is huge thermal activity, it is rare to have more than 4 or 5 fliers get three 180-second maxes and have a large first or any second flyoff round.  In order to get a team medal (three person team), it normally takes an average score of at least 400 seconds per team member and no DQ's or poor flights (less than 2 minutes).

Gyrocopter duration (inaccurately called helicopter duration in the US) is an FAI event where there is significant room for technical innovation and no "standard" design approach among all the top competitors.  The goal is to get a high boost (which takes a light model, therefore small, or at least light blades) then to deploy an unfolding set of blades that immediately spin up to a high rotation velocity and produce significant lift, while remaining upright with the body hanging beneath the rotating rotors.  Current successful designs include both larger, folding blades (such as George Gassaway's design in the files section), and single-piece 12-13 inch blades such as Keith Vinyard's design.  Almost every successful flier today uses only three blades.

Recovery of models is very important.  Only two models may be used for the three standard rounds.  These are flown consecutively, with each being 90 minutes long. Therefore, at least one of the models from the first two flights must be recovered within less than two hours 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.

Vehicle Construction

• Lightweight fiberglass or Kapton body (see Construction page) - can be more flimsy than in S3 or S6 because the folded blades provide longitudinal structural stength during boost.
• Nose cones are typically vacuum-formed plastic (available from Apogee Components).
• The fins are made out of either lightweight 1/20 or 1/16 balsa wood, or 1/32 balsa that is reinforced with tissue, fiberglass, or a vacuum-bagged layer of expoxy.
• Airframe mass, without motor but with blades, should be less than 15 grams; top-end models may be as little as 12 grams.
• Rotor blades are made of 1/32 inch thickness balsa.
  
  

Rotor System Construction

The art and science of this event is in the design and construction of the blades and hub.  See the presentation and the R&D reports in the files section for more on the theory of how gyrocopter recovery works.  In general the blades should be thin and light, and should have a pitch angle that varies along the length of the blade from most at the hub to least (near zero) at the tip.  This "twist" can be achieved by cutting and gluing (with an angle added that decreases as you go outward from the hub) a diagonal across the blade from the hub (widest section) to the mid-chord point.  Or it can be achieved by "curling" a blade that has wider chord at the root than at the tip.  This curling is done by moistening the balsa blade with ammonia and then binding it with a long wound-around strip of cloth to a PVC tube (either 1 1/4 or 1 1/2) with the long axis of the blade along the long axis of the tube.  Once the ammonia dries the stiffness of the softened blade is restored and it holds the new "curled" shape which provide a camber equivalent to the camber of the diagonal cut-and-glue method. The blades should be installed on the hub such that chord line of the tips is perpendicular to the rotation axis.

Blades are attached to the hub with small plastic model airplane hinges (DuBro or Klett) using a variety of fairly intricate techniques that are best described using photos rather than words -- see the example photos in the files section below.  The hub is attached to a piece of solid carbon rod that is 0.040" diameter and slightly longer than the blades.  If the hub does not free-wheel around the carbon rod on a rotary joint (which it should), then at the bottom of the carbon rod a barrel swivel is attached.  Either way it is important to isolate the rotation of the blades and hub from the rotation of the body hanging underneath it during recovery.

It is important that the blades immediately come up to full rotation speed after deployment.  The section of the blades near the hub that has the sharpest "down" pitch angle generally drives this initial spin-up from zero rotation.  Long (folding) blades generally take much longer to reach full rotation speed than single-piece shorter blades, due to their higher rotational moment of inertia.  It is also important that the blade assembly maintain orientation during recovery and neither oscillate while rotating nor fall beside the body.  Put about 15 degrees of "dihedral" in the blades when attaching them to the hub and setting the limiters on the blade deployment mechanism, and keep the blade system light so that it does not want to fall faster than the lighter body during the initial deployment phase.  And use good strong elastic (generally multiple orthodontic rubber bands) to pull the blades open after they have been ejected from the body.  Be careful not to put the blades under so much tension when folded that they deform the cylindrical body of the rocket, as this will lead to non-vertical, coning boosts that reduce altitude.

Almost all competitive designs today use only three blades.  Adding more is only advantageous if rotation speed is not high, but if this speed is not high you're going to lose anyway, and the extra blades on a high-speed system just add unnecesary weight.

Tim Van Milligan, a US Team member who also runs Apogee Components, published an excellent article on S9 model blade/hub construction in his company's "Peak of Flight" newsletter in October 2014.  Click here to go to his company website and read the article.  His "Rotary Revolution" kit is a full-up FAI design.  He followed up in his September 2018 newsletter with a good piece on stiffening 1/32 balsa baldes for S9.  Doug Hillson did a set of excellent detailed drawings of a U.S. S9A model that won a Bronze in 2014 (posted below), and is typical of state-of-the-art model designs for this event.

Propulsion

S9A is not an event where any U.S. motor is competitive. Maximum possible altitude and minimum possible post-burnout mass are critical to performance.  US A motors have far less total impulse than the full 2.5 N-sec that European motors have and their burnout casing mass is significantly heavier, so if you use them in a WSMC you will lose.   All U.S. competitors fly in the WSMC using 10.2mm diameter European motors, generally with 2N average thrust and 4 seconds delay. These provide higher boost altitudes and lighter recovery masses, which increase altiude and duration. Some pre-WSMC experience test-flying with these motors is recommended to make sure that the motor and ejection characteristics are understood.  Depending on how tight the fit of your blades and nose are in the cylindrical body, the as-built ejection charge of European A motors may need to be augmented slightly with extra black powder in order to pop out the fairly large mass of the blades and hub.  All competitors in S9 use piston launchers to increase launch altitude.

Competition Strategy

Rounds are fixed in length and must be shared by all three fliers on a team, as only one can be in the air at a time. Don't procrastinate too long waiting for perfect lift, but don't get caught up in "go fever" and launch too early when conditions are poor.

Finding significant lift is critical; you have little chance to get a max flight if you don't find lift. Methods for finding lift are documented on the Techniques page. At a WSMC, the U.S. team will generally have several Kestrel weather stations plus some thermal streamers on tall poles deployed to help identify wind/temperature patterns that indicate thermal formation. Another strategy is to watch how other models in the air react and to launch accordingly. Frequently, when there is thermal activity, there will be a multiple models in the air at the same time. Timing of the launch here is everything since down air or sink always surrounds a rising up current, and the decision window to piggyback off a competitor's thermal is very short. Experience (or advice from experienced teammates) plays a key role in choosing when or when not to fly.

Each round is fixed in length, with the next round starting exactly when the last one ends. This requires that the modeler be adequately prepared to deal with any situation, be it weather, motor malfunctions or catos, or a misfire.