HS = Horisontal Stabiliser also known as a tailplane in old fixed wing terms the plane at the rear that the elevator attaches to.
Early gyros often did not have one, more recent thinking is they should all have one
Bensen used to fit a stone guard under the prop in an attempt to prevent stones chipping holes out of the prop, not to be confused with an HS. All the modern factory machines have very large HS fitted as part of the design.
A HS reduces the chances of a pitch over, it stabllises the machine.
Early Crickets, bensens and the early RAF2000 did not have them fitted as standard. It is thought that not having one makes them more vulnerable to PIO with catastrophic results. However during the forum debates some who trained without a HS are happy to continue to fly without one as they have been trained to do so.
Note that a number of non UK machines have their HS higher up right square in the prop slip streem, some I believe have different angle of incidence to compensate for engine torque, (my thought is that it's a really neat solution that would also help prevent torqueover.) Some forign RAF owners fit a small moving stab near the top of the mast to increase stability, but i do not think it's approved in the UK.
I believe that the modern factory machines have a large Stab on a long moment arm to act as compensation for not being CLT (centerline thrust). Following an investigation by glasgow universtity all podded single seat machines had to conform to CLT, which in some cases involved a rebuild, in others inverting the gearbox and dropping the keel made the CLT.
I am sure someone will respond to my comments, but I have simply introduced you to the concept. There has been much discussion on the forum regarding this subject. I just hope I have not lit a fire !
(PIO= Pilot Induced Oscilation, which can lead to catastrophic unrecoverable results in a gyro plane unless immediate action is taken... immetiate being critical)
hope this helps
Stability is a design requirement, how that is reached is down to the designer and the approval process.
The design would have to pass the very stringent tests. Because of the stringent testing needed this is no longer something an amater is likely to take on. In the UK variance and evolution is limited to the large companies that can afford the testing. Small modifications can be carried out but even these need approval from the LAA and they will ask for finite details and a test flying plan.
just for example can I fit a motorbike engine to my gyro ? My understanding is this ... and I may be wrong ...... How will the gyro cope with the torque, what prop will you use with what grearbox, how will weight and balance and CLT change ?
what happens in a crash ?, where will the engine go after impact ?... now prove it by crashing one on a rig. will the engine mount brake, will it split the mast, will the fuel supply cut out, will a fire start ? and so much more. cooling and carb ice will it work on a pusher prop, noise and reliability.
The easiest way to gain a mod is to show that someone else has done it. Even something relitavly simple like changing rotor blades requires a mod application to the LAA. That mod requires flight testing.... by a qualified person, and there are not many of those around. It really is an up hill struggle. Best to buy a machine, rebuild it to spec, get a permit on it, get a licence, convert to your machine, then consider any modifications. It is a long torturous route. I am just suggesting ways to get you building and flying without getting weighed down with paperwork. I once heard someone say that "you can only fly your gyro when the paper work weighs more than the gyro."
To clarify, Sandl: Bensen NEVER characterized his HS as a rock guard. Others did that. He insisted that it was, in fact, a horizontal stabilizer. When he introduced his metal "gull wing" H-stab, he touted its improved aerodynamic performance -- as a HS. Not a word about rocks.
It was neither a very good HS nor an effective rock guard. I took mine off to save weight and couldn't tell the difference.
Bensen expressed a theory (to a rather famous gyronaut acquaintance of mine) that a gyro should have equal projected area ahead of, and behind, its CG when viewed from below. Perhaps this was to make it descend flat in a vertical descent.
A gyro with an adequate H-stab will try to nose down and regain airspeed in a vertical descent. It will require considerable stick back pressure to stay in the vertical descent. This "trying to regain speed" behavior is one aspect of a stable aircraft -- and so it is a good thing.
Bensen's "theory" amounts to a prescription for an aircraft with only neutral stability. That's perhaps OK for an aerobatic pilot, but is less than ideal for a weekend pilot, like most of us.
Igor Bensen, while a clever designer, would occasionally "improve" upon the truth when it suited his interest to do so.