I've been away from computers for a few days.
Fara, you don't know me. I don't know you. I do, however, know Raghu.
As a newcomer to this activity, you may not be aware of the tests we conducted at Bensen Days a number of years ago on the airspeed actually experienced by gyro horizontal stabilizers. I designed the experiment, and got several of the guys to lend us gyros and help with testing and recording. I contributed an article detailing the setup, and our results, to the PRA magazine (back when there was one).
The highest airspeed over the H-stab is located at about the 2/3 prop radius point. This airspeed, in a full-throttle static test, varies from a low of about 80 mph for a slow-flying Gyrobee, to over 100 mph for a faster gyro such as an RAF or powerful MAC-powered Bensen.
The propwash tapers down from the full prop diameter, right at the prop, to about this 2/3 diameter by the time it reaches the H-stab; the exact amount of taper is a function of the ratio of slipstream to freestream speed (the taper is less at higher gyro airspeeds and greater at low ones). Especially on gyros with bulky components mounted on the engine block near the prop, there is a significant central "dead air" cone inside the propwash.
Certainly not after that test, but even before it, I would not, did not, and do not demand that a H-stab be centered on the prop hub. If it is to counteract HTL, however, it shouldn't be a full prop radius away from that center, either, because of the tapering effect I just mentioned. At a full prop diameter, the slipstream speed over the HS is little more than the freestream speed. Look up my article for numbers on the three gyro models we tested.
Despite the "dead air" zone, there's an advantage to placing the H-stab near the center of the slipstream, however. A more-or-less centered airfoil can be used to counteract the reaction torque created by the prop, which can roll a gyro in a zero G situation. Cierva developed this technique. Watch the (rare, but they exist) films of gyro PPO's, and you'll notice the craft executes a combined pitchover and rollover. Prop torque is over 100 ft.-lb. at full throttle in even in a small gyro with a redrive. Rolling to inverted is no healthier than pitching to inverted.
No, I do not posit that a heavy, high-powered gyro will need a continuous 600 lb. of thrust to stay airborne. PPO's, however, very frequently occur on climbout or coming out of a climbing pattern turn. In those flight regimes, the gyro will indeed be operating at full throttle at modest airspeed and the HS down-load needed to counteract the PPO moment will be as I described it. It's a significant phantom weight to carry unless you have a 15-foot tail boom.
I don't know the thrustline offset of a Magni; there are none in my part of the country. Based on gyros that I have measured, however, and on test figures given me by people I trust, unless the rotor is extraordinarily heavy or there something else heavy at the top of the mast, a gyro's prop thrustline needs to be at or below the crew's navel(s) for the gyro to be non-HTL. 1-2 inches of HTL is fairly easy to compensate for, however, with a properly designed H-stab.
I devised the "double hang test" in the early 90's to check the thustline location of my lowrider 503 DC Air Command. The test is based on a grade-school science-fair experiment. My gyro's prop thrustline came out about 5-6" above the CoM. This explained a lot of things that had mystified (and worried) me about this gyro's behavior. Adding the stock A.C. H-stab (located down at the 2/3 prop radius point where that good fast air is found) went a long way toward taming this gyro. Again, though, it does not neutralize the roll torque problem, and it may not be adequate to compensate fully for the PPO moment of a 532 engine and and added body pod.
A relatively slow, heavy and overbalanced rotor has a higher level of rotor damping than a light, fast one such as a Bensen rotor. This effect helps to provides a margin of safety against PPO in otherwise vulnerable gyros. I would not choose to depend on it, however.
Look at the summaryI won't make any Archaeology in the forum, I am waiting for the same kind of paper then Jean fourcade's one, I am sure he can do this in order to reassure every body showing that a gyroplane in Zero G at full power with a magni/mto/ela rudder can't flip up-side down on it's rolling axis
My guess is they would pass. Calculations I did years ago suggested that the J-2 and 18A are both CLT, plus they have generous horizontal stabilizers well back, plus articulated rotors that are relatively insensitive to low-g compared to the Bensen derivative style teetering systems. I recall no reports of any PIO or PPO in them. In flight, they are as pitch stable as any Cessna.all certification standards even including Part 23 have provisions for exceptions but they do not apply to something as fundamental as longitudinal stability. Sec T longitudinal stability requirements are more stringent than Part 27 or CAR 4. The type certificates Gyroplanes in the US from the 60’s may not pass those requirements. Though that’s an educated guess only on my part.
I was thinking of the long term stability requirement but you are likely right. I didn't think of the articulated rotor systemMy guess is they would pass. Calculations I did years ago suggested that the J-2 and 18A are both CLT, plus they have generous horizontal stabilizers well back, plus articulated rotors that are relatively insensitive to low-g compared to the Bensen derivative style teetering systems. I recall no reports of any PIO or PPO in them. In flight, they are as pitch stable as any Cessna.
This is the most revealing PPO video that I've seen.
The craft has a H-stab but its volume is insufficient.
The pilot porpoises a little during his fast low pass.
The airspeed at the top of the zoom climb appears not to be great.
At the top of the zoom, the pilot pushes the stick forward, but soon yanks it back. The rotor follows the aft stick command but this has no effect on the developing pitchover.
Doug, the very problem is the word should (not lead to a PPO),This is the most revealing PPO video that I've seen.
Raghu's observation that a downdraft should not lead to a PPO in a gyro with an AOA-stable airframe is correct, with certain assumptions: (1) the H-stab is of sufficient volume (area x moment arm) and receives airflow unobstructed by other components (2) the rotor spindle stays at the same angle to the frame throughout the downdraft encounter (i.e the pilot does not "float" the stick, nor does he push it forward because he has gotten behind the aircraft), and (3) RRPM loss during the event is not great.
You are going to be the long term test pilot for the aircraft you designed jm-urbani and the maker did not do a stability study.Dear Vance,
when a gyroplane enters in a ppo situation , it can't recover this means : death of the pilot
when a gyro get's out of it's flying envelope it can't go back in the envelope like it can be the case for a fixed wing aircraft ( for example : stall limits)
a test pilot can't go to the limit without risking it's life because there are no parachutes on the gyros
hence it is impossible to define the flying envelope of a gyro otherwise then by making a stability study and taking big margins
the fact that most of stock htl gyros have prooved to be stable because there have been zero or very few people that suffered this kind of accident simply means that if the maker does not make a stability study the clients have been the long terms test pilots ... i accept to beta test a soft but not a gyro
but I am sure that the serious makers have conducted serious stability studies they should issue for each of their machines to put an end to the discussion
jean if you mean what will prevent low to zero G if someone flies a parabola to generate low G. Nothing. But that maneuver is a known maneuver to create low G. It should never be done in a Gyroplane of any thrust line. Gyroplanes are not aerobatic Aircraft just as trikes are not. In both if you do a deliberate parabola, and a tumble or torque roll or more likely a combo of both starts it’s not going to be good thing. That case will likely also be fatal for a LTL Gyroplane. In fact in Florida a few years ago a Dominator had a fatal because the pilot was new and got into PIO on takeoff and flew up, down, up, and then down and done. Do I blame Dominator? No. The pilot was not ready. He unloaded the rotor. This idea that somehow the tail configuration and thrust line will save you when your rotor is gone is a figment of imagination that has never been proven. I believe the effort is to make it easier to avoid getting to that situation.When the forward speed is almost zero at the top of the trajectory, how large a not too little plate?
I was remembering the exact same thing... the RC tests. I was also wondering if this could somehow be a cooperative effort with the University Student projects the PRA has organized. I wonder if GyroPedia / Phil Harwood would be interested in being involved in such a study.A few months back, someone suggested flying some gyroplanes using remote control.
That would allow the finding of 'the edge' w/o undue risk.
Accuracy would seem to be high as it was all empirical data, not 'calculated'.
But...…. you'd have to crash at least one of each type of aircraft unless you were confident in applying the results to others.