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**NiqueNaque** · 11-12-2005, 08:44 AM

Barry,I protest!It's very heavy handed of you.. You've deleted all of Birdy's supplementary and explanatory posts on the topic which were interesting responses to Tim's discussions of the mechanics of the thing, not to mention what I and others posted about practice.This thread was in control until someone used the word 'crap' in an as yet unsubstantiated way, you could have had the decency to edit from that point on instead of chopping the topic summarily off at the end of Tim's post, by my reckoning you've wasted half of the topic and deleted some of the knowledge pool by doing this.Please be a little more moderate in your next moderation.Nick.

**Greg Mitchell** · 11-12-2005, 03:10 PM

Nick climb down off your high horse and get into the spirit of the thread. I reckon Barry will be looking forward to catching up with you Nick. For a drink I mean. Ha. Anyone who wants another stab at it can repost. So again, this by Doug Rielly Ex US forum............................................. .................................................. .I'm afraid that some of what has been said here (and elsewhere by fairly respectable gyro people) will lead to needless confusion. Here are two things that are FALSE:1. PIO causes PPO. If a gyro PIO's, it's always in danger of PPO.2. There is something inherent in all gyroplanes that makes them subject to a mysterious malady called "buntover." IOW, gyros would rather be upside down, if we would only let them.Pilot-induced oscillation -- PIO -- happens when pilot and machine, taken together, lack damping. They bob up and down at their natural frequency. All types of aircraft (and, in fact all kinds of machinery operated by humans) can and do PIO. PIO does not necessarily lead to a PPO or any other kind of crash; the system may settle down after awhile, especially if it has at least a LITTLE damping and the operator has the sense to hold the controls still. PIO occurs when the pilot-machine system lacks dynamic stability and has a lag in its response. (Even a non-laggy machine like car steering can PIO if the driver is drunk. It's the familiar "weave," caused by lag in the driver, not in the car. You have to look at the total system, machine plus operator.)OTOH, PPO is an example of STATIC instability. A force on the airframe pushes or pulls the whole craft over. No oscillation is involved in PPO, and no oscillation necessarily precedes it.NO tendency to push over is present in any properly designed gyro rotor. There is NO force generated by the rotor itself that is continually trying to flip the rotor over. Even if the rotor is dipped low enough in the front that the air hits the "disk" on its upper side, the rotor will not "dig in" and continue flipping like a coin. Instead, the rotor will simply generate lift in a downward direction. This may actually push the gyro's tail down and restore positive angle of attack.To borrow an old campaign phrase, "it's the airframe, stupid." PPO happens if, and ONLY if, the (1) prop thrust line is above the CG AND (2) there is no other force that counters the nose-down torque created by the high thrust line.*Pilot skill can often completely compensate for poor dynamic stability -- remember, PIO is a lack of damping in the man-machine SYSTEM. If the pilot has plenty of "training, training, training," than the pilot can sometimes do the damping job with no help from the machine. It's a lot of work, but you can learn to do it automatically with enough practice. All pilots of tailless gyros -- old Bensens included -- develop this skill, if they survive the learning process.The seeming link between PIO and PPO is this: Teeter-rotor, cyclic-controlled gyros, PPC's, trikes and weight-shift hang gliders have as their primary control a device that points the lifting surface in various directions to carry out the pilot's wishes. This setup makes the aircraft simpler. It does, however, deprive the pilot of control during moments of zero "G." HTL machines would all be un-flyable unless something countered the nose-down effect of the prop thrust. Since HTL machines do fly, something does counter the prop thrust. If the "something" is rotor thrust, however, watch out. Rotor thrust is interrupted by zero G events. The "PIO connection" is simply that PIO will produce a zero G event if it persists through a few cycles. The PIO lets the PPO genie out of the bottle. Without a built-in tendency to PPO, PIO will not lead to PPO.OTOH, if a gyro experiences a PPO after PIO-ing, then that gyro was susceptible to PPO all along. This means the gyro became statically unstable at low G -- a frame-layout defect that could have been corrected in the design.*(There is one other kind of pitchover that I know of besides PPO -- the drag-over. If the frame layout is such that the pod, wheels, radiator or what-not tend to pull the nose down, then they can act in a way that mimics HTL, even if there is no HTL. This is most likely to happen at high airspeeds, especially if the nose tends to get lower at such speeds. Note, however, that the airframe, not the rotor, is again the culprit. The vague term "bunt" suggests that there are any number of other varieties of pitchover. I know of no others besides these two.) .................................................. ...........................................www.thebutterflyllc.com

**Greg Mitchell** · 12-12-2005, 03:16 PM

I never met Ken Rehler but I met blokes in Texas that new Ken well. I was and will continue to be deeply affected by his passing. There is much to be read on this subject on the US forum.Greg Grimminger I believe posted this report and whilst some discussion continues as to detail within this report, it seems the basic premise is generally accepted.With respect to Ken, Here it is.Gyroplane Crash ReportNov. 11, 2005 at New Braunfels Airport, TexasGeneral-Gyroplane flight is similar to fixed-wing aircraft in many ways but different in others. Here are a few differences:1) Gyroplanes are much less affected by wind gusts than fixed-wing aircraft. The rotor lift cannot change rapidly since it depends on rotor rpm not airspeed. In addition, the rotor is free to tilt relative to the fuselage. The result is that things usually happen more gradually than with fixed-wing aircraft.2) Gyroplanes cannot stall since lift is created by the rotor rpm, not airspeed. However, the induced drag becomes very high at slower airspeeds. As with any aircraft, when the drag exceeds the thrust, the gyroplane will lose altitude and/or airspeed.3) The fully articulated rotor head, which Ken's gyroplane utilized, provides a great deal of control in pitch and roll at any airspeed, allowing the pilot to easily respond to any upset. Problems are more commonly due to over-reaction by inexperienced or poorly trained pilots rather than inadequate control.Report-After my offer of assistance was accepted by Harry Kifer of the FAA, I inspected the wreckage and could find no evidence of pre-crash mechanical or structural failure.I talked to two eyewitnesses who provided consistent descriptions of this tragic accident (see fig. 1). One (#1) was Ryan Winter, a helicopter instructor, who was on the ground in a helicopter with a student and had a clear view of the entire event. What he saw was the gyroplane descending over runway 13 into a gusty 10 to 20 mph headwind to about 30 feet where it leveled off and began slowing down. Suddenly it yawed rapidly to the left and pitched up until it was headed backwards. The nose continued up and over until the gyroplane was nearly completely inverted at impact. This took only a few seconds (see reenactment video).Probable cause-This accident was probably caused by a combination of circumstances. At low airspeed and low power setting, the yaw stability and rudder response would have been very sluggish. A gust from the right caused airflow to the left rudder to become blocked by the fuselage (see fig. 2). The side force on the nose pod overcame the ability of the right rudder to maintain yaw stability, causing the fuselage to turn rapidly to the left.As the fuselage turned, any or all of the following effects may have caused it to rapidly pitch-up:1) As the rotor disk pivoted around the yaw axis, it would have begun pulling the mast backwards into the wind causing the nose to rise.2) As the fuselage rotated past 90deg, the reverse airflow over the horizontal stabilizer would have had a destabilizing effect on pitch.3) At the slow airspeed, the rotor disk would have been at a high angle-of-attack. The rotor disk may have been unable to keep up with the rapidly turning fuselage, causing it to hit the teeter stops and transferring a pitch-up moment to the fuselage due to precession.4) The horizontal stabilizer was set with a 2 deg leading-edge down angle to provide pitch stability with increased airspeed by creating a pitch-up moment. Any power that may have been applied in response to the sudden left yaw would have added to the pitch-up moment.Conclusion-The primary cause of this accident was probably yaw instability when airflow to the left rudder was blocked by the fuselage. This twin rudder design requires that both rudders receive clean airflow to maintain yaw stability. In a slip, when yaw stability is needed most, it is likely that airflow to one rudder may become blocked by the fuselage, making it ineffective. In this design, the other rudder had insufficient area to maintain yaw stability, causing a rapid rotation around the yaw axis. In addition, the low power setting limited rudder response by the pilot.Most gyroplanes avoid this potential problem by having a single central rudder. This provides reliable yaw stability by placing it where the airflow to it is the cleanest when in a slip. It also places the rudder directly in the prop blast, making it very responsive whenever thrust is present, regardless of airspeed.Larry Lindgren.......................................... ........................Anybody here in OZ have prior experience or knowledge of a similar design craft with said yaw instability?www.thebutterflyllc.com

**NiqueNaque** · 12-12-2005, 03:55 PM

Hm..Thanks for posting that Greg.Ken's passing is a tragedy, I remember his posts to this forum and he was a helpful guy.After reading this I now know for sure why Wal Flaks advises to do a power on approach to land and stuff it onto the deck in gusty conditions, it helps avoid Ken's unfortunate fate.

**Greg Mitchell** · 13-12-2005, 05:09 AM

No Problem Nick.I forwarded this to Allan Wardill, ASRA Opps Mgr and I am hoping Allan will discuss with other OZ accident investigators and comment for us on the content.I guess what bothers me most is that Ken's gyro won awards in the US and there would have been many experienced folk there that had seen this gyro. To come out after the event and indicate possible inadequacies in the design which present a major yaw instability in that particular part of the flight envelope, seems to me like shutting the gate after the horse has bolted.Also, the similarities to the twin tails of the 18A, Hawk and Carter Copter ect gets me to thinking.I hope Tim Mc and others will comment here.I agree with Wal on the power on approach given such conditions BUT Ken's rudders being out board of the prop wash might not have helped in this instance AND what if you had an engine out in said conditions with the same configuration?Now I'm recalling Ozzie Woz has twin tails and would be keen to hear from you Woz about your gyro and whether or not you have experienced any yaw stability issues? I realise you dont have a front enclosure, just curious.Cheers,Mitchwww.thebutterflyllc.com

**Greg Mitchell** · 13-12-2005, 05:32 AM

Here are the two schematics that go to the above Larry Lindgren post.Image Insert: 17.55

**asra** · 13-12-2005, 06:50 AM

Well done Greg, were back on the topic again.The fist time I saw Kens machine, my first impressions were that his twin tails were to small.He had plenty of room above, behind, and below to increase its surface area.I bet he didnt due to the extra rear weight of his machine, typical of this type of configuration. The rear facing head plates, are a give away. The pic you posted further proves it to me.If you were to use the old cutout method and find the centre of pressure, then I bet it is in front of the hang point.For a correctly setup machine it should be located hehind the CofG, or hang point, which is usually located behind the CofG.This ensures the tail will react, and yaw, even without power,instead of pod and pilot. Regards Sam.

**Greg Mitchell** · 13-12-2005, 12:04 PM

Yeah, good explanation Ozy. I also went back and took a look at some of the Cooma video. Nice Flyin! Appreciate your comments.Sam, agreed. I thought the vertical fins could have been larger. Here is a link to the US forum and a post from Doug Rielly about the "old cut out method". The thread develops over three pages and is well worth reading.http://www.rotaryforum.com/forum/showthread.php?t=6706.................................................. .................. Ken Rehler accident

Since the witness reports suggest a yaw-stability problem, I decided to try a crude "paper doll" yaw test. We are fortunate in that Ken was an architect and CAD expert. Scale views of his gyro are posted at his Web site.Using his posted side view, I made a cardboard cutout of Ken's gyro. Following Paul Abbott's technique, I tried balancing it along an approximately vertical axis, using first one rudder and then both. Attached is a scan of the actual cutout (the second rudder is above the first and is detachable). I've drawn in the balance axes for the one-rudder and two-rudder configurations. Ken's posted drawing indicates the CG of his gyro, right behind the pilot's upper arm.For the aircraft to be statically stable around a given axis, the aerodynamic center of pressure (COP) must be aft of that axis, when the axis is centered on the gyro's center of mass (= CG). Within limits, the farther back the COP, the more stable the craft is. A COP right on the CG axis will result in a craft that's equally happy going forward or sideways. COP ahead of the CG axis means the craft will try to flip around backwards unless the rudder pedals are constantly tended.This test is very simplistic; it neglects the real differences in aerodynamic effect among various shapes. It assumes that the gyro is a flat 2-dimensional thing like a weathervane. In the case of an open gyro, however, the irregularities of all the shapes that make up the gyro will tend to some extent to cancel each other out. With one rudder (area about 3.97 sq. ft.), Ken's gyro is almost perfectly yaw-neutral -- the balance line runs just aft of the CG. If I'd been able to carve out the empty spaces under the engine, it would probably be right on the CG. With both rudders working, the COP is about as far aft as it is on other stable gyros. The area of the two rudders together is nearly 8 sq. ft., compared to a Bensen rudder at 5-6 sq. ft. So what? Could a rudder be rendered ineffective?Yes, under some circumstances, though we're guessing here. In a slip, one rudder will be in the wake of the gyro's pod. The energizing effect of the prop may not make up for this, with the engine at idle -- thus that downwind rudder may be in disorganized air and may not produce much of a yawing reaction.In an extreme slip, the upwind rudder may also create a disorganized wake in which the downwind rudder has to work -- same problem.One experienced rotorcraft thinker suggested to me privately that the wake of the rotor tips, angling past the outboard rudders, might also screw up the flow to at least one of them. Meanwhile, if the stick is far back (the witnesses report a mid-air flare) and deflected left (witnesses report a turn or slip to the left), the rotor's thrust line may be ahead of the CG and pulling the nose left. This yaw effect FROM THE ROTOR is rather unusual and might catch any of us off guard. I'm not entirely satisfied with all of this. Neutral stability is not enough by itself at low throttle to produce a "whip yaw," just sloppiness in yaw. The other yaw-precession-stall accidents have involved a deliberate maneuver with throttle added. Ken's gyro yawed left, if I recall. That's the direction that is LESS likely to produce a precession stall. The rotor turns left and is therefore "running away" from the spindle all the time. This means that the rate of cyclic input from the "stirring" of the spindle is slower than in a right yaw. OTOH, a partial rotor bearing seizure or something jamming in the rotor WOULD cause a left yaw. Perhaps the investigation will say something about this.The second maneuver described by the witnesses -- a sort of half loop IN THE AIR -- can't easily be accounted for. The energy to lift the machine like that must come from either the rotor, airspeed or the engine. If the engine was idling, it's out of the picture. There's little airspeed after a flare, even in a 15-mph wind. If rotor energy is starting to transfer into the frame via uncontrolled flapping, you'd think the machine would already have hit the ground thanks to lost RPM/lift. Maybe the machine actually did hit the ground sooner than the witnesses thought. They might have seen the kind of violent somersaults that can accompany a hard blade strike. (Example: The video of Ken Wallis's crash is spectacular in this regard. Cmdr. Wallis was very, very lucky not to end up like Ken Rehler.)Image Insert: 13.79

**NiqueNaque** · 13-12-2005, 01:10 PM

A few extra things to consider:1. These rudders are full flying rudders, which aren't quite as effective as a fin/rudder combination. 2. Ken's drawings show that his tailplane is an elevator as well, with an articulated section - what was happening with this during the event?3. The spacing of Ken's rudders and elevator seems to be in the wake of the prop, but there is a slight contraction in the airflow after the prop (vena contracta) which is about 90% of the prop diameter nominally, could it be that the accelerated airflow off the prop would have counter acted the required accelerated airflow over the outside of the rudder required to correct for the yaw? What I mean here is that the gyro is yawing left, so you give it right rudder, for this to work the airflow on the outside of the rudder must be faster than the inside of the rudder, but because of the effect of the accelerated airflow from the prop being faster on the inside of the rudder it actually serves to accelerate the yaw instead of correcting it, spinning the gyro left even harder, hence the crash. This is a speculation but I have a suspicion that it has a lot to do with Ken's demise. [This contrasts with Ozy's machine where both the fins move into the prop's accelerated air flow when the rudder is turned.]It is a little difficult to understand what I've said but if you think it through you might see what I mean. Either way, I'm not happy this happened.Birdman, please post this on the Yank forum - I don't have a registration so if you could oblige I thank you in advance.No cheers on this occassion,Nick.

**asra** · 13-12-2005, 01:41 PM

Mitch, looking at those drawings of Ken Rehler's machine I have difficulty seeing his twin tails being outside the prop wash, they are inside the prop circle on the drawing, as is also the Hz stab. Perhaps Magni have a good reason for the triple tail on a good length arm.I haven't seen any reference to tail volume, has anyone calculated it?I see there was a suggestion that with the side gust he experienced the all flying tail's AOA may have been increased to the point where one or both may have been stalled. I'm told an all flying airfoil stalls more easily than the usual fin/rudder which acts as a cambered airfoil.This sad event certainly gives those of us flying with pods something to think about.John Think logically and do things well. Think laterally and do things better.John EvansThink logically and do things well, think laterally and do things better.

**asra** · 13-12-2005, 01:52 PM

Nick, I wasn't going to get into "vena contracta" as I have some doubts about this at very low airspeeds, in this situation there appears to be a fairly large component of spanwise flow along the prop airfoil, so in that case the blast diameter can be larger than the prop diameter.John.John EvansThink logically and do things well, think laterally and do things better.

**NiqueNaque** · 13-12-2005, 03:33 PM

John,I'm not with you.Please explain.Nick.

**NiqueNaque** · 13-12-2005, 03:49 PM

Further,If a Vena Contractor was the culprit, if you look at the nose on view you'll find a significant portion of the bottom of the rudders fall on the margin of a 90% arc of the prop diameter. The Hz stab seems to be balanced about it with about half in and half out of the 90% arc [How this effects it I have not worked out yet].If the prop's incoming airflow on the leeward side of the pod is stalled, then that side of the prop has less air flow going through it, and the right side more, so the VC could be mishaped and the relative speed to the inside of the right rudder much greater than thought [allow also for prop vortices], where as the left one could be in relatively still air and totally ineffectual, but what happens when it keeps spinning to the left - surely the left side prop gets a bite sooner or later, but maybe too late if the gyro is spinning on its axis like a jet in a flat spin.I'll think about this some more, it may help.Regards,Nick.

**asra** · 14-12-2005, 12:51 AM

Nick,my reservations about "vena contracta" are of extrermely recent origin, they are based on observations made during videoing the behaviour of the SS hub bar.At very low airspeeds the behaviour of the grass and flying gobs of cow manure demonstrated a much bigger diameter blast from the prop than we expected. Thus "vena contracta" may only apply at higher airspeeds.Please regard these remarks as somewhat tentative. One of my numerous incomplete project was to place a grid with tufts all over it behind the prop at various distances.Tricky stuff air, it doesn't always do what you anticipate! JohnThink logically and do things well. Think laterlly and do things better.

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