My SparrowHawk recommendations

[Mayfield]

In order to give this subject it's own thread I have moved it here and added to it:

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It is my hope that GBA will expedite fielding of no charge recalls as follows:

1. Redesign the roll pivot block plates on the rotor head by increasing them slightly in height. This would enable people to retrofit their rotor head and remove the rotor head torque tube stop bolts. These bolts are redundant; a left over from the modified RAF program. If these bolts are not there, they can not back out and cause a control jam. Retrofitting the entire SH fleet would require about 6 hours of drafting time, a couple of hours revising the CNC program to cut new plates, and about 13 sq/ft of 1/2" aluminum plate. In other words, a couple of thousand dollars.

2. Design and ship to all customers a new primary pitch control rod using 5/16"high quality rod end bearings.

Although neither of these items has been definitely identified as the cause of Terry Eiland's accident, expediting these recalls would ensure they can not cause a similar accident.

These two possible failure modes are particularly critical to an aircraft like the SparrowHawk. If the SH is loaded near the forward CG limit, a control discontinuity, or a jammed stick/rotor head can result in a situation where it may not be possible to recover the aircraft.

A power reduction at the loading condition described above will result in a nose down pitching moment. With a control discontinuity, the trim system will not have the power to bring the nose up once the aircraft has accelerated to ~80 MPH. With a jammed flight control, the nose will also pitch down with a power reduction and since the head is jammed;trim is useless.

R/S

Jim

Note: I am now aware of evidence that indicates the tail support tube bracket on Terry's machine was intact prior to impact. It pleases me to be able to take that possibility "off the table" in the specific case of Terry Eiland's accident.

[Vance]

I remember a thread about the stop bolts backing out on several RAFs. My recollection is that Paul and a few other RAF pilots had frightened themselves, but had been able to land the aircraft without incident.

I used to spend a lot of my preflight time in Buckeye on the rotor head. I could never figure out how to check the stop bolts, but there was a line scribed on them and I would check them for alignment. The rear stop was my concern. I believe it is there on an RAF because of the adjustable mast.

I went to the Freedom fly in with a helicopter enthusiast friend of mine and he determined that a helicopter had too many critical systems to be a viable home built project. That is part of the allure of the autogiro for me. I feel that there are only two critical systems beside the pilot, the rotor system and the control system.

The first time we did an engine out in a modified RAF I had a clear picture that the nose dropped. When I practiced engine outs with Steve in the SparrowHawk it was a little more pronounced because Steve weighs more than the Beast (Terry) and I am no lightweight, so we clearly had a forward-loaded CG. As a relatively inexperienced pilot, I may be over sensitive. I was taught to start my take off roll with the stick full back, but I am not sure I would notice less travel. I always checked the travel before beginning my pre-rotation

I admire the attention to detail in a SparrowHawk and I noticed it the first time I did a pre-flight. I spoke to Stan Foster and he told me that GBA is about to come out with the larger spherical rods ends. I feel that getting rid of the stop bolts would have value as well. If you lock-tite them you can’t check them with a torque wrench and I don’t see a good way to safety wire them. I feel that using a jam nut on a critical part without a secondary form of retention is not best practice.

Please do not hear that I am being critical of GBA, I love the SparrowHawk and think it is a great design. These are two areas that I feel would make the SparrowHawk more mistake proof.

Thank you, Vance

[ptope]

we drilled and safety wired ours.. ill take a picture today and post it..

[Mayfield]

I have stated, in this thread, that failures like those postulated above, can be particularly challenging to SparrowHawk pilots. The following scenario is provided as reinforcement:

As to the rotor stop bolt and jam nut:

If the rear stop bolt backs out just enough the following scenario is possible.

1. Bolt backs out just enough to be felt by the pilot in level flight when making the small corrections we always make when flying. It would feel very strange to hit a hard stop with the stick in level flight.

2. Detecting the abnormality, most pilots would reflexively reduce power. The SparrowHawk, with two on board, and with fuel, would pitch down with the power reduction.

3. If the rear rotor head stop precludes applying back stick to slow down, the aircraft would pitch further nose down and accelerate. Because of the sloping windscreen, among other possible things, The greater the airspeed and the greater the negative pitch attitude of the aircraft, the more nose down force is generated. With an intact and unjammed control system, This characteristic is benign and transparent to the pilot.

4. I am not certain what the result would be to reapply power, but I suspect the nose would not pitch back up once the aircraft accelerated to 80 mph or so.

Feel free to share the above scenario with anyone if you see benefit in doing so.

I am available to all SH owners and pilots at any time to look at or discuss anything pertaining to SH characteristics.

Jim

Note: This corner of the flight envelope makes it even more necessary that we maintain control continuity and prevent control jams; thus my recommendations above.

[Vance]

Hello Jim, I am grateful for your continued efforts to save my life.

I think this is a wonderful example of how to apply your “response to abnormalities” protocol.

Thinking about the situation prioritizes design and maintenance.

The intuitive action is counterproductive, so a different action must be planed in advance.

Jim, what would the best response to this abnormality be?

Thank you, Vance

[Mayfield]

Vance,

I'll do my best to answer concisely. it is important to understand that my answers are very specific.

1. If the abnormality is a control jam I am not aware of a recovery technique that would work.

a. Reduction in power will result in nose drop.

b. The trim system works by pulling on the rear of the rotor head. If the head is physically blocked, trim would be ineffective.

2. If there is a control discontinuity, with aircraft loading at or near the forward CG limit, the current trim system will probably not have the power to raise the nose if airspeed has exceeded 80 MPH or so. I am not certain what effect adding power would have at this CG condition.

3. If there is a control discontinuity, with aircraft loading at, near, or behind the center of the CG envelope, a pilot may be able to manipulate trim and power to set up a mush to the ground.

R/S

Jim

[Udi]

Very good discussion Jim. I hope GBA takes the necessary steps to make the SH even safer.

I am intrigued (and a little concerned) by something you said. You said you are not sure whether a fully-loaded SH (i.e. nose heavy), with the stick jammed at the cruise trim speed, would level out of an idle power descent - if power was brought back on. I have been wondering about that since Terry's accident. I was sure stable gyros were power-controllable in pitch with a locked stick. Here are my thoughts.

1. I feel it would be humanly difficult to add power when your stick is jammed and you are accelerating towards the ground. There is one exception. If this maneuver has been simulated in a similarly loaded healthy aircraft, was shown to be the correct course of action, and was taught and practiced as part of emergency procedures, then it would be a no brainer.

2. If your suspicion is correct, i.e. the SH would not level out of a dive just by adding power - and this can be intentionally tested, then I believe the design should be modified to correct this problem. Pitch control with power is a redundancy that most airplanes posses (what was that Delta DC-10? with no hydraulics). I don't think this redundancy should be compromised for the sake of... what? Power-on pitch stability? PPO bulletproofing?

Greg Gremminger has been arguing for a while that some moderate HTL is actually good because it allows you to use a negatively pitched stab, which, in turn, makes the gyro more airspeed stable. I have never really bought this argument because I felt the same airspeed stability characteristics may be achieved with a CLT or LTL design but now I am starting to wonder if there was a corner of the flight envelope I wasn't considering. I think Greg has a good point. I can't think of a way around it right now.

Udi

[Rotornut]

Jim Mayfield said / I am available to all SH owners and pilots at any time to look at or discuss anything pertaining to SH characteristics.

Jim you are awesome! Thanks for going above and beyond for all SH Owners.
I like the fact that you Step up to the plate. MJ :)

[Vance]

Thank you Jim.

Now I understand the reason for the focus on the rotor head torque tube stop bolts. It is not likely for the bolts to back out, but the consequences are such that eliminating the possibility is worthwhile.

Thank you for helping me understand this, I don’t believe it is something I would have discovered and understood on my own. It gives real incite into the thought process that has kept you alive.

I agree with Udi, it is a very thought provoking discussion.

Thank you, Vance

[Mayfield]

Udi,

Thanks. As usual, you are exactly on point. I am pleased you are providing input.

Thanks MJ. It is my desire to help where I can.

Appreciate it Vance.

I would hope that some more serious testing is accomplished vis-a-vis this area. As I said, I'm not certain about how far you can let the nose drop and still be able to bring it back with power when the controls are jammed. With the increasing cabin download with increasing airspeed and increasing negative angle, coupled with the drag below CG, I just do not know how fast the aircraft can be going before it becomes unrecoverable.

If power application will recover the aircraft at any reasonable airspeed, I do not know how much altitude would be lost before the aircraft could be leveled off.

I believe strongly in trying to achieve longitudinal stability. I'm not sure how much I would compromise to ensure the capability of raising the nose in case of a control jam or control discontinuity.

I believe my current mindset would lean me toward making the control system a little more bullet proof; at least as an interim measure.

However, I agree with you that the redundancy you speak of should be a design goal for making the SH even better.

Jim

[ptope]

here is what we did with regards to safety wireing them.. we drilled a small hole through the square part of the bolt just below the bolt head..i would definalty like to do away with these all together if we could, i can already see where its denting the pivot block where these stop at.. it would be nice to have some sort of bumper to absorb the forward and aft limits and do away with the bolts.. JIM I personally really appreciate the machine you designed, it flys great and i am really impressed by your continued desire to make it better even though youve moved on. Thanks alot, you are a role model to me, a relative newbee to the sport.. Thanks Again, i look foward to meeting you someday and trying to absorb some of your vast knowledge on aviation.. Patrick Tope

[Mayfield]

Patrick,

Thank you for sharing your remedial tactic. It appears that this critical bolt can not "unwind" very much at all.

I, as you do, hope that longer roll pivot block plates can be provided to SH owners soon. As I'm sure you have already surmised, the reason the stop bolts gouge the surface of the roll pivot block plates is a matter of surface area. There is a fair amount of force applied over a pretty small area when the blades are spinning slowly.

When we can get the longer plates, chamfered correctly, the torque tube itself will ride on the plates. This configuration will spread the load a little and reduce pounding damage.

Jim

[mceagle]

Very interesting and constructive Jim.

I have not had a good look at the S/H setup, but would it be possible to drill the holes right through and install a bolt and castle nut?

The way I see it the slider head would work on loosening the existing studs whenever the controls are on one of the stops and the rotors are spinning. This situation would be most prevalent on machines used for training, where they spend more time taxiing with the rotor head on one or the other of its stops.

A simple test to determine the possibility of recovery tecniques would be to try in a "good" S/H with a removable rear control lock (e.g. a removalbe bar between the seat and the joystick. Technically it should be able to be landed with considerable power, albeit with a fast ground roll. Of course you would not have to land it to do the test.

[StanFoster]

Jim: Thanks for what you have been posting. I hope GBA takes it how I know you mean it....and that is of a concerned person seeing some improvements that need to be made on this fine gyrocopter.


I am flying quite a lot....and do so with a hightened sense of awareness to watch more closely everything I have been....and to constantly be thinking of other areas that need improvement.

My teeter stop bolt was throroughly red locktited...and the jam nut as well...along with tiny marks that I check each flight. I did not improperly tighten that jam nut like some have by tighten it while the bolt is turning. I came from a farm and this turns that bolt into an auger....instead of augering corn...its augering aluminum.......not good.:eek:




Stan

[Chopper Reid]

I'm not a technical person but I cant for the life of me understand why SH would carry on from the AAI with a head set up that could in anyway cause an accident ???
The SH should have been well sorted out but that does not look to be the case!!
I bet there are a few nervous SH drivers out there !! [and I dont blame you ]

[Udi]

None of the safety features of the SH have to be compromised. All I am saying is that CLT is not the Holy Grail and HTL is not evil. We want to consider all possible configurations and decide which of them provide an overall safer design. I agree that jammed controls may be a rare occurrence, but apparently they not rare enough. I would like to get a little deeper into the aerodynamics in question.

When a fixed-stick gyro is accelerating with reduction in power it can mean one of three things - the prop thrust line is lower than the CG, there is a negatively pitched stab in the prop wash, or the stab becomes less effective countering other aerodynamic nose-down pitching moments such as sloping windshields, etc., due to the lower airspeed over it (or a combination of any of the three).

When a stabbed gyro is airspeed unstable with the power off it can mean one thing - the stab is not powerful enough to counteract destabilizing factors such as sloped windshields and landing gears. Somehow, I doubt that the SH stab is not large enough but there may be a problem with the airflow getting around the pod at idle which may affect the effectiveness of the stab at idle power. Maybe the stab, being in the wake of the pod, is not doing its job.

The fact that the gyro was nose heavy has little impact on these dynamics. The gyro was flying trimmed and level with cruise power. When the power was taken out, the gyro would maintain airspeed IF the prop was CLT and IF the stab was as effective as it was before the power was taken out.

I am less concerned about the fact that the SH has a higher trim speed at reduced power than if it is indeed airspeed unstable under these conditions. The scenario of airspeed getting out of control is very worrisome to me and should not be accepted.

If the problem is with the stab being in the wake of the pod than possible solutions may be to install vortex generators around the pod to improve flow into the stab, or to move the stab lower into cleaner air.

I am still questioning this scenario as the cause for Terry's accident because even a modest amount of power would have made the stab much more effective, which would have slowed the aircraft down and shallowed the descent. This would have given the pilot the cue he needed to add a little more power, which would have shallowed the descent more, and so on.

I think these issues can be investigated by GBA and corrected, if necessary. After some more thought I still think the overall airspeed stability of a gyro can be improved by the “Greg” method. The consequence of doing so is only a marginal reduction in efficiency (i.e. added drag), without any compromise in safety.

Udi

[Rehan K.Janjua]

Thank You Mr Jim Mayfield.

I dont have a SH. But we all benifit from your experience.

This is extremely thoughtful of you and will definately have positive results and the most important is to save a human life.

This forum certainly contributes to the well being of our fellow gyro pilots.

Appricate your concerns with the highest regards.

[Mayfield]

Hi Udi,

We are in concurrence. As is often the case, there are some things I know to be true, and some things I suspect to be true.

In the first category: Because the engine thrustline is somewhat below CG (a couple of inches) at some loadings, the nose will drop and the aircraft will accelerate when power is reduced/cut. The new trim speed is higher than the entry speed.

During testing, while I was still at Groen, I conducted many throttle chops. Entry speeds from about 60 MPH (IAS) to well over 100 MPH (IAS) were used. At forward CG, The nose would drop and the aircraft would settle at a new, higher, trim speed. I did not conduct any testing to determine if the aircraft would return to level flight, by reapplying power, with the controls jammed or with no direct pitch control.

In the event of a control discontinuity or a control jam, there are several questions that are of interest to me.

1. Will a reapplication of power bring the nose back up beyond some critical speed?

2. How much altitude is consumed before the aircraft again achieves a level pitch attitude?

3. With a control discontinuity, will trim force be adequate to bring the nose up?

4. At what maximum speed will trim force be enough?

Udi, when I was made aware of this horrible event I immediately started to run scenarios. I quickly came up with three.

1. The down elevator scenario. This would require failure of the tail support tube bracket. I now know that the tail support tube bracket was intact at impact.

2. A control discontinuity.

3. A control jam.

I believe we can not yet take one and two above off the table. The SparrowHawk can be made better aerodynamically. I suspect that will be a long process and probably not applicable to the existing fleet.

I am therefore advocating making the likelihood of a control jam or control discontinuity even more unlikely.

Thanks to all for their comments and input. We’ve got a lot of brain power on this forum. I listen and learn every day.

R/S

Jim

[Vance]

Hello Brian, I understand your frustration.

I have been in design meetings and it is remarkable that anything ever happens. If there is a lawyer in the room, nothing does happen. A life is priceless and there is tremendous pressure on everyone at that meeting to imagine every possibility and to give the most thought to those things that could injure someone. At some point you have to move forward. Often you say, “it is as safe or safer than anything else out there” so lets move forward.

I don’t believe that there are many aircraft out there that will have a successful landing with the flight controls jammed. With skillful pilots and a lot of luck this has been accomplished. This does not make it a primary design goal. Making the controls less likely to jam is the primary design goal because we don’t want to depend on luck and pilot skill to prevent accidents. Aircraft must be light so that they fly. This makes everything a compromise between lightweight and strength through mass.

I know Jim well enough to know that he has flown a SparrowHawk into the ground many times to in his mind to try to understand what happened to his friends. He gave Terry his CFI and Commercial check ride. He had a great deal of input into the design of the SparrowHawk. He feels that the rotor stop could have been done better. It is a copy of the RAF system. I don’t know of any accidents attributed to the rotor stop bolt in an RAF. Copying a proven design holds less engineering risk than inventing something new.

If it is designed and built by man it is not perfect. Everything can be made better. We don’t know that this stop bolt had anything to do with the accident. I know for a fact that Terry used locktite on the bolt. I did a preflight his ship more than once. I had complete trust in Terry and I was supposed fly with Terry in that aircraft that morning, so I was much closer than I would like. I loved Terry and I will never know why I won’t enjoy his presence again.

I believe that the SparrowHawk is a wonderful design and remarkably safe. I don’t believe it is perfect. I believe it will get better because people like Jim care.

Thank you, Vance

[Doug Riley]

I would agree with Udi that, if the craft locks into a dive with the controls disconnected, there's a basic problem with static pitch stability in that particular corner of the envelope. If that's the case, and if the present cabin shape is to be retained,* something different must be done with the horizontal tail surfaces.

If the HS's span is long enough to extend outside the propwash, then the HS can be given a strong dose of washout. The non-immersed portion, with the most negative incidence, would respond primarily to free-stream airspeed (not throttle setting), and would add a nose-up moment that increased as the square of this speed.

If necessary, the HS could be either lowered or raised to get more of it OUT of the propwash. From what we know, partial immersion seems to be the gold standard.

Alternatively, a second HS could be placed entirely outside the propwash and given substantial negative incidence. This is not a sanitary-looking option, but I believe it would work.

* People like the RAF/S-Hawk cabin shape, with its upright seating and relatively easy entry-exit. My own weird preference would be something closer to sailplane-style seating -- essentially, sitting on the floor. Such a setup helps you reduce frontal area and get rid of the nose-down "wedge" effect of large sloping windscreens. By the time someone can afford a fancy gyro, however, he or she has usually arrived at a "certain age" and may not have the gymnastic ability to get in or out of a half-reclined seat. We aren't all as agile as Chuck Beaty, our leading exponent of reclined seating.

[Rando]

* People like the RAF/S-Hawk cabin shape, with its upright seating and relatively easy entry-exit. My own weird preference would be something closer to sailplane-style seating -- essentially, sitting on the floor. Such a setup helps you reduce frontal area and get rid of the nose-down "wedge" effect of large sloping windscreens. By the time someone can afford a fancy gyro, however, he or she has usually arrived at a "certain age" and may not have the gymnastic ability to get in or out of a half-reclined seat. We aren't all as agile as Chuck Beaty, our leading exponent of reclined seating.Like Hank Hinchman's H-1 Racer!

[C. Beaty]

I first became interested in reclined seating as a way of improving performance where power is limited. Power to frontal area ratio is just as important as power to weight ratio if traveling more than 30 mph.

Maybe Grandma prefers bolt-upright seating in her Cadillac as well as the guy driving a dump truck but is it really essential in a short haul aircraft?

I introduced myself to some high performance sailplane owners who were gracious enough to let me try out their seating. Didn’t seem bad at all, lying nearly flat on one’s back.

I also learned the VariEze has a seatback angle of 50º which doesn’t seem to upset the people who drive them. The Quickie has a seatback angle of 39º.

I built a wooden seating position mockup from 2x4s and doorskins and decided a 45º seatback angle suited me just fine.

It serves two purposes; cleaner airflow into the prop with an attendant increase of efficiency and a reduction of frontal area.

Cabins like RAFs and SHs, chopped off abruptly at the seatbacks, are horribly draggy and the airflow into the prop is even worse.

[david holmes]

chuck,
and all this time I thought you sat like that because of your arthritis and backy juice LOL

[Vance]

Hello Doug and Udi,

I can count on you both to think outside the box. An aircraft that will fly itself and land itself is a great safety feature.

I am trying to understand your proposal. That is, that a gyroplane should be able to land with a disconnected or jammed control system. Do I understand this correctly?

When I flew in a two-place Dominator it seemed that the nose would drop with a reduction in power, perhaps even more than a SparrowHawk. Dave was not comfortable letting me land after I told him I was blind, so I have no direct comparison. Doug, I know you have a two place Dominator, could you land it from the front seat with the rotor control disconnected or jammed?

When we would practice engine out landings in the SparrowHawk it seemed, with my limited experience, to be particularly docile requiring minimum stick movements.

I don’t know if, in a dive near the ground, I would have the presence of mind to add power in order to pitch the nose up. I don’t know if I could adjust the trim quickly enough to fly anything I have flown with the trim alone. I would expect that the recognition of the control discontinuity would come at some difficult moment when I was commanding a change and near the ground.

Part of the allure of the gyroplane for me is that it has only two critical systems besides the pilot, the rotor and the rotor control. I can see where it would be even better to have only one critical system, the rotor, and really only use the pilot for aiming.

I particular liked Ron Heron’s third control rod, so you could lose any one of the three and still fly. It seemed like such an inexpensive fix for safety.

Compared to the helicopters I have flown, a gyroplane is wonderfully stable aircraft, but there is always room for improvement.

Thank you, Vance

[Doug Riley]

Vance, some of my comments derive from my Dominator experience. The S-Hawk is similar in a number of ways, including its centered HS on the rudder and the underslung prop thust line. The S-Hawk is different in that it has SxS seating and a full enclosure.

The Dom picks up 10-15 mph when you close the throttle, hands-off. I crank in most of the aft trim when entering a descent to landing. This moves the hands-off airspeed back down to cruise speed. The stick design is such that the stick assembly itself doesn't add any trim bias --compare this to a pump stick, which adds a nose-down bias simply by virtue of the weight of the lower stick assembly. Presumably, then, if the Dom's stick became disconnected, the trim would be sufficient to prevent an out-of-control dive, although the resulting "landing" wouldn't be the world's best.

The stick jamming is a different matter. The trim doesn't work in that case.

The Dom's centered HS is great during cruise and climb. A centered HS is even better if you happen to have a bit of a HTL, since the resulting HS force varies with throttle setting -- just what you want.

The centered HS is least good in power-off flight. It's blanketed by the engine and fuselage, and so becomes aerodynamically weak when the propwash goes away. This is a minor issue with an open-frame machine but it can be serious with a large pod, at least if the pod is shaped so that it creates a nose-down aerodynamic moment. I believe Jim Mayfield was speculating that the SH pod (perhaps combined with a forward CG) may have been able to overpower both the HS and the trim at high speeds.

[Mayfield]

I believe Jim Mayfield was speculating that the SH pod (perhaps combined with a forward CG) may have been able to overpower both the HS and the trim at high speeds.

The SH also gains speed when power is reduced. At forward CG conditions. the speed may stabilize as much as 15-20 MPH (IAS) faster than trim cruise speed.

Once it has stabilized at the higher trim speed the current trim system, many times, does not have the autority to lift the nose all the way back to a level pitch attitude.

If the controls are jammed, power application may, over time, be able to rotate the nose upward somewhat, but if so, it would burn a lot of altitude before effect could be seen. I did not test for the jammed stick scenario so, as I indicated, I am uncertain of the effect of reapplying power with a jammed stick.

At this point, I would be comfortable with strengthing control system components, and designing possible control blockage initiators out of the system.

Jim

Note: At this time we do not know what GBA will determine as "probable cause." In the interim, based on the information available to me, I believe the two improvements I suggest will increase the safety of the vehicle.

[Vance]

Thank you Doug, Some how I always feel like I have learned something when you post. You have such a nice way of putting jumbled thoughts in order.

I flew the SparrowHawk at close to 100 miles per hour and it felt very solid, almost boring. I was a little tense so I may not have been aware of just what I was doing with the stick. Most of our engine out practices were from around 70 miles per hour and I never let the airspeed climb. If anything I would unintentionally slow down. It seemed so simple compared to a Robinson 44.

I could tell a real difference in the way it flew with Steve compared to Jim or the Beast. It seemed less responsive with the heavier load. Engine out practice seemed similar, but the speed of the flair was noticeable faster. The only time I felt uncomfortable was when the winds would gust above 20 knots. I had trouble separating my mistakes from the effects of the wind. The SparrowHawk was always a joy to fly, but I do not know enough to have a useful opinion. I never had trouble maintaining a chosen speed and my only real challenge seemed to be the landing flair. That is what Terry was going to help me with at Bensen Days. I have always had more trouble with talking on the radio than flying. I have spent a lot of time inspecting the landing gear in the post flight inspection. I never found any damage.

Thank you, Vance

[StanFoster]

Vance: I love your last two sentences you just posted pertaining to post flight inspections of your landing gear. You make me smile.:)


Stan

[Doug Riley]

Jim, it seems to me that re-working the trim system so that it can pull the nose up to, and even past, cruise stance hands-off at idle power and high speed would be a wise move.

Down the road, the increase-airspeed-as-power-is-cut behavior should be eliminated -- in both the aircraft we're talking about. Greg Gremminger has been carping about this issue in LTL gyros for years. He may turn out to have been a prophet.

[Mayfield]

Jim, it seems to me that re-working the trim system so that it can pull the nose up to, and even past, cruise stance hands-off at idle power and high speed would be a wise move.

I concur. I see some minor challenges, but nothing insurmountable. It would be quite easy to increase trim power on a fixed wing; make the tab bigger or give it more movement. On a gyro, we need the springs soft enough to "pull" through, and stiff enough to transmit trim force. Difficult but attainable.

Down the road, the increase-airspeed-as-power-is-cut behavior should be eliminated -- in both the aircraft we're talking about. Greg Gremminger has been carping about this issue in LTL gyros for years. He may turn out to have been a prophet.

Yes, this should be a design goal that fits in nicely with a continuous improvement program philosophy. Reducing offset a couple of inches would help in this area. It might be a good time for GBA to look at Neil's gear box.

I appreciate your "down the road" comment. I believe that the SH has met it's original design directive, but we are in agreement that this aircraft, like many others, can be improved.

Even though the rotor head stop bolts and the primary pitch control rod ends have not been, and may never be, implicated in the Eiland/Finnegan accident, I do not believe we can deny that these areas should be improved.

Jim

[Doug Riley]

I neglected to mention the possibility of reducing the head offset a little so that such brutal springs aren't required. I believe Paul Bruty has done that with RAF-class gyros...

[Mayfield]

Yes. I reduced the offset to a hair under 5/8th of an inch before I left. David Eiland did the experimentation after switching from RAF blades.

I could not reduce it any more because the pitch pivot bore in the torque tube was getting close to the vertical bore for the spindle bolt.

It could be redesigned. I believe Jimmy Vanek produces a head that gets rid of this concern.

Jim

[Udi]

On the subject of trim range and trim forces - would it be out of the question to try an aerodynamic trim system, similar to the RAF stabilator? I haven't heard any negative feedback about this system, compared to the good ol' spring trim system. Ausie Paul was actually pretty enthusiastic after he flew it on one of his drop-keel modified RAFs. Maybe this should be looked at as an option, despite RAF's IP (http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=6,824,093.PN.&OS=PN/6,824,093&RS=PN/6,824,093)...?

Udi

[mceagle]

I neglected to mention the possibility of reducing the head offset a little so that such brutal springs aren't required. I believe Paul Bruty has done that with RAF-class gyros...
Yes. I reduced the offset to a hair under 5/8th of an inch before I left. David Eiland did the experimentation after switching from RAF blades.
I could not reduce it any more because the pitch pivot bore in the torque tube was getting close to the vertical bore for the spindle bolt....We must be mindful of the fact that considering "blow back angle", reducing the offset could move the rotor thrust vector ahead of the pitch pivot bolt and reverse the stability benifit of the offset gimbal head (unstable), especially with taller head towers.

[Aussie_Paul]

IMHO the subtle stability of an offset gimbal head is far outweighed by the 2" HTLM with an effiective H/stab scenario, if that is what is required with the torque tube offset for a particular brand of rotor blades.

The torque tube offset required for different brands of rotor blades was quite an eye opener to me!!!

With Firebird, the last decision to be made will be the torque tube offset, after all the other stability scenarios have been resolved.

Aussie Paul.:)

[Doug Riley]

Rotor blowback provides a degree of airspeed stability even without an offset. As speed increases, so does blowback, and that tends to increase rotor AOA. More rotor AOA tends to slow a rotorcraft down.

Trouble is, this effect tends to bring the gyro to zero airspeed if left to its own devices (say, on an old spindle head without bias springs). The offset gimbal biases the effect so that it brings you to a pre-selected airspeed instead of zero (well, actually a preslected amount of blowback).

[Aussie_Paul]

Thanks for the explination Doug.

Aussie Paul.:)

[mceagle]

Rotor blowback provides a degree of airspeed stability even without an offset
Doug, I believe that the "degree of airspeed stability" is predictable and it's linier gradient is easily compensated for by the pilot, and as such is not really a stability issue.

In contrast, the spring balanced positive offset supplied by the gimbal head reacts in a positive stability manner to unpredictable gusts and airpockets and tends to stabilize the aircraft. If the rotor thrust vector was to move ahead of the pitch pivot then the offset would be negative and would tend to destabilize the aircraft and cause control pressure V's load to be reversed.

Paul, I believe that compensating for the offset with a spring both reduces the weight on the stick and maintains positive stability. Compensating with less offset will reduce the weight on the stick but may not necessarily maintain positive stability, (especially if a different set of rotors is used). It would all depend on the height of the head towers and the degree of blow-back. This effect would be most noticable at higher forwards speeds (where rotor blow-back is greatest) and with taller towers.

[PW_Plack]

Jim Mayfield says...

It would be quite easy to increase trim power on a fixed wing; make the tab bigger or give it more movement. On a gyro, we need the springs soft enough to "pull" through, and stiff enough to transmit trim force...

Could the RAF "stabilator," which is a trim device, be useful in addressing this issue? RAF uses it to stabilize the rotor plane to the relative wind, but would it not provide a tool in the scenario of an LTL-aggravated power-off dive?

[Doug Riley]

How can a low thrust line aggravate a power-off dive?

[C. Beaty]

We must be mindful of the fact that considering "blow back angle", reducing the offset could move the rotor thrust vector ahead of the pitch pivot bolt and reverse the stability benifit of the offset gimbal head (unstable), especially with taller head towers.
Some rotors “blow” back and some “blow” forward, Tim. It all depends upon pitching moment characteristics.

Bensen wood blades were over reflexed and had a nose up pitching characteristic. So much so that with some blades, no trim spring was required at all for “hands off” flight at 45 mph.

Correctly designed rotor airfoils have a zero pitching moment coefficient and “blow back” with increasing airspeed until at some forward speed, no trim spring tension is required.

Early Cierva C-30s had rotor airfoils with quite a large nosedown pitching moment coefficient; so much so that several were lost in high speed dives. That defect was remedied by the installation of reflexed trailing edge tabs on the outer 1/3 radius of the blades.

Negative pitching coefficient blades tend to twist more nosedown on the advancing side of the rotor disc than on the retreating side; suppressing cyclic flapping. If sever enough, cyclic flapping can actually be reversed, causing the rotor to “blow” forward.

[Aussie_Paul]

Doug, I believe that the "degree of airspeed stability" is predictable and it's linier gradient is easily compensated for by the pilot, and as such is not really a stability issue.

In contrast, the spring balanced positive offset supplied by the gimbal head reacts in a positive stability manner to unpredictable gusts and airpockets and tends to stabilize the aircraft. If the rotor thrust vector was to move ahead of the pitch pivot then the offset would be negative and would tend to destabilize the aircraft and cause control pressure V's load to be reversed.

Paul, I believe that compensating for the offset with a spring both reduces the weight on the stick and maintains positive stability. Compensating with less offset will reduce the weight on the stick but may not necessarily maintain positive stability, (especially if a different set of rotors is used). It would all depend on the height of the head towers and the degree of blow-back. This effect would be most noticable at higher forwards speeds (where rotor blow-back is greatest) and with taller towers.

yes Tim agreed, I have found that one torque tube offset will be ok for some rotor brands and not for others. That is why the final torque tube offset for Firebird will be the last decision in the total equation.

Maybe I have to look at a ground adjustable torque tube offset if I want to offer a variety of rotor systems to the market place.

The one below just requires shifting spacers to change the torque tube offset.

I hope to fly up your way with a Firebird before the year is out.

Aussie Paul.:)

[C. Beaty]

Clever rotorhead but far too much metal "sculpting" for my taste.

That looks to be a somewhat different version of a slider.

[Brent_Brown]

OK I'll bit. Who makes that rotorhead? Looks a slider to me too.

[mceagle]

Negative pitching coefficient blades tend to twist more nosedown on the advancing side of the rotor disc than on the retreating side; suppressing cyclic flapping. If sever enough, cyclic flapping can actually be reversed, causing the rotor to “blow” forward.Thats interesting Chuck. So a negative pitching blade would require more back stick at high speed instead of more forward. Could tend towards a dangerous situation, especially with forward C of G loading.
Are many such airfoils used in gyros today?

Very interisting head Paul. A bit complicated to manufacture though. I'm sure that it could be manufactured a lot more simply and still retain it's slider and adjustable pitch pivot location features.

[Doug Riley]

Tim, I'm sure you know that a basic requirement for sane and stable aircraft is that stick pressures not reverse over the entire airspeed range. The craft must always require more forward pressure to go faster and more aft pressure to go slower (when trim isn't used). This requirement is built into the FAA's certification standards for both Light Sport and heaver aircraft. Since our LSA regs cribbed from yours, something similar is probably in the Oz regs as well.

A few years ago, somebody here very foolishly got an expensive extrusion die made up for rotor blades that had down-drooping trailing edges instead of reflex. This sort of T.E. turns up on sailplane wings and windmills, but it's no good on rotor blades. These blades would certainly have had reversal of stick pressure, and probably a total running-out of back stick, above a certain airspeed. Dangerous.

The sailplane case is interesting. The strong down-pitching moment would normally require lots of tail down-load, creating drag and defeating the purpose of the droopy airfoil. To avoid this, the aggressive sailplane designs instead use very aft CG to balance the pitching moment. This only works at one airspeed, though, and the aircraft is not actually pitch-stable. A trained pilot can handle it (beginning to sound familiar?), but a friend of mine was killed in one such such sailplane some years ago. The manual didn't explain this design quirk, and he just lost airspeed down low and augered in.

A tail-heavy airplane with a strong pitching moment is not unlike a HTL gyro with an under-powered H-stab.

[Aussie_Paul]

OK I'll bit. Who makes that rotorhead? Looks a slider to me too.

I don't know Brent. I just happened to find it on the net.

Aussie Paul.:)