Wind Shear/Gyro/HS/PartII

Doug Riley

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The old thread wasn't taking any more replies, so here's a new one.

Udi, there are reasons other than immersing the HS in the prop blast to use a tall tail. Absence of yaw and (potentially deadly) roll reaction to engine torque are two of them. I don't especially like the looks of tall tails or high-rider gyros in general, and having the HS move with the rudder seems a bit silly, but you can't fight physics.

In the case of centered thrust, the need for an immersed HS diminishes greatly. IF the HS is used to compensate for a high thrustline, I think some degree of HS immersion is necessary. Without it, you have the potential for a low-speed PPO when powering out of a vertical descent. Perhaps not a likely scenario, but clearly possible.
 
Re:Wind Shear/Gyro/HS/PartII

Mike Gaspard - Greg gave you the long answer. The short answer is that we were discussing a specific accident in which a low-thrust-line gyro was lost, apparently, due to a bunt over. We will never know the whole truth, but we think that when the engine stopped, the aircraft pitched nose down faster than the pilot could react, and bunt over (or entered an uncontrolled PIO). This is where more pitch damping would have helped.

Doug - I agree. Maybe the best setup incorporates both a tall tail and a low, and far back, h stabs. A good alternative is using a counter-rotating prop system, and a normal low rudder/stab setup.

Udi
 
I would like to add my support for a couple of points here:

The need for HS immersion is probably mostly related to how much high prop thrustline must be countered. With the HS immersed, it's nose-up moment can be tuned to counter the nose-down moment of the high prop thrustline. This would develop a degree of "Power Stability" by maintaining essentially the same RTV relationship to the CG - with all power levels. "Power Stability" is tested by simply seeing how much the TRIMMED airspeed changes with a change in engine power. The proposed ASTM gyroplane standard sets that airspeed requirement at no more than a 10% change with full power or idle power from the median MPRS (Minimum Power Required Speed) power level - reduce or raise power from MPRS to full or idle power changes the trimmed airspeed by no more than 10%. For a turned off engine, this requirement allows 20% trimmed airspeed changes from MPRS power. This is so the pilot would not have to provide so much compensating stick forces when power is changed, so that G-Load stability (CG/RTV relationship) does not change badly with power changes, and so that there is less tendency to bunt or initiate pilot over-reaction upon sudden power changes!

A lot of HS immersion might be necessary to balance a high prop offset, but high immerwsion can lead to other issues. The DYNAMIC stabilizing characteristics of the HS will change according to the acceleration of the air over the HS - engine power. This means that the dynamic damping rate may significantly diminish - change handling and control characteristics, when power is reduced and the HS is not as DYNAMICALLY effective! This can play a part when such a configuration (highly immersed HS) suffers a sudden power loss. Not only may the nose attitude be changing rapidly, but the dynamic characteristics might suddenly be such that the pilot is not familiar or proficient with - over-control and possible PIO initiation of. I noticed this in my High Command (and in another un-named model highly immersed configuration) when descending in rough air at high speed and low power. To the seat of my pants, this became very uncomfortable in the turbulent air. But, upon restoring power back to cruise, it was again very comfortable flying in the turbulent air! Part of this effect in the un-named model was from the low prop thrustline G-Load stability enhancement with power. But, on the High Command, which was more CLT, the affect was probalby from less dynamic stabilizing effect of the less effective HS with power low.

The "T" tail with the thrustline centered HS does very effectively minimize the engine torque issue. I demonstrated this on my High Command by testing the two HS configurations on it. The torque and nose yaw displacement, and need for pilot rudder inputs was greatly diminished with the "T" Tall Tail (a standard Dominator tail). But, I was less able to adjust for better Power Stability with adjustments of the HS mounting incidence. The enhancement of the HS lift with power prevented me from tuning all of the Power instability out of it by simple HS incidence adjustments. I maintain that all gyro configurations are different, and that only final flight testing can determine if you have the right combinations of things. These combinations include, for the HS, how big, how far back, what airfoil shape, angle of incidence mounting, and the immersion factor. Affecting these parameters are the prop thrustline, the center of pressure relative to the CG, and other airframe aerodynamic moments produced by windscreens, enclosures, etc. There are no single answers for any particular configuration. I also feel that even a keel mounted HS can have some amount of immersion effect from the propwash. The Magni has a keel mounted HS with a slightly high prop thrustline. And the trimmed airspeed changes little from low power to full power. There is noticeable torque effect, but those issues are delt with by a laterally offset roll pivot, a very long and forgiving wheelbase, and a very long tail moment arm, and a bit of rudder trim - no noticeable need for excessive rudder action!

Although two different HSs might be a solution or a "fix" to a particular gyro configuration, it should be possible to have the same effect with a single HS, properly placed, sized and angled! Remember, the absolutely centered HS ("T" Tail configuration) may not be the highest immersion possible. The airflow acceleration around the inner portion of the prop is not so great as the outer 1/3 area. The highest immersion HS might be at about the 1/3 distance from the outer diameter of the prop. The placement of the HS in the propwash may affect the cantilever loads on the HS, as well as the turbulence that the HS sees. I have seen arguments that say the swirl from the prowash actually can add to the AOA of the HS surfaces and the drag. The center of the propwash may necessarily amplify some of the forward fuselage disturbances to the airflow - further turbulating the air that the HS sees. So, cleaner airflow on the HS may be best achieved closer to the keel.

I am trying not to promote any particular configuration, there are always tradeoffs in any design and different designers and pilots desire different characteristics. I do feel that the basic stability criteria should and can be achieved in most configurations - but the proof of what works for that configuration is in actual flight testing. But, discussing configuration impacts is fun and can be instructive.

- Greg Gremminger
 
Greg - I'm a non-pilot (so far), but I am a Mechanical Engineer, so I've had training in fluid flow systems. What you say makes a lot of sense, both from an Engineering point of view and from comon sense approach.

It would seem that an ultimate solution to determining the best design(s) - or alternatively, determining the efficiency and stability of a given design - would be to put them through wind tunnel testing. Your commets about the unknowns about the actual air flow from the prop back past the HS would seem to indicate there is still a lot to be learned about gyro design. (I realize the air flow is further complicated by the actual physical configuration of the gyro). Maybe a program to test existing and proposed designs in wind tunnel situations might go a long way toward resolving some of the unknowns of the gyro design problems. Of course this would be a very expensive and time consuming process, but I'd be surprised if there weren't monies and expertise available out there somewhere for this effort.

Thanks for your expert input - I've read most of your articles (and actually understood part of them :-)) I've only been following gyroplanes for a couple of months, but was fascinated right from the first. I'm definitely hooked on the idea of flying in the open at generally slow speeds so you can actually enjoy the flying experience.

Dave Bohler
 
Greg...

Good point about wind tunnel testing...

But consider this.......given your typical gyro's size and speed....any determined tinkerer already has (nearly) his or her own wind tunnel...

A car/truck....

"All" you have to do is get the gyro higher than the top of the car and truck and hit the back roads....

Now, I'd (almost) never do such a thing with the rotor system attached....but doing such a thing sure would allow for ALOT more scientific way of optimizing the non-rotor part of a gyro rather than tinkering with something, flying a bit in uncontrolled conditions, and then trying to quantify whether its better or worse...

You could measure drag, centers of pressure, centers of lift etc etc.....you could use video cameras and streams of smoke (or little attached strings everywhere) see where you got good flow or turbulent flow or where things needed improving....things that are darn near impossible to do with a flying gyro...

Where you could probably get the most useful results would be in the area of engine cooling....if you did such poor mans wind tunnel testing with the engine and propeller attached and running.....you could run through all kinds of "fixes" to get the engine cooled efficiently and adequately in a fraction of the time it would take with flight testing and probably in a much more methodical way as well....

If if ever get to designing/building/optimizing a gyro I pretty darn sure thats the way I am going to approach the process...

take care

Blll
 
Dave and Bill,

A wind tunnel would be nice, but very expensive, and would have to be conducted for every small configuration change. Remember the airflow on the HS can be highly affected by the disturbing components in front of it and the prop. At different loadings, where the nose perhaps flies lower, the airflow aft of the engine, seat and enclosure can be different. The "poor man's wind tunnel" trailer is also a good idea, and some investigative work might be done such as evaluating the aerodynamics (lift and drag) of an enclosure, and the airflow disturbance characteristics aft of the enclosure.

But, I don't think that is all that necessary! Following some simple guidelines can get you pretty close to desireable results. Leave some flexibility to experiment with different HS configurations. From that point, flight testing can fine tune the design. Some adjustments in fairings and windscreens, etc. might be desireable.

I'm going to go out on a limb here, but IMHO, those simple guidelines include:

1) Keep the prop thrustline at or slightly above the CG in all conditions of loading.
2) If possible, keep the Moment of Inertia (MOI) of the airframe maximized for the weight of the gyroplane (stretched fore and aft).
3) Make tail surfaces as effective as possible consistent with your configuration desires - large surface areas, airfoil shape if possible, as far back as possible. For prototype adjustments and tuning, allow adjustable incidence angle and vertical placement of the HS. The HS will have to balance the prop thrustline offset and any aerodynamic nose-down moments on the airframe. IMHO, you can't make these stabilizing surfaces too large! For instance, on gyros with very large tail volumes, the tail just simply holds the static alignment of the airframe in-line with the relative airflow. That allows the CG to be positioned in flight well forward of the RTV and assures it will statically remain there - don't need to worry much about tuning the HS when it is very large and basically over-powers the other minimized static moments on the airframe - the attitude of the airframe is statically maintained relative to airstream at all times!! For a large HS, start with its chord line level with the airframe (keel) - with a slightly high prop thrustline, this will ensure a slight download on the HS and the positioning of the CG forward of the RTV! If the HS is large and the prop offset is not more than about 3 inches high, it should not need to be in the propwash much! For short wheelbase gyros, a tall tail would take out most of the engine torque effects and reduce the rudder action required on landing.
4) Within design goals, minimize all aerodynamic nose-down moments on the airframe. Down-lift of windscreens, drag of low landing gear, etc.
5) Use an offset gimble WITH SPRINGs to impart some static pitch stability into the rotor. (The offset gimble is not effective without a spring!)
6) If final flight testing verifies all three static stabilities are strongly positive, consider adding a little stick friction to allow the stick to follow the airframe and minimize any pilot required actions to handle a disturbance. (Don't add any friction if the static stabilities are not positive, this would exacerbate the negative stability and force the pilot to apply more skilled reactions.)

Can anyone suggest more guidelines?

I think these simple guidelines would result in a relatively stable gyro. But, flight testing of at least the three static stability characteristics would be needed to assure or tune up the desired results. If the HS is large enough to provide strong static stability characteristics, it is probably well large enough to provide good dynamic damping - don't try to test dynamic stability - this takes professionals and can be dangerous if your ship has some hidden dynamic instability such as dyanamic resonance between the rotor and the airframe! The three static stability tests for one loading and about 3 airspeeds can be accomplished well withing 1/2 hour - it's no big deal! To do it for all the possible loading combinations, will take a few more flights!

- Greg Gremminger
 
Greg - Thanks for the response - Your guidelines make a lot of sense - Could these (and any others knowledgeble gyro pilots could add) be verified, codified and possibly published by a reputable source (PRA, EAA, ?) as guidelines for desinger/builders to use. Maybe also some "rules of thumb" - how large surface area for the HS, a recommended airfoil geometry, etc.?

The rules wouldn't have to be prescriptive, but instead performance related recommendations, to keep from trying to tell designer/builders how to do their work.

Another question - What do you think about the HS as built into the vertical stabilizer (moving with the vertical tail) as opposed to a fixed HS at the keel level? Do you think it makes any difference (except for the height of the HS in the propwash).

Thanks - Dave Bohler
 
Dave,

Mounting the HS on the vertical rudder:

Aerodynamically, I don't think there is much of an issue of mouning and moving the HS with the rudder. But, structurally, if the HS is required to provide much balancing lift forces, it is easier to carry the loads through solid mounting than through the rudder pivots or bearings.

Also, the HS will weigh something, and the more effective HSs are longer. this means its inertia, or more correctly it's MOI, presents some additional lag and over-shoot with rudder inputs. For instance, if there is any slack in the rudder cables (usually is), the MOI of the HS adds delay to the rudder movement with rudder pedals, and causes it to over-shoot the desired position. This could present even a rudder/yaw PIO situation in the extreme, but probably only reduces the accuracy of rudder inputs.

If possible, I prefer solid mounting of the HS to the keel or vertical stabilizer - not to the moving rudder. Attaching the HS high up on a half-height all-flying rudder seems to me to be a bit precarious. There is lot of turbulence in the propwash, and most of it not very symetrical on the HS. This possibly presents a lot of dynamic fatigue stresses on the rudder bearing and attach points.
 
Greg - Thanks - That verifies my own thinking about the higher mounted HS (mounted on the vertical fin). Although I have never personally flown a gyro (just taken a ride), just looking at some of the tail designs, from an engineering viewpoint, some of them seem a little fragile - (really hard to tell from pictures without actually seeing them first hand, I realize) - tails like the butterfy, where the HS seems be supported by a thin sheet metal (?) angular strut fastened to the vertical fin. I would think the strut would have to be a strong support to take the maximum force the HS might see. I don't see any indication that serious engineering has been applied to make these components strong enough, but I suppose (I hope) the designers must have worked everything out to have a multiple safety factors in everything. (It would be good to see the design criteria and calculations the designers used for a given design before actually buying a machine (probably dreaming - I'm sure that info is propriatary). :-)

Am I thinking straight here, or am I way off base?

Thanks - Dave
 
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