Control push-rod length

[Rowdyflyer1903!]

I would like to discuss general philosophy on the fabrication of control rods. Would you level the yoke or center the stick, level the idle arms and level the rotor head before attempting to measure and fabricate the control rods? Level flight on the rotor head would be tilted how many degrees back? How has this been done on other scratch built machines?

Bill Crawford

 

[Gyro28866]

Short answer:
Only if a Hang test has been preformed and the head is mounted in the appropriate fore/aft position for the CG.
block the rotor head fore/aft movement into center, block the control stick to center of fore/aft movement. This will provide you with the maximum movement fore/aft in the stick vs rotor head.
Build a simple jig to hold the Rod ends and you can fabricate both tubes the same.

Level flight on the rotor head would be tilted how many degrees back?

Loaded question, with a variable answer.
The disk to airframe relationship is normally considered to be AOI (angle of incidence)
The angle between the rotors chord line and the angle of the airframe while in flight.
To increase airspeed you must decrease AOI.
so, the AOI at 45mph is greater than the AOI at 85 mph.

 

[Rowdyflyer1903!]

Understand the Angle of Attack decreases with an increase of airspeed. Nevertheless each machine has an angle of attack in which the machine is at its most efficient. This tends to be the best rate of climb and with in a similar speed, it’s best glide. At that theoretical speed, would the torque tube be angled back at XXX? and the stick be centered. There must be some rule of thumb.

I have heard many times, the Hang Test is to center the stick. So during the HT the rotor head centers Itself above the CG of a properly balanced fuselage. More correctly stated the fuselage is balanced under the proper mounting point of the rotor head as it must be moved fore or aft to find that point. Only when this is completed successfully configure the control tubes for a centered stick. This is irregardless of any stick position which might be necessary to achieve an airspeed with a combination of pitch position and power settings.

I have also heard the rotor tower must be free to move XXX? amount of degrees in every direction independent of teeter motion. I expect this is a rotor head design feature and this is not dependent on the HT and subsequent control tube adjustment.

 

[Rowdyflyer1903!]

I certainly did not intend to pose a loaded question

 

[Rowdyflyer1903!]

Short answer:
Only if a Hang test has been preformed and the head is mounted in the appropriate fore/aft position for the CG.
block the rotor head fore/aft movement into center, block the control stick to center of fore/aft movement. This will provide you with the maximum movement fore/aft in the stick vs rotor head.
Build a simple jig to hold the Rod ends and you can fabricate both tubes the same.


Loaded question, with a variable answer.
The disk to airframe relationship is normally considered to be AOI (angle of incidence)
The angle between the rotors chord line and the angle of the airframe while in flight.
To increase airspeed you must decrease AOI.
so, the AOI at 45mph is greater than the AOI at 85 mph.

Most rotor heads have an offset gimbal with the point of rotation one inch behind the gimbal points. When a hang test is performed, is the torque tube level or tilted slightly due to this offset?

 

[Countach]

Head tilted 10 degs. Aft. of keel.Control stick in neutral position. 10-12 degrees side to side movement.

 

[giro5]

Head tilted 10 degs. Aft. of keel.Control stick in neutral position.

I presume that is with a bensen style gyro with the mast tilted back 9-10 deg. Therefore a gyrobee style would be 0 deg. Agree or not??

 

[Doug Riley]

Rowdy: Let's back up a sec'.

Your control stick should have its own stops, independent of those in the rotor head. At any extreme control-stick angle, the stick's stops should engage just before the head's stops.

This arrangement does two helpful things. First, it prevents the application of high compression loads to the pushrods, which are relatively weak in compression. Second, it helps to prevent bending loads on the stems of the rod-end bearings. Bending loads will break the bearing stems in time -- obviously a lethal occurrence.

All of this means that the stick assembly and the rotor head assembly must be harmonized so that, with their respective leverage ratios, the stick has just a little less total range of motion than the head. Again, the stick stops must engage first.

A typical gyro rotor cruises at a SPINDLE angle of about 9 degrees to the horizon in level flight. The rotor disk is actually flying a couple degrees aft of this angle -- say 11 degrees -- which is the reason for the offset. But your spindle, and your torque bar, should be inclined about 9 deg. aft of vertical (spindle) or level (torque bar) when the stick is centered.

Depending on your gyro's tail design, you may need to fly with the spindle tilted slightly left or right to compensate for engine torque (large centered tail surfaces reduce or eliminate this need). It's probably best to start with the spindle vertical in the lateral plane when the stick is vertical, though. You can adjust later if you find you are holding the stick off-center in the lateral plane.

Giro 5: The head angle at stick-neutral would be 9-10 deg. aft relative to the keel on any gyro whose keel is intended to fly level (i.e. keel is the datum). On a Bensen, this would mean that the head would be exactly square to the mast at stick-neutral. On a Gyrobee, the head would be 9-10 degrees aft of the mast with stick neutral.

 

[Rowdyflyer1903!]

Good note on the control stops prior to the rotor-head stops. This is an excellent suggestion. The controls on this machine are the dual rod type patterned similar to more modern not the teeter pump type yoke. My system does not have the stops. This can be designed and fabricated as an attachment to the base of the control stick. This might be a good place to put a control lock but complex complicates.

This machine has been fitted with a tall tail similar to a Dominator style. By most of what I can glean, this should help with roll with the application of power and trim during cruise flight as well as stabilize the fuselage at higher cruise speeds. The rod ends have an inch and a half of thread grip on each. I expect not to need much adjustment and if it exceeds 1/2 inch it’s time to build new push pull tubes.

My rotor head is Chuck and Ernies two place design and the fuselage is loosely fashioned from Bill Parsons’ two place trainer. So an extended Bensen if you will But much stronger. The design furthermore took improvements from Steve McGowens’ ’Black’ with the exception of a raised forward keel.

It is a two seater without dual controls. The nose gear will be extended and full swiveling with Cessna style toe brakes. I am about half way fitting saddle tanks in addition to the seat tank.The fuel line routing has been brought to the gascolator, through to the filter to check valve then to the backup electric fuel pump. No engine fuel lines have been fabricated. As the fuel line route passes close to the aft control yoke and control rod attachments, this essential clearance is what kicked off this premature discussion.

No cowling or instrument much less electrical have been design or installed, so I am at least one year away from a hang test.

I am just working the process though my mind as I go. The more you understand about the design the better the pilot will be. Can’t fault me there.

By the way the name Rowdy came from the final approach fix into KCLL and the 1903 is in honor of the first Rowdy Pilots, the Wright Brothers. Despite the nickname, I am quite the conservative give but pride myself asking a good stick and rudder pilot. I flew a 1946 built wooden airplane for years. I had to slap the wings so all the termites would join hands before I went up. You don’t horse around with a 65 year old wooden airplane. I like flying odd aircraft. I am building a lumbering aerial pick-up truck. To think anything but this would be fanciful.

Thank you all for your patience. Unless you apprenticed under Pitcairn, Kellet, Bensen, HollMann, Brock, Chuck and Ernie or De la Cierva, you guys are all we have to refer to. I expect this has been explained till your are blue in the face but in reality each new generation of pilots both old and new need to be taught this over and over again. This is the cross you bear and you love it. Or you would not be here.

 

[Doug Riley]

The "teeter pump" stick design has a lot going for it -- not the least of which is the ease with which it can be equipped with stick stops. But the Brock type, with a torque tube and a push-pull tube under the seat, should also have stick stops. This is a matter of protecting the more delicate bits that make up the pushrods and rotorhead cross-tube from the large forces that a pilot can apply, amplified by the leverage built into the lower stick mechanism.

Interestingly, I found that, even on my 2-place Dominator, some lateral pressure (not necessarily lateral stick displacement, but maybe a bit of that, too) was needed to avoid gently banking during hands-off flight. I could hold it with my knee, but a more elegant solution was a lateral trim spring installed on the under-seat torque tube (Brock type), near the mast.

 

[Rowdyflyer1903!]

The "teeter pump" stick design has a lot going for it -- not the least of which is the ease with which it can be equipped with stick stops. But the Brock type, with a torque tube and a push-pull tube under the seat, should also have stick stops. This is a matter of protecting the more delicate bits that make up the pushrods and rotorhead cross-tube from the large forces that a pilot can apply, amplified by the leverage built into the lower stick mechanism.

Interestingly, I found that, even on my 2-place Dominator, some lateral pressure (not necessarily lateral stick displacement, but maybe a bit of that, too) was needed to avoid gently banking during hands-off flight. I could hold it with my knee, but a more elegant solution was a lateral trim spring installed on the under-seat torque tube (Brock type), near the mast.

I have often thought the trim mechanisms did not necessarily need to be mounted at the rotor head/ torque tube. After all the trim mechanism is to relieve stick pressures at a given flight configuration. Why not apply that pressure at the control stick or yoke level?
This might even dampen some of the stick shake. Wishful thinking.

Having said this, some of the control push pull tube systems I have observed seem to have a good bit of looseness in them especially in the, what I call, the idler arms.

I do not know if the addition forces, be it spring tension, would not induce wear. I suppose no greater than the forces which the pilot applies to the control system.

This would have the advantage, if the trim apparatus was mounted in front of the joystick, that an additional visual check of the trim status before departure, would be welcomed. Linear actuators, although convenient and accurate, have added complexity and some additional pilot load which has been problematic from time to time.

RAF and Sparrowhawk seem to have trim wheels for pitch and I assume, roll, which the pilot can access although I do not know how their trim influence is applied.

I did not know that Ken Brock was responsible for the dual tube control yoke system. I met him once at Oshkosh years ago only briefly. I have yet to travel to El Mirage. It’s on my bucket list.

 

[Doug Riley]

Brock was the most famous purveyor of the dual-tube joystick system. Others have offered them, and many still do. I don't know which of them was first.

The "pump" got an unfair bad rap during a spate of crashes of early Air Commands in the U.K. Some people speculated (wrongly, it turns out) that pump-type sticks cause PIO and PPO. One theory had it that a pilot or passenger sliding forward could push the stick down and trigger a PPO. But a gyro won't PPO unless it has a high prop thrustline. Blaming the stick was a red herring.

I imagine that Bensen put his trim springs up at the rotorhead because he considered the overhead stick to be the preferable control system for gyros. Almost no one agreed with him -- in the pre-trike era anyway.

 

[Rowdyflyer1903!]

Brock was the most famous purveyor of the dual-tube joystick system. Others have offered them, and many still do. I don't know which of them was first.

The "pump" got an unfair bad rap during a spate of crashes of early Air Commands in the U.K. Some people speculated (wrongly, it turns out) that pump-type sticks cause PIO and PPO. One theory had it that a pilot or passenger sliding forward could push the stick down and trigger a PPO. But a gyro won't PPO unless it has a high prop thrustline. Blaming the stick was a red herring.

I imagine that Bensen put his trim springs up at the rotorhead because he considered the overhead stick to be the preferable control system for gyros. Almost no one agreed with him -- in the pre-trike era anyway.

Bensen had something which worked in a trim system that did not have to be changed when the machine was adapted to joystick control. Pure evolution.

 

[Rowdyflyer1903!]

The degrees of movement of the rotor head does not necessarily correlate to the degrees of movement of the stick ie yoke?. Correct? The length of the torque tube and whether the control tube connections mate each other at the same place on the idler arms will affect total degrees of movement.
Another words are they connected at the same place on the idler arms? (If this is the wrong name for the mid- connection of the control rods, please educate me) I would expect the width and throw of the yoke and it’s pivot points to have a need to be somewhat duplicated with the length and pivot point of the torque tube in order to accurately duplicate movement. Also the width of the torque tube horizontal control tube connection “bar“ should be appropriate that of the width of the yoke.

It is my observation that when disconnected from the control rods, and therefore free of the limits of the movement of the rotor head itself, the control yoke can move far greater than required of the rotor head.

As suggested, I am exploring some method of mechanical limit to the movement of the stick as not to stress the control rod system if the limit of the movement of the head was reached.

I expect the answer is I will not know until the results of the hang test. However, what is your observation of the limits of the yoke movement on a parallel bar control system?

 

[Vance]

Many gyroplanes use a link on the control tubes so that they don’t develop a resonance on the long push/pull tubes. For some it is just a link with the tubes attaching the same distance from the pivot. Some are offset and change the ratio between the cyclic and the rotor head.

Some gyroplanes have multiple holes to adjust the ratio.

I am not aware of a standard ratio.

The goal is to have an acceptable amount of cyclic movement for the rotor head movement you want.

In my opinion having stops on the cyclic is a good idea.

Some people like a very light stick and some prefer a heavier stick.

Some are limited by things that get in the way.

For example in The Predator the ratio is different in the front seat cyclic compared to the back seat to keep the back seater’s hand from hitting the back of the front seat. It makes for a higher effort during taxiing and takeoff.

I prefer the way the ratio in the front feels compared to the back but I can’t move the front seat forward or the back seat back.

If you have clearance you can change the ratio to suit your taste.

You have to be careful about exceeding the maximum misalignment on the spherical rod ends so I recommend after any change or with a new to you gyroplane you move the cyclic to the four corners to see that there is no binding. A spherical rod end will break quickly with very little bending and there have been gyroplane accidents from rod ends breaking.