Collective Linkage

2006 Jul 8

Table of Contents:

Caution: Disconnect all linkages from the servos. Failure to do so will result in mechanical binding which can damage your servos.

Preliminary Information:

The collective pitch will be setup mechanically. It is very important that the radio requirements be accomplished prior to beginning the setup.

The collective setup is written as a step by step guide and the result of each section is dependant on the previous.

The Aileron and Elevator linkage setup MUST also be completed in order to lock the swash plate level. This is done with the transmitter and receiver on. Extend the radio antenna so that the RF energy does not radiate back into the radio when in the on position.

The Main Blade and Fly Bar Paddle orientation is important. Ensure that the blades and paddle's leading edges (thick airfoil) are facing in the correct direction. See photo.

"Hysteresis is a property of systems (usually physical systems) that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state". http://en.wikipedia.org/wiki/Hysterisis.

Radio Requirements:

Verify that your radio meets the following parameters prior to beginning the collective linkage setup.

End Points, ATVs, EPAs, Travel Adj = 100% / 100%

Sub Trims= 0%

Collective Hover Pitch menu- INHIBITED
(The Hover Pitch function in some radios cannot be inhibited. With the collective stick at half stick, 50%, rotate / move all knobs and slider switches. Verify that the collective servo does not move. If this function cannot be inhibited, ensure that the knob is in it's center position and verify it's center position using the digital menu screen. Be careful not to move this knob at anytime as this will adversely affect your mid point pitch curve.)

Pitch Trim menu - INHIBITED
Perform same checks as Collective Hover Pitch. On some radios, holding two buttons down while turning the power on will access this menu.

Pitch Curve Menu - All flight modes (default) 0%, 25%, 50%, 75%, 100%

Hi/Lo-PIT menu - Man / Null (The Futaba 9CHP super has a Hi/Lo-Pit menu that allows the high and low ends of the pitch curves to be limited for each flight modes. Set this both function to Man and Null due to the fact that the assigned switch / slider control will affect our mechanical alignment.. You can activate this function after completion of the mechanical setup.)

Linkages - Set per instruction manual's Linkage Dimensions.
Dimensions of linkage rods connected to any servo are ball park values due to a variety of servo brands available.

Feathering the blades:

The term "feathering the blades" is credited to Ray Hostetler's Authoritative Heli Manual.

Remove both double links from the blade grips and extend the blades outward. Pivot the blades so that it balances level.

This condition of the blades being at a level balance is referred to as "feathering the blades".

This ensure that the blades are running true on it's pivot point, center of the main blade bolt. The blade's center of axis, when rotate, will be the same from the blade root to the blade tip. When the pitch gage is place at any point on the blade, it's pivot point will always rotate on the center of its axis.

Lock the blades down so that it does not pivot out of balance by tightening the main blade bolt.

Determining Fly Bar Ratio

It is important to know if the fly bar has a 1:1 ratio or not.

A fly bar ratio that is NOT 1:1 will not provide repeatable readings when checking the main blade incident.

The pitch readings in a fly bar system that is NOT 1:1 will change as the fly bar is tilted.

Since the fly bar rod is use as a reference point for the pitch measurements, we need to ensure that the pitch readings do not vary as the fly bar is tilted.

To determine if the fly bar has a 1:1 ratio or not, place a pitch gage on either blade.

Eye ball level the the fly bar and align the pitch gage to the fly bar rod by sighting at a reference point on the pitch gage. The pitch gage reference point should be leveled with the fly bar rod.

Lock the pitch gage by tightening the securing knob. The pitch readings are irrelevant since we are concern that the pitch gage reference point remains parallel with the fly bar rod.

Tilt the fly bar as far as it would go in either direction. Verify that the pitch gage is still parallel with the fly bar rod. If the pitch gage STAYS parallel as the fly bar is tilted, then you have a fly bar ratio that is 1:1.

If the pitch gage goes out of coincident as the fly bar is tilted and back in coincident at eye ball level, then you have a fly bar ratio that is not 1:1.

Fly Bar Ratio other then 1:1:

In this setup, our objective is to obtain repeatable pitch readings.

To do this, we need to lock the fly bar so that it does not tilt / move. Also, it is important that we level the helicopter using any reference point on the mechanics to establish a reference with the Datum Line.

Lock the fly bar rod with a Fly Bar Locking Tool or suitable device.

Place a bubble level on the tail boom, frame or any part of the helicopter's mechanics, such that it is running parallel to the Datum LIne.

Prop the helicopter so that the bubble levels are leveled. Verify that the bubble level is leveled by relocating the bubble level to a flat surface running parallel to the Datum Line. Both location should indicate the same level conditions. If not, determine the true level surface running parallel to the Datum Line.

Place another bubble level on the fly bar rod and adjust the Fly Bar Locking Tool so that the bubble level is leveled.

During the course of the pitch adjustments, ensure that the bubble levels remain leveled.

Servo Horn Alignment:

The objective in this section is to set the collective servo horn 90 degrees to the linkage rod when the collective stick is at half stick position (50%).

Turn on the radio and verify that you meet the above Radio Requirements. Move the collective stick to half stick position (50%). Use the radio's digital pitch menu to set the collective stick to the 50% indication. This is the preferred method, since there are no ambiguity when setting the collective stick to half it's total travel.

Also note that at full collective stick deflection, that the collective servo will output a rotation of the servo horn that will cause the pitch control arm to deflect the blades upward (positive pitch).

The use of a homemade tool (collective horn alignment) will simplify this alignment.

Estimate and set the pitch control arm to its 50% mechanical travel. Install a servo horn and using the collective horn alignment tool, rotate the servo horn on the servo's output spline shaft for the best center on center alignment.

Installing two machine screws on the selected servo horn will aid as an alignment guide for the collective horn alignment tool and will yield a stable alignment process.

The intersecting point of the collective horn alignment tool should be superimposed over the selected hole that is approximately 13mm to 17mm from the center.

Just select any hole at the approximate distance of 13mm - 17mm. The final hole selection will be determined during the Linkage Rod Alignment section.

The long arm of the collective horn alignment tool should be exactly over the center of the pitch control arm ball.

Be aware that the collective horn alignment will vary when moving in or out a hole on the horn as the distance from center changes the angle.

Install a ball on the collective servo's horn. Connect the linkage rod's ball link to ball on the servo horn. Do not connect the other end of the linkage rod's ball link to the pitch control arm's ball. The alignment for this part will be describe in the Linkage Rod Alignment section.

The setup above is primarily for ensuring that the linkage rod is 90 degree to the servo horn when the collective stick is at its 50% travel or half stick. This alignment procedure will need to redone because the over mechanical throw will be matched to the radio's electronic throw.

In simple terms, setting the collective linkage rod so that its endpoint value is near 100% / 100% when the pitch control arm is at its full maximum mechanical throw in both direction.

Understanding the Equal Throw Method:

In order to set the collective linkage rod, we must understand the "Equal Throw Method".

The term "Equal Throw" can be define has having an equal amount of throw from each side of center. Study the diagrams below as this will aid in your understanding of a Equal Throw setup and a Not Equal Throw setup and will simplify the collective linkage rod adjustment.

In both diagrams below, you will note that the balls on the pitch control arm are at EQUAL distance from the center of the linkage rod's ball link when the collective stick is at full low stick position and full high stick position.

For this scenario, we would use the radio's end point adjustments to get the linkage rod's ball link to align with the center of the pitch control arm's ball at maximum mechanical and electronic deflection.

The end points have been adjusted EQUALLY, and you will note that the pitch control arm's ball is aligned at the center point of the linkage rod's ball link.

Now let's examine a scenario where the balls are NOT at equal throws. From the diagram below, you can see that the pitch control arm's ball is Not equally center on both side of the linkage rod ball link's center.

If we were to adjust the radio's end point values such that the ball is aligned on the center of the linkage rod's ball link at full low and high stick position, we will have a setup that has different end point values.

Here's another variation of a Not equal throw that will yield radio end point values that are Not equal.

Linkage Rod Alignment:

In order to do the linkage rod alignment we must fully understand the "Equal Throw Method" and perform the "Servo Horn Alignment".

Connect one end of the linkage rod to the collective servo's horn and disconnect the other end of the linkage rod's ball link from the pitch control arm's ball.

The hole and arm selection was accomplished in the Servo Horn Alignment section

With the radio and receiver on, move the collective stick to full low stick position. Apply a constant slight upward pressure to the pitch control arm while superimposing the linkage rod's ball link over the pitch control arm's ball.

Take note of the pitch control arm's ball in relation to the linkage rod's ball link center.

Move the collective stick to the full high stick position. Apply a constant slight downward pressure to the pitch control arm while superimposing the linkage rod's ball link over the pitch control arm's ball. Take note of the pitch control arm's ball in relation to the linkage rod's ball link center.

Using your new found knowledge of the "Equal Throw Method", adjust the linkage rod's ball link to obtain a EQUAL throw setup. Repeat the above steps from full low stick position to full high stick position for a EQUAL throw setup.

Once you have obtained Equal Throws adjustments on the linkage rod, adjust both radio pitch end point values EQUALLY until the linkage rod's ball link is centered over the pitch control arm's ball at full stick deflection on both sides.

There will be a slight out of alignment (not equal throw) when ever the radio's endpoint values are varied so a back and forth check of the Equal Throw method will be necessary. This is a process that refines the adjustments of the linkage control rod's over total length.

I have had no issues of the ball link coming out in flight as a result of a half turn ball link adjustment. What is important, is that you have achieve an equal mechanical and electronic throw at full deflection.

The selection of the best hole out from the center of the servo control horn should be such that the radio endpoints (travel adjust) is at approximately 100%. This part will be based on trial and error.

I strongly suggest that if the hole you select yields an endpoint of 93% / 93% make every attempt to try the next hole out and vice versa. The goal is to use the best hole on the servo horn that will yield an endpoint closer to 100% / 100%.

As your flying skills develop, and you need the quicker collective response, move the ball further out on the collective servo horn and reduce the pitch endpoint values to no less then 80% / 80%. Any further and you will lose resolution.

As a result of achieving the best endpoint value, you will need to rotate the servo horn on the servo output shaft again and reselect the best servo horn alignment for the hole that you re selected. Keep in mind that the angle changes from as you move the ball in or out on the servo horn.

Servo Binding Verification:

This section is redundant as you have verified servo binding during the linkage rod alignment. But it is also important that it be discussed as this can be over looked and result in premature servo failure.

Remove the linkage rod from the pitch control arm.

Turn on the radio transmitter and receiver and move the collective stick to the full low stick position. Superimposed the the linkage rod's ball link over the pitch control arm's ball. The linkage rod's ball link should be aligned with the center of the pitch control arm's ball.

Repeat the above with the collective stick at full stick position.

Make the necessary adjustments such that there are no mechanical binding at full low and high stick position. Refer to the Servo Horn Alignment, Equal Throw Alignment and Linkage Rod Alignment for the necessary adjustments if mechanical binding is evident.

Setting the Master Blade's Pitch Mechanically:

In this setup our objective is to set low and top end pitch so that they are Equal mechanically.

Also note, that we will only be using the radio transmitter to keep the swash plate level.

Verify that the collective linkage rod is disconnected from the collective servo and you meet all the preliminary requirements as stated above.

If you have a fly bar ratio that is not 1:1, then the fly bar must be locked to obtain repeatable pitch readings. Refer to Fly Bar Ratio other then 1:1.

Place an identifying mark on one blade and its blade grip (see photo). We will call this the "Master Blade" and the non marked blade the "Slave Blade".

For whatever reason, we do not want to adjust the Master Blade when we are doing the Blade Tracking Alignment. One of the main reason that we marked this blade the Master Blade, is that we can always revert back to it's original bench reference setup in any event that we tweak the Slave Blade out of alignment.

With the radio and receiver turned on, place a pitch gage on the Master Blade. Do not concern yourself with the Slave Blade at this time.

This adjustment is strictly for the Master Blade as we will use a different technique to mirror the Master Blade's pitch setting to the Slave Blade.

Do not exert excessive pressure when applying a constant upward / downward pressure on the collective pitch control arm. In order to obtain repeatable readings, remember to continue to apply slight constant pressure when taking pitch reading.

Apply a slight constant upward pressure on the collective pitch control arm and adjust the pitch gage so that it's reference point is level with the fly bar control rod. Take note of the pitch readings.

Apply a slight constant downward pressure on the collective pitch control arm and adjust the pitch gage so that it's reference point is level with the fly bar control rod. Take note of the pitch readings.

If the readings goes past the maximum indication on the pitch gage, just note that it goes past max pitch indication.

Total the sum of both max pitch readings and divide the total in half.

For example:

If max positive pitch is +14 degrees and max negative pitch is -7 degrees, then the total mechanical pitch is 21 degrees. Dividing this total in half equates to 10.5 degrees of pitch from either side of center (half stick) or 0 degrees.

So the maximum positive pitch attainable mechanically is +10.5 degrees and the maximum negative pitch attainable mechanically is - 10.5 degrees.

Therefore, we need adjust the long linkage rod (see photo) from the swash plate to the mixing lever in order to obtain an equal mechanical pitch from either side of center.

Remove and adjust the long linkage rod from the swash plate to the mixing lever to obtain "Equal" pitch readings.

Reconnect the linkage rod.

Take note of the number of turns the ball link is adjusted on the long linkage rod. This will be applied to the long linkage rod on the Slave Blade. If you followed the preliminary requirements, both long linkage rods were initially setup with identical lengths.

Repeat the above steps of applying a constant slight upward or constant slight downward pressure on the collective pitch control arm until the top and bottom end mechanical pitch readings are Equal.

The total approximate mechanical pitch travel is 22 degrees or -11 and +11 degrees. Because of hysterisis, double check that the pitch indications are EQUAL at maximum mechanical stops.

Setting the Slave Blade's Pitch Mechanically:

In this setup our objective is to mirror the pitch settings from the Master Blade to the Slave Blade Mechanically. This will be done by connecting the collective servo's linkage rod to the collective pitch control arm and electronically setting the Master Blade's pitch to 0 degrees by moving the transmitter's collective stick until the pitch gage reference point is parallel to the fly bar rod.

To avoid inaccurate pitch measurements, Hysterisis, move the collective stick in the same setting point.

If you completed the "Equal Throw Alignment", then the issue of servo binding will be eliminated. If not, perform the "Equal Throw Alignment" or remember not to move the collective stick to full low stick or high stick deflection.

Disconnect and adjust the long linkage rod for the Slave Blade the same amount of turns as noted earlier during the Master Blade adjustment. Do not be overly concern with this measurement as this will be a ball park estimated adjustment for the Slave Blade. Reconnect the long linkage rod.

If a fly bar lock was use, re-establish the reference point and ensure that the bubble levels are leveled.

Place a pitch gage on the Master Blade and lock it to a 0 degree indication.

Because of the presences of Hysterisis, all pitch measurements will be done with the radio's collective stick moving in the same direction (setting point). In other words, if you move the radio's collective stick from low to mid stick position and you over shoot the pitch gage alignment with the fly bar rod, restart your measurement with the radio's collective stick at the low stick position moving towards the mid stick position.

The direction of movement doesn't matter. What matters is that you take all measurement using the same setting point to avoid inaccurate measurements due to hysterisis.

Move the radio's collective stick until the pitch gage reference point is aligned with the fly bar rod. If you over shoot the alignment restart in the same direction.

Gently remove the pitch gage from the Master Blade and gently place it on the Slave Blade. Sight the pitch gage reference point to the fly bar rod and note if the pitch gage reference point is level with the fly bar rod.

Adjust the Slave Blade's long linkage rod accordingly so that the pitch gage reference point is in alignment with the fly bar rod. Do not be over concern with this part of the alignment of trying to nail it the first shot.

It will take several back and forth adjustment because of the inherent mechanical hysterisis caused by disrupting the mechanical linkages.

Move the radio's collective stick in the same direction as noted above. Sight and align the pitch gage reference(s) point to the fly bar rod.

Gently remove the pitch gage from the Master Blade and place it gently on the Slave Blade. Sight the pitch gage reference point to the fly bar rod and note if the pitch gage reference point is level with the fly bar rod.

If the pitch gage reference point is level with the fly bar rod, the 0 degree reference point of the Master blade has been mirrored to the Slave Blade bringing both blades into coincidence, 0 degrees.

Repeat the above steps until no further alignment is required for both blades to be at 0 degrees.

This method of matching the 0 degree pitch of the Master Blade to the Slave Blade and if done correctly will require no blade tracking on the first flight. Of course, there are those odds of a static bench setup and a dynamic test flight that will throw this part out the door.

Swash Plate Binding Verification:

Setting the Dynamic Top / Bottom End Pitch:

This section describes how to set the top and bottom end pitches based on actual flight characteristics of the blades. Because of the many types of available blades, each blade manufacturers have different air foil designs that make the model react different in flight.

In our bench setup, our top and bottom pitches were set identical. So upright and inverted should have the same response feeling. This is not true because of the different airfoil designs and actual flight characteristics of the model.

If you are able to do a full inverted power climb and compare its response to a full upright power climb and you note a difference from inverted to upright then a quick adjustment of the long linkage rod will correct for this difference.

Adjust BOTH long linkage rods from the swash plate to the mixing levers EQUALLY.

If the model climbs quicker inverted then upright, adjust both long linkage rods equally to reduce the bottom end pitch. And vice versa, if the model climbs quicker upright then inverted, adjust both long linkage rods equally to reduce the top end pitch.

This adjustment sets the overall pitch based on the blade's actual flight characteristic.

References:

Hysteresis is a property of systems (usually physical systems) that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state. http://en.wikipedia.org/wiki/hysteresis.

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