Subject of the Week: Linkage set-ups

Disclaimer: These techniques are how I see it, but definitely NOT the only way to do it!

Select the servo wheel size recommended by the heli mfg'r and place it on the servo so that the pushrod is at 90 deg. to the wheel when the collective stick is 1/2 in the hovering condition. Place the pushrod at the tail rotor gearbox bell crank in the position that is recommended in the instructions and adjust the length for the suggested hovering pitch.

Once the hovering blade speed, stick position and engine settings have been established the tail rotor, gyro, servo arm length and control rates can be fine-tuned. If you change rotor speeds the entire setup can change! Remember that the gyro / servo / and tail rotor is a closed loop control system on the machine and when you change one thing it will affect the others. The transmitter commands just override the loop to make the model rotate in the yaw axis. You should be able to have the gyro set at about 95% in the hover mode without the tail rotor oscillating on it's own. This is controlled by the length of the servo wheel and the efficiency of the tail rotor (disc diameter and rpm's). If the model oscillates, then reduce the servo arm length. (Let's assume that the model has stock t/r blades and gear ratios.) Once the oscillation has stopped then the next step is to adjust the control rate to achieve the desired rotation speed of a t/r turn. This is done by adjusting ATV 's (or AFR's). The transmitter control rate overrides the gyro's ability to stabilize the machine in the yaw axis.

Now check for left / right stopping power of the gyro. Do this by letting the rudder stick "snap" softly back to center from about a 1/2 stick rudder turn. The model should have the same rebound (or "feel") when the gyro stops the rotation. If it does not, then it can be corrected with a combination of t/r servo wheel offset and / or t/r blade length (remember efficiency?). This process is trial end error and best done when wind is calm! A higher gear ratio will allow you to have a smaller t/r disc and have similar efficiency without as much of a tendency to weathervane in cross wind.

Now that the basic mechanical setup and throws are worked out it's time to work on forward flight tuning. I won't get into tail compensation setup here but I will finish with the control loop stuff.

Go into forward flight with a gyro setting no higher than 50% (I’m assuming here that the gyro is at least a dual sensitivity type). Get the blade speed, collective pitch and throttle curves worked out first. (Again, when the rpm's change the t/r setup will too! ) Then slowly bring up the gyro sensitivity until the tail just wags at full speed downwind and back off just a touch. Now attempt a 540-stall turn and see if you can get the spin rate that you want. Adjust the rudder ATV's until the spin rate is acceptable. If you run out of ATV adjustment it's time to increase the efficiency of the t/r and repeat the above processes (including the hover stuff).

It gets a bit more complicated when you have a linear gyro. Where you can either set it up as dual sensitivity by adjusting the gyro sensitivity via ATV, or, as stick priority proportional sensitivity via a program mixer. For the stick priority setup I usually set the rudder stick center position mix point for the max gyro as above and then use a combination of full stick gain reduction and ATV to get the spin rate right. But if you reduce the gain too much, then the spin rate will not be constant with regards to outside conditions. IE: the gyro sensitivity can be useful as a spin rate regulator. Once it seems constant at some reduced sensitivity point, then set the speed with the ATV. The cool thing about these gyros is that you can have so much holding power as well as adjustability.

Can't think of anything else for now,

Cliff Hiatt

Author: Rene' Recinos

As far as tail rotor setup goes, I'd also like to add a couple of things in addition to those you mentioned. Firstly, one should strive to make the tail rotor linkage move as freely as possible, since the more free the movement is, the more efficiently the control loop will function. I used to oil the brass sleeve that slides over the tail rotor axle on Xcells, but I decided to stop doing that because it seemed to attract dust too quickly, which defeated the purpose of oiling in the first place. These days, for example, I keep the tail rotor axle as clean as possible by wiping it off frequently. I don't oil any points in the tail rotor loop. I check the bearings occasionally to make sure they're spinning freely, and clean/oil them if necessary. Secondly, the size/shape of the vertical fin will significantly affect the efficiency of the tail rotor. Personally, I've cut down my vertical fins (I'm part fish) on my 60s, so that they're about the size and shape of a 30 fin. I've noticed a large increase in tail rotor efficiency, which in turn has enabled me to decrease the disk size (tail rotor blade length is currently 95 mm from blade tip to bolt hole). Just be careful to ensure that enough vertical fin is there to prevent the tail rotor blades from striking the ground.

René

Author: Cliff Hiatt responds:

Rene,

I agree with your reasoning for friction free pushrod. I don't consciously oil the pitch slider either, the engine exhaust seems to do that well enough! My thoughts on a vertical fin are a bit different though, I use it to help tracking in forward flight maneuvers. Since I fly primarily fly F3C, which has forward flight maneuvers, the fin helps negate slight changes in t/r compensation while doing the maneuvers. Too big of a fin however, will cause weather vaning problems and require a larger t/r disc. I shoot for good tracking in a straight auto to settle on fin size. 3D will benefit from a smaller fin for backward flight.

Thanks for the input,