The Hardcore RC Helicopter Magazine
The Pushrod: by Mike Fortune
- I make my living as a Lead Aircraft Mechanic for the largest express freight airline in the world. I recently had the opportunity to travel to the Airbus assembly factory in Tolousse, France. The trip was for my employer. While visiting the Airbus A380 assembly factory, I was impressed with the extensive tooling and fixturing used to assemble a modern airliner. The one tool that impressed me the most was the wing to fuse alignment jig. Each wing half is so large that it could hold over 100 automobiles on its upper surface. Obviously, TLAR (That Looks About Right) fixturing for the wing's alignment would not be adequate enough. Airbus developed a series of jacks and strain gauges that together were capable of accuracy to 1/10th of 1 degree.As the wing and fuse were mated, each strain gauge had to be within its tolerance before the wing was considered mated, and then joined. For a guy who likes tools, fixtures and gadgets, this would have to be as close to heaven as he could get. I have been encouraged by MHTjef to share my tooling theories, thoughts and designs with the readers of MHT. I started designing tools for the RC model industry by first making them for myself. I have always wanted to know, with confidence, that if I crashed a model, I could re-build it so it would fly the same, or at least close to the same as it was before the crash. Some of these tools were my own unique designs and some were modifications I felt necessary to make an existing tool more versatile.
One of the “fixture systems” I want to show you in this issue is the one that will help answer the often pondered but seldom asked question, “How long should this pushrod be?” I know that probably does sound a little funny, but there is actually a method that can be applied to make the pushrod the correct length the first time. In the pictures you will see examples of a JR Vigor with the push/pull CCPM control system pushrods and the single throttle rod.
The single push rod:
I cannot think of a single pushrod installation that has not been subjected to the “Rule of 90” regarding the relation of the servo output arm to the thing moved. I’m sure there must have been one, somewhere ... but I just don’t know where it is or why you would have ever wanted to use it. With that said, were going to review the pushrod “Rule of 90” and it’s effect on the servo arm and the thing it’s moving. How long should the pushrod be? The pushrod length should be the same length as is the length from the center of the servo output shaft to the center of the bell crank or lever pivot (for example, the throttle barrel arm mounting screw-hole). Believe it or not ... it’s just that easy! I can hear you asking... OK, now that we know how long the PR is, where does it fasten on the servo arm? And then, how do we know it’s really the right dimension to begin with? Here’s what you do if there is only one pushrod to measure, as in the case of a throttle pushrod. It’s easy: measure the throttle arm from the mounting screw center to the outermost hole and find a servo arm that measures the same. There is an advantage to this. Now, measure from the center of the servo output shaft to the center of the throttle barrel pivot, where the arm screws on, and size the pushrod to that same length - if it was 112mm, then make the pushrod 112mm. Next set throttle stick to 50% - mid stick. Now, install the pushrod on throttle arm and servo. The throttle barrel arm should be loosely affixed on the barrel at this point. Now set the angle on the pushrod-to-throttle-servo-arm to 90 deg (see pic). If 90 deg is not possible at that point, due to the spline to arm arrangement of that servo arm, rotate the throttle servo arm, and keep rechecking until you get a 90. Once the pushrod is 90 deg to the servo arm side of the system, secure the arm onto the servo and install it’s retaining screw. Next, set the carb opening to 1/2 open throttle, align the throttle barrel arm, as you did the servo arm, making sure it is at 90 to the pushrod as well - 90-90!
Why use a servo arm the same length as the throttle arm? It keeps the setting consistent (especially from one model to the next). In other words, when I look into my radio’s throttle curve and I see, for example, that I have the stick at 65 percent and the output is 75 percent, I know then that the throttle is at 75 percent power or 75 percent throttle barrel opening. And I know that is standard for each model I have, or that I will build sometime. Keeping both the servo arm and the throttle arm the same length keeps everything linear. Linear servo response is critical. Why? Because we fly right side up and upside down and we want everything to be the same, no matter what side is up! Now, adjust throttle end points, high and low equally, in the radio as you typically would to obtain full open and full closed carb setting.
With a dual push rod setup, otherwise known as a push/pull setup, or a closed loop configuration, both push rods are required to be the exactly the same length. This is easy to do. Make up one rod measuring the same way as you did above (servo output to bell crank pivot). Place one push rod on the bell crank and the other one on the servo. Make up the other the same way, but here is a trick for checking your work and insuring that you have a 90-90 square setup. Measure diagonally from one sideof the bell crank ball to the opposite side ball on the servo arm. Then measure the opposite diagonal: it should be the same dimension. If so, snap on the remaining pushrod and you’re finished. If the distance is not the same, its most likely caused by the ball links you're using not being 1/2 turn adjustable. (I sourced the FMP Ultra Ball Link specifically to fix this problem. The Ultra links are 1/2 turn adjustable). Now, both pushrods are the same length and the bell crank is 90 deg to the pushrod. This is a "must do" to minimize the interactions that naturally accompany any eCCPM system. I hope that this article is of help to you and that you understand the humble ol' pushrod a little better.
Transfer this dimension to the pushrods/balls
and you've got a square system. Servos will
not bind bellcranks or opposite. Lowers
servo loading too.
REVIEW:
Measure from center of servo output arm to center of
bellcrank. This will be the same length for pushrod.
Note: Measurement from center of each ball is the same
as servo to bellcrank.
Dual Pushrod setup (on the Vigor and similar): Also shown are the 1/2 turn adjustable links.
And here is the linear throttle pushrod setup from the same machine. Notice the required 90 degree alignments. All is "square". It's the Rule of 90 setup.
