Back in the 80’s when I was considerably more active with flying and competing; I published an article in Model Airplane News (MAN Mag) about pushrods in model aircraft. Below is the jest of that article from memory, and expanded upon for new and experienced flyers.
For those people who are new to the hobby, even the simplest things can cause a great deal of confusion. I have always noticed that a lot of newcomers are baffled by the assortment of pushrods, connectors, and hinging advice/illustrations during their new building experiences. Even RTF and ARF models generally do a poor job of designing or illustrating good pushrod, control surface hookups, and hinging. There are RTF kits out there that actually have 1/8-3/16 in. spaces on the hinge lines – This does not mean that it is OK or even correct just because the manufacturer builds it that way.
A new modeller will often take guidance from more experienced modellers. But unfortunately, many of them still have a lot to be desired when setting good and safe standards for the new beginner – because they were never shown good guidance either, so they pass down what they know. So in an attempt to fill this void, I’ve put together a little article in hopes of shedding some light on these subjects.
Your airplane, and how it performs in the air, relies completely on the aerodynamic flows and forces placed upon it. The way it performs is based on the inputs to the control surfaces which in turn enable the airplane to maneuver precisely, smoothly, and safely in the air. How aerodynamically tight those controls surfaces are, how firm they are when forces of flight are applied to them, and how safe they are hooked up – all determine the performance, safety, and longevity of the model.
How many times have you seen a model that shows up to the field with loose or sloppy connections between the servos and control surfaces? Or how about the large gaps in hinge lines of the control surface? Or even hinges that are actually loose or misaligned?
The result can be that you have a model that will be difficult to trim, one that can constantly change attitude when speeds increase or decrease, and I have even seen models that stop responding as speeds increase because of the pressure exerted on the control surface is stronger than the pushrod’s ability to hold it in position because the pushrod is flexing too much, or the connections are too sloppy.
Another noticeable flight condition that is usually noticed in higher performance models, (particularly racers and aerobatic aircraft) but also in less performance models as well; where in a situation of increased G-load on the model (such as in a turn or vertical maneuver) actually causes the pushrods to sag under their own weight, resulting in loss of elevator or an elevator that becomes sluggish. Yes even that 2 oz pushrod! In a 6 G load now weighs 12 oz (¾ lb), or in 8 G load weighs 16 oz (1 lb). So to visualize that – take your pushrod and suspend it between two points – with the control clevis at one end and the connection for your servo at the other. Now hang a 1 lb. weight in the middle and see how much it sags. The length or the pushrod will also make quite a difference, the longer it is the more it will flex. That balsa pushrod likely snapped – and that .75 cents you spent on that pushrod - is that something you want to use to control that several hundred dollar model?
Many modellers figure OK – maybe a stronger pushrod - a Spruce Dowel or a Carbon Fiber Arrow Shaft, are popular. So do the same test and see what happens. There is still some flexing and the steel Rods at either end likely are now sagging, or in some cases if they are not piano wire (piano wire still flexes) – they bend or even kink. A bent pushrod in flight means disaster, as it just shortened the length which changes the throws of the movement on the control, or worse – caused the pushrod to snag… See illustrations below.
So what is a good solution? Well, there is a solution that by itself is subject to issues, but slightly modified works very well. Surprisingly – Nyrod, but with a modification!
A Modified NyRod pushrod:
· Is quick to install
· Is the strongest pushrod solution available
· It is not affected by drastic changes in temperature
· Is virtually frictionless
· Is unaffected by ‘G’ Loads
· Is easy to install and hook-up
NyRod, if modified and installed properly, will solve all your control and pushrod issues!
The NyRod by itself without modification, and installed per instruction, is prone to some issues and has some disadvantages:
1. It is made of plastic, so it is very susceptible to changes in temperature – expansion and contraction. This can be frustrating because during a day of flying, and as temperatures generally warm up during the day - you can be re-trimming constantly as the pushrod expands or contracts with the changes in temperature. In competitions, this can be frustrating as you often do not have time between flights to re-trim a model. In Racing as an example; taking only a second or two to trim could cost you the race. ie: a F1 racer doing close to 200 mph – 293.33 ft/sec X 2 sec = 587 ft. Likely you just lost the race!!
2. The entire length is very flexible and needs to be suspended multiple times, in narrow spaced braces, but even the exit points still remain subject to movement and flexing.
3. In larger or faster models, it simply is not strong enough to push the controls as required, due to its compressibility and flexibility under load.
How to modify it!
1. Measure the required pushrod length and add 2 inches.
2. Cut a 1/16” piano wire to length and insert it through the entire YELLOW NyRod pushrod, and cut the yellow pushrod 1-1/2 in. shorter than the Yellow NyRod.
3. Then using thin CA glue – put a few drops in ONE END only to secure it inside the yellow pushrod. (Generally the end near the servo. Gluing from one end only still allows the plastic insert to expand and contract with temperature by allowing it to slide along the piano wire itself. Leave only enough piano wire on each end to allow servo/control connectors to be attached. (Generally 1-1-1/2 in.) See Figure 1.
4. Insert the Yellow pushrod with the piano wire installed into the outer tube (Red or Blue). (See Figures 1 and 2). That OUTER tube should exit the fuselage near the control surfaces – neatly, and securely. If done properly, it can provide an aerodynamic filet to keep dirt and fuel out of the inside of the pushrod assembly.
5. Secure the entire assembly inside the fuselage with a slight bend/curve during installation. The slight bend/curve removes any slack within the pushrod itself and pre-loads any remaining excess play. Secure it within the fuselage with multiple points within the fuselage to remove any movement or flexing. You can use a bulkhead, or a good quality 1/4” dowel or square strip anchored to each fuselage side, and then use small cable-wrap to secure the pushrod to the dowel. Be sure to coat the cable-wrap and outer pushrod casing with thicker CA so it will not move. On smaller models with narrower fuselages, one secure point half way down the fuselage on the inner side is usually sufficient.
6. The other 2 secure points will be near the servo mounting area and the exit point at the rear of the fuselage by the control surface and control horn. See Figure 3
7. The assembly should be aligned straight with the Servos and the Control Horns at the control surface. This will keep the piano wire straight without bending. Bends in excess of 25-30 degrees weakens the pushrods strength. Completely avoid ‘Z’ bends, even at the servo. ‘Z’ bends change leverage points thereby reducing the push/pull strength of the pushrods and also add a measure of flexibility. Kind of like adding a spring in the pushrod.
8. To hook-up the servo and control horns - Use connectors/clevis that have little or no movement. Solder connectors, E-Z connectors, etc. If threaded metal clevises are used, solder a Threaded Brass connector to the Servo end – and be sure to secure the clevis with a 2-56 jamb nut on all metal clevises – Nylon Clevises generally do not require a Jamb nut. Use fuel-line or other retainer to squeeze the clevis together. See Figure 5 .
NOTE: Many safety officers at a flying field will not allow a model to fly if the clevises are not secured in this fashion – After all - It is all about SAFETY!
A modified NyRod install has advantages:
1. NyRod is readily available.
2. Some of the inner pushrods, such as on brand name “Sullivan”, have eight raised points of contact, thereby reducing friction (Drag) along the shaft to a minimum. See Figure 4
3. The 1/16” PIANO wire installed inside the Yellow shaft is strong. The softer steel threaded shafts that that are supplied are short, and should be discarded. Piano wire is stronger, less resilient to bends and kinks, is easily soldered, and provides an ideal diameter for connectors and other clevises.
4. The entire assembly is generally lighter than other pushrod solutions, and there are many more advantages to its use.
Vince Mulhall MAAC #11134