This article was originally published in Fly RC’s May 2016 issue.
By Kevin Siemonsen
The other day I was chatting it up with a flying buddy and he was ranting on about how much he loved his Hangar 9 P-51, but was having a heck of a time with the radio system. I responded “what kind of problems are we talking about?” He mentioned how he first cooked a servo and then a reversible servo extension. I could understand bad luck and losing a servo, but couldn’t understand why he would even have a reversible servo extension? A little more prying into the situation revealed that he was using a Spektrum DX18 radio, three “Y” harnesses and a reversible extension. This is a perfect example of what can happen when a radio setup can get frustrating and it becomes easier to just toss in a few “Y” harnesses. I told my buddy I would help him out and we got the plane and radio dialed!
This is a back to basics walk through radio setup for primary flight controls. The P-51 has ten servos installed in total. The receiver that comes with the DX18 is a nine channel receiver which was installed in the plane. With that being said one “Y” harness will be required for the radio installation because of the lack of receiver channels.
• Step one: Make sure all the servos are installed and set correctly with appropriate length servo arms and control horns and that the servos are centered along with control surface. Setting up control surfaces is just about as easy as it gets with all the programming power of the Spektrum DX18.
• Step two: Go into setup mode and configure the programming to match the plane. With the P-51 there are two independent elevator servos and a single rudder servo. Set the configuration for one rudder and two elevator servos. When selecting multi servo configurations for primary flight controls, the radio automatically selects and labels the channels on the display, simplifying the process. In doing so you can fine tune each elevator servo independently.
• Step three: Configure for the wing. The P-51 wing is configured with four servos for flight controls (two for ailerons and two for the flaps). Because there are not enough channels in the receiver for all the servos to have their own channel, I enlisted a “Y” harness on the ailerons. For this reason, when I set up the wing configuration, I selected one aileron servo and two flap servos. This only gives me the capability to fine tune each flap servo individually. If I had a 12 channel receiver there would have been enough channels in the receiver for me to of selected two aileron servos and two flap servos.
• Step 4: Verify servo direction using servo direction function.
• Step five: Set up servo center using the sub trim function. Make sure the flight control is streamlined with the airfoil and neutral. Do not use trim for this function. Trim should only be used for flight control fine tuning when in flight.
• Step six: Set up the travel limit using end point adjustment feature. This is especially critical on the elevator. Uneven deflection will induce roll. I used to estimate control deflections by eye, but found it’s worth it to use an inclinometer for best results. Make sure the flight control moves the travel recommended by the plane’s manual.
• Step seven: Dual rates and exponential are ways to change flight control sensitivity and travel with the flick of a switch. With dual rates you are able to increase or decrease control travel depending on mode switch position. When increasing deflections, the flight controls will become more sensitive. Adding exponential will reduce flight control sensitivity. This is whey these two features are grouped together when programming the radio.
You apply the same steps that you would for the primary flight controls when it comes to setting up secondary controls. This is also where you can take advantage of some of the intuitive programming of the DX18.
• When setting up the flaps, the factory preset switch to activate them is on the front of the transmitter above the throttle stick. On the flap screen you are able to choose any of a number of switches or sliders to activate the flaps. In my case I prefer the slider on the left of the transmitter that is fully proportional over the factory preset three position switch.
• Just like the elevator, it is imperative that the flaps deploy symmetrically or it will induce roll. Make sure to set the servo travel limits identically.
• There are a few other options like servo speed that would add to the realism with scale extension and retraction rates. This would be achieved by reducing the servo speed on the servo speed set up menu. You can even set different rates depending on direction of the servo (ex slow down and fast up).
• A very desirable mix on the DX18 is flap/elevator mixing. A trait when deploying the flaps is the increased lift, which in turn makes the plane pitch up. On the flap/elevator mix screen you are able to mix in down elevator when flaps are deployed. A nice part of the feature is that the elevator down trim that is added is proportional to the amount of flaps.
The Hangar 9 P-51 comes with electric servo driven retracts. Each retract is operated independently by its own servo. When setting the retracts up it is most important to have each servo on a separate channel. This is because when the retract locks into the up or down position, the linkage mechanism has a hard stop at the end of the travel. It is imperative that the servo end point travel limit does not exceed the mechanics of the retract. If this occurs, the servo will push the linkage beyond the end of its travel limit, drawing excess current, stripping gears or even failing. The factory default gear switch position is on the top left corner on the transmitter which is a perfect spot for it.
• First: Use a programmable mix. Select an open channel and mix “gear” with an open port on the receiver. In my case I mixed “gear” with “AUX3”. This will enable you to set up servo travels independently.
• Second: Set the servo travel limit at around 75% of what you need for full articulation of the landing gear.
• Third: Operate one gear at a time increasing the travel limit of the servo until the gear fully retracts and deploys without bottoming out the lock mechanisms.
When we were all done with programming, my buddy had a much better level of under-standing transmitter setup with the new generation of radios and the P-51 was dialed! Taking advantage of the DX18 programming does take a little time and there is a learning curve, but remember that the purpose for the computer radios was to make things easier for us modeler. If you have a model with more than four independent servos, it is well worth it to use your transmitter to its fullest potential. In the end, you’ll be left with a much more tidy wiring setup, it will eliminate variables such as reversing y-harnesses that might lead to failure and you might even learn a thing or two!