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Servos
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Your servo selection is one of the most important choices you will make. I spent decades using the Futaba S148 servo on everything I flew. Cheap and reliable, it worked just fine and I was completely satisfied with them.  Later I bought my first digital servo and found out how a good servo can bring your plane alive. Once you’ve used the good stuff, there is no going back.

There are several reasons good servos are better. The first is they all have ball bearings, which keeps them from loosening over time. Next is torque. Metal gears become important with larger planes. Last is the precision - or it’s ability to set and hold the position you ask it to. Let’s briefly look at each of these.

Ball bearings maintain the precise lash between the output shaft gear and the second gear. If you are using our Direct Drive system, you are putting virtually no external load on the servo bearings. The internal gears do, however. Do not even consider a non-ball bearing servo for competition.

You cannot have to much torque for your controls. Many problems you have experienced with your flying are directly related to weak servos…and I guarantee you never realized it. Some examples include snap rolls that enter slow and don’t exit cleanly, spins that do not exit crisply, poor knife edge performance, and flutter. You can chase these problems for years, and never realize it’s just that the controls are flopping in the breeze. It is always best to err on the side of extra torque.

Metal gears are not essential in a pattern plane, but should be standard on any plane scale or larger. The word on the street is that metal gears get sloppy faster than nylon gears, with a lot of guys changing gears every 100 flights or so. We have put as many as 1,000 flights on our metal geared servos with Direct Drive and a soft mount, without changing gears. Some of our servos have gone through four potentiometers on a set of gears. This is just one more way you save money with Direct Drive.

Now the biggy - precision. Hands down the digital servo is the way to go. The only difference between a digital and analogue servo is the aggressiveness with which the digital maintains its position. Try taking two servos of the same torque rating - but one digital and one analogue. Put a good size arm on them and try to shove them out of position. You will find that the digital maintains the exact position to the death. The analogue will give a little, up to several degrees when you push hard on it. If you look at the servos available, you will find the digital lines are increasing almost daily…digital is the future.  The only disadvantage is price, but they are well worth the money.

Now let’s talk specifics. JR got the jump in digital servos early in the game. Their 8411 is the original hard hitting muscle servo with about 180 in-oz. Their current top of the line is the 8611a, which is close to 300 in-oz in a case only slightly larger than the old Futaba S148 I used to use. These are very decent servos I highly recommend. Futaba’s early offerings were directed entirely to the FAI pattern guys. They were decent servos, but mostly nylon geared and low on torque. Their heavy hitter is currently the 9152, which is close to 300 in-oz, but larger and a half ounce heavier than the JR8611a. Hitec is a very good value in digitals. They seem to set their goal to just beat JR in torque and do it at about 2/3 the price. Their current biggy is the 5955, which is rated over 300 in-oz. I have found their reliability to be roughly equivalent to their price, but they do offer the same 3 year warranty as everyone else.

Always run 6 volts. All modern equipment is made to take it - which wasn’t the case just 5 years ago. Keep in mind that digital servos will burn the power about 1/3 faster than the old analogues, or 2/3 faster if you are used to running 4.8 volts. Plan accordingly for your power supply.

A point worth mentioning is that flyers are now running the new Hitec 5955 servos using unregulated Lithium Polymer packs (about 8.4 volts).  Since voltage regulators are one of our weakest links, this is potentially a great setup.  It seems to be working, and increases the rated torque by a third.  I will keep you updated as to reliability as we get more hours on these setups. 

So how many servos do you need? There are always odd exceptions, but this will get you in the ballpark as minimums:

  • Pattern - 1 servo per control of at least 60 in-oz. At least 100 in-oz for the rudder.
  • 25% - 1 servo per aileron and 1 per elevator half of at least 80 in-oz. At least 120 in-oz on the rudder.
  • 35% - Switch to the heavy hitters we talked about above. Use 1 to 2 per aileron (1 per biplane aileron), one per elevator half, and 2 for the rudder.
  • 40% - 3 per aileron (2 per biplane aileron), 2 per elevator half, and 4 for the rudder.

 

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