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Thursday, March 1, 2012

New Life For The Hi Tec HS65 Servo

The Hi Tec HS65MG servo has performed flawlessly for many years in our 45-48" aircraft. It has been the standard, and the servo of choice for as long as I have been flying these planes. It is still an awesome little servo, though the latest generation of airframes is pushing them close to the edge of where they were designed to perform.

With the introduction of the EXP series, airframe performance took a quantum leap. We are running bigger control surfaces, and deflecting them more than ever before, as much as 88 degrees on the elevator! Can you imagine the strain this is putting on these little micro servos?

 In addition to airframe advances, the new high discharge Li Po batteries (especially the Thunder Power 65C series that I use) deliver so much grunt and speed that this puts an incredible load on the entire airplane and it's systems. On 4s the EXPs are so blisteringly fast that the micro HS65MG servos can hardly keep up. It's not the servo's fault. We are just using them in bigger, faster and more powerful airplanes than anyone could have imagined when these servos were designed.

We started seeing a little stalling on the aileron servos at super high speeds a few years ago, but back then 3D was not the post stall to supersonic and back again kind of game that it is today. We did not fly the airplanes fast very often so the problem went largely unnoticed. Unlike a lot of other 3D style planes, the EXP series are so smooth and solid at high speed that you want to fly them fast, and then we started seeing aileron stalling and blow back.

Worse, still, with high speed and high elevator deflections, the elevator servo was taking a beating and just couldn't keep up. We were getting stalling the servos in high speed walls and parachutes and clearly could not reach the potential of the airframe.

As an interim measure, some of us started installing Hi Tec HS85MG servos on the elevators, though those were a bit of overkill, exactly what you like on something as important as the elevator! However, you have to cut the servo opening to get the 85 to fit and a lot of guys didn't want to do that. I have to admit it rubs me the wrong way to cut on a brand new plane too. I like the servo to just drop right in, and so does just about everyone else.

Airboss 45 ESC__Once Again The Solution
Extreme Flight came up with the perfect solution, and like in the past it was also the Airboss 45 speed controller that solved so many problems. All new Airboss speed controllers now come with an on board 6 volt Battery Eliminator Circuit (BEC). Now the servos are operating on more voltage, which gives them more speed, and more torque.

 I had not tried an HS65MG on the elevator with 6 volts before. Since I was building a new Edge 540T EXP, it seemed an opportune time to drop one in and see how it liked the higher voltage. On all my other EXPs, the HS65MG running on 6 volts have been performing brilliantly on ailerons and rudder, so there was no reason to believe things would be different for the elevator.

The only difference between this Edge and the one I am flying with an HS85MG on the elevator is that this Edge gets about 76 degrees of elevator travel and the other one gets 88 degrees. The difference is that the G10 servo arm that comes with the kit is just a little shorter than the Dubro extended arm that I use on my 85 equipped plane. I am planning to cut a new arm out of G10 and match the Dubro arm for length. I just couldn't get it done in time for this report, but 76 degrees turned out to be fine


As you can see, the provided G10 servo arm extension slides over the back of the standard servo arm and secures with a Dubro self tapping screw. Being the obsessive type, I went for two, plus I ran a bead of CA around the arm to make sure there was a really good bond. This baby ain't never coming apart.

Aileron set up is that same as my other Edge EXPs because I use the HS65MG on all of those too. Pictures are provided for continuity and also to show off the sexy new aircraft nuts I am using to secure the ball links. I have found if you over tighten the bolts you can squeeze the ball links and cause them to drag. Since we want these to operate as smoothly as we can, that's no good.

What I like to do is tighten the aircraft nut until the ball link starts to develop a little drag, then back it off and tighten a second nut against it, turning the two nuts together. This really locks things down and they will never, ever vibrate or shake  loose ... ever. To make forever more permanent, I put a drop of thick CA on the thread and these will survive a nuclear holocaust.

One nice thing is that if you ever do want to take it apart, spinning the outside nut off shatters the CA, and then it all just disassembles.

The rudder set is unchanged as well. Basically, this was a standard project with no complications. It actually went so quick, smooth and easy that it was a little disappointing!

We try really hard not to turn these reports into boring regurgitation of specs, numbers and other hooey that doesn't mean much to most people who simply care about the flying. It's all about the flying, and whether or not I can stall or blow these servos back.

As you can see, I laid a really good thrashing on my pretty new Edge, with lots of walls and parachutes. I really rotated the plane hard trying to get the servo to give up, hesitate, or blow back. I experienced nothing but smooth operating servos that gave me plenty of authority and holding power. I experienced no stalling or blow back. In fact, it seems like I didn't give up anything to the HS85MG at all. Well, except for the big servo hole, that is.

In  the final analysis, the HS65MG is still a very solid and capable servo, especially on 6 volts. It has plenty of torque to move the surface, and plenty of holding torque to keep it from blowing back. There is also that little bonus of not having to carve up the plane to get it to fit, which I like a lot. I plan to torture these servos relentlessly and I will let you all know how well they hold up. I don't have much of a worry about that though. I wanted a servo that works well and works reliably, and that is why I stayed with the Hi Tec line, and with the HS65MG that has served us all so well for so long.

Just for reference, here is one of the last vedeos I made with my other Edge 540T EXP, which uses a Hi Tec HS85MG on the elevator and HS65MS on everything else, and of course, a 6 volt Airboss ESC.