96S14
Apr 18, 2005, 02:00 AM
!Introduction
| spec2
| @910603
|> <b>Description:</b> |< Standard Sized, high torque, digital servo
|> <b>Motor Type:</b> |< 3 Pole
|> <b>Bearing Type:</b> |< Dual Ball Bearing
|> <b>Gear Type:</b> |< Karbonite (Reinforced Resin)
|> <b>Size:</b> |< 1.6" x 0.8" x 1.5" (40mm x 20mm x 37mm)
|> <b>Weight:</b> |< 1.83 oz (52g)
|> <b>Torque (4.8v/6.0v):</b> |< 69/83 oz-in
|> <b>Speed (4.8v/6.0v):</b> |< 0.18/0.15 sec.
|> <b>Manufacturer:</b> |< <a href=http://www.hitecrcd.com>Hitec</a>
Servos are one of the most important parts of the Radio Control "chain" between the stick under a pilot's thumb and the safety of the airplane he or she is flying. Over the years, I have used all sizes and shapes of servos from little sub-micro servos all the way up to big sailwinches that took 5 seconds to turn 180 degrees. I can't say that in all my years in the hobby that I have had no problems with the servos I used.
About 5 years ago, I got into flying high performance gliders and one of the first things that struck me was the sheer amount of money that sailplane pilots spent on their servos. It was not unrealistic to spend over $100CAD (about $80USD) per servo. I started flying a fully molded RnR Genesis model with some extremely cheap servos and I quickly learned that there actually was a difference between different brands and models of servos. The quickest things I learned were that servo centering is important, that a servo needs to be rugged, and that it needs to be extremely reliable.
Fast forward to today and I'm still at it in this wonderful hobby. I have been enjoying aerotowing and the latest plane in the fleet is an extremely overpowered Hangar 9 Ultra Stik Lite. There is a Brison 2.4 ci engine hanging off the nose of the heavily reinforced tug and I was very worried about how to safely control this plane in such a high vibration, brutal environment. With an engine that is 50% larger than the maximum recommended engine size, there had to be no compromises in the department of controlling this beast of an airplane.
About a half year before the tug project started, I read about a new type of gear that Hitec was putting in some of their servos. They were called Karbonite gears and were made of a reinforced plastic. They were advertised to have the low backlash of nylon gears while having the strength of metal gears. It seemed like a good concept, and with my background in mechanical engineering, I had no reason not to trust a well chosen plastic to perform as well as metal in a servo gear application.
I contacted Hitec to tell them about the towplane project and they quickly suggested their new HS-6635 HB servo. This servo is built in a standard size case and is sold as one of the lower cost digital servos in the Hitec range. I was interested in the cost of the servo and the seemingly good performance specs, seeing as I needed 6 of them for the flight surfaces on the tug. By keeping the cost down and the performance up, our goal of building an affordable tug would hopefully become a reality. So with that introduction, let's get started with torturing this servo…
!The Servo
The Hitec HS-6635HB servo is built with dual ball bearings supporting the output shaft in a rugged-feeling, standard-sized, servo case. The servo feels very nice, with high quality 60 strand - 22 AWG twisted wire leads and it comes with an assortment of very rugged servo arms. When I first turned the output arm, I was absolutely blown away by the smoothness of the geartrain. It was silky-smooth, with no discernible backlash. So far, I was sold on the Karbonite gears. I took a good look at the specification sheet available <a href=http://www.hitecrcd.com/Servos/spec_sheets/HS6635HB.pdf>here</a> and then decided to get to the testing.
@910606:The Hitec HS-6635HB comes packaged in a handy plastic box that can be used around the workshop for storing all those little bits that tend to create clutter.
@910603:The HS-6635HB is made with a rugged reinforced plastic case and exudes a feeling that it costs much more than it actually does.
@910607:The HS-6635HB comes with a very nice assortment of strong servo horns. They are thick and very stiff...perfect for those large & heavy control surfaces on sport aerobatic airplanes.
@910608:The servo lead that comes with the HS-6635HB is very nice. The wires are 60 strand 22 AWG copper wire with a high quality, very flexible jacket. This lead is more than up to the task of carrying the additional current required by the digital amplifier.
@910609:This is where the HS-6635HB is unique. It uses Karbonite gears which are a reinforced plastic. This gives nearly the strength of metal gears with the smoothness and near-zero backlash of nylon gears. It truly is a perfect choice of material for a servo gear application.
@910610:When you remove the bottom cover, the digital amplifier and motor are visible. The motor is just a plain 3 pole motor, but its performance is very impressive. Note the use of silicone to prevent vibration from causing the motor leads to fail...a nice touch.
!Testing
Since I have never done a servo review before, I decided to take some time to devise some good lab tests for the servo so that I could compare it to other servos in future articles. I wanted to get a quantifiable number for those things that the "pros" seem to always gush over. I wanted to prove things like good centering, powerful holding torque, and get an idea of the current drawn when the servo is pushed to the max.
I started by building a rugged frame to hold the servo securely on the edge of my workbench. Then I proceeded to use my Multiplex Profi 4000 transmitter and a Hitec SuperSlim receiver to drive the servo. I hooked up the receiver to a regulated lab-grade power supply so as to have exactly 4.8 volts (or 6.0 volts as the test dictated) at the servo regardless of the current drawn. I also hooked up a pair of Fluke digital multimeters to get an idea of the current drawn by the servo at the various phases of operation and to ensure that the voltage at the servo was remaining constant. Finally, I used a Mitutoyo dial indicator to be able to measure the repeatability of the servo horn position. This indicator has 0.001” resolution which proved to be more than adequate to see the variation in servos horn position.
@910611:This mess actually does something. The multimeters are used to measure the current and voltage supplied to the servo. The dial indicator allows me to measure the position of the servo horn.
@910612:Here you can see the frame made to hold the servo in place. The dial indicator is pushed by the servo arm.
@910613:This is the amount of weight to get 58 oz-in of torque at the servo arm. It is impressive to think that all that load is being held by a plastic output spline. The HS-6635HB happily lifted these giant nuts and more. See below for more details.
!Test Results
!!Centering Test - No load
This test involved operating the servo with no load. Using a switch input on my transmitter, I cycled the servo from center to one extreme and back to center 20 times and recorded the position of the servo arm after each cycle. It was impressive to see that under no load the servo continually came back to a position within 0.1 degrees of the median position, with only two trials out of 20 having a maximum deviation of 0.3 degrees. Even in this worst case scenario, the total error works out to less than +/-0.005” positional error on the servo horn hole furthest out from the servo pivot. That is pretty good repeatability/centering. Under no load, the servo drew between 4 and 5 mA.
@910614:This graph shows the servo's ability to return to the same home position under no load. 20 trials were performed and the angular deviation from the median position is shown on the vertical axis. As can be seen, this is a very accurate servo!
!!Centering Test - 40 oz-in of torque
I took enough weight and hung it off the servo horn to cause the servo to put out 40 oz-in of torque. Note that by hanging the weight off a single arm, the worst case installation for the servo is being reproduced. This setup puts the maximum load on the output shaft and loads the servo case quite heavily. The only worse way to mount the servo is to have the short dimension of the servo case parallel with the applied load, but this is rarely seen in actual practice. In many cases, it's best to use push-pull linkages if possible, but I wanted to recreate the worst case (and realistically one of the most common) scenarios for servo loading.
With the application of 40 oz-in of torque, it is readily apparent that the servo does not reach the same position as it does under no load. At 4.8V, the servo had a median deflection from the median zero-load position of -4.7 degrees. While this may seem like a lot, it still represents only about 7.8% of the total servo travel, meaning that the control surface deflection is still a very large portion of what the pilot is asking for. At 4.8V under 40 oz-in of torque, the servo was drawing 222 mA.
@910615:This graph shows the "droop" that the servo experiences when putting out 40 oz-in of torque. Note that the deflection from the zero load position is only a few degrees and that it's less at 6.0v than it is at 4.8v...a good reason to use 5 cell RX packs if you can.
It was interesting to note that at higher voltage, the performance of the servo was markedly improved. The angular deflection at 40 oz-in decreased to approximately -4.0 degrees and the servo only drew about 8% more current (239 mA). This makes a good case for using 5 cell receiver packs if you have the option and your servos are rated for it.
!!Centering Test - 58 oz-in of torque
As expected, as the load increased on the servo, the deflection from the zero-load center position increased. At 4.8V, the median deflection was -10.5 degrees and at 6V, it was -6.8 degrees. The currents drawn at 4.8V and 6V were 438mA and 460 mA respectively.
It is very apparent that as the servo is loaded more heavily, the use of 5 cells becomes even more valuable. While the current draw does increase at higher voltage, the price of extra current seems to be a small cost compared to the benefit of far better servo torque, speed and centering.
@910616:At 58 oz-in of torque, the "droop" of the servo arm position is more pronounced, as expected. The overall droop though is still pretty reasonable considering the load, and the benefit of the 5 cell receiver pack is more pronounced.
!!Centering Test - 77 oz-in of torque
At 77 oz-in of torque, it was apparent that the servo could no longer reasonably move the load at 4.8V drive voltage. While it still moved, it was probably only achieving about 50% of the motion that was requested of it. Luckily, Hitec has rated the 4.8V torque of the servo at 69 oz-in which is about where I would agree it ceases to operate in an acceptable manner.
At 6V however, the servo still quite happily pulled up the weight, achieving a deflection of -13.3 degrees from the zero-load position. It was sounding strained, but was still going strong. There was noticeable warmth in the servo, but at this point I had been beating on it for over 15 minutes, and the servo was still not what I would call dangerously warm. I was very impressed. While pilots seem to be obsessed with having the most torque in the world, I have to admit that seeing this standard sized servo pull up 4 massive nuts with confidence truly led me to believe that some people tend to err on the side of excessive torque that they feel they need in their aircraft. Based on the performance of this servo, I would not hesitate to use it in a good sized aerobatic plane (Hitec recommends the servo for aerobatic sport aircraft up to 78” in span). It will easily make a great aileron or flap servo on our tug and will be more than overkill on the rudder or elevator.
@910617:At 77 oz-in of output torque, the servo no longer operates at 4.8 V, but at 6.0 V the servo is still going strong. Note that the "droop" of the servo horn position is significantly more pronounced, however the total error on a 60 degree servo horn motion is only 22% of the total motion. What this means in practical terms is that even if you are in a terminal dive and loading the servo to the maximum, you will still get almost 80% of the zero load control surface deflection. This means you still have an acceptable amount of control.
!!Absolute maximum ratings
One thing that impressed me about the HS-6635 HB was the massive holding torque. Even though the servo sounded strained to move the 77 oz-in load at 6.0 V, once it reached the steady state position, I could still give it a healthy twist with my hand before it totally let go of the intended position. This test shows the massive holding torque that digital servos have over the older analog style. At this fully loaded position, the servo drew 1280 mA at 4.8V and 1530 mA at 6.0 V. This indicates a need for a powerful receiver battery as several digital servos can draw down a reasonable amount of power in a short time if they are worked hard. In practice, the servos do not all draw this high current all at once and it is up to the end user to pick an RX battery capacity that is sufficient for their use. If in doubt, put too much battery in the plane. Our tug uses a 5 cell, 2400 mah sub-C nicad to drive all eight servos (7 of which are the HS-6635HB's). We will carefully monitor the RX battery voltage over the first several flying sessions to get an idea of the average current draw of our tug.
Finally, it should be noted that all the deflection values reported in this review are not simply the positional error in the servo, but include other factors like the small deflection of the servo mounting frame as well as the flex of the servo case, horn and output shaft. Though it is difficult to zero in on the positional error only, the tests done above are more applicable to the real world, since factors such as case and shaft deflection are deformations that will be seen in regular use.
!Performance in the real world
Testing a servo on the bench is all fine and well, but it does not replicate many of the factors that a servo encounters in real flight. Things like the vibration of an engine and impact loading of ground strikes are hard to replicate. For this reason, there is still nothing like putting the servo through its paces in the air.
We installed the Hitec servos in the tug and flew it several times. The tow pilot, Adam Till, was extremely impressed with the performance of the servos on our heavily overpowered tug. Even under the extreme vibration and loads sustained in flight, never did Adam feel that the servos were not up to the task. They were noticeably better than the JR DS-811's that we were using before in the tug. The HS-6635's centered far better, while being significantly faster and more precise. Granted, the DS-811 is not direct competition for the Hitec HS-6635HB, but we were impressed by the improvement in performance that we gained from swapping out the servos. Based on the numerous flights we have had to date, we expect the Hitec HS-6635HB's to perform flawlessly for several years to come.
@910618:The HS-6635HB installed on the aileron of our Ultra Stik Lite. Note the strong 4-40 linkage.
@910619:The HS-6635HB is a standard size servo and hence is pretty easy to fit almost anywhere on a larger airplane.
@910620:The HS-6635HB installed on the rudder of our Ultra Stik Lite. Even when dealing with heavy vibration and massive prop-wash over the control surfaces, the HS-6635HB holds strong, with the majority of the control surface vibration coming from the structure of the surface itself.
!Conclusion
Hitec really has a winner on their hands here. The HS-6635HB is absolutely smooth, precise, and very powerful. There really is not anything more that can be asked of a servo that has a street value of a little over forty dollars. With its strong gears and quality construction, the HS-6635HB is a giant leap ahead of any standard servo. While it does not have the torque of some of the truly monstrous servos out there, it is perfectly suited to larger aerobatic airplanes in the 55-78" range as well as a perfect servo for large scale gliders. It is also ideally suited to any application where a more precise servo than a standard servo is required. I would not hesitate to use these servos in any of my planes and I have a sneaky feeling I will have a whole bunch of the HS-6635HB's in my fleet soon. From a dollars to performance perspective, they simply can't be beat.
| spec2
| @910603
|> <b>Description:</b> |< Standard Sized, high torque, digital servo
|> <b>Motor Type:</b> |< 3 Pole
|> <b>Bearing Type:</b> |< Dual Ball Bearing
|> <b>Gear Type:</b> |< Karbonite (Reinforced Resin)
|> <b>Size:</b> |< 1.6" x 0.8" x 1.5" (40mm x 20mm x 37mm)
|> <b>Weight:</b> |< 1.83 oz (52g)
|> <b>Torque (4.8v/6.0v):</b> |< 69/83 oz-in
|> <b>Speed (4.8v/6.0v):</b> |< 0.18/0.15 sec.
|> <b>Manufacturer:</b> |< <a href=http://www.hitecrcd.com>Hitec</a>
Servos are one of the most important parts of the Radio Control "chain" between the stick under a pilot's thumb and the safety of the airplane he or she is flying. Over the years, I have used all sizes and shapes of servos from little sub-micro servos all the way up to big sailwinches that took 5 seconds to turn 180 degrees. I can't say that in all my years in the hobby that I have had no problems with the servos I used.
About 5 years ago, I got into flying high performance gliders and one of the first things that struck me was the sheer amount of money that sailplane pilots spent on their servos. It was not unrealistic to spend over $100CAD (about $80USD) per servo. I started flying a fully molded RnR Genesis model with some extremely cheap servos and I quickly learned that there actually was a difference between different brands and models of servos. The quickest things I learned were that servo centering is important, that a servo needs to be rugged, and that it needs to be extremely reliable.
Fast forward to today and I'm still at it in this wonderful hobby. I have been enjoying aerotowing and the latest plane in the fleet is an extremely overpowered Hangar 9 Ultra Stik Lite. There is a Brison 2.4 ci engine hanging off the nose of the heavily reinforced tug and I was very worried about how to safely control this plane in such a high vibration, brutal environment. With an engine that is 50% larger than the maximum recommended engine size, there had to be no compromises in the department of controlling this beast of an airplane.
About a half year before the tug project started, I read about a new type of gear that Hitec was putting in some of their servos. They were called Karbonite gears and were made of a reinforced plastic. They were advertised to have the low backlash of nylon gears while having the strength of metal gears. It seemed like a good concept, and with my background in mechanical engineering, I had no reason not to trust a well chosen plastic to perform as well as metal in a servo gear application.
I contacted Hitec to tell them about the towplane project and they quickly suggested their new HS-6635 HB servo. This servo is built in a standard size case and is sold as one of the lower cost digital servos in the Hitec range. I was interested in the cost of the servo and the seemingly good performance specs, seeing as I needed 6 of them for the flight surfaces on the tug. By keeping the cost down and the performance up, our goal of building an affordable tug would hopefully become a reality. So with that introduction, let's get started with torturing this servo…
!The Servo
The Hitec HS-6635HB servo is built with dual ball bearings supporting the output shaft in a rugged-feeling, standard-sized, servo case. The servo feels very nice, with high quality 60 strand - 22 AWG twisted wire leads and it comes with an assortment of very rugged servo arms. When I first turned the output arm, I was absolutely blown away by the smoothness of the geartrain. It was silky-smooth, with no discernible backlash. So far, I was sold on the Karbonite gears. I took a good look at the specification sheet available <a href=http://www.hitecrcd.com/Servos/spec_sheets/HS6635HB.pdf>here</a> and then decided to get to the testing.
@910606:The Hitec HS-6635HB comes packaged in a handy plastic box that can be used around the workshop for storing all those little bits that tend to create clutter.
@910603:The HS-6635HB is made with a rugged reinforced plastic case and exudes a feeling that it costs much more than it actually does.
@910607:The HS-6635HB comes with a very nice assortment of strong servo horns. They are thick and very stiff...perfect for those large & heavy control surfaces on sport aerobatic airplanes.
@910608:The servo lead that comes with the HS-6635HB is very nice. The wires are 60 strand 22 AWG copper wire with a high quality, very flexible jacket. This lead is more than up to the task of carrying the additional current required by the digital amplifier.
@910609:This is where the HS-6635HB is unique. It uses Karbonite gears which are a reinforced plastic. This gives nearly the strength of metal gears with the smoothness and near-zero backlash of nylon gears. It truly is a perfect choice of material for a servo gear application.
@910610:When you remove the bottom cover, the digital amplifier and motor are visible. The motor is just a plain 3 pole motor, but its performance is very impressive. Note the use of silicone to prevent vibration from causing the motor leads to fail...a nice touch.
!Testing
Since I have never done a servo review before, I decided to take some time to devise some good lab tests for the servo so that I could compare it to other servos in future articles. I wanted to get a quantifiable number for those things that the "pros" seem to always gush over. I wanted to prove things like good centering, powerful holding torque, and get an idea of the current drawn when the servo is pushed to the max.
I started by building a rugged frame to hold the servo securely on the edge of my workbench. Then I proceeded to use my Multiplex Profi 4000 transmitter and a Hitec SuperSlim receiver to drive the servo. I hooked up the receiver to a regulated lab-grade power supply so as to have exactly 4.8 volts (or 6.0 volts as the test dictated) at the servo regardless of the current drawn. I also hooked up a pair of Fluke digital multimeters to get an idea of the current drawn by the servo at the various phases of operation and to ensure that the voltage at the servo was remaining constant. Finally, I used a Mitutoyo dial indicator to be able to measure the repeatability of the servo horn position. This indicator has 0.001” resolution which proved to be more than adequate to see the variation in servos horn position.
@910611:This mess actually does something. The multimeters are used to measure the current and voltage supplied to the servo. The dial indicator allows me to measure the position of the servo horn.
@910612:Here you can see the frame made to hold the servo in place. The dial indicator is pushed by the servo arm.
@910613:This is the amount of weight to get 58 oz-in of torque at the servo arm. It is impressive to think that all that load is being held by a plastic output spline. The HS-6635HB happily lifted these giant nuts and more. See below for more details.
!Test Results
!!Centering Test - No load
This test involved operating the servo with no load. Using a switch input on my transmitter, I cycled the servo from center to one extreme and back to center 20 times and recorded the position of the servo arm after each cycle. It was impressive to see that under no load the servo continually came back to a position within 0.1 degrees of the median position, with only two trials out of 20 having a maximum deviation of 0.3 degrees. Even in this worst case scenario, the total error works out to less than +/-0.005” positional error on the servo horn hole furthest out from the servo pivot. That is pretty good repeatability/centering. Under no load, the servo drew between 4 and 5 mA.
@910614:This graph shows the servo's ability to return to the same home position under no load. 20 trials were performed and the angular deviation from the median position is shown on the vertical axis. As can be seen, this is a very accurate servo!
!!Centering Test - 40 oz-in of torque
I took enough weight and hung it off the servo horn to cause the servo to put out 40 oz-in of torque. Note that by hanging the weight off a single arm, the worst case installation for the servo is being reproduced. This setup puts the maximum load on the output shaft and loads the servo case quite heavily. The only worse way to mount the servo is to have the short dimension of the servo case parallel with the applied load, but this is rarely seen in actual practice. In many cases, it's best to use push-pull linkages if possible, but I wanted to recreate the worst case (and realistically one of the most common) scenarios for servo loading.
With the application of 40 oz-in of torque, it is readily apparent that the servo does not reach the same position as it does under no load. At 4.8V, the servo had a median deflection from the median zero-load position of -4.7 degrees. While this may seem like a lot, it still represents only about 7.8% of the total servo travel, meaning that the control surface deflection is still a very large portion of what the pilot is asking for. At 4.8V under 40 oz-in of torque, the servo was drawing 222 mA.
@910615:This graph shows the "droop" that the servo experiences when putting out 40 oz-in of torque. Note that the deflection from the zero load position is only a few degrees and that it's less at 6.0v than it is at 4.8v...a good reason to use 5 cell RX packs if you can.
It was interesting to note that at higher voltage, the performance of the servo was markedly improved. The angular deflection at 40 oz-in decreased to approximately -4.0 degrees and the servo only drew about 8% more current (239 mA). This makes a good case for using 5 cell receiver packs if you have the option and your servos are rated for it.
!!Centering Test - 58 oz-in of torque
As expected, as the load increased on the servo, the deflection from the zero-load center position increased. At 4.8V, the median deflection was -10.5 degrees and at 6V, it was -6.8 degrees. The currents drawn at 4.8V and 6V were 438mA and 460 mA respectively.
It is very apparent that as the servo is loaded more heavily, the use of 5 cells becomes even more valuable. While the current draw does increase at higher voltage, the price of extra current seems to be a small cost compared to the benefit of far better servo torque, speed and centering.
@910616:At 58 oz-in of torque, the "droop" of the servo arm position is more pronounced, as expected. The overall droop though is still pretty reasonable considering the load, and the benefit of the 5 cell receiver pack is more pronounced.
!!Centering Test - 77 oz-in of torque
At 77 oz-in of torque, it was apparent that the servo could no longer reasonably move the load at 4.8V drive voltage. While it still moved, it was probably only achieving about 50% of the motion that was requested of it. Luckily, Hitec has rated the 4.8V torque of the servo at 69 oz-in which is about where I would agree it ceases to operate in an acceptable manner.
At 6V however, the servo still quite happily pulled up the weight, achieving a deflection of -13.3 degrees from the zero-load position. It was sounding strained, but was still going strong. There was noticeable warmth in the servo, but at this point I had been beating on it for over 15 minutes, and the servo was still not what I would call dangerously warm. I was very impressed. While pilots seem to be obsessed with having the most torque in the world, I have to admit that seeing this standard sized servo pull up 4 massive nuts with confidence truly led me to believe that some people tend to err on the side of excessive torque that they feel they need in their aircraft. Based on the performance of this servo, I would not hesitate to use it in a good sized aerobatic plane (Hitec recommends the servo for aerobatic sport aircraft up to 78” in span). It will easily make a great aileron or flap servo on our tug and will be more than overkill on the rudder or elevator.
@910617:At 77 oz-in of output torque, the servo no longer operates at 4.8 V, but at 6.0 V the servo is still going strong. Note that the "droop" of the servo horn position is significantly more pronounced, however the total error on a 60 degree servo horn motion is only 22% of the total motion. What this means in practical terms is that even if you are in a terminal dive and loading the servo to the maximum, you will still get almost 80% of the zero load control surface deflection. This means you still have an acceptable amount of control.
!!Absolute maximum ratings
One thing that impressed me about the HS-6635 HB was the massive holding torque. Even though the servo sounded strained to move the 77 oz-in load at 6.0 V, once it reached the steady state position, I could still give it a healthy twist with my hand before it totally let go of the intended position. This test shows the massive holding torque that digital servos have over the older analog style. At this fully loaded position, the servo drew 1280 mA at 4.8V and 1530 mA at 6.0 V. This indicates a need for a powerful receiver battery as several digital servos can draw down a reasonable amount of power in a short time if they are worked hard. In practice, the servos do not all draw this high current all at once and it is up to the end user to pick an RX battery capacity that is sufficient for their use. If in doubt, put too much battery in the plane. Our tug uses a 5 cell, 2400 mah sub-C nicad to drive all eight servos (7 of which are the HS-6635HB's). We will carefully monitor the RX battery voltage over the first several flying sessions to get an idea of the average current draw of our tug.
Finally, it should be noted that all the deflection values reported in this review are not simply the positional error in the servo, but include other factors like the small deflection of the servo mounting frame as well as the flex of the servo case, horn and output shaft. Though it is difficult to zero in on the positional error only, the tests done above are more applicable to the real world, since factors such as case and shaft deflection are deformations that will be seen in regular use.
!Performance in the real world
Testing a servo on the bench is all fine and well, but it does not replicate many of the factors that a servo encounters in real flight. Things like the vibration of an engine and impact loading of ground strikes are hard to replicate. For this reason, there is still nothing like putting the servo through its paces in the air.
We installed the Hitec servos in the tug and flew it several times. The tow pilot, Adam Till, was extremely impressed with the performance of the servos on our heavily overpowered tug. Even under the extreme vibration and loads sustained in flight, never did Adam feel that the servos were not up to the task. They were noticeably better than the JR DS-811's that we were using before in the tug. The HS-6635's centered far better, while being significantly faster and more precise. Granted, the DS-811 is not direct competition for the Hitec HS-6635HB, but we were impressed by the improvement in performance that we gained from swapping out the servos. Based on the numerous flights we have had to date, we expect the Hitec HS-6635HB's to perform flawlessly for several years to come.
@910618:The HS-6635HB installed on the aileron of our Ultra Stik Lite. Note the strong 4-40 linkage.
@910619:The HS-6635HB is a standard size servo and hence is pretty easy to fit almost anywhere on a larger airplane.
@910620:The HS-6635HB installed on the rudder of our Ultra Stik Lite. Even when dealing with heavy vibration and massive prop-wash over the control surfaces, the HS-6635HB holds strong, with the majority of the control surface vibration coming from the structure of the surface itself.
!Conclusion
Hitec really has a winner on their hands here. The HS-6635HB is absolutely smooth, precise, and very powerful. There really is not anything more that can be asked of a servo that has a street value of a little over forty dollars. With its strong gears and quality construction, the HS-6635HB is a giant leap ahead of any standard servo. While it does not have the torque of some of the truly monstrous servos out there, it is perfectly suited to larger aerobatic airplanes in the 55-78" range as well as a perfect servo for large scale gliders. It is also ideally suited to any application where a more precise servo than a standard servo is required. I would not hesitate to use these servos in any of my planes and I have a sneaky feeling I will have a whole bunch of the HS-6635HB's in my fleet soon. From a dollars to performance perspective, they simply can't be beat.