mdjohnson

Hello All

Just wondering if anyone is using Hitec HS7966 servos in their 2m pattern machine? Here's the link http://www.hitecrcd.com/servos/show?name=HS-7966HB. It looks like a good one to me but I would like some opinions on it.

Cheers

MJ

vbortone

My Feedback: (2)

MJ,

Do you get K-Factor? Check the equipment used by all pilots atending the Nats in the last few years. You can get your own conclusions reading that information.

Good luck,

VB

Jetdesign

My Feedback: (8)

That can also be viewed here:
http://nsrca.us/documents/nats/2008_...tion_Sheet.pdf

Gungadin

The Hitec 7900 series servo's were not available back in 2008 so they would not have been used by anyone at the 2008 Nats.

Ryan Smith

My Feedback: (25)

You can see how many people used Hitec equipment in their airplanes, though.

tIANci

Someone told me the Hitec servos are not as precise as the Futaba ones. Anyone else with such experience?

highfly3D

If you see the link.. it says it all no one uses Hitech servos except 1 or 2 of the total pilots so i wont recommend these servos as they are good for Giant scale applications not for precession aerobatics like F3A. There is no cheap way to compromise on servos selections when it comes to F3A.

http://nsrca.us/documents/nats/2008_...tion_Sheet.pdf

petec

My Feedback: (58)

Speaking from experience, the old Hitec analogs and some of the digitals did not center well and you would wind up chasing the plane around instead of concentrating on the task at hand. Now having said that, some of the newer digitals are much better and actully do function well. You certainly get what you pay for in the case of servos it would seem.

If those servos are what you can get then don't let that stop you from coming out and playing. You can always get better servos down the road if they are less than pecise. By the time you are getting ready for the Masters Finals you'll have a preference anyway.

Jeff Boyd 2

I use JR *SA's in my pattern planes and Hitec 7955TG's in my IMAC plane. The JR digitals have been super reliable and very smooth and accurate. The Hitecs are a bit "steppy" when compared, and the aaccuracy is not quite there.

Have a friend flying F3A who did use Hitec in one Pattern plane. They worked OK, but he didn't use them in subsequent planes . . although, I would never beat this guy no matter what servos he used !

Never had an issue with JR servos in Pattern . . Had a Hitec "burn out" in a IMAC plane (not many flights on it either). I have now seen 3 Hitecs "melt" in IMAC planes in various configurations (being used by top pilots too) . . never seen this with JR or Futaba in IMAC.

Have replaced Hitec with JR DS6301 now, so see how that goes

JB

teookie

Aren't most of the top pilots sponsored by futaba or JR and thusly wouldn't use hitec servo's??

klhoard

My Feedback: (10)

Correct. . . and a couple years ago nobody would touch AirTronics either . . .
.
.
Amazing what some free stuff and a "sponsorship" will do to the "quality" of somebody's equipment!!
.
.

Jeff Boyd 2

True . . but surely not ALL the entry list are "top pilots" . . or sponsored. I am thinking that only a select few get "free" stuff, otherwise who are the "top guys" and the companies promoting to. Companies can't sustain a business giving a LOT of free stuff away . . they are usually very selective with how and where their profit is spent on promotion. Even at the World Champs, a lot of pilots pay for their own gear and these guys are usually the best from their country.

I am sure that many have tried different servos and keep coming back to JR or Futaba for pattern. Apparently, Savox make some nice servos these days that some Chinese, Taiwanese and Japanese pilots are using in Pattern, with a couple having swapped from JR, so I read. They only swapped due to cost, commenting that the performance was EQUAL to JR (others may have swapped from Futaba???).

I use both JR and Hitec Digitals for different planes. I won't use Hitec servos in a Pattern plane, and I pay for my gear . .

JB

Jeff Boyd 2

The Savox thread I was thinking of is here . . http://www.rcuniverse.com/forum/fb.asp?m=9124648

Cheers, JB

mdjohnson

Hello All

To those who have added opinions, thank you very much. I have been reseaching this a bit and have found that there is indeed a compromise in speed vs resolution. JR claims that their 8411 has 5900 steps but a transit speed at 6V of .22 sec. This is in direct contrast to the Hitec 7966 claim of .08 sec @6V. They however have a larger step tothem which woud make sense as the quicker you movethe servo it has totake larger "steps" for the desired travel. I have read that the quicker the servo, the coarser the stepping thus coarser resolution. The slower the servo, the finer the resolution and thus more "step" accuracy. This backs up the majority of opinions here and confirms the choice of many on the NSRCA equipement list. It is my understanding therefore that in pattern the resolution of the servo is more desireable than the speed as accuracy is tantemount and in 3D type flying the speed of the servo would be more desirable as radical and very quick control movements are called for.

Is this sound thinking? Anybody else?

Cheers

MJ

woodie

Speed of the servo has nothing to do with 'accuracy' from an electronic viewpoint. The servo may resolve the receiver signal into x number of steps which might affect the rotational degrees of a 'step', however, the accuracy is determined by the 'dead band' allowed by the servo. The 'dead band' is how close does the servo make the signal from the pot match the signal from the receiver. The broader the dead band, the bigger the window of rotation that the servo considers to be 'correct'. This accuracy is for the servo only, it ignores and linkage or mechnical slop.

Speed is a matter of gear ratio, voltage and motor strength combinations in the servo. You can take a servo at 4.8V and it will have a certain time frame for x degrees of rotation and have a certain amount of torque. Up the voltage and the time to rotate that same x degrees decreases, the torque probably increases, but the accuracy remains the same.

The tighter the dead band, the more the servo works to find 'center', and the closer to center it will return each time. When you hear a servo 'buzzing', that is the servo trying to find center. The weight of a control surface, drag in the linkage, etc, may inhibit the servo ability to land on 'zero' and stay there. It is easier for a servo manufacturer to build a servo with broader deadband, the components can have a little more tolerance, etc.

Woodie

cmoulder

One thing to keep in mind is that the difference might not be as stark as the above scenario suggests.

What is not being taken into account is servo arm length and control surface throws. A pattern plane control surface has a much smaller range of travel, and at the same time uses all (ideally!) of the servo throw available. Therefore the differences in steps are very minimal indeed.

I am using a pair of HS-5955TGs for elevator and rudder in a 2M plane, simply because I had them. I've got only 8 flights on the plane, but using the full servo range available (minus a small cushion for tweaking) they seem pretty precise to me. No shortage of torque! Using a pair of Futaba 9551s on ailerons, and they are excellent as well.

My concern with the Hitec servos you mentioned above is the"HB" designation for Karbonite gears. Early on, I had several gear failures with HB-geared micro servos used in small electrics, where the gears shattered. I don't know if this is an issue with the HB gear trains with the standard-sized servos - or whether Hitec has "reformulated" the Karbonite material - but in any event I personally wouldn't use them in a glow or gas pattern plane, that's for sure. However, not having actually used the servos in question, I stand to be corrected on that opinion!

mdjohnson

Hi Woodie

I am a bit confused by what you say here. If I read you correctly then servo accuracy is tied to the deadband width and the effect of the number of "steps" is relatively inconsequential.My thinking was that the more "steps" there are in the rotational travel of a servo, the more accuracy is attained. This of course is not taking into consideration the mechanical link to the flying surface. Don't get me wrong, I am really leaning toward the JR8411 for my coming 2M but just for my ownedification I would like tounderstand the reasoning of why people are/are not using a certain servo. In the JR literature they state(for the 8411) " increased resolution of 5900 steps per 120 degrees and a less than 1 msdeadband for extreme accuracy". Is the number of steps quoted just another gimmicky selling point ordoes it havemerit when they include it in their information? In practical application is this accuracy noticable? On the Hitec websiteI could not get a number for the steps involved but they give a 1500 usec number for theoperating window. It does state however thattheir deadband is 1ms (this is for the 7966). Both servos are operating at 250hz. I am also a little confused as to which is the best gear train in a servo. Some people say the metal gear train inherently has too much "slop" but the preferred JR 8411 has just that. All a compromise I suppose. Maybe the difference of the metal gear train is inthe quality of a higher priced servo (JR) as opposed to a middle cost servo (Hitec) giving credence to the fellow that said "you get what you pay for". I am just 7 months into the hobby again after a hiatus from it for 15 years. I did compete in pattern almost 25 years ago and amdetermined to get back into itbut everything has obviously changed. Back then there were no computers!! Now even the little servos in our airplanes have them!! At that time the concensus was that electric flight was going nowhere as they were just barely getting off the ground hauling around 12 or 16 nicad cells. Now it seems everything is going electric. The new 2m will be just that. Learning curve is still very steep. There are 'way more choices to make now than there were back then and I would like to wind up with a machine I will be happy with (for the time being). Now to get some more stick time!!

Thanks again for the input.

Cheers

MJ

woodie

MJ, let me explain it a little differently. First, the same servo at 4.8V and then at 6V. The number of 'steps' is the same at both voltages, the gear train and deadband are are also the same. The servo resolution (ability) to come back to the same spot will not change with voltage, only the speed and torque change. Increasing the number of steps will theoretically improve accuracy but only as long as the deadband 'window' is able to resolve pot position to less than or equal to 1 step. Given gear train slop, measuring accuracy of the pots, vibration, etc, we are getting less and less return for increasing resolution. Add to that the linkage slop, and binding or resistance to moving (hinge alignment, drag on linkage, etc) increased resolution doesn't seem to gain much realized advantage.

Now, on the positive side, consider we rotate the servo thru 60+ degrees to get 15-20 degrees of surface deflection in typical pattern planes. This is actually a benefit to accuracy since any error or variation in the centering point is reduced by 50-70%. This becomes more obvious as the deflection at the surface is increased for the same rotation of the servo. Very high deflections give up all sorts of mechanical advantage and amplify any loseness or centering errors.

Woodie

mdjohnson

Thanks Woodie

It is clearer to me now. I was looking at the hitec 7966 and it did seem to be a good servo for this application. The specs are right up there with the best. It operates on 250hz pulse sampling and has a 1 ms deadband. It has dual ball bearing output and 111 oz.in torque at 6V and a transit time of a blistering .08 sec for 60 deg of travel. These are only specs mind you and first hand knowledge would be helpful. The only questionable thing is the karbonite gear train. One fellow has had problems on a smaller hitec  servo with karbonite gears stripping. I definately don't want that to happen. With a saving over the JR8411 of $45 cdn each I thought they might be worth looking at for a 2m machine. It would be nice if I could get an opinion from someone that has had some experience with them.

Thanks for taking the time to explain a couple of things!! Much appreciated.

Cheers

MJ

bjr_93tz

Hi mdJohnson, the deadband for that servo is 1 micro second not 1 millisecond.

A servo that can resolve more than 2048 steps is of no use as the best PCM radio's (including 2.4GHz) input signal to the servo is resolved to no more than 2048 steps. My nasty old 2 channel AM set has better resolution and latency (but this brings larger problems ).

I have heard second-hand that some hitech servos, despite the manufactures spec sheet claims, tend to "step" their travel and this may be more to do with mechanical constraints rather than electrictronic. Most cored servos tend to use a 3-pole motor, some hi-end ones used 5-pole motors. One dodgey old servo I had a 3 pole motor and every 120deg of motor position was 0.7deg of arm movement so the most steps the servo was capable of resolving mechanically under load was about 128 steps for +-45 degrees. Analogue controller or Digital that servo mechanically can't do any better than that.

That Hitech servo you mention is coreless however corless motors still have a pole count but the number of poles is not listed. Pole count and gearbox ratios set the mechanical constraints of the servos performance from an accuracy and resolution perspective but these are very rarely given. The digital controller might be able to resolve the input pulse and feedback pot to some level and provide control outputs to the motor, but if the motor is sitting on a "dead-spot" then it isn't going to move until the control signal (read position error) gets big enough.

Where does this leave us mere end users? Until the manufactures publish real performance data it's up to us to spend out money, try it out for ourselves and tell our mates what seems to work well and what doesn't

Jetdesign

My Feedback: (8)

I just learned in my EE class that 'deadband' is the amount of time between signal pulses - pulses that tell the servo motor what to do, including stay at the center point. The servo motor turns based on the characteristics of the pulse, which is not a function of the deadband. So a servo with a narrower deadband would be more constantly told to center, while a servo with a wider deadband would allow more time between centering commands.

Hope this makes sense. I'm pretty tired.

bjr_93tz

gaRCfield, be careful here, there are a number of systems within the servo model which exhibit "deadband" by definition.

Yes, deadband could be refered to as the time between pulses from the receiver AND the slop in the gearbox BUT in the context used in this thread it refers to the amount of change in the input signal (or error in output position) before the controller will provide a control signal to the motor to get things straight again. The word hysterysis could almost be used but is slightly different again.

With a 5 microsecond "deadband" the signal from the RX would need to deviate 5 microseconds before the controller would try to reposition the servo, 5 microseconds usually being about 1 click of trim depending on the radio. This allows the signal and servo to fluctuate a bit without the servo motor being given a drive signal.

Narrow deadband in this context is not how often the servo is being told to get with the program but refers to how far off track it can get before it gets a kick in the rear

vbortone

My Feedback: (2)

mdjohnson,

Do you have the Budd's laser tool? If you have it just get both servos and try how close the servos go back to center. That red dot has to stop in the same place every time. You will need to set up equivalent mechanical advantage for both servos and radio set up has to be exactly the same. For example ailerons. The second check if you have time and money fly the same plane with both brands and check how easy or difficult is to trim the plane. You will be surprised that the laser test confirms the trimming test. Please don't ask me how I know.

Good luck,

Vicente "Vince" Bortone

Jetdesign

My Feedback: (8)

Just for the record, I have the HiTec 79XX servos in my 120 Yak. I'm just doing some spring maintenance and adjusting the tension in the pull-pull system. As I am doing this, I am noticing that the rudder and the elevators are twitching about a full click's worth of travel as the servos work on finding 'center'.

I would not put this crap in my pattern plane, that's for sure[:@]

Jeff Boyd 2

I use JR 8411's BUT the "SA" version (DS8411SA). I believe it stands for "Special Application"?? These servos have a nylon gear train and I think this range was introduced for Pattern. I understand they are no longer available, however the Nylon gears from similar JR servos fit.

I don't find the standard 8411's sloppy when new, but they do wear faster, particularly with the vibration of Glow power. With EP, I expect they would be fine, although they are heavier .

I use the Hitec "TG" servos on IMAC as the 'Titanium Gears' give very good life with the vibration and load of a 35%'er. JR's new range of high torque servos have a "special" metal gear train . . so I am keen to see how these perform over time, but they are VERY nice servos although nearly twice the price of Hitec.

JB