Alpha 2.8 TJIRC (AKA, TJIRC Great Wall 2.8m)

[Konrad]

I’m one of the guys that likes the idea of a low cost S-Bus servos. But with Corona the cost isn’t as cheap as it first appears. Whenever I start to set up a plane with Corona servos, I start with twice as many as I think I’ll need. Those that fail I think of as donors for case and gear sets. So those $10 servo really are $20 servos.

Now that FrSky has again tried to move into the servo market with the F-Bus servo the need for a $20 Corona servo has fallen. This is assuming that FrSky has a better hold on quality than their last set of servos.

 

[Wayne]

Yeah, quality so far seems to be very good, FrSky doesn't make them. Same basic servo as the Hacker and Jetti, but about 50% less money. Just running FrSky firmware and the outer cases seem to be slightly different.

 

[Konrad]

Back to the Alpha 2.8, I’m using 51mm center to center long push rods for the ailerons and 66mm center to center push rod for the flaps.

 

[slopecorn]

oh man the hunt for cheap and reliable servos in between hxt900 and STD size is very taxing.. I think I found the jackpot and then it burns out 5 minutes later LOL

 

[Konrad]

What? In the 80‘s I happily paid $79 for a quality analog servos in the 35gram class. And this was with 1989 dollars!

What are you looking for today? Today any MKS servo is a better value than that Futaba 3002 even at full price!

 

[slopecorn]

What? In the 80‘s I happily paid $79 for quality analog servos in the 21 gram class. And this was with 1989 dollars!

What are you looking for today? Today any MKS servo is a better value than that Futaba 3002 even at full price!

I just want a 10 dollar MKS haha or hitec 85MG. I want HXT 900 reliability (none of the other blue servos, tower pro, etc. don't work as well), silly as it is to say. I never had a dud and they always center and never break or burn out, and I've been through about 200 at least. I can't say that about most servos. Maybe I've just been lucky with them.

 

[Konrad]

85 and HXT 900 being good servos? This must be in a rather benign application like foam fold foamies?

 

[slopecorn]

85 and HXT 900 being good servos? This must be in a rather benign application like foam fold foamies?

Well the other servos didn't do well in the foamies so I don't know what to tell you! Certainly servos designed for less benign applications would be up to the task. The surprising thing is the number of fancier $10-22 servos I've bought that would fail before the $4 HXT-900s.. PowerHD, and those coronas, for example have really been major hit or miss.

85MG's are great servos for composite pitcherons and wingerons and very reliable, couldn't say the same for 81's - but even 85's are more expensive than they need to be these days. All the ones I use are old though, maybe they don't make them like they used to.

 

[Konrad]

Generally I like Hitec servos. But their metal geared servos show excelerated wear rates compared to comparable servos. The plastic idler shaft journals flex and the brass teeth face wear fast as a result. The plastic geared servo holds up ok in application where metal gears aren’t needed ( low shock loads)

 

[Konrad]

Well the other servos didn't do well in the foamies so I don't know what to tell you! Certainly servos designed for less benign applications would be up to the task. The surprising thing is the number of fancier $10-22 servos I've bought that would fail before the $4 HXT-900s.. PowerHD, and those coronas, for example have really been major hit or miss.

85MG's are great servos for composite pitcherons and wingerons and very reliable, couldn't say the same for 81's - but even 85's are more expensive than they need to be these days. All the ones I use are old though, maybe they don't make them like they used to.

This concerns me a bit, reading PowerHD and Corona in the same sentence. Can you please name the servo model number and time frame you have had these issue with the PowerHD servo ?

I have used a plethora of PowerHD servos. I don’t recall any issues that stand out with the 9 gram and smaller analog servos. I did notice noise but twisting/braiding the servo leads solved those issues. (All servo leads should be twisted. Flat ribbon leads are inappropriate in an RF environment). I do like the small digital servos, great performance.

I love the mid price Storm7!

The mini HD1711MG is what I think of as being in the same class as the HS85. I use this analog servo in a lot of my down scale ships like the larger Fly Fly gliders. The HD1711MG strong points are that it is in the same power class 3.5Kg/cm. It is a moderate speed servo (11ms) and it has replacement gears that are dirt cheap. It has the same limitation as the HS85 in that it is a plastic cased servo with only one output bearing and has an analog amp. The big hit against the HS85 is that it is slow at 14ms@6v and heavy 22grams.

Now a great value servo in this weight class in the EMax ES90MD with her dual ball bearing supported output shaft and good speed. I use this digital servo in all the places I used the analog HS65mg.

I think a lot of the servos mentioned are obsolete, in that they are low voltage (less than 7.4V) and have analog amps.

 

[Wayne]

PowerHD use to make a version of the HS-85 that had a great price. We sold a lot of them. They where solid and reliable. Sadly they went away as they could no longer source the motors from Taiwan. (Or some similar reason.) Think those servos were under $15.

 

[slopecorn]

interestingly enough I don't see the model of powerhd servo I had a problem with on the website. I should be fair and say that I was using them in a mad stab application and it seemed like they burned out or didn't have the torque, but when I used an 85, worked great. Could be a case of analog vs. digital. I had the same thing happen with a slightly bigger corona servo. Other than that I had delivered a flying wing to my friend dominic with two of the 4.4g or I think it might have been 6g powerhd ~$10 which had really smooth operation, but just didn't work at all the next time he turned it on. I'm not even bashing these servos I'm just looking for the 'honda civic' or hxt-900 variant for every size

 

[Wayne]

In general I like the PowerHD brand. They have a good line of products (maybe a few too many offerings) but we generally do not have troubles with them. They strike a good balance of price and performance. Similar to Emax. Not going to set the world on fire, but also not going to drain your bank account.

 

[Konrad]

Fair enough. From my perspective the Civic and HXT can best be called entry level, not Yugos but nothing to write home about.

 

[Konrad]

PowerHD use to make a version of the HS-85 that had a great price. We sold a lot of them. They where solid and reliable. Sadly they went away as they could no longer source the motors from Taiwan. (Or some similar reason.) Think those servos were under $15.

Was that the 1711? Have they dropped the model or just can't supply it at this time?

 

[Wayne]

Yep, that is the one. Could not recall the number. (brain a bit fried from the flu earlier this week)

They told me discontinued. They suggested some other servos, but they didn't fit the niche IMHO. I encouraged them to look at offering a similar price point HV digital. (They probably laughed before closing my email.)

 

[Konrad]

I think you hit the nail on the head!

Unless I’m slumming it, I want my new 21st century servos to be HV (7.4v or higher) and the amp be digital. I see no need for the 4.8v analog servo!

Now in this Alpha 2.8 I’m using the new old stock KST 125 and 135 in the wing. But I did add a voltage regulator in the wing wiring harness To drop down the 2@18650 8.4V to a regulated 5.1V. I needed to do this to run the 125 and 135 on 2 lipos but I also wanted the HV performance for the two X0-8 in the tail.

So there might be some reason to my madness for using the KST 125 and 135 in a new build.

 

[Konrad]

Other than having to make a 3mm high jelly bean flap servo cover I think I'm done with the build of this Great Wall (Alpha 2.8).

The last thing to do was install the wing servos. Now as counter intuitive as it seems I don't try to place the control surface in profile with the wing when the servo is centered. Instead I calculate the total amount of throw a surface needs divide by 2 and use this value to set the control surface when the servo is centered. In the case of this Alpha 2.8 my flaps could move 22° up. Total flap movement was 112° (AKA 90+22=112). So divide by 2 means that I want the surface at 56° with the servo centered. This means that at servo center the flaps should be down 34° using the in profile as zero for the 34° measurement.

For the ailerons I want a 2:1 differntial. Total aileron up was 40° so I would want 20° down aileron. Center would be 10° up aileron.

I took the easy way out and just used what the control wipers would allow. I could have fitted the wipers to get more throw but The numbers as is looked ok to me.

What I like about using the throw center to set the servo center is that if I have to change servo arms the effect will be close to the same at each end of the servo throw.

Now the sharp eyed amongst you will have noticed that my flap servo is not centered well in the flap servo opening. This is a habit I seem to have in that I I almost always choose too short a push rod for my flaps. I do this as I don't want to crash into the wing spar. With the Alpha 2.8 and the Servorahmen IDS the next longer pushrod at 66mm would have been a better fit.

Now I need to give a shout out to OpenTX and Mike Shellim F3F templet. With Mike's templet it is real easy to adjust for any mechanical offset errors in the placement of the control surface spoons and servo arms. This addresses any unwanted servo throw differential or cosine errors. It also allows one to custom match the surface throws on each side. That is the left and right flaps move the same and the same amount. Same goes for the ailerons. This goes along way towards making a well behaved aircraft!

So with the build done all I have to do is balance the model repair the cosmetic damage (paint the black on the fuselage and wing bottom) and set up Mike's templet. Still not sure what I'm going to do with the white wing panel.

 

[Doc James Hammond]

Other than having to make a 3mm high jelly bean flap servo cover I think I'm done with the build of this Great Wall (Alpha 2.8).

The last thing to do was install the wing servos. Now as counter intuitive as it seems I don't try to place the control surface in profile with the wing when the servo is centered. Instead I calculate the total amount of throw a surface needs divide by 2 and use this value to set the control surface when the servo is centered. In the case of this Alpha 2.8 my flaps could move 22° up. Total flap movement was 112° (AKA 90+22=112). So divide by 2 means that I want the surface at 56° with the servo centered. This means that at servo center the flaps should be down 34° using the in profile as zero for the 34° measurement.

For the ailerons I want a 2:1 differntial. Total aileron up was 40° so I would want 20° down aileron. Center would be 10° up aileron.

I took the easy way out and just used what the control wipers would allow. I could have fitted the wipers to get more throw but The numbers as is looked ok to me.

What I like about using the throw center to set the servo center is that if I have to change servo arms the effect will be close to the same at each end of the servo throw.

Now the sharp eyed amongst you will have noticed that my flap servo is not centered well in the flap servo opening. This is a habit I seem to have in that I I almost always choose too short a push rod for my flaps. I do this as I don't want to crash into the wing spar. With the Alpha 2.8 and the Servorahmen IDS the next longer pushrod at 66mm would have been a better fit.

Now I need to give a shout out to OpenTX and Mike Shelium's F3F templet. With Mike's templet it is real easy to adjust for any mechanical offset errors in the placement of the control surface spoons and servo arms. This addresses any unwanted servo throw differential or cosine errors. It also allows one to custom match the surface throws on each side. That is the left and right flaps move the same and the same amount. Same goes for the ailerons. This goes along way towards making a well behaved aircraft!

So with the build done all I have to do is balance the model repair the cosmetic damage (paint the black on the fuselage and wing bottom) and set up Mike's templet. Still not sure what I'm going to do with the white wing panel.

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Konrad...Mike Shelium? I'm still giggling - I'm sure even Mike would find that one funny.

He's bloke, not a feaking rare earth...you, my friend, are going downstairs to the hot place for that one.

Funnier than a barrel of monkeys.

Doc.

 

[Konrad]

Ok, I took care of that issue. As to going to hell that's a given!

All the best,
Konrad