Servos for large mouldies

[Edgeflyer]

I am currently renovating 3 moulded gliders. An old Dogan. a Pike Superior and an XModels Xcalibur.

I am wondering what servos would be suitable for ailerons and flaps. The Pike had elderly Hitec hs81mgs which seem a bit small to me for large surfaces. I plan to replace them with hs125 thin wing servos. The Dogan has hs82mgs in custom mounts. 

What do people with experience of this type of model suggest? I want to utilize crow braking and possibly variable camber.

[Frank Skilbeck]

Variable camber will put very little load on the servos as you are only moving the surface slightly, crow braking could put a large load on the flap servos if you applied them at high speed, but applying crow braking at high speed isn't normally recommended. There are a few online servo torque calculators, like this one, you'll see how little torque is actually required. No harm n changing the servos but probably the existing servos will be adequate.

 

 

[Nigel Dell]

I agree with Frank, I would concentrate on a ballraced output shaft and very little slop as I have found that to be more of a problem for my OCD! 😂 some servos I have used in the past have had slop the transmitted to about 2-3mm movement on the surface, that was in the early days however but the hitec HS 125’s have been used without issues and more recently I have been using KST’s which have been good.

[MattyB]

HS81s (and 2s or that matter) have no output bearing so IMO are unsuitable for any high performance moulded model - they will go sloppy very quickly. However, I personally don't like the 125s; despite being "MG" they have a sacrificial plastic gear that is easy to strip in even a relatively innocuous landing or ground handling incident. I would go for KST myself, they are not cheap but do work very well and have been consistent performers for me in a range of models.

[Nigel Dell]

I think the choice we have with servos is now pretty comprehensive, the 125 was one of very few available, I think I have been lucky with them, I did not know they were that fragile, I still have a couple of models with them I will be especially careful with them now.

You are always learning in this game.

[Gary Binnie]

My Pike Superior has a different servo on each surface! I think there are some HS-81s in there. Was bought like that.

The larger servos in older models were needed for high power winch launches (car starter motor) to prevent blowback of positive camber used in the first part of the launch.

 

Popular F3J wing servos were the Futaba S3150 and its high volt version, the S3172SV.

 

F3F (slope racing) would need higher power servos and carrying their extra weight would be beneficial.

 

The emphasis now with F5J (electric launch thermal soaring) is light weight. Very popular are the MKS 6100 and 6110, these are 9 gramme servos. No problem at all for normal slope soaring and bungee launches. I have just renovated a Vladimir's Graphite and managed to remove almost 200 grammes, I fitted Hitec HS-55 feather servos to the ailerons, they are double-sided taped to the inside face of the servo cover, no mount.

 

There were some problems with early MKS servos burning out if used a smidge over 4.8 volts, this was fixed long ago.

 

Kingmax is another emerging brand, I use them in small gliders/electric sport models and they are excellent quality and cheap.

 

The key with crow braking is the geometry, using the nearest hole in the flap servo arm provides the maximum torque (holding power), it's usually about 9 or 10 mm from the centre. At full crow the horn and linkage should be in line, there is no load on the gears at that point. Using crow brake might set off a low receiver battery voltage warning in the transmitter if you have the telemetry, it's transient as the servos load up, just increase the alarm limit until it doesn't sound or live with it. 

 

It is more efficient to loose height (descending from a monster thermal) by flying fast instead of using crow brake, that leaves the only place to use crow is on the landing approach where the model is flying slowly (minimal air loads). 

 

The Graphite, centre wing panel has been recovered with the original Oracover (small mortgage required for a 2 metre roll!!) 

 

 

 

Edited September 30, 2022 by Gary Binnie

[MattyB]

1 hour ago, Gary Binnie said:

The key with crow braking is the geometry, using the nearest hole in the flap servo arm provides the maximum torque (holding power), it's usually about 9 or 10 mm from the centre. At full crow the horn and linkage should be in line, there is no load on the gears at that point.

 

Completely agree, just one point for clarification in case @Phil B is not aware...

 

On a 4 servo wing thermal model you don't generally utilise quattro flap (coupled ail to flap mix). This means you can set the servo arm such that one end of the servo travel gives just 1-2mm of flap reflex, and the other end is full down flap for crow with the servo arm as near to in line with the linkage as you can get it (aim to get at least 60 degrees down flap). This will seem like a weird setup geometry for power fliers, but is the default in moulded gliders to ensure the minimum slop and, and maximum holding power and resolution.

 

However, in an F3F/B model where you might need quattro you'll need to set the linkage to allow more upgoing flap movement (if the hinge allows it) whcih for geometrical reasons normally means you can't get the servo arm as close to in line with the linkage. This is not generally an problem though as slope models shouldn't need such extreme down flap angles (and you should be tucking the crow away just before landing anyway).

[Edgeflyer]

Very good advice and suggestions here! Thanks to all.

I setup a simple Rx battery discharge checker by bench testing through a motor current tester and servo tester which runs 6 standard analogue servos through y leads.

With no load the current draw is 1.5 amps. I will see how long my c cell 2000mah sanyo pack lasts.

If no good I am thinking of replacing with a 2s2p lion pack with a 6v regulator to give 4000mah.

[Edgeflyer]

PS...

Is it ok to mix digital and analogue 125s eg. Digital 5125 in ailerons, analogue 125s in flaps? Analogue standards rudder and elevator??

[MattyB]

41 minutes ago, Phil B said:

Very good advice and suggestions here! Thanks to all.

I setup a simple Rx battery discharge checker by bench testing through a motor current tester and servo tester which runs 6 standard analogue servos through y leads.

With no load the current draw is 1.5 amps. I will see how long my c cell 2000mah sanyo pack lasts.

If no good I am thinking of replacing with a 2s2p lion pack with a 6v regulator to give 4000mah.

 

What exactly do you mean "With no load the current draw is 1.5 amps"? I can only assume you mean with all six servos moving but not attached to the surfaces, correct? If so I would say anything over (say) 1h15 is probably ok, given you will not move the surfaces anything like that often in use.

 

37 minutes ago, Phil B said:

PS...

Is it ok to mix digital and analogue 125s eg. Digital 5125 in ailerons, analogue 125s in flaps? Analogue standards rudder and elevator??

 

Yes, absolutely fine. The only thing to watch with servos is their max voltage tolerance; some servos (for instance the digital S3150s from Futaba) are not even rated for a 5 cell NiMH let alone a LiFe. 

Edited September 30, 2022 by MattyB

[MattyB]

3 hours ago, Nigel Dell said:

I think the choice we have with servos is now pretty comprehensive, the 125 was one of very few available, I think I have been lucky with them, I did not know they were that fragile, I still have a couple of models with them I will be especially careful with them now.

You are always learning in this game.

 

I haven't had any since ~2007 after I had about 4 failed in my Baudis Banana in a year on perfectly acceptable landings. Basically the output gear was a rubbish sintered alloy which stripped very easily, and (at least whilst I used them) Hitec never did anything to fix it, so everyone in the UK slope pylon community just stopped buying them.

 

https://www.rcgroups.com/forums/showthread.php?601719-Hitec-5125-servos

 

https://www.rcgroups.com/forums/showthread.php?395734-hs125-5125-new-gears

Edited September 30, 2022 by MattyB

[Wookman]

If you are running at 4.8 volts Futaba 3150s are good. Old but good. Very reliable but a little prone to developing some slop in the gearbox.

KST 125 and 135 wing servos are excellent at 4.8 to 6 volts. The 145 is a fuz version of the 135. My Extreme runs a 6 volt eneloop pack with KST 125 in the wing and Hitec HS 85s in the fuz. This model has flown in 55mph with a full load of ballast at 3.8Kg auw. Set 2 new PBs on the F3f course that day!

If you are going HV with a Li-on or Lipo KST x10 series or Kingmax is the way to go. This is what lots of the top F3f guys are using.

MKS also do some excellent servos but the they are expensive or very expensive comparatively. 

Personally I would not use an 81 or an 82 for any of those planes on anything. Just not up to it. They are for foamies or ailerons on 60" woodies. I would also steer clear of the HS5125 digital as you will for ever be re-centering the as neutral drifts. I have a pair in a Falcon and the need re-centering almost every flight. They also hunt like crazy on 4.8 volts, they are better on 6 volts. The HS125 anologue is ok but the KST125 digital is better and cheaper