40% sailplane servo selection

[pete914]

Hey guys,
So I'm finally starting the servo install on my 6.5 meter DG 303!
I was wondering if I could pick the communities' brain on servo selection.

Paritech recommends Hitec 5645 servos for the wing surfaces (so I bought 10 of those) but they aren't that clear on the elevator and rudder servo selections.
They suggest running two servos for the elevator. I'm wondering if that's really neeed or not? Redundancy is good but I'm not sure a failed servo will always move freely if it burns out. Simple is best IMO.

Anyway, what do you recommend in terms of torque for a 40% sailplane for elevator and rudder? What about the tow release?

I can't imagine the elevator control surface needing more than 140 oz's. The rudder seems like it's going to need some power though.
Does anyone have empirical data on actual current draw in flight via telemetry? (LEN!?)

I would like to stay with Hitec's but open to all suggestions.
Thanks guys!.


Thanks!
Pete

[Stew2]

I think it is most common to split the elevator halves when running two elevator servos. That will eliminate the burned servo from stopping the working servo.. As for rudder and tow release? Rudder can take a beating from toe kicks to huge side forces from a tail whip. Tow release, I never skimp on tow release servos, you can need the grunt at the worst of times, when things go wrong on tow and you need to get off the line, it will never be under a load less than normal, but it will be with a load X times greater than normal. On my 6m ships I use servos in the area of 300 in-oz torque levels for rudder and tow release,

[Steve P]

+1 to Scot's comment about the rudder needing to be durable more so to bumps, handling and such than just flying deflection. Metal gear, 250 ounce + for the tow release is cheap insurance for such a nice/big sailplane when all goes to hell. Also, in normal times we see too many people have trouble getting off the line just with normal tow tension due to employing slow/weak servos.

One thought for you to mull is if you can find a buyer for the 4.8-6.0V Hitec 5645's, opt instead for High Voltage servos. Hitec has a range of digital HV MG offerings with more torque (even the HS-5585MH which is a modest step-up compared to the more spendy TH offerings) if you want to stick with Hitec. The operable voltage range of the HV servos and ability to use a variety of pack chemistry are a big plus. Also, HV somewhat ameliorates your current draw concern. The Paritech servo spec details are quite a bit out of date given the vast improvement in servo options over just the past few years. (I recall yours is a sailplane created about 9-12 years ago that Paritech no longer sells? Or, did they special order one for you?).

Finally, if you do have current concerns across so many servos and long leads, a power buss and redundant battery circuits is more cheap insurance on such a nice sailplane.

Whatever you do, enjoy your cool new axe! Share some assembly photos.

[pete914]

Thanks for the info!
Indeed, this is the old-school paritech acro sailplane. They still have the mold and were willing to make me one. I have always loved the look of the DG's and saved up for some time for this one. It took them about a year from order to it arriving.
I will shoot for 300+ oz on the rudder and tow release. I'm not trying to skimp out on anything! I like the Hitec's because they are programmable which I think makes matching control throws easier (two servos per flap/aileron).

Doesn't Len run a single elevator servo on his huge DG 1000 i.e. the "Lindenburg"?

As soon as I make some initial headway I will start a thread. The canopy frame on this glider does not fit very well. I regret not having paritech build canopy but I think they were just trying to get the thing shipped to me...

[Stew2]

Len's big DG 1000 uses 2 elevator servos. What radio do you use?

[pete914]

Stew, I run a Taranis with Open TX 2.2

[lenb]

Hi Pete.
Thats great you're getting started on the new glass slipper. I was thrilled to hear you talk about it in Cumberland.
Listen to the words of the wise, as stated so expertly above.

Sell the Hitec 5645 servos ( that should not be hard at all, a sport flyer will scoop them right up) and get yourself some HV servos. The 5645 is a great sport servo, but run a smaller amplifier than a more expensive brand. That leads to overheating and burnout, this is not an industry secret, its just a fact. The Hitec 7955 or 7954 are fantastic servos. Its the only one I will use for retracts on every ship I have. Programable is a must for that. Plus its HV even though its not shown that way, we've been using them for years. If you want to keep it simple, then go HV, you need to at some point.

I use dual elevator servos on most of the bigger ships, otherwise I use a super quality servo like the Futaba BLS 173, or MKS servo, or Spectrum 5060, all of which are mid size and strong.

Its kinda funny, we were all sitting around one night at dinner talking about servo selection and when you think about how much these glass ships cost, its worth putting high dollar stuff on the control surfaces, after all, thats what will bring the plane back to you.

Goods luck and please show us your progress.


len

[pete914]

Thanks for the great feedback guys! Exactly what I was looking for.
I suppose research and planning is one of the fun parts but I've let it drag on too long...

MKS has a really nice spread of options with good torque ratings. I'm leaning toward using them.
Len good call on the Hitec 7955. I'll order that for the LG.

Uwi at Paritech told me they still run the Hitecs on the wings and 2 standard servos for the tail/rudder but couldn't remember what they had installed.

[Stew2]

Does the Taranis programming offer channel balancing to match the output curves for multiple channels? That works for multiple servos on a common control surface so they do not fight each other.

[pete914]

That's a nice feature! As far as I know, it does not. This glider has internal linkage and I like the idea that after programming the servo itself, regardless of radio setup, the servo won't bind or break something. Especially if I'm running HV high torque servos on the wings. Speaking of which, I returned the servo's to tower hobbies. I will take everyone's recommendation on running HV on this glider. Sweet! I have made some progress on the canopy installation this weekend. I will start a separate post with a loose and spotty build thread

[Stew2]

You probably have the capability of making a million mixes in the Taranis though. Balancing is basically a curve mix of one servo to the next for the servos on the same control surface. There are a few theories on multiple servos on 1 surface. one of which is purposely build in the slightest amount of "fight" between the two servos to actually help control flutter initiation. An interesting idea, but i have not tried it myself. I prefer to fight flutter with really sound mechanical set ups in the control linkages and save all the servo power to use in the surface control. I can see how doing this could result in super accurate control surface deflection though. Fun mechanical stuff really, but really I doubt our gliders need anything that fancy.

[pete914]

Stew,
Thanks for the great input! Indeed Open Tx will let you do just about anything. The mixing options are endless and it's pretty straightforward to match control throws between two servos on a control surface. I will have mixes for flaps etc but it's so nice to know that with the individual servos programmed, I cannot mechanically bind the servo with erroneous programming. With 16 channels it's easy to get a bit mixed up at times! My servo power distro board has current limiting and telemetry for current draw would should also help me determine if I'm having any binding issues. Do you run any regulation between your 2 cell batteries and your Rx/Power distro boards on your large gliders or do you just plug in directly?

[Mark9]

About two years ago I started using MKS servos in my competition gliders, they are much above other brands IMHO. If they have what fits your requirements I highly recommend them. I have put them on all surfaces and tow release on the 4.8 meter Ventus I am assembling. The only non-MKS is on the retract...they don't make a low profile servo as far as I am aware.

[Stew2]

Pete,

I run Powerbox power distribution boxes in all of my gliders. I fly Spectrum radio systems, so I use the Competition SRS model Powerbox. I use the servo match function through the Powerbox to match any servos I need to. I don't have any gliders with multiple servos on a single control surface, but I do use the match function to allow my spoilers to only require a single transmitter channel. This allows me to run the typical 13 servo glider on 12 channels. I really like the Powerbox systems hardware, it is truly redundant and isolating for dual battery power and I believe the regulators help greatly with the life of my servos.

[stevekremer]

Are the Hitec 7955 or 7954 both high voltage? Any reason to get one over the other? They are both the same price.

Steve K