Fitting Digital Servos To My Entry For La Coupe Des Barons.

[David Davis]

Those who are regular contributors to this forum will know that every summer I participate in La Coupe Des Barons (Barons' Cup in English.) This is a competition for a 5 foot or 1.55 metre wingspan French three channel trainer which looks vaguely like a Great War monoplane fighter or more accurately, a cartoon scale model of such an aircraft. If you're unfamiliar with the event I will attach a link to the 2017 event. There are other videos of the event in other years but the 2017 video is the best. The event consists of a concours d'elegance and four five-minute flying rounds.

 

In recent years I have always campaigned with a model powered by a four stroke engine. For 2023 I will be using a Thunder Tiger 54. You are unlikely to win the event with a model powered by a four stroke engine so to give myself a chance I plan to save a little weight by fitting smaller servos and having been informed of the advantages of using digital servos with 9 or 10 millisecond updates on another thread I have looked through my spares drawer and found several Hitec HS5085MG and HS5055MG servos.

 

I plan to fit two small servos at the rear of the fuselage to operate the elevators via carbon rods and to connect them to the reciever with a Y lead. Each elevator has an area of about 22 sq ins or 142 sq. cms. Given that one of the events is a pylon race in which the trusty Thunder Tiger will be running flat out for five minutes, is the HS5055MG servo strong enough to operate an elevator half or should I use the HS5085MG?

 

Am I right in assuming that the HS5085MG will be strong enough to operate the rudder which has an area of 36 sq ins or 232sq cms? 

 

Picture of my Baron below the link to the 2017 competition.

 

https://www.youtube.com/watch?v=vYlLvJC-MuE

 

PS. When I last looked there were still several places available if anyone fancies a go!

 

 

[john stones 1 - Moderator]

I would opt for the 5085 myself, matches pretty much what many standards offer.

[Paul De Tourtoulon]

I have corona 929 and 939 servos in mine, and an HP 21vt which makes it a bit under powered, a shame the Coups des Barons is so far away as I have acquired an OS.30fs to go in it.

[John Wagg]

If you start putting servos at the rear how will that effect the CoG ?

And how much weight are you going to save ?  ( Will this be 4 servos instead of 3.)

 

Saving a few grams in weight I wouldn't expect it to make the plane faster. It might help with acceleration a little bit though.

 

I am using mini digital servos on my Morane.

These for comparison :- https://www.4-max.co.uk/servo-4max-4M-175DMG-030.html

And only need one on my all moving elevator.

 

 

Edited January 25, 2023 by John Wagg

[Nigel R]

I wouldn't want to use 9g servos with any IC. Even metal gear. I've killed a handful with a 1lb foamie. And they don't suffer the vibration of a moderate sized four stroke. Or the airspeed, with increasing possibility of flutter.

 

5085MG would probably be ok, but don't quote me on that.

 

That said, I don't think the weight you save will be worth the effort. You might save an oz, perhaps two, all told. 

 

What benefit are you expecting to derive from the faster frame rate of a digital servo can work at?

[J D 8]

              Can not see any benefit for this type of aircraft doing what you suggest. The model is a bag of drag with all those flying/landing wires, are they just for show/rules ?  Another thing may be to experiment with different props on your TT54 and fine the best for the job.

[john stones 1 - Moderator]

I took Davids weight saving comment as the fierce competitor wanting to win the C de B trophy, never paid it too much mind myself. However is losing some weight now a bad thing ? If David wants to go to the trouble why rain on his parade ?

Flutter, I've never had, keep your gaps small and effective control linkages in place, servos, never a stripped gear unless in a crash, had I.C most of my RC life, now all petrol, gears stripped via vibration ? Zero, just my opinion but we're in old wives tales area for me, (someone crashes, examines wreckage, ah gears are stripped, not pilot error then).

3D helis, some feisty slope soarers with fair size control surfaces and some reach a fair speed, can't get standards in these, and owners don't bat an eyelid at going smaller, a plethora of small quick foamies out there, most have servos nowhere near the specs on a 5085 mg.

[Nigel R]

I've had flutter.

I've had gears stripped (with and without flutter) and landed afterward (an amount of luck involved)... and not landed in one piece, if luck determined so.

Vibration won't instantly strip gears but will cause accelerated wear.

Hangar rash is another reason to avoid tiny servos in all but smallest of airframes.

 

YMMV

Edited January 26, 2023 by Nigel R

[David Davis]

Thank you for your advice gentlemen. I have worked out that if if I fit three HS5085 to control the elevator and rudder and an HS5055 to the throttle I will have saved about 2 ozs (54 grammes) from the weight of the aircraft compared with using three HS311 standard servos. Whether I would detect any difference i n the performace of the aircraft is open to question. However, by using digital servos and uprating the frame rate, I may end up with a model which is easier to fly accurately.

 

Another reason for wanting to use two servos in the rear is that in one of my previous competitions I was obliged to land early in the pylon race because of elevator flutter. There was no gap in the hinges because the elevators were top-hinged, however, the two elevator halves were joined together by a piece of dowel and the glue joint had failed. I was able to epoxy the elevator back into place to the judge's satisfaction and I participated in subsequent rounds. I suppose that a servo mounted in the fuselage operating a rod attached to two pushrods would achieve the same end. This would save the weight of an HS5085 which is 22 grammes  or about 3/4oz.

 

JD8 the rules call for wing warping wires at least on the top of the wing,  some kind of gun, a pilot and a cowling. Most use rotary cowlings but mine will use an "inline" cowling which I designed myself!

 

Paul de Tourtoulon you live a lot closer to Jonages than I do and there are still some places left!

 

PS. I could save a lot of weight by using lighter wheels!

PPS. The winner of the 2018 event used tiny seros in his OS 35 powered model.

 

 

Edited January 26, 2023 by David Davis

[Don Fry]

To minimise weight, use a two cell LiPo for aircraft power. Use HV servos to push the controls. I have 20 gram Corona units, same power output as a Futaba 3001, a 10 gram servo does for the throttle, and if a more powerful servo is needed, a 30 gram servo delivers 50% more power than the Futaba 3001 ( weight 45 grams)
The entire aircraft runs off a LiPo, another weight saving over NmHh batteries.

I seem to remember you use Spektrum kit, so the RX is happy with the increased voltage.

No idea if it’s a game changer in the quest for victory, but in my experience, winners win cos they attend to the details of not being first loser, as my rugby coach used to refer to coming second. 

[David Davis]

Thanks for the advice Don.

 

I doubt that anyone could win the event using a fourstroke but I'd like to win the four-stroke class again as I did in 2021.

[David Davis]

I  plead guilty to reviving an old thread!

 

My Baron just requires the installation of the throttle servo, fuel tank and receiver before it's airworthy and I hope to fly it later in the week. In the end I mounted both the rudder and elevator servos, Hitec HS-5085s, midships using a Y shaped rod for the elevator and a closed loop for the rudder.

 

Which leads us on to the throttle servo.  I have the choice of fitting either a Hitec HS 55 or a Hitec HS 5055 MG digital servo to operate the throttle on my Thunder Tiger 54 FS if I want to save weight. I have a vague recollection that you should not mix digital and analogue servos. Is this correct?

 

Secondly, do you think that both of these servos are too small for the job and would break up under the vibrations of a four stroke? In that case I'll fit a standard servo like an HS 311.

[Chris Walby]

IMO either would be ok, its not like the servo is going to work really hard against anything...unless the throttle ices up 😉

 

PS not sure what RX battery set up you are using, but before committing to the sky, TX and RX on and just stir the sticks in every direction possible all at once....just once I was bench testing a set up and 1 or 2 digital servos were ok but 3 and stick stir and the RX would brown out.

[Steve Colman]

I've always quite fancied one of these, or something similar. How does it fair as a slow flyer just for pottering about on a balmy summer evening?

Edited April 3, 2023 by Steve Colman

[David Davis]

29 minutes ago, Steve Colman said:

I've always quite fancied one of these, or something similar. How does it fair as a slow flyer just for pottering about on a balmy summer evening?

 

Dunno Steve.

 

They were originally intended as a basic trainer and the prototype was powered by a 3.5cc Micron two stroke but everybody fits 35AXs or big four-strokes for La Coupe. 

 

I dare say they'll fly with a 15. Paul de Toutoulon has an HP VT 20 in his and they've never won a rice pudding skin pulling contest!

[Nigel R]

55 is very small and is really a foamie servo.

 

Myself, Id use the smallest hitec that has rubber grommets. Hs81 or Hs85?

 

Or if you have a standard size 311 handy, there's your low cost option.

 

 

 

 

[David Davis 2]

7 hours ago, Nigel R said:

55 is very small and is really a foamie servo.

 

Myself, Id use the smallest hitec that has rubber grommets. Hs81 or Hs85?

 

Or if you have a standard size 311 handy, there's your low cost option.

 

Thank you Nigel. That makes sense. I don't know why I bought the 55! I have several unemployed 311s so I'll fit one of those and just put up with the slight weight increase.

 

 

 

[Paul De Tourtoulon]

10 hours ago, David Davis said:

 

 

I dare say they'll fly with a 15. Paul de Toutoulon has an HP VT 20 in his and they've never won a rice pudding skin pulling contest!

VT .21 is out, and an OS .30fs is in and now flown, it's now a rocket ship, I am sure that Elon Musk has his eyes on it.

 

Ps, it's a really light build, silk covering and mini 939 servos, maybe I should weigh it.

Edited April 4, 2023 by Paul De Tourtoulon
ps

[Steve Colman]

10 hours ago, David Davis said:

 

Dunno Steve.

 

They were originally intended as a basic trainer and the prototype was powered by a 3.5cc Micron two stroke but everybody fits 35AXs or big four-strokes for La Coupe. 

 

I dare say they'll fly with a 15. Paul de Toutoulon has an HP VT 20 in his and they've never won a rice pudding skin pulling contest!

I was thinking more along the lines of the electric ARTF version from FlashRC, formerly Topmodel.

[Frank Skilbeck]

14 hours ago, David Davis said:

 

Which leads us on to the throttle servo.  I have the choice of fitting either a Hitec HS 55 or a Hitec HS 5055 MG digital servo to operate the throttle on my Thunder Tiger 54 FS if I want to save weight. I have a vague recollection that you should not mix digital and analogue servos. Is this correct?

Digital servos will work at a higher frame rate than analogue servos (which may burn out at a higher frame rate). So if you are mixing them then you need to make sure that the Tx/Rx is not operating at the higher frame rate.

[David Davis 2]

1 hour ago, Steve Colman said:

I was thinking more along the lines of the electric ARTF version from FlashRC, formerly Topmodel.

 

Yes they fly pretty well. We have had several at my club.

[David Davis 2]

27 minutes ago, Frank Skilbeck said:

Digital servos will work at a higher frame rate than analogue servos (which may burn out at a higher frame rate). So if you are mixing them then you need to make sure that the Tx/Rx is not operating at the higher frame rate.

 

How do I check the frame rate Frank on my Spektrum DX9 Frank?

[Nigel R]

I think it tells you when you bind the RX? There is a message like 'DSMX 22ms' pops up.

[Nigel R]

Seems like a system setting option for each model.

[David Davis]

Problem solved. I am going to borrow a HS 5085 from my reserve Baron and fit that. Then I'll  order another 5085. The reserve Baron is in need of repair anyway.