servos and giant scale

[richard armstrong]

hi all

i have plans for a giant scale DC3 140" span 2 x 50cc petrol trying to keep the weight below 20kg but might not be able to keep this under think this would come under the LMA/CAA rules but that's another story

i will look also for a pilot have not flown for 2-3 years due to funds

i would like to know what the best servos are for large scale models, prepared to spend £35-50 mark per servo (less if i get it from hobby king)

also under-cart servos

i also would like to know if their is a an twin engine controller other than a Y-lead with auto cut out on engine failure

[Martin Waller]

Richard,

If you are serious I would reccommend you speak to Tony Hooper who will nominate a LMA inspector to discuss your project with you. I my view the aircraftyou propose will be well over 20kgs. I have a smaller single engine plane and that is 56lbs. The subject is far more complex that just the servos as servo choice will dictate the power supplies (regulators) and battery specification. Your project will result in a plane less electronics costing around £4,000. 3W 55cc engines are £550each. You will have possible specially made retracts £800 and wheels £50 to £100.

Would you risk all this on cheap servos. You need expert one to one advice. Servos I would considerwould be JR DS8425 13kg cm at 6V; JR DS8411 13.5kg cm at6V; JR8511 15kgcm at 4.8V; JR DS8711 25kg cm these are all high grade digital coreless servos with top grade amplifiers and high vibration resistance specially if double rubber mounted in Futaba single servo holders these are all aprox £100 each depending on supplier. Please note other than the DS8425 which JR rate at 6V the others are rated at 4.8V. I find it OK using 6V on the 4.8V servos providing the 6V is regulated. If youconnect 5 nmhi batteries at 7.2V fully charged or 2 Lipos at 8.4V fully chareged then the servos may fail prematurely. Jet pilots have been using these on Power Boxes and big Weatronic receivers (12-22R)for years. The big Weatriniv would give you the cheapest and best in my view receiver and regulator combination as they are only £420 approx. A suitable power box for dual redundancy receivers will cost around £400 then you need two top quality receivers. The Weatronic gives you two receivers, 8 regulators so you can set your servos to different voltages in 8 groups and you have 22 servos outputs all independant with automatic matchboxing of up to 8 servos per channel. This automatically plots the servos matchbox graphs of the slave servos to the master whenyou have up to 8 servos on one control serface. You should not need this but you can plot the flap curves to give you the same angle of deflection across the whole movement of the flaps. I find this most usefull.

Hope this helps.

Martin

[Ken Bones]

Wow! Martin you dont have to scare Richard off!
I am guessing that this is the Ziroli DC3? If so it is poss to get this under 20 kg. But, it will be close so why not take advantage of the service offerd by the LMA and get it registerd incase it does go over the 20kg mark. As Martin says Tony would help in getting a freindly inspector to help and advise.
This could quite easilly (and safley) be built for less than half the figure that Martin is quoting.


Bonzey

[TimCrose]

Richard i hope you are progressing with the project, i think a confusion between Kg's and Lb's in the two earlier posts...mine is is closer to the 40lb mark.

the rest of martins advise is solid though. IMHO any model of this sort of scale and weight needs the failsafe and reassurance of quality gear, you get what you pay for in terms of quality and why skimp if you are spending the sort of money needed on a model of this magnitude. To get one for half the price you would need it second hand, the retracts and engines alone when silenced properly will cost you close to £1500, materials, covering, finish etc etc will take it over the 'half' price so Bonzey must be really skimping on avionics if he could get one in the air for less and safely...when you start cutting corners on servos especially then failure is more common as they cannot take the stresses. Its not just about the model weight, at speed the effective weight increases and forces on surfaces rise exponentially, stry sticking a hand out of the car window at 20 MPH and imagine your aileron is expected to do that comfortably....

It really worries me as these models increase in size and complexity when corners are cut on safety

[Andy Boylett]

Just a thought re you get what you pay for......

A Gucci handbag will cost you £1000 but an identical (and I mean absolutley idential) unbranded one will cost £150.

A Ford GT40 will cost you £112,000, but an unbranded identical one will cost you £56,000 (that cost is based on a higher bhp of 580!)

A branded JR/Futaba digital, coreless, smt, anti-vibration mounted one will cost around £100. The 'unbranded' equivalent is around £55.

The point of course is that you pay for the "known piece of mind" by paying extra for the branded versions. It can be VERY difficult to tell if a cheaper item really is the same, or whether it is inferior. Just because it says it is the same, does not mean that it is!!!

So the issue is that the 'unbranded' equivalents always have to prove themselves, which is why we have been wacking the hell out of the Turnigy 33kg.cm, digital, coreless, high voltage, titanium gearded servos (£55) for the last year to try them out before using them in a larger model. So far not a twitch that was not asked for! We have also been following reviews on these servos - still good. Only time will tell.

[Alan Cantwell 1131]

i used to think futaba futaba futaba until i met hi-tec 645 MGs, never looked back, if they had been 60quid, i would still have bought them, as it is, they are under 30quid, and do the job well on every one of my larger aircraft, the DC3 isnt exactly a 3d model is it? 2 on the rudder, 2 on the elevator, and one on each aileron, i would even put them on the flaps--on 6volt, i dont call this skimping on the avionics, i call it using a good servo, which has a great name,

[TimCrose]

Good points and its about reliable and having confidence - nothing to say a futaba or a JR wont fail just as any other for less but when you are putting all the multiple control systems in with a sensible redundancy sharing load to multiple servos etc etc then you are getting the realms of distribution boards like the powerboxes of this world...at what ever level you pitch your modelling the amount of gear will be more than a 40 sized trainer and deserving of something robust...i will happily use a £5 servo on a non essential function but on the primaries i prefer a piece of mind - and no i am not minted i just don't like rebuilding....at the end of the day i would like to see anybody finish a Dak properly for £2k as suggested....if he can i will have him build me one it would save a shed of time...my missus still thinks they cost £100

[BILL MARSHALL]

nice thread guys
Bill

[Andy Boylett]

Hey guys if my wife sees this I am in trouble. Can we just divide by 10 and use numbers like that? So a DC3 might cost between £200 and £400 to build say?

[stewart clifford]

Are we expecting Andy to turn up at Kirkby with a Gucci handbag(big enough to put a cat in) and using a GT40 as the get away vehicle?

As for the servo's, if it's a large aircraft and you're looking for something powerfull but not bank breaking take a look at the Futaba 3306, it's a 1/4 scale servo, metal gears and puts out 24kg and cost about £40. the beauty of these is because they are bigger they're less likely to fail.

Stewart

[TimCrose]

3306 will do the job, but its a buggy servo not a 1/4 scale servo. good and strong and with high torque though....probably not the fastest either but not gonna 3d a DAK

[stewart clifford]

Sorry, meant 1/5th scale.

[Andy Boylett]

Are we expecting Andy to turn up at Kirkby with a Gucci handbag(big enough to put a cat in) and using a GT40 as the get away vehicle?

As for the servo's, if it's a large aircraft and you're looking for something powerfull but not bank breaking take a look at the Futaba 3306, it's a 1/4 scale servo, metal gears and puts out 24kg and cost about £40. the beauty of these is because they are bigger they're less likely to fail.

Stewart

I always wanted a GT40 but I made the mistake of costing it out before I started building myself one and £56k is a real price. I ended up building a GTM Coupe, the mid engined one based on a mini and looked like a Ferarri 308GTB - but much smaller. I built a 1480cc full race engine, 120bhp at the wheels, in a car weighing 730kg. But, the engine idle was 2800rpm and at 30mph you were still in first gear - plus you needed ear plugs. It only ever did 4000miles and then got sold. Now I have a Volvo .