Servos mounted in the tail???

[stuart knowles 1611]

Hello All

A friend and I have a pair of Bf 110 airframes under construction from the Don Smith plan. The 118 inch span version to be powered by 2 x 26cc petrol engines. The wings are nearly done and soon work begins on the fuselage & tail. Transport considerations suggest that the tailplane ought to be made demountable.

Now for the questions. In an ideal world, I would put a servo on each surface. As a standard for a model of this size I would probably use the HS 645 servo, so 2 x rudder and 2 x elevator. However the weight of four servos way, WAY down the end of a very long fus and we are worried about the balance issues.

Does anyone have practical experience of using smaller servos for a similar application. Servos are available with 3kg/cm torque which should be enough for the rudders (quite small) but might be a bit limited for the elevators.

What do you think?? Elevator servos under the cockpit? but then the faff with linkages when mounting the tail. or just a sackful of lead in the nose to offset the best solution to direct controls with some redundancy?? or lighter weight servos but can they put up with the vibration??

Your thoughts would be appreciated.
Stu knowles

[Alan Cantwell 1131]

Its the metal gears that are your enemy, and thats a long back end, there is nothing wrong with a decent nylon geared servo, also, have a look at slim glider aileron wing servos, some of these will go in very slim wings, and have good torque, these may do for the rudders

[Phil Clark]

Put the servos in the tail....simple as that. At this size, you should be able to get 2 standard size servos in the underside of the stab. Rudders are more difficult I agree as the fins are slim.

Slimline servos have VERY small gear trains which are normally metal as well. Small gears = small contact area which means thay cannot take the load and are poor when subjected to vibration. Metal gears also wear faster than a good quality nylon geared servo when subject to vibration, so I'm with Alan as I'm a fan of nylon geared servos as well. These slimline servos are normally associated with gliders & jet models....neither vibrate!! Subject them to vibration and you are using a servo for something it's not intended to be used for.....so you do this at your own risk. Don't even think about scrimping on servo spec, just put the best servo in for the job (and that you can afford) and put up with the fact you may need nose weight.......but then again, even an extra couple of lb in a 118" twin is neither here nor there.

Short direct linkages are preferable over long pushrods & bellcranks every time......they are stiffer and slop free and pretty much standard practise on large models these days.

I built a Ziroli Beechcraft D-18 a couple of years back for a customer, similar size span wise to your 110. This had servos in the tail, batteries right in the nose and no nose weight was required (it did have 2 x ZG38's which are a lot heavier than your chosen engines though). The D-18 does have a MUCH bulkier fuz than the 110, so with careful building elsewhere, no nose weight is achievable.

http://www.nottingham.ac.uk/~svzimb/fighteraces/custombuild/beechcraft/beechcraft.htm

Phil

[stuart knowles 1611]

Thanks Phil and Alan, interesting and sound advice. I have to say that I haven't had the need to use much beyond 3001 and 645 servos. From just browing the spec of various servos, I have been looking at metal geared servos thinking that these would be a better option so what you say is an interesting slant on that idea.

Is there anyone out there in LaMA land who has used slim / small servos on a large petrol?? suggestions invited. I think that you are right - stick to known good servos for the critical controls, i.e. 2 x 645 on the elevators. The rudders have a small area so a good slim servo installed in each fin or in the tailplane next to the fin should be up to the job.
stu k

[ian redshaw]

Is there more depth in the tailplane tips for the rudder servos? Mounted flat in the underside of the tailplane, with the horn closest to the fin, a neat and positive solution can be found without adding extra weight, therefore structural loads on the fin mountings. This is how John Greenfield of the Ghost Squadron's Bf 110 rudders were controlled, a very neat and practical solution. Standard torque servos would be more than adequate on the rudders. The build looks superb too, I don't know how Simon does it so quick!!

Ian

[Mike altham]

If its slimline servos you are after I have used these on my jets and some aero model

Very small with 8.2kg pull on them.

http://www.t9hobbysport.com/item.asp?ca ... prodid=514

Mike

[stuart knowles 1611]

Thanks Mike, they are impressive.

good points there Ian, thanks, will have a look at TP mounting, that seems a sensible option.

[frederick barlow]

hi stuart id suggest you save some money and go for kst ds125mg servos the problem i know about the mks servos is the dont like 6v and these ksts are an exact copy of them and there have been no reports of them been overloaded at 6v.
the specs are simular 7kg@ 6v and it also comes with a alloy arm if you look at contacting tony fu at sloperacer on 07505456160 he can tel you the pros and cons and possibly supplt the ksts

[Mike Booth]

Beg to differ but I use large scale good quality servos as far forward as I can mount.
I have carbon push rod system into well engineered closed loop.
Have a look at any scale control system of the period or modern light aircraft.
There is no need to have any slop at all, five minutes of Faff is better than adding two pounds of lead to anything.

[stuart knowles 1611]

Thanks Mike, a good point well noted at this end. I have been working on the tail set of this build today. Apart from a few issues with the plans I am rather alarmed at the way the weight builds up when you pile on 2 x 645 servos for the elevator and two minis for the rudder. Add a few bits of ply for mounts - It soon starts to feel weighty. The easy option would be to fix the tail permanently and take the two 645s up into the nose. If I did, I know that we would regret it everytime that it got bashed on the hanger door and shelf. They are going to have to go into the tail on this one.

Still, if its not hassle its not scale!!
cheers
stu k

[Alan Cantwell 1131]

use a good nylon geared servo, several suggestions have been made, do a bit of research, and you will find your best option, metal gears at the back are not needed, and are too much weight

[Mike Booth]

Stuart if you give the removable tail some thought you can still have detachable drive system without the servos at the back.
A good test is to weigh all the servos and bits of ply; and the linkages.
Then, multiply that weight by four and that is what you are going to need in lead up front to compensate.
The other factor is the 110 fine flyer as it is, has a long tail moment , exaggerating the weight in the rear when it comes to achieving the correct CG.
One way or the other I'm sure they'll be fine models of what is a very under modelled aircraft.

[Peter Clare]

What is to stop you having the join in the fuselage just rear of the wings with the servos just rear of this joint ? This would avoid the servo weight in the tail and any complication in the linkages. With no joint at the tail end the tail itself would also be lighter.

[Mike Booth]

Or better still keep the servos right up front and have bell crank closed loop system in the rear half.
Then connect the carbon pushrods to the bell cranks after the two halves are bolted together before the wing goes on.

[Chris Lane]

Please also consider torque drives as they are particularly easy to auto-couple! A good source of inspiration is to look at the back of a manual pipe organ. These use "stickers and trackers", often with ingenious joints and runs, to connect the keyboards to the wind chest pallets. But don't use carbon rod or tube for torque drives as they are not torsionally stiff (try them!). I use small diameter thin-walled aluminium alloy tubes from B&Q.

Attached photo shows self-coupling torsion rod ends for a small electric model aileron. A fork end was made and attached directly to a servo mounted on its side and the torsion tube in the wing was provided with a trunnion to match.

[Chris Lane]

More on pipe organ mechanisms.

This picture shows a "roller board", in a Flair Fokker DVII fuselage, to take a throttle linkage vertically from a servo at the bottom to the carburetter height near the top. In this case the torque rod is hardwood dowel which is light and stiff enough in this short run. Spare servo arms are epoxy glued to the ends (the unused arms cut off) which are drilled for pivot rods. Although these torque tubes are on modest models they can be readily scaled up to large model needs.

Across it can be seen the conventional elevator and rudder push rods which are balsa dowel supported half way along the fuselage by passing them through apertures to constrain whip.