cocobear

My Feedback: (2)

You can certainly keep an airplane alive on one half of an elevator.Flying BUSA's 33%N28 at a noon show this year, the hinging on one half of the elevator failed. I was able to safely bring the aircraft down despite the flapping half knoking off the rudder by the time it touched down. Lots of aileron:-)

valleyk

Wind loading should determine size of servo. Lets just look at the rudder. My 1/3 scale fokker dr1 rudder has an surface area of 0.55 sq feet. Rudder length = 9 inches and rudder height = 11 inches.

wind loading force = area * .00256 * mph * mph* drag coeff.

drag for flat plates = 1.28

so lets say im flying my plane in a side wind of 20 mph will the wind load deflect my rudder?

wind force = .55*.00256*20*20*1.28 = 0.73 lbs of force. My moment arm or center of the rudder to hinge  =rudder length/2 = 9/2 =4.5 inches.

Total moment on rudder or inch -ounces = 0.73*16*4.5 = 53 inch-ounces. So if my servo is stronger then 53 inch-ounces of torgue a twenty mph side load will not deflect rudder and I will have control.

Now lets say this planes top speed is 65 mph and my servo can deflect the rudder twenty degrees full stroke. What size servo do I need?

Surface area of my rudder deflected 20 degrees into the wind =sine(20)* rudder length* rudder height = .342*9*11= 33.8 sq inches or .24 sq feet

wind force = .24*.00256*65*65*1.28 = 3.3 lbs

Moment arm = (sine(20)*rudder length)/2 = (.342*9)/2 = 1.54 inches to hinge line

Total moment on rudder or inch-ounces = 3.3*16*1.54 = 82 in ounces. Great I put a 133 in ounce servo and can control plane up to 65 mph.

abufletcher

Wow, my brain just went into number-lock!

I'm going to go with the Futaba S3305 servos, as per DJ's recommendation, because Futaba parts are more readily available in Japan. The HS-645MG also looks like a great servo. Either would do the job.

pimmnz

Geezer, I know that the modern idea is to go to multiple failsafe on everything, battery redundancy, servos that'll pull the surface off the model etc, etc, but these are light, low loaded models, mine have working rigging and yes, I use a bellcrank between servo and cable, just to keep the tension off the servo output bearing and the servo fixing screws, but like I say, good standard servos, usual maintenance practices and an eye on your battery, and my models have been flying for twenty years. How lucky do I feel? It ain't luck, just conservative builds and maintenance. Heck, the original battery was a 500 mAh, and that's all there was...the big 1100 pack is twice the capacity of what used to be, a day's flying hardly takes the top off the charge...I know what I am saying ain't popular belief, but I've had the things for a good long time and I can only point to my own experience. Others will have had a different experience, so you read what they all say an' you makes yer choice, I'm just stating that there is another way, and it works just as well, and for just as long, maybe even longer...
Evan, WB #12.

abufletcher

The question to my mind is WHY NOT use high torque servos? On a 1/4 scale WWI model that's only using 5-6 servos there would only be a 10-15g total weight saving by using Futaba Standard S3001 servos over Futaba high-torque S3305 servos. So weight really isn't an issue. Price? Well, you can pick up 6 standard servos for maybe $70. Six high-torque servos will cost maybe twice that. So for some, that's going to be an issue. Since I don't have an enormous fleet of 1/4 scale models (in fact, I don't have even one at this point) putting some additional money into a servo set isn't a key consideration.

On batteries...well...I hate them. Hate them, hate them, hate them. I see batteries as the weakest link in my RC models. Over the last couple of years I've improved my battery charging and maintenance routines (thanks to some very good guidance by Steve P.) but I still see batteries as more likely to fail than any other component. I used to fly with the 600mAh and 700mAh packs that came with my radios. (They are all dead now from my poor charging procedures.) Now I've moved up to 1000+ packs and the idea of having a 2000mAh 6V pack on a 1/4 scale model doesn't sound large at all...particularly when I'll be using a much larger pack for on-board glow with my Seidel 770. Come to think of it, that's starting to be a LOT of batteries! Two for the Rx and one for on-board glow! [X(]

foodstick

I vote for the hitec 645 if they are cheap there.. the control horns are so nice and beefy on that servo you will never need replacements.. Also as frail as the WW1 tail is on even a 1/4 scale .. there can't be that high of a load on it !

I used to just worry all the time about how little structure is back on the tail.. I usually beef them up to my own satisfaction.

abufletcher

I just placed an order with Tower which included 5 of the Futaba S3305 servos. I applied an email discount for international orders with gave me a $60 discount on an order of $250. (The servos themself accounted for about $170 of that.) So the cost for the order was $190, plus about $30 in international shipping. I decided that was better than worrying about whether it may or may not make to my US address during the time I'll be in the US. Furthermore, the discount for international orders was $60, while the discount for US orders was only $30. So that effectively cancels out the additional cost of shipping to Japan...and saves me having to haul it back with me.

abufletcher

The number designation for the S3305 standard high torque servo seems to be different in Japan. At least I can't find it listed in the Japanese RC magazine ads. But a comparable torque servo looks like it would be 5,400 yen...or $70!

Frank Sopwith

Hi Don

actually I fly my 1/3 BUSA Pup (for almost ten years now..) with S3001 servo's: each aileron got one and both elevator halves, only the rudder got a stronger one,

Frank

abufletcher

Frank, that's good to know. Can I ask, why didn't you use higher torque servos? Was it just a matter of cost? Ten years ago, high torque servos might have been quite expensive (particularly in Europe). The S3305 servos I just ordered were about $30 a piece (actually a bit cheaper with the discount). But if I had to paid what they cost in Japan (about $60-70 each!) I would have used standard servos for sure! As is, I paid about $150 for 5 servos when I could have gotten 5 standard servos for $60. Paying $100 more (which seems even less to me when I think of it in yen) isn't a big additional cost. But no way I would have paid $300-350 for servos!

cocobear

My Feedback: (2)

Abu, they may be listed as car servos. They were here until I raise a stink that I was using them in both GP product development and my own larger aircraft. What no one has mentioned is the reason for big servos and the ABSOLUTE NEED FOR METAL GEARS, flutter. Pimms, with no disrespect, do you drive a Model T? That is the basis of your argument. One loose pull cable and those servos are gone now the first time your plane picks up a little steam. I do not as of yet run redundant batteries but have considered it. The biggest killer of planes is not the battery but the switch. More people need to worry about these. By the biggest, highest quality one you can afford. I am contemplating using Deans connector with the male side shorted for my new projects. That provides a positive connection that cannot fail if I did a half decent solder joint. Valley, it was nice to see some science applied. My only point is as stated look out for flutter. In testing literally hundreds of aircraft, I have only seen one lost to servo failure with out flutter. Again, over serving is great insurance for your tremendous investment of time and money. BTW, scared the heck out of my self last summer at Hoosier Dawn Patrol. I had just built up a bunch of speed to roll my 1/3rd DVII and as it went over the top on its back, I saw the left elevator half fluttering away. Without metal gears it would have been gone for sure. Upon landing, I tightened a turnbuckle and went back up and did it again with no issue. Had I cheaped out, I probably still might have been walking around the field with a garbage bag.

DJ

TFF

http://www3.towerhobbies.com/cgi-bin...?&I=LXBXA8&P=7 is my pick. If the servos drop because of the weight of the controls, turn the radio on for pics.

Joe Westrich

My Feedback: (95)

Yo Coco, I agree with the switch as a weak link too. The redundant battery setup gives you two batteries and two switches. It covers both.

To the standard servo comment. If you plan on getting more than just a few flights a year, you need not use plastic gears. I have 100flights in 3 years on my Fokker DVI and there is no chance I would have made it with standard servos.

geezeraviation

My Feedback: (3)

Not all hi torque servos have metal gears and I agree with Dave that the metal geartrain is as important as the amount of torque maybe more so. Also a good point on switches, the first time I had a switch fail it was sort of in slow motion, I would check the batt state with an ESV post charging and over a period of about a month it seemed to me like the battery wasnt coming to full charge then one day I checked after a charge and the power was way down so I unplugged the batt and tested it direct and sure enough the batt was fine and the switch was building resistance as time went by. I use the big JR switch with the charge port built into it and at 20 bucks I swap them out every couple of years.
Doc

rctech2k7

My Feedback: (1)

I believe you've got plenty of information by now. Actually, you can make your own calculation base on the size of control surface, airspeed and weight of an aircraft. You can use wing loading or dynamic pressure to get surface load in the air then combine it to static load when you have heavy surface control. Load or pressure must be multiply with the surface area to get force. Using the net of aerodynamic force and weight acted on the surface then multiply that to the distance from the pivot hinge you'll get torque load. Divide that with control arm distance to get the force acted on a linkage. Now using your servo arm you can get your required torque for your servo. To prevent your servo from overloading, you must operate within 80% of your servo capacity. Last but not the least, your servo speed determines power and its electrical equivalent.

abufletcher

As I said above I have already ordered the five of Futaba S3305 servos. If they're good enough for Dave's models, they're good enough for mine! They have metal gears and at about $35 a piece they are a good compromise between standard servos (at around $11) and the expensive ones ($65) that TFF proposes. The torque numbers for the S3305 seem more than adequate for a observation aircraft that isn't going to be doing loops and barrel rolls (though Mick Reeves has done that with Strutter). Honestly, I just don't think I could spend $65 for a single servo. That might be enough to send me back to 1/6 scale models.

Now that I've gotten used to the idea, using one servo for each of the elevator halves seems like good advice. I'm still getting used to how "cavernous" the fuselage of a 1/4 scale model is compared to my 1/6 scale fighters were the servos really to be wedged in carefully. I also think Dave's right about the switch often being the weak link in the chain.

abufletcher

That's a BIG assumption!

semeraro

oops, ya Iuse the 3305 servos. And Ishould clarify that it is not because of any great insight on my part or due to a great deal of experience. I use them because Dave Johnson told me that is what he uses. Dave has way more experience than Ido and I respect his opinion. He is a professional after all. My personal mantra has become "dave is always right". Say it with me... .

Seriously though, Itend to agonize over every decision. Listening to Dave has helped me reduce that by quite a bit. Again, the great thing about these forums is the ability to get advice from all sorts of experienced builders and fliers. I am getting mushy. Must be the holiday season.

Dave S.

cocobear

My Feedback: (2)

OK, calculations are always a great starting point, but..... they are based upon assumptions and models of prescribed behavior. I have all the appropriate math and I KNOW that with out real word testing my calcs are a starting point. That is why testing occurs. When we build a one off, we don't have that luxury. So why play on the edge of a cliff?

abufletcher

By the way, I also can't find these Futaba S9405 servos listed in the magazine. But a servos with comparable spec seems to be about $100+ a piece.

TFF

$65 is not bad for a servo that should out live a regular servo. The hot ticket giant scale aerobatic guys need about 12 of these for one airplane http://www.horizonhobby.com/products...rvo-JRPS8711HV

geezeraviation

My Feedback: (3)

I have quite a few JR 4721 servos that I think were about 75 bucks a pop and the coreless Hi tec servos I have in the 1/3 N 28 were around 65 a copy. But like TFF said at least I dont need a dozen.
Doc

abufletcher

"Dave is always right."


Which is just one of many reasons I will never be a hot ticket giant scale aerobatic guy. And, yeah, you're probably right that $65 isn't bad if it outlasts a cheap $11 standard servo. But is there any reason to be believe it will outlast a $35 high-torque servo with metal gears (from the same company)? I have to admit that I'm completely turned off by the "tech" part of the RC hobby. I just don't care at all about radios and servos and batteries and all that. They are all just necessary evils but not in the slightest bit "sexy" to me. I think I went through my "tech obsession" phrase with photography.

On the other hand, a hand-carved laminated prop...oooh...now THAT gets my blood pumping!

abufletcher

OK. Now you've got to explain that (to me using little words). I though you just plugged the extra battery into an empty slot on the Rx. So what I'd really need is two switches with the two switch plugs hooked up to the Rx.

And I hate myself for even asking this but "what's the difference with a coreless servo?"

geezeraviation

My Feedback: (3)

Speed mostly, they have less armature mass to accelerate and decelerate.