Pull pull vs. push pull
 

Im building my first giant scale 3D airplane and Iv been trying to do some research on the rudder setup. What I want to know is if a pull pull system using 3 servoes and a bell crank is better or worse then a push pull system with 2 of the same strengthservoes in the back of the airplane??? Also I have really only seen the 3 servo set up, so is a push pull even legal for IMAC?

Thanks

RE: Pull pull vs. push pull
 

I don't think there's a rule in IMAC regarding push pull pull pull


only thing i don't like about push pull with ( servro's on one side ) is it seem's the end points are never equall if you max them out




for 3d flying I'd say main question is center of gravity ( if your puting weight in the tail and having your servro under the canapee then that's somthing you could change )

just wondering how big of plane are we talking here 3 servro's on the rudder ??? i think that older servo's that's the way it was but now i day's i dont think so


like for example 3 8911's on lion's would be over kill for a 40% plane i think if you had enough force on the rudder for blow back with that set up the rudder would physical fail before the servo's blow back

for IMAC i think they want the whatever set up has the most resloution at the center point and toqure with the least amount of Fade . the 3d guy probly doesn't care about center point resllution

the popluar set up i see is push pull two servo's miorring eachother mounted under the horizontal Stab .. note that you should use singlechannl's and end point's for each rudder servo if mirrored and you'll have to reverse one of the channal's mechaincaly or inside the radio

main thing you don't want end point's fighting eachother and cuaseing exessive load you could burn up a servo or get some nasty binding

another option is the two servo's and a joinig rod on one single side

RE: Pull pull vs. push pull
 

2 300oz+ servos will be fine for rudder..... most planes are designed to have them under the hatch pull pull style... however if you are using a bigger engine than recommeded or a really heavy engine, push pull might be the right thing to do to help in balancing

what size plane is this? hopefully 50cc that's the perfect size to start off with to begin the giant scale journey..... you might want to get a hold of some locals to guide you in the right direction

RE: Pull pull vs. push pull
 

I have a new 42% Pilot Edge I will be assembling soon; been reading all summer about setups. I too found the recommendations for 3 servos in pull/pull, and two push/pull in the tail. Haven't exactly figured out why, other than the short, direct linkage maybe loses less "power" to the surface. My Pilot edge comes with a plywood servo tray for pull/pull, but I discovered after reading threads on F G that there are push/pull cutouts in the tail. I'm going to be using two Futaba 9157's on my rudder...

RE: Pull pull vs. push pull
 

Anything that removes mass from extremities of an airplane is always a good thing. Its kinda like reducing unsprung weight in a race car. With a pull-pull, you also eliminate several feet of (failure prone) servo extensions/connectors.

RE: Pull pull vs. push pull
 

Both have their advantages. The biggest reason to pick one over the other is for CG reasons. The fact that you stated a 3 servo pull pull setup leads me to think we are dealing with a 40% bird. Be careful as most 40% airplanes can't tollerate alot of weight in the tail. Most have 2 servos per elevator so adding another 2 back there can create an issue. I would build up the airplane leaving the rudder servo install for last and do a CG check then work from there.


As far as performance, IMO it's a wash. Pull Pull done correctly has great response and is a little more forgiving in setup and is lighter. Even in a 40% airplane two servos @ 300 oz is plenty, no need to go with three. Putting them in the tail should give better power to the rudder however it is near impossible to get them 100% in sync. The result with that is lost power and then even more lost power through the 4 feet of wire. Not to mention that wire is going to put another 3 oz of weight aft of the CG. As you can see I suggest you stick with Pull Pull unless you are coming out nose heavy.

RE: Pull pull vs. push pull
 

One thing I would recommend with a push pull on the rudder, is to mount the servo on the left side. For flat spins, the engine torque naturally wants to pull the plane to the left, so I usually flat spin left for that reason. But the servo would be pulling on the rudder arm verses pushing, where the push rod could flex a little. It's a high stress situation on the rudder and linkage. It's just a thought. My CAP232 27% has an option in the manual for both setups but the previous owner of the plane messed up the pull-pull setup pretty bad so I scrapped it and mounted the servo in the tail. Good luck.

RE: Pull pull vs. push pull
 

local guy from my club flys for chief aircraft he has the pilot 40 precent 300 amont other big planes but i know that on his 40 percent pilot he uses push pull 2 servo's miorring eachother mounted in the fuse and mounted right under teh horizontal stab...


I'd give Chief aircraft a call and to ask question's surely they have done it both way's and can give you intel

RE: Pull pull vs. push pull
 

I've also got a nice pulley/belt drive servo tray setup I picked up off RCG; I could do it either way, but listening to the "experts", I'm gonna go push/pull. If it doesn't provide the proper power, I can always put 'em in the tray and add a servo if needed...

RE: Pull pull vs. push pull
 

Thanks for all the input guys and yes its a 40% extra 330 that im building and I have had some 25-27% stuff so im not completely new to this stuff and I think Im going to go with the idea to build the plane and see what the cg is and base my decision on that.<div>
</div><div>Thanks guys</div><div> </div>

RE: Pull pull vs. push pull
 

You are making the right choice.

David

RE: Pull pull vs. push pull
 

Good desision, Just for grins? what make Extra and what engine? I may have run across the combo and can save you some guesswork.

RE: Pull pull vs. push pull
 

just to put my two cents worth in the servos in the tail can be synced up using hitech programmable servos and a programmer to set the center and endpoints.

RE: Pull pull vs. push pull
 

True, but most likely the OP does not have either. And it will work on centers and endpoint but what about the rest? I dont know about you but when I fly I usually use the full range of rudder and not just center and endpoints.

RE: Pull pull vs. push pull
 

Yes, you can synch them up.

But unless you have programmable servos, a programmer, and a need to have weight in the tail, I wouldn't bother.

RE: Pull pull vs. push pull
 

You can MATCH center and end points only. They can not be put in sync. This was cut and pasted directly from Hitec's site. However, I do agree that unless you need a CG shift stay with pull pull.



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RE: Pull pull vs. push pull
 

Unless you have a geometry problem, then matching the center and end points should be enough to synch the servos.

I've done it before for someone, we used a programmer and went back with an inline amp meter to make sure the current wasnt spiking up anywhere indicating a bind.

RE: Pull pull vs. push pull
 

i picked up the programmer for 20 bucks on ebay. The price of the servos are insignificant to all the the other prices you are spending on the aircraft. How big is the model if it needs between 2 or three servos to operate a rudder. once properly program and setup right you do not get any appreciable current drains along the entire movement

RE: Pull pull vs. push pull
 

Depends on a couple things. Not all servos are the same. Some are better then others but I have never seen two servos track the same. They are always a couple degrees off. The mechanical setup of the control horns have to be spot on as well. The mix used or the matching device can affect the servo track as well. The best you could do with a push pull setup in the tail is to use a multi point mix.


The reason to go with 2 servos rather then 3 is to reduce the complexity. Ihave set up 3 and 4 servo rudder trays with matchboxes, equalizers, programmers and power expanders and no matter how much time I spend I simply cant get them to draw less then 1.5 amps at some point of rotation. The fewer servos mean less binding.

RE: Pull pull vs. push pull
 

you have never dealt with hitech programmable servos. you program the servo itself. you can set the centerpoint anywhere in 360 rotation so the centerpoints of all servos would be spot on. you then can set how many degrees you want to rotate, I believe up to 180 degrees lock to lock but the resolution suffers past 120. the programmer does not remain in aircraft, unlike a servo mixer

RE: Pull pull vs. push pull
 

Yes Ihave used the Hitec programmer and I will repeat myself again LOL All these devices do is match up the center and end points, they do NOTHING for the sweep between adjustable points. So when you figure in manufacturing tolerences of the pot, driver IC and controller board components you will get servos that degree slightly different. The only way to get away from this is to use higher grade components and thus drive the cost of servos even higher. I can get virtually zero amp draw at the adjustable points, that is easy. It's the getting no bind at 1/4, 1/2, 3/4 stick deflection and anywhere else in between that is a pain in the butt. This is the main reason SWB came up with their floating servo tray set up. Not as critical with ailerons and elevators as usually there is enough flex in the surface to compensate. For rudder there really is no need for a combined torque more then 600 OZ and nowadays that can be accomplished with 2 servos. I remember having do do this with 4 JR 8411 servos, so glad those days are over.

RE: Pull pull vs. push pull
 

I believe you also need to set the speed of the servos the same, with a Hitec programmer. The fine adjustment should be made on the radio.

RE: Pull pull vs. push pull
 

well I used the system the first time. the servos were ganged together with solid links to the push rod arms. I programmed them then I set the programmer to step them and I had very low to almost no current draw between the steps. Me and my friend mark were impressed by how the two servos moved so well together and how easy it was.

RE: Pull pull vs. push pull
 

Except with the programmer you can only adjust the servo slower. I agree that rather then use any matching device it should be mixed via the TX with a multi point mix. This is the best one can do but is limited to two servos. Then agin, even with a 40% two servos would give enough power as long as the correct servos are used. The best I have been able to do is with a Powerbox royal. It had the RX and alll the servo matching built in. The matching was done via a removable LCD and adjustment box. The LCD gave info on amp draw and direction of adjustment. What we need is a system that senses the current draw and automatically makes the adjustment until lowest draw is accomplished.

Rlipsett, Yes with two servos especially Hitecs you can get really close. Hitecs right out of the box are fairly good as far as being matched is concerned. The worse I have ever had to deal with was a batch of 8611a servos. I actually had to sit down and match them up, the difference between the best and worse was around 8 degrees. When I got down to the last surface, I had to order another two servos as the difference between the two I had was too much.

RE: Pull pull vs. push pull
 

Yes the balance function on high end radios is great for that but I've never not been able to get it right with a programer and inline amp meter.