Why Pull Pull on Rudder, not Elevator
 

Giant scale folks make an effort to make the rudder pull-pull with 4 servos ganged up for rudder. Then for control "goodness" they go to an aft mounted servo or two servos on each stabilizer half and a short pushrod to the elevator.

Why the difference? Certainly a tight pull-pull system is just as tight as a direct servo connection. Or the reverse, if direct is so good why not use it on the rudder?

Another question. Why use a pull-pull when a big diameter carbon fiber tube push rod would be rigid and let you put the servos up in the equipment bay?

Granted dual elevator servos per elevator half gives some reduncy in failure analysis.

RE: Why Pull Pull on Rudder, not Elevator
 

I use pull-pull on both elevator and rudder on all my 1/4 scale birds. Much better that push rods as there is a low mass so moment of inertia (read hard landings etc.) do not strip servo gears nor cause unwanted damage. It is also easier to route, I use guide tubes and can easily route around structure to allow easy installation.

RE: Why Pull Pull on Rudder, not Elevator
 

Ben,

I have the same questions and have for years. We used to get four servos for primary plus one for retracts in a 3" wide fuselage. Next we almost doubled the size and started hanging them ouside.

Alot I fear has to do with a new generation who just won't spend the time and design effort for tight control set ups. It can be done and is still done by a few of us but the 'newer group' seems to be content to mount servos anywhere. I shouldn't just point out the 'newer group' as many who followed (note, I did not say progressed) to giant scale from the old days do so also and question it not.

True, part of it is 'free' tail weight to balance gassers but that weight is slowly coming down. I've heard 'tight control surfaces' as an excuse 'ad nauseum' and in irritation asked the "servo in the breeze crowd" to come over and show me where my control surfaces were sloppy-----end result----I stopped hearing about 'tight direct drive'. Of course not one of them has made any attempt to get them out of sight-----I just don't hear the 'tight' excuse any more.

On the BARFS it is simply way easier to 'non engineer' external mounts.

I would like to see a return to clean airframes. The Jet guys do it in a lot more restricted areas than we routinely see today. Their speed and control surface loadings also eliminate the 'to prevent flutter' argument. It CAN be done.

Summation, I think we've got Lazy Design Illness among us hereinafter 'LDI'. With a little effort we could probably come up with some Gov't. $$$ to eliminate the pending LDI scourge ;-).

RE: Why Pull Pull on Rudder, not Elevator
 

Ben -you missed the easy answer -
Too many modelers simply can't make the setup operate properly.
I still have to patiently explain big arm/little arm on direct servo linkages
as soon as the terms "torque and moment arm" are mentioned- the receptors can be seen shutting down.
In my earlier life - I designed linkages , including spring balanced linkages -which can be really tricky.
Also, I did control and pressure linkages for industrial clothing manufacturing machines .
So with similar backgrounds- - some of us see no problem in doing a pull pull setup .
For others it is simply a very daunting task -to get throw, ratio and friction all in proper order.
The first clue on all of this is the goofy 4 servos doing one job setups --
the wrong tools for the job - but cheap and available and easy to do.

RE: Why Pull Pull on Rudder, not Elevator
 

I'm afraid this isn't really an Aerodynamics topic but I'm not sure where it should be moved to. Ben, since you started it can you find a spot to move this too that fits? Giant Models, Pattern or Scratch Building seem to be the obvious choices. Thanks

RE: Why Pull Pull on Rudder, not Elevator
 

I have a 60 size plane that has both rudder & elevator on pull-pull, that's an in-house bash. I also converted a Funtana 90 to use all pull-pull for the tail All the tail servos are under the hatch The kit was meant for pushrods. On the Funtana, each elevator half gets its own servo and the rudder gets a beefy one. Separate elevator servos make for some interesting mix possibilities.

I think the symetry of the rudder generally lends itself to pull-pull. While folks that really don't understand the geometry, like Dick says, won't know where to start on elevator pull-pull.

Check the Funtana set up at:

http://www.rcuniverse.com/forum/fb.asp?m=2227711

RE: Why Pull Pull on Rudder, not Elevator
 

Like Dick I spent the first couple of years at McDonnell Douglas (before Boeing and before transferring into the aero dept.) designing control systems on the F-4K&M. Push rods went the length of the fuselage with lots of bell cranks, etc. and worked fine. I certainly think the airplanes would look better with the servos enclosed and it would stop the (granted small) effect of servos sticking out on the stab part.

Bruce, I should have added at the front I guess - I had been looking at a lot of the big monsters and thinking about control effectiveness, high deflections and high alpha work (Dick started it with his question earlier, let's blame him :-) and going through the mental gymnastics involved and there was a good shot of the elevator. So I started thinking about what made one control system be better than the other from all standpoints, aero, mechanical etc. but primarily aero. So I posted it here knowing there was some big airplane-aero-experienced guys here.

Asking the guys who do it because the other guys did it wasn't going to get a good answer. Really anyone that does the typical setup hasn't really thoght it all the way through unless like John mentioned it is done for some aero effects, I wonder if they have done that on the big monsters?

I pretty well got answers that agreed with what I thought. You can certainly put the question in wherever large scale aerobatic is spoken. No problem. Probably get somebody mad!!

RE: Why Pull Pull on Rudder, not Elevator
 

Ben, probably one of the things that makes pull pull rudder installations easy is that the fuselage is usually symmetrical from side to side so the installation is a mirror image of components. Plus most of the time only one control surface is involved.
Elevators usually are split plus the top and bottom cable exits most likely will be in different locations fore and aft due to asymmetry of the fuselage top to bottom.
These are not show stoppers in the least but I think that non scale and semiscale flyers find the scattering of servos easier than the routing of control cables.
I have large scale planes flying with servos direct, pushrod elevators, and cable controlled elevators, depending on the scale requirements. Non out perform the other. In the end probably design and mechanical skills probably are the deciding factors.

RE: Why Pull Pull on Rudder, not Elevator
 

In giant scale, the stabs are removable and the rudder is not. Much easier to unplug a wire than pull pull cables for the stabs... For me that is enough to convice me to use direct linkages on elevators.

RE: Why Pull Pull on Rudder, not Elevator
 

Excellent point and one that works for me!! Pushrods with a good reliable repeatable connection to a horn would do the same thing though and not have the wire and plug and unplug of electronics issue.

RE: Why Pull Pull on Rudder, not Elevator
 

my rudders are removable

RE: Why Pull Pull on Rudder, not Elevator
 

Yes Dick, but what about your airplane's rudders? .........
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Sorry about that, well not too sorry :-)

RE: Why Pull Pull on Rudder, not Elevator
 

That's gotta hurt though... :D:D:D

RE: Why Pull Pull on Rudder, not Elevator
 

you have never heard of a ship without a rudder ?

RE: Why Pull Pull on Rudder, not Elevator
 

Would any of it have to do with the possibility of a cable snapping? With direct drive, there is a very slim chance a low tension rod will break. Maybe planes have pull pull on the rudder only since, if one cable snaps you can most likely still land without a rudder. Where if a pull pull elevator setup snapped, you'd be screwed. I don't really know, this is just a speculation.

MIke

RE: Why Pull Pull on Rudder, not Elevator
 

Well I can point to a specific time in the pattern/F3A world, mid->late 80's (gee has it been that long?) when weight was a big factor. It was quicly realized that 2 lengths of thin cable or kevlar cord was lighter than a CF pushrod & essentially did the same job on teh rudder control, so out it went. Yes it was also done on elevators, but to a lesser extent. Pattern types want discreet control over both elevator halves, which meant getting the geometry right & keeping it that way over hundred of vibrating flights. It also meant 4 cables vs 1 pushrod & ideally 2 servos to slave each elevator half without it becoming a rats nest in there. When the 4 bangers came along it basically meant gobs of power for all, so the weight thing could be relaxed a bit. With bigger fuses & smaller more powerful servo's comes the opportunity to sink the servo very close to the deflection surface which is probably the best from a slop free standpoint. But in the bigger scale stuff when you have to gang servos together for power, the current setup might be a function of where servos can best be accomodated. So to answer your question, probably some practical reasons & probably just old habits! <g>