YellowAircraft

Hi,

I'm trying to demonstrate to a certain engineer that a rigid pull-pull steering setup (3mm pushrods) is problematic when the geometry isn't exactly symmetrical. In this case, the servo output shaft is offset from the center of the strut pivot point AND the steering arm is forward of the strut pivot. Additionally, the span of the steering arm is shorter than the span of the servo arm. His belief is that the fact that everything is on ball joints mitigates all this. That's not correct.

Anyway, I wonder if anyone has any links to any videos showing a pull-pull setup where one cable is going slack upon deflection? Or maybe someone could post a clip of such a setup if you have one? I'm in Asia right now and don't have access to my own stuff, but I kinda need to short-circuit this issue for this fella before he goes into production with this built-in servo-burner-upper.

mongo

My Feedback: (15)

well,
sometimes experience is the best teacher, and bad experience teaches the best lessons...

Shaun Evans

Haha

Yes, indeed. But THIS time, I'd just like to circumvent all that. Every once in a while, we gotta be that parent that prevents the child from touching the hot stove. So... got anything for me (besides that nugget of Yoda-ish wisdom)?

causeitflies

A little sketch of the setup would help. It doesn't have to be precise. It sounds like a double push-pull system instead of pull-pull. As you stated, symmetry is the key. What kind of engineer is he?

BarracudaHockey

My Feedback: (11)

Correct me if I'm wrong (i'm not an engineer) but rigid means push/pull correct? Pull pull would be wires not pushrods each putting tension on when it pulls. I'd think the easiest way to eliminate binding would be to use 1 pushrod otherwise anything less than perfect geometry would cause binding.

FlyJ

I put this in CAD for you and it shows the geometry problem well.

Setup:
4" servo arm
2" control horn
18" wires

When both arms are horizontal, hole-to-hole lengths are 18" (this is set by the wire length). When you turn the servo arm 20°, one side remains 18" (set by the wire in tension), but the opposing side increases to 18.02". The reason for this is because of the pivot arm length. If the arms are the same length, then they will remain parallel through all angles. If the arms are different lengths, they will not remain parallel through their travel and this will result in slack on the non-tension side

mongo

My Feedback: (15)

then,
my next step would be to ask him why he does not use a rigid dual push/pull on his elevator, rudder and even ailerons. make the point that if a single pull push is ok for flight controls, it is more than adequate for nose steering.

mongo

My Feedback: (15)

i guess, in order to be certain,
is this one of those set ups where the steering servo "rides" through the retraction motion with the gear leg?

in that situation, i can see a rigid dual push/pull being able to work with minimal binding at the ends of the throws. probably produce excess current draw and most likely premature servo failure. with short links and the arms kept closer to the same length, it could work, for a while.

YellowAircraft

Hi,

Yeah, you're correct. Double push-pull. Aero-Engr. What's not clear from the photo is that the servo output shaft is also offset from center relative to the center of the strut pin.

YellowAircraft

THIS!

Sadly, the problem I'm trying to solve isn't how to eliminate the natural binding this asymmetry is causing. It's trying to convince the boss that the problem exists.

mongo

My Feedback: (15)

if he will believe in electronic measurements:

you can put an amp meter on the steering servo and show him the amp load increase as the system is moved about.
how it stays relatively constant around center and climbs as the deflection increases

speedracerntrixie

My Feedback: (29)

Simple, let him set it up his way but disconnect one ball link. Demonstrate that at neutral everything lines up, then show him that it doesn’t line up at the end points. Hopefully then he will be more receptive to the solution.