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Just a sanity check, what kind of force does the tie rod that connects to the elevator bellcrank see? I'm designing a tie rod to connect to the bell crank lower cable attach point and I'm figuring there's not really all that much force involved. Cable tension is 30-40lbs if I recall, so I would think the momentary forces on the tie rod wouldn't be much more than that, being as the cables basically act as long springs if you're reefing back and forth in pitch. My tie rod has to be on the order of 3ft or so and rather small on the OD so buckling is my concern.
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It depends. I’m guessing this is for a auto pilot servo so forces will be small. In cruise flight the stick forces are pleasantly light. However, on some occasions (usually during a takeoff with a load) I won’t have the trim set properly and it will take approximately 30lbs of push on the stick to keep the nose down. On one occasion it took two hands; not fun. Considering the stick is about 2 feet tall that is a reasonably significant load the bell crank is seeing. I’ll let you do the actual calculation😉Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop. -
Wow, 30lbs is quite a bit of forward pressure. I suppose I could look at how Vans does theirs and try to keep the same moment of inertia, but I need a better idea of the loads. Thanks for the report Whee. I am looking to size this for all flight loads.Comment
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2ft of arm and 30lbs of force is 60ft-lbs. Easy math. Any 4130 tube of any size will take that. 3/16 .035 will even do that load by a massive margin.
Use this once you find the actual force
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If this is autopilot connection there is a bit more to the discussion.
Compression buckling is most likely the critical condition.
The rod and connections should be designed to 15% greater column load than can be placed on the rod with one end fixed rigid and the servo at current limit stall torque or shear pin shear torque.
Those are the factors that are define and constrain the problem. The least weight solution will most likely be aluminum tube 5/8 in x .045 wall.
Then the geometry of the travel of the arms at each end must be matched. ( hint ) simplest linkage would be to have the bell crank pivot point and the AP servo actuating arms be the same distance pivot to pushrod attach point.
Then the stick or elevator travel must not be limited by the servo hitting the stops........ at the elevator travel limit.
Then to be functional, the servo must apply enough torque ( push force ) to overcome the aerodynamic loads imposed via the elevator.
The neat thing is all of that still matters whether the rod is 6 in or 6 ft.
Bearhawk272
KDComment
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The bell crank isn’t symmetrical and there’s some other geometry to consider when calculating the loads. I think 30lbs would be a good start, you need to design for worst case scenarios. Maybe ask Bob too so your sure to consider max loads.
I’m curious what this tie rod is for.
Possibly interesting note: I installed pushrod steering in my boat. I didn’t bother with any real engineering calculations, just kinda farmered it based on previous experience and looking at factory installed systems. I used 3/4†cold rolled steel rod. It’s supported every 24†except the last 3 feet where it attaches to a bell crank via a rod end bearing. The steering is very light, one finger steering, but once it hits a either stop it takes very little force to bow the rod at its unsupported end. It’s elastic deformation so it not an issue but something less robust could quite easily fail. I had to make sure it was tough due to my brother being the type that could break an anvil.Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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Hmm, Euler says a 5/8"X.028 4130 tube will take 530lbs at 36" before deflecting, the benefit of Ti is less than I thought. Might be the way to go.Comment
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Quite teasing and show us fellow nerds what your designing and how you defined your end conditions. Eulers is fun.Originally posted by Archer39J View PostScratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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What about harmonics? Is it possible that the diameter and wall thickness over the length will be strong enough but will it be rigid enough to withstand the frequencies that may be involved with the engine? In my work (race cars), there was a optimum ratio bestrewn length and diameter. I apologist for not remembering what that number was.Comment
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We had a race car crash because a brake line failed from vibration. In developing one model of my cars, I had increased the torsional rigidity of the chassis to where the vibration would foam the brake fluid in the master cylinder reservoirs and that would cause brake failure. There is a big difference in an engine turning 2300 vs 8000 rpm.
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Dave:
Just interested. I haven't really thought about it, and I will fly without an autopilot at first. Why the 3 ft pushrod? Just interested. Might be a good idea. Sounds like you want the servo up by the firewall, or behind the pilot seats. Why not under one of the front seats, actuated on the left or right stick assembly?
I am pretty sure you put more thought into this than me. Looking for ideas.
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I'm looking at a yoke conversion, having done the cost/benefit analysis specific to my mission. I have a Cherokee control column on its way, either to use directly or as an example. I got to looking and it seems the geometry may just work out for a fairly painless conversion. Try as I might I don't think I'm comfortable with my bolt on only solution so I'll have to weld, only one part so far though. Very early stages but looks promising, I'll make a separate post when I'm farther along.Originally posted by svyolo View PostComment
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Take a look at the Maule setup as well. The yoke setup is pretty straightforward there as well. The center section beneath the dash to the floor needs to be open for it to work appropriately.Originally posted by Archer39J View PostChristopher Owens
Bearhawk 4-Place Scratch Built, Plans 991
Bearhawk Patrol Scratch Built, Plans P313
Germantown, Wisconsin, USAComment
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I was thinking a good video would be a demonstration of the controls in action before the fuselage gets covered... from the stick back to the tail surfaces...range of montion and travel...Last edited by way_up_north; 09-28-2019, 07:54 AM.Comment
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LOL. I haven't flown with a proper stick in a long time. I am looking forward to going back to one. Different strokes.Originally posted by Archer39J View PostComment
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