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  #1  
Old 04-14-2021, 04:43 PM
BealeStAviator BealeStAviator is offline
 
Join Date: Mar 2020
Location: Clovis, NM
Posts: 11
Default Throttle Quadrant/Mixture Issue

Hey all,

Bottom line up front: My mixture control has full range of motion at the engine, but the lever in the cockpit stops 1" short of forward when running full rich.

Long version: I've noticed a kind of odd setup on my RV-8's throttle quadrant. The mixture lever doesn't go fully to the forward stop. It seems like it runs out of cable an inch or so from what would otherwise be full rich. I say "otherwise" because after checking the engine side of the cable, the mixture control arm moves through its full range of motion and hits both of its stops. The engine appears to be properly calibrated and runs perfectly. It's like I get to full rich but the lever doesn't show it. Idle/cutoff works fine too. Is this a problem then? My thought was that either it was just a sloppy build (I bought it, didn't build it) or it was intentional to get it out of the way of the prop lever, offset it a bit. Alternatively I'm concerned I might be missing something that is potentially dangerous at worst or is costing me some performance at best.
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  #2  
Old 04-14-2021, 09:23 PM
agent4573 agent4573 is offline
 
Join Date: Aug 2019
Location: Mountain view
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If it goes full rich to idle cutoff on the engine side then there's not really an issue. You can fix it if you want to by either putting a longer arm on the engine side, or drilling a hole closer to the pivot point for the cable on the quadrant side. Either option will move the motion ratio in the correct direction. If you want exact numbers for where to drill the hole, post up the following information:

1. Distance from pivot to cable attachment on the engine side.
2. Difference in length of exposed cable when in idle cutoff and full rich (need to know how much throw is required in your setup to move the engine side arm from idle cutoff to full rich.
3. Distance from pivot to cable attachment on the quadrant side.
4. How much further does the quadrant side arm have to move? This should be measured at the flat part of the quadrant with the writing on it to the lever at that point.
5. Height from pivot to the flat part of the quadrant with the writing on it.

Drilling new holes to fix the motion ratios may mean having to adjust cable mounts in order to maintain both idle cutoff and full rich. Make sure at least one side of your cable mount is the adjustable bulkhead kind and not the rivet in place clamp type.
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  #3  
Old 04-14-2021, 09:56 PM
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fl-mike fl-mike is offline
 
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It’s a geometry thing and can drive you crazy if you chase perfection.
I have the same thing, sort of, and mentioned it to my buddy who claimed he had full mixture travel on his quadrant Until I checked. He doesn’t
I settled on having a bit of “sweep” on the mixture/prop/throttle full-forward positions so my palm can push prop and mixture to their stops if I have my fingers around the (full) throttle handle.

For sure you want some extra travel available at each extreme for all the controls. The important travel is at the engine. You must hit the stops there
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Last edited by fl-mike : 04-14-2021 at 09:58 PM.
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  #4  
Old 04-16-2021, 12:11 PM
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Saville Saville is offline
 
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Location: KBVY Massachusetts
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I have the same problem:

You can adjust things so that you get to idle cutoff....

Or you can adjust them so that you get full rich.

An A&P tried to fine tune the cable to get both but was unable.

I have a CT83F.

I would have thought that the bolts holding the quadrant together (which limits the throw of the lever) and/or the position of the connector hole in the lever (or both) would be more than sufficient to move the engine mixture control through it's full range.

So I'm wondering if there's some sort of "loss" along the system that limits the movement of the cable.
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  #5  
Old 04-18-2021, 12:47 PM
Desert Rat Desert Rat is offline
 
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Standard safety practice (it may be in the FARs somewhere too, but darned if I know where) is for the control to hit the stops on whatever is being controlled before it hits the end of available travel on the cockpit side.
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  #6  
Old 04-18-2021, 12:58 PM
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Saville Saville is offline
 
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Quote:
Originally Posted by Desert Rat View Post
Standard safety practice (it may be in the FARs somewhere too, but darned if I know where) is for the control to hit the stops on whatever is being controlled before it hits the end of available travel on the cockpit side.
Yes of course.

Problem is that with the trow available on common quadrants, and the position of the clevis hole on the quadrant lever, sometimes this isn't possible.

And the question is: why?

Why does it work for some people but not others?
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Old 04-18-2021, 05:00 PM
Desert Rat Desert Rat is offline
 
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Quote:
Originally Posted by Saville View Post
Yes of course.

Problem is that with the trow available on common quadrants, and the position of the clevis hole on the quadrant lever, sometimes this isn't possible.

And the question is: why?

Why does it work for some people but not others?
I don't have the answer. I was responding to the OP who was questioning whether it was okay to hit the stops up front while still having lots of throw available on the cabin side.
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  #8  
Old 04-18-2021, 06:08 PM
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n82rb n82rb is offline
 
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The answer is because over the god knows how many years they have been building carbs, the have made god knows how many arms. Each one has a different geometry. The length of the arm determines how much linear movement you need for stop to stop. Itís a matter of getting the right lengths on the carb arm and the hole in the right spot on the lever arm.
As itís been said, as long as the carb arm hits the stops on both ends your good to go.

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  #9  
Old 04-18-2021, 06:53 PM
agent4573 agent4573 is offline
 
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The only thing I don't like about hitting the stops on the engine side while there's still throw left on the quadrant side is you can overstress the system. If you go all angry gorilla on the control, once the engine side hits the stop, all the extra force gets put into the cable and into the small arm on the engine side controls. It's always better to have a hard stop on the pilot side so that any extra force goes into the quadrant and not to the engine.
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  #10  
Old 04-18-2021, 07:27 PM
TimO TimO is offline
 
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Quote:
Originally Posted by Saville View Post
Problem is that with the trow available on common quadrants, and the position of the clevis hole on the quadrant lever, sometimes this isn't possible.
There's nothing stopping you from drilling a new hole on the quadrant lever, if you are careful that it won't interfere. Usually by a combination of drilling the lever on the engine and/or the quadrant, you can come up with an ideal solution that gives your quadrant nearly full travel (with a tiny cushion) and still hit stop to stop on the engine side. The benefit is that at that point you have a little finer control over whatever control it is you're fixing. i.e. you can get more precise mixture if you spread it out to nearly full throw on the quadrant.

You can even remove the cable from each end and test various combinations if you're careful.
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