Throttle/Mixture/RPM Cable throws?

[claycookiemonster]

I'm measuring carefully to get the required length for the outer sleeve of these cables as well as the length needed for the actuator within. So far, so good.

The challenge is that the "throw" distance at the quadrant isn't the same as the "throw" possible at the actuator end on the engine.

The Throttle lever in the cockpit has a throw of 2.25" at the quadrant, however the actuator on the Servo has a throw of 1.8"
The Mixture lever in the cockpit has a throw of 2.2" at the quadrant, but the throw at the engine is 2.5"
The RPM (CS Prop) throw in the cockpit is 1.6" while the throw available at the engine is 2.1"

So, the Throttle lever has more movement than the engine allows, while both the Mixture and RPM levers do not move enough to actuate the engine controls to their full range.

Several questions arise:
1. Can I, should I assume that the controls in the cockpit will need to actuate the levers at the engine to their full range?
2. It seems obvious that altering the throws could be done by drilling new holes in the levers in the cockpit more easily than altering things at the engine end.
3. It also seems possible to add some sort of intervening bell crank between cockpit and engine, but that feels like going down a rabbit hole and hoping to find a way out later.

Thoughts?

 

[Mike S]

3. It also seems possible to add some sort of intervening bell crank between cockpit and engine, but that feels like going down a rabbit hole and hoping to find a way out later.

Thoughts?

That could work, could reverse the push/pull also if needed.

Might be easier to add throw at the quadrant by drilling a new hole ???

 

[rv8ch]

...
Several questions arise:
1. Can I, should I assume that the controls in the cockpit will need to actuate the levers at the engine to their full range?
2. It seems obvious that altering the throws could be done by drilling new holes in the levers in the cockpit more easily than altering things at the engine end.
3. It also seems possible to add some sort of intervening bell crank between cockpit and engine, but that feels like going down a rabbit hole and hoping to find a way out later.

Thoughts?

I think you are right on all three - full actuation at the engine side, new holes in the levers can be drilled if needed, and a bellcrank is not needed.

You don't need to have the throws at the quadrant to be exactly stop to stop as long as the movement on the engine side does go fully stop to stop.

 

[pilotpat]

I found that the bends in the cable from the quadrant to the engine absorbed significant travel, the prop being the worst at about 270 degrees in bends and over 1/2 inch lost. I could not get full travel at the engine side, and had the cables reworked at MacFarlane. This was for an RV9A and IO-320.

 

[Drum]

Hi Clay - Good article on this in Kitplanes article by Dave Prizio Dec 2012 entitled “Firewall Forward: Engine Controls.” I copied a relevant section for you. Godspeed.

Best practice calls for a throttle control that has a range of motion that slightly exceeds the full travel of the carburetor or fuel-injection servo. In other words, the range of motion of the throttle should be limited by the stops on the carburetor, not the limits of travel of the throttle control. This extra travel is called “control bounce” or “cushion.” This is in keeping with standard aviation practice, wherein the thing being controlled limits the travel of the control, rather than the thing doing the moving setting the limits. When checking control travel, work the actual controls from the cockpit, not at the engine-compartment end. This is the best way to ensure that everything will work properly in flight.

Warm regards,

 

[BobTurner]

Yes, best practice is for cockpit controls to have some ‘cushion’ showing. The way, if some day you see the cockpit control going all the way to its stop, you know something is wrong.
You can slightly increase engine arm movement by mounting the arm to the cable ‘off center’, e.g., middle of the cable extension is not when the arm to cable angle is 90 deg. Don’t go over-center, though!