What did I do wrong with my McFarlane Cables?

[claycookiemonster]

These things are beautiful, but they are not cheap. So, I measured the set up compulsively and carefully several times. I mocked up the cable runs with 3/8" hose. I measured the throw required at Prop Governor and Throttle and Mixture arms many times. I transferred this distance to the Throttle Quadrant (Acft Spruce SMCT3L) and redrilled several holes to get the needed throw. It was going to be PERFECT.

And then it wasn't. Despite all of the above, all three levers at the quadrant are restricted from going full aft by something we cannot see. We are finding ourselves considering drilling new holes in the quadrant levers to change the throw again. While there are some curves required in the cables, for example to route the prop cable to the rear mounted governor or to the throttle arm on the right side of the engine, none of the curves are very sharp. I'd expect everyone to have to do similar curves to get the job done.

We discovered this yesterday, Friday, and McFarlane was closed before I could call for help. So, I'm calling here. HELP!

 

[mburch]

I think the most obvious next step would be to disconnect each clevis at the throttle quadrant end, to reduce the number of variables.

If you then still can't get the travel you want at the engine end, disconnect that end too and measure the actual amount of travel you're able to get at the engine end, just to make sure the travel isn't somehow being limited by the cable itself.

If you can get the travel you want when the cable clevis is not connected to the lever, then probably what is happening is that the lever is hitting the inside of the clevis fork at some point in its travel. I had this same problem and solved it by using a different set of clevises with a deeper "throat". You can also improve this situation by biasing the hole in the lever towards the engine, rather than centering the hole in the middle of the lever arm.

More details here: https://www.rv7blog.com/2008/04/20/finished-engine-controls-and-throttle-quadrant/

 

[RV7A Flyer]

If you can get the travel you want when the cable clevis is not connected to the lever, then probably what is happening is that the lever is hitting the inside of the clevis fork at some point in its travel. I had this same problem and solved it by using a different set of clevises with a deeper "throat". You can also improve this situation by biasing the hole in the lever towards the engine, rather than centering the hole in the middle of the lever arm.

More details here: https://www.rv7blog.com/2008/04/20/finished-engine-controls-and-throttle-quadrant/

Spot on! I had exactly the same issue, solved in exactly the same way. Excellent advice!

 

[pilotpat]

Could be the curves

I had a similar problem (RV-9A with throttle quadrant), and realized the travel loss was caused by the installed cable not being straight.
On the bench, check the travel in the unbent cable, then do it again with a 90 degree bend. I lost at least 1/8" travel, presumably from the cable not being tight against the inside of the sheath. The prop cable bends the most, and lost enough to be unusable. McFarlane reworked the cables to add 1/2" travel for $100 each, and all is good.
My eyes still hurt from when they popped out during the bench test!

 

[JDA_BTR]

I’m putting an IO-390 in my RV8 and I don’t know how to measure the cable length at all. Is there a stepwise guide anywhere?

I can’t do it yet because my engine is in the crate, the cable connect points in the engine aren’t in yet, etc. But I’ve been looking for how it’s done for a long while.

 

[rv8ch]

I’m putting an IO-390 in my RV8 and I don’t know how to measure the cable length at all. Is there a stepwise guide anywhere?

I can’t do it yet because my engine is in the crate, the cable connect points in the engine aren’t in yet, etc. But I’ve been looking for how it’s done for a long while.

There is another thread on this somewhere. My advice is to take plenty of time to do this, and be very careful. I had to go a couple of rounds of ordering new cables due to mistakes, and they are pretty expensive.

If you can find another similar cable to do a test install, or some stiff wire, or something else, that will save a lot time and money. Even with this knowledge and technique, I found a way to make a mistake.

 

[Ironflight]

I’m putting an IO-390 in my RV8 and I don’t know how to measure the cable length at all. Is there a stepwise guide anywhere?

I can’t do it yet because my engine is in the crate, the cable connect points in the engine aren’t in yet, etc. But I’ve been looking for how it’s done for a long while.

1/4” poly tubing (pitot/static tubing) is inexpensive and a good stand-in for cables. Route it the way it needs to go, then measure the length.

Paul

 

[Ed_Wischmeyer]

I got my cables from California Push-Pull, IIRC, and they already knew all the lengths.

 

[claycookiemonster]

OP here. Yes, I carefully mocked up the cable from throttle quadrant bracket to the brackets on the engine with some 3/8" poly tubing. Remember this is measuring for the OUTER SHEATH of the cable which will have threaded ends which will be fastened to brackets on both ends. That came out correct. Then I carefully measured the full travel of the mechanism (Prop, Mixture and Throttle) and drilled holes in the quadrant levers to achieve the necessary travel needed. I will check to see if the clevis forks are hitting the levers at the extreme travel and relieve the side of the lever if they are. I've already done that and thought it was no longer an issue, but maybe things changed.

I can't quite wrap my head around the inner cable "losing" travel within it's sheath because of curves, but enough people have said so that I'll grant it possible. My question about that is how the heck could you ever anticipate such travel loss before putting the final cable in place? Is there some general guidance, like "add an extra 1/8" for every 90 degrees of bend?" And, extra what? Extra length of the inner cable? Extra travel beyond what I measured at the prop governor or fuel servo?

Seems like quite a lot of art has crept into the simple science of measuring distances.

 

[agent4573]

My question about that is how the heck could you ever anticipate such travel loss before putting the final cable in place? Is there some general guidance, like "add an extra 1/8" for every 90 degrees of bend?" And, extra what? Extra length of the inner cable? Extra travel beyond what I measured at the prop governor or fuel servo?

Seems like quite a lot of art has crept into the simple science of measuring distances.

If you order the exact amount of travel that's required, you leave yourself zero room for adjustment. The bulkhead mount has to be perfectly distanced on both ends in order to not limit the throw in one direction and your only adjustment will be in the clevis ends. my general rule of thumb is order at least 1/2" more throw than required, 1" extra if it will physically fit. That gives you adjustment room on the bulkheads and clevis pins.

I also hit the clevis on a djm throttle quadrant and had to redrill a hole closer to the edge in order to make it work. The quadrant also didn't have enough throw for the SDS throttle body and there wasn't room on the throttle body to redrill, so the quadrant throw had to be increased.

If you design the system to be perfect, perfection is the minimum required product. If you have a little extra throw in the quadrant and a little extra in the cable you give yourself wiggle room to have a functional system even if other tolerances don't stack up to perfection.

 

[rv8ch]

...

Seems like quite a lot of art has crept into the simple science of measuring distances.

I could not agree more. The travel is what bit me. I finally figured out that adding more travel to the cable does not hurt anything, and doesn't cost anything. So mine are not limited by travel on either end - they hit the mechanical stops at the device being controlled.