Throttle tugs

[wadeg]

Wondering how others have approached this:

Being that the throttles are under spring tension to toss them to full if the cables break, how does one overcome the desire for the engine to pull itself off of idle? Of course my throttle cable has a 'twist-lock' but of course I cannot be always locking the throttle. Is there a secret?

Thanks!
Wade

[84KF]

I have the same question..., and problem.
I'm thinking of just finding some lighter springs. Just enough to pull the arm open if the cable(s) break, but not so strong as to move the throttle from off the idle position.

I think the main reason for the springs is to keep the slack out of the cables and ensure that the both open equally when the throttle is advanced. (Just a personal opinion...nothing to back it up.) But...I've also run mine with no springs at all and it works fine....just no tension pulling foward.

Another thought.... what happens when (if) just one cable breaks so as to allow just that carb to go full open...? Would you have to open the other with the throttle and finish the flight at full power just to keep the engine running smooth?

[RandyL]

My understanding is that the reason for the spring pulling the throttle to full open is that in the event of a cable break you'd rather be at full throttle using ignition to modulate the power than at idle with no power, so it's a safety thing. Having two carbs does complicate things but getting on the ground safely is the priority. I know many certified planes have full-throttle springs installed on the their carbs for this very reason. With an experimental of course that is up to the builder.

True story: a friend of mine built an RV-8A in Hawaii. He got transferred to the mainland so he sold the plane to an airline pilot living in Hawaii. The pilot really enjoyed the plane and was doing an inter-island flight one day when all of a sudden the engine just lost power, the prop remained windmilling. Nothing he did would bring it back to life. He was on flight following so dispatching the rescue birds was not a problem. He did a normal water landing and although get got knocked around a bit he was fine. While he was treading water waiting for the chopper he watched his RV-8A sink to the bottom in over 1,000' of water. He had opted to not buy hull insurance, ouch! He was picked up by the Coast Guard in 20 minutes, but the plane could not be retrieved. Post accident conclusion was that the throttle cable had detached, the last A&P to work on remembered a bit of an issue with the rod bearing that attached to the throttle arm. Best guess is that the cable detached, the engine's suction pulled the throttle plate closed and the engined idled happily all the way to the water.

So, the springs are there for a reason. Lighter springs might be ok to substitute but I'd keep the same design to make sure that they are able to stand up to the vibration.

[84KF]

The "broken throttle cable" is referred to in Denney Aerocraft Company SERVICE BULLETIN # 12, December 24 1991
http://kitfox.lazair.com/skystar/ser...etins/sb12.htm
Notice that at the time it was common to use a solid wire type... and they broke for the reasons mentioned.

I have never seen, or heard of, a properly maintained and inspected braided cable "just" breaking due to "fracture". Ain't gonna happen.
And a properly swaged fitting is stronger then the cable it's attached to.

What I don't want is a uncommanded power surge at a critical landing or roll out phase due to over aggressive springs rapidly pulling the throttle open if your hand is removed from the control. Been there\done that.

[wadeg]

Watching a RV8 sink away...very sad but even without insurance on the hull, it sure did its job in getting him into the water safely. Amazing how many die trying to save the airplane (ie stall/spin at low altitude).

Saw Rare Bear deal with a stuck throttle after the gold race at Reno last year. Stuck at power for a while circling the field overhead until it lightened up. Cut the engine and settled in without a problem. Full power is a good place to be if you don't have any control over the engine. Time to plan is a friend.

If it hits the fan, use the plane for nothing but you're survival.