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olderthandirt

Well Known Member
Today I noticed the passenger side carb spring had broken, so went flying anyway and did not notice any difference..hence my question: why do we need these springs??? I purchased a spring from Rotax dealer: ($14.00) then on way home stopped at hardware store and purchase a like spring for $1.29...glad we have experimental airplanes and do not have to meet stringent standards like certified...by the way the engine has 12 hours on it...is there anything else in this world that only lasts 12 hours????
 
Does you engine run super smooth (I.E. are you sure the carbs are well balanced/synched). Do you know for sure the pitch on both of your blades is within .1 degree of each other.
Have you had a dynamic prop balance done? If not I would highly recommend you do so.

Breaking of the light weight springs supplied by McFarlane has happened to some, but in 12 hrs is ridiculously short time. It is very likely that you have some anomaly with your airplane that is inducing undesirable vibration.
 
spring broke

Do not have another plane to compare with but would say my plane engine runs quite smoothly...I have new props with pins that line up props so guessed that was o.k.... to my way of thinking for a spring to break at twelve hours the engine would have to be vibrating off firewall...seems to me to be less than quality spring??
 
Prop pitch

I have the new style prop with the setup pins. I use the pin to get the pitch in the neighborhood and then the final pitch is set with the procedure outlined by Van's. Just using the pin setting resulted in about a .3 degree difference between blades which is enough to cause your problem. I find that the same .3 degree pitch change makes about 500 rpm static RPM difference. I try to get both blades exactly the same pitch, .1 degree max diff. But I'm a machinist and precise setups are my trade. 55 hours on my bird and no broken throttle springs.......yet.
 
DBRuth said:
I have the new style prop with the setup pins. I use the pin to get the pitch in the neighborhood and then the final pitch is set with the procedure outlined by Van's. Just using the pin setting resulted in about a .3 degree difference between blades which is enough to cause your problem. I find that the same .3 degree pitch change makes about 500 rpm static RPM difference. I try to get both blades exactly the same pitch, .1 degree max diff. But I'm a machinist and precise setups are my trade. 55 hours on my bird and no broken throttle springs.......yet.
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olderthandirt said:
Do not have another plane to compare with but would say my plane engine runs quite smoothly...I have new props with pins that line up props so guessed that was o.k.... to my way of thinking for a spring to break at twelve hours the engine would have to be vibrating off firewall...seems to me to be less than quality spring??
Click to expand...

Like Doug said, the pins just get you in the ball park but typically don't set the blades accurately enough. A difference of .3 deg between blades can definitely be felt.
It does not take severe vibration to cause fatigue failures in parts.
This type of failure is often caused by a vibration freq. inducing a vibration in the failed part which is causing it to vibrate (flex) many times (possibly many thousands depending on the vibration freq.) per minute at a specific location which causes it to fatigue and break.
This is no different than slightly bending a piece of wire back and forth with your fingers until it breaks. If you were able to do it very quickly (at a high frequency) it would break in a very short time.

As you said you are in the same boat as everyone else new to the Rotax. You have nothing to compare it too. That is why having a dynamic balance done is highly recommended so that you can determine if it is running the way it should. But doing the balance is a waste of time and money without confirming both the blades are at the same pitch (or no more than .1 deg difference) and that the carbs synch is well balanced.

A properly running Rotax 912 feels almost turbine smooth compared to a Continental or Lycoming.
 
After arriving at my cross country destination I found that both of my McFarlane springs had broken. I called McFarlane and they asked if I had used any tools to install the springs. I fessed up that I had used a needle nose plies, and the tech said that my installation probably caused a stress mark which worked into a break.

Knowing that I had caused the problem, McFarlane overnighted the springs to me in Oklahoma at NO CHARGE on a Saturday. They said to use only fingers, no tools when installing the new springs.

Following their caution, I now have 50+ hours on the new springs.

Mitch
N2ET
 
That definitely can cause a rapid failure but unfortunately using only fingers to install doesn't assure the will never break.
 
Throttle springs.

Postaltwo said:
I would think it's a safety incase the cable breaks the default will be idle
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Incorrect, Defiantly defaults to wide open throttle (WOT). In the event of a broken throttle cable, An idling Engine is as useful as the altitude above you or runway behind you! ::D
 
olderthandirt said:
So I took airplane to shop and had dynamic prop balance...it was not out of balance and nothing was needed...except to pay $750.00 now tell me how the spring on carb broke at twelve hours due to vibration??? These springs have broken in past and I believe it is due to faulty material/Bends... Scott is just covering for Van's and I for one am tired of all faulty products being blamed on builder/owner...let's just own up to the real problem product or otherwise...I just spent enormorse amount of money and fuel and time to prove nothing was wrong...because Scott said it was vibration...
Click to expand...

Did they sell you the prop balancer at that price?
 
olderthandirt said:
Scott is just covering for Van's and I for one am tired of all faulty products being blamed on builder/owner
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I'm just shy of 200 hours on the same set of springs, so if yours are faulty it is not a statistically proven fact that they are all faulty, nor is it a proven fact that anyone is covering up anything. That's a pretty harsh accusation based on so little actual fact, in my opinion.
 
Prop balance

Did the shop also confirm or set the pitch of both blades in that price? Even with that, the price seems way out of line. I would have checked everything I could have before doing the prop balance, like checking the pitch as outlined by Van's procedure. Not sure but there is a possibility the dynamic balance could be good and the blades not equally pitched. Anyway,springs are a highly stressed part and subject to vibrations . Cars use " shock absorbers" , really spring dampers , to prevent springs from vibrating. Not making excuses for the manufacturer,but there are many factors involved, some of them owner induced. The most likely time for any part to fail is when new, infant mortality. Breaking a $14 spring is not the end of the world,aggravating yes. If that spring was on a C172 it would be$50.
 
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