What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Question on the Rotax engine...

D

David-aviator

Well Known Member
I was flying with a friend in his Rans Coyote yesterday and had a devil of a time with the Rotax engine.

This particular engine set up has rather strong springs pulling the throttle to wide open and unless the friction brake is constantly being adjusted, the engine accelerates when least desired - like in the flare. Is this a normal feature of the Rotax?
 
"This particular engine set up has rather strong springs pulling the throttle to wide open and unless the friction brake is constantly being adjusted, the engine accelerates when least desired - like in the flare. Is this a normal feature of the Rotax?"

Aloha DD the Rotax engines are setup with the carb springs to pull them wide open in the event of a failure.. throttle fiction lock is my friend
 
A complete solution...

To add to what Robert posted, there is an RV12 specific Push/pull vernier throttle cable assembly that includes the new springs from the same supplier. Part #MCRV12-T
http://www.mcfarlane-aviation.com/Products/?ID=73756587&PartNumber=MCRV12-TV&

They also can supply the cable for other Rotax powered aircraft as well.

This cable has been installed and reported on in the forums here, and the users report it works well. I have mine on the workbench to be installed along with a (long, and getting longer) list of other upgrades and SBs at next months annual inspection.

Tony
 
Last edited:
Tell your friend to order the less-strong springs. He won't regret it. They still have the desired fail-safe effect, but won't yank the throttle out of his hand like the original ones. JMHO
 
Before my First Flight this week in N737G, I flew a bunch of circuits and bumps in Tony's airplane N477RV. He has the OEM strong springs - I immediately noticed the trouble I was having keeping the power at Idle, even after cranking down the friction lock. On each final approach, I was mostly controlling the airplane with one hand and pulling like the dickens on the throttle with the other.

I also flew the Vans Demonstrator extensively last summer, and I believe it has the OEM strong springs. I remember pulling on it a lot too, but not as much as Tony's. Still, I do remember a few hot, long landings where it turns out I had substantial power on that I was not aware of.

I installed the McFarlane 5 lb springs prior to my First Flight. These are described as "less than half the Rotax OEM spring force." They also offer a set of 3.5 lb springs. Granted, my experience with these springs is limited, but I noticed immediately that my fighting with the throttle was much diminished - maybe even gone away.

This subject has been hashed and re-hashed endlessly on this Forum, but I personally do NOT like the Rotax schema, and don't really understand it. Fighting the throttle during landing is a major distraction, and going uncommanded to full throttle is reminiscent of the (supposed) Toyota problem of throttles going wide open and causing car crashes. It's hard to imagine the Vans supplied throttle cables, which are pretty doggone stout, failing and requiring the throttles to go full open. It looks to me much more likely that those springs will fail before the cable.

And, if that really happened, and the engine went to wide open - THEN what would you do??? How would you shoot an approach? Blip it like the rotary engine in a WW I fighter?

Bob Bogash
N737G
Flying
 
Yep, Bob

I'm not particularly fond of the Rotax setup and have installed the weaker McFarlane springs, but I understand the reasoning. If the cable breaks and you go to wide open throttle, you can always use the mag switches. If it fails and you go closed throttle, you ARE going noplace but down.

Wayne 120241/143WM
 
NASA515 said:
Before my First Flight this week in N737G, I flew a bunch of circuits and bumps in Tony's airplane N477RV. He has the OEM strong springs - I immediately noticed the trouble I was having keeping the power at Idle, even after cranking down the friction lock. On each final approach, I was mostly controlling the airplane with one hand and pulling like the dickens on the throttle with the other.

I also flew the Vans Demonstrator extensively last summer, and I believe it has the OEM strong springs. I remember pulling on it a lot too, but not as much as Tony's. Still, I do remember a few hot, long landings where it turns out I had substantial power on that I was not aware of.

I installed the McFarlane 5 lb springs prior to my First Flight. These are described as "less than half the Rotax OEM spring force." They also offer a set of 3.5 lb springs. Granted, my experience with these springs is limited, but I noticed immediately that my fighting with the throttle was much diminished - maybe even gone away.

This subject has been hashed and re-hashed endlessly on this Forum, but I personally do NOT like the Rotax schema, and don't really understand it. Fighting the throttle during landing is a major distraction, and going uncommanded to full throttle is reminiscent of the (supposed) Toyota problem of throttles going wide open and causing car crashes. It's hard to imagine the Vans supplied throttle cables, which are pretty doggone stout, failing and requiring the throttles to go full open. It looks to me much more likely that those springs will fail before the cable.

And, if that really happened, and the engine went to wide open - THEN what would you do??? How would you shoot an approach? Blip it like the rotary engine in a WW I fighter?

Bob Bogash
N737G
Flying
Click to expand...

Thanks to all for the information, I passed it on to my friend.

The reasoning behind the springs seems a bit paranoid. If the cable system is properly designed, it won't fail.

As Mr. Bogash says, this has been discussed here (and elsewhere) before. I remember 30 years ago Burt Rutan had a spring on the carb throttle arm but it was passive and did not interfere with day to day flying. It assumed a failure at the arm. Today I know of no springs with Lycoming engines, the cables and fittings, if properly installed, are very reliable. In the early days when a push-pull wire was used with screw clamp fittings, a spring made sense.

With the Rotax, what would happen if the cable to one carb failed but not the other? Will the engine run with one carb at WOT?

As a post script to this discussion, I do believe Rotax ordered and paid for the development of a AFP FI system for their 912/914 engines but nothing came of it. I remember seeing the completed manifold and system on one of their engines while attending the Fuel Injection 101 course a couple years ago.

Rotax is indeed married to carburetors, nothing wrong with that, they've worked well for many years. Just like magnetos. :)
 
FYI. The throttle cable in my Cherokee frayed inside the housing. It was getting harder and harder to move. No one had any idea what was wrong until the mechanic parted it while trying to work lube down the cable. So if your throttle is getting stiffer and stiffer don't assume it just needs lube. Wires from the cable may have parted and be interfering with cable movement.
 
David-aviator said:
With the Rotax, what would happen if the cable to one carb failed but not the other? Will the engine run with one carb at WOT?
Click to expand...

You ask an excellent and very germane question, David. One I have also pondered. The RV-12 Rotax installation isn't really one throttle cable - it's two. So it's highly unlikely both cables would fail at the same time. That means, as you noted, that one carb and its two cylinders would be affected while the opposite carb and its cylinders were not.

The situation would then be two cylinders running WOT due to the throttle spring, and the opposite cylinders running at some other power setting - perhaps Idle.

I have no idea what the engine would do under those circumstances, or if it would like that situation at all. Some really bad and asymmetric stresses could be created along the mechanical power train.

It DOES raise the question, however, about the Rotax throttle spring philosophy. By that, I mean, if the engine can run, to some degree, satisfactorily on the power produced by just two cylinders, then why have this back-up WOT throttle spring scheme at all. If one of the two throttle cables were to fail, and that carb were to go to Idle instead of WOT, the power produced by the other carb and its cylinders would be enough to get you to an airport.

If that is NOT the case, and the engine will not run satisfactorily with a large power asymmetry LH vs RH, then you may have your hands full controlling the engine (and airplane) when one of the carbs goes WOT. Granted - if you are thinking fast on your feet - and can get the engine to run smoothly by advancing the throttle and the remaining good carb to WOT, you could proceed to an airport at WOT.

Maybe some questions with answers from the more knowledgeable engine experts on the Forum.

Bob Bogash
N737G
 
A lot of people who have had a cyl failure on a Lycoming in flight will tell you that it felt like they lost more like 50% power, not 25%.

I think you would find similar results with the Rotax (though I have no personal experience).
So, if you lost 2 cyl, how much power would you have? I guess it would all depend on what position the throttle was on the stuck carb.

BTW, power (thrust) output is very non-linear when factored against RPM.
THIS ACCIDENT had the engine running smoothly at 4000 RPM but that wasn't enough to climb. Somethign to think about....
 
I think we're all familiar with this accident, Scott - it has been referenced many times in the past and has become the poster child for this situation. It certainly raises excellent questions. But, it also begs a few others - like what happened to his throttle cable that forced the system to revert to the WOT springs? And - what constituted the actual force of his "weaker" springs and how they compared to the Rotax OEM springs.

Has Vans ever received a report of a throttle cable failure on the Rotax installation? On the other Vans series airplanes? What is the failure mode and what pre-failure warning signs were evidenced?

What would be the effect of going to WOT on one side while at Cruise power on the other? Idle power on the other side? (like on Approach.) I think if a severe engine vibration resulted, most pilots would react by pulling the throttle to Idle, not going to WOT.

How would the airplane perform during the Approach and Landing phase with one side WOT and the other at Idle? It seems the positive side of the WOT spring situation is during the Take-off/Climb, while the negative side may be during the Approach and Landing.

I realize that you probably don't have the answer to some (or all) of these questions, but they are the other side of the spring question.

Bob Bogash
N737G
 
NASA515 said:
I think we're all familiar with this accident, Scott - it has been referenced many times in the past and has become the poster child for this situation. It certainly raises excellent questions. But, it also begs a few others - like what happened to his throttle cable that forced the system to revert to the WOT springs? And - what constituted the actual force of his "weaker" springs and how they compared to the Rotax OEM springs.

Has Vans ever received a report of a throttle cable failure on the Rotax installation? On the other Vans series airplanes? What is the failure mode and what pre-failure warning signs were evidenced?

What would be the effect of going to WOT on one side while at Cruise power on the other? Idle power on the other side? (like on Approach.) I think if a severe engine vibration resulted, most pilots would react by pulling the throttle to Idle, not going to WOT.

How would the airplane perform during the Approach and Landing phase with one side WOT and the other at Idle? It seems the positive side of the WOT spring situation is during the Take-off/Climb, while the negative side may be during the Approach and Landing.

I realize that you probably don't have the answer to some (or all) of these questions, but they are the other side of the spring question.

Bob Bogash
N737G
Click to expand...

It seems obvious that you would have to ask the builder of the 701, the answers to the questions related to that accident. One detail that seems obvious to me though is that it wasn't caused by degradation due to age.

I am not sure of the relevance of your cable failure questions... since there are likely many different failure modes/causes possible.
I am not aware of any RV-12 throttle control failures. I am aware of one RV-6 throttle failure (froze up at partial power setting). The pilot simply flew over the airport, shut down the engine, and flew a power off approach and landed.

As for the Rotax engine performance questions... I gave my opinion in my previous post... If there was a very significant difference between the two throttle settings, I think the engine will be running pretty badly and airplane will be coming down.
 
Question - Since there is a balancing tube running between the two intake manifolds, would this allow the engine to continue to run in somewhat of a normal manner if one carb were to go to either idle or full speed?
 
According to Jim Scott of AirCore Aviation, that balancing tube accommodates only minimal pressure differences between the two carbs. In the event of a large imbalance - say one side goes WOT while the other is at Idle - during an Approach, the pilot will experience such heavy vibration that he may think the engine is going to shake itself off the airplane.

Bob Bogash
N737G
 
NASA515 said:
That means, as you noted, that one carb and its two cylinders would be affected while the opposite carb and its cylinders were not.

The situation would then be two cylinders running WOT due to the throttle spring, and the opposite cylinders running at some other power setting - perhaps Idle.
Click to expand...
This situation actually happened to me while receiving training as a student pilot in a 912S powered RANS S-12XL. One of the throttle cables had worn out and broke while cruising at 3000ft AGL. The result was a very rough running engine with noiticable (but not scary) vibrations. After trying a few tests for clogged carb bowls, fuel flow problem or carb icing (it was November) we decided to land controlled at the nearby home field and check the cables. We were able to do a field repair and take off again 10 minutes later.
My point is, the plane was absolutely controllable, did still climb fine (with no vibrations) at full trottle and was landable at idle with slightly increased vibrations but no serious issue. We killed the engine on safe final when it was apparent that we would make the runway as it ran at much higher RPM than usual. The engine had the OEM springs in it.
 
McFarlane's answer

Talking with Mr. Mc. at McFarlane's regarding the throttle springs in OSH, he felt that the spring strength selected by Rotax was based on using the cheapest/un-slickest cables ever conceived, and that, with fine-quality/slick-on-the-insides McFarlane cables, the excessively strong springs were not needed to achieve WOT on a cable break. My favorite pilot wife, doing taxiing tests with the stock throttle/springs combination, was not too pleased with the feel of it, since she was used to Jetguy's plane (for excellent transition training, BTW), which has the weaker springs. I replaced both the springs and the throttle (the vernier throttle mentioned above), and she is now a happy camper.
 
You must log in or register to reply here.
Back
Top