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m.tucciarone(at)hotmail.c
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 Posted: Sat Nov 20, 2010 4:34 am Post subject: Rotax 912 throttle return springs |
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I think it is a good idea to have the throttle wide open in case of a cable
break. But I have always wondered how long the 912S would go on full
throttle. I was taught to use full throttle on take-off for a few minutes
and then 5500 max cruise. Would not the engine just blow up?
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Sat Nov 20, 2010 4:59 am Post subject: Re: Rotax 912 throttle return springs |
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If your prop pitch is set properly (5,500 rpm with WOT in straight and level flight), then the only time you would overspeed the engine would be in descent. The engine is good for 5500 rpm all day long.
When it comes time to descend, if you want to save your engine, then you turn the fuel off and when the engine dies, you cut off the mags and do a dead-stick approach and landing. You will have to cut the engine off anyway at some point or you won't be able to get your speed down to normal approach airspeed, so you might as well do it before you start your descent.
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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m.tucciarone(at)hotmail.c
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| Posted: Sat Nov 20, 2010 5:15 am Post subject: Rotax 912 throttle return springs |
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I never tried strait and level at full throttle. My engine goes to 5800 on
take off. It was set up that way when I bought it. I fly low, so I would
imagine it would go past 5800 in strait and level. What are your engines
doing on take off?
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Sat Nov 20, 2010 5:32 am Post subject: Re: Rotax 912 throttle return springs |
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m.tucciarone,
You are losing a lot of cruise speed with the prop pitch set for 5800 rpm during climb but getting better climb performance than you will with a little more pitch.
I've owned three airplanes with 912 engines. With the prop pitch set for 5500 rpm at WOT in S&L flight, the climb-out rpm at Vy depends upon which prop you have and on what airframe. Mine varied from about 4800 rpm to 5100 rpm at Vy climb at WOT.
With a given prop, the faster the airframe, the lower the Vy climb WOT engine rpm will be, and vice versa.
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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jlatimer1(at)cox.net
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| Posted: Sat Nov 20, 2010 7:34 am Post subject: Rotax 912 throttle return springs |
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I actually had a throttle cable break on a Super Cub. It also has a
throttle spring that went full throttle. After a little thought I used
the mixture control to control the rpm and was able to land with no
issue.
Because of this incident, I've given some thought of what to do if the
same thing happens with my 912 and you're in flight. The first thing to
do is to turn one of the ignitions off. The rpm will be reduced by a
couple of hundred rpm. Then you turn the other mag on and off to
control the rpm, similar to what was done in WWI with one of the motors
used back then. A little inconvenient but doable. If your WOT setting
is set above 5500 you should be able to keep the motor from being
damaged while finding a place to make a safe landing.
Jerry
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b.carl@sympatico.ca
Joined: 20 May 2008
Posts: 77
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| Posted: Sat Nov 20, 2010 7:45 am Post subject: Rotax 912 throttle return springs |
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Listers.
Interesting discussion.
I'm squarely in the" don't screw around with the return springs" group but will add a few extra thoughts to ponder.
Depending where the linkage breaks the situation will vary from both carbs fully open to an out of sync situation. The first case should become obvious fairly quickly when power is adjusted .
The second may not be so obvious and present and engine that is fairly happy with throttle at near full power, to one that is running very rough and developing no power when throttle is at idle.
Given the above we may want to review our check list for rough a running engine?
When only one carb is affected it should be possible to modulate power at the top end with the one controlled carb to prevent over speed and complete a controlled descent to an approach then shutting the engine when the landing is assured?
Happy landings
Carl
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Sat Nov 20, 2010 3:47 pm Post subject: Re: Rotax 912 throttle return springs |
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Thom has it right.
Your WOT rpm should be right around 5500 rpm. 5450-5550 rpm WOT flat and level. The proper check if you have a sudden large vibration and power issue should be to try WOT and see if it smooths out. If it does then somewhere in your cable setup it has disengaged from one carb and it is now wide open. If it doesn't then it's time to land very quickly. If your rpm is set to 5500 WOT then you can fly anywhere and into the pattern. When you are on finial and know you have the field made just shut down and glide in dead stick. Dead stick really is no big deal and should be practiced once in a while. It isn't any different than with power so long as you keep your speed up. Yes I know there is no go around at that point, so pay extra attention to your approach and landing. When I'm dead stick I keep 5-10 mph over my normal approach to help make sure I don't get any surprises at the bottom with wind or failure to watch my speed and dip too far below a safe approach speed. I never approach too slow any way so 5-10 extra for me gives me a healthy safety margin. You can always bleed extra speed off down on the runway, but you may not be high enough to get it back if you are one of those that like to drag it in. If you are from the old Ultralight days then you are used to doing dead stick landings.
p.s.
Leave your springs alone. They are the way they are for a very good reason. If yours isn't acting right then fix the problem in your throttle system and leave the springs alone. Don't try and reinvent the wheel, it already works fine. This goes back to my mantra. The worst thing that happens to a Rotax engine is it's owner.
Work safe, Be safe so you can fly safe and come home to hug the wife, girlfriend and I have to include significant other, too. 
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AmphibFlyer
Joined: 29 Dec 2009
Posts: 25
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| Posted: Mon Nov 22, 2010 7:42 am Post subject: Re: Rotax 912 throttle return springs |
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| Thom Riddle wrote: | It is standard aviation practice to spring load the throttle arm to wide open.
Several years ago, one my partners in a Cherokee 140 had the throttle cable break during flight and it went to full throttle which enabled him to get home and landed deadstick after turning off the fuel and then the mags. Cable replaced and we were back to flying. |
I had exactly the opposite problem once in a SeaRey used for transition training that had been parked outdoors for several years. SeaReys have the throttle springs reversed, so that the spring pulls the throttle closed, not open. The reasoning for this is that the throttle cables (which are pull-only) are much less likely to fail than the springs. When I throttled back on downwind one day, the engine wouldn't go to idle--kept running at about half power. The remedy was easy: switch the engine off and land dead-stick, then re-start to taxi in. The problem was that one spring had broken during the flight. So I got a little bonus training that day. And I was thankful that the cables, not the springs, were set to pull the throttles open.
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