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JohnF
Joined: 13 May 2010
Posts: 124
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 Posted: Tue May 31, 2011 8:31 am Post subject: Static RPM Oddity (?) |
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I just reinstalled the hub "clamp" for my Sensenich composite ground adjustable prop on my RV-12 that had been "recalled" to machine it so it clamped the blades tighter (there were some reports of blade angle changing because they were not clamped enough).
I was setting the blades to achieve about 5,000 static RPM when I noticed something that is not entirely clear to me: The 'speed' or haste used to go to full throttle affected the static rpm that was achieved. A rather good shove produced more top rpm than a slow methodical application of full throttle.
I ended up doing ten 'runs' and averaging them to get the 5,010 rpm that I settled for on the Rotax 921ULS in the plane.
I'm happy but not certain why the variabtion; my guess is that it has something to do with the fuel fed to the carbs, or into the combustion chambers.
JohnF
[quote][b]
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Tue May 31, 2011 12:53 pm Post subject: Re: Static RPM Oddity (?) |
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Most likely a throttle cable and or a throttle lever adjustment. Evidently something can stretch or even be pulled so much as to make a little 2-4mm change and that's all it would take. My guess is the change is so small you can't feel it, but does affect the carb.
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Wed Jun 01, 2011 4:05 am Post subject: Re: Static RPM Oddity (?) |
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I think Roger is on the right track. However, it should be examined closely. If it is a throttle cable binding a bit in the sheath it will only get worse. It could also be a weak throttle advance spring. I ran into both of these situations this past week during annual and condition inspections.
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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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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Wed Jun 01, 2011 9:33 am Post subject: Re: Static RPM Oddity (?) |
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Hi John,
Thom is right. double check those throttle arm springs on each carb. If one is a little weak it could be this slight issue.
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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JohnF
Joined: 13 May 2010
Posts: 124
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| Posted: Wed Jun 01, 2011 10:01 am Post subject: Static RPM Oddity (?) |
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Thanks, Roger. I think I will just replace both of them..since I can't
really check their strength with any accuracy. Again, thank you fellows.'
John
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ronlee
Joined: 25 Dec 2006
Posts: 141
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| Posted: Wed Jun 01, 2011 3:21 pm Post subject: Re: Static RPM Oddity (?) |
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This two cable throttle system on the Rotax 912 series engines is kind of hokey in my book. I highly suspect both carbs nearly never advance and retard at the same rate and especially on retarding the throttle. The cables are almost never the same length. I would think this to be more likely after a bit of wear and stretching of the cables. It seems to me that someone out there could come up with a better system to to tie the carbs together to keep them in absolute sync. Somehow with a solid rod between them or what have you. I have gotten in the habit of when coming back on the throttle to then advancing it a bit to bring the RPM up to where I want. This puts a little stretch on the cables so if there was any slack they are now pulling the throttle arms of both carbs equally. Does it work, I like to think so. I guess the only real sure way of knowing would be to have a vacuum gage on each carb but then the crossover may even it out anyway.
I have seen pictures of solid rods from the firewall an airplane but when I think about it the engine is always moving around and the firewall isn't, so the carb levers are in constant motion. Anyone remember the old cars that had a solid rod to the carb, the gas pedal was going up and down in sync with the engine moving on the motor mounts. It was annoying and wore the carb throttle bushings out quickly.
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Ron Lee
Tucson, Arizona |
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Thu Jun 02, 2011 2:29 am Post subject: Re: Static RPM Oddity (?) |
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Ron,
Though I've never had any problem getting the two carb throttle and enricher cables to operate simultaneously, it does take patience and some perseverance to get it just right on some airplanes. Most folks use commercially available cable splitters but I owned a Rans S6S that had a home made one that I was able to get to work perfectly well.
Zenith has a really good way to avoid the cable splitter entirely. The cockpit throttle controls directly operate a pivot arm mechanism mounted on the engine side of the firewall. The arms on the pivot mechanism are rigidly interconnected to each other and move the carb throttle arms via short straight cables. Much simpler and less tortuous than long sheathed cables that can start to bind after many hours years of service.
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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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