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gmcjetpilot(at)yahoo.com
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 Posted: Thu May 25, 2006 9:34 am Post subject: Low RPM Harmful Re: Engine Life |
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Engine life if based on DIS-USE. Engines flown everyday, hard, medium or soft will make TBO and have less maintenance issues. Engines that SIT collect dust and RUST will not make TBO no matter how much you BABY IT. Don't baby your engine. There are so so so many factors besides RPM that control the life of an engine. I recommend the book by sacramento sky ranch, Lyc engineering manual. I am not saying I fly at 2,700 rpm all day, in fact I throttle/prop back to 25 square for climb. I have been told by many to leave it WOT full high RPM from take off to climb and let the altitude take care of MAP. Its a habit to go 25" and than maintain that with the throttle as long as possible. RPM wise 2,700 is just more noise. HOWEVER if you are trying to get to altitude faster WOT & RPM makes sense. Obviously for cruise if you have a C/S prop lower RPM is just less noise and actually better efficiency from the props aerodynamic stand point (more efficency).
Low RPM is NOT harmful if the Temps and Pressures are in the green, period. There is a practical limit on how low you can go RPM wise and still make headway. At altitude you may need the RPM to make power and airspeed. A side benifit is lower RPM increases prop efficency, which means more of the HP your engine makes gets turned into thrust. As far as low RPM and MAP that is a differnt topic.........
Yes you can run OVER square. Just look at FAA APPROVED flight manuals for Lycoming powered planes. You see plenty of over square power settings. Over square is a throw back to some radial engines.
http://www.lycoming.textron.com/main.jsp?bodyPage=support/publications/keyReprints/operation/powerSettings.html
AND SECOND
http://www.lycoming.textron.com/main.jsp?bodyPage=support/publications/keyReprints/operation/oldWivesTales.html
(second old wives tail)
One caveat is this chart is 65% POWER. Keep it simple get a manual for your engine and follow it. Using higher MAP and lower RPM has advantages.
I usually fly at altitude so M.P. is usually lower anyway. However when given the opportunity of two combos of RPM/MAP which produce the same power, one being OVER SQUARE. I'll pick over square.
Every time I take off I am over square, 2700/29".
Just give lycoming a call and they can reassure you. They have run engines on test stands for 10's of thousands of hours like this.
There is no need to get with all up with the urban legend, rumor, fear and prejudice about it. The data is out there.
Cheers George
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n395v
Joined: 10 Jan 2006
Posts: 450
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| Posted: Fri May 26, 2006 3:40 am Post subject: Re: Low RPM Harmful Re: Engine Life |
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Great post George.
You might consider repeating it on VAF as I recollect this issue has come up over there too.
Do not archive
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