From kls Thu Apr 7 13:55:17 1994 Newsgroups: sci.aeronautics.airliners Path: bounce-back From: kbarr@nyx10.cs.du.edu (Keith Barr) Subject: Re: Engine rotation References: Message-ID: Approved: kls@orchard.Chicago.COM Organization: Nyx, Public Access Unix at U. of Denver Math/CS dept. Sender: kls@orchard.Chicago.COM Date: 07 Apr 94 13:55:17 PDT In article , Terrell D. Drinkard wrote: >Making them all turn the same way has more >than economic implications. It also eliminates the 'critical' engine I imagine further follow-ups should be directed to sci.aeronautics... Actually, a critical engine only exists when the engines turn the same way. Numerous light twins, such as the Beechcraft Duchess BE-76, have counter- rotating propellers to eliminate the critical engine. First some background. Before one can understand why there is a critical engine, one must first understand P-factor. Basically P-factor is a non- symmetrical distribution of thrust over the rotating disk formed by the propeller due to varying speeds and angles of attack of the prop blades at various positions. More simply, you can just take it as gospel that while in any sort of nose-high condition there is more thrust on the side of the propeller that is travelling down. I could try to ASCII-art it, but I think it would be too difficult. If you are interested in the fine points, find an aerodynamics text. When viewed from the top, a propeller attached to a plane in a nose-high configuration will have a lift distribution as shown below: Normal Engine...rotates clockwise as viewed from behind. ( | represents the thrust vectors ) ||||||||| |||||||| ^ ||||||||||||| |||||||||||||||||| / \ |||||||||||||||||| ==================/hub\================== Ascending Descending This can be more easily represented/simplified to: . /|\ | ^ | / \ | ==================/hub\================== With that said, let me give a quick introduction on flying a multi-engine aircraft with a dead engine (not counting the Cessna 337). When an engine dies there is a yawing moment created that tends to "move" the nose away from the good engine. This yawing must be counteracted, if the pilot is to retain control of the aircraft, with rudder, which yaws the nose back into the proper direction. In this configuration, the aircraft is pointing in the right direction, but it is slipping slightly in the direction of the dead engine, so normal procedures call for banking away from the dead engine slightly (in pilot circles we "raise the dead" so to speak). On with the explanation... So, if you have a twin with both engines rotating the same way, you can see that the right engine creates a larger moment than the left engine. This means in an engine out situation, you will have to counteract more moment with the rudder. The critical engine is the engine you do not want to lose, so the left engine is the critical engine. | ^ | | / \ | ----- | | ----- __________+++___| C |___+++__________ | | G | | |________________| |________________| | | | | | | | | ___| |___ |____.____| In a well designed aircraft with counter-rotating props, you have the followng situation, and neither engine is considered to be critical. | ^ | | / \ | ----- | | ----- __________+++___| C |___+++__________ | | G | | |________________| |________________| | | | | | | | | ___| |___ |____.____| Note: There have been some aircraft designed with the counter-rotating engines switched, so both engines are effectively critical engines. Enough rambling...hopefully this wasn't too confusing. _____________________________ _____ | Keith Barr \ \ \__ _____ | barr@ncar.ucar.edu \___________\ \/_______\___\_____________ | Commercial/AS&MEL/Inst/A&IGI / ( /_/ ..................... `-. |_____________________________/ `-----------,----,--------------' _/____/