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# counter rotating props

 foamhead (Mechanical) (OP) 7 Jan 04 12:30
 What are the advantages and disadvantages to counter rotating props (one just in front of the other)besides complexity and cost. I am considering using such a setup to have a twin engine setup in a single engine airplane. Bob
 Nigel (Aeronautics) 18 Jan 04 22:57
 If you don't know anything about it, why are you considering it? Nigel Waterhouse & AssociatesAeronautical Consulting EngineersTransport Canada and F.A.A approval & certification of fixed and rotor wing aircraft alterations: Structures, Systems, Powerplants and electrical.  FAA PMA, TC PDA.n_a_waterhouse@hotmail.com
 CLmax (Aeronautics) 19 Jan 04 9:33
 Co-axial counter-rotating propellers are an interesting mechanical and aerodynamic problem with many complexities. Normally they are only used when absolutely necessary, for example, to harness the full power of a large engine.  The Russian Bear bomber comes to mind.  The reasons to avoid, I think, are the complexity of the power transmission system and the marked loss of aerodynamic efficiency, not to mention the vibration modes, of the props operating in close proximity. The folks who know the most about designing and building these are probably from the UK.  See the Fairy Gannet aircraft for instance.  Also recognise that the previous contributor is asking an important rhetoric question. We hopefully assume that you are not going to assemble anything until you can answer a few design questions yourself.  Regards,
 foamhead (Mechanical) (OP) 19 Jan 04 13:59
 Thanks for the responses. I guess stating my purpose better at the onset would have helped, so here it is.I am not very concerned with the design complexity as I am a machine designer with solid modeling and FEA software. The drive I can handle. I was hoping to gain some knowledge of the aerodynamics involved.All I have been able to find on the internet is this article about a Cozy with 2 engines. It cruises at 172 MPH on 192 cubic inches. Sounds pretty efficient to me. Anyone know of anything else. Take a look at this article (link below) and see what you think.http://www.infortel.com/cozy/article_english.htmThanks againBob
 NeilRoshier (Automotive) 20 Jan 04 3:36
 I have a feeling that the arrangement was utilised on some aircraft due to convenience in carrier operations...it would allow a much shorter undercarrage with the attendant packaging benefits plus the landing/take-off advantages.The aerodynamic issues I have no idea
 Rollerblades (Mechanical) 20 Jan 04 13:28
 Thats a good question because I dont know how a single engine airplane could be balance in terms of rotating inertia!
 wilg (Mechanical) 21 Jan 04 10:49
 CLmax    Excellent answers, you are absolutely correctabout the Gannet and Bear, the Gannet seems tobe one of the only Western examples of asuccessful Contra Rotating prop military design,the Douglas Skyshark failed because of thegear box complexity. One real advantage of usingContra Rotating props is the abscence of torque.If your in a single seater with an R-3350 orR-4360, torque can be difficult to control,a lot of rudder and aileron is used to controlprop torque. The gearbox on Contra Rotating designscould be a maintenance headache too.
 jetmaker (Aeronautics) 21 Jan 04 14:28
 My understanding of counter-rotating props is this:1) they are more efficient at high Mach numbers than a single propeller configuration;2) they allow smaller diameter blades, which allows them to spin at higher rpms without a decrease in aerodynamic efficiency.I'm sure there are others.Regards,jetmaker
 wilg (Mechanical) 22 Jan 04 9:01
 naveenshastri (Mechanical) 19 May 04 2:06
 I have heard of a counter rotating system used in small rec boats - made by Volvo Penta. Counter rotating props would be the best choice for paraglider backpack motors, as the effect of torque is so bad, pilots can turn in only one direction while climbing

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