Which way 'round?

Discussion in 'Aircraft & Aviation' started by TableTopAirCrew, Oct 22, 2007.

  1. TableTopAirCrew

    TableTopAirCrew New Member

    As I was looking at a photo of some prop-driven aircraft or other (don't remember which), it occurred to me that the engine on any given airplane could turn in either direction even though we probably live in a clockwise world. It also occurred to me after some days of rumination that most single engine craft would turn the prop in a standard direction (whatever that might be) so that a pilot would not have to adjust for the engine torque from plane to plane.

    Could someone who is familiar with 1:1 aircraft straighten me out on this? Also what direction did the individual engines on WWII multiengine bombers turn?

  2. 46rob

    46rob Member

    Most propellers turn in a clockwise direction, as viewed from the aft end of the aircraft.....But I say most, because there are exceptions. Early model P-38's, for example, had one prop rotating in each direction to counteract torque. I beleive later models had the same rotation on each side, as a matter of expediency. Then there are those aircraft with contra-rotating propellers, like the Tupolev Bear and Convair Tradewind. Tandem configured aircraft, like the Dornier Arrow and Cessna 210 had one prop turn in each direction. The direction of rotation, in many cases is a gearbox function, rather than the engine itself.
  3. TableTopAirCrew

    TableTopAirCrew New Member

    Thanks for the info...gives me an idea how to "twist" my props...
  4. sakrison

    sakrison Member

    A few civil twin-engine aircraft have counter-rotating (C-R) props. Piper's PA-39 Twin Comanche, introduced in the early '70s, was one of the first; earlier Twin Comanches (PA-30) did not have counter-rotating props.

    C-R props always rotate toward the fuselage (clockwise on the left, counterclockwise on the right, from the pilot's perspective). In a climb, a propeller generates more thrust on the down-stroke (right side for clockwise) than on the up-stroke, which pulls the airplane to the left. That's why you need to apply right rudder on takeoff in a single-engine prop plane.

    When you yaw an airplane in either direction in flight, one wing is moving forward faster than the other and thus generating more lift, so an uncorrected yaw will also result in a roll in the same direction as the yaw. Leave ailerons neutral, push hard on either rudder, and the airplane will roll in the same direction.

    On a twin, with the engines on either side of the center line, if one engine fails or loses power on takeoff, the asymmetrical thrust yaws the airplane and causes it to roll "into" the "dead" engine. If the remaining engine rotates toward the centerline, the center of thrust remains close the center of the wing and the yaw and roll are minimized.

    On the other hand, if one engine fails and the remaining engine rotates away from the fuselage, that puts the center of thrust way out on one wing and gives the aircraft a strong tendency to roll into the dead engine.

    If both props rotate clockwise, the left engine is called the "critical engine"-- the one you don't want to lose. Lose it in level flight and you'll have a very stiff right leg by the time you get the airplane on the ground and parked. If you lose the critical engine on takeoff, a few hundred feet off the ground, at full power, and with speed just above stall (a typical takeoff), the resulting sharp roll into the dead engine can screw up your whole afternoon.

    Cessna's 337 SkyMaster, with its push-pull props, was designed to eliminate the asymmetrical thrust if one engine failed, and it worked. Unfortunately, the 337 had other problems, the biggest one being inadequate cooling for the rear (air-cooled) engine. With modifications to correct the cooling problem and cut down on cabin noise, it's a fine airplane, but it never caught on commercially. I guess its appearance was just a bit radical for its day.


    Attached Files:

  5. aphelion16

    aphelion16 Member

    And the Brits, Godbless them, have many aircraft piston engines that rotate anti-clockwise. Just cause I guess . Should you attempt to start an aircraft engine by propping it, i.e., turning it by hand this is a good thing to know. Severing of limbs is a nasty business.
  6. shrike

    shrike Guest

    You'll have to check on a case by case basis. Most American engines turn clockwise when viewed from the rear, as do most German. Some British engines turn the opposite direction. Russian inlines are counterclockwise as are their French forebears (VK-100 -108 derived from the Hispano-Suiza 12Y series)

    P-38s had, through their life engines that turned inwards, outwards and in the case of the 'Castrated Lightning" operated by the RAF that both turned the same way.

    Most multi-engine bombers had engines that all turned the same direction to simplify things from a maintainence point of view.
  7. Paladin

    Paladin Member

    Sakrison has it absolutely correct. Either you are an avid aviation scholar, or a pilot, or both. I myself am a pilot, and fly a Piper Seminole, (I am about to have my FAA check-ride for my multi-engine commercial license. What is nice about the Seminole, is that it is not a "conventional twin" (both props rotating in the same direction). We have to master the ability to fly the aircraft with only one engine, and make both visual landings, and instrument landings on whichever engine the instructor chooses. Anyway, the vast majority of aircraft have clockwise rotating engines, (as seen from the rear).
  8. TableTopAirCrew

    TableTopAirCrew New Member

    Thanks for the detailed info...also appreciate the photos--I like the pushme-pullme.

  9. TableTopAirCrew

    TableTopAirCrew New Member

    I hadn't thought about the hand starting problem--although I've many a hand-started tractors "kick" back.

  10. TableTopAirCrew

    TableTopAirCrew New Member

    Just 'cause is an excuse I used with my children many a time; it worked about as good as any other.

  11. TableTopAirCrew

    TableTopAirCrew New Member

    ...case by case basis...Is this something that would be in a spec sheet?

  12. TableTopAirCrew

    TableTopAirCrew New Member

    Thanks for the info...
  13. David H

    David H Member

    British piston Engines... erm well the Merlin was a "standard" clockwise affair. The much larger capacity Griffin, which replaced the Merlin on later Spitfires turned Counter-clockwise.

    I have always understood this to be a result of the Griffin's origins as a power plant for Royal Navy aircraft. Counter clockwise would have swung to port and away from the carrier's superstructure, so it would have been safer for the ships crew?

    So what about US naval aircraft?

  14. Rick Thomson

    Rick Thomson Member

    Hmm...I've cranked a Tiger Moth a few times, and it seemed obvious that you pulled on the thinner edge of the prop, but then Murphy does show up when you least expect him.
  15. 46rob

    46rob Member

    USN aircraft propellers all turn in the conventional cw direction.
  16. shrike

    shrike Guest

    I'm not sure that there ever was a real reason for the original choice of rotation, just that it has to be accounted for, especially in powerful aircraft. The vertical fin is often offset by several degrees to counteract the torque.
    Certain Russian lend lease P-40's were fitted with replacement Klimov engines when their original Allisons ran out of spares. The Klimovs turn the opposite direction and combined with the fin offset to make them an utter handful on take-off.
  17. TableTopAirCrew

    TableTopAirCrew New Member

    Thanks to all who replied to my query. I guess there are no simple answers, just simple questions.

    Thanks again,

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