Pusher/Puller Props.

Discussion in 'Scratchbuilding!' started by ANB31, Jun 2, 2011.

  1. ANB31

    ANB31 Rookie

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    Hi,
    I am totally new to the hobby, so I want to clear my doubts before I get to the build my first RC plane. I was going through your video on the puller/pusher props and as I understand, it’s by simply reversing the direction of rotation of the motor we can convert either of the props to behave like a puller or pusher, only thing to note is that the lettering on the prop should always face outwards.

    I assume that when we have a puller prop its generally in the front of the model and the prop wash is towards the motor, however when the motor is mid/rear mount the prop wash has to be away from the motor or else the craft will fly backwards thanks to 3rd law of motion. This prop wash direction can be controlled adjusting the direction in which the motor rotates but how about rotating the prop itself otherwise in one of the case(Motor direction reversing) the training edge of the prop will be meeting the air before the leading edge, wouldn’t it affect the lift and performance?
     
  2. Argonath

    Argonath Top Gun

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    a prop always needs to have the lettered side facing the direction of travel of the plane. You can reverse the direction of rotation of the motor by swapping 2 of the 3 wires from the ESC to the motor.

    If you put the lettered side facing the rear of the plane then you lose a great deal of thrust from the prop.
     
  3. ANB31

    ANB31 Rookie

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    thanks for the response. So we just have to ensure the lettering should always be in the same direction as that of the intended direction of the plane. It’s not at all an issue if the trailing edge of the prop cuts the air before the leading edge? Also my assumption on the direction of the prop wash correct for the various motor mount positions as posted earlier?
     
  4. Argonath

    Argonath Top Gun

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    if the letter side always faces the FRONT of the plane then the leading edge of the prop is always cutting the air before the trailing edge. There are no exceptions exemptions, variations to this rule. The prop is made to face 1 direction only and that is the front of the plane. If you have to make a pusher you take 2 of the 3 wires on the ESC -> motor and swap them to make the motor spin in the opposite direction and you still put the prop on so that the prop faces the FRONT of the plane.

    So if the motor is on the nose, the lettered edge is facing away from the motor towards the direction of flight. If it's a pusher then the lettered edge is facing the motor which is also facing the front of the plane. So the LE always cuts the air first.
     
  5. kaptondave

    kaptondave Rookie

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    It might help to remember that a propeller is a miniature wing or airfoil. It makes no sense to have the trailing edge hitting the air first. A reversed prop will actually generate thrust is the forward direction - just not very much of it.

    Generally the only time you really need a pusher pop is on a twin-engine plane where you want counter-rotating props to cancel torque roll.
     
  6. rchawk

    rchawk Cadet

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    So is it correct to say that most pusher planes purchased ( RTF ) the wires on the ESC have already been swaped , so that means that the propeller would go on facing the same direction on a front or rear mounted motor , right ???
     
  7. archie

    archie Cadet

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    Some of the RTF planes come with everything installed apart from the prop due to packaging. Even if the ESC wires are not inserted you would be able to swap them around until they are the right way. The prop is installed the same direction if it is Rear or front mounted
     
  8. kaptondave

    kaptondave Rookie

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    Yes BUT... Never assume that anything on a new plane is correct. Before your first flight:

    1. Inspect all screws and bolts for tightness.

    2. Inspect the prop to verify that it is facing the right way and is on tight.

    3. Whiie hanging on to the plane and keeping clear of the prop, advance the throttle to verify that it is moving air in the right direction.

    4. Check the direction of control surface deflection. When you command right turn the rudder should move RIGHT (as seen from the tail). If it has ailerons the one on the right wing should deflect UP while the one on the left deflects DOWN.

    5. Perform a Range Check of your radio link.
     
  9. rchawk

    rchawk Cadet

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    Thanks Guys
     
  10. Argonath

    Argonath Top Gun

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    Exactly. The lettered side of the prop always faces the front of the plane.

    and as KaptonDave has already pointed out, always check everything RTF or not.
     
  11. ANB31

    ANB31 Rookie

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    Thanks every one, I'm clear with the concept now. Cheers!!!!
     
  12. Jbirky

    Jbirky Ace Pilot

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    The forum members above already did an excellent job answering this question, but I have something to add.

    The reason for pusher props/reverse rotation pros:

    i.e. There are forward rotation props such as Dave's typically used prop, the 6x4 by APC (LP06040E). This is basically a standard (Puller Prop)... then there is an identical 6x4 prop that is basically a mirror-image and serves as a (Pusher Prop/Reverse rotation prop) from APC (LP06040EP).


    The old reason was that the only power-source guys used to have is Nitro or Gas (fuel-powered), and the engines would run in ONLY one direction. If the engine shaft was in front of the plane, they had to use a standard puller prop. If they flipped the engine around and put it behind the wing or in any other oriantation that had the engine shaft facing toward the rear of the plane, they had to use a Reverse-Rotation (Pusher) prop to make the plane fly forward. :)


    Now, as the forum members already said, in electric RC you have motors and reversing any 2 of 2 (brushed) or 2 of 3 (brushless motors have 3 wires), the motor spends in the opposite direction. Hence in electric RC, if you take a motor that is pulling (the shaft facing forward) and then move it behind the wing or in some other oriantation to where the shaft faces the rear of the plane, you can do one (1) of two (2) things:

    1. You can remove the puller prop and put on a pusher prop just like you would with a fuel-powered engine without reversing the wiring.
    OR
    2. You can remove the puller prop and flip it so that the lettering now faces the front of the plane and then swap any 2 wires to reverse the driection of the motor.

    Either way, if the lettering is facing forward and the thrust is going backwards, you have it setup right. Since electric motors can run either direction, you can mix and match pusher or puller props and swap wires to get the thrust going in the right diretion.


    Now you are wondering... "Why in the age of electric motors if I can use either type of prop just be facing it in the right direction and getting the thrust moving in the right direction, why would anyone want to buy two different types of props (pusher and puller) if either type of prop can be used in a puller or pusher configureation (by flipping the wries)???"


    The reason is that if you have two motors pushing the plane (or pulling the plane) and they both run in the same direction, you have even more torque-roll than if you had a single-motor plane. Hence, acclerating or decellerating the motors (adjusting the throttle) will cause the plane to roll left or right causing you to have to compensate with the stick controlling the aierlons. Not only that, even when running at speed the motor is always going to be trying to get the plane to "unwind" in the oppsoite direction of the props though the aircraft is much larger than the prop (except in the case of helicopters, which have tail-rotors for this "unwinding" reason)...

    What Dave Powers, Scott Lott, and dozens of other people do when they have two (2) motors pushing or pulling is set them up to counter-rotate. That is, one motor is running one direction and the other motor is running the opposite direction... This means one of the two motors has two (2) wires flipped to run in the other directon. Now, you simply need both motors to produce forward thrust, so you slap a Puller Prop on one motor and a Pusher Prop on the other motor so as to make both motors push thrust behind the plane to make it go forward.

    Now, when you jam the throttle up, or down fast, both motors create torque-roll in opposite directions and the net torque on the fusalage/chassis of the plane is zero (0). The plane does not roll right or left... Clever right? This is also how co-axial helicopters and Whinook helicopters work without a tail rotor... they cancel this torque-roll by having two props running in the opposite direction.


    Take this in addition to what everybody else already said... No matter what configureation you come up with the text on the prop always faces forward (toward the directon of travel)... as already stated in this tread... reverse-rotation or counter-rotating setups still follow this rule; since, props are only efficient in one direction.
     
  13. peckair

    peckair Cadet

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    This is actuly not true, two stroke engines(glow or gas) can run in both directions.:D So you could use a normal prop for a pusher. And the only reason for the reverse prop for two strokes(ports) is for a dual motor planes(counter rotating). Four strokes engines(mini cams, push rods, and rockers) can only run in one direction(won't start if you try the other way), so reverse prop are nessary for an pusher. And for those who don't even know the difference I attached some pics. The first pic is an four stroke(O.S. FS-200-p glow), and the second is an two stroke(DLE 55cc gas).
     

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