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Author Topic: Props on electric - tractor (clockwise) or Pusher (counter-clockwise) using?  (Read 909 times)

Offline Dennis Toth

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Am taking a break from cooking chores and thought I would pose a question about props on electric. For a while we were trending toward pusher (counter-clockwise) props because they gave a bit more secure tension on outside maneuvers and a side benefit was you could install the bellcrank with the front line down and the pushrod connection clear of the leadouts.

Lately, I think the conventional tractor props have been used. Why the change? Seems that it might be that the pushers didn't have as much selection and the benefits of the pusher just not that great over the selection of sizes available to the tractors. What are you using?

Best,    DennisT

Offline Mark wood

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The difference between the two is the same as a question I posed a while back regarding the impact of precession. The CW turning propeller precesses in one direction and the CCW the other direction. In the world of CL this becomes, is it better to have the nose point in on the inside corners or the outside corners. See my thread where I asked the precession question about how significant it is. It's not much, I find there is a difference. A small difference. I've done a lot of A-B-A testing going from one propeller and back on a given airplane there is a slight difference but not so much as to really say the impact is significant. The end of the day, I fly the normal propellers primarily because they do everything I've learned to anticipate. I have the technology in hand to to virtually remove the precession but the difference just isn't that big of a deal. What I really learned / determined is that a well trimmed airplane with good yaw stability really doesn't much care. Run a higher pitch propeller so it runs slower, less angular momentum and higher efficiency, for a specific lap time then the impact becomes even less and your battery will like you for it.

 
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Offline Shorts,David

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My electric oriental was a little loose, I think on the outsides, so I switched to pusher props and that solved it. Unless I have it backwards, but I just went with how the plane performed, like motorman said.

Online Ken Culbertson

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Am taking a break from cooking chores and thought I would pose a question about props on electric. For a while we were trending toward pusher (counter-clockwise) props because they gave a bit more secure tension on outside maneuvers and a side benefit was you could install the bellcrank with the front line down and the pushrod connection clear of the leadouts.

Lately, I think the conventional tractor props have been used. Why the change? Seems that it might be that the pushers didn't have as much selection and the benefits of the pusher just not that great over the selection of sizes available to the tractors. What are you using?

Best,    DennisT
For me it was the lack of inventory for props and spinners.  I got tired of having to find them and the benefit is minimal.  Takeoff's a bit easier, hourglass a bit easier but not that much.  Active timer pretty much made those benefits moot.

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Online Howard Rush

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I flew a backwards prop for awhile after Dennis Adamisin had APC make some.  My plane at the time didn't mind.  My next plane would do perverted things at the top of the hourglass with the backwards prop, so I switched back. It's the devil we know.  The backwards prop also takes off funny. 
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Online Ken Culbertson

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I flew a backwards prop for awhile after Dennis Adamisin had APC make some.  My plane at the time didn't mind.  My next plane would do perverted things at the top of the hourglass with the backwards prop, so I switched back. It's the devil we know.  The backwards prop also takes off funny.
You have to change the direction in the ESC or swap the motor leads.  I have seen one of those takeoffs, they are funny.  LL~

Ken

Interesting that you found the takeoff not to your liking.  Do you power up fast or slow? 
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Offline Dennis Toth

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I know the take-off is different with the pusher. If you have a ship trimmed for tractor props the pusher will tend to hug the ground and wait for a bit of up elevator to get airborne. Some build in a bit of positive stab angle for the tractor prop that needs to be overcome when doing a straight switch to pusher.  I think if you had a ship designed for the pusher it would be less of an issue. My problem is selection of prop size and blade shape. For tractor props there are a dozen or so different props in wood, glass reinforced, carbon to choose from. For pushers Zinger did make some wood, but I don't know how to order them off their web site. You are left with APC and some custom carbon props but in a small range. There seems to be a better selection for large diameter pusher props (11" +) but for the smaller 35 size ships it is very limited.

The other big issue for me with pushers is re-pitching. There are no pitch gauges available for the pusher props. You can reverse the sides on a Prather (holes are drilled and taped for the same bolts except for one) and get a "relative" pitch position gauge but you can't get a readout as the numbers are on the other side of the gauge plate. You can kinda guess at it but this is about the same as Windy's motor mount/tip plate tool. You can see where a starting pitch is but you guess on the change, which can be a very small angle change but you don't see about what to expect.

I like Ken's comment about the active timers reducing the need and will maybe need to try this on one ship. I am working to reduce amp draw and save 1 3/4 oz of battery weight but finding the prop to keep the speed up and amps down is a challenge without a box of 9x6 pushers to cut up.

Best,    DennisT
« Last Edit: November 26, 2021, 08:01:53 AM by Dennis Toth »

Online Ken Culbertson

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I know the take-off is different with the pusher. If you have a ship trimmed for tractor props the pusher will tend to hug the ground and wait for a bit of up elevator to get airborne.
Best,    DennisT
It was that exact thing that I liked.  I have used the long tail wheel setup that Gieseke recommended to me back in the 70's.  I set up with about 2 degrees positive AOA.  With IC you had to be careful to not take off too soon.  With a slow ramp up using electric and a pusher prop the plane will take off on it's own without popping up as soon as it gets enough lift and the pusher setup keeps the lines tight.
Not that big of a deal.  With a slightly faster ramp up the tractor is just fine.   The other place that a pusher helps, top of the hourglass, has not been an issue for my planes.  Can't put a finger on it, may be the CAM rudder or the twin tails I have on most of my ships.  They just do not make obscene jestures overhead on their own, they leave that to me!

Ken

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Online Howard Rush

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Interesting that you found the takeoff not to your liking.  Do you power up fast or slow? 

Fast to the takeoff power setting.  Then the plane is released.  After 7 seconds or so, power increased to the cruise setting.  The TUT enables the two-step takeoff power.  It probably helps with either prop rotation direction.
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Offline Mark wood

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With either prop the plane goes nose out as soon as it gets line tension after release. There's no aero drag on the lines then.


Motorman 8)

That's been my experience. Except on the one wheeled rat racers and formula 40's.
Life is good AMA 1488
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Online Ken Culbertson

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Fast to the takeoff power setting.  Then the plane is released.  After 7 seconds or so, power increased to the cruise setting.  The TUT enables the two-step takeoff power.  It probably helps with either prop rotation direction.
Now I could learn to like that.  I am still trying to get a slow smooth ramp up on the Fiorotti. Since the takeoff counts the same as the square 8 you just can't be giving away points!

Ken
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Offline Mark wood

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Now I could learn to like that.  I am still trying to get a slow smooth ramp up on the Fiorotti. Since the takeoff counts the same as the square 8 you just can't be giving away points!

Ken

The ramp up is a function within your ESC. It's usually called acceleration or helicopter hi / low, 1 / 2. Big helicopter rotors need to start off more slowly. On the Jeti Spin the acceleration is time based and access through the Jeti Box.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Online Dennis Adamisin

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I use pushers, almost exclusively.  I only use tractors if/when I cannot find an equivalent size pusher.  APC has the best selection, including a handful of "F2B" selections.  I have also built a couple twins and used opposite rotation props on each side.

APC has the best selection, including a handful of "F2B" selections.  As Howard noted, APC has made few props directly to our requests, my role was typically to gather interest via SH to determine if a threshold could be met.  I would then order (200 piece minimum) and distribute.  Thankfully I was able to sell all the sizes except one so did not go broke on it.  With my profits I was able to take my wife out for a Happy Meal!  In most cases APC would keep the sizes available after that threshold order.  The 10x5.8EP, 11x5.5EP, 12x6EP-F2B, 12x6.5WEP, 13x4.5EP, and 13x5.5EP all were produced as a direct result of this effort.

The APC's are good props, but they are a wee bit heavy! The light weight alternative appears to be wood.  Xoar advertises them but supply is dubious.  I have written Xoar directly about buying some, but did not get a response.  Brodak got some through their Chinese supplier but tht supply has now dried up too.  Inov8tive Designs (same place to get Cobra & Bad Ass motors) is supposed to be bringing out a series of pushers as part of their Bad Ass prop line.  These are rumored to be Xoars, I guess we'll see.

I have encountered a single event of "weird hourglass" syndrome.  In my case I determined it was pilot error - overcontrol.  Why? There is higher (usually much higher) line tension in the top of the verticals & hourglass when using a pusher prop.  ALL of the handle input shows up in the elevator, unlike a normal prop.  Put another way if you fly an hourglass with the same control input (i.e., leading and pulling on the control) that you have learned flying normal props then you WILL overcontrol the top of the hourglass because you have more line tension and thus more control input into the airplane.  That is not a good place to stall the airplane!


TRIMMING SUGGESTION - Up-thrust:
In the past year and a half or so I have started adding upthrust to my pusher equipped airplanes with good results.  Pay-off is generally better groove, and in one case correction of an inside/outside problem.  Why is that?  I was put onto this trajectory after reading an article (see link below) by "Joe Supercool" (Stuart Sherlock) about stability of FAI speed models!  Joe (Stuart) is a physicist and went through the math to predict the magnitude of the propellor gyro force and its effect on the stability of a speed model.  In a nutshell, the gyroscopic effect of a normal rotation prop has the effect of holding the nose of a speed model UP.  It follows that the gyroscopic action of a pusher prop would be to push the nose down, causing an unequal turn (turns easier outsides), or in less extreme cases, hunting.  In the case of a stunt model, I think the effect is also caused by the fact that most of our airplanes still have the thrust line above the centerline of the wing, causing a pitch-down.  That's (probably) OK for a "normal" prop because it has a nose-up input due to the prop-gyro counteracting the pitch-down.  However, with a pusher the gyro effect is nose down, thus a double whammy!  Of note, the only airplane I have that did not respond positively to up thrust has the thrust line only 1/4" above the wing.  There are advocates for building in positive incidence (down elevator) into stunters to counteract the gyro, I speculate that building in down-thrust might be the alternative (& better?) solution.

Long and short of it, I am using a #4 washer at the motor mounting bolts to induce about 1 degree upthrust.  The next round of airplanes will likely have that 1 degree built-in.

The article is here: http://www.supercoolprops.com/home/articles/gyroscope.html

Enjoy...

Denny Adamisin
Fort Wayne, IN

"Dear Lord, please keep your arm around my shoulder - and your hand over my mouth..."

Online Mike Alimov

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Dennis, interesting. Last year we had to do exactly the opposite: remove 1 degree of up thrust (#4 washer) to achieve decent level tracking. Pusher prop.
There must be something else to it besides the gyro forces.

Online Howard Rush

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I have encountered a single event of "weird hourglass" syndrome.  In my case I determined it was pilot error - overcontrol.  Why? There is higher (usually much higher) line tension in the top of the verticals & hourglass when using a pusher prop.  ALL of the handle input shows up in the elevator, unlike a normal prop.  Put another way if you fly an hourglass with the same control input (i.e., leading and pulling on the control) that you have learned flying normal props then you WILL overcontrol the top of the hourglass because you have more line tension and thus more control input into the airplane.  That is not a good place to stall the airplane!

Maybe that's what happened.  There is embarrassingly more to the story.  My first electric plane had the up line in back, as one would, I think, for a backwards prop.  I attempted to do the same on the second one, but I did not notice that the Igor flap system reverses the sign.  The up line is in front on the second plane.  I found happiness with a normal prop.
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Offline Tim Wescott

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Maybe that's what happened.  There is embarrassingly more to the story.  My first electric plane had the up line in back, as one would, I think, for a backwards prop.  I attempted to do the same on the second one, but I did not notice that the Igor flap system reverses the sign.  The up line is in front on the second plane.  I found happiness with a normal prop.

Some random guy at a content told me that if you use eyelets for line adjusters it's easy to swap the front and back lines, and as long as the leadouts just cross inside the wing rather than actually getting wrapped up, it's OK.  And that way you can make that part of your trim.

Old retired fart.  Paul -- Wannabe, Stalker, Pacer, something like that.
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Online Ken Culbertson

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Some random guy at a content told me that if you use eyelets for line adjusters it's easy to swap the front and back lines, and as long as the leadouts just cross inside the wing rather than actually getting wrapped up, it's OK.  And that way you can make that part of your trim.

Old retired fart.  Paul -- Wannabe, Stalker, Pacer, something like that.
And when you swap them back did you go under or over?  Oh well, what's wrap or two.  It's inside the wing, nobody will notice. LL~

Never heed random advice!

Ken
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