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P-Effect

Old 08-24-2016, 11:46 PM
  #51  
serious power
 
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Hi Scott,
You didn't get the point I was trying to make - it was a thought experiment - my bad !

A new thought experiment ;
Let us build a new model.
- A standard fuz crutch with decks added top and bottom.
- Wing and tail mounted but with the wing set a little -ve in incidence.
- Let's put the vertical stabiliser on the bottom (like a predator - but a single one)

What motor thrust set up do you think will be req'd so that will pull from s&l horizontal to vertical and track straight ??
Why ??

Brian

Last edited by serious power; 08-25-2016 at 09:40 AM. Reason: typo
Old 08-25-2016, 09:38 AM
  #52  
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Originally Posted by drac1
I'm not the one having trimming issues am I.
Hi,
Scott, I think Jim's comment was sent my way.

Jim, when I read this my mind went to the 'nothing' scene where Luke wins with nothing and Kennedy said about the fight ' he kept coming at me with a whole lot of nothing '.
However you are most likely referring to the line ' what we have here is a failure to communicate '.
If so you were correct - as said already 'my bad'

' I'm shaken it Boss, I'm shaken it '

It also put me off hard boiled eggs for a long tome

Brian
Old 08-25-2016, 01:52 PM
  #53  
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Originally Posted by serious power
Hi,
Scott, I think Jim's comment was sent my way.

Jim, when I read this my mind went to the 'nothing' scene where Luke wins with nothing and Kennedy said about the fight ' he kept coming at me with a whole lot of nothing '.
However you are most likely referring to the line ' what we have here is a failure to communicate '.
If so you were correct - as said already 'my bad'

' I'm shaken it Boss, I'm shaken it '

It also put me off hard boiled eggs for a long tome

Brian
Yes, it was the line ' what we have here is a failure to communicate '.

Scott, If there is such a thing as P-factor and it causes the plane to veer to the left (the airplanes left) when you pull to vertical, and it veers to the left (the airplanes right) when you push from inverted to vertical what do you do to trim it so it goes straight in both cases, and if you can't achieve that, what rudder input will you use to correct it?

I'm sure some planes are worse than others and maybe it can't be detected on some. Gyroscopic precession and perhaps the thrust line settings could magically cancel out the P-effect but it would seem to me to be a case of luck.

Jim O

Last edited by OhD; 08-25-2016 at 01:57 PM.
Old 08-25-2016, 02:02 PM
  #54  
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Interesting thread. So if P effect is at a high AOA what is dragging the airplane to the left during take off?
Old 08-25-2016, 02:44 PM
  #55  
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Originally Posted by OhD
Yes, it was the line ' what we have here is a failure to communicate '.

Scott, If there is such a thing as P-factor and it causes the plane to veer to the left (the airplanes left) when you pull to vertical, and it veers to the left (the airplanes right) when you push from inverted to vertical what do you do to trim it so it goes straight in both cases, and if you can't achieve that, what rudder input will you use to correct it?

I'm sure some planes are worse than others and maybe it can't be detected on some. Gyroscopic precession and perhaps the thrust line settings could magically cancel out the P-effect but it would seem to me to be a case of luck.

Jim O
Scott, If there is such a thing as P-factor and it causes the plane to veer to the left (the airplanes left) when you pull to vertical, and it veers to the left (the airplanes right) when you push from inverted to vertical what do you do to trim it so it goes straight in both cases, and if you can't achieve that, what rudder input will you use to correct it?

P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, that is responsible for the asymmetrical relocation of the propeller's center of thrust when an aircraft is at a high angle of attack.

P-factor - Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/P-factor

If the plane is veering the same direction, (relative to you), whether inverted or upright, then the wings aren't level to start with. It sounds like you may be flying with the inside wing down.




I'm sure some planes are worse than others and maybe it can't be detected on some. Gyroscopic precession and perhaps the thrust line settings could magically cancel out the P-effect but it would seem to me to be a case of luck.

No magic or luck involved. It's called trimming and flying with the wings level.




Last edited by drac1; 08-25-2016 at 02:47 PM.
Old 08-25-2016, 02:49 PM
  #56  
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Originally Posted by serious power
Hi Scott,
You didn't get the point I was trying to make - it was a thought experiment - my bad !

A new thought experiment ;
Let us build a new model.
- A standard fuz crutch with decks added top and bottom.
- Wing and tail mounted but with the wing set a little -ve in incidence.
- Let's put the vertical stabiliser on the bottom (like a predator - but a single one)

What motor thrust set up do you think will be req'd so that will pull from s&l horizontal to vertical and track straight ??
Why ??

Brian
One question - Why??
Old 08-25-2016, 02:50 PM
  #57  
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Originally Posted by speedracerntrixie
Interesting thread. So if P effect is at a high AOA what is dragging the airplane to the left during take off?
If ??

P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, that is responsible for the asymmetrical relocation of the propeller's center of thrust when an aircraft is at a high angle of attack.

[h=3]P-factor - Wikipedia, the free encyclopedia[/h]https://en.wikipedia.org/wiki/P-factor



Old 08-25-2016, 04:10 PM
  #58  
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According to your Wiki definition, P effect happens when the aircraft is at high AOA, so question remains. What is the force that pulls the airplane left on takeoff and why would it not be the same force as when we push or pull?
Old 08-25-2016, 05:55 PM
  #59  
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Originally Posted by speedracerntrixie
According to your Wiki definition, P effect happens when the aircraft is at high AOA, so question remains. What is the force that pulls the airplane left on takeoff and why would it not be the same force as when we push or pull?

It's not my definition.

My plane tracks straight on take off and through pulls and pushes.

As I've previously said, from the description of the problem, I think it is a thrust, stab alignment issue, or a combination of both.
Old 08-25-2016, 06:53 PM
  #60  
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I'm leaning towards torque myself. We can trim the airplane for a certain spot along the torque curve but once we load the airplane and increase the power required to maintain speed the trim changes. On takeoff when we are climbing up the torque curve it is noticeable, when we power up and pull to vertical the load is increased and the trim changes. Granted the more we spread out this load the less pronounced the change is. Running a pylon airplane that weighs 6.5 lbs and has a YS 115 14x14 power combo requires a ton of right rudder and slow throttle application to keep straight on takeoff. Once up to full speed and constant torque loading it tracks straight through the course and through turns.
Old 08-25-2016, 09:15 PM
  #61  
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Originally Posted by speedracerntrixie
According to your Wiki definition, P effect happens when the aircraft is at high AOA, so question remains. What is the force that pulls the airplane left on takeoff and why would it not be the same force as when we push or pull?
A tail dragger is at high angle of attack on the runway during takeoff until it rotates (tail lifts off). It is really the angle between the prop shaft and the flight path that causes P-effect. Hold a prop horizontal and look at it in what would be a top view and then rotate it to simulate a nose up. You will see the pitch of the blade on the right increases and the one on the left decreases with respect to the horizontal. This couple causes a pull to the left and requires right rudder.

Test: What if the engine/motor rotated in the opposite direction? I seem to remember some old British engines did rotate opposite. Anyone confirm this?

Jim O
Old 08-25-2016, 10:24 PM
  #62  
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Hi Jim,
I'm giving up on the theory and hypothetical here as it seems a little pointless.

On the more practical side I suggest you try adding some sub-fin for a test or two. With more sub-fin less right thrust should also be req'd. If you do let us know the outcome and if it works we can discuss why. The three S's come to mind
Also/alternately, though it is a single prop set up, try a flare on the rudder TE.
I believe the fin design on the Nuance is inadequate to begin with !!!!!!!

Brian
Old 08-26-2016, 07:13 AM
  #63  
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Jim, I understand how P effect is supposed to work, I just don't see how it applies to our models. Most pattern airplanes sit at rest maybe 15 degrees but the tail rises pretty darned quick. After the tail rises there is still rudder input required to the right. During flight our light models react to inputs very quickly and are flying quite fast for their size. As a result I tend to beleive that for the most part the airflow hitting the prop is almost always at 90 degrees to the prop disk. The only time I see this changing is during final approach to touch down. Even then unless there is a cross wind I don't hold rudder to the right during flare to counteract the P effect created from flying 10-15 degrees nose up from flight path. Granted at that point we are at very low power setting. Have a look at the first part of this video. You can clearly see rudder input on the take off roll. You can also see it being tapered off as the torque curve flattens out as speed builds

https://www.youtube.com/watch?v=GQTLS6l28fo
Old 08-26-2016, 01:09 PM
  #64  
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Originally Posted by drac1
The old "Spiral Slipstream" myth.


Slipstream swirl is a corkscrewing airflow that travels around the fuselage.
This airflow strikes the vertical stabilizer, increasing its AOA. The increase in AOA of the vertical stabilizer pulls the tail to the right and yaws the nose to the left.
This situation is more prevalent at high power settings and low airspeeds.
To compensate, right rudder is required.
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Old 08-26-2016, 02:42 PM
  #65  
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Originally Posted by Cruiser1970


Slipstream swirl is a corkscrewing airflow that travels around the fuselage.
This airflow strikes the vertical stabilizer, increasing its AOA. The increase in AOA of the vertical stabilizer pulls the tail to the right and yaws the nose to the left.
This situation is more prevalent at high power settings and low airspeeds.
To compensate, right rudder is required.
Yes, I know the definition of the myth. That doesn't mean it's a real phenomenon though.

As far as I am aware, it's never been proven to actually exist, just a theory. If you have evidence, feel free to share.
Old 08-26-2016, 03:45 PM
  #66  
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Originally Posted by Cruiser1970


Slipstream swirl is a corkscrewing airflow that travels around the fuselage.
This airflow strikes the vertical stabilizer, increasing its AOA. The increase in AOA of the vertical stabilizer pulls the tail to the right and yaws the nose to the left.
This situation is more prevalent at high power settings and low airspeeds.
To compensate, right rudder is required.
You are probably right, the spiral slipstream has more influence than the P-effect during takeoff, but don't let Bryan Hebert hear you say that. He and his devotee's say it doesn't exist. I think what they really mean to say is that it doesn't affect our models. I believe it is the reason we add right thrust.

Jim O
Old 08-26-2016, 04:55 PM
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Originally Posted by drac1
Yes, I know the definition of the myth. That doesn't mean it's a real phenomenon though.

As far as I am aware, it's never been proven to actually exist, just a theory. If you have evidence, feel free to share.
Myth or not, that’s what they teach USAF student pilots. I pulled that quote right out of the JointPrimary Aircraft Training System (JPATS) Aerodynamics Student Study Guide.
Old 08-26-2016, 05:09 PM
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Originally Posted by Cruiser1970
Myth or not, that’s what they teach USAF student pilots. I pulled that quote right out of the JointPrimary Aircraft Training System (JPATS) Aerodynamics Student Study Guide.
Doesn't mean it's true though.

Maybe the Abominable Snowman, Yeti or Loch Ness Monster can verify
Old 08-26-2016, 05:12 PM
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Originally Posted by OhD
You are probably right, the spiral slipstream has more influence than the P-effect during takeoff, but don't let Bryan Hebert hear you say that. He and his devotee's say it doesn't exist. I think what they really mean to say is that it doesn't affect our models. I believe it is the reason we add right thrust.

Jim O
Believing is one thing. Having proof to back it up? Well.............
Old 08-26-2016, 05:16 PM
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Originally Posted by Cruiser1970
Myth or not, that’s what they teach USAF student pilots. I pulled that quote right out of the JointPrimary Aircraft Training System (JPATS) Aerodynamics Student Study Guide.
With all the resources and technology available to the USAF, if it exists, don't you think they would be able to prove it?

I'm sure they've tried.
Old 08-26-2016, 06:35 PM
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I think the size of a pattern plane, spiral is too small of a force. Precession force is 90 deg after the input. Pull up with normal motor rotation and the precession force is to the right wing until stable again. Whole point to need nose rings on soft mounts.
Old 08-26-2016, 08:22 PM
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I don't know if this is proof, but it looks to me like evidence. I corresponded with the pilot. The photo is from Joe Nall a few years ago. The prop did not touch the water, but the wheels did. What do you think made the spiral pattern in the spray aft of the tail?

Bill

Originally Posted by drac1
Yes, I know the definition of the myth. That doesn't mean it's a real phenomenon though.

As far as I am aware, it's never been proven to actually exist, just a theory. If you have evidence, feel free to share.
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Old 08-26-2016, 09:56 PM
  #73  
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Originally Posted by n233w


I don't know if this is proof, but it looks to me like evidence. I corresponded with the pilot. The photo is from Joe Nall a few years ago. The prop did not touch the water, but the wheels did. What do you think made the spiral pattern in the spray aft of the tail?

Bill
Correct, not proof. If it was that easy to prove, the SSS advocates would be all over it in a flash.
If you put streamers down the sides of the fuse, I bet they would blow straight back.
Also how could it spiral around the fuse when there is a big fence called a wing and the stab which would prevent it from spiraling anyway.
Old 08-27-2016, 05:36 AM
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http://www.boldmethod.com/learn-to-f...ng-tendencies/

A little Googlefoo found this.



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Old 08-27-2016, 08:23 AM
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Originally Posted by drac1
Correct, not proof. If it was that easy to prove, the SSS advocates would be all over it in a flash.
If you put streamers down the sides of the fuse, I bet they would blow straight back.
Also how could it spiral around the fuse when there is a big fence called a wing and the stab which would prevent it from spiraling anyway.
drac1, please don't misapprehend: the air, aft of the wings and tail, is spiraling.

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