P-factor?

[popaviator]

This is probably a simple answer question, but is P-factor opposite in a descent? In a climb the descending right blade takes a larger "bite" of air due the greater angle of attack, but in a descent is the opposite true, as in "does the descending blade now take a smaller bite than the ascending". I was listening to another instructor explain it to a student and it left me alittle confused.

 

[Cessna414JJB]

This is probably a simple answer question, but is P-factor opposite in a descent? In a climb the descending right blade takes a larger "bite" of air due the greater angle of attack, but in a descent is the opposite true, as in "does the descending blade now take a smaller bite than the ascending". I was listening to another instructor explain it to a student and it left me alittle confused.

It lessens with a decrease in AoA and lower power setting.
P-Factor is at it's greatest at high AoA, low airspeed and high power setting. It never goes away completely but you may find yourself compensating with left rudder instead of right rudder in fast descending right turns depending on plane, to keep the ball centered.

The text on wiki sucks - but anyways, here's the link.
http://en.wikipedia.org/wiki/P-factor

and another one...
http://wiki.flightgear.org/index.php/Understanding_Propeller_Torque_and_P-Factor

 

[wjmiller3]

This is probably a simple answer question, but is P-factor opposite in a descent? In a climb the descending right blade takes a larger "bite" of air due the greater angle of attack, but in a descent is the opposite true, as in "does the descending blade now take a smaller bite than the ascending". I was listening to another instructor explain it to a student and it left me alittle confused.

You are not at high power and not at a slow airspeed in a descent normally. If you could keep the airplane slowed down, at a high negative AoA, and at high power setting, then yes p-factor would be the opposite. But that just broke about all the laws of physics.

 

[Firebird2XC]

It lessens with a decrease in AoA and lower power setting.
P-Factor is at it's greatest at high AoA, low airspeed and high power setting. It never goes away completely but you may find yourself compensating with left rudder instead of right rudder in fast descending right turns depending on plane, to keep the ball centered.

:yeahthat:

Sounds good, there. To elaborate, P-factor is most notable when prop thrust is highest at low airspeeds. At higher speeds, especially in the flight regime, mitigating things such as the design offset of the vertical stabilizer, etc, help offset left-hand turning tendencies so that P-factor is less prevalent.

Slip, skid, and torque (in a single engine/multi scenario) are more immediate concerns when keeping the ball centered. Even in a jet, with the yaw-damper off, you'd better be using the pedals, or you're making the passengers sick.

 

[tgrayson]

but in a descent is the opposite true, as in "does the descending blade now take a smaller bite than the ascending".

No, but I've heard that explanation from many pilots; I laughed the first time. They're implicitly associating a climb with a large AoA and a descent with a negative one.

Note that some aircraft compensate for p-factor by offsetting the vertical stabilizer; in a descent at a constant airspeed and low power, this offset stabilizer may overcompensate and require some left rudder.

 

[fish314]

You are not at high power and not at a slow airspeed in a descent normally. If you could keep the airplane slowed down, at a high negative AoA, and at high power setting, then yes p-factor would be the opposite. But that just broke about all the laws of physics.

That scenario is possible, but not common. It doesn't break all the laws of physics, but it would probably be a tough one to get into accidently. And it certainly doesn't occur during a normal descent.

The best example I can think of where it DOES occur would be during an outside loop, especially at the top of the outside loop. Imagine flying an aerobatic aircraft. You start the manuever at the bottom by rolling inverted first and then once upside down, you push negative 3-4 g. The airplane has a negative angle of attack, and it begins climbing upside down. By the time you get to the top of the inverted loop, you are right-side up, slowed down, with the power still set high. You'd have bled off most of the airspeed (and you wouldn't have as much g available), but you'd still be pushing forward on the yoke or stick (high negative AOA), still high power setting, and low speed. At this point, you'd have a high p-factor and in the opposite direction of the one that you commonly experience.

The key to popaviator's question is this. P-factor is opposite during negative angles of attack, but descents don't necessarily (or even typically) equate to a negative angle of attack. In fact, most maneuvers in flying don't involve a high negative angle of attack unless you start talking about some aerobatic maneuvers.

 

[mshunter]

You are all wrong. Very very wrong. P-factor is only present when I'm on downwind after a long x-country flight with a student, and request a short approach, keep the speed up to the numbers, and taxi at 20mph..... because it was an early call and I drank wwaaaaayyyyy to much coffe! Thats what P-factor is.

 

[fish314]

You are all wrong. Very very wrong. P-factor is only present when I'm on downwind after a long x-country flight with a student, and request a short approach, keep the speed up to the numbers, and taxi at 20mph..... because it was an early call and I drank wwaaaaayyyyy to much coffe! Thats what P-factor is.

:rotfl:

 

[MikeD]

You are all wrong. Very very wrong. P-factor is only present when I'm on downwind after a long x-country flight with a student, and request a short approach, keep the speed up to the numbers, and taxi at 20mph..... because it was an early call and I drank wwaaaaayyyyy to much coffe! Thats what P-factor is.

P(ee)-Factor?

 

[wjmiller3]

That scenario is possible, but not common. It doesn't break all the laws of physics, but it would probably be a tough one to get into accidently. And it certainly doesn't occur during a normal descent.

The best example I can think of where it DOES occur would be during an outside loop, especially at the top of the outside loop. Imagine flying an aerobatic aircraft. You start the manuever at the bottom by rolling inverted first and then once upside down, you push negative 3-4 g. The airplane has a negative angle of attack, and it begins climbing upside down. By the time you get to the top of the inverted loop, you are right-side up, slowed down, with the power still set high. You'd have bled off most of the airspeed (and you wouldn't have as much g available), but you'd still be pushing forward on the yoke or stick (high negative AOA), still high power setting, and low speed. At this point, you'd have a high p-factor and in the opposite direction of the one that you commonly experience.

The key to popaviator's question is this. P-factor is opposite during negative angles of attack, but descents don't necessarily (or even typically) equate to a negative angle of attack. In fact, most maneuvers in flying don't involve a high negative angle of attack unless you start talking about some aerobatic maneuvers.

I dont fly acrobatic aircraft so I have no idea. I was speaking of a normal descent as the OP stated. All I know that if you are pitched down, and at full power, you are gonna be accelerating. Barring a inverted 4g negative dive with Goose and Maverick.

 

[escapepilot]

What I would be confused about is why an instructor would even mention p-factor in a descent to a student. :dunno: That just seems confusing.

You COULD have a descent w/ a high angle of attack, low airspeed and high power, but then the p-factor would be the nearly the same so why confuse a student w/ it?

 

[shdw]

I dont fly acrobatic aircraft so I have no idea. I was speaking of a normal descent as the OP stated. All I know that if you are pitched down, and at full power, you are gonna be accelerating. Barring a inverted 4g negative dive with Goose and Maverick.

This descent was explained by tgrayson earlier, even in a dive descent the opposite rudder is not because P-factor reversed. P factor will only go the other direction if you fly upside down, it is relative to the AOA of the wings and as long as that AOA is positive P factor will always act as a left yawing tendency.

To understand it, tape a string to the nose of a prop that is horizontal (parallel to the ground) and tilt the string up and down to change the relative wind. As you tilt the string down (representing a higher AOA) you will see the AOA on the downward blade would be increasing and the upward blade would be decreasing. Since lift is a function of speed and AOA then if AOA went up the lift on that blade has gone up. You can move this string to straight on to simulate a low AOA (such as your dive) and see the AOA is about equal on each blade so P factor is basically non existent. Tilt the string up and observe the AOA change for the will give now a reverse pull from P factor. Hope that helps.


On a clarification note, isn't the rudder angled for the purpose of spiraling slipstream as it is arguably the most predominant of the turning tendencies? I was told it was angled to change how the air hits the rudder from this stream to reduce the effect but I didn't dig in or ask anymore details.

 

[Crockrocket94]

MS that killed me man...

Good one


Btw, I heard and saw that in the PA-28-161 the engine is actually canted.. like offset at an angle slightly...
Look at one head on sometime... its a little offset. Heard thats to help with all those left turning tendencies...

 

[tgrayson]

On a clarification note, isn't the rudder angled for the purpose of spiraling slipstream as it is arguably the most predominant of the turning tendencies?

I've read that the spiraling slipstream is relatively more powerful than p-factor (Kershner, I think), but I don't have any authoritative data. A canted vertical stabilizer could help counter both effects, so I'm skeptical even the designers know which effect they're countering.

And as Crockrocket94 mentioned, a canted engine is another common means of countering left-yawing tendencies, perhaps more common than a canted vertical stabilizer.

 

[beasly]

This is probably a simple answer question, but is P-factor opposite in a descent? In a climb the descending right blade takes a larger "bite" of air due the greater angle of attack, but in a descent is the opposite true, as in "does the descending blade now take a smaller bite than the ascending". I was listening to another instructor explain it to a student and it left me alittle confused.

My gut reaction is from Wolfgang Langewiesche's (sheeeesh, that's a long name) classic "Stick and Rudder" where he brings up two topics: 1 the effect of wind on an airplane in flight and 2. Angle of Attach vs pitch attitude and stalls


1.Effect of wind on an airplane in the air: None. (effect of wind on airplane transitioning from air to ground "see volumes on how to do x-wind landings"

2. Relationship of Angle of Attack to a stall--constant in all attitudes.

Wolfie baby had the first (?) insight that a stall was independent of pitch attitude. I.E the critical angle of attack in a power on stall is exactly that as the critical angle of attack in a steep turn......etc.

Now, AOA is always in relation to the Relative wind---tits up, tits down, balls to the wall, upside down, sideways...the question becomes....where is the airfoil in relation to the tits...er wind passing over it?

Since a prop "is" a "wing"

1. you are in a pitch down attitude in a very strong updraft where even with full pitch down attitude, the thermals are kicking you up.


2. .......


screw it.

Wolfie was right. It don't matter. the up prop has it--unless you are moving backward.

b.



b.

 

[beasly]

My gut reaction is from Wolfgang Langewiesche's (sheeeesh, that's a long name) classic "Stick and Rudder" where he brings up two topics: 1 the effect of wind on an airplane in flight and 2. Angle of Attach vs pitch attitude and stalls


1.Effect of wind on an airplane in the air: None. (effect of wind on airplane transitioning from air to ground "see volumes on how to do x-wind landings"

2. Relationship of Angle of Attack to a stall--constant in all attitudes.

Wolfie baby had the first (?) insight that a stall was independent of pitch attitude. I.E the critical angle of attack in a power on stall is exactly that as the critical angle of attack in a steep turn......etc.

Now, AOA is always in relation to the Relative wind---tits up, tits down, balls to the wall, upside down, sideways...the question becomes....where is the airfoil in relation to the tits...er wind passing over it?

Since a prop "is" a "wing"

1. you are in a pitch down attitude in a very strong updraft where even with full pitch down attitude, the thermals are kicking you up.


2. .......


screw it.

Wolfie was right. It don't matter. the up prop has it--unless you are moving backward.

b.



b.

but is P-Factor "relevant" in a descent?

no.

Can any airfoil take a "bigger bite out of the relative wind" when the wind is moving faster than the airfoil?

No...Vne on a helicopter is where the backward blade can't generate lift because the relative wind to that blade is decreasing...
(according to my ex-girlfriends army helicopter brother)

But!

do we know that relative wind does not act as a "reverse airfoil" against an an airfoil?

I don't know.

but you raised a Great question. Thanks! you got me thinking!

b.

 

[beasly]

That scenario is possible, but not common. It doesn't break all the laws of physics, but it would probably be a tough one to get into accidently. And it certainly doesn't occur during a normal descent.

The best example I can think of where it DOES occur would be during an outside loop, especially at the top of the outside loop. Imagine flying an aerobatic aircraft. You start the manuever at the bottom by rolling inverted first and then once upside down, you push negative 3-4 g. The airplane has a negative angle of attack, and it begins climbing upside down. By the time you get to the top of the inverted loop, you are right-side up, slowed down, with the power still set high. You'd have bled off most of the airspeed (and you wouldn't have as much g available), but you'd still be pushing forward on the yoke or stick (high negative AOA), still high power setting, and low speed. At this point, you'd have a high p-factor and in the opposite direction of the one that you commonly experience.

The key to popaviator's question is this. P-factor is opposite during negative angles of attack, but descents don't necessarily (or even typically) equate to a negative angle of attack. In fact, most maneuvers in flying don't involve a high negative angle of attack unless you start talking about some aerobatic maneuvers.

Wow.

b.

 

[beasly]

MS that killed me man...

Good one


Btw, I heard and saw that in the PA-28-161 the engine is actually canted.. like offset at an angle slightly...
Look at one head on sometime... its a little offset. Heard thats to help with all those left turning tendencies...

Heard the same about C-172's .

 

[beasly]

What I would be confused about is why an instructor would even mention p-factor in a descent to a student. :dunno: That just seems confusing.

You COULD have a descent w/ a high angle of attack, low airspeed and high power, but then the p-factor would be the nearly the same so why confuse a student w/ it?

What about this....out practicing at 5000 and heading back to the airport and you want to make time because that day's subject matter is over (although learning and teaching never stops). You have the student pitch for top of green line and hold it.

Have you ever heard the props cavitating because the relative wind was faster than the prop speed? (pa -28)

Shouldn't the UP prop have a "negative" P-factor?

Kind of like in a variable pitch prop scenario where one of the tools you can use to slow down is prop levers forward.


b.

 

[tgrayson]

Shouldn't the UP prop have a "negative" P-factor?

It will be providing reverse thrust, so it's still a left turning tendency.