Teaching P-factor

djh1007

djh1007

Flying Squirrel Trainee
Ok guys, I've got my initial CFI ride Wednesday, and although I feel prepared, I need a little help from new sources cleaning this up.

Here is a page from my PHAK on P-factor:

ghb7j.jpg


I understand P-factor, but need help teaching it to a FED :)

WHY is the area of resultant velocity greater on the right side? Or is it simply enough to know and teach that it is greater on theright side, just because. And that is that, don't dig any deeper.

I'm probably over thinking this, they probably don't expect me to have an aeronautical engineering degree level of understanding.

I also realize this would be 100% overkill for a private student with 10 hours.
 
Saying that thrust is greater from the right side and leaving it at that is absolutely not the subject matter expertise expected of a CFI. The right side of the prop is biting down into the relative wind, and the left side is pulling up and away from the relative wind. Think of slow flight. Your pitch angle is 15 degrees (arbitrary figure), but your flight path is level. When the prop goes down, it is biting 15 degrees into the relative wind and generating extra thrust because the "airspeed" over the blade is higher. The blade that comes up is biting 15 degrees away from the relative wind, and therefore has less "airspeed" and generates less thrust.

If you want an extreme examlple, think of a helicopter blade traveling forward.
 
Easiest way to visualize what happens with p-factor is to hold one of these at a 45° angle, move it through the air like an airplane at a high angle of attack and see how the relative wind is affecting the ascending and descending propellers.... much easier than trying to explain it with words or drawings.
plastic_prop_toys_600.jpg
 
I started my early students out on the "bite" expanation. When the entire prop is at an upward angle (high AOA) the descending blade takes a bigger "bite" out of the air than the ascending blade. It's pretty easy to sketch out on a whiteboard by drawing a rediculously high AOA wing, the relative wind, and the AOAs of the ascending and descending blade. Point out the different AOAs on the two blades. For advanced students it's a similar explantion but leave out the "bite" bit and go into how the AOA between the two blades differs with the changes in overall airplane AOA. This change in prop AOA is what actually causes a higher resultant velocity on the descending blade and a lower resultant velocity on the ascending blade. Believe me, pictures are a heck of a lot easier.
 
rframe said:
Easiest way to visualize what happens with p-factor is to hold one of these at a 45° angle, move it through the air like an airplane at a high angle of attack and see how the relative wind is affecting the ascending and descending propellers.... much easier than trying to explain it with words or drawings.
Click to expand...

That's great, I could teach it with this prop in my sleep. Any ideas where to buy one from a non-internet source?
 
djh1007 said:
That's great, I could teach it with this prop in my sleep. Any ideas where to buy one from a non-internet source?
Click to expand...

See if there are any "smart" toy stores around, the kind that sell overpriced wooden puzzles, chemistry sets, etc to yuppies who think their kid will be the next Einstein.

Or, just go to an R/C hobby shop and buy a large 10+" wooden propeller and glue it to the end of a 1/4" wooden dowel (Home Depot).

By the way, the inspectors want to see creative use of visual aids like this. My CFI inspector was like "man, I dont get it, I want to be excited and entertained when I learn and no CFI candidates come in with anything cool... I want someone to come in with a huge RC airplane some day, and some videos on a DVD, and other cool illustrations".
 
You have to seperate the relative wind of the airplane to that of the propeller. If you just think about the relative wind of the prop only its a bit easier to understand and teach. That being said, if you increase the airplane's AoA, you are opening the blade angle on the decending blade and closing the blade on the acending blade. The resultant effect will cause greater thrust on the decending blade causing asymmetrical thrust...aka p-factor.
 
I always taught P-factor in terms of lift because a propeller is just a rotating wing providing lift in a forward direction rather than an upward direction like the aircraft's wing does. Since lift is typically a simple concept for students to understand it is easy to explain that greater angle of attack = more lift. When an aircraft is climbing the descending prop blade is at a greater AoA which provides more lift (thrust in the case of a prop) which causes an asymmetry of thrust and a turning tendency towards the left. Clear as mud? A white board and some markers make it a simple topic to teach.
Good luck on the check ride.
 
The way I was taught p-factor was think of a helicopter moving forward, the blade thats moving forward is biting into more relative wind than the blade moving away from the relative wind.
 
you can hold two pieces of paper together twisted at an angle to form a prop. have them sit and view the angle of attack on the right blade. pitch your make believe prop up as if you were to climb. this is something you can even have them do themselves.
 
Wow you guys are making this way harder than it needs to be. The right blade has a higher pitch angle than the much flatter pitched left blade. More angle means more "bite" and more thrust.
 
Pay2 said:
I always taught P-factor in terms of lift because a propeller is just a rotating wing providing lift in a forward direction rather than an upward direction like the aircraft's wing does. Since lift is typically a simple concept for students to understand it is easy to explain that greater angle of attack = more lift. When an aircraft is climbing the descending prop blade is at a greater AoA which provides more lift (thrust in the case of a prop) which causes an asymmetry of thrust and a turning tendency towards the left. Clear as mud? A white board and some markers make it a simple topic to teach.
Good luck on the check ride.
Click to expand...
I like that. Nice and simple.
 
Teaching that the descending blade has a higher angle of attack is how it use to be explained but I believe that it is insufficient. Max1594's explanation accounts for the greater effect of P-factor. The descending blade is traveling at a faster airspeed because of the forward motion of the aircraft. The descending blade's resultant airspeed is equal to the relative wind plus the speed of the descending blade into the relative wind. The ascending blade's resultant airspeed is equal to the relative wind minus the speed of the ascending blade away from the relative wind.

This greater resultant airspeed of the descending blade means more lift.
 
As I was taught the "greater bite of air" is a factor, but less of a factor than the downward swinging blades greater velocity through the air, which generates more lift. P-factor finally made sense to me when a helicopter was drawn on a board (top down or plan view) with the relative wind coming from the front. As the helicopter moves forward the blade swinging forward (against relative wind) has more velocity and lift versus the blade swinging back (with relative wind). A helicopter is just an extreme example of angle of attack.

Also on my CFI checkride my examiner asked me if a C-172 taxiing down the ramp as P-factor. No because there is no AOA. Same examiner, does the same C-172 have P-factor in the run up, no because no AOA. Examiner "ok applicant, when does the aircraft begin to have P-factor?" ... when the airplane rotates on the runway, because now you have a AoA. Examiner, "Correct!"
 
Just out of curiosity, since the descending blade is generating greater 'lift,' and greater lift = greater drag, how does that play into all of this?
 
dakrt23 said:
Also on my CFI checkride my examiner asked me if a C-172 taxiing down the ramp as P-factor. No because there is no AOA. Same examiner, does the same C-172 have P-factor in the run up, no because no AOA. Examiner "ok applicant, when does the aircraft begin to have P-factor?" ... when the airplane rotates on the runway, because now you have a AoA. Examiner, "Correct!"
Click to expand...

Not true. On the ground, in a crosswind, there will be a difference in relative airspeed for the top or bottom blade. Therefore the airplane will have a pitching force as the result of p-factor (while we normally think of yawing forces exclusively)

If a propeller is turning 300 knots at mid-rpm taxi and encounters a 30 knot direct crosswind the top blade will encounter a 330 knot relative airspeed while the bottom blade will encounter a 270 knot relative airspeed, a 60 knot difference. Since lift is equal to the square of airspeed, this 20% variation in speed will have a substantial difference in thrust from the top blade vs the bottom blade... and the net result is that extended operations with a right crosswind will wear out your nosewheel tire much faster than extended operations with a left crosswind (assuming of course a standard American-made non-geared engine).

Bazinga!
 
rframe said:
... and the net result is that extended operations with a right crosswind will wear out your nosewheel tire much faster than extended operations with a left crosswind (assuming of course a standard American-made non-geared engine).

Bazinga!
Click to expand...
Good to know!
 
So it's my understanding that to have P-Factor at all, some form of relative wind must be necessary?
 
I struggled with p factor in my primary training until my instructor took me out to the 152 had me look at the prop, then pushed the tail down and told me to look at the prop again. Everything clicked and I enjoyed the lesson.
 
Rudabega said:
I struggled with p factor in my primary training until my instructor took me out to the 152 had me look at the prop, then pushed the tail down and told me to look at the prop again. Everything clicked and I enjoyed the lesson.
Click to expand...

Glad somebody finally mentioned this. Give the explanation and then go out and show them!
 
You must log in or register to reply here.
Back
Top