Toujours

If an airplane is in straight and level flight we know that the relative wind if parallel to the flight path but in the opposite direction. However where is the relative wind for the propeller? Is it the plane of rotation???

SNS3Guppy

When the aircraft is stationary, such as sitting on the ramp with no breeze, then the relativewind is found in the plane of rotation. A vector is established when the aircraft moves forward or wind flows through the propeller arc in which the relative wind is a combined direction based on the plane of rotation, modified by that forward speed.

The value varies with the propeller RPM, and airspeed.

airfoilmod

p-factor, similar to lead lag in helo. As a propellor climbs, (and with a counterclockwise rotation), the descending blade has more lift than the ascending blade, which will exacerbate torque issues, requiring more rudder input. (Single). It is exceedingly rare to find Relative Wind at ninety degrees to Plane of Rotation in Propop. Each Blade is an airfoil, and experiences different Relative Airflow depending on its position in the Propellor "Disc". (When Rela. Wind is other than 90 degrees)

Airfoil

Oktas8

When teaching instructors how to draw the relative wind acting on a propeller blade, I teach the following:

• Draw the propeller from side on, showing the downgoing blade. Draw an arrow pointing down from the blade, representing plane of rotation.
• Draw an arrow from the bottom of the plane-of-rotation arrow, pointing forwards. This represents the forward speed of the aircraft.
• Draw an arrow diagonally from the forwards-pointing arrow to the propeller blade.
• You should now have a right-angled triangle, consisting of Plane of Rotation, Forward Speed and finally Relative Wind.

It's easy to draw variations showing the effect of higher forward speed, increased or decreased RPM etc. The only time the relative wind is the same as plane of rotation is when forward speed is zero. It is also easy to see that it is not possible for the relative wind to be the same as the wing's relative wind (that is, straight forwards) unless the propeller is not rotating at all.

Secondly, there are several corrections to be made with this statement...

The effect you're describing occurs when the aircraft is operating at high angles of attack. The aircraft need not be climbing, and often is not (e.g. on rotation during take-off, but before a significant ROC has developed).

It doesn't matter whether the propeller rotates counterclockwise or not.

This effect is not directly related to torque issues. The thrust developed by the entire propeller slightly favors (as you said) the downgoing blade, tending to cause yaw. Torque causes roll, which may lead to yaw...

The effect is not limited to single engine aircraft. I fly a light twin which has a pair of clockwise rotating propellers, and tends to yaw quite strongly during / after rotation.

All the best,
O8

airfoilmod

The Rotation is important or one would not know which side of the A/C was affected by the additional thrust.

Torque does cause Yaw, or one would need Aileron on T/O rather than opposite Rudder. This applies to single engine A/C and twins with similar rotation.

R/W can be 90 degrees to Propellor Plane at times other than on the ramp. Consider windmilling or engines configured so at particular cruise.
Just as the wing needs an AoI, the Propellor can as well, or be "tuned" to cruise in the air mass at 90 degrees, while the wing is not.

Sir, in "exacerbating torque issues" I intended to suggest an enhanced Yaw to the left (counterclockwise propellor), not that the Propellor was creating or adding to torque.

I would also disagree that the discussion requires a "high angle of attack"
Any AoA at all creates the departure from ninety degrees by definition.

barit1

If you draw the relative wind vector using only the aircraft's velocity vector to represent the incoming airflow, I think you are overstating the P-factor.

The low-pressure field upstream of the propeller disc means that the air receives substantial acceleration before it reaches the plane of rotation. This has two components:

1. It enters the propeller plane at a higher speed than the IAS says. (In static runup, there is a positive incoming wind component, not a zero incoming wind)

2. The angle of attack, if any, is reduced by the above acceleration. (The stream tube is bent to nearly align with the prop axis)

airfoilmod

I agree completely. Why discuss Relative wind at rest on the ramp? By definition, there is none. The Tube is created by the propellor, hence dependent upon it. The discussion is deceptive at its outset, comparing "driven" airfoils (wing) to "driving" airfoils can be problematic. Trying to create a situation in which a discussion can occur needs careful setup.

barit1, in propellor 101 it is advanced that the most efficient propellor is one with a single Blade, (Obviously counterweighted) do you remember why?

Airfoil

Aside: During runup, the Stream Tube resembles a Helix, as the incoming and soon to be thrashed air mass tries to align with each Blade to oppose its AoA. This is a Hint to my question, above

barit1

It's because there's no wake interference between blades (for the same reason a biplane or triplane is aerodynamically less efficient than a monoplane)

Incidentally I've seen a photo of an experimental single-blade prop on a prewar light plane - a Cessna Airmaster I believe. The problem I believe is that it would need to be larger diameter, and thus turn lower RPM, to match the thrust of a multiblade prop.

cwatters

To provide the same load a single bladed prop is larger than one blade of a twin bladed prop. Generally one big wing is more efficient than two small ones.

Model flyers who compete in the control line speed competitions frequently use single blade props.

Wait till it gets "on the pipe"..
http://www.youtube.com/watch?v=JGBHips72TA
...208mph.

inbalance

.... and itīs got only one wake