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Old 03-10-2012, 10:26 AM   #20  
Gets Weekends Off
Joined APC: Jun 2009
Posts: 317

Slipstream, with regards to high powered military single engine propeller aircraft, in particular the navy fleet, was a critical design consideration. Being most critical at low speed and high power, such as landing on a carrier.

It's not a theory; we know it to exist. Here's a quote from a design book written in the late 40s:

Originally Posted by book
Slipstream rotation -- The slipstream behind the propeller has a rotational component which changes the angle of attack of the vertical tail and will create a sideslip if uncorrected by the rudder. The critical condition for slipstream rotation is for high power at low speed.
On the flight instructor level, aerodynamics for naval aviators chapter on direction stability and control covers the subject well. Noting that slipstream rotation and/or spin recovery are the typical critical criteria placed on rudder control in a single. Though I suspect they too are referring to high powered singles used for carrier landings.


FWIW, this is how I teach it:

Torque: A left rolling tendency, not yawing, that is the result of the airplane reacting to the spinning propeller. It's effects are small enough to be ignored unless viewed at full power and at, or very near, stall speed.

Gyroscopic Precession: A left turning tendency during the days where tail draggers ruled the fleet. In todays tricycle aircraft it too can be ignored. Though I depict it visually using a stick, held vertically, to represent the propeller/yoke, having the student grab the top with one hand and bottom with the other, and then push on the top/pull the bottom. We then rotate the stick/propeller 90 degrees and see the right side is pushed forward, left back -- a left yaw. (This is the classic raising of the tail in a tail dragger aircraft)

Slipstream and P-factor: This is what we contend with during takeoff roll and departure climb out. At cruise design corrects for them or our legs would get quite tired.

P-factor is of no factor until we leave the ground and the fuselage has an angle of attack to the relative wind. Slipstream is what requires use of the rudder during the takeoff roll. In flight a combination of these two require rudder corrections during full power climbs, go arounds, or any other high powered low speed flight.


You may notice the particular interest I've put into mentioning when a pilot must contend with these conditions. That is because it is my position that we are not trying to teach the pilot a complete theory behind these 4 conditions. Instead, we try to show them enough to 1) not be confused/scared and 2) understand and accept their existence.

Once their existence is accepted it will be of far greater value to have a complete understanding of when these conditions require particular attention and input from them. And that is during takeoff roll, climbs, go arounds, and any other slow speed and high power condition.
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