The aerodynamic sideforce generated by a slip depends on the shape of the aircraft
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This page was last modified on August 6, 2006
In "Aerodynamic sideforce during slips and skids", we noted that the sideways airflow in a slip or a skid impacts the side of the fuselage and other surfaces of the aircraft and generates an "aerodynamic sideforce". We noted that this "aerodynamic sideforce" is really a form of lift, and acts perpendicular to the flight path and relative wind, and parallel to the wingspan. We described how this "aerodynamic sideforce" increases the turn rate (for a given bank angle) in a skid, and decreases the turn rate (for a given bank angle) in a slip, and also makes it possible for an aircraft to fly in a straight-line, non-turning slip. We noted that during vertically banked, sustained, "knife-edge" flight, this aerodynamic sideforce acts in the upward direction and must support the entire weight of the aircraft. We noted that this "aerodynamic sideforce" is what makes the slip-skid ball, the pilot's body, and the other contents of the aircraft deflect toward the "high side" or the "low side" of the fuselage during a skid or a slip.
For a given "yaw angle" or "slip angle"--i.e. for a given angular difference between the aircraft's heading and the actual direction of the flight path and relative wind--an aircraft with a large, slab-sided fuselage will generate much more "aerodynamic sideforce" than will an aircraft with a slender, streamlined fuselage. This is why RC model aircraft that are intended to perform well in sustained, knife-edge flight often have flat "planks" for fuselages (photo to be inserted). Conversely, a flying-wing aircraft will generate very little aerodynamic sideforce for a given yaw or slip angle. This means that for a given deflection of the yaw string (if present), there will be little deflection of the slip-skid ball, and of the pilot's body. This also means that very little "upwind" bank will be required when a pilot uses the rudder to yaw the aircraft's nose out of alignment with the flight path and relative wind, and into alignment with the ground track, during a slipping crosswind landing. In other words, in a flying-wing aircraft the pilot is free to yaw the nose from side to side to adjust for crosswinds and align the landing gear with the runway during a crosswind landing, and these yaw adjustments will create only a very gradual acceleration ("drift") in the downwind direction, and very little opposite bank will be needed to completely cancel out the sideways drift arising from these yaw adjustments. This means that both the "sustained slip" and "kick out the crab" methods of crosswind landing should be very easy to execute.
When I held a full-deflection left rudder input in this aircraft (view 1, view 2), no discernable bank either to the left or to the right was required to prevent the flight path from curving. This is in marked contrast to a non-turning slip in a "conventional" aircraft, where a left rudder input generates a significant aerodynamic sideforce to the left, which must be counteracted with a right bank if the goal is a straight-line, non-curving flight path. (Another difference between the aircraft illustrated here and a more "conventional" aircraft: when I yawed the nose of this aircraft to the left, I also had to give a left roll input to counteract the roll torque created by the interaction between the sideways airflow and the anhedral wing, so that the bank angle remained constant. In a more "conventional" aircraft, when the nose is yawed to the left of the actual direction of the flight path and relative wind, the resulting sideways airflow interacts with the dihedral wing to create a roll torque toward left, so a right aileron input is needed to hold the bank angle constant. The issue of how much an aircraft must be banked to counteract any aerodynamic sideforce that is present, so that the flight path remains a straight line, and the issue of which direction the ailerons must be moved to counteract any roll torque that is present and keep the bank angle constant, are entirely seperate.)
As another example, I added a controllable rudder to these flex-wing hang gliders (Spectrum, Blade, Skyhawk, Raven) and monitored a set of slip-skid bubbles (earlier version) as I yawed the nose to one side in relation to the actual direction of the flight path and relative wind. I found that yawing the nose in the manner created virtually no aerodynamic sideforce or deflection of the slip-skid bubble. I also found that no significant bank in either direction was required to keep the flight path from curving to either side when I deflected the rudder and yawed the nose in this manner. In fact, when I yawed the nose in this manner the slip-skid bubbles actually deflected very slightly in the "wrong" direction: yawing the nose to point to the left of the actual direction of the flight path and relative wind moved the bubbles to the right (and a slip-skid ball would have moved to the left). This showed that when the rudder was deflected to the left, this actually created a net aerodynamic sideforce toward the right rather than toward the left, and a very slight left bank was actually needed to keep the flight path moving in a straight line! The reason for this bizarre state of affairs will be explored in more detail elsewhere in this tutorial; for now the main lesson is that an aircraft with minimal cross-sectional area, as viewed from the side, will generate minimal aerodynamic sideforce when the nose is yawed to one side in relation to the actual direction of the flight path and airflow.
If an aircraft develops only a small amount of aerodynamic sideforce in a slip, this also suggests that only a small amount of drag will be generated, although even an all-wing aircraft will be somewhat less efficient (lower L/D ratio) when the airflow over the wing has a sideways component. The less drag an aircraft generates during a slip, the less useful a slip will be to increase the descent rate on final approach. For example, a slip will be more effective in a Piper Cub or Schweizer 2-22, with their slab-sided fuselages, than in a sleek high-performance sailplane. (In the case of a very efficient aircraft, the rudder itself may be the largest source of drag when a large rudder deflection is applied to yaw the nose away from the actual direction of the flight path and relative wind.)
For some more interesting notes about aerodynamic sideforce and drag, see the text of this lecture by John Northrop entitled "The Development of All-Wing Aircraft". In particular, Mr. Northrop made the following points (paraphrased):
1) A conventional rudder is relatively ineffective on an all-wing aircraft because it acts at such a short moment-arm from the C.G. Drag rudders near the wingtips are much more effective.
2) Drag rudders are feasible because there is no big performance (drag) penalty associated with allowing the aircraft to fly in a yawed (slipping) condition, where the nose is not aligned with the actual direction of the flight path and relative wind. In particular if there is a failure of one engine, which will make the aircraft fly in a yawed (slipping) condition, this yaw (slip) will not create a lot of drag, so the yaw (slip) need not be counteracted with the drag rudder. For a long-range all-wing aircraft it may even be efficient to shut off one engine in flight to extend range, even if this causes the nose to yaw (slip) to one side.
3) When a failure of one engine, or some other event, causes the aircraft to fly in a yawed (slipping) condition, this yaw (slip) will not create a lot of aerodynamic sideforce, so little opposite bank will be needed allow the aircraft to fly in a straight line. This is one of the reasons why the overall performance (drag) penalty associated with yawed (slipping) flight is so low.
4) The lack of aerodynamic sideforce ("very low crosswind derivative") associated with yawed (slipping) flight in all-wing aircraft, particularly those with no vertical fin of any kind, does have some disadvantages for military applications including close formation flying and gun-aiming.
5) The lack of aerodynamic sideforce ("very low crosswind derivative") associated with yawed (slipping) flight in all-wing aircraft means that it is very difficult for a pilot to "feel" when the aircraft is slipping, i.e. when the nose has yawed to point in a different direction than the actual direction of the flight path and relative wind at any given moment. Therefore a sideslip meter (e.g. a yaw string!) is needed to supplement the conventional slip-skid ball.
(This last point anticipates the difficulties that flex-wing hang glider pilots have had throughout the history of modern hang gliding in correctly analyzing the dynamics of slips in their all-wing aircraft--this is covered in detail elsewhere on the Aeroexperiments website!)
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