Is a glider on a winch directionally stable?

Question

Let's say I were to fly a glider with the stick and rudder exactly centered laterally, while making the appropriate pitch inputs, during a winch launch.

Would the glider remain wings level throughout the launch or will it diverge to one side? My intuition says it will be stable, because on the flip side you need to apply constant stick and rudder inputs to steer into a crosswind during winch launch. Then again, that is for coordinated flight, so perhaps stability suffers if we allow a sideslip to develop. Also, I've only flown a very gentle ASK21 trainer, perhaps it's different for less stable aircraft.

I'm looking for an answer that relates the forces acting on the airframe to a stability argument: so both the forces and moments due to the winch, as well as the sideslip / roll coupled forces. Free body diagrams and/or equations of motion are highly encouraged.

Answer 1

If you want the short answer, skip to the bottom.

If we look at a side-view of a winch launch, we can get a sense of the forces on the glider during launch. The below is a representative example taken from a numerical study of a winch launch, including a dynamic model of the aircraft, cable, and winch:

Looking at the angle of the cable with respect to the aircraft, we can infer that the cable, while providing a non-negligible downward force (relative to the aircraft), it is primarily providing forward velocity to the aircraft. Per the study (and experience flying a winch), the primary job the pilot is to maintain a desirable dynamic pressure (read: airspeed), which we do via managing pitch angle. This is verified if we look at our experience as well as results from the paper (copied below). Please excuse the first few seconds of the experiment as it doesn't include ground roll, but note how the pitch angle, $\theta$, and the climb angle, $\gamma$, trend with glider's true airspeed, $V_a$ (not to be confused with the published maneuvering speed for the aircraft...I can't help their notation, sorry). In short, the winch is giving energy to the aircraft, which is then converted into altitude via management of the glider's pitch angle.

All this to say: a winch launch doesn't naturally produce lateral or directional moments. The aircraft is receiving energy from an outside force, but the cable giving that force is nominally attached at the aircraft's CG and is allowed to pivot freely. In other words, it is not contributing to the stability of the aircraft save through its influence on the aircraft's speed and angle of attack. This does end up producing an opportunity for the pilot to excite the aircraft's phugoid mode (a pitching response) while pitching up into the climb, but that isn't going to generate a yawing/rolling motion. While on the ground, the rope can produce highly de-stabilizing yaw moments (should a wingtip catch the ground), but, once in the air, the winch cable isn't intuitively stabilizing or destabilizing the aircraft (as opposed to an aerotow cable, which provides strong directional or pitching moments due to where it is attached). Hence, if all is perfect we can consider a glider on a winch launch as a glider in the air, albeit with highly loaded wings. And, if everything is perfect, that is all it is.

However, let us consider what isn't perfect.

THE SHORT ANSWER

A glider is designed to be laterally stable -- if only because their long, slender wings flex upward as they are loaded (thus giving the glider some more dihedral) -- and the extra energy given by the winch lets us load the wings more than normal as we pull back during a winch launch to gain altitude. Lateral stability is typically expressed as a rolling moment that corrects for sideslip. If the aircraft is maintained in perfectly coordinated flight through the initial phases of launch (i.e., where it is generating airspeed and safety margin above stall speed), the aircraft will not experience sideslip and will not roll. However, the key assumption is that the aircraft's ground track is parallel with the winch cable.

The winch will continue to pull the glider toward itself throughout the launch, regardless of the yaw angle of the glider. As mentioned before, this is particularly problematic on the ground (as it can produce an aggressive ground loop), but, in the air, this results in an increased sideslip angle instead. A laterally stable glider's natural response to a sideslip is to roll opposite the direction of the slip to correct for it. Hence, if the ground track of the glider is not parallel to the direction of the cable pulling on the aircraft, the winch cable will give the glider some sideslip. The aircraft's response will be roll, and it will be up to the pilot to maintain a safe attitude.

Long story short: don't let go of the stick and rudder during a winch launch.

Answer 2

The winch cable adds a force which acts in the vertical plane of symmetry at a point below and a bit ahead of the center of gravity. In zero crosswind there is no lateral force component added by the cable and lateral stability is not affected. A laterally stable glider will still be laterally stable during the winch launch.

This is not to say that longitudinal stability is not affected: The cable adds a pitch-up moment in the early stages of the launch and a pitch-down moment in the later stages. Both cases need to be considered when sizing the horizontal tail during design. All-flying tail designs are known for the risk of pitching up violently before the tail can become effective (because of high deflection and low speed). In a Cirrus or LS-1 always keep the stick centered during lift-off to ensure that the tail can do its job from the start!

In a crosswind the winch cable force is still acting in the plane of symmetry but the glider will drift sideways if no corrective action is taken. This is equivalent to flying a kite - eventually the glider will be on a different course determined by the direction of the wind. The glider's lateral stability will turn it into the wind while the wind pushes it sideways until the crosswind is reduced to zero.

Only when the glider pilot corrects for the crosswind will lateral cable forces emerge. By applying rudder during acceleration (aileron deflections only to keep the wings level!), the glider will fly at an angle to the winch cable during lift-off which adds a lateral force below the center of gravity. Now we have both a side force and a rolling moment added by the cable. If no further action is taken by the pilot, the airplane will develop a sideslip from the side force and a windward wing down rolling moment. However, depending on the initial rudder application, this added side force might

• reduce the sideslip because the initial rudder overcompensated for the crosswind. Now the rolling moment from the sideslip due to overcompensation and from the cable force will add up and drop the windward wing. The continuing acceleration of the glider during the initial stage of the launch will over time reduce the sideslip angle caused by the crosswind, so this scenario is rather likely. Now the glider will roll into a bank angle where the side force of the winch cable is again in the plane of symmetry and the glider nose is pointing into the wind. Lateral stability will drive the sideslip to zero and the glider ends up in a state similar to the one at zero crosswind, only with a bank angle which will eliminate the sideways component of the winch cable. As the glider climbs higher, this bank angle will diminish and any side force from the cable will pull the glider into a sideslip which in turn will let dihedral take care of the wrong bank angle.
• add to the sideslip because too little rudder was applied initially. This will now look similar to the case with no crosswind and the glider will drift sideways but slower than if no correction had been applied.

The considerate pilot will overcompensate and fly the glider to a point above but laterally offset from the winch in windward direction so the cable will not drift too far leeward during the final stage after the winch cable has been released. This makes the first scenario more likely. At all times the lateral stability of the glider will be maintained and the cable force will have little influence because this stability will drive the cable force into the plane of symmetry.

Answer 3

Perhaps one way to look at it would be to use the attachment point of the winch to the airframe as a thrust vector.

In a winch launch with no wind, with the attachment ahead of CG, you have an arrangement curiously like a kite. As the aircraft speed increases and it lifts off, there will be downforce on the nose working against the pitch up tendency of the glider. This keeps the line in tension and allows the aircraft to accelerate by lowering wing AoA. Effectively, the thrust line is now down as well as forward. The aircraft is stable in roll and yaw, but not free flying in pitch.

As for lateral stability in a cross-wind, it might be best not to even attempt it with a winch. Simply turn the winch so it pulls directly into the wind.

The placement of the winch attachment under the CG would introduce a destabilizing rolling component that would work against the righting tendency of the aircraft but ...

the aircraft is accelerating in the direction of the winch pull and has it aerodynmic stability to increasingly dominate thrust line deviations. Rolling torque forces of the "thrust line" and crosswind could be calculated against the aileron torque theoretically. There also may be crosswind launch specifications in the POH or known among experienced pilots or instructors.

In short, find an experienced winch operator and stay in the safe part of the envelope.