4. LESSON 30 - AEROTOW - BOXING THE SLIPSTREAM
- Be aware of the aerotow limits and always maintain your position
- An iron rule: if sight of the tug is lost, release immediately
The boxing the slipstream exercise has been invented to reinforce positionkeeping skills. You will not be allowed to attempt this advanced exercise until your position keeping skills are already good. As with low tow, it should not be flown unless the tug pilot is prepared.
The aim of this exercise is to practice all the positions a glider pilot can reasonably get into. The aim is to perform a square box outside the slipstream, pausing at each corner. Getting familiar with these positions will help you to restore and maintain a normal position behind the tug quicky. We will first describe the high, low, left and right limits, which you need to be aware of, and then the exercise itself.
Limit – High
It’s important to stress once more that there is a clear limit for how high a glider can be on tow – i.e. how low the tug can be on the canopy. You should always keep the tug in view and prevent a tug upset; you should always be able to deal with slack rope during a subsequent correction, and you should take the tug’s available elevator authority (the tug’s ability to pitch) into account. Don’t climb too high and maintain sensible margins before reaching any of these limits.
A high position needs careful management:
- Stabilise and don’t climb any further
- Pause! Stop for a think
- Move back down as slowly as you can possibly manage. A glider is equipped with two different ways to make that happen: gentle (very gentle) forward stick pressure, or a small amount of airbrake
- Whichever method is used, the up/down movement needs to be halted just before the picture is correct and the rope tightens again
Limit – Low
The lower limit is normally defined by the tug’s slipstream and is easy to detect – the propwash can be lumpy with a noticeable tendency to roll the glider. It causes the tug pilot less grief than a glider that is too high. Flying in the slipstream is not dangerous, just uncomfortable and inefficient. Dealing with it is also easy: a little back-pressure on the stick to move up into the correct place. Once up and out of the slipstream the undesirable roll and lumpy air will swiftly disappear. Slipstream effects are more marked with shorter ropes.
Instead of moving back up, you can also move down to low tow position. Though normal in Australia, the low tow position is unusual for UK gliding. It should not be used unless the tug pilot is prepared. Below the slipstream the air also gets smoother again.
Limit – Left/Right
As with up/down, small errors left/right can easily be corrected. Go further, though, and rope/ yaw/ bank can become an issue. Flying well to one side can also make it difficult for the tug pilot to maintain direction. A guideline for the limit is half the tug’s span beyond its wing tip, but it’s much better to aim no further than the tips themselves.
BOXING THE SLIPSTREAM
We start this exercise by descending from normal tow position through the slipstream until just below it (1), clear of the buffet (the tug’s turbulent propwash) into low tow position. You now exert a slight downward pressure on the tail of the tug, and the tug pilot will respond to the downward pull by lowering his or her pitch attitude. This will slightly change the direction of the slipstream. Re-establish your position by making a slight climb back to the slipstream, and descend back down to just below it.
Now carefully move at least one wingspan sideways (2, left or right). Keep in mind that if you move too far sideways, you will start overtaking the tug. Maintain the same sideways position and slowly climb back to the normal tow position (3). There shouldn’t be a buffet if you’re far enough away. As you slowly climb back to the normal tow position, you may need some offset ailerons and rudder because the rope will pull you back to the centre. After a pause and while keeping the tug on the horizon, now let the glider be pulled back into line by the towrope (4). Keep your wings level. Complete the box in the opposite direction if desired and if there is time left.