What is slow flying and why should you practise it?

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the gremline digest — slow flying

We can see that the total drag is high at low airspeed (under the influence of lift induced drag), decreases as speed is increased until it reaches a minimum (at min. drag speed!) and then increases again (under the influence of profile drag) as the speed continues to increase. The most obvious point on the total drag curve is that it has a minimum value where profile drag and lift induced drag are equal. This is our minimum thrust required point and so our theoretical endurance speed.

      If we fly faster than the endurance speed, at Point A on Figure 1 for example, and the aircraft is disturbed by the slightest turbulence then the airspeed will decrease slightly, moving us to the left along the total drag curve. Notice that the total drag also DECREASES as we move left from Point A. We still have the same amount of power applied so the aircraft will have more power than is required at the new lower drag value, and will automatically accelerate back to the original speed. This is called ‘speed stability’ and exists whenever we are flying straight and level at airspeeds greater than the minimum drag speed.

      What happens if we are flying more slowly than the minimum drag speed, at Point B on Figure 1 for example, and our aircraft is disturbed by a little bump? Once again the airspeed decreases and we move to the left on the total drag curve, but now the total drag INCREASES, the aircraft loses more airspeed, the total drag continues to rise and the speed continues to fall. Unless we apply more power and/or lower the nose, the speed will continue to decay right down to the stall. This is called ‘speed instability’ and is the first thing to understand about slow flight.
      At speeds less than the minimum drag speed the airspeed is unstable and will continue to decay if allowed to do so.
      We need to look at two more small graphs and then we can think about getting into the air and having a go at controlled slow flight. Most GA aircraft have flaps, of greater or lesser effectiveness. These have two effects; they increase the coefficient of lift and they increase the profile drag. Figure 2 is a redrawn version of Figure 1 to illustrate the large increase in profile drag as we fully lower the flaps. We can then consider what effect this has on the shape and position of the total drag curve and what this means in practical terms.

 

 

Slow Flying

What is ‘slow flying’ and why would anyone wish to practise flying slowly? Regular and structured slow flying practice will enhance your skills as a pilot and increase your safety during take-offs and landings, particularly from short strips. Just a few minutes at a safe altitude spent brushing up on your slow speed flying skills is better use of your flying time than bumbling around looking at the scenery. This will teach you to avoid the traps waiting during a go-around from a botched landing attempt. You will learn to safely control the aircraft close to the stall and to recognise the early symptoms. You will learn to avoid an unplanned spin due to lack of appreciation of what the aircraft is trying to tell you during a low speed manoeuvre.

 

A useful definition of ‘slow flying’ for the average GA aircraft is ‘flight at any airspeed less than endurance speed.’ Most pilots will recall that endurance speed is also the speed at which, in level flight, the aircraft’s total drag is at a minimum and thus requires minimum thrust (and fuel consumption) to maintain level flight.

 


Several interesting points will emerge if we look at the classic drag v. airspeed graph showing profile drag, lift induced drag and total drag. Profile drag is the sum of form drag and skin friction and increases as the square of the airspeed. This appears as a green line in Figure 1 below. Lift induced drag is not quite so simple to explain but for the purposes of this article all we need to know is that lift induced drag is at a maximum at minimum airspeed and DECREASES as the airspeed increases – a mirror image of profile drag and plotted as a blue line in Figure 1. Adding profile drag and lift induced drag together gives us total drag, which appears as a red line in Figure 1.

Notice that, compared to Figure 1, the red total drag curve has changed shape and the minimum drag is now, as might be expected, greater than it was before we lowered the flaps. We will need more power to maintain level flight. What may not be so obvious is that the minimum drag speed is now lower than it was without flaps. This means that we can now fly at a lower airspeed without suffering ‘speed instability.’ This explains why some aircraft without flaps, or with insufficient drag from fully lowered flaps, have tricky speed control on the approach.
      The final graph is a plot of ‘power required’ against ‘power available’, shown in Figure 3.
Gremline article on Slow Flying: Figure 3 illustrates the relationship between 'Power Required' and 'Power Available' in level flight.

The shapes of these curves are not realistic, but they allow me to make the point that as the airspeed is reduced the power available also REDUCES, but the power required to overcome the increasing total drag INCREASES. There comes a point at which the ‘power available’ curve (blue in Figure 3) crosses the ‘power required’ curve (red in Figure 3) at low airspeed. This is the minimum airspeed at which level flight can be maintained with maximum power applied. If you allow the speed to reduce further you are going to descend or stall. You cannot climb out of this situation without converting height to speed. If you don’t have the height to trade for speed then you will find the next few minutes quite exciting.

 


Now we can think about practising slow flight so that we can become familiar with the feel, attitude and performance of our own aircraft when operating at speeds below the minimum drag airspeed. We all engage in ‘slow flying’ every time we get airborne. Remember my definition of slow flying is flying at any speed less than the min. drag/endurance speed for your particular aircraft, so we will be slow flying during each take-off, initial climb and approach to land.
      Operating on the ‘back side’ of the total drag curve (below min drag speed) poses several problems for the pilot, particularly during a STOL approach to a short strip. The worst situation here is getting into the low and slow trap. It may be instinctive for the inexperienced pilot to ease back on the pitch control to raise the nose when low. This will increase the angle of attack, lower the airspeed even further, greatly increase the lift induced drag, increase the rate of descent and lead to either a stall on the approach or a touchdown short of the strip.
      Elevators control airspeed and power controls the rate of descent on the back side of the drag curve. You must remember and practise this to keep yourself out of trouble during slow flight at low altitudes. Slow flying should be practised, preferably with a qualified flying instructor, at a safe altitude until you become totally familiar with the feel and performance of your own aircraft while operating on the back side of the drag curve. The actual minimum airspeed attainable varies with all-up weight, manoeuvring loads, aircraft configuration and density altitude. Turbulence and pilot ability also have affects.

 


As airspeed is reduced the flying controls become less effective and the normal nose-down tendency is reduced. The elevators become less responsive and coarse control movements become necessary to retain control of the aircraft. The slipstream effect produces a strong yaw so the application of rudder is required to maintain balanced flight. The secondary effect of applied rudder is to induce a roll, so aileron is required to keep the wings level. This can result in you flying with ‘crossed controls’ at low airspeed. A wing can drop very rapidly and an incipient spin is a real possibility.
      Your actual flight profile for practising slow flying should be provided by your qualified flying instructor and should certainly begin with a HASELL check, every time. Try flying slowly while maintaining straight and level flight. Try a steady rate of descent at a constant low airspeed and then add a few gentle turns either way. Notice what happens to your rate of descent as you apply bank. Try a climb at low airspeed and then add a few gentle turns. Don’t forget that your stalling speed will increase as soon as you begin to turn.


PS. I hope you will already have noticed that if we fly our aircraft at exactly the minimum drag speed while seeking maximum endurance the aircraft will suffer from speed instability. The slightest speed reduction will result in increased total drag and thus a further reduction in speed. It is for this reason that the recommended speed to fly for endurance is always slightly faster than the theoretical minimum drag speed, thus giving speed stability and making control easier.

 

 

 

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