Gremline Flight Safety Report: Accidents on Takeoff.

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Accidents on Takeoff — Introduction

General Aviation aircraft suffer far too many accidents during the takeoff and landing phases of flight. Many reasons are advanced to explain these accidents, and many of these reasons are valid. That’s another way of saying that there is no single reason for these accidents. They happen because of poor instruction at the basic flying stage. They happen because of lack of flying currency. They happen because of lack of knowledge about aircraft performance. They happen because of inadequate pre-flight planning making allowances for the aircraft’s performance limitations on the day of the accident. We read in accident reports of an apparent sudden and unexplained reduction of thrust soon after rotation on takeoff. We read of aircraft floating off the end of a strip during landing. Sadly, we read of pilots killing themselves when they lose control of an aircraft during the takeoff phase of flight.

 

I suggest that a lack of understanding of the relationship between lift induced drag and ground effect is a contributory factor in many of these incidents and accidents. The aim of this article is to remind pilots about lift induced drag and to explain, as simply as possible, how ground effect alters the behaviour of your aircraft when it is flying close to the surface.

First, we need to refresh the memory about lift, drag and airspeed.

 


Drag Versus Airspeed
We were all taught about ‘drag’ during our early training days. Most pilots will recall that ‘total drag’ is a combination of ‘profile drag’ and ‘induced drag.’ Profile drag is the sum of ‘form drag’ and ‘skin friction’ and increases as the square of the speed. Double the speed and your profile drag will increase by a factor of four. ‘Induced drag’ is not quite so simple to explain or to understand. The vast majority of instructors and training manuals speak of ‘induced drag’ when I believe it is more correct, and makes things easier to understand, if we refer to this component of total drag as ‘lift induced drag.’ If your eyes have glazed over and your brain has slipped into neutral then take a break while you remember that an understanding of this topic could save your life.
      A very simple graph (Figure 1 below) will make things clearer. Let’s plot profile drag and lift induced drag against airspeed and then add these two curves together to show the total drag of a typical light aircraft in level unaccelerated flight. By doing this we can see several interesting facts about drag and about the performance of our aircraft. Please note that the shape of the curves in the graphs are not mathematically accurate and have been distorted from reality to, I hope, make some points more obvious.

Fig 1. Drag v Airspeed

Looking at the easy one first, we can see that the green line representing profile drag begins at a low drag value at low airspeed and then increases as the square of the airspeed, so the faster we go the more thrust we require to overcome profile drag. The blue line representing lift induced drag is quite the opposite. It is at a maximum at the lowest airspeed and then reduces (in inverse proportion to the square of the airspeed) as the airspeed is increased.
      Let’s leave lift induced drag for the moment and examine the red ‘total drag’ curve. This red curve gives up several useful pieces of information. Probably the most obvious fact is that total drag decreases as we increase airspeed until it reaches a minimum where the line AB touches the red curve at point E, and then increases again as the airspeed continues to increase. Recalling that thrust equals drag in level flight, we can see that the minimum total drag speed is also the speed at which we require minimum thrust in level flight. Minimum thrust required means minimum fuel consumption so the minimum total drag speed is our theoretical best endurance (gallons per hour) speed. The next point to look at is that point on the red curve where a tangent OD from the origin touches the total drag curve at point R. If we fly at the speed that places us at point R on the total drag curve we will achieve the greatest specific air range (greatest air miles per gallon) available, so this is our best range speed. To fly at point R on the total drag curve involves operating at a speed with slightly increased total drag compared to the minimum drag speed, requiring a slight increase in thrust, but this is more than compensated for by the increase in airspeed.

 


Speed Instability
Another factor that becomes apparent is
‘speed stability.’ If we are flying at the minimum total drag speed (Point E on the total drag curve), or at any speed less than the minimum drag speed, and the aircraft meets the slightest turbulence the airspeed will reduce slightly. This moves us to the left on the total drag curve and the drag increases, reducing the airspeed again, which increase the drag still further. Thus the airspeed will continue to decay until the aircraft stalls, descends, or the pilot increases thrust. You have allowed the aircraft to get ‘on the wrong side of the drag curve’ because of ‘speed instability.’ On the other hand, if we are flying at range speed (Point R) and the airspeed is reduced by transient turbulence we still move to the left because of the decreased airspeed but the total drag is reduced. The thrust required is also reduced (thrust = drag in level flight) but we have thrust applied to overcome the drag at the higher speed before the turbulence. So we now have excess thrust already applied and the aircraft will accelerate back to point R without any power adjustment by the pilot. This condition is known as ‘speed stability’ and makes flying a much more relaxed operation.
      Now it should be apparent that at speeds faster than the minimum total drag speed the profile drag is the dominant component of total drag – and the airspeed is
‘stable.’ At speeds lower than the minimum total drag speed the lift induced drag is the dominant component of total drag – and the airspeed is ‘unstable.’

 

Lift Induced Drag

It’s time to begin to look more closely at lift induced drag. Some aerodynamic text books use the term ‘vortex drag’ instead of ‘lift induced drag.’ It is easy to visualise the wingtip vortices that are generated as soon as an aircraft starts to move at the beginning of its takeoff run. These vortices are the result of the pressure differential about a wing as the aerofoil section generates lift. There is a higher pressure below the wing and a lower pressure above the wing. These two areas of differing pressure meet at the wingtips (and at the trailing edge). The higher pressure below the wing tends to flow around the wingtip towards the lower pressure above the wing and this mixing detaches from the wingtip in a circular (‘vortex’) motion. When viewed from ahead of the aircraft, the vortex from the starboard wingtip rotates in a clockwise direction while the vortex from the port wingtip rotates anti-clockwise. Figure 2 gives an idea of the formation of wingtip vortices

Fig 2. Development of Wingtip Vortices

The usual picture of these vortices shows a neat vortex at each wingtip, but it is important to recognise that the effect of these vortices is felt far from the wingtips. Remember that air is a viscose fluid and that a disturbance at one place in the air is felt, to a greater or lesser extent, throughout the air mass surrounding the aircraft. The whole cross-section of the mass of air surrounding the wing is affected by the rotational flow around the tips.

      All very simple so far. Unfortunately, things become more complicated as we look more closely at vortices and lift induced drag. There are two ways in which to explain lift induced drag. One is quite simple, but incorrect and misleading. The other seems quite complicated and involves a number of formulae and suchlike. I will keep these to a minimum.
      So far, I have said that lift induced drag has something to do with wingtip vortices. All aerofoils generate tip vortices as soon as they begin to generate lift. If you could eliminate tip vortices you could be confident of surpassing Bill Gates in the annual income stakes. Now is the time to grasp the nettle and try to produce a simple and easily understood explanation of how these wingtip vortices lead to lift induced drag.
      When viewed from behind the wings, the tip vortices always rotate anti-clockwise around the starboard wingtip and clockwise around the port wingtip. So, if we position ourselves behind the trailing edge of the port wing, close to the wingtip, the port tip vortex (rotating clockwise) will be pushing air downwards onto our head. Now move to a similar position behind the trailing edge of the starboard wing. The starboard tip vortex (rotating anti-clockwise) will also be pushing the airflow downwards onto our head. We have discovered ‘downwash’! All wings have ‘downwash’ because they have tip vortices. Don’t confuse ‘downwash’ with ‘washout’; they are quite different. ‘Washout’ is an intentional twist built into some wings to reduce the angle of incidence (and hence, the angle of attack) towards the wingtip. This is a design feature to inhibit tip stalling. For this discussion we are interested in ‘downwash’ which is a directional airflow behind the trailing edge of a wing.
      This ‘downwash’ is obviously strongest right at the inner edge of each wingtip, but the down-flow affects the airstream leaving the trailing edge all the way along the span of each wing. This downwash has a considerable effect on both the lift and the drag of the wing. To clearly visualise the effect downwash has on drag it is necessary to look at the various flow angles around an aerofoil in level flight. The angles in Figure 3 below are greatly exaggerated for clarity. I have also not included the actual outline of an aerofoil section in an attempt to reduce clutter in the diagram. Visualise any aerofoil shape you fancy aligned along the blue chord line in Figure 3.

Fig 3. Lift Induced Drag

If you don’t like geometry just ignore Figure 3 and the following paragraph and accept the fact that downwash causes the lift force generated by the wing to be inclined backwards from the vertical by a small angle S. This inclination of the lift force from the vertical may be likened to a car parked facing up a hill. The reaction of the road on the car acts at right angles (‘normal’) to the road surface. It therefore has a rearwards component that will cause the car to roll backwards when we release the parking brake. On a wing, the size of this rearwards inclination varies with the amount of lift being generated. This is because the lift acts at right angles to the direction of the airflow causing the lift. The effective free stream that is generating the lift has its angle to the aerofoil reduced by the downwash. The greater the angle of attack the greater is the downwash and thus the greater is the lift induced drag.
      For those peculiar people like me who enjoy graphs and diagrams, I will try for a simple explanation of the angles shown in Figure 3 above.
       The upstream free stream airflow direction is shown as a horizontal black line because the aircraft is in level flight, despite the exaggerated size of the angle of attack (a). The downstream flow (in magenta) far to the right is shown deflected downwards by the downwash effect. The size of this deflection is shown as 2S, which is the overall downwash angle. The angle S itself is defined as half the overall downwash angle because the airflow around the wing is not actually straight. It is easiest to think of the local flow having an effective free stream direction that is the
mean of the far upstream and the far downstream flow directions. ‘Mean’ means halfway between, so the downwash angle close to the wing is at an angle S, which is half way between zero and 2S. This means that the effective angle of attack aeff  is less than the free stream angle of attack a by the angle S. ( aeff = a – S ).
     Let’s get back to the basic subject. Please don’t let the previous paragraph put you off. We are coming to the interesting bit that has a practical implication for all pilots of light aircraft.

 

 

Ground Effect

‘Ground effect’ is something that you may have heard about, but never really noticed in operation. This effect is caused by ground interference with the airflow patterns around an aircraft when the aircraft is close to the ground. By ‘close’ I mean within one wingspan of the surface. This effect is probably most noticeable to pilots landing larger delta-winged types such as the old Gloster ‘Javelin’ and the Dassault ‘Mirage’ series of fighters (which I have flown) or the Avro ‘Vulcan’ and the ‘Concorde’ (which I have not). Ground effect partly explains why delta-winged aircraft exhibit a nice tendency to flare by themselves and produce a gentle touchdown on landing. Ground effect may not be so obvious in the average GA light aircraft, but it does have particular significance to light aircraft taking off from a short strip. It applies to ALL fixed-wing aircraft, including sailplanes and microlights.
      The wingtip vortices (See Figure 2) streaming behind an aircraft follow a downward inclined path for some distance behind the aircraft and then gradually level out to follow a path at a lower level than the aircraft. They also drift apart. The pressure pattern about an aircraft flying outside ground effect becomes almost cylindrical, with positive pressure below the wings and negative pressure above the wings. These pressure differentials are felt quite a distance from the airframe, and the cylinder of affected air has a diameter close to the wingspan of the aircraft. This is shown in Figure 4A.

Fig 4A. Pressure Pattern Outside Ground Effect

Now, let us consider what happens when the aircraft is close to the surface.
      The almost cylindrical vortex-induced circulation around the wing outside ground effect (Figure 4A) is modified by coming into contact with the surface. This flattens the cylindrical circulation pattern as well as reducing the downflow angle of the airflow behind the wing. The flattening on the circulation pattern spreads the pattern outwards below the wing, thus increasing both the effective span of the wing and its aerodynamic aspect ratio. The aerodynamic aspect ratio is measured between the
cores of the wingtip vortices that occur at about 80% of the geometric wingspan when the wing is outside ground effect. The aerodynamic aspect ratio of a wing has a strong inverse effect on lift induced drag.
      So, if we imagine the aircraft being so close to the surface that the sag of the wingtip vortices is restricted by coming into contact with the surface then we can see that the total downward deflection effect of the vortices is reduced. This reduction in the total downward deflection reduces the angle of the downward deflection of the downstream flow (refer to Figure 3). Because the lift induced drag is a product of the downward deflection of the airflow behind the wing a reduction in this downward deflection as the vortices contact the surface reduces the lift induced drag.

Fig 4B. Pressure Pattern In Ground Effect

The combination of the reduction in the downflow angle of the airflow behind the wing and the increase in both effective wingspan and aerodynamic aspect ratio of the wing occur when the wing is within one wingspan of the surface. This increases the aerodynamic efficiency of the wing. That’s ‘ground effect’.
      Research has shown how the lift induced drag of a wing varies as the aircraft leaves the ground and then moves out of the shallow layer of air close to the surface.
      The reduction in lift induced drag due to ground effect varies in inverse proportion to the aircraft’s altitude above the surface, measure as a percentage of the aircraft’s wingspan. Thus if the aircraft is at an altitude equal to 10% of its wingspan then the lift induced drag is 48% less than the value out of ground effect. At an altitude of 25% wingspan the reduction is 24% and at one span altitude the lift induced drag is reduced by about 1.5%. Remember that at any airspeed below your minimum drag speed the
lift induced drag is the predominant component of total drag, so these reductions in induced drag at low airspeeds while operating in ground effect will provide a dramatic reduction in total drag.
     Using ballpark figures in a practical situation this means that the lift induced drag (which is very much the dominant component of total drag during takeoff and landing) of your 30 foot span low-winged aircraft will be halved while you are at 3 feet altitude compared to what it will be at 30 feet altitude.
     
Another way of looking at the numbers above is to realise that your aircraft’s lift induced drag (and thus total drag) is going to INCREASE quite dramatically as it leaves the ground and climbs out of ground effect. This is a factor to consider, particularly when taking off from a short strip. Your instinct may be to haul the aircraft off the ground and into its climbing attitude as soon as possible. This technique works alright with something like a F15 ‘Eagle’ in full reheat with a power-to-weight ratio of better than one, but is exactly the wrong technique if you are flying a light GA aircraft with a poor power-to-weight ratio. If you ease the aircraft into the air when it’s ready to fly and then hold the aircraft level by hugging the ground just a few feet above the surface it will accelerate to Vx, the best angle of climb speed, more rapidly and in less distance than if you try to haul it away from the ground.

 


An additional gain by adopting this technique is that the reduction in the effect of wingtip vortices due to ground effect means that the wing will need a lower angle of attack in ground effect to produce the same amount of lift – or will produce more lift for the same angle of attack. However, be aware that hauling the aircraft off the ground and out of ground effect at something very close to the ‘in ground effect’ stalling angle of attack will almost certainly induce a loss of lift and a possible stall as the aircraft leaves ground effect, with a consequent rapid return to the ground. What some pilots perceive as a sudden loss of thrust on takeoff may well have been a sudden increase in total drag as the aircraft is hauled out of ground effect.

      That idea may require a few moments thought, but I believe that it explains quite a few ‘failure to get airborne from a strip’ accidents. An aircraft is hauled off the ground at minimum speed in a steep attitude, climbs to tens of feet and then flops back onto the ground to repeat this frog-hopping until the far hedge puts an end to this demonstration of the pilot’s lack of understanding of ground effect.
      Ground effect also comes into play when landing. As the aircraft entered ground effect during the flare (or round-out or whatever you like to call it) the aircraft will tend to float along the strip because the lift induced drag (and therefore, total drag) is reduced quite dramatically as the aircraft drops below one wingspan distance from the surface. Any excess speed will exacerbate the float and may cause an inexperienced pilot to begin to grope for the ground. Pilot induced pitch oscillations then lead to another untidy landing, another bent nose leg or a trip into the far hedge.
      I believe that the majority of landing accidents that result in nose legs being bent are due to pilots (and instructors?) not understanding the relationship between ground effect and lift induced drag. If you fly the approach at the correct stabilised airspeed while aiming to touch down at the beginning of the strip, and then gently rotate the aircraft into the correct touchdown attitude, all will be well. If you have not got the approach speed, approach slope and aircraft attitude all correct then
GO AROUND for another attempt. Excess speed on the approach, and aiming to touch down well beyond the threshold, is of no use to anybody except tyre and brake manufacturers and aircraft repair agencies.

 

 

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Text and Photographs © 2007 Gremline & Hill House Publications, unless otherwise stated.

 

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