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the gremline digest — accidents on takeoff
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.
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.
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
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.
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.
‘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.
Now, let us consider what happens when the
aircraft is close to the surface.
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’.
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
Text and Photographs © 2007 Gremline & Hill House
Publications, unless otherwise stated.
Text and Photographs © 2007 Gremline & Hill House Publications, unless otherwise stated.