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The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

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Monday
Oct082012

Ground Effect - Historical Perspective & Technical Explanation

During the Second World War, a crippled Boeing B17 was struggling to maintain altitude.  The aircraft and eleven crew members were over occupied Europe, returning to England, after a successful bombing mission.

Searchlights, Flak & Enemy Fighters

After negotiating the enemy searchlights that probed the darkness over their target, and then being struck by several pieces of shell fragment from anti-aircraft flak, they had been pounced upon by German fighters on their homeward leg.  The ensuring fight was dramatic and left the damaged bomber with only two engines running and third engine having difficulty.  As the bomber approached France, the enemy fighters, starved of fuel, aborted their repetitive attacks, but the damage had been done.  Loosing airspeed and altitude the aircraft could not maintain contact with the Bomb Group; soon they were alone.

The captain, in an attempt to maintain altitude, requested that everything heavy be jettisoned from the aircraft.  This included machine guns, ammunition and damaged radio equipment; soon the B17 was a flying Skeleton if its former self.

The Captain was concerned that a  fire may develop in engine number three as it was spluttering due to a fuel problem.  The Captain did not need to concern himself much longer as the engine began to cough uncontrollably before vibrating and ceasing to function.   The aircraft was now only flying on one engine – something that was not recommended as it placed great strain on the engine and aircraft superstructure.  

The aircraft continued to loose altitude despite the jettisoning of unwanted equipment.  The Captain decided it was better to ditch into the English Channel rather than land in occupied France.  His thinking was that Air Sea Rescue maybe able to pick them up, if their repeated Morse code had been received by England; the power of one engine was nowhere enough to maintain such a large and heavy aircraft, as the B17 aloft for much longer.  The crew prepared for the unenviable ditch into the freezing cold water of the channel.

We’re Going In – Good Luck Boys!

“Get ready guys, we’re 300 feet above the water” yelled the Captain into his intercom system.  “As soon as we hit bust them bubbles and get out.  Try to get a raft afloat”.  “Link up in the water  – Good Luck!”  

Everyone expected the worse.  Surviving a ditching was one thing, but surviving in the cold water of the English Channel in winter was another!  The rear gunner, since moving forward sat close to escape hatch and gingerly rubbed his rabbit’s foot; he had carried this on every mission.  The side gunner fumbled repeatedly with his “lucky” rubber band, the bombardier sat wrapt in private thoughts, a photograph of his loved one held tightly in his hand, and the navigator frantically punched his Morse set trying to get the last message out before fate took command of the situation.

The aircraft, although trimmed correctly, slowly began to dip towards the sea.  But at 60 odd feet above the waves, the aircraft began to float  – it felt as if the aircraft was gliding on a thermal.  For some reason the aircraft didn't wish to descend.  The remaining engine screamed its protest at being run at full throttle, however the glide continued. 

The Captain was amazed and thankful for whatever was keeping this large aircraft from crashing into the sea.  It was as if the B17 was cruising on a magic carpet of air – why didn’t it crash.  

A tail wind assisted in pushing the B17 toward England and safety; seeing the English coast in sight, the navigator quickly calculated a route to the nearest airfield closest to the coast.  Twenty minutes later the bomber lumbered over the runway where the only way to land was to actually reduce power to the remaining engine and push the control wheel forward lowering the pitch angle.  They were home and safe!

Divine Interaction, Luck, or Skill ?

The crew thought it was divine interaction that the bomber had not crashed – or perhaps luck!

Aviation engineers were baffled to what had occurred.  The aircraft had glided many miles above the surface of the English Channel and had not crashed.  Boeing, in an attempt to unravel what had occurred, repeated the event in the confines of a wind tunnel to realize that what had maintained the large aircraft airborne was not divine interaction, but the interaction of what has since been termed Ground Effect.

The above account, although embellished in detail, did occur and the mishaps of this bomber during the Second World War demonstrated a previously unknown phenomenon - ground effect.

Ground Effect – Technical Explanation

Ground effect" refers to the increased lift and decreased drag that an aircraft wing generates when an aircraft is about one wing-spans length or less over the ground (or surface).  Ground effect often gives pilots and/or passengers of light aircraft the feeling that the aircraft is "floating", especially when landing.

When an aircraft is flying at an altitude that is approximately at or below the same distance as the aircrafts wingspan, there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices and down wash behind the wing.  LEFT:  Diagram depicting ground effect with aircraft in flight.

 

When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed and lift of the aircraft.

LEFT:  Diagram depicting aircraft in ground effect whilst on the ground.

A wing generates lift, in part, due to the difference in air pressure gradients between the upper and lower wing surfaces. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static air pressure. These air pressure differences also accelerate the mass of air downwards.  Flying close to a surface increases air pressure on the lower wing surface, known as the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio.  As the wing gets lower to the surface (the ground), the ground effect becomes more pronounced.

While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result, which accounts for the "floating" effect. Ground effect will also alter thrust versus velocity, in that reducing induced drag will require less thrust to maintain the same velocity.

The best way to describe ground effect and which many people, both pilots and passengers, have encountered is the floating effect during the landing flare.
Low winged aircraft are more affected by ground effect than high wing aircraft. Due to the change in up-wash, down-wash, and wingtip vortices there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the static source.
Another important issue regarding ground effect is that the makeup of the surface directly affects the intensity; this is to say that a concrete or other hard surface will produce more interference than a grass or water surface.

Problems Associated With Ground Effect

Take Off

Ground effect should be taken into account when a take-off from a short runway is planned.

Although ground effect may allow the airplane to become airborne at a speed that is below the recommended take-off speed, climb performance will be less than optimal.  Ground effect may allow an overloaded aircraft to fly at shorter take off distances and at lower engine thrust than normal.  However, the aircraft will not have the ability to climb out of ground effect and eventually will cease to fly, or hit something after the runway length is exceeded.

Approach and Landing

As the airplane descends on approach and enters ground effect, the pilot experiences a floating sensation which is a result from the increased lift and decreased induced drag value. Less drag also means a lack of deceleration and could become a problem on short runways were roll-out distance is limited.

Therefore, it's important that power is throttled back as soon as the airplane is flared over the threshold and the weight of the airplane is transferred from the wings to the wheels as soon as possible.

How to Counter Ground Effect

To minimise ground effect on landing, the following must be addressed:

  • Pitch angle should be reduced to maintain a shallow decent (reduces ability of the wing to produce more lift).
  • Thrust should be decreased.
  • Power should be throttled back as you flare across the threshold at RA 50 feet.
  • Land the aircraft onto the runway with purpose and determination.  Do not try and grease the aircraft to the runway (often called a carpet landing).  The weight of the aircraft must be transferred to the wheels as soon as possible to aid in tyre adhesion to the runway (also important when landing in wet conditions).

Does Ground Effect Occur in Flight Simulator?

If FSX is set up correctly, ground effect will definitely be experienced in flight simulator. 

If you have ever wondered why, after reducing speed on an otherwise perfect approach, your aircraft appears to be floating down the runway, then you have already experienced ground effect.

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