Aspect ratio (aeronautics)

For other uses, see Aspect ratio (disambiguation).

In aeronautics, the aspect ratio of a wing is the ratio of its span to its aerodynamic breadth or chord. A long, narrow wing has a high aspect ratio, whereas a short, stubby wing has a low aspect ratio.[1]

For a constant-chord wing of chord c and span b, the aspect ratio is given by:

AR = {b\over c}

If the wing is swept, c is measured parallel to the direction of forward flight.

For most wings the length of the chord is not a constant but varies along the wing, so the aspect ratio AR is defined as the square of the wingspan b divided by the wing area S.[2][3] In symbols,

AR = {b^2 \over S}.

For such a wing with varying chord, the standard mean chord SMC is defined as

SMC = {S\over b} = {b\over AR}

Aspect ratio of aircraft wings

Low aspect ratio wing (AR=5.6) of a Piper PA-28 Cherokee
High aspect ratio wing (AR=12.8) of the Bombardier Dash 8 Q400
Very low aspect ratio wing (AR=1.55) of the Concorde
Very high aspect ratio wing of the Schleicher ASH 31 glider (AR=33.5)

Aspect ratio and other features of the planform can be used to predict the aerodynamic performance of a wing.

For a given wing area, the aspect ratio, which is proportional to the square of the wingspan, is of particular significance in determining the performance. Roughly, an airplane in flight can be imagined to affect a circular cylinder of air with a diameter equal to the wingspan.[4] A large wingspan is working on a large cylinder of air, and a small wingspan is working on a small cylinder of air. For two aircraft of the same weight but different wingspans the small cylinder of air must be pushed downward by a greater amount than the large cylinder in order to produce an equal upward force. The aft-leaning component of this change in velocity is proportional to the induced drag. Therefore the larger downward velocity produces a larger aft-leaning component and this leads to larger induced drag on the aircraft with the smaller wingspan and lower aspect ratio.

The interaction between undisturbed air outside the circular cylinder of air, and the downward-moving cylinder of air occurs at the wingtips and can be seen as wingtip vortices.

This property of aspect ratio AR is illustrated in the formula used to calculate the drag coefficient of an aircraft C_d\;[5][6][7]

C_d =C_{d0} + \frac{(C_L)^2}{\pi e AR}

where

C_d\; is the aircraft drag coefficient
C_{d0}\;   is the aircraft zero-lift drag coefficient,
C_L\; is the aircraft lift coefficient,
\pi\; is the circumference-to-diameter ratio of a circle, pi,
e\; is the Oswald efficiency number
AR is the aspect ratio.

There are several reasons why not all aircraft have high aspect wings:

Variable aspect ratio

Aircraft which approach or exceed the speed of sound sometimes incorporate variable-sweep wings. These wings give a high aspect ratio when unswept and a low aspect ratio at maximum sweep.

In subsonic flow, steeply swept and narrow wings are inefficient compared to a high-aspect-ratio wing. However, as the flow becomes transonic and then supersonic, the shock wave first generated along the wing's upper surface causes wave drag on the aircraft, and this drag is proportional to the span of the wing. Thus a long span, valuable at low speeds, causes excessive drag at transonic and supersonic speeds.

By varying the sweep the wing can be optimised for the current flight speed. However the extra weight and complexity of a moveable wing mean that it is not often used.

Wetted aspect ratio

The wetted aspect ratio considers the whole wetted surface area of the airframe, S_w, rather than just the wing. It is a better measure of the aerodynamic efficiency of an aircraft than the wing aspect ratio. It is defined as:

\mathit{AR}_{\mathrm{wet}} = {b^2 \over S_w}

where b is span and S_w is the wetted surface.

Illustrative examples are provided by the Boeing B-47 and Avro Vulcan. Both aircraft have very similar performance although they are radically different. The B-47 has a high aspect ratio wing, while the Avro Vulcan is a low aspect ratio tailless aircraft. They have, however, a very similar wetted aspect ratio.[10]

Aspect ratio of bird wings

See also: Bird flight

The aspect ratios of birds' wings vary considerably. Birds that fly long distances or spend long periods soaring such as albatrosses and eagles often have wings of high aspect ratio. By contrast, birds which require good maneuverability, such as the Eurasian sparrowhawk, have wings of low aspect ratio.

See also

Notes

  1. Kermode, A.C. (1972), Mechanics of Flight, Chapter 3, (p.103, eighth edition), Pitman Publishing Limited, London ISBN 0-273-31623-0
  2. Anderson, John D. Jr, Introduction to Flight, Equation 5.26
  3. Clancy, L.J., Aerodynamics, sub-section 5.13(f)
  4. Clancy, L.J., Aerodynamics, section 5.15
  5. Anderson, John D. Jr, Introduction to Flight, section 5.14
  6. Clancy, L.J., Aerodynamics, sub-equation 5.8
  7. Anderson, John D. Jr, Fundamentals of Aerodynamics, Equation 5.63 (4th edition)
  8. Dommasch, D.O., Sherby, S.S., and Connolly, T.F. (1961), Airplane Aerodynamics, page 128, Pitman Publishing Corp. New York
  9. Hamilton, Scott. "Updating the A380: the prospect of a neo version and what’s involved" Leehamnews.com, 3 February 2014. Accessed: 21 June 2014. Archived on 8 April 2014.
  10. "The Lifting Fuselage Body". Meridian-int-res.com. Retrieved 2012-10-10.

References

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