A symmetrical airfoil is a cross-section whose upper and lower surface are mirror images, so its mean camber line lies exactly on the chord line and it generates no lift at zero angle of attack. Because the geometry is identical above and below, the section delivers balanced performance in both upright and inverted flight, a characteristic especially valued on rotorcraft blades and other control surfaces. Although it only produces lift once the angle of attack becomes positive, its lift-to-drag ratio is generally lower than that of a comparable cambered airfoil. The stall onset is delayed, giving the pilot extra margin before separation occurs.
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Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a symmetrical airfoil in aviation?
A symmetrical airfoil has identical upper and lower surfaces, and it generates no lift at a zero angle of attack. It produces lift at a positive angle of attack by creating a pressure difference. Because the two sides of a symmetrical airfoil are identical, the mean camber line - an imaginary line that runs midway between the upper and lower surfaces - coincides with the chord line that joins the leading edge to the trailing edge. This uncambered contour gives equal distances for the air to travel over both the top and bottom surfaces, so at zero angle of attack the pressure distribution is the same above and below the airfoil.
What is an asymmetrical airfoil?
An asymmetrical airfoil is designed with camber and generates lift due to curvature differences on its top and bottom surfaces. An asymmetrical airfoil is a streamlined body whose mean camber line lies above the chord line. This positive camber produces the characteristic curves of an asymmetric profile. In a semi-symmetrical airfoil, the camber is reduced, so the mean camber line stays closer to the chord line than in a fully cambered section. This partial camber gives semi-symmetrical airfoils gentler pitching moment and lower torque created on the wing by the net lift and drag forces, combining moderate lift with lower structural stress.
What is the difference between symmetrical and asymmetrical airfoil?
Symmetrical airfoils have identical curves above and below the chord line. Because there is zero camber, lift at zero angle of attack is zero, and the stall angle is normally near the same positive and negative values. Asymmetrical airfoils, which include positively cambered sections, place the mean camber line above the chord line. This built-in curvature lets the wing make useful lift even at small positive incidence, and it lets the section reach a stall angle at a higher absolute value on the positive side while stalling sooner on the negative side.
The two types differ mainly in where the curved line - the mean camber line - lies relative to the chord line: directly on it for symmetrical and offset above it for positively cambered. The offset brings higher design-lift coefficients and gentler pressure recovery, but it also means negative stall occurs sooner than positive stall. Picking one form over the other therefore balances the need for equal upside-down performance against the need for extra lift at low, positive angles.
How does a symmetrical airfoil work?
A symmetrical airfoil works because the airflow splits in two at the nose and the shape and angle of attack causes airflow to accelerate over the curved upper surface. This acceleration produces lower pressure on the upper surface and higher pressure on the lower surface. Pressure difference creates lift, and the greater the angle of attack, the larger the pressure difference and therefore greater the lift.
The curved path forces air above to move faster, making the top side have lower pressure than bottom. Simultaneously, the lower portion of the airflow is deflected downward, adding to the reaction force. The aerodynamic center of pressure on this profile remains nearly fixed, so pitching moments stay small as lift rises.
Does symmetrical airfoil produce lift?
A symmetrical airfoil does produce lift, but only when it is set to a positive angle of attack. At zero angle of attack it produces no lift, because no net turning is present. As the angle increases, the airfoil turns the airflow downward. Bending air produces lift, and the amount of turning decides the magnitude of lift. The lift coefficient, a dimensionless quantity, relates the lift generated to fluid density, fluid velocity, and reference area. For the symmetric section it is zero at zero angle and grows almost linearly with angle of attack until the flow over the trailing edge becomes non-smooth. Any airfoil producing lift also produces induced drag. The symmetrical shape, once it is lifting, produces induced drag.
What is the angle of attack for a symmetrical airfoil?
Angle of attack is the angle between flow direction and chord line. For a symmetric section that line is straight, so zero geometric angle gives zero lift. A positive angle of attack is required if lift is to be generated. The lift coefficient varies almost linearly with angle of attack within 10 degrees, and larger angle of attack produces greater lift until stall occurs when angle of attack exceeds critical angle, usually between 12 and 15 degrees for low subsonic Mach numbers. At that stall angle the lift coefficient peaks and then falls, while drag rises sharply.
Because the profile is mirrored about the chord, the centre of pressure remains fixed at the quarter-chord point regardless of angle of attack. Pitching moment coefficient stays zero and no nose-down or nose-up couple is produced. This constant zero moment simplifies control-surface design and is the reason why symmetrical airfoils are preferred for tail-planes and aerobatic wings.
What happens when a symmetrical airfoil is scaled up?
Scaling of a symmetrical airfoil is characterized by the Reynolds number. When the chord is doubled the Reynolds number doubles, viscosity forces become half as large relative to inertial forces, the boundary layer stays thinner longer, and the laminar separation bubble shrinks. The result is a later stall, a slightly higher maximum lift coefficient, and lower pressure drag in the low-drag range. Beyond about fifteen-percent thickness these gains taper off, yet for any thickness a larger wing area still lifts more because lift is proportional to dynamic pressure.
What is a low-Reynolds-number symmetrical airfoil? A low-Reynolds-number symmetrical airfoil is the same profile operating at half chord length, where the Reynolds number is halved. Viscosity forces are then twice as big relative to inertial forces, the bubble thickens and stalls the upper surface earlier, so the tip airfoil stalls at a slightly lower angle of attack and lift coefficient than the root airfoil when scaled. Wing-tip airfoil scale plays a vital part in stall characteristics for aileron control.
What are the benefits of a symmetrical airfoil?
The benefits of a symmetrical airfoil are explained below.
- Symmetrical airfoil has angle of incidence of 0 degrees which helps the plane as a whole perform in a similar way
- Symmetrical airfoil allows optimization of lift-to-drag ratio
- Symmetric airfoil provides good lift-drag ratio
Ahead and behind the hub, the rotor needs no cyclic pitch correction for profile mismatch: the blade pressure distribution stays alike on the retreating and advancing sides. The penalty is that the section works best only near its single ideal point. Away from that point the symmetric airfoil gives less extra lift than a cambered airfoil, so the pilot must accept a lower ceiling or a larger rotor disk for the same weight.
What are the advantages of an asymmetrical airfoil?
The advantages of an asymmetrical airfoil are outlined below.
- Asymmetrical airfoils provide increased lift-drag ratios and more desirable stall characteristics
- Asymmetrical airfoil shape is advantageous for a wider range of angles can be traversed without the threat of boundary layer separation
- Asymmetric wing is designed to create more lift and less drag
- Asymmetric airfoil produces a favorable pressure gradient over the leading edge
- Asymmetric airfoils are used at the wing tip to avoid loss of cruise speed compared to washout
What type of aircraft uses symmetrical airfoil?
Aerobatic aircraft, exemplified by the Extra 300, carry symmetrical airfoils at 0 angle of incidence so that the wing offers identical lift whether the airplane is upright or inverted. Aerobatic model helicopters rely on symmetrical airfoils, and full-scale helicopters use them on rotor blades when the mission demands equal performance in every flight attitude. Most helicopters employ asymmetrical airfoils. The symmetrical airfoil family is selected chiefly for vehicles that must fly equally well right-side up and upside-down.
Is a symmetrical airfoil used for the vertical stabilizer?
Yes, a symmetrical airfoil is used for the vertical stabilizer. The vertical stabilizer and rudder create a symmetric airfoil. This profile is always used for both the horizontal and vertical stabilizer because, in normal flight, the vertical tail does not produce any lift to maintain directional trim. Instead, the vertical stabilizer points upward and works in the same manner as the rear fin on a weather vane, producing side forces, not up and down. The vertical tail airfoil must be symmetric to guarantee aircraft symmetry about the x-z plane. To meet these requirements, NACA 0009 or NACA 0012 are employed for the vertical tail, sections that generate the required lift coefficient with minimum drag when the rudder changes the effective shape of the airfoil of the vertical stabilizer.
The upright fin is an ideal use for a balanced airfoil, because its main purpose is not to create aerodynamic force but to give constant directional steadiness. The bilateral configuration makes aerodynamic pressures the same on each side, so aerodynamic effects are the same and regulating pressures are uniform. This intrinsic equilibrium gives a foreseeable returning reaction and a concentrated reviving power, keeping the aircraft steady during crosswind landings and engine-out scenes.
What is the best symmetrical airfoil?
NACA 0015 is generally considered the best symmetric airfoil. NACA 0012, NACA 0018 and NACA 0009 are symmetric airfoil sections frequently listed in the same catalogue, yet NACA 0015 remains the default choice because it offers different thicknesses while still giving lift and accepting moderate drag. A symmetrical biconvex airfoil is the family shape these sections share: both upper and lower surfaces bulge equally, giving zero design-lift coefficient and identical moment characteristics when inverted. A thin symmetrical airfoil is the low-thickness variant - NACA 0009 is the 9% chord example - favoured when surface friction drag must be minimised and structural depth is less important. A laminar symmetrical airfoil is the section whose favourable pressure gradient extends farther aft, letting the boundary layer stay laminar. Eppler E387 is known for good performance at such low Reynolds numbers, although the catalogue still lists NACA 0012H as the symmetrical laminar candidate. For aerobatic airplanes and for tail surfaces that must produce the same lift upright or inverted, NACA 0015 is used.
Did John Roncz design a symmetrical airfoil?
Yes, John Roncz designed an airfoil with concave surfaces symmetrical from leading edge to trailing edge. Roncz's airfoil design includes symmetrical 4 digit series airfoils with maximum thickness at 30% of chord. Within that series NACA 0015 being symmetrical is explicitly noted. The chosen profile is driven by the equation for a symmetrical 4-digit NACA airfoil: y = (t/c) [0.2969 x - 0.1260 x - 0.3516 x + 0.2843 x - 0.1015 x ], where t is the thickness expressed as a fraction of chord and x is the normalized chord position.
Roncz designed R1145MS airfoil for canard application, a symmetrical replacement that produced abrupt stall, undesirable pitch-up post-stall, and lower maximum lift coefficient than the original cambered section it replaced. Roncz designed GOLA airfoil, OSPITE airfoil, and RONCZ 1046 airfoil prototype, yet these later sections are cambered. Only his rotor and canard work explicitly retains the mirror-image, zero-camber form.
