Streamline tube is round tube reshaped into a teardrop or airfoil section, so that air flows around it with a small fraction of the drag of a round tube. In aluminium it is used for aircraft lift and jury struts, ultralight and gyroplane frames, sailing spreaders, roof bars, drone arms and fairings. This guide covers how streamline tube is made, how its size relates to the round tube it comes from, which aluminium alloys to use, and how to check a strut.
A round tube moving through air leaves a wide, turbulent wake behind it, and that wake is where most of its drag comes from. Streamline tube has a rounded leading edge and a tapering trailing edge, so the flow stays attached around the section and closes behind it with a narrow wake. The shape is usually described by its major axis (chord, front to back) and minor axis (thickness, side to side).
Streamline tube is also called streamlined tube, airfoil tube, aerofoil tube, teardrop tube or flow-line tube. True aircraft streamline tube is a symmetric section with a fineness ratio (major ÷ minor axis) of about 2.3 – 2.4 : 1. Oval and egg-shaped tubes are related but less efficient: an ellipse is symmetric front to back, so its rear half does not let the flow close as cleanly.
Its first home was aviation. Biplanes and high-wing monoplanes from the 1920s onwards used streamline steel and aluminium for wing struts, wires and landing-gear legs, and many light aircraft, homebuilts, ultralights and gyroplanes still do. Outside aviation the same benefit is used on sailing-boat spreaders, vehicle roof bars, bicycle and wheelchair racing frames, wind-tunnel stings, antenna and sensor booms, and on structures where low wind load or low noise matters.
A round cylinder in the speed range of light aircraft and road vehicles has a drag coefficient of roughly 1.0 – 1.2 based on its frontal area. A well-shaped symmetric streamline section of around 2.5 : 1 fineness has a drag coefficient in the region of 0.05 – 0.1 on the same basis, according to classic wind-tunnel data compiled in references such as Hoerner's Fluid-Dynamic Drag.
Because a streamline tube made from a given round tube is also narrower than that tube (its minor axis is a little over half the original diameter), the drag per metre of strut falls further still. In practice, replacing a round strut with streamline tube of similar strength cuts the drag of that member by about an order of magnitude, which is why the claim "ten times less drag" is commonly made.
| Factor | Round tube | Streamline tube |
|---|---|---|
| Drag coefficient (frontal area, typical) | About 1.0 – 1.2 | About 0.05 – 0.1 |
| Frontal width for the same perimeter | 1.0 × D | About 0.57 × D |
| Wake and vibration | Wide wake, vortex shedding, possible strut flutter and whistle | Narrow wake, much less vortex shedding |
| Sensitivity to flow direction | None | Best aligned with the flow; drag rises with yaw angle |
| Bending stiffness | Same in all directions | High about the minor axis plane, lower in the thin direction |
Drag figures are indicative ranges for comparison. Actual values depend on Reynolds number, surface finish, section shape and alignment.
Alignment matters. A streamline strut is efficient only when it points into the airflow; a strut turned 10 – 15° off the flow loses much of its advantage. Fittings at the ends should fix the strut at the correct angle, and drawings should specify the orientation of the major axis.
Streamline tube is normally made by drawing or rolling a round tube into shape, so the perimeter of the streamline section is the same as the circumference of the original round tube, and wall thickness stays about the same. Sizes are therefore quoted as an "equivalent round" diameter plus the major and minor axes. For the classic aircraft profile, the major axis is about 1.35 times and the minor axis about 0.57 times the round diameter.
| Equivalent round (OD) | Streamline, major × minor (in) | Streamline, major × minor (mm) | Perimeter |
|---|---|---|---|
| 1/2 in (12.7 mm) | 0.67 × 0.29 in | 17.1 × 7.3 mm | 1.57 in (39.9 mm) |
| 5/8 in (15.9 mm) | 0.84 × 0.36 in | 21.4 × 9.1 mm | 1.96 in (49.9 mm) |
| 3/4 in (19.0 mm) | 1.01 × 0.43 in | 25.7 × 10.9 mm | 2.36 in (59.8 mm) |
| 7/8 in (22.2 mm) | 1.18 × 0.50 in | 30.0 × 12.7 mm | 2.75 in (69.8 mm) |
| 1 in (25.4 mm) | 1.35 × 0.57 in | 34.3 × 14.5 mm | 3.14 in (79.8 mm) |
| 1 1/8 in (28.6 mm) | 1.52 × 0.64 in | 38.5 × 16.3 mm | 3.53 in (89.8 mm) |
| 1 1/4 in (31.8 mm) | 1.69 × 0.71 in | 42.8 × 18.1 mm | 3.93 in (99.7 mm) |
| 1 1/2 in (38.1 mm) | 2.02 × 0.86 in | 51.4 × 21.8 mm | 4.71 in (119.7 mm) |
| 1 3/4 in (44.4 mm) | 2.36 × 1.00 in | 60.0 × 25.4 mm | 5.50 in (139.6 mm) |
| 2 in (50.8 mm) | 2.70 × 1.14 in | 68.5 × 29.0 mm | 6.28 in (159.6 mm) |
| 2 1/2 in (63.5 mm) | 3.37 × 1.43 in | 85.7 × 36.3 mm | 7.85 in (199.5 mm) |
Calculated from the 2.36 : 1 profile used for many standard streamline shapes. Actual axes vary slightly by drawing die and maker; confirm the exact section and its tolerance before ordering fittings.
Common wall thicknesses follow aircraft round tube: 0.035 in (0.89 mm), 0.049 in (1.24 mm), 0.058 in (1.47 mm) and 0.065 in (1.65 mm), with heavier walls for larger lift struts. Flatter "blade" shapes with ratios of 3 : 1 or more, and fuller egg shapes nearer 2 : 1, are made by changing the die. Extruded aluminium airfoil profiles, often with internal webs or a sail track, are an alternative where a non-standard shape or larger chord is needed; see our custom aluminium tube guide.
Most aluminium streamline tube is 6061-T6. The alloy draws well, is heat-treatable to good strength, resists corrosion and welds. Higher-strength and lower-strength alloys are used for special cases. Minimum values below are from ASTM B210 (drawn seamless tube) where stated; for other alloys the comparison is qualitative and the certificate gives the actual figures.
| Alloy and temper | Strength | Characteristics | Typical streamline uses |
|---|---|---|---|
| 6061-T6 | ASTM B210 drawn tube minimum 290 MPa (42 ksi) tensile, 241 MPa (35 ksi) yield | Good strength, corrosion resistance, anodising and weldability; the standard choice | Lift and jury struts on light and homebuilt aircraft, ultralight frames, roof bars, booms |
| 2024-T3 | Substantially stronger than 6061-T6 | High strength and fatigue resistance; needs anodising or primer; not normally welded | Highly loaded struts and aerospace fittings where weight is critical |
| 7075-T6 | Very high (typical about 570 MPa tensile) | Highest strength; not weldable; sensitive to stress corrosion in some tempers | Racing and sport equipment, compact high-load members |
| 6063-T5 / T6 | Moderate | Excellent surface and anodising; easy to extrude into airfoil profiles | Architectural fins, fairings, furniture, decorative aero profiles |
| 5052 / 3003 | Low to moderate (non-heat-treatable) | Very formable and corrosion resistant | Fairings, trailing-edge covers, non-structural streamline covers |
A useful rule: if the strut carries flight or structural loads, start from 6061-T6 or 2024-T3 drawn tube made to an aerospace or ASTM tube specification. If the part is a fairing or decorative profile, 6063 extrusions or 5052 formed sheet are often simpler and cheaper. For comparison, steel streamline tube to 4130 specifications is stronger but almost three times as dense; aluminium wins where stiffness per weight and corrosion resistance matter more than outright strength.
A seamless round tube is pulled through a shaped die, sometimes over a shaped mandrel, in one or more passes. This gives accurate sections, a smooth surface and good wall control. Drawing is often done in a softer condition and the tube is then solution heat-treated and aged to T6.
Rolls progressively flatten and shape a round tube. Suited to longer production runs of simple shapes and to tubes too large for drawing benches.
An airfoil-section hollow is extruded through a dedicated die, often in 6061, 6063 or 6082. Allows internal webs, thicker leading edges, tracks and non-standard chords, at the cost of a die.
Sheet is formed around a shape and seam-welded or riveted. Used for large fairings and covers rather than load-bearing struts.
For load-bearing aviation struts, seamless drawn tube is the usual route, because it gives uniform wall and properties around the section with no seam. Straightness and twist matter as much as cross-section: a strut with twist along its length is misaligned with the flow at one end. Ask for straightness and twist limits on the drawing and inspect each length. The general principles of drawn tube are covered in drawing tube and wire.
Lift struts work in tension in normal flight and in compression under negative loads or on the ground, so both cases must be checked. Tension is simple: allowable load is cross-sectional area times the allowable stress. For example, a section with 0.554 in² of metal in 6061-T6 drawn tube at the 42 ksi minimum tensile strength has an ultimate tensile capacity of about 23,300 lbf before any fittings are considered. Compression is different, because a long, thin strut buckles long before the metal reaches its strength.
Aircraft struts are primary structure. Their design, materials and repairs must follow the aircraft's approved data or be approved by the relevant aviation authority. This guide is general engineering background, not approval data.
| Sector | Typical parts | Notes |
|---|---|---|
| Light and experimental aircraft | Wing lift struts, jury struts, tail braces, landing-gear fairings | Usually 6061-T6 or 2024-T3 drawn tube with certified material |
| Ultralights, gyroplanes, paramotors | Frame members, mast fairings, rotor-head supports | Weight and drag are both critical |
| Sailing and marine | Mast spreaders, boom vangs, bimini and canopy frames | Anodised 6061 or 6082; extruded airfoil profiles common |
| Automotive and transport | Roof-rack crossbars, wing and spoiler supports, aero fairings | Reduces drag and wind noise; often extruded 6063 or 6061 |
| Cycling and sport | Aero handlebars, seat posts, racing wheelchair and handcycle frames | Often 7xxx alloys or 6061 profiles |
| Drones and research | UAV arms and booms, wind-tunnel stings, sensor masts | Low drag and low vortex shedding improve measurements |
| Architecture | Sun-shading fins, decorative columns and canopies | Mainly 6063 extruded aerofoil profiles |
For related shapes, see oval aluminium tubing for elliptical sections and flat-oval tube for heat-exchanger use.
Metline supplies aluminium streamline and airfoil tube in 6061, 2024, 7075 and 6063 to customers worldwide, together with the round drawn tube it is made from. We supply aluminium only; for steel streamline tube, see the relevant steel specifications.
Mainly for members exposed to airflow where drag or wind load matters: aircraft lift and jury struts, ultralight frames, sailing spreaders, roof bars, drone booms, aero bicycle parts and architectural fins.
By the major and minor axis and wall thickness, often together with the equivalent round diameter it was drawn from. For the standard profile the major axis is about 1.35 × and the minor axis about 0.57 × the round diameter; a 1 in round becomes about 1.35 × 0.57 in.
6061-T6 drawn seamless tube is the usual choice for its balance of strength, corrosion resistance and weldability. 2024-T3 is used where higher strength is needed; 6063 and 5052 suit fairings and decorative profiles.
Typically about a tenth or less for the same strength, because the drag coefficient falls from around 1.0 – 1.2 to roughly 0.05 – 0.1 and the frontal width is also smaller. The advantage depends on keeping the strut aligned with the flow.
6061-T6 can be TIG welded, though the weld zone loses strength unless re-heat-treated. Bending a streamline section without distortion is difficult; form it in a softer temper and heat-treat afterwards, or design straight members with end fittings.
No. Metline supplies aluminium streamline and airfoil tube and the aluminium round tube it is drawn from. Steel and titanium are mentioned on this page only for comparison.
Send the section, wall, alloy, length and quantity, or the round tube you want converted. Metline will reply with supply options, certification and delivery worldwide.
