Drag.

DRAG..Where Does it Come From?
by Geoff Burdon
The Sensational Adelaide Air Races revealed some interesting observations. Firstly, the larger more powerful open class Golden Era aircraft were not necessarily faster than the smaller, lighter and lower powered AT6 class (with only a couple of exceptions). The fastest aircraft had low frontal area (inline engine cowls) and were smaller in overall dimensions. The heavier of the AT6 class were clearly faster in a straight line than the lighter models.

So what makes a fast flying aircraft? – simply low drag. But how do we get it?

Drag is divided between profile or form, parasitic and induced.

PROFILE Drag…
…relates to the size of the frontal area including attached fittings (undercarriage, struts etc.). Minimise frontal area and you minimise the profile drag.

PARASITIC Drag…
…relates to surface features on the airframe. Any protrusion disturbs the airflow and results in drag-creating turbulence. Small items such as foot steps, panel joints, surface finish, junctions of airframe components (undercarriage legs, struts, tail surfaces, wings, control surfaces, canopy, aerials), rivets and even paint lines all contribute to parasitic drag. Parasitic drag can be minimised by attention to detail and careful craftsmanship. All airframe junctions should be filleted and the fillets should be as large as practical. Any change in surface profile should be in the thickening direction rather than thinning, i.e. at a control surface joint, the control surface itself should be slightly thicker than the frame it is hinged on. All airframe components should be streamlined, i.e. tear drop sections. Control surface gaps should be carefully constructed to maintain smooth airflow.

INDUCED Drag…
…is that drag component generated by the lifting effort of the wing. Such drag is divided between section and planform. Section drag can be minimised by using the thinnest possible (in the order of 6 – 9 percent thickness) together with a wing loading that utilizes the ‘Drag Bucket’ between 3 and 5 degrees angle of attack. Lightly loaded sections will operate well below these angles at speed and suffer a very poor lift/drag ratio. Flat bottom sections are not at all suitable. Symmetrical section is perhaps the best for model use. Wing loadings will need to be as high as possible (40 or 50 oz/sq.ft.). Such wing loadings will result in higher than usual stall speeds so that landing and takeoff may become difficult without high lift devices (flaps or slots). Planform governs the strength of tip vortices which contribute to induced drag, particularly at high angles of attack. High angles of attack will occur at slower speeds and high G manoeuvres. This may not be a serious concern during a speed event. Tip vortices can be reduced by high aspect ratios and tapered wing forms at the expense of manoeuvrability. Tip shape will also contribute to the strength of vortices generated.

So if you want to go fast, build it slim and clean with a high wing loading and maybe with a powerful engine, although the latter is the least important.

PROFILE Drag PARASITIC Drag… INDUCED Drag…