BIA: revising lift and drag

Lift is the force that opposes the aircraft's weight and keeps it airborne; drag is the force that opposes its forward motion and that the engine must overcome. Both arise from the same physical phenomenon: the flow of air around the wing's airfoil. Understanding their common origin stops you treating them as two separate topics, when in fact both result from the pressure distribution around the airfoil.

An airfoil isn't asymmetrical by accident. Its shape, with a curved upper surface (extrados) and a flatter lower surface (intrados), creates a difference in air speed between the top and bottom, and therefore a pressure difference. It's this difference that produces lift. The angle of attack — the angle between the chord line and the relative airflow — then modulates this force.

For the BIA, the goal isn't to memorise a full physical derivation, but to know how to link the parameters together: speed, angle of attack, wing area and air density all influence lift, and you need to know in which direction without hesitation. Drag, meanwhile, breaks down into several sources: friction drag from air moving over the surface, form (or profile) drag, and induced drag, which is linked to lift itself.

This foundation underpins other topics in the syllabus, such as the stall or the lift-to-drag ratio (finesse). A wing that exceeds a certain angle of attack suddenly loses lift: this is the stall, a concept that flows directly from this chapter and is often tested from a different angle.

The first mix-up concerns the origin of lift. Many candidates remember a simplified explanation linked solely to airspeed, without factoring in the role of angle of attack. Yet an airfoil can generate lift even at moderate speed if the angle of attack increases, right up to the stall. You therefore need to think of lift as the result of several combined parameters, not a single isolated factor.

Another classic mix-up confuses angle of attack with pitch attitude. Pitch attitude describes the aircraft's orientation relative to the horizon, whereas angle of attack is defined relative to the relative airflow — that is, the actual direction of the air meeting the wing. An aircraft can have a nose-down pitch attitude while still maintaining a high angle of attack, for example during a steep climb followed by a power reduction. These are two different angles, and they don't always vary in the same direction.

Finally, total drag is often confused with induced drag. Induced drag is linked to lift and decreases as speed increases, whereas parasite drag increases with speed. Failing to distinguish between these two components makes it impossible to understand why there is a speed for maximum lift-to-drag ratio, at which total drag is at its minimum.

The first mistake is memorising a formula without understanding what each term actually represents. Knowing that a formula exists is useless if you can't predict how a force will vary when a parameter changes. Questions in this chapter often require you to reason about the effect of a change, not to recite a statement.

The second mistake is learning lift and drag as isolated concepts, without linking them to the stall, the lift-to-drag ratio, or high-lift devices such as flaps. These topics are in fact direct extensions of the same physical reasoning, and syllabus questions often chain them together within the same series of topics.

Finally, many candidates neglect diagrams. This chapter is easier to understand visually — by following the pressure or relative airflow arrows on an airfoil — than by reading text alone. Working without ever redrawing an airfoil and its forces deprives you of an effective memorisation tool, especially for a subject that relies as much on physical intuition as on vocabulary.

Start by firmly fixing the basic vocabulary: extrados, intrados, chord line, relative airflow, angle of attack. Without these reference points, any more advanced explanation becomes confusing. Take the time to redraw an airfoil from memory, with its main forces, before even moving on to formulas or question sets.

Next, focus on cause-and-effect relationships: how lift varies as speed increases, as angle of attack increases, as wing area changes. Do the same for drag, carefully distinguishing its different sources. It's this ability to predict the direction of a change that's being tested, more than the ability to recite a raw formula.

Then move on to the stall and high-lift devices, which directly extend this same logic. Once these foundations are in place, broaden out to related topics in the aerodynamics syllabus, so as to place lift and drag within a coherent whole rather than treating them in isolation.

Finish your revision with varied sets of practice questions, systematically reviewing your mistakes to understand why an answer was wrong, not just which one was correct.

On ForAllPilots, you can work on this subject with MCQ sets organised by topic, letting you train specifically on lift and drag before broadening out to aerodynamics as a whole. These practice questions follow the format of the real exams, without ever claiming to reproduce actual exam questions.

Flashcards are particularly well suited to fixing vocabulary and the cause-and-effect relationships between parameters — a useful format for this chapter, where every term has a precise meaning. The glossary, which is free, lets you quickly check a definition without breaking the flow of a revision session for long.

If a concept remains unclear despite the course explanations, the AI Instructor lets you ask a precise question in writing and get an explanation suited to your level, with the option to dictate your question rather than type it. This is useful for clarifying a point like the difference between angle of attack and pitch attitude, which often resists a simple re-read of the course material.

Finally, timed mock exams let you place this subject within the conditions of a full exam sitting, alongside the other chapters of the syllabus, and check from the results whether this topic remains a weak point to work on further.