Private Pilot course sample
Four Forces & Steady Flight
Read a real course lesson, try three knowledge checks and explain the idea in your own words.
Look at an airplane in straight, level flight. Four forces are acting at the same time: lift, weight, thrust and drag. You do not compare all four together. Lift is compared with weight; thrust is compared with drag. This lesson will show where each force comes from, which way it acts and what changes when one pair no longer balances.
Four forces. Two balanced pairs.
Select a force to see what it does.
Opposing pairs are balanced in this example. Arrow lengths are not to scale.
Lift balances weight
In this straight-and-level example, lift acts upward. Its magnitude equals weight, so the airplane does not accelerate vertically.
Straight, level, unaccelerated flight with horizontal thrust. Arrows share a diagram origin; their physical application points differ.
Illustration by The Pilot Method. Force relationships based on FAA PHAK, Chapter 5, Figure 5-1 ↗.
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Two pairs, not four rivals
Four forces act on the airplane in flight, and they are not four rivals in one contest. They are two pairs. Lift is compared with weight. Thrust is compared with drag. That is the whole organizing idea, and everything later in aerodynamics leans on it.
Why the pairing matters
In this straight, level example, lift acts up and weight acts down. Thrust acts forward and drag acts backward. Compare each force with the force pointing in the opposite direction.
So when you look at the airplane, look twice. Once up and down. Once fore and aft. Two comparisons, made separately, and neither one tells you anything about the other.
Check your understanding.
Question 1 of 3
Put the idea into your own words.
Source: FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 5. Aircraft-specific procedures and limits come from your airplane’s AFM/POH.
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