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I am having the following graphic from one of the question for the EASA PPL(A) exam.

While I know that:
(1) represents "induced drag", as its decreasing with higher speed
(2) represents "parasite drag", as its increasing with higher speed

But I wonder what the following represent:
(3) Total Drag??? as we know total drag is at its minimum when induced drag and parasite drag are equal?
(4) Form Drag???

enter image description here

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    $\begingroup$ 3 is definitely the sum of 1 and 2 and might be termed as total drag. Form drag should look like 2. 4 looks like a drag term going very high very fast... 🤔 $\endgroup$
    – sophit
    Feb 5, 2023 at 10:57
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    $\begingroup$ Whatever 4 is, it isn't representative of the same object as 1-3. $\endgroup$ Feb 5, 2023 at 16:31
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    $\begingroup$ As others have said, form drag is a component of parasite drag, it is reasonably a part of 2. If you want to get subtle, 1 and 2 should cross at the minimum point for 3 -- and of course 3 should be at 2x the 1/2 crossing point. $\endgroup$ Feb 5, 2023 at 19:29
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    $\begingroup$ Looking at the answers to the EASA question bank, nothing is answered by item 4. In other words, it's just thrown in as a red herring. $\endgroup$
    – tedder42
    Apr 4, 2023 at 11:00
  • $\begingroup$ @tedder42: this sounds like "the" answer $\endgroup$
    – sophit
    Apr 5, 2023 at 7:12

3 Answers 3

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You are correct about 1, 2 & 3.

As Michael Hall has commented, 4 is not representative of the same object that the other three curves represent. To figure out what 4 represents, let's look at the drag equation:

$$Drag = ½ρV² \cdot C_D \cdot S$$

Now the "speed" in the chart is most probably IAS (dynamic pressure). We observe in the chart that 4 attains a drag equal to 2 at a much lower "speed". This can be a result of two factors: 4 either has a high $C_D$ or a large surface area, or both.

In my opinion, 4 represents a parachute or a similar object. A parachute produces a lot of drag at relatively low speeds, And that's precisely what the object in 4 is doing.

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The graph above assumes, (is only valid for), LEVEL flight, i.e. when at 1 G, or when the lift being generated is equal to the aircraft weight. The actual distinction between induced drag and parasite drag is simply a matter of definition. Parasite drag is that drag that the aircraft would experience at zero Lift, - at that AOA where the coefficient of Lift is zero, i.e., the drag due only to the aircraft's velocity. Induced drag is just, well, everything other than the parasite drag, or the extra drag caused by Lift.

So, your statment (1) above, to wit, " ... represents "induced drag", as its decreasing with higher speed" is also only true for 1 G LEVEL flight.
To be accurate it should say:
"Induced drag increases as Angle of Attack increases."

Form drag is a euphonism for parasite drag. Sometimes it is used to separate, (or make a distinction), between the parasite drag from a clean aircraft, and the extra parasite drag created by interference drag between external stores (like wing tanks or bombs), although I've also seen the term Interference Drag used for that purpose.

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Number 3 is total drag and it is the sum of number 1 and 2, the lowest point of the total drag curve is where minimum drag exists.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Aviation Meta, or in Aviation Chat. Comments continuing discussion may be removed. $\endgroup$
    – Ralph J
    Apr 7, 2023 at 1:30

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