Profile Drag?

[wanabe]

Somebody please help me understand this:

6526 The resistance, or skin friction, due to the viscosity of the air as it passes along the surface of a wing is called:

A- form drag
B- profile drag
C- parsite drag


My understanding is that C- Parasite drag is the answer. The Handbook of Aeronautical Knowledge backs this up. However, 2 different study guides I am using for the FIA state that B-Profile drag is the answer. Ironically, they both reference said handbook for the answer.

Am I missing something?

Thanks in advance.

[Realms09]

Different books tend to make different semantical distinctions.

From Mechanics of Flight by A.C. Kermode:

We may divide the resistance of a body passing through a fluid into two parts -

1. Form drag or Pressure drag
2. Skin friction or Surface friction

These two between them form a large part of the total drag of an aeroplane - in the high subsonic range, the major part. The sum of the two is sometimes called profile drag but this term will be avoided since it is apt to give an impression of being another name for form drag, whereas it really includes skin friction.

The total drag of an aeroplane is sometimes divided in another way in which drag of the wings or lifting surfaces, wing drag, is separated from the drag of those parts which do not contribute towards the lift, the drag of the latter being called parasite drag.

Since your questions askes about the skin friction acting over the wing, the answer based on this text is none of the above. If by chance they mean drag due to all viscous effects and not just skin friction, then the answer is B. A is incorrect in all cases because it negates skin friction. C is incorrect because it pertains only to non-lifting surfaces, and the question refers specifically to the wing.

This question is simplying asking about definitions, and unfortunately different people define and use these terms differently. I do not have my heavy-duty aerodynamics books with me at the moment, so for now, I would operate on the text I quoted for your general knowledge. As far as the FAA test goes, use FAA publications. However, the more you learn, the more you will realize the FAA is wrong and sloppy about a great many technical matters.

[flyguy]

I've never heard of profile drag. It sounds like another word for form drag. There are 3 kinds of parasite drag. Form drag (or I guess you could call it profile drag) Skin friction drag, and Interference drag. The question is asking about skin friction drag, which is a type of parasite drag, so I'm pretty sure the answer should be parasite drag.

[MidlifeFlyer]

Originally Posted by flyguy

I've never heard of profile drag. .

PROFILE DRAG - The total of the skin friction drag and form drag for a two-dimensional airfoil section.

That's from the Airplane Flying Handbook. Unfortunately, it's the same as the definition of parasite drag from the PHAK:

"Parasite drag is composed of two basic elements: form drag, resulting from the disruption of the streamline flow; and the resistance of skin friction."

To make things worse, the AFH defines parasite drag differently than the PHAK From the AFH:

"PARASITE DRAG - That part of total drag created by the design or shape of airplane parts. Parasite drag increases with an increase in airspeed."

Make sense now?

[wanabe]

Originally Posted by Realms09

A is incorrect in all cases because it negates skin friction. C is incorrect because it pertains only to non-lifting surfaces, and the question refers specifically to the wing.

ASA's explanation: A is incorrect because form drag is caused by the frontal area of the aircraft. Answer C is incorrect because parasite drag includes both form drag and profile drag.

Thanks for the replies, it makes sense now. I should have dug a little deeper than the PHAK but that was the reference given for the answer and I was tired.

[meritflyer]

Originally Posted by MidlifeFlyer

PROFILE DRAG - The total of the skin friction drag and form drag for a two-dimensional airfoil section.

That's from the Airplane Flying Handbook. Unfortunately, it's the same as the definition of parasite drag from the PHAK:

"Parasite drag is composed of two basic elements: form drag, resulting from the disruption of the streamline flow; and the resistance of skin friction."

Profile drag is simply the parasite drag of the wing. Most of the wings parasite drag is skin friction drag because of its surface area. The parasite drag on the wing (profile drag) is somewhat dependent on the shape of the airfoil or profile shape hence the word "profile" drag is attached to it.

In summary, the profile drag is the parasite drag of the wing.