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Teaching students about Drag


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So going through a CFI students' lessons today I discovered something interesting, in the Rotrocraft Flying Handbook as well as the Helicopter Flying Handbook it lists Form, Interference and Skin Friction as sub-types of Profile Drag. In the Pilots Handbook of Aeronautical Knowledge as well as the Jeppesen Private Pilot book it lists these three sub types as types of Parasite Drag.Of course the PHAK and JPPHB don't even recognize Profile drag as a type of drag saying that there are only Parasite and Induced.

 

Just curious how do/did you teach it / learn it?

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I teach, in a nutshell, that form= profile+ skin. Parasite is totally diff. The only reason skin gets brought up is that there is no such thing as a perfectly smooth material from which an airfoil may be made. Profile is the height, basically, of the leading edge and with simple examples will show how much harder it is to move a huge wing through the air than a thin one.

 

Parasite is any drag created by a non-aerodynamic surface. Pitch links, skids, doorknobs, etc.

 

Sorry for the short weird post... On the iPhone.

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Simply tell them that drag is friction (shear stress). Friction is present in the production of aerodynamic force on any type of surface immersed in a fluid; in fact it is necessary for the production of lift. Research the Kutta Condition for more information. Friction causes all the drag experienced on an aircraft including skin friction drag and drag from flow separation also called pressure drag. Aircraft designers classify drag in different schemas such as external store drag, protuberance drag, leakage drag, landing gear drag – you get the picture.

 

Anderson, J. D. (1999). Aircraft performance and design. New York, NY : McGraw-Hill.

 

Anderson, J. D. (2007). Fundamentals of aerodynamics . (4 ed.). New York, NY : McGraw-Hill.

Edited by Tom22
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Take the P51 racing Mustang for example. Some have brush aluminum skin surfaces to reduce skin friction. Plus, the tolerances (space) between moving control surfaces and non- moving surfaces are extremely tight. Cowlings, rivets, access holes, control surfaces, wings are all minimized to reduce parasite drag. Any reduction of parasite drag equals an increase in performance. Now compare the P51 to the average helicopter. The helicopter has a ton of stuff hanging out into the wind creating a ton of parasite drag. Parasite drag is one minor force which needs to be overcome by the performance of the machine.

 

Profile drag is what it is and can’t be minimized. At least not by the average Joe. It is what it is….

 

Parasite drag can be minimized by an operator. A clean polished airframe and rotor blades with minimal accoutrements hanging of the machine is one way to decrease parasite drag.

 

Edited by Spike
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Just curious how do/did you teach it / learn it?

 

We're pilots not engineers, so I don't think we have to get real deep into drag. I just give the three types stating; Profile drag is simply the frictional resistance of the blades as they pass through the air (just lick your hand and feel the wind against it as you swing it around). It increases with airspeed, but not as significantly as Parasite drag.

 

Then I combine them into the total drag curve, mentioning that the base, or ld/max is the speed at which drag is the least, otherwise known as the best rate of climb speed. I then draw a line through that and talk about the power curve, from there I go to the HV diagram...

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I learned the 3 basic types...

 

Profile drag caused by the blades (skin friction and form)

Parasite drag caused by any NON lift producing surface (ie skids, pitch links... oh and the fuselage itself)

Induced drag caused by induced flow

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