Parasite Drag ???

[AIRWAY]

Hello,

While reading my manual "The Aeroplane- Technical" i came accross a form of drag called "Parasite drag" :eek: , can someone explain me this type of drag? I mean the book only states a single line explaining Parasite drag.

Many Thanks

[LD Max]

Author's note. This post turned out to be wrong. I have described Induced Drag - not Parasitic Drag. However I'll leave it up a) as a lesson in induced drag and b) as a reminder that the ATPLs are full of little traps like this to trip up the unwary... and I certainly just fell for it!


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You are probably aware of the concept of Drag, being that force which opposes thrust. The Engines Push, (or pull), the aircraft forwards and the drag tries to hold the aircraft back.

The TOTAL drag is comprised of two elements:

Form Drag and Parasitic Drag.

-----------------------------

Form Drag is, (as the name suggests), due entirely to the effects of a solid body ploughing through the air. The precise components which make up form drag are mainly to do with displacement of the airflow, friction across the skin and localised turbulence around protrusions or other irregularities on the skin surface.

Parasitic Drag is a bit more complex, but is entirely a function of the LIFT generated by the aerodynamic surfaces, (primarily the wings of course). There is no such thing as a perfect wing, because there are so many different factors which determine its design. It can only be described as the best compromise for all the factors required.

But one thing they all have in common is that they are joined to the airframe at the root and end in a tip which is essentially in "Free Air". In general terms, Lift is generated by the contour of the upper surface which is curved in an "Aerofoil" shape.

In the cruise, the wing presents itself to the oncoming air in cross section, with the airflow meeting the leading edge at an optimum angle, determined as the point where the ratio between lift and total drag is at a maximum. LD Max in fact, (hence my name!) :8

As the airflow accelerates over the top surface faster than the bottom surface, it creates a pressure differential where the air pressure on the top of the wing is lower than on the bottom. This is LIFT. Ideally, the Lift will equal the Weight, (Straight and Level Flight), and the Thrust will Equal the Drag, (Constant Speed).

However, since the wing is unattached at one end, the higher air pressure UNDER the wing will be drawn towards the lower air pressure ABOVE the wing, and this encourages something called "Spanwise Flow", where a proportion of the air flows from the root towards the tip. As it traverses the wing tip, it causes turbulent flow called "Wing Tip Vortices", which are MASSIVE currents of spiralling air which trail behind the aircraft much like a wake.

These vortices contain a LOT of energy, (enough to turn another aircraft over if too close behind!), and by the law of conservation of energy, we can see that any nett loss of energy must come from the engines! Thus the mechanics of LIFT conspire to oppose the thrust - in other words it is a type of DRAG.

Thus when an aircraft is HEAVY, it needs more lift from the wings AND when it is SLOW, it needs a greater angle of attack to generate the required amount of lift, and which departs from the optimum angle required for LD Max.

This type of drag is called "Parasitic" because it is dependant entirely on the amount of lift being generated, and is at the very worst when the aircraft is slow and heavy, i.e. during takeoff! Next worst is when landing!

There are some devices employed to minimise parasitic drag, the most obvious being the current trend for upturned wingtips. Other devices include "wing fences", which are exactly as the name suggests, and which are maybe a couple of centimetres high, running from the leading to trailing edge. Several of these can be deployed in different positions along the span of the wing. Whatever device is used, the objective is to discourage the spanwise airflow, minimise wingtip vortices and reduce the amount of parasitic drag.

Some simplifications were necessary for a concise answer, so don't anyone "slam" me for a few generalisations!

Hope that helps

[FE Hoppy]

"(lift)Induced(dependent) drag" see above.
"Parasite drag"

"Zero lift drag (Form drag)+skin friction drag" = parasite drag.


Dp = CDp x S x ½ r V2

Dp = Parasite Drag
CDp= Coefficient of Parasite Drag
S = Wing area
r = Air density
V = Velocity

[SawThe Light]

I seem to recall one of my old lecturers telling me (over and over) that any drag caused by any part of the aircraft that was not contributing to the lift was indeed parasite drag.

[ROB-x38]

It's my understanding that the description of parasite drag above is actually a description of induced drag.

Parasite drag = form drag + skin friction drag + interference drag.

Parasite increases with the sqaure of the aispeed - ie: the faster you go the more parasite drag.

However everything is backwards in the northern hemisphere ;)

[SawThe Light]

No, induced drag is a by-product of lift if I recall my studies from all those years ago.

Parasite drag is generally accepted to be caused by those things such as exposed landing gear, antennae, struts etc and all those other protrusions and odds & sods and things that are not involved in the production of lift.

Some form drag is produced by the the wing in the airflow, but the wing is producing lift so we cannot consider that drag to be parasitic. We get form drag off the fuselage but in some cases it is also part of the lift production.

Anyway, I'm going to crack another tinnie now.

[mstram]

LD Max,

Quite a nice eloquent description of induced drag.

The "two forms of drag" are actually parasitic and induced.

And parasitic is further composed of form drag and skin friction drag

Mike

[Tinstaafl]

As others have indicated, a bit of an error in terminology in FE's explanation (my error: that should be LD's explanation). Correct for where 'Induced' should have been used instead of 'Parasite' and it would be more correct.


All objects in a moving fluid (or moving through a fluid) - including air - experience 'drag'. Drag is the term used to describe forces that act in opposition to the direction of motion.

The total amount of drag experienced is conventionally divided into two major categories:

'Induced' drag: A retarding force caused as a by-product of producing 'Lift' ie it is induced by producing lift. Nothing is free so causing a force to be produced must have a cost somewhere; and

'Parasite' drag: The remainder of the drag experienced by the body. This is present even when no lift is being produced. Parasite drag, in turn, is subdivided into several components:

'Form' drag: relates to the frontal area being shoved through the fluid.

'Skin friction': Any fluid flowing past a surface experiences frictional forces.

'Intersection drag': Where surfaces intersect they interfere with the ability of the air to flow around the body. In a sense the other surface 'blocks' where the fluid would have gone in order to allow space for the first surface to move through.

[AIRWAY]

Wow, :ooh:

Thanks very much for all the replies, now i just have to digest all this info :ok: Thanks once again

:}

[FE Hoppy]

It was 1:15 and I was a little tippsy!!

what I meant to say was that the description by LD MAX was good for "Induced" or "Lift dependant" drag

and that "Parasite" drag or "zero lift drag" was the sum of "form drag" and skin friction.

I believe that "inteference drag" is part of "form drag"

[LD Max]

Mstram (and others)...

What can I say. I got it completely muddled. You're right. I was wrong. Anyone want to know about INDUCED drag - go read my last post.

Sorry for the confusion.

:O

LD Max

[AVIATOR757]

Any drag that is not induced is parasite. Its as simple as that ,or you could get into cooling, wave, interference, form , skinfriction, profile and I could go on .But lets keep it simple.;)

[Tinstaafl]

My apologies, FE! I meant LD. :O I was on call so can't even have an alcohol related excuse....