# Drag associated with Foils

Discussion in 'Boat Design' started by dalebirrell, Apr 10, 2010.

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### dalebirrellJunior Member

Say you were analysing the total resistance of a keel or rudder,

in addition to profile drag (viscous pressure), residuary drag (wavemaking etc) and induced drag... where does the drag you would associate with the lift come in to this?

i.e. if I look at the lift and drag coefficients of a NACA foil for example, does this drag that is directly a result of producing lift become an additional drag component?

Thanks

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### markdrelaSenior Member

I think it's simplest to look at a device's drag breakdown in terms of the dissipation of the fluid's kinetic energy (KE) by the device, plus the addition of KE by the device. The drag can then be defined at two levels of detail:

1) Overall, we have
Drag * V = KE dissipation rate + KE deposition rate
where "V" is the vehicle speed through the undisturbed fluid. For any given flow, the two terms on the right can be precisely defined for any chosen control volume or box that you draw around the device. The resulting Drag you get is always the same.

2) For most typical flows, the Reynolds number is high enough so that the viscous and inviscid parts of the flow are clearly distinct. And with a large enough control volume, the waves and trailing vortices of the inviscid flow also become distinct. A further breakdown of the two KE terms is then possible by location, giving the more familiar drag components:
Profile_drag * V = KE dissipation in surface boundary layers and viscous wakes
Interference_drag * V = KE dissipation from separation at the rudder/hull intersection
Induced_drag * V = KE deposition in the fluid motion of traling vortices resulting from lift
Wave_drag * V = KE deposition in the fluid motion of waves resulting from lift and volume

As an illustration of the issues, consider a hull+keel with massive separation, operating in a confined water channel. In this case the viscous region, the trailing vortices, and the waves are all in close proximity and are not distinct.
In that case step 1) is still valid and there certainly is an overall drag. But step 2) is not "obvious" and the familiar drag components cannot be rigorously separated out.

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### tspeerSenior Member

Induced drag is the drag due to lift. It is a function of the span and the planform shape.

There is a variation in the profile drag with angle of attack, but it comes from changes in the skin friction as the velocity increases at the leading edge and transition shifts on the two surfaces.

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### dalebirrellJunior Member

Thank you both for the replies, very helpful.

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Dale, saw your post on SA-if you haven't already you need to see this.

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• ###### CSYSPaperFeb09 Beaver paper on Moth.pdf
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6. Joined: Nov 2009
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### dalebirrellJunior Member

thanks doug, yeh i've used some of the aero drag stuff in the VPP!

cheers

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