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Saturday, October 25, 2014

Working the Angles

Our Paradox Version 3 A Class cat platform design is complete and tooling is underway.
The foil housing arrangement in the new boat is designed to accommodate virtually any shape with full interchangeability of parts using a new version of our proven system of hull and deck bearings.

Now focus is on foil design.
The plan is to offer the boat with a foil package that prioritises ease of use. 
Design constraints were imposed to keep the overall arrangement symmetrical (so the foils need not be raised/lowered/trimmed at every tack or jibe) and to minimise part count.

An alternative foil package with flaps to control heave is being developed in parallel.
Owners will have the option of fitting either foil package depending on preference.
Full interchangeability is being implemented from the earliest stages of design.

Looking at the simple 'no moving parts' option, the most promising concept is the Z foil, itself a development of our 'comma' foils, in turn inspired by Hydroptere.

Studying the Z foil in detail reveals some interesting tradeoffs that the reader will appreciate.

Since the A Class has a maximum beam limit and an inboard limit for all immersed portions of the boat, there is a theoretical maximum available horizontal (projected) span.

To take advantage of the full available width, the 'working' part of any lifting foil should ideally start at maximum beam and end at the inboard limit.
This can be achieved in a number of ways including:
a) Mount the supporting strut right out at max beam.
b) Use a T foil.
c) Cant the strut outward so it exits the canoe body somewhere inboard of the hull maximum width, goes down and outboard until it hits maximum beam, then connects to the lifting span.

Option a) has the drawback of poor interference drag characteristics at the junction between hull and foil. Since the foil leaves the hull tangentially where the topsides roll into the 'shoulders' of the bilge, the included angle between the inboard face of the foil and the bottom of the hull is very acute.

Option b) could potentially be promising but it is difficult to overcome the drag of the T junction. The two free tips of the lifting element also give higher lift-induced drag.

Option c) leaves us with some interesting trades to make.
Moving the junction inboard gives better 'end plating' and less interference drag. 
These two factors also discourage ventilation when transitioning to full flight.
However moving the exit point inboard requires either more outward cant or more depth of the vertical strut to achieve the same span of working foil.

More outward cant means less draught and less overall foil area. But in some conditions the outward canted strut can generate downforce, negating some of the gains and adding induced drag.
Less outward cant means more draught and more overall foil area. But also more total lift.

Overall characteristics of lateral resistance (and optimum effective toe-in) are also affected by the above tradeoffs.
Cant angle of the upper strut also has an effect on the rate of change of effective dihedral with heave.
Which is a measure of heave stability (decreasing dihedral angle with ride height gives positive heave stability).


Surprisingly the best combination may well be to give up some horizontal span in order to limit outward cant and/or draught without moving the exit point too far outboard.

Other considerations are the 'droop' angle of the main lifting segment and the shape of the tips.
Interactions of these parts are quite complex as there is significant 'wraparound' of the pressure fields.

Fascinating as always.

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