Figures 1, 2, 3, 4, 5, 6,7, 8, 9
(n
A 3cce roct ?or Schoo
by.
These foils were made for sailing .
T1HERE are two ways of controlling the lift froin a iL foil and each has given birth to its own type of hydro-foil craft. In the simpler of the two a Vee (seen from the front) is used and this will obviously give more lift the deeper it travels; but, unfortunately, since it cannot "know" whether this means a loss of altitude or the beginning of a wave there is no way to make this system "platform" the
sea.
The other idea is to have a foil that never thnerges but whose anglé of attack is controlled so as to make lift a functión of depth, and since this can offer both positive and negative lift it can fill a long-felt want of the sailing vessel and so create a completely new method of sailing.
Lab.
y. Scheepsbouwkunde
Technische Hogeschool
It
is convenient to think of sailing as a bird turned
through 900 so that the water interface is at the plane of symmetry and the lower side wing has vanished. The "bird" now has a stability problem which man has still not solved properly, but first let us note that sailing to windward may
be likened to a bird soaring without effort in a rising
current (Fig 1): So long as there is. a forward resultant to L and W, flight will be in the direction of DF, and W is filling the role of the keel in Fig 3.
Man's first idôa of a mäss M to provide a recovery
moment is not a very bright one because for zero heel angle the righting moment is also zero while the weight penalty remains, so the next idea (copied from the Poly-nesians) is to widen the statiò flotation base and sit the pilot as far ut to windward as possible, using his weight
ARCHIEF
12
on a wire sling to counteract the wind force F: just
another way to try to replace the missing wing (Fig 1) bt far from comfortable or practical for anything except racing. Furthermore, the compensating weight W will not scale up for larger sizes.
Next comes Nigg's Exocoetus öl a split Vee hydrofoil but still using the pilot as live ballast, and again such a foil cannot platform waves; the gain in speed, however, is considerable and. an understanding of the new method is beginning to be acquired, Gordon Baker having preceded Nigg with his 30-knot Monitor built under a US Navy development contract in September 1955. The writer was fortunate enough to be able to follow Baker closely in his earlier 16 ft Flyiig Sailboat in Miami in 1952.
We now come to the concept of both positive and nega-tive lift as provided by the fully-submerged and incidence-controlled hydrofoil as extensively developed for sports craft of the Hydrofin type. These foils are primarily con-trolled by nonlinear input signals of water level provided by mechanical sensors so arranged as. to provide informa-tion both in pitch.and roll. The roll system signal is the difference signal of the port and starboard sensor. In addi tion to this the pilot is provided with a control column and steering wheel and, by moving this fore and aft, he can shift the zero position of the relative setting of sensor and foil to control flight altitude and/or unstall for take-off. By rocking his control column sideways he can feed in a difference signal that applies banking.
It follows that provided with such a sensing system the craft can rise easily onto its foils and fly at a predetermined height by reference to the local water surface, while the pilot is equipped to feed in an incidence bias such as to provide the necessary larger foil incidence to leeward and (possibly) negative incidence to windward, despite the fact
that the craft is level (le zero heel angle). On ging about this bias is, of course, reversed by the pilot.
-One Of the most difficult problems with the sailing hydrofoil is the provision of adequate static stability in a craft that must be built as light as a glider if flying in light winds is to be achieved; another is the length of leg that is required (if full advantage is to be taken of the craft's ability really to platform the seas without having to sell a crane with each boat, for lifting in and out of the water !). Many years of designing and redesigning as well as some second-hand experiénce in various parts of the world have brought replies to these problems, so that satisfactory solutions can now be worked out.
Let us begin by examining the transverse forces system. Fig 6 shows a three-force co-planar system that is stable and does not rotate (ie heel). Obviously it matters not at all if the upward force in Fig 6 is moved to a new position as in Fig 7, and, we recognise the pattern in Fig 9. For no heeling, this supposes an adequate negative lift to wind-ward depending on the wind force. By drawing the polygon of forces (Fig 8) it is possible to see exactly at what wind force the windward foil goes negative since, if W represents,
the weight and F, the wind force, then F.,1 calls for a positive. F2 a neutral and F,3 a negative incidence.
The problem of extreme lightness consistent with strength
is solved by converting the usual mast, deck and stays (wherein the stays are only tensioh members) into a rigid structure by replacing the stays with profiled sections into which the lower ends of the struts proper retract to bring the foil into the beaching position at R.
This will require the installation of a hydraulics system. so that retraction or extension (once foiling speed as a displacement craft has been attained) can be carried out by pilot-operated push-button control, but hydraulics are now
-ydrof n
D. (Pury schvriatic crrcngmnt
PATENTED
s.'
'New
.Hydrofin
1966THE CASCAD! PRINCIPLE'
Figure 10. Purely schemwic representation of the old and the new controls system. All the Hdro fin parts appear again in a new form in the New Hydro fin and have been given similar letters for identification. P indicates pitch and R roll in the new system
4
off-the-shelf items. Test models for schools need not install this feature but radio control will be essential instead.
The problem of the transition from static to dynami force systems as speed increases is solved by water ballast to windward in the tubular floats (tubular for strength with lightness), retained by storage above the static water line so that evacuation is assured by gravity on opening a tap by radio. Steering might be managed by wind vane instead of radio control but probably only in calm water which, by definition, has no wind. It is the whole point of this exer cise that wind produces waves and to escape these the high-flying hydrofói] (je with incidence control) is the oNix answer short of real flight. The simpler fixed-incidence foil will always be restricted by its geometry, and its inability to provide negative lift means that live ballast will always have to be used to prevent the capsize, with all the limita-tions thereby implied. Clearly, the fixed foil has come to the end of its possibilities.
THE CONTROLS
Some people have difficulty in understanding a controls loop. the need for feedback, etc, so the following pre
liminary remarks will possibly be helpful.
All control systems that use power need a feedback control to moderate the source and avoid over- as well as under-shoot. The extreme simplicity of a water-contacting sensor is remarkable, since it controls power (ie that powet produced by the relative motions of boat and water sur-face) and is, at the same time, its own feedback system since boat motion in response annuls the signal. If a very light trailing sensor is used it is necessary to spring-load it onto the water to provide the foil incidence urge in the negative direction, that in the positive direction being given by the dynamic reaction of the water.
There is a big difference between the pitch and the roll signals, the former being of very high frequency when the boat.is running against the seas - in fact far too high for
it to be possible to rise and fall to them:. the g would
be totally unacceptable. This is because the frequency of encounter is boat speed plus wave speed, but for side seas this is not so and these "arrive" at wave speed only, which is 2.26 It follows that whereas the former require to be "thrown away" mainly, the latter can be fed directly to the foils even without damping. It is therefore convenient to take the two signals on board by separate collectors, damp them down where required and then unite them before feeding to the foils, while the pilot too must be able to add his zero shift or roll differential to produce banking. Because experirnentation on the sea is about ten times more difficult than on an airfield - because the interface effects introduce entirely new problems of ventilation to the negative lift side of the hydrofoils - and because the whole construction must be very light and very strong and still pass smoothly from the static to the dynamic mode with thrust points placed incredibly awkwardly on the craft, this is nò amateur's realm of experimentation.
Essentially all I have written above can be found ex-pounded in far more scientific language by H. M. Barkia, MA, BSc, in his paper "The Physics of. Sailing" (the Inst of Physics and the Physical Society) of October 23, 1964; and in his article "Hydrofoil Sailiòg" in Yachts and Yacht-ing of November 8, 1968; he ends up as follows:
"It should be clear why this is no undertaking for the amateur. The problem requires the intensity of study and the effort and skill in design which only a rofessional organisation of some size could give. It remains only to find such an organisation which is also open to the con-viction of the value of doing it."
Figure Ii. Not so much a yacht, more a wind träp. Suggested form for the sailing model for schools based on some experi-ence and a lot of theory. For the stability see Fig 9 and for the control system. Fig JO
With this view I am completely in agreement, mainly because I have already had many years of experience of hydrofoil development and in seven different countries. I have resolved to offer my designs and, skill to science masters who are willing to encourage their classes to par-ticipate in 'a methodical attack on this problem and ori an international scale that ill lead to interesting student exchanges as progress is made.
lla
Stratford & Assocaes
Consultants in Aviation
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