18' Whisper Production Foiling Cat

jetboy

Super Anarchist
1,595
0
My issues with downward foil force is waves. You need to have the foil very deep to avoid the possibility of the foil exiting the water passing through a wave form. If you're relying on the downforce, the effect would be similar to cutting the trap wires - instant capsize. I think the range of conditions in which you would achieve sufficient speed to maximize the use of downward foiling force and yet have flat enough sea state not to risk the windward foil breaching the surface is quite small. Add to that the fact that a capsize at that speed could very well be in the zone of possibly killing the crew - 40 or 50+ knots? and we're entering a design issue that IMO isn't safe enough or useful enough to be worth spending a lot of effort on.

Now if you had a completely computer controlled system that could react instantly to identifying such conditions - maybe it could work. We can make inherently unstable airplane forms fly. We can probably do this, but I don't think it's likely to be worth the effort with the rudimentary tech. like wands.

 

Doug Lord

Super Anarchist
11,483
21
Cocoa Beach, FL
You do realize that foil RM(including windward foil downforce) was standard on several hundred Raves and on every Hobie Trifoiler including the record setting Longshot version and on the new Osprey, right? In my personal experience with a Rave that never even came close to happening. In the years I knew Doctor Sam I never heard of a windward foil popping loose. But a crossarm broke when the never exceed speed was exceeded.......

 

Doug Halsey

Member
349
107
Full flying lift foils, no flap, have the theoricial advantage of more lift with less drag.
What theory is telling you that? Foils with flaps can generally produce more lift than those without, but usually with more drag.

 

Doug Halsey

Member
349
107
No Chris, I wasn't agreeing with Jim. At least not with my interpretation of what he wrote (which I found a little confusing).

I was agreeing that foils without flaps typically have lower minimum drag coefficients than those with flaps, but I was disagreeing with the part about higher lift.

But, I'm using words like generally, usually & typically - meaning that I'm aware that there can be special-purpose single-element foil sections that have higher lift & lower drag in certain conditions. (For example - some Liebeck airfoils have incredibly high lift, with low drag). The challenge is to be able to achieve those characteristics over a broad enough range of conditions to be useful in sailing applications.

I'll be very interested to see what Jim can show us in a couple of months. Maybe he can give us some more hints in the meantime.

 

Mizzmo

Anarchist
700
125
Monterey, CA
Getting inreased RM from foils also adds hugely to total drag.

Beach cats with crew on trapeze have the centre of gravity of boat+crew combo at about the widward hull c/l or about where the windward foil is. If the windward foil pulls down hard enough to double RM, the leeward foil has to carry double the lift. So total vertcal foil force is now twice upward from the leeward foil plus once downward from the windward foil. The total induced drag is now 3 times what it was without the down force, or what the drag would be if the boat weighed 3 times as much. Probably more if the windward foil is a cambered section designed to lift only one way.

Sounds like a bad idea to me if the purpose is to go faster.
+1

Induced drag is a function of Lift^2. By increasing the downward lift on the windward foil, and the required upwards lift on the leeward foil, drag grows exponentially. Meanwhile while RM only grows linearly. This should mean as a rule that using the windward foil to provide RM should be slower in the end. I may be wrong, but I think the Rave and Trifoiler did this to increase the wind range that the boat could effectively sail in.

OTH Maybe Doug is right, with a wide platform it doesn't take much windward foil lift to produce a lot of RM. There may be a wind range where foil induced RM could be faster, I would actually have to do the calculations.

 

Phil S

Super Anarchist
2,608
233
Sydney
Flaps or no flaps?

The amount of lift any boat ever needs exactly equals the total mass of the boat and crew. Get more lift than you need and you crash, not getting enough and you low ride. Do not argue with Isaac Newton.

A well cambered rigid foil or a foil with the flap lowered will provide the required lift at lower speed than a low camber foil or one with the flap raised so they take off at a lower speed than the low camber options.

At higher speeds and the same angle of attack the cambered foil or lowered flap will generate excess lift. To manage the excess the rigid foil needs to have its angle of attack reduced, by moving the whole board or by trimming the boat bow down. The flapped foil only needs the flap to be raised. The high camber foil at reduced angle of attack has higher drag than a thin low cambered foil or a flapped foil with the flap in neutral at the slightly larger angle of attack needed. The difference greatly exceeds the small amount of extra drag from the hinge line.

Moths have spent 10 years experimenting with foil shapes. Early foils had reasonably high camber and quite narrow trailing edge flaps, this meant that the camber line of the section had two bends when the flap was deflected. They could be made to take off reasonably early but even with the flap raised the drag was enough to limit speeds to just over 20kts. Foil sections changed with less camber in the front part and more in the back, and bigger flaps with the hinge line close to the line of maximum camber. So now when the flap is raised or lowered the camber line is more subtly bent which greatly reduced drag in all angles of deflection. Top speeds have gone up over 50%.

Raising the flap also reduced the angle of attack of the foil as the trailing edge comes up, so retrimming the whole boat is not needed.

Even if automatic wand control is deemed to be banned in most classes I believe the advantage of manual controlled flaps has not yet been appreciated and developed. All with in reach if canted foil type height control can be combined.

 

Doug Lord

Super Anarchist
11,483
21
Cocoa Beach, FL
Phil, don't know if you're familiar with Doran C's work with the Rave but he and another guy replaced the wands on two Raves with manually controlled joysticks. They won many races with those boats-definitely faster than a "normal" wand equipped Rave.

For those that don't know, David Lugg flew his canard configured I-14 with manual flap control of the main(rudder) foil.

see the PDF-

Rave-Hand-Controls-for-Rave.pdf

 

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Phil S

Super Anarchist
2,608
233
Sydney
Long long ago I tried a moth with TFoil and no wand or flap, exciting times, fast with sudden stops. I understand Lugg's rides were pretty scarey too. Sitting in the Rave cockpit, hands on the levers, with all the extra inertia of the heavy boat might have been possible in smooth water, and the AC72s did it with 8 men on pumps, but I can not see manual contol as a sensible way to go for height control.

But putting flaps on sloping surface piercing foils with the inherent height/area control available could make changing gears/drag and increasing RM from assymetry much simpler.

 

Doug Halsey

Member
349
107
Induced drag is a function of Lift^2. By increasing the downward lift on the windward foil, and the required upwards lift on the leeward foil, drag grows exponentially. Meanwhile while RM only grows linearly. This should mean as a rule that using the windward foil to provide RM should be slower in the end. I may be wrong, but I think the Rave and Trifoiler did this to increase the wind range that the boat could effectively sail in.

OTH Maybe Doug is right, with a wide platform it doesn't take much windward foil lift to produce a lot of RM. There may be a wind range where foil induced RM could be faster, I would actually have to do the calculations.
Mizzmo : What you are missing is that induced drag is also proportional to 1 / Speed^2. As a result, profile drag dominates at high speeds & inefficiencies in induced drag can be better tolerated.

Do the calculations & you will find that the extra righting moment is more important than the increased induced drag at high speeds.

 

Doug Halsey

Member
349
107
But putting flaps on sloping surface piercing foils with the inherent height/area control available could make changing gears/drag and increasing RM from assymetry much simpler.
The trouble with this is that surface piercing foils tend to be self-leveling when they are lifting upward, but not when they are lifting downward.

When they're lifting up, an increase in lift causes them to rise, resulting in a loss in area & hence also a loss in lift.

But when they're lifting down, an increase in downward lift causes them to sink, resulting in a gain in area & hence also a gain in downward lift (ie a crash). I learned this the hard way!

Broken-FoilCrash_6-17-09.jpg

 

P Flados

Anarchist
716
282
North Carolina
Flaps or no flaps?

The amount of lift any boat ever needs exactly equals the total mass of the boat and crew. Get more lift than you need and you crash, not getting enough and you low ride. Do not argue with Isaac Newton.

A well cambered rigid foil or a foil with the flap lowered will provide the required lift at lower speed than a low camber foil or one with the flap raised so they take off at a lower speed than the low camber options.

At higher speeds and the same angle of attack the cambered foil or lowered flap will generate excess lift. To manage the excess the rigid foil needs to have its angle of attack reduced, by moving the whole board or by trimming the boat bow down. The flapped foil only needs the flap to be raised. The high camber foil at reduced angle of attack has higher drag than a thin low cambered foil or a flapped foil with the flap in neutral at the slightly larger angle of attack needed. The difference greatly exceeds the small amount of extra drag from the hinge line.

Moths have spent 10 years experimenting with foil shapes. Early foils had reasonably high camber and quite narrow trailing edge flaps, this meant that the camber line of the section had two bends when the flap was deflected. They could be made to take off reasonably early but even with the flap raised the drag was enough to limit speeds to just over 20kts. Foil sections changed with less camber in the front part and more in the back, and bigger flaps with the hinge line close to the line of maximum camber. So now when the flap is raised or lowered the camber line is more subtly bent which greatly reduced drag in all angles of deflection. Top speeds have gone up over 50%.

Raising the flap also reduced the angle of attack of the foil as the trailing edge comes up, so retrimming the whole boat is not needed.

Even if automatic wand control is deemed to be banned in most classes I believe the advantage of manual controlled flaps has not yet been appreciated and developed. All with in reach if canted foil type height control can be combined.
Phil provides an excellent discussion above. Most main foils need to provide a near fixed amount of lift over the widest possible range of speeds, Flaps greatly improve performance in this area.

This is actually one of the few good cross-overs from airplane wing technology. All standard large aircraft use flaps for lift. The "best" performing wings are quite complex. They go from a single element low camber wing at cruise to an increased cord, 3 element and very high camber wing to get the same lift at drastically reduced speeds.

For a foiler without wands (or other active control equivalent), flaps will allow lower take offs combined with less drag at higher speeds. The flap drive is the biggest area that is not fully figured out yet (I envision a "speedometer" cable). From the top of the foil to the crew, the manual flap controls for this application should be easy enough with multiple acceptable choices available.

I do expect experiments with a manual flap on a L foil or an Uptip foil for the function described above. The payoff is just too big to ignore.

 

dacarls

Anarchist
602
10
FL
Which of this group has read Steve Killing's published report on the C-class foiler OffYerRocker? I read it, especially the part about having lifting foils on both sides. Blunted got upset with me when I wrote him that they had not explored "all possibilities" of making OYR work properly. Blunted told me that they "had done everything". I asked re "changing AoA of weather foil?" Blunted said "EVERYTHING!" Killing's report says specifically that they had to sheet out the wingmast going to weather because the weather lifters were too strong and would not stop lifting.

Guess what THAT did to the OYR polars.

 
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Doug Lord

Super Anarchist
11,483
21
Cocoa Beach, FL
For those that haven't read Killings analysis here it is: Whups! too big for this forum. If you want it ,e-mail me and I'll send it.......

 


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