Why no negative lift foils for greater righting moment?

Discussion in 'Multihulls' started by Jetboy, Jul 19, 2012.

  1. upchurchmr
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    upchurchmr Senior Member

    I believe you are right about not being able to find a safer substitute.

    But that is where performance demands can drive you.

    It only takes one to succeed then the rest have to follow or drop behind.
     
  2. mtroyka
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    mtroyka Junior Member

    @CutOnce: I'm familiar with Dr. Speer's work, including the H105 and the basiliscus. I still think some of his arguments here were nonsense, though. However, TBH I'm not sure that the trifoiler/longshot actually include negative lift in the range of motion of their foils. Thinking about it that's what I was looking for when I stumbled across this. I can say that from what I've seen the trifoiler doesn't have nearly as much tendency to roll as the rave does, for whatever reasons.

    Assuming that the trifoiler doesn't use negative lift, there are still numerous ways you could improve on existing foiling trimerans.

    @El_Guero: Speer mentions collisions elsewhere. That would indeed be a bad combination, especially if you've maxed out your rig.

    As for military aircraft, instability means greater maneuverability for aircraft, and technically dodging a missile/bullet vs not dodging a missile/bullet is actually the safer design choice in that case. Fewer control surfaces also generally translate into a smaller radar cross section. (another safe decision)

    The situation is very different for hydrofoils, where more stability means better overall sailability, ex. modern foiling moths which can do flying tacks riding on only two foils. That also (partly) explains the better foil-borne performance of the trifoiler vs the rave, since on the trifoiler the entire foil was used as a control surface.
     
  3. El_Guero

    El_Guero Previous Member

    Test pilots often push that limit, but I think entirely too many thrill seekers push for the same adrenaline rush in sailing that test pilots look for with huge differences:

    Test pilots are usually intelligent, well trained (often with graduate degrees), and have already led a successful career in flying before they begin testing ....
     
  4. Doug Lord
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    Doug Lord Flight Ready

    =================
    In post 8 of this thread I quoted Greg Ketterman where he discusses using vertical lift on the lee foil in combination with downforce to provide all the RM for the trifoiler/longshot--he calls it "Dynamic Levelling Affect". Both Ketterman and Bradfield used the foils to develop all the boats righting moment up to the structural strength of the boat. Bradfield, years after designing the Rave felt that a faster foiler would be oversquare as compared to the Rave and use movable ballast(crew) to reduce foil loading which is what he did on the Osprey. That boat takes off in a 7knot breeze and is a couple times faster than a Rave.
     
  5. Corley
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    Corley epoxy coated

  6. mtroyka
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    mtroyka Junior Member

    @Doug: I'll believe it when I understand how it works. :| Saying "downforce" and producing it are two different things, and given the way the foils were mechanically controlled on the trifoiler it wouldn't have been simple to produce downforce when actually required. The particular system on the trifoiler produced a range of ride heights, and it would have taken considerable rolling in order to swing a foil through it's whole positive lift range and then into the negatives, depending on the section used and its ZLL angle. At low speeds the trifoiler also has a low ride height, so the slower it went the further it would have to roll before any downforce would be produced. The rave had a fixed angle of attack and was more or less incapable of producing negative lift, and therefore incapable of producing RM on the windward foil. The trifoiler might have, but I'm doubting it the more I consider the complication.

    If you really want to compare the rave and the osprey, it'd be relevant to mention that the osprey's foils are further forward toward the bow than the rave's, set wider than the rave's, AND the osprey's foils have an aspect ratio of 10, whereas the rave's foils had an A/R of only 6.4. The rave also had undersized flaps, and to make things worse it used a bungee cord in the linkage between the wand and the foil flap, which delayed and distorted the surface feedback. Whether or not those issues were addressed on the osprey isn't something they advertise.

    As for "a couple times faster", the "redline" on the rave was listed at 30kts, exactly the same as the osprey. The fastest actually measured speed I've heard of for a rave was ~34kts, and if you went even twice as fast as that you'd be challenging the Sailrocket. The osprey might be faster on average, but if it could even foil upwind it'd already be doing better than the rave.


    @Corley: From what I saw, GC32 didn't look like it really flew on foils (although it might lift one side), and adjusting the foils that way = more human intervention and less stability in actual flight. The stuff they had connected to the bottom of that boat made it look kinda like a giant lawn chair. I've never seen a foil/strut setup anything like that, and it doesn't really make much sense to me. Maybe it was one of those races where real foilers are banned?
     
  7. Doug Lord
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    Doug Lord Flight Ready

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    If you don't believe Greg Kettermans description of his own system then there is no hope.
    The Rave main foils were set at +2.5 degrees and the rudder foil was set at zero degrees relative to the static waterline.
    The Rave foils most certainly developed all the RM for the boat-limimited by the structural limit of the boat. This is a serious consideration: Dr. Sam told me of an incident in Spain (I think) where the warning sticker(Do Not Exceed 30 knots! I think thats what it said) in the cockpit was ignored and the enormous aluminum main beam broke.
    ========
    I should have explained better: the Osprey is at least twice as fast as the Rave when they're both foiling in the lightest wind they are capable of.
    The Rave foils that I measured were 7/1 aspect ratio if I remember correctly.(6.75 actually) And I'm pretty sure the mainfoil flap was close to 30%-which is not undersized by any measure.

    https://sites.google.com/site/hydrosail/HydroSail-Home
     
  8. mtroyka
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    mtroyka Junior Member

    Correct, but you can't produce negative lift with +2.5 degrees of angle of attack! Greg Ketterman and Sam Bradfield both brag about how great their designs are, but that isn't the same as actual figures; ex the exact range of AoA used on the trifoiler or longshot. Reducing lift (which is what the rave does) is not the same as "producing downforce".

    "Producing all of the RM for the boat" is more of a play on words here. If the boat is flying on foils, all of the weight of the boat is supported by them and thus all the counterforces must be provided by the foils (or by weight). That doesn't mean that the foils generate counterforces for all of the errant forces the sail produces, however (as with any hull design). The rave in particular would roll considerably on foils, which you can see obviously in any (non-advertising) video of a rave flying. Both the rave and the trifoiler would produce extra lift on the lee foil and reduced lift on the windward foil to counter sail forces, but it isn't clear whether the trifoiler would actually produce negative lift on the windward side under any conditions.

    I've heard of plenty of people who've pushed the speed limits on their raves. I've never once heard of the crossbeam breaking! If anything, there are weaker components which would fail first. That sounds to me to be more likely a manufacturing error that weakened the aluminum in some way, and/or the product of a collision with something (or BS). That's surprising/hard to believe since the raves are known for being quite durable, even against bashing into things. Merely exceeding 30 kts shouldn't have been nearly enough to cause that, esp given that the less sturdy trifoiler could and would do 36kts without breaking (not counting collisions).

    Higher aspect foils = better lift/drag ratio, so that's not too surprising, but it's not exactly a measurement with real numbers, either. The osprey only does ~13kts at its minimum wind conditions, but the rave can't even take off until 13kts, so I don't see where "twice as fast" comes from unless they mean under the same wind conditions (video or it didn't happen!).

    Eh, you're right it's 6.75 not 6.4. It's possible that it was the bungees alone which caused the rave to underperform so badly on foils, but I don't really have any numbers for the flaps used on moth foils for comparison. I do know that the aspect ratio on the first moth foils was higher than the rave's foils though, and every subsequent generation of moth foils has used a higher A/R than the last.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    With a symetrical main foil , even with 2.5 degrees angle of incidence but max up flap you sure can produce downforce-I designed and built the F3 RC foiler with my own foils set up like this and it definitely produced down force on the windward foil as does the Rave-no question about it-I discussed this with Dr. Sam at length on many occaisions. Sam experimented a lot with using the shroud to twist the windward main foil to reduce the AOI(relative to the lee foil).
     
  10. El_Guero

    El_Guero Previous Member

    @mtroyka,

    Not trying to get off topic, but sensitive fighter aircraft are not for increased responsiveness .... g-forces were beyond human levels long before.

    Control sensitivity is due to other modern technologies.

    The same is true in foiling a boat. IF you do not have a very good reason to introduce instability (RISK) into your design, don't introduce extra risk.
     
  11. mtroyka
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    mtroyka Junior Member

    @DL: It turns out I had it backwards. You're right that the rave could produce negative lift, which they used for getting the foils to lower themselves into the water while moving forward. Whether it could produce negative lift at high speed is another matter.

    The trifoiler's foils were actually fixed, probably at +2.5 degrees, but it could adjust them +/- 2 degrees by rotating the amas pitch up or down. At the minimum angles it was likely to run at it couldn't have produced negative lift.

    @Guero: Well, my thought is that if it's already been proven on the water, then why not? On the other hand, if the longshot didn't need it, then why bother? Seeing how it didn't roll much or flip even running wing sails in 20-25kt winds, I'd say downforce is unnecessary.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ============
    mtroyka, if you look at a Hobie trifoiler or even the patent drawings you would see that the foils are "all-moving" and pivotably attached to the boat(either the ama pivots or the foil pivots-I forget how it was done). The "feelers" sticking out forward are attached to the foil and serve as dual independent altitude control(one on each side), and therefore, can develop all the lift and righting moment for the boat-using downforce on the windward foil automatically and as required.
     

  13. Gary Baigent
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    Gary Baigent Senior Member

    Pretty sure Trifoiler pivoted float and foil as one unit - so the foil was actually fixed and probably at 2.5 degrees, with float level and at rest; once sailing the feelers would alter float/foil angles from wave movement and height.
     
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