Winglet
What is the effect of adding winglets? The major effect is to reduce downwash, make it harder for the high pressure side to leak around to the low pressure side.
This increases apparent aspect ratio of wing reducing induced drag. But it comes with the penalty of adding parasitic drag without adding any extra lift.
As a consiquence of the above you should always add wing span in preference to winglets.
The induced drag effect is much more significant at high lift coefficients (slow speeds, when your foils are struggling to provide enough lift) so you could see the effect of adding winglets (or extra span) as helping you take off sooner, but it will reduce your top speed.
Wingtip Fence
When are winglets used? Always when wing span is limited
ie by structural considerations
ie by outside influences, such as 15m class in gliders or aerobridges for airliners.
So should winglets be added to your moth?
Only if you need to take off at a slower speed and
1. You are using the same production hydrofoil as everyone else.
2. Your proto foil design is aready solid carbon and at its structural limit.
3. If your moth doesn't look enough like the bat boat yet (but I suggest adding winglets in a more visible spot).
In all other cases, add span, not wingtips, and add more reinforcing if necessary.
Spiroid Wingtip
yep.
ReplyDeleteso i would definitely go for actual span and a nice planform. my winglets do help me take off earlier. but as they get nailed on sandbanks the parasitic drag is going up with each sail! and my top speed has dropped from just over 20 to just over 18. sailed annas bladerider and my boat today and the proper foil is much much quicker, controlable etc.
interestingly because the new bladerider planform is less elliptical, they have twisted the section and the flap to mainatin the elliptical lift distribution but increase the flap area and allow it to extend right to the tip. consequently the bladerider gets better flap response with half the range of motion.
Nick don't you think the better responsiveness of the BR is just due to having a larger flap? Interesting that it is twisted I have not taken a close look at it before.
ReplyDeletei think eliptical planform for our hydrofoils is over-rated anyway, but that is for another post...
ReplyDeletewasn't Nick referring to elliptical lift distribution rather than planform shape, the latter can lead to the former, but there are other ways to achieve elliptical lift distribution.
ReplyDeletecan you elaborate/clarify point 1 of your conclusions?
ReplyDelete"So should winglets be added to your moth?
Only if you need to take off at a slower speed and
1. You are using the same production hydrofoil as everyone else."
would this include the situation when you are a fat bastard with the same foil as everyone else, hence are taking off later, so you should consider adding winglets, if you don't want to build a whole new larger lifting foil.
also, I still think that any winglets should have the trailing edge toed inwards a little to take advantage of the leak from high to low pressure, as opposed to trying to stop it.
ReplyDeletefinding the best angle is tricky because of the large range of operating speeds for the foil, but the principle is the inverse/opposite of a propeller blade spinning in a straight flow...a straight/fixed blade in a spinning flow. this needs to be taken advantage of to "optimise" (whatever that is) the effect of the winglet.
trickier said than done without really cool programs and experience in them...or a trial and error full scale testing program, which wouldn't really be that hard, but results need to be carefully considered.
as nick's anecdotal evidence suggests, take-off speed may drop with the addition of winglets, but from what he's written here and on his bloog, it may not be faster around the course due to a drag penalty. win some, lose some.
Eliptical lift distributions are also overrated. I believe that you can achieve a higher aspect ratio with a trapazoidal foil(with trap lift distro) than an eliptical one with the same root chord profile, but more on this once I have proved it one way or another.
ReplyDeleteRE: n4rkla's clarification to point 1.
It is probably easier to add winglets (or particulary a wingtip fence) to a production foil than extend its span. And it adds less bending moment then adding extra span so you can sleep better at night.
Less down wash will lead to a higher lift coefficient at the same angle of attack so it will help take offs but will reduce top end speed. The same is true no matter what your weight is. The heavier guy will always be at a disadvantage.
Re: Narkla's later statement with mumbo jumbo, I would keep winglet inline with centreline. There is too much going on about to try to calculate optimal toe angles. But I don't know why you even care. It is obvious from looking at a majority of aircraft outthere that winglets are just about worthless anyway.
so why not winglets on the sail headboard
ReplyDelete