Wingtip Vortex: Heavy, Clean, Slow - Why?

"steve.t" wrote in message
oups.com...
Is this why there are fences on wings? [I am assuming that the over
sized chord shaped flat metal blade on a wing is a fence]. That is to
keep the air flow straighter than it would otherwise flow?

And speaking of winglets, would a winglet provide sufficient efficiency
increase for piston singles to make it worth the cost of the
modification (field approval or STC)?

Later,
Steve.T
PP ASEL/Instrument


Without getting into a bunch of searching for proof, what I recall reading
is that winglets can be thought of as more wing, to keep the vortices from
coming off the wing tip. Vortices still come off the winglet. Some of the
energy is reclaimed by making the vortices from the winglet flow over the
airfoil, adding lift. Angle and size are important, as being slightly wrong
can soon destroy any gains.

They are most efficient at speeds above what piston singles can attain.
Fences and other trick wing tips (drooped, angled) are better at low speeds.
You are better off adding more wingspan to increase the aspect ratio, at our
size.

Airliners can't simply add more wing length, and still fit in the gates and
hangars, and keep the spars strong enough, so they put on the winglets.

I'm sure that some of this is not quite right, but I think the general
concepts are correct.
--
Jim in NC

If I'm not mistaken fences prevent the spanwise flow across the wing,
delaying the onset of a stall. For the airplanes that we typically fly
the wing stalls from the root first then spanwise to the tips, the
fences help to reduce this. Vortex generators on the other hand do
exactly what their name imply, create vortices that re-energize the
boundary layer on the wing. Slots also serve a similar purpose, except
that higher energy flow from beneath the wing is allowed to travel to
the upper surface and re-energize the boundary layer.

Dave

But the reason that you can fly slower in the first place is because
the flaps are down. :-) If you look at a CL curve for a wing with and
without flaps you will notice that with flaps extended the curve moves
up and to the left. Up and to the left. (Sorry about the JFK movie
bit). So while the AOA with flaps is reduced for the overall wing, the
local lift coefficient in the section with flaps extended is higher.
The common belief is that downwash causes vortices which in turn
increase drag. Actually, it's the wing tip vortices and vortices bound
to the wing surface that deflects the airflow in the vicinity of the
wing downwards, contributing to the creation of downwash and induced
drag.

Dave

Actually, the reason that a dirty plane has vortices that are less
intense than a clean airplane is that the vortices coming off the flaps
themselves act destructively with the vortices coming off the wingtips.
Same principal is in play on a turbulent day when the vortices are
broken up more quickly than on a day with smooth air.

Dave
Eric Nospam wrote:
You get the strongest wingtip vortices when an aircraft is flying
heavy,
clean and slow. But why?

It has to do with the amount of lift being generated by the wing.
HEAVY I
can understand. Heavier aircraft - you need more lift to keep it
aloft.

But CLEAN? SLOW? Why do you get less vortex with the flaps down?
Why does
a slow-moving aircraft generate a stronger vortex than a fast-moving
aircraft generating the same amount of lift? Is it because of the
larger
angle of attack necessary to generate the same lift at a lower
airspeed? If
so, why?

Eric