I found an interesting letter in mailing list archive that explains something I always thought looked weird. The attached e-mail is long so here's the thing I learned: The winglets on some Rutan airplanes are canted inward to reduce the dihedral effect. It's a neat trick that never occurred to me. So if you have a dutch roll problem, Tom, you can lower the speed that it starts by canting the fins inward a few degrees.
Date: Thu, 18 Sep 2003 13:20:21 -0500
From: "Andy Amendala" <email@example.com>
Subject: Re: [Canards] Winglets (Long Long Reply)
WARNING: If you don't like reading, ignore this. It's a long one.
Greetings John -
I figured I would take the time to educate you on some of the functions the
winglets perform and how they do so. I'm not sure what you know about
aerodynamics so if I sound like I'm patronizing your knowledge, *please*
don't take it that way, I simply wish to inform. I thought you'd be
surprised to find out just how much those giant fins you have on your
Long-EZ are doing out there.
First off, let me say that you are 100% correct in your assumptions about
the forces applied to the wing structure on the whole. The resultant forces
of the winglets do resolve to a tension in the high-pressure surface of the
wing (the lower side obviously), and of course if we're in tension on the
bottom side, the top side of the structure must be feeling some compressive
load. I'll give you an explanation of why this is the case and the function
of the winglets as a whole. I'll try to be as detailed as I can but keep it
relatively simple as well.
First and foremost, the reason the wing feels those forces is simply because
the winglets on the Long-EZ produce lift. In fact, they produce a
tremendous amount of it. I haven't done the precise lift calculations but
cursory calculations indicate right off that you'd have a hard time
accelerating your Long-EZ on the ground to a high enough speed to even lift
off if you had a symmetric airfoil on the winglet (as opposed to the lifting
airfoil we have). So the winglets on the Long-EZ pull inward toward the
fuselage really hard. Much harder than you might imagine... Take notice of
the substantial amount of glass applied when attaching them to the wing.
So let's get technical... Why all this force pulling inwards from the
creation of lift? Why do we have these structures as opposed to just a
longer main wing? Why are they angled inwards toward the wing? Why are
they canted toward the nose? And so on.
The higher pressure air under a wing wants to spill around the wingtip to
try to fill in the low pressure area on top of the wing. This flow results
in a tip vortex trailing aft from the wingtip, like a horizontal tornado.
You can see these vortices at the wingtips of a jet fighter during a
high-lift maneuver in sufficiently humid air, or at the tips of an
airliner's flaps during a landing approach in wet weather. The energy
extracted continuously from the aircraft to make the air swirl like that is
a direct result of the creation of lift and is dubbed 'induced drag'. These
vortices are at their worst when we're trying to make lots of lift with
relatively little airflow. This means that slow flight (low speed, low mass
flow, high lift coefficient) is one of the worst cases. This also means
that the intensity of the tip vortices will be highest at these kinds of
flight conditions. The higher the intensity of these vortices, the higher
the induced drag on the aircraft, and thus, a greater amount of wasted
energy. If you trace back how your airplane is really flying, you get to
one source of energy, the fuel in your tanks. Extracting every ounce of
energy from that fuel in every respect is a challenge of aircraft design.
So the more energy we waste on things like wingtip drag, the less energy for
the airplane to use for other means. I won't go into it here but you can
read about a coefficient that you can calculate that will tell you in
general how efficient your aircraft is... this is known as the Oswald
So back to winglets specifically, there are generally two families of
winglets you'll find on aircraft today. Simply put, lacking many specifics
of course, one family has the production of lift as one of its primary jobs,
and the other does not. The winglet style on the Long-EZ is of the
lift-producing family and was designed by Richard T. Whitcomb. Our winglets
are hence called Whitcomb Winglets. A small historical fact, the first
aircraft to ever fly with these winglets was Varieze N4EZ.
So we need to talk about "helix angle". If you understand the pitch of a
prop, you're already familiar with it. Helix angle is one way to measure
how far something rotates compared to how far it travels forward in the same
time. The blade angle of a propeller blade is nearly the same (minus its
efficiency effects and local angle of attack) as its helix angle. A wingtip
vortex has a helix angle as well. This angle will be nearly parallel to the
airplane's direction of flight when induced drag is low, but twist up into
increasingly greater angles relative to the flight direction as we slow down
or pull more "G".
If we have a significant amount of induced drag, and a correspondingly
stronger tip vortex, then the flow at the wingtip will not be parallel to
it, but rather at an inward angle on the top and an outward angle on the
bottom. This is where the winglets come in.
If we park a lifting surface in the middle of this angled air flow, it will
develop lift perpendicular to the angled air flow. The resulting lift will
be angled forward, and the forward component of that lift will be producing
thrust. The lifting surface (the winglet) will also be producing drag of
its own, including both parasite and induced drag. So essentially, the
winglets on the Long-EZ are producing lift not only due to their
high-pressure-on-the-outside airfoil, but also due to the energy they are
harnessing from the tip vortices. So the winglets, being an effective wall
in the middle of the tip vortices, don't just waste the energy there, they
utilize it for lift and thrust and in the end, you have a highly diminished
vortex trailing behind the aircraft and that means lower induced drag at the
But recall I said that the winglet makes drag of its own too... If the drag
the winglet produces is less than the forward component of its lift, then
there will be a net thrust applied from the winglet to the aircraft. Yes,
your Long-EZ winglets actually provide some thrust to the aircraft! This
thrust actually represents some of the energy in the tip vortex, harvested
from the vortex by the winglet and given back to the aircraft. That's it.
That's all there is to it, quite simple really.
Ok, now the catch... How do we maximize that thrust? This is where it gets
complicated. Let me quickly define a couple of geometry terms I'll refer
to. When I say "toe-in", I'm referring to the angle of the leading edge of
the winglet with respect to the absolute tip of the Long-EZ's nose. So if
you stared at the winglets from the FRONT of the airplane, the more of the
"outside" of the winglet you can see, the greater the toe-in angle. I'll
also refer to winglet "cant". The "cant" I'm referring to is the tilt
inwards of the winglets toward the wing. If you look at a Long-EZ, you'll
notice its winglets tilt inwards slightly (the top of the winglets point
If you increase the angle of attack of the winglet by increasing the toe-in
angle, then it makes more lift force (which should theoretically increase
the forward component of that lift), but it also makes more drag of course.
Depending on the specific situation, this could increase, decrease, or not
change the net thrust of the winglet. It's going to depend on a lot of
factors, including the flight condition.
The last item is particularly critical. Because the amount of induced drag,
and the helix angle of the vortex decrease as you increase airspeed, the
energy available for "harvesting" by the winglet decreases as you fly
faster. Meanwhile, the parasite drag of the winglet is increasing.
Eventually you get to a point where the total drag of the winglet is equal
to the forward component of its lift, and at that point the winglet produces
zero thrust. This is called the "crossover velocity". At airspeeds higher
than the crossover velocity, the winglet adds to the aircraft's total drag
and you'd be better off without it.
Thankfully, we don't have to worry about most of this with the Long-EZ since
the aircraft is already superbly aerodynamically engineered. I just thought
you'd find it informative. So I covered why the high pressure side is on
the outside, and what the toe-in does for lift, but what about twist and
The process of "unwinding" the tip vortex that the winglets perform is
accomplished both because they are a physical wall in the way of the vortex,
but also due to the effective aerodynamic twist of the airfoil. The
orientation of the upper and lower winglets provide effective aerodynamic
twist to assist in this function. I'll leave this alone unless you desire
As far as the "inward-cant" of the winglets is concerned, when you think
about it, you might think they're detrimental to the design to some degree.
If lift is created perpendicular to the airfoil body, and the winglets on
the Long-EZ are canted slightly inwards, don't we end up with a slight
portion of that lift pointing towards the ground (I.e. adding to the weight
of the aircraft)? Yes, we do. However, it is entirely negligible, it's
that small. Burt ran me through some quick calculations a ways back just to
show me how negligible it is. So why do they point inwards at all then?
They reduce the effective dihedral of the wing.
You know of course that the Long-EZ main wings have sweep to them and, duh,
they have winglets. Adding wing sweep and a winglet to a wing both make the
wing feel as if it has dihedral. Since they don't actually have dihedral
physically, we call it 'effective dihedral'. When Burt designed the Long-EZ
with a larger wing, he needed wingtip clearance for crosswind landings...
Because of this, he needed to do away with the anhedral design of the
Varieze. Think of the consequence... The Varieze's effective dihedral from
both adding the winglet and from sweeping the wing is counteracted by the
anhedral in the main wing surface. Reducing this anhedral to zero, as was
done on the Long-EZ for tip clearance, would obviously bring the effective
dihedral back up and make the craft more stable, however, more difficult to
turn. So to reduce this effect as much as possible, Burt canted the
winglets on the Long-EZ inward slightly.
So now you can run off and think about all that's happening out there on
those fins. I think I've dragged you on long enough, but think about how
the rudders on the Long-EZ might work given your knowledge of winglets now.
They function differently than conventional rudders. Also think about what
happens to roll rate if you cant the winglets outward instead of inward?
Think about how changing the toe-in angle would seriously change things?
Also think about my favorite modification that I still fail to agree with,
cutting off the lower winglet...
If you think about what all those changes do, you'll better understand the
function and design of the winglet. If you've got any questions, write me
back privately, I'd be happy to respond however I can.