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<< According to my engineering analysis, which must still be verified through
flight test on Derek Hine's Lancair IV, is that at cruise speed they will
cost us from 0 to 2 kts at max and mid weights respectively. They should
also have a pronounced
improvement in increased climb rate and reduced stall speed. >>
Let me get this straight... The American Eagle winglet is going to make the
Lancair IV as much as 2 knots slower for the sake of an unquantified stall
speed and climb improvement. It seems that any engineer capable of such
accurate analysis of negative performance impact could give us a quantifiable
estimate of what we get for this penalty. <LOL>
Winglets have been in use for decades and are the subject of much rumor and
myth. I investigated the design rules of Whitcomb style and blended winglets
years ago, consulting with the Chief Scientist of Boeing (who was the
professor that signed off on Bernie's doctorate), half a dozen of their PhD
aerodynamicists and the top aeronautics professors in academia. The results
were interesting.
Other than the fashion statement, there is no case where a winglet is better
than a straight aspect ratio increase. Not in stall, not in climb, not in
cruise.
Wings are structural. The higher the aspect ratio (span squared / area), the
heavier the structure. Winglets are an aerodynamic fix that extract some
energy from the wing tip vortices in the thrust axis and pushes the vortices
farther outboard than they would normally be. This reduces induced drag.
The penalty for this is increasing the weight at the wing tip and behind the
elastic axis of the wing as well as an increase in parasitic drag from the
additional wetted area of the winglet. This costs us top speed (based on
parasitic drag), range (in useful load), and flutter margin (which reduces Vd
and Vne). Whether the reduction in induced drag makes up for the increased
parasitic drag, as well as the weight and flutter margin penalties is a
function of how slow you want to fly and how bad the wing was to begin with.
Winglets work best when the wing tip vortex is strong. Things that
strengthen the vortex are low aspect ratio, high lift coefficients (low IAS
and high wing loading), aft wing sweep, and poor wing tip design. Modern
airliners cruise at high lift coefficients and use swept wings, so winglets
are worth examining for them. Even so, the only case where they win out over
simply increasing the span of the wing is when wing span is artificially
limited by things like airport gate spacing. Hardly an issue for our
Lancairs!
Like winglets, increasing the aspect ratio reduces the lift coefficient
(slightly) and the induced drag by reducing span loading. Like a winglet,
the penalty is more weight and reduced margins. The difference is that an
aspect ratio increase makes more of a difference to stall speed than a
structurally equivalent winglet can; and so for our Lancairs, it would be a
better choice.
Blended winglets are nothing new either. The idea was around before the ink
was dry on Whitcomb's report on the "sharp corner" winglet. The first
production use of the "blended winglet" was on the Lear 55 "longhorn" wing.
It goes back a little farther than the AP GII blended winglets but was later
than the original GIII Whitcomb style winglets. Lear has continued with the
blended winglet and is convinced that if not better, they are at least sexier
than a longer wing.
Once an aircraft is built and certificated, increasing aspect ratio is
difficult without inducing handling problems such as tip stall. It is easier
to add a winglet, but it still has to pay for itself.
The Lancair IV uses a high aspect ratio, straight wing, that cruises at low
lift coefficients. While the wing tips could be better, they aren't bad.
All of this means that the wing tip vortex at TSIO-550 cruise power levels is
weak. Therefore, our trade studies showed that winglets were not worth the
penalties at and above stock cruise speeds. Increasing cruise power with the
Walter, AE, or Zehrbach engines will increase cruise IAS even more; reduce
cruise lift coefficient even more; and thereby reduce the case for winglets
even more.
To date, there has been no case where a winglet was better than an aspect
ratio increase. None. And that's for aircraft that fly at the high lift
coefficients that winglets are supposed to be best suited for. They have
made their way onto aircraft that are restricted by gate spacing and have
excess wing structural margin. In other cases, they are there to satisfy
marketing.
The factory winglets on the Lancair IV do create a marginal yaw stability
improvement that shows up above 180 KIAS. A lower drag fix for this would be
to add strakes to the tail. This is planned by AE, and will probably be used
by anyone adding power to a Lancair IV; much like many twin turboprops added
them when they were stretched into commuter airliners.
This brings up the question of flutter. The original flutter analysis of the
Lancair IV wing contained assumptions that lowered the Vd (and Vne) to less
than was probably true. Adding anything to the tips, especially winglets,
would reduce flutter margins by unknown amounts and require a new analysis.
I hope that before anyone hands me something to hang on the end of my wings,
they do this. Blithely saying that there's plenty of margin without doing
the work is really asking for trouble.
If the objective is to lower stall speed and increase climb rate, there are
better ways. The flap system on the Lancair IV is a small chord, single slot
flap. There are several augmentation devices that can be added to improve
its performance and thereby reduce the stall speed. Care would have to be
taken to insure that the unaugmented ailerons maintain authority with the
higher inboard lift coefficient of the augmented flap. A better wing tip
design with a minimal aspect ratio extension would gain everything the
winglet could and have less negative impact. As far as climb rate goes, AE
is already bragging about how much more power their engine will have than a
TSIO-550. Since the climb rate is already pretty good, this will increase
the vertical speed to jet-like numbers; so why use winglets?
The only answer remaining is that they look cool. Let's face it, certain
things on aircraft look sexy and winglets are one of those. While I can
appreciate this, I prefer function over form.
Eric Ahlstrom
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Please send your photos and drawings to marvkaye@olsusa.com.
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