From: AFE12@aol.com
Message-ID: <f8.9cbec71.2824c9ab@aol.com>
Date: Fri, 4 May 2001 23:12:43 EDT
Subject: Re: American Eagle LIV Winglet
To: lancair.list@olsusa.com
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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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