From: highflyer <firstname.lastname@example.org>
Subject: Re: Wash out (was GROW UP!)
Date: Wed, 30 Sep 1998 14:19:48 -0500
Robert Chilcoat wrote:
> On the subject of washout (ON TOPIC, please note), does anyone know if
> the nasty stall characteristics of the Spitfire were due to the
> eliptical wing, or to a lack of washout (assuming it had none due to a
> desire for maximum speed - but perhaps it did have some)? If due to the
> wing plan form, why would this give a bad stall characteristic? Just
> curious, I'm not planning to build a Spitfire.
That is a reasonable question, Bob. Back in the thirties we had
learned that the lift distribution on an ideal wing was elliptical.
The easiest way to be sure of attaining an elliptical wing without
haveing to use washout to reduce the lift near the tips, is to use
an elliptical planform. Of course, if you use an elliptical planform,
you do not need to use any washout. In fact, if you do, you merely
louse up that beautiful elliptic lift distribution you obtained by
spending the time and effort to build a wing where every curve is
compound and every rig is different.
Unfortunatly you get eaten by Reynolds number. The key input in the
Reynolds number, that brings in the scale of what you are working
with so that it compensates for the scale effect aerodynamically
and defines the flow you see is a length. For a wing, the critical
length is the wing chord at the point under consideration. The
smaller the chord, the smaller the Reynolds number. The smaller the
Reynolds number, the smaller the angle of attack where the stall
occurs. As a result, with any tapered wing, whether that taper is
linear or elliptical, the tips of the wing stall at a lower angle
of attack than the root.
Since the lovely elliptical wing of the spitfire could NOT and DID not
have any washout, the wing TIPs always stalled first, causing a loss
of roll control near stall and the unpredictable stall behavior that
we associate with a "wicked" stall characteristic.
In modern designs we use a straight taper and then wash out the tip
to reduce the lift near the tips to approximate the elliptical lift
distribution with a straight tapered wing. This gives us close to
the aerodynamic ideal distribution along with a more docile stall
characteristic. However, some aircraft with straight taper wings and
washout, such at the Beechcraft Bonanza and those aircraft based on
its design and wing, that use the 23012 airfoil still tend to be a
bit twitchy near the stall.
This is a fault of the 23012 airfoil section. Whereas most of the
airfoils we are accustomed to using have a lift curve that increases
as a decreasing rate beyond a certain critical angle of attack, stalling
at a point where the lift begins to decrease with increasing angle of
attack. At no time is there an abrupt CHANGE in lift. It merely stops
increasing with angle of attack. The 23012 airfoil does NOT stall in
this way. When you reach the stall angle of attack there is a sudden
and abrupt LOSS of lift. This makes for a very abrupt stall. If the
stall angle of attack is not reached in s gradual and progressive
manner, the stall will be "twitchy." This is good for aerobatics, but
not so good for careless pilots.