Attributes, simpleContent, etc.

I am just beginning to work with schemas. I'm having a little trouble authoring a schema.
(I'm using the oXygenXML editor, if that matters). If there is a better place for me to
post this, please let me know.

I have an element in my XML document that looks as follows:

 <wingarea unit="FT2"> 174.0 </wingarea>

The element is required, the content must be greater than 0, and the unit can be one of
FT2 or M2 (the unit is optional, with FT2 being the default). I've read up on this, but so
far I've been unsuccessful at making the correct schema, according to the oXygen XML error
messages I get.

Here's what I've got so far:

  <xs:element name="wingarea">
    <xs:complexType>
      <xs:simpleContent>
        <xs:restriction base="xs:double">
          <xs:minExclusive value="0"/>
          <xs:attribute name="unit" use="optional" default="FT2">
            <xs:simpleType>
              <xs:restriction base="xs:string">
                <xs:enumeration value="M2"/>
                <xs:enumeration value="FT2"/>
              </xs:restriction>
            </xs:simpleType>
          </xs:attribute>
        </xs:restriction>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

I get an error message when I try and validate a document using this schema:

--- start error message ---

"Complex Type Definition Representation Error for type '#AnonType_wingarea'.  When
<simpleContent> is used, the base type must be a complexType whose content type is simple,
or, only if restriction is specified, a complex type with mixed content and emptiable
particle, or, only if extension is specified, a simple type. 'double' satisfies none of
these conditions. URL: http://www.w3.org/TR/xmlschema-1/#src-ct"

--- end error message ---

I'd be grateful if someone can give me a "simpleType" of explanation of where I've gone
wrong! Thanks.

Jon Berndt
League City, TX