Re: Dihedral on Horizontal Tail Surfaces

Date:         07 Dec 98 23:19:40 
From:         arch6@mail.inlink.com (arch)
Organization: McK&A
References:   1 2
Next article
View raw article
  or MIME structure

In article <airliners.1998.1810@ohare.Chicago.COM>, stevek1957@aol.com (SteveK1957) wrote:
> Dihedral in both wing and tail increase the tendency for the airplane to roll
> due to sideslip which can agrivate Dutch Roll.  The same tendency is caused by
> wing and tail sweep.  I suspect the reason for dihedral in the airliner
> horizontal tails has more to do with getting the tails out of the wing wake to
> reduce tail buffet than increasing the roll tendency.  There is usualy plenty
> of roll due to sideslip in most airliners.  The F-4 has the anhedral in the
> tail to counter the dihedral in the outboard panels of the wing which were bent
> up for ground clearance on carrier decks.

I disagree with the implication that if one has dihedral the other should
counter with anhedral.

The tail sections are often a combination of trade-offs in active area,
coupling with the wing vortices and stall, and strong hinge-point
availability. Anhedral and dihedral are just one of many trades. Dihedral,
while increasing dutch roll tendencies at cruise, does increase overall
stability of the system compared to horizontal or anhedral. Anhedral is
more "risky" but, in the case of the F-4, allowed for a smaller vertical
stabilizer and took the place of lower fuselage strakes for stability at
speed. That is, the vertical projection of the combined surfaces (wingtips
bent up, anhedral horizontal stab, and vertical stab) simulated a very
large vertical stabilizer area and allowed the same F-4 to be short enough
to "go below" on the carrier hangar deck where vertical clearance is an
issue.

The F-4 wingtips were bent up for supersonic stability. If there were a
horizontal area clearance thing with the Navy then the whole wing would
have been elevated. Additionally, weapons loading at such a low height
made slinging missiles and bombs up from deck racks to weapon racks
easier, except where the wing did not allow the ordies to stand up and the
had to hump a weapon up while bent over. As it was the bent section of
wing tip folded vertical to increase storage availability on the carrier
deck and the wing tips were only dropped for the launcha and recovery
phases. The F/A-18 wingtips also fold up for the same space saving reasons
but the tips go full horizontal for flight. Unfortunately the F/A-18 also
has missile racks on the ends of those wingtips and folding wings puts the
missile and rack way up in the air (try pinning that rack or lifting up a
missile to load it there).