A Tail-Less Raven by Bernie Willis

My good wife is always pleased when I ask something about her interests. Last week
she was walking with a friend and saw a tail-less raven flying along with a normal
looking raven. They both saw it confirming that it really had no tail. So it was fair to
ask, how can it fly? I had some ideas but upon research I learned more than I
previously knew about “tail volume.” Thought you might be interested too.
Every student pilot soon learns that the elevator changes the pitch of the wing which
allows it to fly slower with less power or climb if more power is applied. The wing has
a range of angle of attack from near zero to about 14 degrees in a Super Cub. As the
angle relative to the passing air is increased the lift increases and so does the drag.
This angle of attack is controlled by the elevator or elivon in conventional airplanes.
But these devices have limitations.
Engineers use the term “tail volume” to describe the effectiveness of the horizontal
stabilizer/elevator. Basically, in an effort to reduce the size and drag of the tail its
surfaces are made as small as possible. The further aft of the center of gravity they
are the more effective they become. Look at Bob Reeves Fairchild 71. The tail looks
miniature but the fuselage is rather long in proportion to a Cub. It’s like the big kid
balanced by the little fellow further out on the teeter totter in grade school.
Oh, but there is something else hidden in the aerodynamic mystery. Most wing
airfoils have a center of lift about 25% of the distance from the leading edge to the
trailing edge until the elevator pitches them nose up. Then the center of lift typically
moves toward the trailing edge up to about 30% from the leading edge. This makes
pitching up not so much a matter of movement of the stick or yoke as it is a pressure
on the control. Its something you can feel and get tired of, hence the trim crank or
wheel. This is the basic reason why every time you change pitch you’ll likely trim or if
you’re in “heavy iron” and the autopilot is flying the pitch trim automatically changes
from time to time to maintain altitude. Can you describe what happens when the
pitch is held with the elevator until the wing’s angle of attack is such that it stops
lifting the aircraft?

Don’t just think to yourself that the stall warning horn wakes you from your stupor
and since you’re the aviator your friends think you are everything is fine. What
happens next all depends upon a few other factors. I’ll try and list them.

  1. In a Cub the inner part of the wing quits flying first and the flow over it
    becomes turbulent and shakes the tail. Your instructor called if pre-stall
    buffeting. The outer part of the wing is twisted nose down so it still flies. A
    Cirrus inner wing has a purpose installed “defect” on the leading edge that
    initiates an aerodynamic stall warning.
  2. Assuming you do nothing the whole wing quits lifting and the airplane noses
    down. It then recovers all by itself if you have enough altitude. Or it should,
    but only if it is loaded within the aft CG limits. Back in the 1920’s the Jenny,
    JN4C WWI trainer, was being evaluated by NACA and it was discovered that in
    level flight the horizontal stabilizer was acting as a lifting surface. Instead of
    balancing against a CG forward of the center of lift of the wing by pulling
    opposite, it was pushing the same direction as the wing to overcome a CG aft
    of the center of lift. Actually wings since the Jenny is a biplane. So if the wings
    stalled completely instead of pitching down, assuming level flight, it remained
    level or pitched up with a rapid descent rate. Stall recovery, if it was to be, was
    a matter of power and careful elevator control. I’m told the Russian AN-2 has
    similar characteristics.
  3. Stall awareness has been drilled into us by design of the FAA. Practice is
    usually limited to approach, power off and departure, power on stalls in more
    or less wings level flight. If and when a wing drops when in a stalled condition
    it seems normal to try and raise it with ailerons. However nothing could be
    worse, because the lowered aileron will increase drag while increasing lift.
    This drag will tend to pull the wing back further decreasing its lift and lowering
    it even more. If you’re guessing this is the beginning of a spin entry, you’d be
    correct. If the plane also has an aft CG you’d have the “delight” of doing a
    “flat” spin. If recovery is possible it will require a lot of power and altitude.
    Neither of which you’re likely to have in a general aviation aircraft. So rudder
    control is what you’ve got left to lift that low wing. The wing can quit providing
    lift if you’re flying straight and level dirty side up or down or in any bank angle,
    or flying vertically up or down. Your wing relates to the air moving over it, not
    gravity. The CG relates to gravity. It’s up to the designer, builder, loader and
    pilot to keep these elements in balance. You can’t break the laws of physics.

So with a fixed tail volume it is our CG range that keeps us from having a CG too far
aft to be controlled with the elevator. When that aft limit is exceeded especially at
low speed like take off and landing, pitch control becomes very sensitive. Its possible
to get in a situation where forward pressure on the stick is necessary for both take
off and landing. Should an airplane actually lose its tail like the raven there would be
no down pressure balancing the normal CG forward of the center of lift. So the plane
would be very nose heavy and uncontrollable. If it was an airworthy airplane except
for loading and the CG was too far forward it would be very stable in flight but
slower than normal because of the excess weight the wing would be lifting because
of the excessive down pressure of the horizontal stabilizer/elevator. The real
problem comes when trying to land. Tricycle geared planes with a too far forward CG
may touch down nose wheel first bending the firewall, while conventional geared
aircraft may get a prop in the ground and everything associated with that. Instead of
a typical stall warning, the plane when slowed starts a rapid descent interrupted by
the ground. Only extra speed will make the tail more effective and salvage the nose
heaviness. I hope this has prompted you to consider your airplane’s CG and keep it
within limits. As mechanics we can remove and add equipment that will change the
aircrafts empty CG but we can’t change its limits for flight. Staying within those limits
are up to the pilot. What about flying overweight? Some say it’s against the law while
others do it almost every flight. That’s an interesting discussion for next time we
My wife’s eyes have glazed over, so she asks one more time, how can the raven fly
without a tail? So I try again. It’s like the military V-22 Osprey that can change its
center of lift without changing it’s center of gravity by tilting its wing tip rotors.
Watch the wing tip feathers, pinion, of the raven. They can spread out, twist
differently from one side to the other and the whole wing can be straight, swept
back or forward as needed to change the center of lift as needed to compensate for
the CG. Without a tail I bet the landings were fast and a few muscles were sore. But
since they’re not man made, I’m not sure. I also have no idea why the magpie
plucked the raven’s tail.
The raven is known for its intelligence, it’s one bird that really loves to fly. Their
aerobatic skills are unmatched by the best man can do. Now if they would just enjoy
themselves and stay out of the trash. No wonder, when just before he died, Charles
Lindberg was asked, If you had to choose between airplanes and birds, what would it
be? He chose, birds!

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