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An extensive program of research and development in the field of disk flight, which was started
in 1952 is being conducted by Avro Aircraft Limited at Malton, Ontario. Early studies
on behalf of the United States Airforce proved the feasibility of a circular planform vertical
take-off aircraft utilizing a system of peripheral jets for propulsion, stabilization and control.
What you are witnessing is one of the most bizarre technologies of the mid 20th century.
The year was 1960. The Cold War was looming over America and within 5 years American troops
will be entering Vietnam. Many of them by helicopter. The helicopter’s role was clear.
A vertical take-off vehicle capable of rapidly deploying behind enemy lines with the need
for runways. It could carry both men and weapons and serve a multitude of roll’s, but it’s
speed and maneuverability left it vulnerable to attack and it could not intercept enemy
planes. The Avrocar was envisioned as a solution. A new type of airframe that could fulfill
both the role of a helicopter and a supersonic jet fighter.
And it all began life as Project 1794 [1] with the goal of creating a vertical take
off and landing aircraft capable of flying at three times the speed of sound with a service
ceiling of 100,000 feet.
This all sounds like it was born from the imagination of alien conspiracy theorists,
but the design was perfectly logical in the mind of John Frost, a renowned and respected
design engineer.
John’s concept for the Avrocar was seeded from his experimentation with combining two
physical phenomena. The Coanda Effect and Ground Effect.
The coanda effect is simply the tendency of a fluid to follow the curve of convex shapes.[2]
Modern planes like the C-17 take advantage of the effect with flaps which can descend
into the exhaust flow of the engines during landings. This would redirect some of the
exhaust downwards to provide additional lift, allowing the C-17 to slow it’s approach
speed and land on shorter runways. [2]
Most online publications on this vehicle overstate the importance of the Coanda Effect, and in
some cases completely confuse it for ground effect. While there are many test videos of
John Frost experimenting with the effect with small prototypes, the effect does not get
a single mention in the initial secret white paper, where early designs were presented.
There were multiple concepts drawn up for the Avrocar, but they all centred around on
basic idea. Direct high pressure air 360 degrees outwards and use shutters and control surfaces
to redirect it for control. The coanda effect was not a feature for these kind of devices.
The initial blue sky design called for a completely novel engine layout which would not use conventional
jet engines for power. Instead there would be one large diameter engine surrounding the
pilot. With the compressor stage mounted on the inner diameter, the turbine mounted on
the outer diameter with a combustion chamber in between. A major change from typical jet
engines which have these stages mounted sequentially over a common axel.
This new engine would be an ideal use of space. With fuel tanks mounted in a ring around the
pilot, which presented a massive fire hazard, but did keep the centre of gravity as close
to the centre of pressure as possible.
However, designing an entirely novel engine for the prototype would have been far two
costly, so a compromise design was created for the early prototype designs.
Commercially available turbo jets would be mounted radially around the disk in a repeating
pattern. This created a radially symmetric design, that would allow for repeating sections
identical in construction. This was a huge advantage of the flying disk design as it
would reduce the number of parts needed thanks to the radially symmetric design resulting
in repeating parts. This would drastically reduce manufacturing costs.
These turbojet engines would be mounted around a central turbine. The outlet pressure of
these turbojets would drive this turbine in the middle section, and it’s remaining kinetic
energy would be used to provide lift by exhausting it directly downwards.
Meanwhile the rotation of this turbine was powering two impeller levels mounted above
and below the turbine. These centrifugal impellers would draw air from intakes above and below
the disk and accelerate it radially outwards through ducts. Some of this air would be redirected
back into the inlet of the turbojet engines to maintain a constant ram pressure, which
is the pressure we typically associate at the inlet of a jet engine as an aircraft forces
itself through the air.
For this design I don’t see much of the coanda effect being used as the air has no
other option than to exhaust in the direction the shutters allow them to.
This was the general concept behind the Avrocar, but the two prototypes that were actually
built were much lower in spec. With 3 turbojet engines instead of 6. Here they were mounted
tangentially, rather than perpendicularly to the inner turbine which would drive the
inner turborotor, which would work similarly to the F-35’s direct lift rotor by directing
some of this air directly downwards, however it differs from the F-35 as some of that air
was expelled outwards.
For these prototypes the coanda effect may have played a larger role, as some air was
directed over the outer surface of the aircraft.
However, this prototype was underpowered. With a huge amount of energy lost through
friction drag in the ducting system and through powering the turborotor. Using a geared drive
system, similar to the F-35 would result in far less friction and energy loss.
This prototype relied completely on the lift boosting effects of ground effect to achieve
lift off. Ground Effect is simply a boost in lift when an aircraft is close enough to
the ground to create a high pressure cushion of air between the vehicle and the ground.
[Reference Image 1]
But this created some issues for the prototype. Here you can see John Frost, maybe unintentionally,
demonstrating the first with his early tests. He lifts the prototype out of the ground effect
zone where it no longer has enough lift to keep it aloft and then let’s go, allowing
the scale model to drop back into the ground effect zone where the high pressure air acts
like a bouncy castle. The vehicle begins to bob up and down until the motion has finally
been dampened out.
This is a bobbing motion can be an annoyance, but a far larger issue was an aerodynamic
instability the designers of the avrocar dubbed hub-capping that it could cause. [3]
It was named after the motion a hubcap, or any circular shape, would make as it rotated
around it’s rim. You can see the pilots of the Avrocar struggling to control the vehicle
in most of the archival footage of it’s testing. They likened it to trying to balance
on a beach ball. A constant physically and demanding task.
This was again caused by how ground effect interacted with the aircraft at various altitudes.
Ground effect means that lift rises the closer to the ground you are, but this causes issues
with aerodynamic stability when the Avrocar was tilted. Here the right side of the vehicle
comes closer to the ground and the left moves further away. This causes the center of pressure
to shift to the right as the lift shifts to the right, this however did not act as a restoring
force to keep the avrocar level, as the vehicle needed to keep its nose down in order to move
forward which shifted it’s centre of weight forward. Instead it resulted in the avrocar
swinging around it’s pitch and roll axis in that hubcapping motion.
Over the next year the team worked on fixing this issue. The turbofan itself was intended
to provide stabilization. [4] The turbofan would desire to remain in a horizontal position
through gyroscopic action. So the engineers developed a system where they could couple
this action to the controls at the outer rim of the aircraft. They did this by mounting
the turbofan to a gimbal, allowing the avrocar to freely rotate around the turbofan. If the
avrocar deviated form the horizontal position it would cause linkages connected to the gimbal
system to move and shift the control surfaces to correct the tilt. As clever as this system
was, before the age of fly by wire technology, it could not compensate for the inherent instability
of the aircraft.
The prototype was modified over the testing period with different control mechanisms,
but none of them could solve this instability problem.
They gyroscopic effects were having negative effects elsewhere too. The Avrocar took 5
seconds to rotate 90 degrees counterclockwise, which is too long to begin with, but it took
over twice as long at 11 seconds to turn it clockwise against the rotation of the turborotor.
John Frost attempted to address some of the problems in 1961. Designing this variation
of the Avrocar which included a wing and a tail to provide lateral and pitch stability,
while moving the cockpit to the front of the vehicle.
Ultimately he was too late. The design was ditched as it was simply converging into a
plane with VTOL capabilities which were in research at that time. Like the Harrier Jump
Jet which would fly it’s first flight in 1967 and enter service in 1969.
Today we can see aspects of it’s design being used in the F-35 with the turborotor,
while instability issues with hovering in ground effect are barely an issue for this
aircraft with advanced computer control reacting to wobbles before the pilot has time to even
notice them.
Who knows, if the design was revisited today the flying saucer could truly exit the realm
of science fiction. After all, most of science fiction is based on true science potential.
We can learn a lot about science through the exploration of hypotheticals in audiobooks
like We Are Legion (We Are Bob). Easily one of the most enjoyable science-fiction series
I have ever listened to on Audible.
We Are Legion (We Are Bob) which explores what artificial intelligence combined with
Von Neumann Probes, or self replicating spacecraft would look like. This book tells the story
of a young software engineer who dies and finds his personality has been uploaded to
a space probe destined to explore the universe and battle with space probes from competing
nations. The book is really funny as we witness a man trying to keep himself entertained with
only ever stranger versions of himself to keep him company.
Easily one of the most entertaining and informative audiobooks I listened to last year.
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