The cold war was a hotbed of innovation and
technological development that gave us incredible planes like the SR-71 and Fairchild Republic
A-10 Warthog. But it also gave us some of the worst planes
to ever get past the drawing board. The cold war gave rise to several bizarre
aircraft like Avro Canada’s take on the modern day UFO, Northrop’s TACIT Blue stealth
aircraft, which looked like it was designed by a 3-year-old child, or the peculiar Thunderceptor with its booster
rockets motors which was obsolete before it even hit the runway. But perhaps the worst of all, was the relatively
‘normal-looking’ XF-84 Thunderscreech. While it looked normal, it did not sound normal. The Thunderscreech was so loud that it often
made its ground crew physically ill. So loud that the power of the sound waves
emitting from the plane were capable of knocking people over, and on one occasion caused an
engineer working on the plane to suffer a seizure. This incredible noise this plane created is
what gave the thunderscreech its infamous name and to this day, holds the world record
for the loudest aircraft ever made. [1] Today, we are going to learn why this
abomination was created. Sharing what the plane sounded like here is
practically impossible. Recordings simply give us this fairly standard
sounding droning noise. It’s impossible to emulate over digital
audio because the Thunderscreech ungodly noise was caused by shock waves being thrown outwards
by the strange propeller design. The Thunderscreech was a unique plane, born
out of a unique need during the early development years of the jet engine. The navy needed aircraft with short take off
runs so that they could take off from aircraft carrier runways to give them a competitive
advantage near enemy territory. This required something called climb performance,
which jets at the time were not particularly good at, the propeller planes were superior. This means the propeller design allows these
aircraft to take off from aircraft carriers with much shorter runways and climb faster. However when competing with other more powerful
jet aircraft, speed was the obvious limiting factor. That higher maximum power allowed jet engines
to go fast, really fast, and when building an interceptor aircraft speed is important. Republic designed this abomination in an attempt
to get the best of both worlds. Great climb rate, and great top speed. The Thunderscreech was a supersonic propeller
design. A propeller designed to spin faster and achieve
much higher top speeds than a conventional propeller. Creating a propeller capable of providing
the thrust needed at these higher speeds is not easy. The velocity of air around a propeller blade
travelling at constant rotational speed increases steadily from the root of the blade to the
tip. At the tip of the blade, air will be leaving
the propeller disc at a much faster speed than at the root of the blade. Even though the rotational velocity is the
same for all parts of the propellor, the tip of the blade travels a much further distance
than a point near the root of the propellor, and therefore, the linear velocity is much
higher at the tip. As the propeller tips approach the speed of
sound, air becomes highly compressible, and large pressure differences result in bubbles
of unstable air and shock waves around the propeller tips. The disturbed air results in an increase in
drag and a reduction in propeller efficiency and could lead to other complications like
propeller stall. Keeping the propeller efficiency as high as
possible at the expected operating speeds is thus an important criterion in propeller
design. The propeller efficiency can also be thought
of as the ratio of power in the form of thrust produced by the propeller to the power actually
produced by the engine. This graph shows the typical propeller efficiency
profile of a fixed-pitch WWII aircraft relative to speed. [2] We can see that the propeller efficiency
rapidly plummets at higher speeds as air becomes more unstable over the propeller. Propellers spinning at supersonic speeds,
with this kind of efficiency curve, are untenable. If propeller planes were to compete with jets,
engineers need a way to prevent this drop often in efficiency. For this reason, the supersonic propeller
of the Thunderscreech was much shorter in length. Effectively delaying the shock wave formation
for as long as possible, by reducing the circumference the tip needs to travel during a single rotation. This reduces the tip velocity, a propeller
with half the radius will have half the tip velocity, even if it is spinning at the same
angular velocity. (tip velocity = radius x angular velocity)
However, this alone is not enough to prevent the efficiency issues of a supersonic propeller. The propeller of the thunderscreech was rotating
at supersonic speeds even when it was on the runway. The next big difference between conventional
propellers and supersonic propellers is the angle at which the blade is placed relative
to the air striking it . Supersonic propellers are fitted with a much larger angle to the
axis of rotation relative to conventional propellers, and critically the thunderscreech
had a variable pitch propeller, meaning the propeller can change it’s angle of attack
at different speeds to optimize efficiency. To understand this, let’s look at a normal
aerofoil cross section. When it travels forward, it will provide lift
perpendicular to the chord line. If the aerofoil increases it’s angle of
attack, which is the angle between the direction of movement and the chord line, the lift and
drag of the aerofoil will change in magnitude. We can map this with the lift drag ratio,
which is essentially our efficiency curve for our aerofoil, we can see that the wing
generates the most lift, for the least drag penalty at an angle of attack of around 2-4
degrees.[3] A propellor is essentially an aerofoil, but
it’s free body diagram is a little more complicated, because not only is the propellor
aerofoil moving forward relative to the plane's direction of movement, but it is also rotating
around the axis of the drive shaft. Let’s name these two separate velocities. Propeller velocity is the speed of the propeller
rotation, and plane velocity (aeroplane) will be the forward velocity of the plane. These two velocities combine into a new vector
called the blade path. [4] This blade path angle is not at the same
angle as the angle of attack of the propellor blade, but critically, the blade path is the
ACTUAL angle of attack of air travelling over the propellor. To understand this, let’s see what happens
to a fixed pitch propeller when we increase our propeller velocity. Right now, we are at the optimum angle of
attack of around 3 degrees, but if we increase the propeller velocity the angle of attack
begins to increase, reducing the propeller efficiency, the same happens if we lower the
propeller velocity. Both move us away from the optimum angle of
attack. We can set the rotation speed, but remember
when we said that the propeller velocity changes along the length of the propeller, with the
tip having much higher speeds than the root? That means if the propellor was just one fixed
angle of attack only one section of the propellor would be at the optimum angle of attack. So, the propeller blade twists along its length
to ensure each part is working efficiently, but the forward movement of the plane also
affects this angle of attack. For early planes, who’s top speed did not
get that high, having a fixed pitch propeller didn’t matter that much. The forward movement of the plane only marginally
affected the angle of attack on the propeller, but as speeds increased variable pitch propellers
began to become commonplace. Allowing the propellers to rotate and adjust
to higher speeds and maintain the optimum angle. This was essential for the Thunderscreech,
and you can see it has a very large range of rotation for it’s short propellers, to
ensure it can operate at the large range of speeds it was expected to encounter. The blade profile of supersonic aerofoils
are different from normal propellers too. Research into this began in World War 2, as
the engines were getting continually more powerful the prospect of propellers operating
in transonic and supersonic ranges was becoming more and more likely. With little to no empirical data available
researchers were forced to build specialized wind tunnels and measurement technologies
to research how different propellers would operate. They tested swept back tips, after data obtained
from Germany showed its merit, but manufacturing difficulties and marginal improvements discredited
the idea. The engineers, after much trial and error,
landed on making the propeller blades as thin as possible. Tapering from a thickness ratio of 5% at the
base to just 2% at the tip. [5] That is the ratio of the aerofoil depth,
divided by its width. The propeller was also to have sharp leading
edges with little to no chamber. When tested, these propellers were capable
of maintaining their efficiency much higher and over a wider range of speeds. However, even with the propellor efficiency
improved, the drag associated with the formation of shock waves over the entire propeller demanded
an obscenely powerful engine. The XF-84 had an engine horsepower of a whopping
5850hp, nearly 4 times more powerful than a typical Supermarine Spitfire. [6] The engine installed in the Thunderscreech
was unusual. The engine was actually two engines connected
together to power a common drive shaft. This is called a coupled engine. If a suitably powerful engine is not available,
it’s often easier to just couple two engines to reach the total power needed, rather than
developing a completely new engine. The practice does have some other advantages. [7]
The coupled T-40-A-1 engines of the Thunderscreech were placed inside the fuselage. Typically uncoupled engines are wing mounted,
which creates a lot of additional drag, which is avoided when the engines are placed with
the fuselage. Wing mounted engines also come with some control
issues in the event of an engine failure. If one engine stops working our working engine
creates asymmetric thrust, which will cause the plane to yaw in the direction of the faulty
engine. With two coupled engines driving a single
propeller our thrust remains on the centreline, just at half the power. Finally, coupled engines had some fuel consumption
advantages. Turbine engines typically reach their highest
efficiency at higher power settings, but military planes often operate well below their highest
thrust setting. This graph plots the specific fuel consumption
vs horsepower for a single engine. We can see that at 3400 horsepower the engine
has a lower specific fuel consumption than at half power of 1700, meaning it is operating
more efficiently. One of the advantages of coupled engines is
that each engine can run the propeller independently, so if we wanted to run at half power, we could
just turn off one of the engines and run one of the coupled engines at the more efficient
peak power of 3400. Whereas, if we have two separate engines mounted
on the wings, because of that asymmetric thrust issue, we would have to run each at the less
efficient half power setting. Increasing fuel consumption and decreasing
range and loiter time. The powerful coupled engines did come with
some added complexity that caused a lot of issues. The engine was mounted behind the pilot, with
air intakes placed in the root of the wings. This wouldn’t be weird for a modern day
jet engine which derives its thrust from a nozzle at the rear of the aircraft, but this
is a prop plane. The Thunderscreech had to somehow transfer
power from the back of the plane to the front, and it did this with two drive shafts running
on either side of the pilot. Which is a bit terrifying considering these
driveshafts were turning 14,300 revolutions per minute, and caused the plane to violently
vibrate. This RPM had to be reduced before reaching
the propeller, so the driveshafts fed into a 6.8:1 two stage reduction gear. To bring the propeller RPM down to 2100 revolutions
per minute. Much slower, but that speed still caused some
serious issues for control. When the propeller spins clockwise, it creates
an opposite reaction force that could cause the actual plane body to spin anticlockwise. [8]
With the extremely high rotation speeds of this propeller this effect was particularly
pronounced, and became even worse as the plane picked up speed. According to Hank Beaird, a test pilot who
flew 12 flights with the Thunderscreech: It became a handful…when you got it out
around 400 knots,” “The propeller governor would start surging, and the airplane would
roll rather violently.” “The entire airplane, in fact, was trying
to torque itself around the propshaft, rotating left while the prop spun to the right.” [9]
The propeller governor is the mechanical device designed to sense the speed of the propeller,
and control its pitch. At a particular speed it seems that the sheer
inertia of the heavy fast spinning propeller overloaded the control system and caused it
to malfunction, causing the propeller to increase in speed and cause even more torquing motion
of the aircraft. This torque effect was present at all speeds
of course, and the designers of the Thunderscreech were well aware of the problem. This affected many propeller fighters of WW2,
it affected the rotary engines of WW1 even more. Rotary engines rotated around a fixed crankshaft,
meaning the entire engine rotated with the propellor, which helped tremendously with
cooling and helped bring down the weight of the engine for these underpowered aircraft,
but the gyroscopic precession was so strong that the Sopwith Camel had to use left rudder
for both left AND right turns. The torque effect is usually most prominent
at lower speeds where the air travelling over the plane has less power to resist the rolling
motion. Without counter action the torque could roll
the Thunderscreech up to 30 degrees off centre line. Using the ailerons to counteract roll is possible,
but that introduces adverse yaw, where the increased drag caused by the actuated aileron
causes the plane to yaw, so the designers of the Thunderscreech included this anti-torque
vane just behind the cockpit. On take off and landings the vane was deflected
45 degrees, which caused a counteracting lift that acted around the rolling axis of the
plane. Another unique design feature of the Thunderscreech
is it’s T-tail configuration, which particularly stands out because it required a modification
from the F-84F Thunderstreak jet fighter which the Thunderscreech was based on. This was done to get the stabiliser out of
the way of prop wash and the turbulence associated with it, which could reduce authority of the
stabiliser during pitch. The plane was still a serious handful to handle. Leading to one hilarious quote from Lin Hendricks
to Jim Rust, Republic’s chief engineer “You aren’t big enough and there aren’t
enough of you to get me in that thing again.” Test pilots are a unique breed of crazy, yet
this one knew this plane was not safe to fly. 10 out of eleven flights for Hank Beaird ended
in an emergency landing. The engine was so unreliable the plane flew
with it’s ram air turbine constantly deployed just as a precaution. The turbine could use ram air to power it’s
electronics if the engines failed. Between this, the plane violently vibrating
from the two drive shafts spinning at 14,000 RPM on both sides of the pilot, the sudden
rolling due to propeller surges, and ofcourse the immense sound that was capable of knocking
people over, the plane never got past the testing phase. The program was soon cancelled Weird experimental projects like this are
often the only way we can learn new things. Just trying something way outside of what
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