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
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