10 things you need to know about the P-47 Thunderbolt
Top Ten Things about the P-47
The P-47 Thunderbolt was the most versatile fighter aircraft of the Second World War. It was not “best in class” in any category, however, it was very good in just about every category. Fast, long-legged and well armed, it was also the most survivable fighter of the entire war. In scale and concept, the P-47 was a design outlier, and was different from every other fighter of the war. Somehow, the unusual Thunderbolt came together in a magical way that resulted its outstanding operational record. Myth and misconception surround the aircraft, and much of this leads to an under-appreciation of what the aircraft actually accomplished. This article will dive into some of those misconceptions and look at the real capabilities of those ferocious flying juggernaut.
Before we get started, I will address the elephant* in the room: the P-47 is ugly. There, I said it. Many often state that if an aircraft l***s good, it will f*y good. No one ever talks about the corollary, that if an aircraft is ugly, it will not fly well. This is never mentioned because it is not true. And besides, beauty is in the eye of the beholder. There are plenty who love the P-47 due to its rugged, badass looks. It’s in a similar category to the Westland Wyvern and the A-10 Thunderbolt II (the P-47’s grandchild!). Unfortunately I think the (lack of) sex appeal of each of these aircraft has limited their popularity in a wider public (‘P-47 Thunderbolt’ will garner 8 million results of Google search results, compared to over 20 million for the P-51 Mustang).
But aircraft performance and effectiveness is not a popularity contest. The facts stand on their own. So Let’s dig in!
*Note that the heaviest elephant in the room is a metric ton lighter than the P-47.
- The Size is the Prize: Big doesn’t mean sluggish
The P-47 is the largest single piston-engined fighter ever built. And it is indeed large and heavy! Its empty weight is similar to or greater than every other contemporary single-engine fighter’s maximum weight. It is over twice as heavy as the original Spitfire and Me 109 fighters. There is a common misconception that large size and weight correlates with low performance. The perception is that a larger aircraft will be slow, lumbering, unmanoeuvrable, and ineffective. I believe this idea comes from comparison to other large aircraft. And indeed, most larger aircraft (bombers and transports for instance) do fit this description. But that is not specifically due to their size; it is due to their design attributes.
There are two primary metrics that affect a fighter’s performance: wing loading and power to weight ratio. The first of these states how large the wing is relative to the weight of the aircraft, and is measured in pounds per square foot or kilograms per square meter. A larger wing can produce more lift and it determines how aggressively a fighter can turn at a given speed. In general, the lower the wing loading (i.e. the larger the wing), the more “manoeuvrable” a fighter will be. Wing loading is a compromise though. The larger the wing, the more drag it produces. So a really low wing loading limits the top speed of an aircraft. Both manoeuvrability and speed are the key performance qualities of a good fighter.
The second parameter, power-to-weight ratio, states the power of the engine relative to the weight of the aircraft, measured in horsepower per pound or kilowatts per kilogram. This parameter is directly related to the top speed of the aircraft, but also to the takeoff and climb performance. A high power to weight ratio results in an aircraft that will climb and cruise faster than a competitor with a lower ratio.
The bottom line is that size and weight of an aircraft do not determine the aircraft performance. It is quite possible to build a large and heavy high-performance fighter, as long as it has the right sized wing, and a big engine. Let’s look a little deeper at how the P-47 used each of these metrics to its advantage.
- A Beast of a Fighter needs a Beast of an Engine
““It will be a Dinosaur, but will be a dinosaur with good proportions” – Alexander Kartveli, Chief Designer of the P-47
The P-47 had much smaller and humbler beginnings. The concept started in the late 1930s as two different evolutions of the successful, but dated, P-35 design. The company delivered a small number of P-43’s with a similar design and layout, but using a much smaller R-1830 engine. The next iteration, the P-44, was to use the liquid-cooled Allison V-1710, but was never built. In late 1939, after Germany started its rampage in Europe, the US Army Air Corp had a meeting to discuss the noncompetitive state of the current and coming US fighters. The conclusions that emerged from the meeting was that the Air Corp desperately needed a fighter with more speed, more armor, and more firepower. Alexander Katveli, the Chief Designer of Seversky Aircraft happened to be in attendance. On the train ride home, he sketched an up-scaled version of the P-43 with the goal of meeting this newly desired aircraft specification.
More armour and armament means more weight. To be a competitive fighter, the engine power would have to grow as well. Katvelli turned to the new R-2800 twin-row radial engine being developed by Pratt & Whitney as the only viable powerplant. This engine had over 50% more displacement than the Allison, and initially had 33% more horsepower. A second problem that plagued US fighters of the day was the lack of sufficient power and performance at higher altitudes. Most of the contemporary American engines used a single-stage, single-speed supercharger. This design leads to significant falloff in power at altitudes starting around 15,000 feet. The combat experience in Europe was showing that the air battles were often going to be fought much higher than this.
The R-2800 engine had a single-stage supercharger which was integral to the engine. The P-47 design added an exhaust-driven turbo-supercharger to create a second stage. A turbo-supercharger is effectively a variable speed device, being engine exhaust-driven and having a wastegate control system. This allows peak engine power throughout a wide range of altitudes. By comparison, the famous Rolls-Royce Merlin engine used a 2-speed, fixed-ratio supercharger which provided peak power at only two specific altitudes.
The downside of a turbo-supercharger is that the installation occupies a lot of space. Fortunately the P-47 was already being drawn as a large fighter, and Kartveli was able to squeeze the turbo and even larger intercooler into the belly of the aircraft. This packaging is the primary reason the aircraft looks more like a pregnant whale rather than a sleek fighter, thus contributing to the perception of it being a dog.
Later versions of the R-2800 engine increased the boost pressure of the supercharger system and, along with the use of water-methanol injection, could deliver over 2,800 HP.
- Great, you got a big engine – but can you turn?
Manoeuvrability is a term that gets tossed around a lot. It’s a simple concept. But when applied to a fighter, it quickly becomes complex. There are at least four aspects that affect what we call “maneuverability.” First is the aircraft’s roll rate, or how fast the pilot can bank the wings when choosing to fly in a different direction. Several aircraft details determine this parameter including the wing span, the size and design of the ailerons, and the weight distribution of the aircraft. (Aircraft with twin-engines on the wings have reduced roll rate.) I think roll rate is somewhat overrated in assessing maneuverability. In most dogfights, a majority of the time is spent in sustained turns, not switching turn direction. But a pilot will always be happier with an aircraft that responds quicker to control inputs.
The most important manoeuvrability aspect is the turn rate. However, there are really two versions of this: instantaneous and sustained. Instantaneous turn rate is the maximum rate at which you can turn as soon as you roll to a given bank angle and pull back on the stick. The force to create this turn is created by wing lift, and in a fighter it can be a LOT of lift. The aircraft wing is what determines instantaneous turn rate. A large wing and/or a well-designed wing can produce a lot of lift, and therefore a high turn rate. Unfortunately this lift creates a lot of extra drag. As soon as you start this maximum performance turn, you start slowing down. This leads us to the second turn rate parameter: maximum sustained turn. As you slow down in a turn, the drag will decrease and eventually you will reach an equilibrium point where drag matches the thrust your engine can produce, and you can sit here turning until your fuel runs out. This turn rate is lower (usually much lower) than the instantaneous rate. But this is where many dogfights culminate – slower, sustained turning battles with the better aircraft and/or pilot out-turning the opponent and claiming victory.
A fourth manoeuvrability aspect is the difference between turn rate and turn radius. These are related, but not the same thing. The relation is affected by the speed of the aircraft. When flying slower, a given turn rate will result in a smaller turning radius. The relationship between these three (speed, rate, radius) complicates a dogfight. This can be used to an advantage if you can force your opponent into a speed range where your aircraft is superior in either rate or radius and you maneuver accordingly.
A final comment on manoeuvrability – it is greatly affected by altitude. Both the aircraft aerodynamics and the engine performance change with altitude. The balance of these determines the aircraft manoeuvrability. An ace aircraft at one altitude might be a complete dog at a different altitude.
Let’s get back to the P-47. How did it rate in the manoeuvrability category? The basic answer is “pretty good.” Many fighters could outturn it, especially at lower altitudes. But the margin was small – small enough that a better pilot in a P-47 could often outturn a lesser pilot in a Me109 or Fw190. (It takes a lot of skill to precisely fly a fighter at its maximum turn performance.) The P-47 wing loading was not as low as some of the higher-performance fighters and its instantaneous turning ability was a little less because of this. However, the big R-2800 engine really helped its sustained turn performance. This is particularly true at higher altitudes. The turbo-supercharger provided tremendous power at 25,000 feet and higher. This allowed the aircraft to be on par or better than any of its opponents in a sustained fight at these altitudes.
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A quick note on the P-47 wing. You will often hear this called the “famous Seversky wing.” The lineage of Seversky aircraft dating back to the early 1930s shared a common wing design in both planform and a proprietary airfoil. The planform is elliptical, which is a theoretical optimum for reducing drag due to lift–a critical factor for fighter turning performance. However, you can get an almost equal performance with other wing shapes that might have other benefits such as easier manufacturing or better stall characteristics. At first glance, the Seversky wing looks similar to the elliptical Spitfire wing. The primary difference is the chord layout in the planform view. The P-47 has a straight leading edge, which is much easier to manufacture. All of the elliptical curvature is pushed to the trailing edge control surfaces. These are easier to build with curved edges. (The aerodynamics performance is essentially the same.) The P-47 and Spitfire designers have both commented that the elliptical wing is not just about aerodynamic efficiency. It also provides a thickness distribution that is convenient for both installing large guns, and providing structural efficiency. For all of these reasons, the P-47 did indeed have a very good wing.
- Range – What good is all of this if you can’t get to the battle?
The range of the P-47 is probably the most misunderstood part of its performance. You often hear about how Mustangs could exclusively escort bombers all the way to Berlin, and thus turned the tide of the war. The real story is much more complicated and steeped in Air Corps politics. The reality is that the P-47 could also escort to Berlin and further much earlier than many people think.
Aircraft range is a simple parameter affected primarily by two things. First is how efficient the aircraft can fly at given speed and altitude measured by how much fuel it needs per hour. Second is how much fuel it can carry. The P-47 does burn a lot of fuel with its large size and weight. However, this larger size also allows it to carry proportionately more fuel. Per the pilot’s manual, the P-47D with maximum internal fuel could fly 890 miles (compared to 1,120 for the Mustang). With two drop tanks, the Thunderbolt range increases to 1,360 miles, leaving plenty of margin for the 1,000 mile round trip from England to Berlin.
So why does everyone think the P-47 can’t reach Berlin? Much of this misconception comes from some range maps that were published at various times by the US Army Air Corp which show the Mustang having superior range. I highly recommend watching the “Greg’s Airplanes and Automobiles” YouTube episode on the range of the P-47. Piecing together a lot of historical data, he tells a compelling story about how this myth evolved. Early in the war, the US bomber doctrine was anchored on the idea of bomber self-defence. It was believed the combination of speed, altitude, and defensive firepower would ensure the bombers would get through to the target without the need for fighter protection. Because of this, the Air Corp leadership downplayed the need for long-range fighters in an effort to spend more funds on bomber development. This even went as far as showing no interest in the development of fighter drop tanks.
Reality prevailed once the bombing effort commenced in Europe and it was quickly learned that the bomber did indeed require fighter escort to minimize bomber losses. The Air Corp leadership was slow to learn or at least admit this lesson which likely explains the existence of these range maps with misleading information. Eventually all of this caught up, drop tanks were used, and long range escort mission were flown by both the P-51 and P-47.
The final P-47N version had a redesigned wing with internal fuel and larger drop tanks, enabling an escort range in excess of 1,000 miles. This version was developed to escort B-29s in the Pacific theater. Given this extended range, and the consistent performance at high altitudes, the aircraft was incredibly effective in this role.
- But how fast can it go?
In level flight, the Thunderbolt top speed performance is much like the rest of its performance envelope. It was not the fastest aircraft at any altitude. But it was very fast and was quite competitive in top speed. This is particularly true at high altitudes, where its engine produces superior horsepower. At 30,000 feet, the P-47D was within 7 mph of catching a P-51D, and significantly faster than an FW190A-8 or Bf109G.
The final P-47N version was extremely fast, again, especially at high altitude. Its R-2800 engine could produce 2,800 horsepower with water-methanol injection and this resulted in a top speed of 470mph.
To fully appreciate the speed capabilities of the P-47, you have to talk about its ability to dive. We will dig into that next.
- Dive Dive Dive!
Donald Balkeslee, the first Thunderbolt ace is quoted saying “It ought to be able to dive, because it certainly can’t climb.” We will begin this topic with a brief discussion of the physics of diving.
Just like manoeuvrability, the ability to dive is complex and affected by many parameters. At the beginning of a dive you have to manoeuvre the aircraft into a diving attitude, either by pushing the nose over, or rolling upside down and pulling the nose down. Once you are in a nose-down diving attitude, the aircraft will accelerate due to the relative balance of aircraft weight, drag, and thrust available. All of these affect the acceleration rate, and your ability to pull away from (or catch) an opponent who is also diving. Depending on the dive angle, at some point your aircraft will reach a speed limit, which requires you to reduce the dive angle to prevent exceeding the limit. At any given altitude, this limit might be set by the indicated airspeed or the Mach number (the speed relative to the speed of sound). The relationship between these two limits is complex, and it changes as your altitude changes. Finally, a dive is affected by the controllability of the aircraft as it reaches higher speed, and the ability for the pilot to maneuver out of the dive before impacting the ground.
The P-47 was excellent in every one of these dive aspects, which makes it one of the best diving aircraft of the period. With its throttle-body fuel injection, the engine did not suffer from negative-g fuel starvation like the early Merlin engines. So a pilot could freely push the nose over to initiate a dive without the fear of the engine cutting out. Once in the dive, the P-47 accelerated quickly due its engine power and reasonably low drag. Best of all, the Thunderbolt had one of the highest dive speed and Mach limits of any fighter of the era. So the aircraft could keep accelerating away from an opponent. Finally, the P-47 was highly controllable in a dive, with little concern about Mach effects reducing the control authority. Many fighters of the era had Mach limits set with margin to a flight condition from which you could not recover. Most models of the P-47 also had a dive brake, which allowed an even steeper dive angle, and more recovery margin at the end of the dive.
“The top-10 Thunderbolt Aces all survived the war. This is a statistic, not shared by any other aircraft in World War 2”
This dive ability was a real benefit for P-47 pilots and contributed to its high survivability rate. If a pilot was found in a condition where the aircraft performance was less than an opponents, diving away from the opponent was almost always an option. This would usually result in escaping the immediate threat.
- Big Aircraft Can Pack a Punch
We have talked about how the size and weight of the P-47 didn’t really hurt its performance due to the well-chosen design parameters and a large engine. Now let’s talk about what that extra weight allows you to do. The short answer is you can carry a lot of armament!
The P-47 had eight Browning M2 50-calibre machine-guns, four in each wing, whereas most contemporary fighters had no more than six fifty cals. The Thunderbolt sometimes gets criticised for not having any larger-calibre cannon. The aircraft certainly could have carried them. However, for its intended mission of destroying enemy fighters, the 50-calibre round is quite effective, and the combined firing rate of its eight guns meant a much higher probability of getting hits than with a slower-firing cannon.
The aircraft could carry 3,400 rounds of ammunition which, for comparison, was 65% more than a P-51. This high quantity of ammunition resulted in an interesting evolutionary role for the aircraft as the war progressed. At the end of an escort mission, the pilots often found they had significant unused ammunition. A tactic was developed for the pilots to then drop to a lower altitude and strafe ground targets of opportunity on the flight home. With eight guns and good diving qualities, the aircraft proved highly effective in this role. As the Luftwaffe became less of a threat late in the war, P-47s were more often assigned to ground attack as a primary mission because of their effectiveness in this role.
In addition to the guns, the P-47 could carry a heavy load on its external hard points. Between two wing racks and a belly rack, the aircraft could carry up to 2,500 lbs of external weapons. Initially this mostly consisted of bombs. Later, the aircraft was equipped to carry ten “High Velocity Aircraft Rockets” or HVAR. These unguided rockets did not have pinpoint accuracy, but were quite effective against larger ground targets. Each had a 45-pound warhead which was equivalent to a 105-mm Howitzer artillery round.
The P-47 was the first aircraft to drop a Napalm munition in combat. The original version of this was fabricated in the Pacific theater using drop tanks. This of course went on to infamous use in later wars. The aircraft could also carry a chemical dispersal pod. This is rarely discussed for political reasons. But the capability was in place for use of such a weapon late in the Pacific war if it came to that.
- Protect the aircraft, Protect the Pilot
One of the original goals for a future fighter, as defined by the Army Air Corp in that 1939 meeting, was an increase in armour and aircraft systems to improve the survivability of both the aircraft and the pilot. The P-47’s size and weight made this feasible.
The P-47 had thicker armour in more locations around the pilot than any other contemporary fighter. There were a lot of other features on the aircraft that offered protection and made the aircraft more survivable in a crash landing. With a smooth belly free of cooling features, the aircraft responded well in a gear-up landing with minimal tendency to overturn. Additionally the fuselage contained an internal belly skid structure that contributed to survivability in a crash. It also made the aircraft more repairable after the crash. The original razorback P-47 had a substantial roll-over structure behind the pilot. This was omitted in later versions with the bubble canopy due to a low occurrence of roll-over accidents and the added benefit of better pilot visibility.
An aircraft with an aircooled-engine is generally considered more survivable than a liquid-cooled counterpart. The cooling lines and radiators are particularly vulnerable in that the engine will likely quit soon after a single hit to one of these systems. By comparison, an engine can typically run much longer after a hit to air cooling ducts, or even the turbocharger (at reduced performance).
All of these resulted in a very survivable aircraft. The top-10 Thunderbolt Aces all survived the war. This is a statistic, not shared by any other aircraft in World War 2.
- Combat Effectiveness
All of these factors contributed to the P-47 being a very effective fighter in the Second World War, against both air and ground targets. The overall aircraft destroyed and the kill ratio for the P-47 was impressive. By the end of the war, the Mustang had beat the Thunderbolt in both of these categories, by a healthy margin. However, these metrics alone don’t tell the complete story. You have to factor in both how and when the aircraft was used.
The Thunderbolt started flying significant combat missions in Europe in the Spring of 1943. This was around 9 months before the Mustang arrived. During this time, the Luftwaffe was at the peak of its experience and effectiveness. This provided a very challenging opponent for the Thunderbolt pilots. By the time the Mustang was flying in significant numbers, the establishment of Allied air superiority was setting in, and many of the more experienced Luftwaffe pilots were no longer in the fight. Additionally, the German aircraft had a more difficult time keeping up with the performance advances of the Allied aircraft due to a shortage of certain materials and high-octane fuel. Before the Allied invasion on D-Day, The P-47 had amassed over three times the combat flight hours of the P-51, and 50% more than the P-38.
A second factor is how the aircraft was used. Due to its superior firepower (and a reduced threat from the Luftwaffe) more P-47 sorties were ground attack missions in the later stages of the war. While these missions were highly effective and greatly appreciated by the ground forces, they don’t contribute to air-to-air statistics, which is how most people judge a fighter’s effectiveness. However, there is much more to the story than these simplified statistics.
- Closing Thoughts and Legacy
The above facts and commentary make the case for the P-47 being one of the most well-balanced, versatile, and effective fighters of the Second World War. It excelled at every mission it was assigned, and proved to be a well-loved aircraft by both the pilots and the mechanics supporting the aircraft.
Between its size, and complexity of the turbo-supercharging system, the P-47 was an expensive aircraft to produce. It cost over 50% more to build than the P-51. Despite this, the P-47 was (and likely will remain) the highest-produced fighter in US history. It is a strong contender on the world-wide fighter production rankings as well, coming in at number 6.
`The P-47 left quite a legacy in years to come. Most aircraft companies (like musicians and authors) tend to attempt a repeat of a previous winning formula when embarking on an updated endeavour. This was certainly true of Republic Aviation. After the war the company switched over to the development of jet aircraft. The first-generation F-84 was only partially successful, but the company learned a lot about this new form of propulsion and how to properly design a fighter around a jet engine. The next aircraft, the F-105 was a very successful ground attack aircraft. Following the P-47 family heritage, it became the largest single-engine jet fighter ever produced, and had similar traits of heavy firepower and strong armour. The final success story with the same lineage was the A-10 Thunderbolt II, for much the same reason.`The P-47 left quite a legacy in years to come. Most aircraft companies (like musicians and authors) tend to attempt a repeat of a previous winning formula when embarking on an updated endeavor. This was certainly true of Republic Aviation. After the war the company switched over to the development of jet aircraft. The first-generation F-84 was only partially successful, but the company learned a lot about this new form of propulsion and how to properly design a fighter around a jet engine. The next aircraft, the F-105 was a very successful ground attack aircraft. Following the P-47 family heritage, it became the largest single-engine jet fighter ever produced, and had similar traits of heavy firepower and strong armor. The final success story with the same lineage was the A-10 Thunderbolt II, for much the same reason.
Joe Wilding was the co-founder of Boom Supersonic, an independent company attempting to build a supersonic transport aircraft. He is Chief Engineer at Cosmic Aerospace, Engineering Mentor and Coach.