Aerospace Engineer Joe Wilding examines why the Mustang was so good.
“I want to start with an embarrassing confession. With a lifelong obsession with aircraft and a few decades of experience engineering them, you might think I’d have an insightful and obscure favourite airplane (or ‘aeroplane’ as Hush-Kit would have me say). But the answer will disappoint you. Like many, I absolutely love the P-51 Mustang. I have tried hard to come up with a more sophisticated answer.
Despite some strong contenders, I keep coming back to the Mustang. The Mustang is the Jimmy Stewart of airplanes. Both are solid performers with dashing good looks, and few have a negative comment for either one. The Mustang’s graceful curves draw you in. The performance and technical details are what keep me coming back. It’s the best of both worlds: a stunning piece of art and a technical marvel. While the art claim is uncontestable, let’s dive a little deeper into the technical claim.
10. The Wing is the Thing
Almost any discussion about the Mustang starts with its wing. But the real advantage of this wing might be different from what you’ve heard. Because of its mid-war development timing, the Mustang was the first aircraft to implement a new aerodynamic theory called “laminar flow”. This effect, contrasted to turbulent flow, is an elusive condition that is possible with the right wing shaping and attention to detail. All wings have a small amount of laminar flow at their leading edges. A laminar flow design extends this region to a majority of the wing surface and can reduce the wing drag by an incredible 25%-50%. Unfortunately, the practical application usually falls short of the theoretical promise. Manufacturing flaws, battle damage, hangar rash, and bug guts alter the surface of a wing, and the laminar flow benefits fade away when the shape is not pristine.
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However, the Mustang wing has another aerodynamic trick up its sleeve. By complete serendipity, the shaping of a laminar flow airfoil is also very good for low drag at high speeds due to Mach effects. The airflow over a wing accelerates even faster than the airspeed of the aircraft. At high enough aircraft speed, this local wing airflow will exceed the speed of sound. When this happens, drag increases to really high levels, really fast. To keep accelerating you need a lot more engine power. The shaping of the Mustang wing lessened this airflow acceleration and allowed it to fly a little faster before this drag started its dramatic rise. In modern terminology, the Mustang had a great transonic wing. This effect was not fully understood in the early 1940s, but the science quickly followed up. Nearly all jet aircraft that followed (and a few piston-engined fighters) capitalized on this idea.
A final note about wing design relates to its size. Going fast and far generally favors a small wing. Maneuvering and turning hard favors a larger wing. If you look at the wing loading (aircraft weight divided by its wing area) of various contemporary fighters, you’ll find that the Mustang was in the middle of the pack. This was the perfect choice given its balanced mission of dog fighting performance, at long range.
9. Very Low Drag
In addition to the great wing design, the Mustang also had a remarkably low overall drag. The performance of the airplane is the summation of many little things and a close attention to detail. A close inspection of the Mustang will show its remarkable cleanliness. There are virtually no bumps, bulges, or inlet scoops anywhere on the plane. Most other planes of the era have a multitude of these to address various cooling or system details. The landing gear is a great example. The main gear is fully enclosed by gear doors that fit well. The tailwheel is also retractable, unlike most aircraft of this era.
Cooling drag is another area of excellence for the Mustang. Many sources will talk about the belly radiator system producing thrust by adding heat energy to the airflow. The test data shows that very little positive thrust is ever produced. However, in most high-speed conditions, the system is producing no net drag (i.e. the thrust produced is offsetting the drag of the scoop and the airflow through the radiator). This effect is important, as cooling drag at high power can be quite large. Several clever details contribute to this high-efficiency cooling system. First, the cooling duct contains the combined heat exchangers for the engine cooling, the aftercooler, and the oil cooler. This maximizes the energy transfer with minimal impact to the aircraft. Secondly, the installation of the cooling system is nearly perfect. The inlet is below the wing, maximizing the inlet air pressure. It is also offset from the wing surface which bypasses the low energy boundary layer airflow, which would decrease efficiency. The aft fuselage allowed for a large radiator, which reduces internal flow losses. The airflow exit is behind the wing where the fuselage starts to taper back, placing it in a region of low local air pressure. Finally, the system has variable outlet doors, controlled by an automated system in all phases of flight. Together all these details promote ideal flow through the system and maximize efficiency.
One last detail which results in low drag / increased thrust is optimal shaping of the engine exhaust stacks such that the exhaust airflow produces net positive thrust. Test data shows that at high-speed cruise, this effect increases the net thrust by 20% or more.
Low drag is not just about going fast. At any speed, lower drag means lower engine power, and lower fuel burn. This translates into an increase in aircraft range, which we will talk about next.
8. It Goes the Distance
Most fighters before the Mustang were not designed for significant range. Early in the war, fighters were used mostly as bomber interceptors and the air battles would take place near the fighter’s home airfield. The exception to this was fighters that were used in a ground attack role or for other missions such as photo-reconnaissance. This was the initial design mission for the Mustang, and hence the aircraft was designed for greater range. As the Mustang was entering service in 1943, the US Army Air Corps was executing a daylight bombing strategy and was starting to suffer heavy bomber losses to enemy fighters. It began using the Mustang for long-range bomber escort. The Mustang’s low drag meant it could cruise with lower fuel burn than other contemporary fighters, and thus fly even further on the same fuel. Range was further increased with an additional fuselage tank and wing drop tanks All of this gave the Mustang sufficient range to escort bombers round trip anywhere in Germany and even into Eastern Europe. Several other long-range fighters like the P-38 and P-47 approached this capability. Although the Mustang has less maneuverability compared to some of the lighter fighters, like the Spitfire and Bf 109, it was superior to the P-38 and P-47 which were larger and heavier. The Mustang had the right balance.
7. Amazing Development Story
The Mustang has a great origin story. Prior to World War Two, North American Aviation had developed a family of successful training aircraft. They desperately wanted to get into the lucrative fighter business, so they started building a team to accomplish that. At the outbreak of the war, they had the early concepts for a fighter that incorporated much of the latest thinking in fighter optimization, including the radical new wing concept. In early 1940, with the war starting to heat up, the British urgently wanted more fighter aircraft, and asked North American to build P-40’s under license. North American, seeing this as the chance to finish their original fighter design and have an immediate customer, essentially told the British, “Hold my beer.” The British took a gamble on the offer, and magic ensued. There are three common ingredients for a successful rapid aircraft development program: a team with the right expertise, the necessary funding and tools, and a challenging but realistic deadline. The North American team had all three and it was off to the races! Based on the team’s prior research and design work, they rolled out the prototype aircraft in an unprecedented three and a half months, and it flew a few months later. To be clear, the design wasn’t perfect, including an early crash of the prototype. But it was very good. The team was able to work out the bugs and was delivering the initial aircraft to the British in October 1941, roughly 18 months after the contract was signed.
- 6. The Merlin!
If the Mustang is Jimmy Stewart, then the Merlin engine is June Allyson. A match made in heaven! (And yes, I get the irony in that analogy.) It is an engine that is loved by everyone, and for good reason. Don’t worry Allison fans (the engine, not the actress), you’ll get your due credit shortly. As the Mustang entered service, everyone was impressed with its solid performance, particularly at low altitudes. However, the original Allison engine had a single-speed, single-stage supercharger, and its power output dropped off quickly above 15,000 feet. The most recently upgraded Merlin engine had a two-speed, two-stage supercharger that could shift gears and continue producing high levels of power through much higher altitudes. Both the British and Americans started speculating on what the aircraft could do with the Merlin engine, and teams in both countries started projects to retrofit a Mustang with the engine. The British were slightly ahead, and their prototype flew a month before the Americans in October of 1942. Both teams quickly realized it was the right combination, with high-altitude performance that exceeded any other fighter of the day. A vast majority of the Mustangs built used the Merlin engine, and its amazing success as a high-altitude bomber escort is because of this engine pairing.
- 5. The Allison!
I don’t know the best comparison for the Allison engine, but maybe Cinderella? It certainly gets overshadowed by its boosted step-sister, the Merlin. The base engines are remarkably similar, with the later Merlin supercharger being the primary difference. And the Merlin did have a truly world-class supercharger design that the Allison lacked. However, the benefits of the Merlin supercharger are best at higher altitudes. For any missions where low altitude was required, the Allison was a worthy contender. In fact, at lower altitudes, the Allison was slightly superior, due to the pressure mapping of the supercharger. For missions like ground attack (the original role for the early Mustangs), the Allison was a nearly perfect engine and the aircraft operated exceedingly well in that role. This is reflected in low-altitude speed and climb rate data. Although, these comparisons are hard because some later Merlin-powered Mustangs with higher boost could beat the Allison at all altitudes.
The Allison probably doesn’t get proper credit because the P-47 entered service soon after the Mustang and, with more armament, it was an even better ground attack aircraft. (The-P-47 was also an effective high-altitude aircraft with its unique engine / turbocharger installation.) Due to the urgent need for long-range, high-altitude fighters, most Mustang production transitioned over to the Merlin-powered model which quickly eclipsed the Allison-powered Mustang. And that is a shame. I am a fan of both aircraft, for different reasons, and I wish the Allison got a little more love.
- 4. Bubble canopy
The Mustang was one of the first aircraft to sport a fully unobstructed bubble canopy. This innovation came to reality mid-war as polymer plastic material science matured. One of the first iterations of this technology pioneered on the Spitfire, was called the “Malcolm hood.” This was a hybrid solution that replaced the centre canopy section with a slight bubble Perspex canopy that contained no obstructions. This gave better visibility both laterally and vertically. But since the basic airframe was not modified, the aft view still suffered. This Malcolm canopy was retrofitted to B and C model Mustangs soon after the Spitfire. Both aircraft were going through a rapid evolutionary cycle and the next version of each aircraft (the Spitfire Mk VIII and the P-51D) took this idea a step further. In both cases, the canopy was enlarged, and the aft fuselage lowered to give an unobstructed view in nearly all directions. This idea did increase aircraft drag slightly. (Test data shows an increase of roughly 2%.) But that is a small trade for better pilot situational awareness. This canopy design became the norm for nearly all fighter aircraft that came afterwards and remains the standard today. The clean bubble canopy strongly contributed the P-51D’s “modern” look as compared to the razorback fuselage and glazed canopy design of previous fighters.
- 3. Designed for Mass Production
Often engineering marvels are the equivalent of a Swiss watch. They might exhibit extreme performance, but they give up practicality in the process. They are either prohibitively expensive, difficult to maintain, or they compromise useability to maximize their primary performance metric. An example of this type of design philosophy would be a racing vehicle. They are typically hand-built in low volumes, expected to last for just a few races, and are expertly maintained, often by their creators. The Mustang is the opposite of this. It was designed for profitable mass production and operational effectiveness from the beginning. An example of this philosophy is the wing and the tail. Despite the Mustang arguably having the best performing wing of the war, it was also amongst the simplest. It had a trapezoidal planform with straight leading and trailing edges. This not only is simple and cost-effective to build, but it contributes to the higher manufacturing accuracy needed to achieve its aerodynamic efficiency. Straight lines are easier to align when building tooling, especially in the 1940s without the use of modern laser projection and measurement tools.
Is it possible to quantify this design emphasis for production? Comparing aircraft production costs is challenging, especially between different countries. Labor hours per aircraft is more comparable, but the data is mostly anecdotal. Production efficiency also evolved drastically during the war, with hours per plane in 1945 being much lower than in 1940. All of that considered, most data shows the Mustang as having one of the lowest hours to build, in some cases by as much as half when compared to comparable fighters.
2. Landing gear
There were several arrangements for main landing gear on fighters of this era. Most were conventional taildraggers, with a few tricycle exceptions like the P-38 and P-39. The primary design difference between the taildragger aircraft is the direction of gear retraction: inward, outward, or aft. Each layout has pros and cons.
Landing gear generates high loads and outward retracting gear connects to the wing inboard, where it is stronger. This offers weight savings and was used on the Spitfire and Me-109. This layout also has a compromise: it tends to make the gear width somewhat narrow, which can lead to stability issues when landing. The mustang had inward retracting gear which allows for a wider gear stance. If a pilot lands with a wing low, a wider stance gear has a better ability to correct the roll angle on touchdown, without over-turning. Additionally, a wider stance gear allows for higher turning forces on the ground before the aircraft pivots on the outer gear and drags a wingtip. Spitfire pilots tended to cope well with its gear arrangement, probably due to superior training. However, more than 10% of all Me-109s were lost in landing and takeoff accidents. This landing gear likely has a strong influence on this statistic.
A final advantage of inward retracting gear is it places the retracted wheel in the center portion of the wing where it is thickest. This allows the wheel to be fully enclosed within the wing surface with no drag-producing bumps or blisters. Many contemporary fighters contained these (or exposed wheels), including the Spitfire and the Me-109.
- Conical Lofting
This might be the most obscure detail that contributes to the Mustang’s greatness. As an aircraft designer though, it is one of my favourites. The Mustang was one of the first aircraft to use a geometric design process referred to as “Conic Lofting.” Before we get into why this matters, let’s start with some definitions. “Lofting” is the process of creating the external shape of the aircraft, and the “Loft” refers to this final shape. The term loft comes from the shipbuilding industry where the shapes of ship hulls were drafted (often at full scale) in a loft above the shipyard. Early aircraft development borrowed much from shipbuilding, including this term. In more modern language, the loft represents the collection of external aircraft surfaces.
“Conic” refers to a type of mathematically defined curve. A conic curve is generated by slicing a plane through a cone, hence the name. A circle and an ellipse are both conic curves, among others. The advantage of a conic curve is that it can be described with high mathematical precision, and it is guaranteed to be a smooth curve with no reversal of curvature (assuming it is built per the definition). This type of lofting provides two related advantages. First, it tends to produce aircraft surfaces that are very smooth and continuous, and thus promotes low drag. Secondly, these surfaces tend to look really pleasing to the eye. If you look at a Mustang fuselage closely, you can see evidence of this. The forward cowling is an exceptionally well-designed shape in both cross section and the way the shapes flow from spinner to canopy. The aft fuselage between the canopy and tail is similar.
Conic lofting is a minor detail on a practical level, but it is one more feature that puts the Mustang on a different level. As a footnote, modern lofts are generated by computer aided design (CAD) programs and conic lofting is one of several techniques still implemented today.
Bonus: It Just Looks Freakin’ Amazing.
There are exceptions to the endlessly repeated adage noting the correlation between good looks and good handling qualities. For instance, the American century series fighters were stunning, yet most had questionable handling characteristics. In the other direction, some would say that two aircraft named ‘Thunderbolt’ have questionable looks, yet no one would question their incredible effectiveness as combat aircraft. Few would argue that the Mustang is questionable in either its style or its performance.
In summary, the Mustang stood out in its performance, its practicality, and its design. It was the right airplane at the right time. The incorporation of the best ideas from several decades of fighter evolution allowed it to become the pinnacle of piston-engine fighter design. Very few of its individual features are exceptional by themselves. But the Mustang brings them together in a very balanced and effective way. And it doesn’t hurt that the designers had an eye for good aesthetic design as well. This formula describes most of the greatest aircraft ever built.
Joe Wilding was the co-founder of Boom Supersonic, an independent company attempting to build a supersonic transport aircraft.
Mustang, The Story of the P-51 Fighter, by Robert Gruenhagen
Mustang, the Untold Story, by Matthew Willis
Aircraft Design, a Conceptual Approach, by Daniel Raymer
The Secret Horsepower Race, by Calum Douglas
A History of Aerodynamics, by John D. Anderson
Various performance papers from the 1940s authored by North American Aviation and N.A.C.A