April/May Dakota Territory Air Museum P-47 Update

The flaps, elevators, cowling, and wingtips were areas of restoration progress this month. The fuselage work was concentrated on the cockpit, where the pilot’s seat has been installed and finishing the turbosupercharger ducting.  The wings were attached near the end of the month. We will also examine some of the ‘teething problems” the early P-47s experienced as they were introduced to combat.

Driving rivets in tight places, like the trailing edges of control surfaces, requires some ingenuity and special tooling.

Mike continues fabrication work on the cowl for the P-47. It is a precision task to be sure all the latches work smoothly and securely.

The wing work this month concentrated on finishing up the gun bays and installing the wingtips. The wings were attached to the fuselage, complete with the landing gear, hydraulics, and electrical systems.

Jacob installed the completed wingtips this month.

Newly classified as an escort fighter, the first P-47s were sent to England to support bomber missions. Even though the Thunderbolt’s range was limited to a combat radius of 200 miles, that was still about 50 miles more than the Spitfires of the time. P-38s had a longer range, but were not available in sufficient numbers to completely meet the need.¹ Even with the range limitations of the P-38 and P-47’s they were still better than no escort at all. However, once the fighters had to turn back, the bombing forces often suffered heavy losses at the hands of the Luftwaffe.

The first fighter groups to receive P-47Cs were the 4th, 78th, and 56th Fighter Groups. These P-47Cs arrived in Great Britain a few days before Christmas of 1942. Further shipments were planned to equip the three fighter groups with enough Thunderbolts to fill three squadrons in each Group.

The 8th Air Force hoped to have all three Fighter Groups operational by mid-February 1943. Unfortunately, radio and engine problems during high altitude testing made that goal unattainable.

The cause of the engine problems was found to be ignition breakdown and distributor leakage fouling plugs, resulting in dangerous power loss. Besides the power loss, radio noise made communications nearly impossible. Both issues had to be corrected before the new P-47s could be flown in combat.

At sea level air pressures, the total voltage the magneto is required to produce to break down both the spark plug and distributor gaps is approximately 11,000 volts. This level of voltage reliably makes a spark at the spark plug.

As altitude is increased, the air density drops, and the insulating properties of the air with fewer molecules drop. This means that the necessary voltage to jump the high voltage conductor spacing inside the magneto drops along with the air density. Because of this phenomenon, the magneto sends lower voltage through the ignition harness to the spark plugs at higher altitudes. That works fine with normally aspirated engines at higher altitudes because cylinder pressure at the time of the ignition spark event also drops. It is easier for the current to jump the plug gap because less voltage is required to fire the spark plug, and the ignition works as designed.

High altitude ignition breakdown is a greater problem for turbosupercharged engines like the R-2800 installation in the P-47.

In a turbosupercharged engine, the necessary spark plug voltage remains high because the cylinder pressure is maintained at near sea-level conditions by the turbosupercharger². Therefore, the required voltage to successfully and reliably produce a spark at the spark plug remains high. But, inside the unpressurized magneto, the voltage needed to break down and jump the point and distributor gaps decreases, so the unpressurized magneto puts out less voltage than the spark plugs need, as explained earlier.

As pre-combat testing progressed, it was found that the humidity levels at 25,000 feet over the United Kingdom were much higher than over New England where the previous testing had occurred. That increased moisture level, along with reduced insulating properties of thinner high altitude air made the problem of low voltage from the magneto worse, and the ignition failed to reliably deliver a spark in the cylinder. It was found that tapping pressure from the turbosupercharger in order to also maintain sea-level pressure within the magnetos was the cure for this problem. The higher internal pressure inside the magneto kept the output voltage high, and the spark plugs fired reliably. The rest of the ignition harness was also sealed against moisture intrusion.

^{1 Roger Freeman, Thunderbolt, A Documentary History of the Republic P-47, Scribners & Sons, Great Britain, 1978, page 28-31
2 Harry Fenton, Magnetos Under Pressure, AviationPros website accessed 5-017-2022, https://www.aviationpros.com/home/article/10388584/magnetos-under-pressure}

After finally solving the ignition problems, the first Thunderbolt combat mission was flown on March 10, 1943. The 4th Fighter Group sent 14 P-47s across the English Channel on a fighter sweep. Unfortunately, the mission turned into a fiasco because static from the ignition system rendered radio communications nearly impossible. No more missions could be flown until the communications problem was solved.

The radio static issues turned out to be a problem related to the ignition system issues. Noise from the ignition system caused by static electricity escaping from corroded magneto seals was what prevented radio communications. The problem was temporarily reduced by adding bonded insulation to the magneto distributor caps. The permanent fix came later with the installation of pressurized magnetos.

With a solution to the communication problem achieved, a sweep of 24 Thunderbolts over the Pas De Calais region marked the real beginning of P-47 combat missions on April 8, 1943.