Link to: “Testing the Republic F-84 Thunderjet" - Part One

Editor's Note: We know the Thunderjet as the F-84 but in 1947 we still used the "P" (pursuit) designation used in this article. The museum has both an early straight wing version and a later swept wing version of the F-84.

TESTING THE THUNDERJET
By Carl Bellinger, Test Pilot, Republic P-84 Airplane

LATEST JET FIGHTER TO JOIN AAF OPERATIONS PUT THROUGH ITS PACES BY REPUBLIC’S TEST PILOT

The Republic P-84 Thunderjet over Long Island.

(continued from Part One) ...Those are some of the philosophical differences. Now for some of the actual technical differences.

Just as in all flight testing, no pilot takes a ship up and tries to learn all there is to know in his first flight. Certain flights are made in order to get certain types of information and different planes are assigned for specific test purposes and are not used for any other test flights. During the war, we had 17 P-47’s assigned to the company for experimental uses. That was when every idea with any possibility was tested to see if it would work. Nowadays all ideas must be filtered and only those which are decided on as absolutely necessary tests, are run. Also, more tests have to be run with fewer test planes.

The P-84s J-35 Allison (GE TG-180) being “fired up” by auxiliary starting unit mounted on these carts.

Take-off: A take-off run is, of course, longer in a jet plane than in the conventional types and is more affected by varying temperatures. The taxiing differs in that there is no slipstream over the rudder , therefore, it is necessary to use brakes entirely up to 70 mph, at which time the rudder becomes effective. The tricycle gear makes the Thunderjet easier to handle than the P-47 was. One thing a pilot must watch in ground handling is to keep his tail away from people in order to prevent hitting them with the blast of the jet from the nozzle. This is the reverse of avoiding propeller accidents. On take-off, the commitment is greater, due to the slower acceleration.

Instrumentation for take-off is simpler. Instead of having to work the mixture control, the propeller control, the supercharger, and the throttle, one needs only to use the throttle—one lever instead of four.

The best climbing speed of a P-47 was about 160 mph at sea level, decreasing the climbing speed one mile per hour for each 1,000 feet for best rate of climb. For the Thunderjet, the best climbing speed is 400 miles per hour true air speed at sea level.

Flight Test: Speed runs do not differ, as between reciprocating and jet types, except, of course, for the faster speeds and the necessity to make decisions from ten to twenty miles, or one to two minutes earlier than we used to. The method of making speed runs involves only the attempt to hold at a certain altitude, with no more than plus or minus ten feet while calibrating the speeds sought. Incidentally, we’re all wearing crash helmets these days, not that they’d be too much use in an actual crash, but because you take a bumping around at such high speeds, and with the canopy inches from the top of your noggin, it’s nice to have something in between, absorbing the blows.

Stability and control tests also are about the same. Much greater care must he taken, of course, because, again, of the high speeds encountered and the fact that stresses are greater in maneuvering. Also, since it is obviously so much easier to approach compressibility, a pilot must resist tendencies, in acrobatic flying, to roll or drop into a high speed dive which puts him in an unscheduled attempt to fly through the speed of sound. Dive brakes are installed, which enables slowing down under such circumstances, and they are used frequently in various tests. But even with the dive brake, it is necessary to throttle down, if you want to keep your respect for the laws of gravity.

Instrument panel of the Thunderjet. Flight group is on right, engine instruments on the left. Also, special test instruments are carried.

Instrumentation: The use of the photo panel to record instrument readings has relieved the pilot of much of the responsibility of trying to write down his reactions when he’s flying at ten miles a minute. The photo panel does not entirely replace the writing pad on the knee, hut it does photograph such instruments as the air speed indicator, sensitive-altimeter, free air temperature, engine r.p.m, fuel counter (at 2 gallon increments), clock and, (according to the tests being run that day), pressures and temperatures throughout the fuselage plus internal and external fuel tank pressures. Tufts are also installed, at times, to indicate smoothness of airflow’ over aerodynamic surfaces. The photo panel is put together before each flight to record the proper instruments required for the specific tests involved.

Landing. Final touchdown. Note nose high attitude and smoke from right wheel which touched first.

Landing: With the P-47, we used to come in on our downwind leg at 180 mph indicated air speed, turn into our final at about 150, come in over the fence at 120 and settle at 95 to 100 mph.

The downwind leg in a P-84 is made at 200, we turn on our final about 160 to 170, pull up the nose to brake down and settle with normal gross load of 13,000 pounds at approximately 120 I.A.S. Both landings are figured with no wind conditions. Also, just as the jet needs more take-off runway, we use up more runway coming in. If you make up your mind to go around for another pass instead of landing the first time, you do so a little sooner, due to the lower rate of acceleration.

Bellinger, in cockpit discusses result of test flight with Williamson and Capt. Yeager, AAF acceptance test pilot.

On the ground, both before and after the flight, sessions are held with the company flight test engineers. Prior to the flight, the purpose of the test is explained, as is the way the engineers want it run. Afterwards, the result of the flight is gone over, calculated, and the data is observed to determine necessary changes or additional tests. The crew chief has been notified, before the flight, of any plane changes requiring alteration of operating procedure, and the crew shapes up the plane for specific types of flights. Results of tests are sent on as a matter of information to the Air Materiel Command, Flight Test section.

Test planes carry a duplicate set of most important instruments which are photographed by a camera.

Since early this year, we have been flying all company tests at our Farmingdale plant, the project having been moved from Muroc Army air base. Now, the Fourteenth Fighter Group is assigned to AMC accelerated flight tests at Muroc. The Fourteenth, first group to be assigned to the Thunderjet, will operate their airplanes from Dow Field, Bangor, Maine, when the accelerated flight tests have been completed.

P-84s at Republic’s ramp ready to be delivered. 14th Fighter Group is first tactical organization to use them.

Meantime, we’re hard at work readying a production model P-84 for that new assault on the world’s speed record. All of us at Republic look forward to that one. And when we turn the plane over to the Army pilot who will make the actual record run, it will have been proved, beyond question, by Republic’s own test crew. And we’ll go to work planning for our part of the job to get the next plane in the Thunderbolt, Thunderjet, Thunder--- series in flying condition to maintain America’s supremacy in the sky.