A White Paper on XF-84H Propulsion

By John Leonard — Rolls-Royce Heritage Trust, Allison Branch



The XF-84H, a fighter aircraft with a supersonic propeller, was powered by an Allison XT40-A-1 engine. Figure 1 shows this aircraft. This aircraft made its first flight on July 22, 1955. There have been a number of articles written about this aircraft, and several books written on aviation subjects include short discussions of it. Information can also be found on various web sites. There is general (though not 100%) agreement that:


However, there are several issues for which there is not general agreement or for which there has been little or no information provided in the popular press. These issues are:

Research being done within the Rolls-Royce Heritage Trust, Allison Branch in Indianapolis has uncovered generally unknown information relevant to these questions. This white paper is intended to summarize this additional material.



Interest in supersonic propellers resulted from the loss in propeller efficiency in typical World War II type propellers as speed increased. This is illustrated in Figure 2, which was taken from NACA Misc Report L57C14-15. This loss of efficiency resulted in a limit on the speed of aircraft using these propellers. Increases in power resulted in only small increases in speed because as the speed increased a smaller percent of the power was being converted into thrust. Research suggested that a supersonic propeller using a thin blade turning at high speed would provide superior performance as suggested by Figure 2.


A supersonic propeller is one where the blade tips are supersonic even while the aircraft is sitting on the ground with the engine running. The blade tips may approach Mach 2 when the aircraft is in high-speed, but subsonic, flight. And at high forward speeds the whole length of the propeller will be subsonic. Because of the high rotation speed of the propeller, it is made of high strength steel.


The YT54-A-1 Engine

The suggestion that the T54 was used to power the XF-84H is contained in two articles in Flight magazine. In the 1 July 1955 issue there is a picture of the XF-84H with the caption, "The turboprop is an Allison T54, and the airscrew by Aeroproducts." In the 26 July 1957 issue in a discussion of the T54 engine it says, "It is scheduled to power an XF-84H test-bed, in which it will be matched with a supersonic propeller." So what is the deal with the T54 engine?

In addition to building the T40 engine, Allison also developed a similar, but more powerful, T54 engine. The T54 began as the XT40-A-8 in March of 1948 when the original spec was written for a 7,500 hp version of the T40. With the adoption of a T56 power section in late 1951, which used a 14-stage compressor rather than the T40s 19-stage compressor, and other changes it became the T54. Figure 3 shows a comparison of these two engines; however, the engines in this figure are not the versions used in the XF-84H. This is because no picture from the 1950s has yet been found of the version of either engine used by the XF-84H. It is expected that these photographs were classified when the program was active. The engines in Figure 3 both have contra-rotating propeller shafts rather than the single rotating propeller shaft used on the XF-84H. Another major difference between the engines shown in Figure 3 and the XF-84H engines is that the gearboxes for the Figure 3 engines are rigidly mounted to the power-sections. However, the shafts between the gearboxes and the power-sections for the XF-84H engines are about 16 feet long and both gearbox and power-sections are mounted to the airframe.


Figure 4 is a sketch of what the YT54-A-1 looked like. The extension shaft was made up of segments with several bearing supports to maintain the alignment of the shafts. The XT40-A-1 would have looked very similar with just slight variations to the power sections.


These two engines are very nearly the same size as demonstrated by the fact that the compressor blade tip diameter for both engines is 14.7 inches. The easiest way to tell them apart is by looking at the combustion section. On the T40 the eight cans can be seen, but on the T54, the six cans are covered by a cylindrical housing. Another major distinguishing difference is that there are eight bleed valves on the compressor housing of the T54, but there are none on the T40. The T54 uses T56 power-sections which makes it about 28% more powerful than a T40. Table 1 compares some of the characteristics of these two engines for the versions that were associated with the XF-84H.


Table 1 – XF-84H Engine Characteristics
Military Engine Designation XT40-A-1 YT54-A-1
Allison Engine Designation 500-B2 500-C3
Allison Model Spec. No. 349 335
Number of Compressor Stages 19 14
Number of Combustor Cans 8 6
Number of Turbine Stages 4 4
Takeoff Power - ESHP 5,850 7,500
Takeoff SFC – ESFC 0.620 0.540
Takeoff Turbine Inlet Temperature - °R 2,130 2,240
Engine Length – inches 339.25 337.59
Compressor Rotor Diameter – inches 14.7 14.7
Turbine Rotor Diameter – inches 19.2
Engine Speed - rpm 14,300 13,820
Propeller Gear Ratio 6.8:1 6.8:1
Propeller Speed - rpm 2,103 2032
Dry Weight – pounds 2,860 3,225
Reference 1 1


This information was found in the archives of the Allison Branch in a report that was missing the title page. But this information came from pages titled Allison Engine Model Analysis. The T40 page was dated 3-9-54 and the T54 page was dated 4-20-56. An interesting note in this reference says that Allison had a contract (Contract No. AF 33(038)-22106) to build eight YT54-A-1s for use in the XF-84H. Allison Memo by C. M. Hawkins titled "Overrunning Clutches for Allison T54 Dual Turboprop Engines" dated 31 May 1963 contains a statement that also bears on the subject. It says, "Overrunning clutches were scheduled to be flight tested with YT54-A-1 engines in the Republic XF-84H airplane under an Air Force propeller development contract."

This same report stated that Allison delivered 2 XT40-A-1s under contract AF 33(038)-22106. This is suspected to be in error because it appears that 4 XT40-A-1s can be accounted for. These are the two engines in the XF-84Hs that flew, one used by the Navy to test their supersonic propeller (described in the following section on ‘The Third XF-84H’) and one used by the Air Force to test an afterburner (described in the following section, An Afterburning Turboprop).

Another source of data on the YT54-A-1 was found in the old card catalog in the Allison (now Rolls-Royce) engineering library. Although the actual reports have not been found (except as noted) the cards contain short abstracts, which contain some interesting information. Table 2 summarizes this information with the reports being arranged chronologically by the report date.


Table 2 – Summary of Information from the Allison Engineering Library Card Catalog
Report No. and Title Report Date Abstracted Information
EDR 770, Model YT54-A-1 Reduction Gear Component 50-hr Back-to-Back Flight Substantiation Test 23 Sep 1955 The object of this test is to obtain flight clearance for the Model YT54-A-1 reduction gear assy for operation in an experimental aircraft by subjecting it to a 50-hr flight substantiation test. Found on microfilm)
SIR 8030, Lubrication & Scavenging Qualification Test of Model YT54-A-1, 2,100 rpm Reduction Gear 11 Oct 1955 The Model XT40-A-1 (Model 500-B2-1) reduction gear assyembly when modified to the 9,000 hp version for the YT54-A-1 incorporates an increased oil flow rate, EEL-3 oil, and overrunning clutches instead of the T40 friction clutches previously used. An altitude calibration to define the scavenging characteristics was completed on the first delivery Model 500-B2-1 reduction gear assembly to be modified to the 9,000 hp Model 500-C3-1 version
TSIR 7096, 50-Hour Flight Substantiation Test of the Model YT54-A-1 Reduction Gear Assembly Using MIL-L-7808 Oil 25 Oct 1955 This 50-hr Flight Qualification Test was performed so as to have a 2,100 rpm reduction gear assembly compatible with the 9,000 hp military ram power rating of the YT54-A-1.
EDR 821, Allison Model 500 High Speed Reduction Gear Assembly Component 50-Hour Back-to-Back Flight Substantiation Test 19 Apr 1956 The object of this test was to obtain flight clearance for the Allison Model 500 high speed reduction gear assembly for operation in experimental aircraft as a component of the XT40-A-1 or YT54-A-1 engines by subjecting the reduction gear to a 50-hr flight substantiation test.
TSIR 7971, 50-Hour Test on Engine A-200091 After Modification to the YT54-A-1 Configuration 21 Jun 1956 The purpose of this test was a complete a 50-hr test prior to the official 50-hr qualification test, to evaluate Model 500-C3 parts and to evaluate the fuel controls and control system and engine functioning.


All of the material reported on in Table 2 was done under contract AF 33(038)-22106) except TDR 821 which was done under Contract NOAS 54-828-C, which appears to be a USN contract. (TSIR is a Test Section Informal Report; and EDR is an Engineering Department Report.)

The main information that can be derived from these abstracts includes: (1) Allison was testing YT54-A-1 gearboxes in 1955 and 1956 with the intent of obtaining flight clearance. (2) This gearbox was a modification of the XT40-A-1 gearbox uprated to 9,000 hp. (3) The T54 was using a slightly modified T56 control system

Since EDR 770 has been discovered on microfilm, the following additional information is available on the gearbox for the YT54-A-1. (The power sections were similar to those of a T56-A-1, which is Allison Model 501-D4.) Photographs of the gearbox were found in the report, and, although not of high quality, they do define the basic exterior of the gearbox. Figure 5 shows two views of the gearbox.


The side view shows the flanged single-rotation propeller shaft. The rear view shows, toward the bottom, the two input shaft locations at the outer edges with the starter-mounting flange in the center. At the top are the smaller hydraulic pump pad on the left and the larger generator pad on the right. Below these two pads on the left side are two square pads for mounding the tachometer pickups.

EDR 770 also contains the following description of the gearbox. The gearbox is capable of delivering the power of either one or both power-section to the propeller, which turns clock-wise when viewed from the rear. The speed reduction is accomplished with a single stage herringbone gear set. The two herringbone pinions are connected through overrunning clutches to the power section input shafts. The main gear, driven by both pinions, is connected to the propeller shaft. Because of this arrangement, the power-sections and the propeller rotate in opposite directions. The propeller is a three-bladed Aeroproducts A39SFN-125.

The overrunning clutches are of the multi-disk type consisting of 8 bronze clutch plates splined at their inner diameters to the clutch shaft and 7 steel plates alternately spaced between the bronze plates and splined at their outer diameter to the pinion. This clutch design eliminates the need for safety couplings, because the overrunning clutch provides for automatic mechanical disengagement of the power-section from the gearbox in the event that the propeller tries to drive the engines, as would be the case if there were a flame out. The YT54-A-1 gearbox does not have a propeller brake. The total dry weight of the YT54-A-1 gearbox used in the qualification test was 578 pounds. The pinions are located with 19.5 inches between their centerlines, and the propeller shaft is midway between the pinions and 9.95 inches above the line between the centers of the pinions.

This report also states that the 50-hour component flight substantiation test was completed on 30 June 1955.

Figure 5 was taken in a T40 assembly area, but the sign on the parts table says, "Parts Being Drawn for Assembly of 2,100 rpm Reduction Gear Assembly XT54-A-1 Model." Since this photograph was taken in September of 1952 this indicates that some XT54-A-1 work was being done at least three years earlier than the date of the reports listed in Table 2.



Conclusion: All of this new information confirms that a version of the T54 was being developed for the XF-84H. However, it appears that the program was cancelled before the T54 was actually installed in the aircraft. To date nothing has been found to indicate whether engines were actually delivered to the Air Force from the order for 8 engines that were under contract.


The Third XF-84H

In February of 1951 someone in the Allison Engineering Department summarized the major accomplishments since the last AMC-Allison Engineering Review Conference on the XT40 and XT38 Turboprop Engine Program. One item was work on reduction gears for supersonic propeller development. Three bullets itemized these three activities related to supersonic propeller work:

This third project is work for the third XF-84H, and the effort is farther along than the other two projects. In 1989 Victor Peterson, a gearbox designer who worked on the YT54-A-1 gearboxes wrote an unpublished paper titled, "Allison Evolution in Gas Turbines." In this document he said, "About this same time the Navy sponsored a 3,000 rpm Aeroproducts propeller. This was a six-bladed (two staggered rows of three) single rotation. The gear box for this test program was a single spur gear stage of 4.771:1 (2,997 prop rpm at 14,300 turbine rpm) with design point of 9,000 hp. Two pinions meshing with a single large ‘bull gear’ were driven through overrunning clutches by the two T40 power sections. The propeller mounting arrangement was unusual. The propeller assembly had its own bearings on a non-rotating propeller shaft. The shaft was assembled through the ‘bull gear’ and supported by the gear box structure with a clamp nut on the back of the box. The torque to the propeller was transmitted through a female spline on the ‘bull gear’. The gear boxes were qualified with a back-to-back rig and all the propeller engine testing was performed at the Navy facility."

No identified photo of this propeller has been discovered. However, a propeller that fits the description shows up in the background of two similar pictures taken in Plant 2 in September of 1952. Figure 6 contains the parts of these photographs showing this propeller. It will be noticed that these are 6-bladed props in two staggered rows of three, although the second row of blades is partially hidden behind a group of combustor cans. The upper picture in Figure 6 shows some kind of protective material covering the edges of the blades, which would be typical of a supersonic propeller because of the sharpness of the edges. Also the insignia on the propeller blades indicate that these are Aeroproducts propellers.



The only other reference to a 3,000 rpm gearbox found to date was in the old card catalog in the Allison Engineering Library. TSIR 7136 dated 9 November 1955 makes reference to a 3,000 rpm gearbox for a Model 500-C3 (the Allison designation for a T54) in the title, but the abstract did not contain any illuminating information.

There was an interesting short article in Flight magazine dated 23 November 1950. It says; "That supersonic airscrews are receiving a good deal of consideration in the U. S. A. is implicit in the announcement that the world’s first supersonic-airscrew spin-pit will be in operation early in 1951 at the Aeroproducts division of the General Motors Corporation, Dayton, Ohio. The pit takes the form of a steel cylinder of 12 ft 10 in diameter, 7 ft 10 in high, which is sunk below ground level with an airtight seal to a concrete base, and a steel-dome roof. By evacuating the chamber down to 0.01 atmosphere (i.e. 99 percent vacuum) an engine of only 450 h.p. is sufficient to rotate test airscrews at relative high r.p.m. The test specimen is rotated in a horizontal plane, by means of a vertical driveshaft through the dome from a right-angled gearbox."

At this point it can only be presumed that this 6-bladed supersonic propeller as well as the 3-bladed supersonic propeller flown on the XF-84H were tested in this facility.

Conclusion: Although the No. 3 XF-84H was cancelled, it appears that the propeller had been built before the Navy cancelled its participation in the program. And the comment by Vic Peterson that "all the propeller engine testing was performed at the Navy facility" indicates that a complete engine was built and tested.


An Afterburning Turboprop

A common claim in the popular literature on the XF-84H is that it contained an afterburner. Two references indicate that the afterburner was for the Navy version of the aircraft. One reference even suggested that if you went to the National Museum of the US Air Force and looked in the tailpipe of the XF-84H on display, you could see the spray bars and plumbing. I did that, but I did not see any spray bars. I did see instrumentation probes in the tail pipe. There are two things I would expect to see if that aircraft had an afterburner: (1) A variable area nozzle, and (2) a flame holder, typically as a set of V-shaped gutters, near the afterburner fuel nozzles. I did not see either. All afterburning engines I have ever seen have a variable area nozzle. Variable area nozzles are required because as temperature rises in the tailpipe due to afterburning, the density drops; thus the nozzle area needs to be larger to pass the flow through the choked nozzle. If the area is not increased the engine will surge. V-gutters or some similar device are needed to keep the afterburner flame from blowing out in the high velocity in the tailpipe. So I do not think the XF-84H in the museum has an afterburner. Since the other XF-84H has apparently been destroyed it is not possible to check it for an afterburner.

However, an Allison Model Designation List dated January 1952 identifies the Allison Model 500-C7 as an afterburning version of the Model 500-C3 (military designation YT54-A-1) and that it was intended for high-speed propeller research. But nothing has been found to suggest this engine was built. Allison Model 500-C2, which did not have a military designator, was also identified as an afterburning turboprop. Allison Engine Spec 315 gives a comparison of the power and thrust available from this engine in afterburning and non-afterburning modes. Table 3 shows this comparison.


Table 3 – Effect of Afterburning on Allison Model 500-C2
Power and Thrust
Condition Engine RPM Shaft Horsepower Jet Thrust - lb
Military Power without Afterburning 14,300 6,746 1,383
Military Power with Afterburning 14,300 4,710 2,740


Because this data was prepared in 1951, it is reasonable to assume that it is based on engine cycle analysis rather than test data. But it would seem to be a poor tradeoff to give up 2036 horsepower in order to gain 1,357 pounds of jet thrust.

It is interesting that the T54 model numbers identify afterburning versions, but no afterburning T40 model was identified in similar Model Designation Lists, particularly in light of the pictures shown in Figure 7 of an afterburning T40. These pictures are identified as a T40 with afterburner at the Air Force Flight Test Center, and they were taken in March of 1955, about 4 months before the XF-84H made its first flight.



Although to date no report has been found describing this test engine or any results of testing, the following can be deduced from the pictures:



Conclusion: Although afterburning turboprops were obviously studied, designed and tested, it does not appear that the XF-84H was equipped with an afterburner. Since no other afterburning turboprops have been identified and since the calculated loss of power hardly justifies the increase in thrust due to afterburning, it may be that an afterburning turboprop is not a good idea.


Variable Propeller Speed

In the August 5, 1955 AllisoNews it was noted that "As a research aircraft, the XF-84H has been built to split just forward of the canopy in order to take several different forward sections, one of which mounts a 3-bladed supersonic propeller built by Aeroproducts Operations." The 1956-1957 Jane’s All the World’s Aircraft and Air Enthusiast No. 48 say much the same thing. This notion is supported by comments on an Allison Engine Model Analysis sheet for the YT54-A-1 dated 4-20-56 which say that one of the design details of the YT54-A-1 is that there are several interchangeable gearboxes that can be used for testing different props, but that the basic engine gearbox is a single-rotation flange-coupling type with a 6.8 to 1 ratio. Air Enthusiast No. 48 also states that Curtiss and Hamilton Standard designed supersonic propellers for the XF-84H, although neither of them were ever mounted on the aircraft. A curious comment is found on the Air Force Flight Test Center web site, which says, "Once at the flight Test Center, the two XF-84Hs began the evaluation of several supersonic propeller designs, including one six-bladed contra-rotating small-diameter airscrew."

Conclusion: Although there are several references to interchangeable noses, interchangeable gearboxes, and alternate propellers being available for or being tested on the XF-84H, no photograph has surfaced showing any alternate nose or propeller configurations. So although it is possible that the aircraft was designed to incorporate various nose and propeller configurations, it seems unlikely that this flexibility was used in the flight test phase. This feature is known to exist on the McDonnell XF-88B, which was also a supersonic propeller test aircraft, because it had a gearbox that could provide three propeller shaft speeds by physically changing the gears in the gearbox. The engine and gearbox for the XF-88B were also designed and built by Allison.