AVmail: February 29, 2016

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Letter of the Week:
Supersonic Adjustments

Just wanted to correct Paul Bertorelli on Capt. Eric “Winkle” Brown’s obit, where you repeat the widespread myth that the XS-1 had to be modified to incorporate the M.52 tail in order to break the sound barrier.

Brown RN was the pilot designate for the M.52 and wrote a book, Miles M.52, Gateway To Supersonic Flight. He makes a number of claims in his book regarding the X-1.

To wit, quote #1:

The X-1 had a tailplane adjustable in incidence for trim purposes, with a conventional trailing edge elevator.

My comment: Correct. The specification supplied to Bell specified that the XS-1 be equipped with a movable horizontal stabilizer to provide pitch (nose up or down) control when shock waves made the elevators ineffective and spelled out also the rate of movement (one degree/second). Brown also notes the X-1 as built “incorporated none of the design features of the Miles M.52.”

Quote #2:

Yeager lost control when flying at Mach 0.94 … . Both he and the engineers thought that was the end of the road. Magically a solution was provided by Bell almost instantaneously, in the form of a “field fix” variation of the “flying tail” encountered on the U.K. visit to Miles. Bell’s control solution was to use the pre-existing tailplane trimmer to modify the incidence of the tailplane in flight, by means of a switch in the cockpit. Two flights after evaluating their ad hoc “flying tail,” Yeager attained supersonic flight.

My comment: Not so. Bell did not change anything; see comment to quote #1. Yeager has always given the project flight engineer Jack Ridley the credit with coming up with the idea to use the tailplane trim as a means of pitch control during the period when the elevator lost effectiveness. Ridley, however, just had his finger on the pulse with respect to the papers crossing his desk, IMHO. The ability to control pitch with stabilizer in the transonic regime was investigated by Axel T. Mattson in his paper NACA RM No. L7A03 “FORCE AND LONGITUDINAL CONTROL CHARACTERJSTICS OF A 1/16-SCALE MODEL OF THE BELL XS-1 TRANSONIC RESEARCH AIRPLANE AT HIGH MACH NUMBERS” (May 21, 1947).

Quote #3:

When the X-1 ran into compressibility problems in 1947 it was rescued by fitment of the “flying tail.”

4:

The British aviation fraternity is firmly convinced that the Bell X-1 owed its success to what the Americans gleaned from their visit to the Miles factory in Autumn 1944, and in particular what Bell learned from the M.52’s “flying tail.” Certainly, in spite of American protestations that they had their own original ideas on a “flying tail,” there does not appear to be any solid evidence of its appearance on test before the X-1 ran into transonic trouble in 1947.

5:

On seventh flight, after “flying tail” has been fitted to the X-1, Yeager attains controlled supersonic level flight, Mach 1.02.

6:

The Americans will not admit that the flying tail which was on the M.52 was what let the Bell X-1 break the sound barrier. Chuck Yeager had run into severe compressibility trouble at Mach .94. In fact, General Albert Boyd, head of the Flight Test Division at Wright Patterson AFB, had said, “Well, fellers, this is the end of the road.” Then three days later Bell came charging down with this all-flying tail, designed by a guy who had been to Woodley and had seen the tail on the M.52. He’d done some work on it, but basically it was the M.52’s tail, which he admitted more or less on his deathbed. They popped it straight onto the X-1 there and then, and it did the trick.

My comment: There is nothing for the Americans to admit to. It’s interesting how Brown’s narrative changes from that given in #2 to that in #6.

Yeager’s flight test report following the first supersonic flight:

Date: 14 October 1947
Pilot: Capt. Charles E. Yeager
Time: 14 Minutes
9th Powered Flight

1. After normal pilot entry and the subsequent climb, the XS-1 was dropped from the B-29 at 20,000′ and at 250 mph IAS. This was slower than desired.

2. Immediately after drop, all four cylinders were turned on in rapid sequence, their operation stabilizing at the chamber and line pressures reported in the last flight. The ensuing climb was made at .85 – .88 Mach, and, as usual, it was necessary to change the stabilizer setting to 2 degrees nose down from its pre-drop setting of 1 degree nose down. Two cylinders were turned off between 35,000′ and 40,000′, but speed had increased to .92 Mach as the airplane was levelled off at 42,000′. Incidentally, during the slight pushover at this altitude, the lox line pressure dropped perhaps 40 psi, and the resultant rich mixture caused the chamber pressures to decrease slightly. The effect was only momentary, occurring at 0.6 Gs, and all pressures returned to normal at 1 G.

3. ln anticipation of the decrease in elevator effectiveness at speeds above .93 Mach, longitudinal control by means of the stabilizer was tried during the climb at .83, .88, and .92 Mach. The stabilizer was moved in increments of 1/4 – 1/3 degree and proved to be very effective; also, no change in effectiveness was noticed at the different speeds.

4. At 42,000′ in approximately level flight, a third cylinder was turned on. Acceleration was rapid, and speed increased to .98 Mach. The needle of the machmeter fluctuated at this reading momentarily, then passed off the scale. Assuming that the off-scale reading remained linear, it is estimated that 1.05 Mach was attained at this time. Approximately 30% of fuel and lox remained when this speed was reached and the motor was turned off.

5. While the usual light buffet and instability characteristics were encountered in the .88 – .90 Mach range and elevator effectiveness was very greatly decreased at .94 Mach, stability about all three axes was good as speed increased and elevator effectiveness was regained above .97 Mach. As speed decreased after turning off the motor, the various phenomena occurred in reverse sequence at the usual speeds, and in addition, a slight longitudinal porpoising was noticed from .98 – .96 Mach which (was) controllable by the elevators alone. Incidentally, the stabilizer setting was not changed from its two-degree nose-down position after trial at .92 Mach.

6. After jettisoning the remaining fuel and lox, a 1 G stall was performed at 45,000′. The flight was concluded by the subsequent glide and a normal landing on the lake bed.

Yeager’s presentation at the first XS-1 conference, 9 January 1948:

As the Mach number was increased from .87, the buffeting became more severe, and a nose down trim change was noted. The forces were quite light, and the movement of the control column remained the best means of indicating the trim change. At approximately .90 Mach number trim change previously mentioned reversed, and the tendency was for the nose to rise and, in the range of approximately .92 Mach number, the buffeting became quite severe.

At this point in the program it was decided from a correlation of model test data that the one degree per second actuator for the stabilizer might prove to be too slow for proper control during subsequent flights, and an interruption in the program was made to install a faster motor. In the first flight after the new stabilizer actuator was installed, the Mach number was increased to .94. At this point, the trim change again reversed to a nose-down tendency, but it was still easily controllable and approximately 3degree of up elevator provided level flight. From .94 to .96. the elevators and rudder became increasingly ineffective until, at the latter figure, they could be moved throughout their range of displacement with very slight response from the aircraft. At approximately .95, the buffeting decreased rapidly and became non existent at .96.

Up to this time a stabilizer setting of 2degree leading edge up was used in all of the high-speed test runs. The next flight was therefore initiated to investigate the effectiveness of control by the stabilizer at the higher speeds above .96 since the setting had only been varied in climbs up to this time. As the speed was increased on this flight, the stabilizer was changed to 1degree leading edge up and returned to 2degree leading edge up successively at .84, .88, and .95 Mach numbers. The acceleration experienced in the cockpit was approximately the same for all speeds, and it was decided that the stabilizer was still effective, even though the elevator and rudder had lost their effectiveness. The ailerons remained effective throughout the range. With the stabilizer setting of 2degree, the speed was allowed to increase to approximately .98 to .99 Mach number, where elevator and rudder effectiveness were regained and the airplane seemed to smooth out to normal flying characteristics. This development lent added confidence, and the airplane was allowed to continue to accelerate until an indication of 1.02 on the cockpit Mach meter was obtained. At this indication, the meter momentarily stopped and then jumped to 1.06, and this hesitation was assumed to be caused by the effect of shock waves on the static source. At this time the power units were cut and the airplane allowed to decelerate back to the subsonic flight condition.

My comment: Of particular note is that the program was interrupted in order to install a faster trim motor, not to install a “flying tail,” as insisted by Brown. The X-1 retained the stabilizer/elevator system as spelled out by Brown in quote #1 throughout its life and was never modified for the supersonic flight, as a reading of Yeager’s reports above confirms.

Brown, in his book, calls the M.52 tail a “flying tail,” and it was what would be called a “slab” today, a unitary piece with no separate stabilizer/elevator. Here is where I feel the problem lies. NASA and Yeager, even to this day, refer to the X-1 trimmable stabilizer and elevator setup as a, you guessed it, “flying tail.” Two contemporaneous aircraft, the D558 and the F-86A, both had the same setup as the X-1 (trimmable tailplane with elevator), and Yeager credits the F-86 with its “flying tail” as giving it the edge over the MiG-15. It seems to me to be another case of two countries separated by a common language. Has Brown merely misunderstood the U.S.A.’s use of terminology? I can think of no other explanation, because the plain fact is, the X-1 tail never underwent modification from the day it was built, other than the trim motor.

It would be interesting to have interviewed Brown to gain an insight into why he had the idea that the X-1 tail was modified.

Brian Abraham

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