Fourth Page of Career Insights; Sonic Fatigue Tests

The numbered contours are overall sound pressure levels on a B-52 wing during takeoff. The timeframe: 1958-62.

Companies such as Boeing build airframes. Engineers match a type of engine to an airframe.
Airframe + Engines = Aircraft
Learn about engine-airframe matching.

Aircraft engines are either turbojets or turbofans. You see a J-75 turbojet mounted in a test stand at the north end of Boeing Field. On the left a vertical structure is holding a test panel.
Boeing conducted a Supersonic Transport (SST) research and development program during 1963-65.

A Boeing SST sonic fatigue test panel. Two vertical elements represent fuselage transport sidewall frames. Horizontal elements are stringers or stiffeners. Stringer cross sections resemble the letter "Z". The term Z-stiffener is used. A fundamental vibration mode occurred where the panel and stiffener flexed or bent between frames. The mode was called oil-canning. Because of the asymmetric cross section, stiffeners twisted when they bent.

Boeing travelling wave duct, 1964-65 SST Phase II Program. The frame at center held the test panel. This is where I used a strobe to observe coupled torsion-bending of stringers.

Click the image to hear 250 Hz online tone. This is the panel oil canning mode frequency observed in 1964-65. I guess you hear it near 80 dB. During a test on a panel, I heard and felt a 250 Hz tone at 145 dB, without protection. I felt ill for a time. This action, one of the dumbest of my career, influenced much of my work for the next two decades.

The sound was generated by a Tone Generator, courtesy of NCH Swift Sound.

Get serious about building your own set of acoustic tools.

On 09/21/2010, the author suggested installing the WINAMP 5.58 toolbar in order to play a 3 second duration 250 Hz tone at the travelling wave duct image.

Boeing SST engineers worked in Renton, Washington. Many of them had moved from Wichita, Kansas. Wichita engineers had the advantage of having had sonic fatigue experience from the B-47 program. The example is from a B-47 test performed in 1952.

I worked in the Boeing SST Stress Group in 1964-65. During one sonic fatigue test, I used a stroboscope to observe coupled torsion-bending behavior of stringers. I studied ways to analyze the phenomenon.

Stressman Jim Maclin was responsible for sonic fatigue tests. Brian Menon had immigrated from England. He had studied analysis methods developed by J. H. Argyris, and kindly shared them with me. Jim Higgins was responsible for determining allowable compression loads on skin-stringer-frame panels. Stringer roll or torsion occurred when compression panels bent.

The SST Stress Group Engineer was Joel Alldredge. He had Wichita experience. To give you an impression of his deep, practical insights, here is a quotation from notes he wrote in April 1964:

Full scale testing is required to adequately predict the total structure responses and should include systems such as wiring, tubing, brackets, etc., since a large part of our failure experience is with these detailed items.

From B-52 and KC-135 service and test experience, a general sonic structure design knowledge has evolved and is basically directed toward providing minimum structure deflection or response with minimum weight. Primary structure has caused few problems since it is generally rigid enough that the sonic pressures do not excite it. Exceptions have been in the thin fuselage skin panels, and even here, the problem has not been excessive. At least some portion of the wing trailing edge and flap structure problems have resulted from a structural vibration environment associated with the high dB levels (in particular, equipment support bracketry, plumbing, and wiring supports, and improperly supported structural mass). Almost all sonic fatigue failures in structure are caused from bending stresses, thus the cure is to eliminate as many connections which depend on bending strength as possible, and replace with a shear transfer. The alternatives are to provide sufficient rigidity so that the bending stress is negligible, to provide structural dampening or to repair the structure on a periodic basis.

THINGS FOR YOU TO DO IN THINKING ABOUT CAREER OPTIONS

Use a search engine.

1. Search Boeing and Lockheed Martin for engine-airframe matching challenges. For example, find the B-777-300 transport. Look for a picture of technicians mounting an engine below a wing.
2. Visit Pratt & Whitney, Rolls Royce, and GE aircraft engines. Study histories of turbojet and turbofan engines.
3. Would you rather be a propulsion engineer at an engine company or an aircraft company?
4. Study the B-727 aircraft engine-airframe match. How could the current engines be replaced to reduce community noise levels significantly?
5. From your understanding of current international events, deduce that the B-52 aircraft are important US national peace-keeping resources. Use your search skills to find information about a proposed re-engine program.
6. Search the TU-144. How long is since this Russian SST has flown passengers? Look for a proposal to acquire a TU-144 for US research purposes.
7. While Boeing was gaining SST transport design experience in 1963-65, Lockheed was active with development of a supersonic surveillance aircraft. Search Blackbird. Learn about A-1 through A-12 aircraft. The background image is a photo of a blackbird who guards my coffee cup.