By Tommy H. Thomason

Saturday, September 26, 2009

When Rube Goldberg Isn't Enough

Jets weren't compatible with the barrier cable system used on axial deck carriers to protect the people and planes forward of the landing area if the tail hook of the arriving airplane didn't pick up any of the arresting wires. A Rube Goldberg creation, the Davis barrier, provided the same function by having the cables lie flat on the deck and adding an actuation strap where they were located for propeller-driven aircraft. When the jet's nose gear hit the actuation strap, vertical straps tied to both the actuation strap and the cable would jerk the cable off the deck where it would snag the main gear. A barrier guard was added in front of the wind shield to snag the strap in the event of a nose landing gear collapse.

Unfortunately, this arrangement only worked at a specific range of speeds. If the airplane was going a bit fast when it hit the actuation strap, the main gear would pass by before the cable had come up far enough to engage it; this resulted in the addition of the big barricade beyond the barriers, still used on angled deck carriers to land an airplane when the arrested landing isn't possible for some reason. If the airplane was a bit slow, the cable would come up off the deck and fall back down before the main gear arrived. Although the barricade would stop the airplane in this case as well, it was an event to be avoided since disentangling the airplane and resetting the barricade were time consuming. Instead, a barrier cable pickup was to be added to airplanes between the nose gear and main landing gear. It would insure that the cable would stay up off the deck and do its job in a low speed barrier engagement.

Only a few new aircraft were equipped with a barrier cable pickup, like the XFJ-2 shown above, before the introduction of the angled deck eliminated the barrier requirement.

Sunday, September 20, 2009

The Tail Hook

The tail hook is one of the critical elements of navy carrier operations. It's a simple device, which when coupled with a massive, largely unseen, and very complicated ship-based arresting gear mechanism, allows non-STOL airplanes to land in impossibly short distances. The drawback of carrying all the STOL (Short Takeoff and Landing) capability on the airplane itself is that it's heavy and only needed for a brief portion of an airplane's mission. The other half of carrier-based STOL is provided by the ship's catapults, also massive, largely unseen, and very complicated but left behind after takeoff.

Two terms of art in carrier landings are "trap," which means a successful landing, and "bolter," which means the airplane touched down but the hook did not engage a wire. For an excellent and illustrated description of the event, see here.

The tail hook, however, is deceptively simple. It has to take the full weight of the airplane times two or three and transmit that load into the airplane's structure. It also has to be mounted so that it doesn't cause excessive yaw or pitching moments during the trap. It has to be resistant to bouncing off the deck on contact (which might result in it skipping over each wire, causing a bolter, but yet be able to swivel up and back to minimize the moments and loads. It can neither be too long (risking an in-flight engagement, which makes a normally hard landing even harder) nor too short (risking no engagement at all).

The Grumman F11F-1 tail hook in the picture above is one of the more unusual installations. Grumman's practice had been to locate the hook at the extreme end of the fuselage. Prior to the F11F, it had been housed within the fuselage. When it was released for landing, it slid aft and then pivoted downward. After landing, the pilot could raise it to the "stinger" position so it was clear of the wires for taxiing forward. The deck crew would then stow it after shutdown.

For simplicity and weight reduction, the F11F hook did not slide aft, but was stowed upside down with the hook point forward.

When released, it simply dropped into position. After landing the pilot would raise it to the stinger position so the Tiger could be taxied forward into parking.

For stowage, the attach point was triple-jointed so the deck crew  could swing it out to the side and then rotate it upside down.

It was innovative, but it didn't catch on. For one thing, the tail hook couldn't be extended after the airplane was on the ground, precluding it from being dropped to engage the emergency field arresting gear to help abort a takeoff or to terminate a unexpectedly long landing roll.

Wednesday, September 9, 2009

Missed It by That Much IV

In 1954, McDonnell proposed what was to become the F4H Phantom with two different engines, the Wright J65 or the General Electric J79. In this F3H-G/H mockup, the J65 is represented on the left side and the J79, on the right. The former was a license-built Armstrong Siddeley Sapphire, which was not only flight rated, but flying in new aircraft, including the Navy's F11F. The latter had only run for the first time in June 1954 and would not be flight rated for more than a year. The J65-powered F3H-G was projected to have a top speed of Mach 1.52 and the J79-powered F3H-H, Mach 1.97.

In late 1954 or early 1955, the admirals selected the J79 for the new F3H, in spite of their (and McDonnell's) recent and horrible experience with a not only new but unproven engine, the Westinghouse J40, which caused major delays and cost overruns in the Navy's early 1950s fighter programs. As it happened, the J79 was a success, meeting or exceeding expectations. Meanwhile, the proven (except for the Wright-furnished afterburner) J65 proved a huge disappointment for Grumman. The F11F, instead of reaching its guaranteed top speed of Mach 1.2 or better, was barely supersonic in level flight due to lower than projected thrust in afterburner. Subsequently evaluated with the J79, it could slightly exceed Mach 2.

In late 1958, the Navy was forced by Congress to choose between the F4H and the F8U-3 for its fleet air defense fighter requirement. Both were demonstrated to have near Mach 2 performance. The decision came down to the belief that the radar operator in the two-seat F4H would be able to acquire and begin the process of launching a Sparrow air-to-air missile at an incoming Soviet bomber just a few seconds quicker than the pilot of the single-seat F8U-3, possibly making the difference in keeping the bomber from accomplishing its mission to destroy an aircraft carrier. However, if the F4H had been saddled with the anemic J65, the decision might well have gone the other way: it's possible that the F8U-3 getting in range for a missile launch sooner would have been found to more than make up for the quicker target acquisition provided by the radar operator in the F4H. If so, the F4H would now only be known to aviation historians like me rather than as an iconic jet fighter of the 1960s and 1970s and the holder of several speed and time-to-climb records.

Monday, September 7, 2009

Why Laser-Guided Bombs

Above is bombing accuracy data from the service trials of the Vought A-7E Corsair II used to compute its CEP, Circular Error Probable, the circle within which 50% of the bombs hit when dropped one per attack. Its bombing system was probably as accurate as could be achieved with unguided Mk 80 series bombs. Even so, the actual hits relative to the target were not as close as those on the plot. They were tweaked a bit because the bombing computer was programed for a 200 millisecond delay in bomb release instead of the measured 50 millisecond delay of the bomb rack used. (For reference, the average human's reaction time to an anticipated action is about 200 milliseconds.)

The blue rectangle is an American football field. You wouldn't want to be standing in the middle of one being targeted by an A-7E dropping a 500-lb Mk 82 general purpose bomb—the lethal and effective casualty circles are shown in red. You were more likely to be killed by one bomb than not because the fragmentation/blast effect was 100% fatal in the lethal circle and fragmentation would result in 50% fatalities in the effective casualty circle. However, bunkers, tanks, buildings, and certainly bridges were a lot less likely to damaged by one bomb. It therefore might take a lot of them to do the job.

The accuracy of precision-guided weapons is far better. Theoretically, a laser-guided bomb will hit within 10 feet of the targeted point and one with GPS capability, within 40 feet. Far fewer bombs and missions were required for a given target. The requirement imposed by stealth to limit the number of bombs to those that could be carried internally was less of a penalty. The Navy benefited even more than the Air Force because of the logistics involved in resupplying an aircraft carrier with bombs and aircraft.