S-3 Viking Flight Quality Improvement Program

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Lockheed S-3 Viking

The Lockheed S-3 Viking was a carrier borne antisubmarine aircraft that entered service with the US Navy 1974 and ended it’s front line service in 2009.

The War Hoover (as it’s known due to the sound of it’s engines) was one the few carrier borne aircraft from Lockheed and as such they partnered with Ling-Temco-Vought (LTV) who had a long line of success of building carrier based aircraft. Lockheed put LTV’s experience in the S-3 to work as it uses the main landing gear from the F-8/A-7 stable of Vought carrier based aircraft.

Before all naval aircraft enter service they all undergo testing to evaluate how they handle when coming aboard the carrier for landing and how they behave when launched from the carrier’s catapult. The testing for the S-3, conducted in 1973 was no different. The case study of the S-3 illustrates the design complexity that all naval aircraft undergo to safely operate from the carrier environment.

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Vikings from VS-21 “Fighting Redtails” aboard an aircraft carrier. Note the wing fold for stowage aboard the carrier.

The S-3 has a high aspect ratio wing the aircraft makes for a good glider and large turbofan engines that have 22:11 bypass ratio big that don’t respond to power changes quickly. Now you have an aircraft that uses low engine power to maintain position on the glide slope (with very low RPM on the engines). Not a huge problem on land but on a ship:

“If all the sudden your starting a settle coming into the carrier, you add power” to regain altitude, but nothing happens because of the delay in getting the engines to respond. “Then you find yourself sitting there looking at the ramp,” the wall of steel below the deck of the carrier. Hitting the ramp means dying. “In fact I almost hit the ramp” when testing the S-3 on the carrier, Webb said. “The combination of a very clean and very slow power response was a major problem.”

S-3 Viking 9.5 Engines
An S-3B illustrates how the engines are tilted 9.5 degress from the aircraft longitudinal axis.

Another problem was the S-3 would pitch up when power was added and pitch down when power was reduced. This was because the thrust line of the engines was below the center of gravity of the aircraft. This always placed the aircraft “out of trim” whenever a change to power was made. The remaining problem was that during simulation, Lockheed didn’t account for the burble of air coming from island and flowing across the landing area. Lockheed assumed the flow was horizontal behind the carrier.

“This gust responsiveness makes it considerably more difficult to bring aboard under wind conditions which create a strong ‘burble’ of distrubed air behind the carrier. In fact the aircraft failed its initial carrier suitability testing largely due to its gust responsiveness”

After additional flight testing, Lockheed implemented a number fixes to address these problems. The first was called “thrust trim compensation.” Whenever the pilot increased or decreased power, the elevators would automatically down or up to neutralize the pitching. “With that fix, a pilot trying to stay on glide slope while coming in to trap “does not have to fight the pitch with power all the time.””

Viking Pitch Trim System
The S-3 Viking’s Pitch Trim System as schemtically illustrated from the S-3 NATOPS Manual.

Another fix was applied to the S-3 spoilers. A spoilers is a control surface at the top of the wing, hinged on the wing’s leading edge. The spoiler is designed to disrupt or “spoil” the airflow on the top of the wing to dump lift. Normally, in the S-3, the spoilers are activated one wing at a time with movement of the stick left or right to assist the ailerons in control of the aircraft’s roll. With the press of a button the spoilers on the S-3 rise on both wings simultaneously. This allowed the pilot to reduce lift and descend faster without the pilot having to pull back on the throttles to reduce power. This “direct lift control” meant that the pilot could keep the engines at a relatively high power and not back to the unsafe “low-rpm” low power regime. The pilot needs the engines to main at a high power level in case he needs go around and try for another landing.

Viking DLC
An S-3 on launch from a carrier. You can see the DLC spoilers on the upper surface of the wings just forward of the flaps (seen in red and captured mid retraction).

These improvements took almost 10 years to apply and were collectively known as the FQIP (Flight Quality Improvement Program) Mod. The S-3 eventually became a very successful carrier borne aircraft and had a reputation the fleet as being an aircraft with relatively benign carrier landing characteristics. The Viking FQIP is one of many example of the performence constraints that naval aircraft must operate in.

S-3 Viking on the landing rollout after catching a wire.
S-3 Viking on the landing rollout after catching a wire.

Sources:

Flying the Edge by George C Wilson.

World Airpower Journal Volume 34 Autumn/Falll 1998.

S-3 Viking NATOPS.

Rockin’ in the LMT

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I’m not a big fan of defense powerhouse Lockheed Martin, but I’ll say this, they’re the only major defense contractor I know of that has a house band:

[youtube=http://www.youtube.com/watch?v=PVQ7tuIYOlA&feature=player_embedded]

It’s got a catchy beat and you can dance bomb to it.

H/T: The DEW Line

BTW, I wonder if CDR Salamander will notice just how diverse the band is…