Piggybacking on Spill’s post on the S-3, I should mention that he and I discussed Direct Lift Control quite a bit the other day.
DLC is used on several different aircraft. And while there are various ways of achieving the effect (the F-35C apparently programs the flaps) let’s take a look at the F-14 Tomcat’s version.
On a carrier approach, you have to balance several issues simultaneously. Airspeed, angle of attack and attitude, and lineup.
Lineup is the left or right displacement of the aircraft from the extended centerline of the landing area. Now, since the landing area of a carrier is canted to port 8-10 degrees, and the ship is moving forward, lineup is never static for the approaching aircraft. The landing area appears to be continuously crawling to the right. So a series of corrections for lineup have to be made throughout the approach. The amount of correction varies due to the variances in just how much ambient wind there is, and the actual speed of the carrier through the water.
Airspeed is critical as well. The lift generated by the wings of a plane is directly related to the speed of the plane, obviously. Similarly, attitude, that is, the amount of nose up pitch, and angle of attack, are critical with respect to the rate of descent. The two are related. AoA is very roughly the angle at which the wings are biting in the air. Obviously, attitude is related to this. But so is airspeed.
Changing any one of the three, airspeed, attitude, or angle of attack changes the other two factors. Given that precision needed for a successful carrier approach, that places an enormous workload on the aviator. And so, tools to reduce that workload are prized.
Here’s the other odd thing. You’d expect airspeed on an approach to be controlled by the throttle, and the angle of attack to be controlled by the control stick. In fact, it’s just the opposite.
When a carrier jet settles into the groove for its final approach, jet is supposed to be at a given airspeed (generally about 130 knots, but varying by type), and a specific angle of attack (again, varying by type) and at a specific rate of descent (again, varying by type, but aligning with the standard 3.5 degree glideslope used on a carrier approach). The jet would ideally maintain this slight nose up attitude all the way to touchdown. There’s no “flare” to stop the rate of descent just before touchdown.
When in this approach configuration, the jet is said to be on the back side of the power curve. You normally think of jets, pull back on the stick, the nose goes up, and the plane climbs, right? On the back side of the power curve, the increase in induced drag from the increase in angle of attack actually causes the plane to slow down, and in fact, increase the rate of descent! Pushing the stick forward lowers the nose, increases the speed, and reduces the rate of descent.
In the cockpit of every carrier jet, there’s a quick visual aid to tell the pilot his angle of attack- the AoA indexer. What it is really telling the pilot is if he is fast or slow. The pilot simply cannot glance down to his airspeed indicator. Even in HUD equipped aircraft, an AoA indexer is a faster way of imparting information to the pilot than a digital airspeed indication).
If you’re slow, pitch the nose down slightly. If you’re fast, pitch the nose up slightly. Helpfully, the “arrows” point the way you should go. If you’re seeing the green donut, you’re on speed. While the picture shows all three symbols illuminated, in operation, only one would show (or on some, two, for instance red and green, indicating slightly slow).
Having this tool to show his airspeed, the carrier aviator also needs information on his glideslope. As noted, there’s a notional 3.5 degree glideslope reaching from the ideal touchdown spot aft into space along the approach path. To give the pilot a visual reference, mounted on the port side of the carrier is “the meatball.” The IFOLS, or Improved Fresnel lens Optical Landing System shines a beam of light along that 3.5 degree slope. That beam is centered between datum lights that show the proper glide slope. If a pilot is high, the “ball” climbs above the datum lights. If the pilot is low, the ball sinks. Sink to far and the datum lights turn red, because landing short on a carrier approach means smacking into the aft end of the carrier.
When you hear Maverick at three quarters of a mile, call the ball, that’s what he’s seeing- confirming to the Landing Signal Officer that he in fact sees the IFOLS.
If our intrepid aviator is on speed, but a bit high, he would squeeze off just a touch of power. That increases the rate of descent. As he approaches the correct glideslope, he’d add on a bit of power. If our aviator is low, he would goose the throttles a bit, and then pull off a bit before climbing through the glideslope.
The problem is, it’s very rare to only have to make one correction. Instead, our aviator would end up having to jockey the throttle virtually to touchdown. All while trying to maintain the perfect speed, attitude, and angle of attack.
So back to DLC. If there is a way to suddenly increase or decrease the rate of descent, without having to jockey the throttles, that’s a boon. And that’s what DLC does.
On the F-14, on carrier approach, the spoilers were partially deployed. That inefficient use of the wing raised approach speed by about 10 knots. That’s the downside. On the plus side, if our aviator is high on his approach, simply using a thumbwheel on the control column allows him to add a bit more spoiler deployment. That instantaneously increases the rate of descent. Coming to the proper glideslope, releasing the thumbwheel puts the spoilers back in the default position, and instantaneously puts the Tomcat back to the normal rate of descent. The converse is also at work. Low? A little thumbwheel lowers the spoilers, increasing the efficiency of the wing, and decreasing the rate of descent.
Spill also mentioned the poor response time of the S-3’s engines at approach power. The lower the power a jet engine is producing, the lower its RPM. Inertia being what it is, it takes time for jet engines to spool up to produce more power.
For this reason, most carrier jets fly the approach with their speed brakes deployed. The higher drag means they need considerably more power to maintain their approach speed. That higher RPM also tends to improve throttle response times, as there is less inertia to overcome. If also means that if a pilot suddenly needs quite a bit more airspeed, all he has to do is pull in the speed brakes.
When Spill and I first talked about DLC, I was a bit surprised to learn one of the very first uses of it was on the Lockheed L-1011 TriStar jetliner. Apparently, it was rather highly thought of by the crews.