The F-8 variable incidence wing
In September 1952, the Navy issued specifications for a new carrier-based fighter capable of exceeding the speed of sound in routine-level flight. Chance-Vought won the bid with its F8U Crusader design (later re-designated the F-8).
One of the most capable fighters of the post-World War II era, the F-8 Crusader was a sleek design that featured a gaping jet intake beneath the fuselage and a variable-incidence wing that could be raised to enable the aircraft to land and take off at slow speeds while maintaining excellent visibility for the pilot.
Robert James, former US Navy F-8 Crusader pilot, explains on Quora:
‘The F-8 was unique because it had a variable incidence wing. Rather than tilt the entire aircraft to achieve the desired high angle of attack required for slow flight, the F-8 tilted the wing up 7 degrees (both for takeoff and landing). That made the fuselage angle 7 degrees lower than “normal,” giving good over-the-nose visibility and letting the design engineers fit the aircraft with short landing gear that could be strong and light. It was a good system, and whoever flew the F-8 loved it.
‘However, the relatively flat fuselage angle meant that the thrust vector was more horizontal than any other carrier aircraft. So adding thrust to correct a low condition when on final approach didn’t have quite the same effect as it did for, let’s say, the F-4 Phantom. In addition, the aerodynamic thrust-required curve for the F-8 had a little bit of a flat spot at the bottom, right where you operate when making a carrier approach.
LSO hard time judging the Crusader energy state just by looking at it
‘The result was that the F-8 could fly at a range of different air speeds just above and just below the target airspeed without changing its angle of attack as much as “normal”. In other words, the overall energy state of the airplane could be a bit high or a bit low while everything looked pretty good.
‘In the real world, it meant that you could be very close to the desired perfect angle of attack for landing, yet you could be slightly underpowered or slightly overpowered, and those conditions were not as easy to recognize as they are in other aircraft. It also meant that the Landing Signal Officer [LSO] had a harder time judging the overall energy state of your airplane just by looking at it.’
He continues;
‘So why was that a problem, you might ask. Here’s why. Let’s take a typical F-8 approach. Say you make a slightly high start and correct by reducing power a bit. As you increase your sink rate to correct, you watch the Optical Landing Aid and see the “high” ball (meat-ball) coming down to the middle, in line with the green datum lights. Good, now you add a touch of power, but it’s not quite enough, and/or you waited one or two seconds too long to make that power adjustment, and you sag a bit low, let’s say half a ball low.
Not F-8 qualified LSO
‘A bit more power easily corrects that and you see that the meat ball is rising back up to the center so you ease the throttle back a tiny bit, hoping to find that sweet spot where you’re producing exactly the correct amount of thrust to keep you on glide path at the correct angle of attack. You’re doing a good job flying right on the perfect angle of attack and you’re working off that tiny bit of a high ball as you approach the “in close” part of the carrier pass. You don’t want to go “over the top,” and you’re happy to see that the ball is just barely high; you’re on angle of attack and your line-up is good too. But, in fact, you’re carrying less thrust than ideal because you’re working on that half-ball-high situation. Nevertheless, things are looking good.
‘You look good to the LSO too, but this time the guy waving is an A-4 pilot. He’s a good LSO just like all LSOs, but having never flown the F-8, he may not have an appreciation of what is about to happen. A salty old F-8-qualified LSO might say “don’t go low” or he might softly say “a little power” as he sees the pilot working off that very slightly high ball. But this time the A-4 LSO just sees a good pilot making a pretty good pass. Meanwhile, the Naval Aviator in the F-8 is easing down to that perfectly centered ball and maintaining the correct angle of attack, but he is doing all this while slightly underpowered. He’s operating in that quirky flat part of the drag-required curve where things can suddenly go wrong, and they do.
F-8 variable incidence wing leads to ramp strike
‘At this point, the F-8 is just a couple of seconds from touching down, and the pilot has successfully worked off that slightly high ball in close while keeping his angle of attack indicator centered and maintaining the center line. But now that underpowered condition he’s been carrying for a handful of seconds catches up with him. The Crusader at this moment isn’t very far from perfect, but it’s enough underpowered so that the low energy condition causes the airspeed to decay and the sink rate to increase, and they can do so at a surprising rate.
‘If the pilot and the LSO aren’t right on top of this situation, bad things happen. With luck, the pilot adds enough power to quickly correct the situation and engages the one wire, which is always frowned on. Worse, he might get a “taxi-to-One Wire” grade in the LSO log book. Shame on him and his fellow Naval Aviators will give him the “side eye” look when he gets back to the Ready Room.’
James concludes;
‘Or, as has happened many times even to the best pilots, he might be a second (literally only one second) too late noticing the hole he’s about to fall into, and he hits the ramp in a big ball of fire. How embarrassing. And of course, his poor mother won’t like it when they ship his remains home in a rubber bag.’
Photo by U.S. Navy
