The F-15's maneuverability is derived from low wing loading (weight to wing area ratio) with a high thrust-to-weight ratio, enabling the aircraft to turn tightly without losing airspeed. The F-15 can climb to in around 60 seconds. At certain speeds, the dynamic thrust output of the dual engines is greater than the aircraft's combat weight and drag, so it has the ability to accelerate vertically. The weapons and flight-control systems are designed so that one person can safely and effectively perform air-to-air combat. The A and C models are single-seat variants; these were the main air-superiority versions produced. B and D models add a second seat behind the pilot for training. E models use the second seat for a weapon systems officer. Visibly, the F-15 has a unique feature" vis-à-vis "other modern fighter aircraft; it does not have the distinctive "turkey feather" aerodynamic exhaust petals covering its engine nozzles, because the petal design on the F-15 was problematic and could fall off in flight; therefore, they were removed, resulting in a 3% aerodynamic drag increase.