Decoding the Mystery of Jets Speed: Thrust, Drag, and Beyond

Have you ever wondered why some jets can fly faster than others despite having similar engines and weights?

It's a common question, and one that might seem akin to the mysteries of why some cars are faster than others or why some people can run faster than others. But the answer lies in a deeper understanding of the interplay between thrust, drag, and the design of the aircraft.

Basic Principles of Thrust and Drag

In level flight, speed is determined by the balance between thrust and drag. Simply put, the faster you go, the more drag you create, and thus the more thrust you need to maintain that speed. Additionally, size and weight generate drag, so the larger and heavier the aircraft, the more thrust is required.

Think of it in terms of a seesaw. The more weight you add to one end, the more force you need to keep the other end in balance. Similarly, the more thrust an aircraft needs to overcome, the faster it can go, but only up to a point. Beyond that, the design of the aircraft will start to limit its speed.

Why Some Jets Fly Faster: Less Drag, More Speed

Some jets can achieve higher speeds with less thrust because they inherently experience less drag. This is exemplified by the F-5 Tiger II, a light fighter with only 10,000 lbs of thrust in full afterburner but capable of supersonic speeds exceeding Mach 1.5. This is due to its size, weight, and low drag design. In contrast, the Boeing 777X, with 220,000 lbs of thrust (22 times more), can only maintain a speed around Mach 0.9. Its enormous size generates significant drag, limiting its speed.

Imagine comparing the speed of a jet ski with a 30-hp engine and a supertanker with a 30,000-hp engine. Despite the supertanker having much more power, it is significantly slower because it has an enormous amount of drag. The same principle applies to aircraft.

Supersonic Speeds and Intuitive Challenges

When it comes to supersonic aircraft, the relationship between thrust and drag becomes even more complex. Some fighters are limited to subsonic speeds due to their aerodynamic design. For example, the A-4SU, with 10,800 lbs of thrust (about the same as the F-5), has a lower top speed. This is because the A-4 is limited to subsonic speeds by its aerodynamics.

Some fighters, like the F-22 Raptor, have their supersonic top speed limited by their intake systems. Despite having more thrust than the F-15 Eagle, the F-22 Raptor is slower because it uses fixed bypass ramps instead of moving ramps, which significantly reduce radar reflectivity but cause the engine to lose thrust at high supersonic speeds. In contrast, the F-15 Eagle, with moving ramps that allow the intake to efficiently direct air to the engine, can maintain full thrust at a higher Mach number and operate at higher speeds without overheating.

Conclusion

The speed of jets is a fascinating topic that combines elements of aerodynamics, engine performance, and design. By understanding the principles of thrust and drag, we can appreciate the design choices and engineering challenges that allow some jets to fly at incredible speeds, while others are limited by their design.

So, the next time you see a jet flying past, remember the intricate balance of thrust and drag that makes it all possible.