Welcome everybody to our new series, “Dissecting the X”. In this series, we will take a closer look at different types of airplanes, be it military or civil. We will discuss the characteristics of this type of airplane, what makes it special and different in regards to aerodynamics, mission and equipment. I will try to be thorough but not lost in the detail, so you can get an overview of how each component plays its role as part of the airplane. At the end of each post I’d appreciate it if you could leave a little comment, telling me what you liked or didn’t like, and which airplane you’d like to see next time.
We’ll start this series with the F-15. The McDonell Douglas F-15 Eagle entered service in 1976 and has been in service since. It is expected to stay in service well beyond 2025. This has several reasons. First of all, the F-15 airframe is a true powerhouse. It has been very well designed and several service updates have kept it…well, up to date. It can stil compete with newer generation aircraft, like the Eurofighter (the Eurofighter has the advantage though, as it is a newer design). Another reason is the fact that due to current budget restraints the US government (the main operator of the F-15) cannot afford to field a large number of more modern aircraft (like the F-22) and therefore has to rely on the F-15.
The following video is a pretty impressive proof of the F-15’s capabilities.
Let me recap this: the F-115 in this video flew with only one wing! The other one was missing, it was torn off during a maneuver accident! Now, how could the F-15 do that? Let’s find out!
Fuslage – The F-15 has a very thin fuselage (compared to other aircraft) that also generates lift. See, a special form is not the only way of generating lift for a body. If you stick your hand out of the window of a car in motion and there is an angle between your hand and the wind, you will feel a lifting force on your hand. The same goes with all kinds of bodies. Even a flat board will generate lift if it has been angled relative to the wind. Now, why go through all the trouble of manufacturing and designing wings, if flat boards would do the job, too, you might ask. A relevant question. The reason for the use of special wing “forms” (called wing sections), is the fact that the bigger the angle gets, the harder it is for the air to follow the board. Eventually the flow seperates from the surface of the board, which leads to a dead area downstream of the seperation point and leads to lower lift, and higher drag (the force acting in the opposite direction of motion…which means drag is bad!).
The F-15 has a high wing configuration. One advantage is that a shorter landing gear can be used. But the flow over the wing can also negatively influence the vertical stabilizers, as there is not the fuselage inbetween to prevent this.
Control Augmentation System (CAS) – When the F-15 was designed, fly-by-wire technology (that is, electronic controls) were not in use. In fact, the F-16 was the first aircraft that relied solely on fly-by-wire. So at first the F-15 had mechanical control systems. Later it was upgraded with a so called Control Augmentation System (CAS). The CAS is an electronic control system that translates and coordinates the inputs of the pilot to achieve superb performance. The mechanical systems were still kept in the F-15, so it could still be flown in case the system went inoperable. How could this have contributed to flying the F-15 with only one wing? Well, the CAS saw that something was wrong with the missing wing (it was missing) and translated all the control inputs of the pilot in such a way that the airplane could still be flown.
Wing Configuration – The F-15 has a swept back wing, that somehow resembles a delta wing. To enable to F-15 to fly several times the speed of sounds without producing exessive drag, the sweep angle of the leading edge is very important. Also notice the big wing area of the F-15. The lift a wing can create is directly related to its’ wing area, the bigger the area, the more lift can be generated at a given speed. A high wing area means the F-15 can fly at lower speeds and have a smaller turn radius when maneuvering, which is very important for fighter planes.
Empennage – The empennage of an airplane is the sum of its vertical and horizontal stabilizers. As a fighter airplane, the F-15’s empennage needs to be as light as possible, but still generate enough force to make the F-15 highly dangerous in aerial combat. A big problem especially fighter planes have is the integration of the empennage and the engines with the fuselage (the body of the airplane) in such a way that the different parts influence each other as minimal as possible. The F-15 is a good example of this integration.
It has two vertical stabilizers as a result of this integration. Two vertical stabilizers mean that the area of each one can be smaller than the area of a single vertical stabilizer. That also means that each one of those two stabilizers doesn’t have to endure as much force as a single one and can therefore be built lighter. Everything has its drawbacks though. Two stabilizers means more interference between the different parts of the empennage and higher drag! An important thing to understand in aerospace engineering is the fact that it basically is a big game of trade-offs. There is nothing one can get without having to give up something else!
The F-15’s horizontal stabilizers are all-moving. This gives the pilot more control over the aircraft, but it also means heavier and more complicated controls. An interesting thing to remark about the F-15’s horizontal stabilizer is the distinct sawtooth on the leading edge. This sawtooth is supposed to generate extra lift via vortices. It is usually used when late in the design phase experiments and calculations show that the body is not generating enough force, but a total redesign would be too expensive. It is sort of a quick and dirty fix.
Variable geometry intake – The F-15 is supposed to fly at a speed higher than twice the speed of sound, which makes every aspect of its design more complicated. Everything has to be custom tailored for these special situations. For that reason the air intakes for the two turbines have variable geometry, to adapt to the changing conditions of supersonic flight. (In the following picture you can see the intake of the F-14, but the F-15’s intakes look pretty similar.)
The intakes are positioned on the shoulders of the airplane. This is kind of unfortunate, because positioning the intakes under the center of the airplane leads to a better airflow into the intake at all angles of attack, in contrast to the position chosen for the F-15. Wind tunnel experiments and CFD calculations have shown that the two intakes, if centered side by side under the center of the fuselage would influence each other in an unacceptable way.
Mission – The F-15 is a tactical fighter and was mainly designed to gain and mantain air superiority. Air superiority basically means controlling ones own aerial territory and having a distincting advantage over the opposing air force. So the F-15 was basically designed for air-to-air combat (also called dogfight). How do all the different components fit into this? Well, the powerful engines allow a very high thrust-to-weight ratio, that allows the F-15 to have a very high acceleration. Combined with the variable geometry air intakes, this allows the F-15 to reach supersonic speed. The big wing area and low wing loading (the ratio of the wing area to the weight of the aircraft) give it superb maneuverability and a small turn radius, which allows the F-15 to easily turn into an enemy airplanes turn.
Little Copyright Info:
Some of the pictures were taken by members of the US Armed Forces while on duty, so they are in the public domain. One schematic was taken from wwwf-15e.info, they have all the rights to that schematic. All other schematics were taken from Klaus Hünecke’s book “Modern Combat Aircraft Design”. I do not own those copyrights!