I have noticed this on several aircraft like the SR-71 as well. It appears, based only on initial looks, that the center of gravity on aircraft like these is farther forward than the center of lift. The wing is almost completely behind the center of the aircraft. So what is balancing the aircraft?
I made some guesses at the reasoning but they are purely guesses:
1- The supersonic nature of the aircraft have something to do with it since it doesn’t seem subsonic aircraft use designs like these.
2- I am vaguely aware there is a concept that the cross-section of supersonic aircraft should be a consistent area throughout the aircraft. I’m not sure if this is part of it or not
3- Maybe it’s just as simple as the heavy engines are in the back?
In: Engineering
Combination of heavy engines in the back and the fact that the center of lift of an airfoil is near the quarter chord line (25% of the way from the leading to the trailing edges) not the geometric center of the airfoil
The Concorde and SR-71 are both supersonic aircraft, and their design reflects the challenges of flying faster than the speed of sound. You’re right in noticing that their wings seem to be positioned further back than on subsonic planes, and there are some important reasons for this.
First, supersonic aircraft have to deal with different aerodynamic forces than subsonic planes. At supersonic speeds, shockwaves and drag become much more intense. The wings are placed further back to help balance the aircraft when it’s flying at these speeds. This is because the center of lift shifts rearward at supersonic speeds, so the designers need to make sure the plane stays balanced by having the center of gravity closer to the nose.
Second, the “consistent cross-section” concept you mentioned is the **area rule**. This is important in reducing drag at high speeds. It basically says that the cross-sectional area of the aircraft should change as smoothly as possible from front to back. So, designers shape the fuselage (body) and wings to follow this rule, which affects where the wings are placed.
Lastly, you’re right that engine placement matters too. On planes like the Concorde and SR-71, the engines are at the back, which helps keep the aircraft balanced despite the wings being further back. The tail also helps balance the aircraft by providing downward force to keep the nose from pitching too far forward.
So, in short, it’s a mix of dealing with the physics of supersonic flight, managing drag, and making sure the plane stays balanced with the engine placement.
The SR-71’s fuselage chines (the knife shaped edge) produce something like 20% of the aircraft’s lift at cruising speed. Supersonic aerodynamics are not very intuitive.
I happen to have an original copy of “Concorde 001 Flying Qualities Tests” from 1973.
Numerous tests were made and described, including all speed, altitude, and maneuvering ranges (climb, descent, level, accelerating, decelerating, turning, …)
It’s way too much to summarize, but some things stand out.
– Fuel movement is used to adjust center of mass in flight.
– Pitch moments during acceleration and deceleration vary a lot depending on altitude, even varying from positive to negative.
– The aircraft is equipped with automatic trim capability, but pilots describe the aircraft as well behaved with it turned off. Low stick loads.
– Several pitch moment “discontinuities” were found accelerating through transonic speeds.
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