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I have created the outer mold line of a blended wing body drone and have put it through Ansys Fluent. Using SST k-omega turbulence model and 40 m/s of airspeed. However, the output from the program tells me that the center of pressure of my aircraft is not on my aircraft, but above it and behind it. Is it possible that the center of pressure of an aircraft is not on it?

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  • $\begingroup$ Likely related -- aviation.stackexchange.com/questions/90803/… $\endgroup$ Jan 15 at 17:01
  • $\begingroup$ @quietflyer the grammer of that question is messy and explaination is hard to understand. So I still can't understand what they are talking about. But does that question states that the center of pressure can be out of the wing? If that is so, then how do we calculate the static margin? $\endgroup$
    – Faito Dayo
    Jan 15 at 20:08

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If I understand the details of your question (the first part), the center of pressure should never be completely behind every part of the aircraft structure. That would not be possible.

However to answer the question as you pose it in the last sentence, the CP does not have to be on a part of the design where there is actual aircraft structure. If the aircraft is designed (kinda like a P-38 or a Cessna 337 Skymaster), where there are aerodynamic surfaces which contribute to the overall aerodynamic center, both ahead of and behind an open area where there is no aircraft structure, the CP could be in between pieces of the aircraft. Think Center of Mass, (or Center of Gravity), of a donut-shaped object...

The static margin is the distance between the Center of Mass and the aerodynamic center (or center of pressure - I can never remember which) , Once these two points are located mathematically, the fact that one or both is not coincident with any part of the actual aircraft structure does not affect the accuracy or validity of the calculation of static margin. Think about a seesaw built from 2 seesaws that are bolted together with their seats, at the ends of the two beams that cross over the fulcrum. The center of gravity could be in empty space between the two beams, but the calculations that determine where the balance point is (where to mount the structure on the fulcrum), will still be valid.

Remember, all the mathematical equations, or relationships, we use to calculate aerodynamic forces, and the impact of them on an aircraft are very accurate mathematical models that represent what is actually happening to the aircraft, in a way that allows us to do calculations that result in correspondingly accurate answers to important questions, and can therefore allow accurate predictions about the aerodynamic behavior of the aircraft. They are not what is actually happening! What is actually happening is that molecules within the atmosphere are bouncing off the entire surface of the aircraft, and each collision results in a transfer of momentum (think FORCE), that pushes on the surface at that point, normal, (or perpendicular), to the surface at that point. All the things we think of or use in our aerodynamic equations (like LIFT, DARG, etc.) are aggregate constructs that do not really exist in the real world, they are mathematical fictions constructed to very accurately represent the vector sum of all the individual forces pushing on every single point of the airplane. LIFT, for example, is the component of the sum of all forces that is perpendicular to the flight path of the airplane. Drag, that component that is parallel to the flight path.

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