November 30, 2018, ©. Leeham News: Last week we started a series on pitch stability of aircraft. It has actuality as the reason Boeing introduced the now well-known MCAS (Maneuver Characteristic Augmentation System) was to improve the pitch stability of the 737 MAX.
We discussed the pitch stability of the basic wing last week. This week we add the fuselage and see what happens.
Pitch stability of fuselage with wing
We could see the pitch stability of a wing could be both positive (the pitching moment around the center of gravity is decreasing with increased Angle of Attack (AoA)) and negative (an increasing pitch up moment with AoA).
We now add a fuselage to the wing. If we look at an airliner fuselage alone it’s destabilizing in pitch. It depends on where the center of gravity of the fuselage is, but if it’s at the middle it’s unstable. The reason is the flow over the rounded nose creates a low-pressure area directly behind the cockpit, Figure 2.
The light blue area directly behind the curved part of the 787 nose shows the airflow creating a low-pressure area on the top front of the fuselage at the typical cruise angle of attack of 3°.
We can also see a low-pressure area over the wing join with the fuselage. This is a spillover from the wing’s low-pressure and not generated by the fuselage in itself. Further back we have no low-pressure areas like on the front of the fuselage.
This is a pressure map of the 787 but other airliners look the same. Aircraft designers are carefully designing the nose curves to create as little low-pressure over the front of the cabin as possible. It creates a pocket of high airspeed (low-pressure and high airspeed go together) which often reaches supersonic levels, by it creating wave drag when the airflow goes back to subsonic flow after the area.
If we add a fuselage which has a strong pitching up moment to a wing which does not have a strong pitch down moment, the combination is unstable.
In Figure 3 the moment curve of the fuselage and wing of an early variant of what became the DC9 is shown. It’s a pitch moment curve which has been measured on a wind tunnel model which has the pitch axis at 40% wing chord.
The combination is unstable. The nose up pitching moment increases with AoA for the reasons we discussed. The change of the curve at 17° AoA is interesting.
The classical wing profile of the DC9 candidate had a pitch up wing moment. When the wings stall, the suction peak at 25% wing chord collapses and the wing moment changes to pitch down. The area directly after a stall is stable (nose down with increasing AoA).
The combination is stable until 25° AoA when it pitches up again if the AoA increases further. The totally separated flow on the wing is once again not stabilizing in pitch.
The bump in the pitch moment curve as the flow around the parts changes shows the problems an aircraft designer is challenged with. The design has predictable characteristic in the flight envelope for normal use. But when approaching the extremes of the envelope, moment curves and lift changes and measures to handle the initial part of the curve must now be adapted to the new situation.
Note how the pitch up moment with increasing AoA decreases when horizontally protruding objects like DC9 style rear pylons and nacelles are added to the fuselage. These add aerodynamic area behind the Center of Gravity and this adds stability.
This curve has a lower angle. It means the pitch up moment grows slower than for the original curve.
In the next Corner, we discuss how we introduce a horizontal stabilizer to turn this unstable curve into a stable curve.