Where does the lift come from when the plane flies upside down?

In many flight performances, in order to show the good mobility of the aircraft, we can often see the aircraft flying backward.

This flight mode does not mean that the aircraft retreats like a car reversing, but the aircraft turns over 180 degrees, that is, the pilot is sitting and driving the aircraft, and suddenly turns over 180 degrees and drives the aircraft upside down.

In this case, How is the lift provided by the aircraft? Doesn’t it push the aircraft down.

Inverted flight: a stunt flight in which the aircraft’s belly is up, back is down and the pilot’s head is down.

In order to keep the lift up and maintain the weight during inverted flight, a negative angle of attack must be adopted.

In order to realize reverse flight, we must also overcome the problems of abnormal fuel and lubricating oil flow, the pilot’s head down and out of his seat, and abnormal aircraft stability and maneuverability.

Method for realizing reverse flight

When the aircraft needs to fly backwards, one method is to make the fuselage fly backward by making a semicircle arc upward in the vertical plane;

The other is to manipulate the aileron of the aircraft to rotate, so that one wing is up and the other wing is down to rotate around the fuselage, forming an inverted state of the aircraft. This phenomenon can often be seen in stunts.

During reverse flight, the aircraft tilts the fuselage obliquely upward at the nose and downward at the tail to form an appropriate angle of attack (or negative angle of attack) between the original upper surface of the reverse wing (with flaps, ailerons and tail wings) and the flight direction, so as to generate lift and maintain the reverse flight state of the aircraft.

Principle of inverted flight

First, let’s look at the principle that aircraft can fly.

The direct reason why the wing can produce lift is the pressure difference between the upper and lower surfaces of the wing.

For low-speed aircraft with flat convex airfoil, the upper surface of the wing will increase the velocity and reduce the pressure, resulting in airfoil lift. Another important factor in the lift generated by the wing is the angle of attack.

When the wing has a certain angle of attack with the air flow direction, not only the negative pressure zone will be generated on the upper surface of the wing because the air is accelerated by the curved airfoil, but also the positive pressure zone will be generated on the lower surface, and the overall lift will be greatly increased.

The aircraft maintains level flight not only by airfoil lift, but also by angle of attack lift. However, this angle of attack cannot be increased indefinitely. When the angle of attack is too large, the air flow on the upper surface will separate due to friction effect and viscosity (the air flow cannot flow continuously from the leading edge to the trailing edge of the wing), and the lift will drop sharply. In addition, the positive pressure zone on the lower surface of the wing will produce huge resistance, and the aircraft will soon enter a stall state.

To sum up, the aircraft can fly in the air not only by the special shape of the aircraft wing, but also by the fact that the aircraft wing has an angle of attack with the moving direction.

Let’s talk about flying backwards. Even if the plane turns upside down, if the wing still maintains a positive angle of attack with the forward direction, it can have enough angle of attack lift. The nose of the aircraft in reverse flight should be tilted upward than in normal flight, in order to have a large enough angle of attack to generate enough lift on the wing to maintain the reverse flight state of the aircraft.

Inverted fuel supply device

When flying backwards, what the pilot really needs to pay attention to is to be careful not to turn off the engine. For ordinary light aircraft, most of the oil storage and supply systems are supplied by gravity. It is easy to stop the oil supply when the aircraft flies upside down, because the position of the oil supply valve has changed and is already at the top of the fuel tank.

When the aircraft is flying with negative overload, only with the help of special devices can it ensure the normal oil supply to the engine. The negative overload fuel supply device of aircraft fuel system is mainly divided into the following categories according to the common structural form:

Double-end oil supply pump type negative overload oil supply device

Usually, the impeller at both ends is driven by a motor, and the oil supply device is installed in an oil supply box.

Multi pump high low negative overload oil supply device

Two or more separate oil supply pumps are respectively installed on the oil supply device at a specific position in the oil supply tank.

Counterweight type negative overload oil supply device

When the aircraft flies with negative overload, the device installed in an oil supply tank, its counterweight is operated by a lever to close the fuel suction port at the lower part of the oil supply pump, and the fuel enters the oil supply device of the oil pump from the suction port at another high position.

Pressure storage tank type negative overload oil supply device

A rubber capsule is installed in the oil tank to separate the oil tank into oil chamber and air chamber, and there is a separate oil supply device for pressurizing the air chamber.