# What aerodynamics forces are acting on a jet fighter during slow flight demonstration?

I have long been contemplating the forces involved in certain maneuvers in an aeroplane and some of those maneuvers begin to break down my understanding of the forces that specifically oppose gravity. Typically this is described as lift and that is how it is explained in ground school, but they did not go over other forces that can be involved keeping a plane in the air (i.e. a non-horizontal component of thrust etc.). They probably didn't go over this as it doesn't apply (significantly) to the type of planes you're one would fly right after basic ground school.

One of those maneuvers I've thought about is a "slow flight" when performed at an air show by a jet where the jet appears to be going slower than their stall speed and at a very high angle of attack. It seems that a portion, if not all of the force keeping the airplane airborne is coming not from the wings in the form of lift but rather the thrust produced by the turbine. (Example:

)

1. Is my understanding accurate?
2. Is there a good, well known reference to aerodynamics that does not gloss over the entire gamut of forces involved during a wide range of flight situations?
• Ryan, pardon my English but I couldn't understand what the second question is asking. Can you clarify a bit? Oct 27 '17 at 19:00
• @GürkanÇetin, to me it sounds perfectly clear, though off-topic, because requests for external resources are generally frowned upon at the stack exchange (but can still be answered by simply providing suitable introduction instead). Oct 27 '17 at 19:16
• I think the question is asking for a particular kind of aerodynamics reference. But what :) Oct 27 '17 at 19:19
• @GürkanÇetin, I think elementary kinematics would be actually quite enough. Oct 27 '17 at 19:20
• Not sure about this video but sometimes as the plane slows down you can hear the after burner turned on. I remember in 2010 Zhuhai Airshow a Chengdu J10 did a 160km/h slow pass with the afterburner turned on half way, after that the plane were basically "standing" on its thrust. Oct 27 '17 at 19:36

break down my understanding of the forces that specifically oppose gravity.

There are no separate forces that oppose specific other forces. There is simply a set of forces and they all add up and the sum divided by the inertial mass of the plane equals acceleration.

Gravity always points down, so to prevent downward acceleration, the sum of the other forces must point up. But whatever force points up can do that.

1. Is my understanding accurate?

Mostly yes. Thrust always points forward along the engine axis, so if the engine axis points up, so does thrust.

1. Is there a good, well known reference to aerodynamics that does not gloss over the entire gamut of forces involved during a wide range of flight situations?

There is really just a few forces, always:

1. The airfoils produce force that is approximately perpendicular to them. It grows with angle of attack (angle between the airfoil and the air flow) with one peak at the at the stall angle of attack (usually 10–15°) and another at 90°.

This force is normally described as two components, lift, which is perpendicular to the air flow, and drag, that is parallel to it. That means the second maximum of lift is at 45°, because at 90° the force is parallel to the flow and thus all drag.

2. Form (parasite) drag. This is parallel to the flow.

3. Thrust. This is always approximately forward along the engine axis.

4. Weight. This always points down. It is actually composed of gravity, which points towards the centre of Earth, and small contribution of centrifugal force that points away from the Earth axis of rotation, but that's just nitpicking.

And then you just need to do vector addition.

So when the aircraft flies with high angle of attack, the thrust points diagonally up, cancelling some of the weight, while the force on the wing is tilted aft, cancelling the other part of weight, but also causing huge drag, so the thrust needs to be correspondingly large. That's why only fighters and aerobatic aircraft with their large thrust/weight ratios can do this kind of stunt.

For more details I would suggest See How It Flies by John S. Denker. It covers basic aerodynamics and explains reasons behind common piloting techniques. Chapter 4 discusses the forces.

• Good explanation! Maybe it would be beneficial to ensure the distinction between pitch angle and angle of attack is maintained (even though a detailed definition might be overdone in the depth required so far). If of interest, another great source of good explanations of the phenomena of flight physics, with very little math required for understanding, is the „Flightwise“ series of books by Chris Carpenter. Oct 27 '17 at 23:28