# What is this red canister on the tail of the YF-22?

On the tail section of the YF-22 further back in the image below, there is a red canister. What is that canister for? I suppose it is for some flight testing purposes from what I can see.

I can't pinpoint the image source.

• Similar system on a X-29: What is the 'arm' at the rear of the X29? – mins Apr 4 '16 at 7:39
• This is a good question, but a duplicate of the X29 question. – J Walters Apr 4 '16 at 10:08
• Same answer, but entirely different QUESTION. Not a duplicate at all. Different airplane, and the add-on equipment doesn't look all that similar, even if the purpose is the same. – Ralph J Apr 4 '16 at 12:21
• IF the same answer can be used to answer this question, it is by definition a duplicate ... Two completely different questions, if both served by the same answer, are duplicates. – CGCampbell Apr 4 '16 at 15:18
• @CGCampbell The answers to two different questions being the same is not the same situation as an answer to one question including the answer to another. Both questions have the same answer, but those answers don't inherently answer the other question. You wouldn't (or, at least, shouldn't) close "What is the integral of x^2 from 0 to 3?" as a duplicate of "What is 5 + 4?" for example. – reirab Apr 4 '16 at 15:46

It is a spin recovery/stabilization chute, installed during testing for high AoA test flights. The following image shows it being deployed on the ground.

Image from fas.org

• Should it/ Is it usually deployed on ground or inflight? – Alex S Apr 4 '16 at 5:54
• @AlexS its an emergency system designed to deploy on command if the aircraft gets into a maneuver that the pilot cannot recover from during testing of the aircrafts envelope. – Moo Apr 4 '16 at 7:12

The canister contains a small parachute and either a spring or an explosive charge to kick the chute out when it is needed. It is a precaution for spin flight tests, that's why it is called a spin chute. See here for a video of the spin chute of the F-35.

I guess now I should explain how a spin chute works.

When an aircraft spins, it will rotate around a vertical axis which is somewhere between slightly ahead of the aircraft (regular spin) or near the leading edge (flat spin). The pitch angle $$\Theta$$ is nose down and the angle of attack $$\alpha$$ is high (around 45° in a regular spin, up to 90° in a flat spin), so the airflow over most of the wing and horizontal tail is separated. But the rotation, which is a mix of rolling and yawing, will induce an angle of attack variation over span, so a part of one wing will operate in the normal angle of attack range, where lift is high and drag is low.

Now we have a receding wing with separated flow, at high angle of attack, with low lift and high drag so the resulting air force vector R (green, below) points mostly into the direction of local flow (which is up). The other wing has partially attached flow, a lower angle of attack, high lift and moderate drag, so the resulting air force vector is almost perpendicular to the local flow, pointing up and forward. This difference in lift and drag propels the rotation.

Speeds and resulting force on spinning wing sections. $$\omega_z$$ is the rotation speed around the vertical axis, y is the local wing station and the green v shows the local flow direction.

Without the rotation the aircraft would immediately pitch nose down, pick up speed and can be pulled out of the resulting dive. With the rotation, however, we get an inertial pitch moment due to the fuselage masses. All parts of the aircraft rotate with the same yaw rate, and the centrifugal force from this yawing motion grows linearly with distance from the spin axis. This difference in centrifugal force along the lengthwise coordinate of the aircraft adds a powerful nose-up moment which in some cases cannot be overcome by the tail surfaces - remember, they have less effectivity in separated flow. Flat spins are almost impossible to escape from.

This can be tested in spin tunnels, but Murphy's Law is very relevant to spin tests. So it is a good precaution to add a spin chute for the first tests: If the aircraft cannot end the spin with control surface deflections, it is in a trap from which it can only escape with something that will add a strong nose-down moment. In the chaotic airflow behind a rapidly spinning airplane. That is why the spin chute is not bolted to the skin of the airplane, but sits in an elevated position hanging out at the back. From there it will not be caught by the tail surfaces but will work as intended in all conceivable situations (save an inverted spin ...)

• 1+ for actually explaining what a spin chute is for – Mels Apr 4 '16 at 11:29
• In the last paragraph, didn't you mean a nose-up moment? The chute would have to be below rather than above the plane's COG to pull the nose down, right? – Mels Apr 4 '16 at 11:32
• @Mels: No, airflow is coming predominantly from below, so the chute will fly above the aircraft and pull the tail up. – Peter Kämpf Apr 4 '16 at 11:49
• Ah, I get it. Thanks for the clarification :) – Mels Apr 4 '16 at 12:03
• @PeterKämpf because I don't want to give you an excuse to write us a text book. :) I just wanted to be sure I understood the basics of what you were explaining - I think a full description would go beyond my ability to comprehend. (There's more than one of your answers that I've read through, nodded my head, and said, "Yeah, I get that (no, I really don't)") – FreeMan Apr 4 '16 at 17:05