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.
Asked
Active
Viewed 7,523 times
28
-
7Similar system on a X-29: What is the 'arm' at the rear of the X29? – mins Apr 04 '16 at 07:39
-
This is a good question, but a duplicate of the X29 question. – J W Apr 04 '16 at 10:08
-
6Same 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 04 '16 at 12:21
-
1IF 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 04 '16 at 15:18
-
@CGCampbell I think the answers are similar, but not the same because the piece of equipment itself in question is different. – SMS von der Tann Apr 04 '16 at 15:26
-
5@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 04 '16 at 15:46
-
@reirab but here we are comparing "What is the integral of x^2 from 0 to 3?" to "What is the integral of y^2 from 0 to 3?". It is the same object, simply with a different colour and mounted on a different aircraft. Or we will have to accept all future questions asking about this same object when mounted on all aircraft that will undergo (or have undergone) this kind of tests. – Federico Apr 05 '16 at 06:48
2 Answers
47
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 ...)
Peter Kämpf
- 231,832
- 17
- 588
- 929
-
10
-
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 04 '16 at 11:32
-
2@Mels: No, airflow is coming predominantly from below, so the chute will fly above the aircraft and pull the tail up. – Peter Kämpf Apr 04 '16 at 11:49
-
-
I'm very confused by the spin mechanics you've described. The plane starts with a pitch down (para 3), which is good (para 5), except for the rotation which causes pitch up (later in p5), which needs to be overcome by the tail (end of p5). If the tail cannot overcome the pitch up, you pop the 'chute which adds nose down (p6). Are you saying that the spin initially starts nose down, but then converts to nose up (because physics), and if that nose up is not counteracted by the 'chute, the ground will handle that task for you? – FreeMan Apr 04 '16 at 15:10
-
@FreeMan: I tried to describe a spin as briefly as possible and left out many details. The pitch attitude is always nose down (regular spin) to horizontal (flat spin). High pitch is better because it is easier to end. This is all true for a fully developed spin, so there is no sequence. A description of the development of the spin would include the incipient spin, which is characterized by pitch oscillations and rapidly changing flow conditions. I felt it would be too much to include here. Then we should also talk about oscillating spins ... why don't you ask specifically? – Peter Kämpf Apr 04 '16 at 16:52
-
1@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 04 '16 at 17:05
-
@FreeMan: You know, once at University I remarked to a friend "I am too stupid to understand this", whereupon he replied: "No, the professor is not able to explain it well enough. It's his job, after all". I had to agree. Now I want to explain things well enough so anyone interested can follow, and I need your help to improve my answers. Please point out where you got lost and give me a chance to rewrite stuff where I expected too much and need to slow dow. – Peter Kämpf Apr 04 '16 at 19:11
-
Thanks, Peter, I'm sure I could understand it if I wanted to, I'm just not interested enough to invest the time & mental energy to make heads or tails out of all the math you tend to post. Getting the general understanding is more than sufficient for me (as a non-pilot). – FreeMan Apr 05 '16 at 13:02
-
For what it's worth, as an engineer, I appreciate seeing the math and physics. :) – reirab Apr 05 '16 at 15:08
30
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
-
-
1@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 04 '16 at 07:12