Spin question

[Toby]

Aerodynamics question: why is it that sometimes when you initiate a spin, the airplane doesn't spin?

This happened to me Saturday. We did many spins to the left, and most of the time the plane went into it easily. But twice the wing dropped and we just dove straight down. I wonder what is happening that prevents it from spinning if the inputs are the same.

Thanks.

 

[BruceAir]

You'll find an excellent, readable overview of spins on the NASA Web site at:

http://oea.larc.nasa.gov/PAIS/Concept2Reality/spin_technology.html

As for your specific question, I recommend Chapter 4 of FAA-H-8083-3A, "The Airplane Flying Handbook" (available for free download from http://afs600.faa.gov/srchFolder.asp?Category=traininghandbook), which notes:

Many airplanes have to be forced to spin and require
considerable judgment and technique to get the spin
started. These same airplanes that have to be forced to
spin, may be accidentally put into a spin by
mishandling the controls in turns, stalls, and flight at
minimum controllable airspeeds. This fact is additional
evidence of the necessity for the practice of stalls until
the ability to recognize and recover from them
is developed.

​

The problem you're describing is fairly common. If the wings aren't stalled or if the angle of attack becomes less than the critical angle of attack as the incipient spin begins, a developed spin won't occur, and the aircraft typically enters a steep descending spiral.

For example, the AFM for the C172 (which can be spun legally when operated in the utility category) notes that's often difficult to achieve clean spin entries and makes recommendations such as using a brief increase of power at the stall break (depending on the entry) to help get things going.

You might also find this article on Transport Canada's Web site interesting. It notes in part:

Early Cessna aircraft did not include a procedure for the spin manoeuvre and Cessna has progressively updated the spin procedures included in the POHs since 1976 for C150 and since 1973 for C172 aircraft. A supplementary booklet titled ‘Spin Characteristics of Cessna Models 150, A150, 152, A152, 172, R172 and 177’ was issued in 1980 and sent to registered Cessna owners. This booklet outlines the differences in spin characteristics between models and even between some model years.
​

Spin guru Rich Stowell has posted many informative articles on his Web site. One of the best is Innovations in Stall/Spin Awareness Training.

You can also find many more references to information about spins and spin training on my Web site at http://www.bruceair.com/spins.htm

​

 

[Toby]

Thanks for all the info, Bruce. It's a little bit of an overload for me. I've read the chapter in "The Airplane Flying Handbook" and I just looked briefly at your other links. What you said about entering a steep descending spiral was interesting, and, strangely, was not what I think happened. From where I sat it looked like I was going straight down. I was looking at the water, and it wasn't turning. But the wings must not have been fully stalled or maybe they got unstalled a bit in the incipient phase, as you mentioned.

 

[BruceAir]

It's very common to be confused and disoriented by spins or spin-like phenomena. I always ask students to tell me how many turns we did after I demo a spin. Most think we've gone around several more times that we actually did. Many people confuse an incipient spin with a developed spin, and so forth.

That's why we need training that gradually exposes us to spins and helps us develop our awareness of what's going on. The situation is similar to that involving any new, intense experience. Remember how exciting and disorienting your first power-on stall seemed?

Note that regardless of pitch attitude, a true, developed spin is characterized by low airspeed (it may oscillate around a value near the power-off stall speed). Regardless of rotation, pitch angle, etc., if the airspeed is high (and increasing), you're not in a stablized, developed spin. Do you remember what the airspeed was during your maneuver?

The feeling that you're pointing straight down is also common, although typically the nose is actually down about 60 degrees (the value varies with aircraft, configuration, type of spin, etc.). For one example of how a spin develops, see the video at http://www.bruceair.com/spins.htm. You'll also find excellent videos and more information at Fighter Combat International (http://www.fcitraining.com/newsletterlist.htm).

Take the information a bit at a time, and gradually you'll gain understanding. The main points of all the references I posted are that spins are complex, difficult to model and analyze, and vary considerably among aircraft.

 

[Toby]

I'll check out those video links. I've seen some videos of spins at safety seminars. It's always good to watch them again and again and, as you say, gain a gradual understanding.

I don't know what my airspeed was! I will certainly check when we go up again on Thursday. All I can say is that time slowed down to NOTHING. Everything happened in extremely slow motion. But that may have been a trick of my mind. We were power off when we stalled, so fairly slow.

We counted our turns out loud. That helped immensely to anchor me. These were one-and-three-quarter spins, losing around 900 feet each time, more or less. I'm sure you're right, that we were not pointed straight down, it only looked that way. Thanks for the further clarification.

 

[gismo]

I'm no aerodynamicist, but a spin is basically a condition where one wing is generating lift and the other wing is not.

Actually that's a very outdated theory of spin aerodynamics. If this were true, the airplane would roll inverted. From what I've read a spin requires that both wings are stalled with the wing on the inside in a deeper stall than the outside wing.

Toby, in answer to your question, if you had the rudder hard over, you should generally get rotation whether the plane spins or not. Remember though, there has to be a yaw moment when the stall breaks or you won't get a spin.

Also many airplanes (especially non-aerobatic ones) have a fair amount of washout (reduction in angle of incidence at the wingtips) which means that the outer portion of the wing where the ailerons are may not be stalled when enough of the wing is stalled to cause the nose to drop. I suspect that you are not letting the entire wing stall and are using ailerons to keep the airplane from spinning.