Jetstream Rider

Can you spin from a slip? what about a skid?

I remember reading a book about spinning a while ago saying you could not spin from a slip, but you can from a skid. Why? Surely you can stall from a slip, crossed controls are pro spin, so why no spin? I am not sure the author was right, although I can see how it might be right in SOME aeroplanes as I tried to stall a Super Cub from a slip one day and couldn't. However a Cub is difficult to stall at times anyway. I know different aircraft have very different spin characteristics, but generally is a slip safe? what about fin stall? Is this likely in a certificated aircraft and if so, what is the recovery?

Secondly, do you have any good recommendations for books about spinning? There seems to be a lot of folklore around and it is difficult to separate the wheat from the chaff, some people say for instance never use power in spin recoveries, others say it helps get out of a spin and some people say it depends on the aircraft. A good book would certainly help.

I hope you do not mind me using this forum, perhaps this post should be on questions, but I have come here to have a good discussion and hopefully avoid some folklore. My background is a degree in Aero Eng and some limited aerobatic experience, but definitely not enough to be able to answer my own question in one type of aircraft, let alone the general case.

Regards,
J-Rider

john_tullamarine

Definitely the better forum for the question .... now, just sit back for a day or so and wait for the TPs to spot it and the specialist commentaries will come in ..

If the source came from a published book, then it is fair comment for you to identify the book's title and author which might help refine the answers to your question. I shall be surprised, though, if the answers are not along the line that the story is a bit wide of the mark.

The SuperCub is a bit different to many lighties in that it can be sideslipped very aggressively without, in my recollection, any problems developing or even being indicated .. but, then again, my experience on Type was limited to three S/N examples only in routine towing ops. A long time since I have flown one but my recollection is that we routinely used full rudder and near full stick deflection to maximise descent angle ... with the cowardice which comes with age, I suspect that it might have been more prudent had we discontinued the slip sometime before the landing flare .. but that, I guess, was a consequence of youthful exuberance and stupidity ... no-one ever came to grief or even had a fright that I know of ...although in a strong crosswind, the gable markers periodically came to an abrupt end of useful life ...

If I might add a subsidiary question, what is the difference between a slip and a skid ? ... the same thing I would have thought, except for "skid" probably being the typical term used if the aircraft's attitude is substantially level ?

Jetstream Rider

As far as I am aware, a slip is too little rudder (in the right direction) for the bank angle in order to keep balanced flight and a skid is too much rudder . Perhaps that is a bit of a simple way of looking at it. Both are sideslips I suppose as the relative airflow has a sideways component in both?

Have just found the book again and itis called "Stalls, Spins and Safety" by Sammy Mason.

He says "One of the most difficult manoeuvres from which to enter a stall or spin is during a slip, particulary a severe slip. Not that a stall cannot be entered if there is enough elevator authority; it most certainly can. But during a true wing down slip, stalling is more difficult."

I would agree with this from experience, but would like to hear any comments about stalling and spinning that the readers of this forum may have.

Genghis the Engineer

My preferred text on spinning is "Flying Qualities and Flight Testing of the Aeroplane", by Darrol Stinton. Expensive, but worth every penny.

Disturbing isn't it that no aero-eng degree mentions the spin in any depth on the aerodynamics courses. Pretty much everything I've learned on the subject was after I graduated.

G

mike_beresford

A bit off-topic, but Genghis's comment about spinning not being covered properly on his university course is quite interesting. I'm not a flight test professional, but I originally trained as an aero engineer and recently completed my PPL. When I did my degree I had only rudimentary "intro flight" type experience, and I found that several aspects of flying and handling had not been covered by my university training

- Spins: We covered the aerodynamics of a spin, but only briefly discussed recovery. We were simply told "stick forward and simultaneous opposite rudder". In practice my flying instructors placed great emphasis on using rudder first then stick forward, and also on the need to minimise height loss during spin and recovery. The latter aspect was totally ignored on my university course. Now that I am doing some basic aerobatics, the situation changes again: emphasis on precise amount of rotation and exit attitude.

- Trim: The action of trim controls was discussed, but the extent to which they are used during flight came as a surprise. Even at the time of my degree I had the feeling that trim was not emphasised properly. I had obtained a copy of "Handling the Big Jets", where Davies discussed trim changes for swept wing aircraft with fuel burn. This was not touched on in our course.

- Rudders: We covered the basic aerodynamics behind rudders, but I hadn't appreciated the extent of usage. It seemed that instructors were forever saying "use your feet".

I might be back at university next year doing an M.Sc, and it would be interesting to see if the undergraduate course has now changed to include more information on real flight.

Blue skies
Mike
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honey badger

My answer is no!
If you are in a slip, than it's possible that you get in an a-symmetrical stall, one of your wings gets in the wake of the fuselage and stalls, the other wing keeps its lift and puts the plane on it's side. If the plane gets to it's side, the nose get's down, and so the other wing will comes out of the wake so there's no spin.
Maybe in specific ways the slip gets to a spin, but that should be very rare. I have a lot of experience in such things, and I never got in a spin when I was in a slip.

Badger,
f-16, Dutch airforce

LOMCEVAK

I think that much of the confusion here arises from phraseology and consideration of whether we are talking about the direction of sideslip when we apply the entry contols or once stabilised in the spin, and whether we are talking about attempting to induce a spin or one occurring inadvertently. The words "slip" and "skid" are old terms referring to the direction of any sideslip that is present during a turn. "Slip" is sideslip in the same direction as the turn ( i.e. relative airflow from the left of the nose in a left turn and vice versa) and "skid" is sideslip in the opposite direction to the turn. In, say, a left turn, left sideslip will invariably be present (certainly in light, piston-engined aircraft) if the turn is flown rudder free (i.e. without rudder co-ordination). So, the question is can you enter an intentional left spin from a left turn? Some aircraft you can, some you may not be able to. It depends on two factors: a. Is the rudder power available at entry sufficient to overcome the sideslip inherent in the turn and generate enough sideslip in the opposite direction to trigger autorotation? b. More importantly, if rudder is applied in the direction of turn, does the increase in IAS that results from the increased nose down pitch attitude due to the yaw rate resolved through the bank angle prevent the wing(s) from stalling? If the applied rudder generates a reasonable roll rate (due to lateral stability and Lr) the aeroplane probably will spin. If the resultant motion is a high yaw rate with roll, it may not. Remember not to confuse the direction of sideslip during a spin with the direction of sideslip at the point of pro-spin control application. Also, if in wings level flight there is no difference between slip and skid!

If you consider inadvertent spins, the slipstream effects from the prop and possibly gyroscopic moments and torque effects must be considered. Again, it will vary from aircraft to aircraft but it could occur.

Out of interest, the most stable intentionally entered spins in the Hawk (which has somewhat inconsistent spin characteristics) generally are achieved by entering from a 30 deg AoB turn in the direction of the spin.

Radhaz

The Hawk certainly is spun (intentionally) from a 30deg bank turn, according to the Aircrew Manual, to help it go as intended (left bank = left rudder = spin to left).
I would say that individual Hawks will often only spin in one direction and will often merely sit in a stall, with the speed increasing, as you plumment earthwards. Folklore says it spins better to the left.

Previous experience would say that the spin direction is not dependant on bank direction (does this help?), but upon what is happening at the time. I would guess it depends upon the strength of the rudder to control the roll, and when/whether the wings stall. I can't see why you cannot enter a stall from either skid or slip, since autorotation is, I believe, the key.

Having said all that I'm not a TP, only a QFI, so my normal answer would be "good question Bloggs - go away and find out, then come and tell me"

I would also agree that Darrol Stinton's book is rather useful. I shall go and refer to it now...

So, anyway, what about inverted spinning?

Jetstream Rider

Thank you everyone for your messages. Amazon has Darryl Stintons book listed, so I will try to get hold of a copy to satisfy my curiosity.

Regards