Spin-entry: an old question revisited.

[Garfly]

The air-speed difference in a turn between the upper and lower wing as a contributor to spin entry is a debate that's been around for quite a while. (As basic - and controversial - as the 'what makes lift?' one.)

 

It's come up again here recently in another thread: sea-plane crash hawkesbury river nsw (posts #150-151)

 

Personally, I'm persuaded to the view that for most practical purposes that speed through the air difference is negligible and that AoA differential alone explains the flick phenomenon.

 

But, methinks, in slow tight turns - where it matters - that difference maybe does play a role. But how slow and how tight and how much of a role in the overall scheme of things?

 

The aeronautically correct answer - whilst feeding the inner nerd - probably has little practical value. Still, I've read that if a turn is perfectly co-ordinated, then the stall, when it comes, should be as benign and manageable as you'd expect in a wings level situation. Given that that's correct, it has me wondering some more about the implications of the wing speed difference in the turn theory.

 

Anyway, I'd be interested to hear from our resident experts about it even though it's doubtless been covered before.

 

BTW: This is just one random web-site I pulled up which sets out the battle lines. There are heaps more out there.

 

Why do aircraft inner wings lose lift when turning?

 

Some brief excerpts:

 

"In any case the inner wing has lower airspeed (simple geometry!) and thus lower lift, if the aircraft turns. Prerequisite is a certain amount of roll." – Georg Nov 26 '11 at 12:24

 

 

 

 

 

"In normal turns, whether banked or not, the speed differential between inner and outer wing is negligible. To get noticeable difference in lift between inner and outer wing, you either need to 1) have very long wings, like a glider, or 2) be moving slowly so as to have a small turning radius." – Mike Dunlavey Dec 28 '11 at 22:52

 

 

 

 

 

 

 

"With respect to the difference in speed over the wings, let's run some numbers and see:

 

 

• small aircraft flying at 100knots/100mph (50m/s in round terms)
   
   
  
• A comfortable turn rate - 180 degrees in one minute
   
   
  
• wingspan 33'/10m (so 5m from a/c centreline to each wing tip)
   
   
  

 

 

Turning 3 degrees per second means that the wingtips have a speed difference of tan(3 degrees)/sec * 10m, which is 0.52m/s or 1%. Even with the fact that lift is proportional to the square of the airspeed (so the lift at the wingtips differs by 2%), this is still a pretty small and pretty negligible effect in cruising-like maneuvers like this." Daniel Chisholm

 

 

[djpacro]

I don’t see why it should be regarded as controversial.

 

This site has Tutorial: loss of control in turns

 

I recommend Skidded Turn Traffic Pattern Stall Discussion and other pages at APS for my students as well as my own material.

 

Must go, got a full day of teaching turning stalls and spins.

 

 

[facthunter]

You don't get as much lift near the wing tips but even using the full wingspan the distance difference travelled or speed difference is very small. In the real world it could be ignored. Planes stalling in a turn don't always drop the inner wing. They can almost as easily flick over the top and go the other way. If a plane is misrigged it will often demonstrate a tendency to drop one wing first when the plane is stalled.

 

I would suggest tightening the turn with a little (or a lot ) of rudder therefore making the plane imbalanced is where the problem arises. PLUS you have dynamically loaded the wing by requiring extra lift causing more drag so you need more power or to lose height in a steep turn. Any time you NEED a steep turn in a critical situation you should have full power on or be prepared to lose a bit of height and make sure the ball is centred and you have your margin above stall speed for that angle of bank. Flap will reduce the radius of turn but has more drag involved with it's use. The individual type should be analysed before just using flap on a principle. You would have to do actual performance evaluation of the type.. Nev

 

 

[dsam]

Just a small additional point to add... many aircraft are designed with a dihedral wing. The lift vector of each wing (L & R) is therefore slightly different, and becomes an additional consideration in a full-detail vector diagram of a banked turning aircraft.

 

Skidding or slipping turns with a dihedral wing bring even more asymmetric forces into play. A mathematician’s picnic! Insane!

 

For my sanity when low or in circuit, I prefer to only think about keeping the damn ball centred, and never forgetting the three S’s (speed, speed, and speed). Hit your correct speeds in coordinated flight, and avoid being a statistic - no vector diagrams required:unhappy composer:

 

 

[Garfly]

Thanks for the replies. Getting one's head properly around these basic - but crucial - aero facts is an ongoing process. Though I know it'd be far better to get one's hand/eye/feet coordination around them first. (Maybe with some of that spin training DJP offers ;-)

 

Anyway, as to why that particular bit of theory might be regarded as controversial (if without practical ramifications), I remember reading an authoritative article a few years back in an Aussie flying mag (I think) wherein the speed difference between wings in a turn idea was explicitly disputed as a significant factor in spin entry. That is, the writer set out - by definition, controversially - to oppose that part of the accepted explanation. If I manage to track it down I'll link to it here. But it's not hard to find other instances of this particular dispute. The one I posted above was just the first that turned up when I went searching.

 

I see that whilst that (accepted) idea is part of the explanation in the tutorial on this site, it wasn't included in the fairly comprehensive analysis of the "Otter" MacNeace video (both linked to above in #2). Do you have a strong position either way on that part of the theory DJP?

 

While it's practical savvy we mostly need, theory debates between them-what-know are always instructive for the rest of us.

 

 

[onetrack]

Dsam - Dihedral diagrams are right here, in the link below. Pictures along with text, makes for comprehensive understanding of the subject.

 

How Dihedral Keeps Your Wings Level

 

 

[facthunter]

Dihedral should make no difference. The lift vector acts at right angles to a line drawn from each tip, if the plane is flown balanced , the projected area of each wing along the lift line is equal, even when a bank is inclining the lift to the centre of the turn..

 

. Dihedral works when the plane slips as does sweepback. Sweepback can cause roll/pitch oscillations that are a bit hard to control. Often called "Dutch Roll". Nev

 

 

[Happyflyer]

If you want to enter a spin. Just stall the wings and add a boot full of rudder ( yaw ) and get ready for spin recovery. No need to stall one wing only.

 

 

[djpacro]

Maybe with some of that spin training DJP offers ;-)

There are quite a few others around. Always happy to chat over coffee or red wine if around Melbourne.

 

.. an Aussie flying mag (I think) wherein the speed difference between wings in a turn idea was explicitly disputed as a significant factor in spin entry. That is, the writer set out - by definition, controversially - to oppose that part of the accepted explanation. If I manage to track it down I'll link to it here. ..... I see that whilst that (accepted) idea is part of the explanation in the tutorial on this site, it wasn't included in the fairly comprehensive analysis of the "Otter" MacNeace video (both linked to above in #2). Do you have a strong position either way on that part of the theory DJP?.

I like to keep things fairly straightforward for pilot theory just referencing my standard sources then fly and (hopefully) show that the theory applies in practice. Having said that I can readily revert to being an engineer in which case I'd want to show and see data ... if you find that magazine article I will read it.I like this guy's stuff in my transition from pilot to engineer: eg http://www.flightlab.net/Flightlab.net/Download_Course_Notes_files/9_RollingDynamics.pdf

 

 

[Ayecapt]

Why if your low and slow would you do a tight turn! Answer ... your worried about flying a nice rectangular circuit.

 

Pulling into the turn to get aligned ! Must have been a lousy set up in the first place. Better to go around .

 

 

[facthunter]

Low and Slow is where you lack any margins for error and also where illusions of slipping/skidding and groundspeed/airspeed judgement gets affected the most.. Get Ball off centre and a bit more back stick and you are history. With the adequate speed margin above stall for the bank you have and enough engine power added to maintain it, the plane doesn't know it's low so it will fly the same as at height, but YOU must get it RIGHT, every time.. Most people's reaction to the nose dropping is to pull the stick further back , guaranteeing disaster. Rolling out of the turn and easing the stick forward "enough" is the way out of it. The BEST thing is to not put yourself in this position at all.

 

Encountering a downwind on base (or just misjudging) and trying to pull the nose around with extra rudder so as not to exceed the (imaginary) Dangerous 30 degrees has crept into the thinking, I feel. lately. While I don't encourage steeper turns is the circuit, you might not have much choice on an outlanding and you should be able to do this sort of thing safely if you HAVE to.. Steep Gliding turns used to be practiced well before solo for all students.. If you practice doing level figure 8's accurately you will get the rolling into and out of turns a lot more coordinated and safer. Change power when you should so it becomes second nature to add power to cover the extra drag and have the right speed and take some of when exiting.. Do a clearing turn before any of this practice to check for other traffic. and do it in a training area (airmanship). Nev

 

 

[Garfly]

Yes, or you might be in a steep climbing turn and have the upper wing stall first, flicking you over the top:

 

Investigation: AO-2014-192 - Collision with terrain Cessna 172, VH-PFT, Maingon Bay (9 km south of Port Arthur), Tasmania, on 29 December 2014

 

It's worth reading the collection of cautionary tales in this report's "Related/Previous Occurrences" in Appendix B.

 

 

[Stoney]

Thanks for the reminder Gaz.

 

I have spent the last hour revising this important topic.

 

The rules we use for safe flying really are written in blood. It would be disrespectful not to take heed.

 

 

[facthunter]

Learn by other's mistakes. You won't live long enough to make them all yourself. No consideration of aileron position has been done in this assessment /judgement. Why not? It does affect the situation. Nev

 

 

[boleropilot]

I just made a comment in another thread here re. doing a big 180 from the end of downwind to short final - any comments?

 

During training my instructor picked up I wasn't 'centering the ball' in some turns, so he showed me what can happen in a Drifter when you do that...the aircraft violently flicked inverted and it took 700 feet for recovery

 

He just said "now imagine what would happen if a Drifter pilot does that at 500 feet in the circuit"

 

which is why I LOVE that little piece of red wool taped to the windscreen of my Drifter - I actually have to look past it to see through the screen...

 

BP

 

 

[facthunter]

He must have "snuck" a bit of back stick on as well. Racetrack pattern is OK. Services and Airlines do it. Works on a tight circuit as well. Height and angle to runway alignment judgement all the way to stabilised height.. I've never taught an ab initio to do it straight off.. Square might give a few points of reference more to adjust in earlier stages.. Nev

 

 

[M61A1]

Been posted before but may belong here....

 

 

 

[M61A1]

He must have "snuck" a bit of back stick on as well. Racetrack pattern is OK. Services and Airlines do it. Works on a tight circuit as well. Height and angle to runway alignment judgement all the way to stabilised height.. I've never taught an ab initio to do it straight off.. Square might give a few points of reference more to adjust in earlier stages.. Nev

You'd have to be pretty sneaky with that backstick......as I mentioned elsewhere, even my non-pilot backseater knows what happens when it stops getting breezy, and that's long before stuff goes bad.

Also no reason why the "racetrack" can't be two continuous rate1 90 deg turns, they just don't have any "straight" between them.

 

I like tight circuits.

 

 

[boleropilot]

well I seem to recall I was deliberately turning un-coordinated and the nose went up gently and the airspeed just washed off nice and gently...and then all hell broke loose - so there probably was a little back stick in there, not sure where the throttle was at the time - all a blur really and one of the biggest lessons ever

 

He loved to pull the power off at the most inopportune times, the funniest one was when we were at around 5,000 over the field and he said "can you see the field?"

 

I replied "yes, it's right below us" and he just said "oh good - engine failure" and pulled the throttle

 

longest and most relaxing engine failure drill I ever did.....and I pulled off a perfect deadstick landing to boot...not surprising really when you consider the time I had to plan it...

 

BP

 

 

[Garfly]

This article by Dick Collins of Air Facts Journal covers a lot of what we're discussing here:

 

Arriving at the VFR sweet spot - without colliding or spinning in | Air Facts Journal

 

In this excerpt he talks about the continuous-turn-to-final technique:

 

"My concern about this procedure was based on being able to judge a proper approach path while turning. Gordon, whose father Art was tower chief at LGA and an old friend of my father’s, tried to explain this in the article as well as to me on the phone. That wasn’t working so he arranged a rendezvous and took me flying in a Navy T-28C for a little show and tell.

 

I got it, and from then on when I did a downwind pattern entry I’d make that close-in continuous turn to final starting just after I was abeam the approach end of the runway. I could tell, from halfway through the turn, whether or not the end of the runway was coming at me properly just as well as I could tell on a straight-in approach and any adjustment was easy to make. I think the circle to final made it easier to anticipate any developing problem as long as I was thinking well ahead of the airplane, something I always tried hard to do. If your thinking gets behind the airplane then a successful conclusion depends more on luck than anything else.

 

In a low-wing airplane the circle to final would block the view of any airplane that might be on a long final so it was proper to take a look when about half way around in the turn. In a high-wing, the view of anyone on a long final was unobstructed.

 

It might be suggested that this reduces the risk of a mid-air collision by limiting the time in the pattern but that’s a stretch. To me, the main advantage was in ease and efficiency. I always flew the downwind at a distance that allowed that final turn to be at about 20-degrees of bank so there was no steep turn at low altitude.

 

I know that some folks want to teach long and stabilized straight-in approaches and they are perfectly free to do so. But the continuous turn to final was always pretty fun to me and after I had done it for a while I couldn’t imagine doing it any other way."

 

 

[facthunter]

There's nothing mysterious about it, but I would start with the more common (initially) practice of the squared circuit but like slipping or crab on approach with a crosswind, don't limit yourself to just one way of doing things but still PLAN what you do. When you fly from A to B (and back) or do a lot of training away from the aerodrome you rarely do a complete circuit. You leave and return with a partial circuit, or even sometimes a straight in.. The "standard " circuit is what you relate to but not always do later with your increased experience. Nev

 

 

[turboplanner]

 

This is what the Barons and RPT expect to see; all aircraft compliant to CAAP 166-01, unless you are flying out of your unmarked paddock somewhere.

 

Note that in a Drifter or Trike you are not being pushed into long finals, but you are in a formal position for safety reason.

 

For example, if you are a Drifter at 500 feet which is turning final, and a second or tow later the RPT turns final, the pilot knows exactly what you intend doing and what he needs to do, and that might include going round, but if you are somewhere near the apex of the final turn point here, you could be doing anything.

 

In an oval pattern at 1000 or 1500', as someone mentioned, you are blind for the whole of the Base and Final legs, and as we know, a lot of people fly fast and wide, and suddenly appear at that part of the circuit. If you are flying a 500' circuit in a high wind ultralight you are blind to someone coming in from a correctly executed rectangular tight circuit from behind.

 

The continuous turn to landing was adopted by the military for a sound strategic reason. At the end of missions, fuel was often short, and to get all planes down as fast as possible, this was the quickest way. In war time, the odd collision was acceptable; same applies for military training now, because if they are going to do it in military ops, they need to train for it now, which is why you see it in the miltary.

 

But you aren't the military, and departing from the CAAP is likely to see you crossing into the criminally negligent if someone hits you when you are doing your own circuit pattern.

 

The advice on spin entry in this thread is very good, and there is no reason why you would need to depart from standard circuits if you have studied, and been trained correctly. It appears from a couple of comments that some have not, and are probably making steeper turns or jagged corrections, which are both voluntary.

 

 

[boleropilot]

fabulous stuff, Garfly - the thought just occurred to me (sitting at my computer and going through the 'round' turn in my head) is that your view of approaching aircraft on a straight-in approach would be improved because of the variation of the view as the turn progressed - but not if you are in a low wing aircraft

 

when I say 'improved' I am talking about the fact that your aircraft is constantly changing it's position in relation (height and direction) in relation to any approaching aircraft flying in a straight line towards the runway, keeping in mind that if they are on a long final they may not be descending at that time and if they have not heard or seen you, you may well be positioned below the cowl of their aircraft

 

just seems to me that the Rhino Turn has a hell of a lot going for it. btw, I HATE straight in approaches....as far as I am concerned they are bluddy dangerous - the only positive in them is no chance of stall/spin in a turn !!!

 

BP

 

 

[boleropilot]

I've been on a 500 foot downwind when passed by a Cessna on a 1,000 foot downwind, high to my right and we were both exactly where we should be

 

if I had turned base at my normal point for a glide approach I would have placed my aircraft directly in front of the Cessna when he turned final, forcing him to go around

 

I simply said "Cessna on base, Drifter 455 is extending downwind to land number two behind you" and I had a perfect view of him as he completed his circuit

 

the reply was "thank you 455, much appreciated"

 

it's not rocket surgery, folks....and for the record, I won't be doing any full circle combos of base/final - my turns are just going to be a bit rounder...

 

 

[Yenn]

Boleropilot. Your view may be improved, but do you think it would stay improved if we all did circling entries? I think not.

 

I did once meet a plane doing a RH circuit into a strip which we normally did left circuits on. the other plane being low performance was just about to turn final, while I was already on final. I saw him coming from my right and just ducked down and right, overtook him and landed very long.

 

His radio wasn't working and he was doing right circuits to keep away from skydivers.