Myth busting! I MIGHT STALL IF I TURN DOWNWIND!

[Happyflyer]

• If you took a plane with a set power and attitude, and 100 knots headwind, then very rapidly changed that to 100 knots tailwind, its IAS would drop sharply and it would be in trouble.
   
  

 

 

I disagree. If the wind was steady the plane would be subject to the same change in IAS as in still air. The analogy of the train carriage is a good one. The model in the carriage can easily do a sharp u turn if the train is moving or not. Just think of the wind as a moving parcel of air, just like the air in the train carrriage.

 

 

[dutchroll]

You quoted this:

 

• If you took a plane with a set power and attitude, and 100 knots headwind, then very rapidly changed that to 100 knots tailwind, its IAS would drop sharply and it would be in trouble.
   
   
  

If you're disagreeing with that particular dot point, then you're disagreeing with reality.

I can't argue the point any further.

 

 

[Happyflyer]

http://www.flyingmag.com/pilots-places/pilots-adventures-more/last-word-downwind-turns-really

[Happyflyer]

I tried to explain the down wind turn myth to a student once but he wasn't having a bar of it. Someone came in and said the wind at 4000 ft was 35 knots but it was smooth as silk up there. I had a bit of time for this student so we took a Decathlon up. Sure enough it was smooth as silk and the GPS confirmed 35 knots of wind from the west.

 

We then cranked over into a very steep, level, continuous turn, circle after circle, much steeper and much more G than anyone would ever pull in a downwind turn. The GPS speed was up and down like a yo yo but much to his surprise the airspeed indicator was steady as a rock. Try it sometime. Sometimes it's better to show reality rather than explain it.

 

 

[kasper]

Since I am the only one who made analogies perhaps you will elaborate as to why they are not relevent or are POOR. You concentrated your argument on windshear which sidesteps the issue of turning downwind. Windshear is a problem mainly with changing level usually quite close to the ground. Nev

I think the quote from Dutch Roll here is the centre of the isseu:

"A constant wind velocity during a normal downwind turn simply doesn't result in an adverse lift change that quickly, at least not one which can't easily be controlled by a modest adjustment in attitude. Some people on message boards have said "yeah well what if you did a 180 degree turn instantaneously in a 60 knot headwind?" WTF? Ok yeah sure, you're probably "simulating" the equivalent of a severe windshear then, and yeah theoretically you could be in trouble if you want to maintain level flight (if you haven't already g-stalled it). However this discussion doesn't involve advanced aerobatics. We're talking "normal" flight."

 

Practically - as you point out - the aircraft is flying relative to the airmass it is in...and as DutchRoll points out the rate of change of heading as you turn is NOT sufficiently fast to change the effective headwind sufficiently for the attitude change to hold height to be noticeable.

 

Heaps of vector maths can be put forward to calculate the effective loss of headwind for various airspeed/wind speed/rate of turn BUT as Dutch points out if your margin of airspeed over stall speed is anywhere near where it should be (1.3 stall) you should have a reserve of around 15-20knts for more aircraft ... and to get a 15knt airspeed shift whilst doing any turn onto downwind is near on impossible.

 

Working an example ... stall = 40, airspeed = 60 wind = 10 so theoretically turning around instantly AND HAVING PHYSICS SUSPEND DISBELIEF AT YOU SO MOMENTUM CANCELLED INSTANTLY AND ALL THOSE G's THAT IT ENTAILS EVAPORATE you would experience airspeed reduction of 20 bringing airspeed to 40 and a stall

 

BUT that 180deg instantaneously turn to get the full cancellation and reversal of wind speed will inflict death on you from G load ... lets see you are experiencing 50knts (airspeed- wind) upwind relative to the earth and you instantly reverse and are experiencing 70knts downwind relative to the earth you get infinite G as no time elapsed ... even if I give you 0.5 seconds to do that reversal you are going from 25.722m/s one direction to 36.01m/s in the opposite over 1 second ... that's 61.73m/s change and 1 G is 9.8m/s so that's 12.7G

 

... and if you can arse about your plane that quickly and do not cancel the physics your momentum will be having you going 60knts backwards after your instant turn till your thrust can start you going the other direction ... most planes can deliver at best 1/2G acceleration and you have to cancel 61.73m/s to get back to the crusie airspeed ... and at 1/2G that will take you 12.5 seconds ... the fist 6 of which your airflow is from tail to nose then reversing nose to tail ... those first 6 seconds are going to be more fun than just stalling.

 

The reality is that most turns add well under 1/2 G to the load ... and that's not going to allow your airspeed vs wind speed to vary greatly and will be SHOULD BE unnoticed by the pilot ... unless they are flying by visual reference to the bits of fixed ground shrubbery when you will adjust attitude by incorrect reference to a perception of speed rather than by attitude of aircraft relative to the earth ...

 

 

[Rastus]

Makes me wonder why all the gliders that have been thermalling in a 45 degree bank a few knots above stall speed in a strong wind haven't been falling out of the sky all these years.

 

 

[facthunter]

I did a lot of work on this about 20 years ago. I agree with Happy Flyer's actual flying situation. The linked article is BS. Whichever way you are facing makes no difference whatever. The earth itself is speeding through space as is the solar system and you are blissfully unaware of it and unaffected unless you want to break free of the earth's gravity it pays to use the 1,000 mph at the equator with the earth turning towards the east as a launch spot for your rocket and head east. You are 1/22 of the velocity you need already. Nev

 

 

[dutchroll]

I agree the linked article is ridiculous.

 

My entire point here is that, going by online discussions I've seen recently, there are a swag of small plane pilots who actually believe that a headwind changing to a tailwind has no effect on them. Ever. Articles like the "Flying Magazine" one are not helping to distinguish the different scenarios.

 

This is due to poor quality discussions about the "downwind turn myth". And yeah, I agree in a steady state wind it is a myth. There is no issue turning onto downwind which would be caused by the increasing tailwind. It is not the same as windshear. I agree. I agree. Did I mention that I agree?

 

It is just a demonstrated fact from various message boards that some pilots reckon if they're sitting there on final approach and the headwind turns to a tailwind, the plane will look after it because it doesn't care what its frame of reference is. All that will change is the groundspeed.

 

Best of luck with that..........

 

Disclaimer for something I wrote earlier: this mythical "instantaneous 180 degree turn" is a different scenario. It's not really the same thing a experiencing a wind change. I shouldn't have even written that because it's irrelevant.

 

 

[M61A1]

The flying magazine article was clearly satirical. I fly a Drifter, it's pretty light also a 95:10. I am quite familiar with gradient and must make allowances for it where I know it will be (not difficult) I have experienced shear, fortunately mild enough to get away with. I do downwind turns frequently with no effect other than increased ground speed and decreased climb angle (rate stays the same). I think both Happy and DR are saying pretty much the same thing, but misunderstand each other.

 

 

[Happyflyer]

The flying magazine article was clearly satirical. I fly a Drifter, it's pretty light also a 95:10. I am quite familiar with gradient and must make allowances for it where I know it will be (not difficult) I have experienced shear, fortunately mild enough to get away with. I do downwind turns frequently with no effect other than increased ground speed and decreased climb angle (rate stays the same). I think both Happy and DR are saying pretty much the same thing, but misunderstand each other.

Yeah, isn't satire wonderful when well done. The thing that can catch you out on downwind turns are optical illusions if you look at the ground too much especially if it is a low level circuit.

This, combined with the fact that most small aircraft have no more power in reserve and are flying slowly can get you into trouble, even more so if the wind is gusty.

 

 

[facthunter]

Wind shear can be bad for any aircraft. It's brought heavies down easily especially to do with microbursts, which YOU should never be anywhere near.. If you transition into a tailwind you will get an immediate airspeed loss needing quick and large corrections depending on the severity of the change. One of the worst hazards likely to be encountered is a dust devil or willy willy and if there is no dust or crass cuttings you may not see it till it hits you. Nev