Why do we Lose Airspeed in a Turn and What Causes This?

[India Four Two]

4L3X

I am so impressed. A PPRuNe newbie and you've got yourself a 5 page thread in fewer days!

I'm voting for the Lift Pixies. Obviously if your turn is not precisely co-ordinated, they slide off the wings and you lose lift.

[[email protected]]

Chaps, I think what has been forgotten in this protracted argument is the good old seat to stick interface which has a very well developed mechanism for assessing accelerations. The problem is that ones inner ear seems to respond to accelerations relative to the earth regardless of whether you are downwind or into wind hence a turn downwind (no visual cues) is sensed as an acceleration and the brain causes a small imperceptible response from the pilot's hands which becomes a slight change in airspeed. This is probably why an autopilot doesn't display the same effects.

[[email protected]]

Load of tosh yourself - the inner ear senses change ie acceleration - the reason you become accustomed to a steady state turn is because the fluid in the inner ear stops moving (steady state you see) then when you move your head, you change the position of the inner ear and the fluid experiences an acceleration.
I note you have edited your post immediately to remove the reference to steady state turns

[Wizofoz]

The problem is that ones inner ear seems to respond to accelerations relative to the earth

Nope, deliverence is right Crabs, load of old tosh.

Once at a particular speed for a while, your vestibular system detects acceleration relative to your head, and whatever your head is attached to.. Do you think an astrounaght half way to Mars feels things relative to earth?

[Arm out the window]

The vestibular organs sense local acceleration, including gravity, so the astronaut on the way to Mars isn't going to feel the same as on Earth.

However, the gravitational field we feel in our own little part of the world is basically a uniform force pointing straight down.
What we'd feel in a steady turn in nil wind would be exactly the same as in a steady turn in a constant-speed moving air mass, so even though the body's sensing systems are relevant, crab, I don't believe they could tell the difference between the two situations.
It's the eyes and the slip-skid illusion that do it, for mine.

[[email protected]]

A fair hit, I hadn't thought that one through fully before I posted

[John Farley]

Sorry to interrupt all the laughs but my name has been mentioned earlier so I hope you won't mind me returning closer to the original topic.

I realise that everybody considers Mike Oxmells and me to be wrong and personally I look forward to the day when somebody can rid me of the wrong notion in my head. That should not be taken as an invitation to extend this thread because I can assure you that some VERY bright guys have tried hard – including one former Dunsfold Harrier tp who has the physics and maths to lecture at Unis on the issues people have today with some of Einstein’s work

Perhaps some good can come out of the error in my ways. Try this for size:

Need the theory of flying fit the practice of flying? My experience suggests that the answer is no. It does not matter if the theories that we believe in are in error, providing our actions in flight are correct. It is incorrect pilot actions, not incorrect pilot theories, that cause accidents.

It is impossible to be around aviation for long without realising that many pilots hold different theoretical views from your own. Indeed some individuals occasionally push ‘their’ theories with a conviction that allows no other possible view of the topic being discussed. Very often the nature of such crew room arguments is that both sides cannot be right and yet both sides clearly manage to fly safely.

I suspect all these debates and arguments in the club house and on the web may surprise non-pilots when they realise people are flying around with so many diverse ideas about how they do it.

As a result I feel the important thing for pilots is to have a theory - any theory - which assists them to do the right things in the sky. If it works for them as an individual then it is fine. Just fine.

When it was blowing a gale I never wanted the wind to get under my Harrier tailplane from behind when I was close to the hover and I felt it could if I suddenly yawed from facing into wind to facing downwind. That I saw as a modern equivalent of the Edwardian fliers problems. So I took great care when turning downwind. If I was worrying unecessarily so what? It worked for me.

End of broadcast.

[Keef]

Fascinating stuff!

As a graduate theologian who has no qualification in aeronautics (other than the bit I did for the PPL), I think I can see the difference.

John Farley and MO are talking about flight relative to the ground, and the effects of wind thereon. Hovering with the wind in front of you implies stable over one spot on the ground, with a headwind of X. Turn in the hover to face downwind, still over the same spot, and you have a tailwind of X - with potentially "interesting" effects.

Flying through the air, not looking at the ground and not in turbulence, you are in a stable air mass and that is your "frame of reference". Turn 180 degrees (and let your speed stabilise) and airspeed will be the same as it was before. The groundspeed will change by approx double your previous headwind component. If you can't see the ground, and don't have any navaids running to tell you, you'll never know about it.

Here endeth Keef's lesson

[Wizofoz]

Keef,

You'll be pleased to know you have a better handle on how god made the universe work than some very qualified Aeronautical types!!

[PTT]

In IMC I turn and make no changes in power. My airspeed remains the same. I have no idea what my groundspeed is at that time as the actual (as opposed to reported) wind could be doing anything.
Therefore Mike and John are talking a bunch of hoop.
Therefore the pixies win (barring false dichotomies).
QED.

[Capt Pit Bull]

John,

I agree with the basic thrust of what you are saying, in that models of the universe don't have to be perfect, they just have to be 'fit for purpose'.

Certainly, I often preface part of a lesson by saying something like:

"this is a simplification, we haven't taken account of x,y,z... etc, but its good enough for our purpose and gives us a working approximation of what actually happens"

Additionally, there certainly are a bunch of dangerous things about turning downwind at low level, but these are related to gusts, shear, and incorrect perception of balance.

But this muddling of the frame of reference that is the crux of this thread is not needed to explain these hazards. Rather it provides a model that suggests behaviour which does NOT actually happen in smooth air, and suggests that turning downwind ALWAYS needs less power. This is not only wrong but it is dangerous.

As such it fails your own criteria.

Incidentally, you've said you don't want to get into the theories, fair enough. But I have to say that we are not talking about a complex application here. It doesn't need a degree in aero eng or Einstein. It is basic Physics; namely definitions for displacement and velocity. Thats all. Educationally speaking, long held basic misunderstandings are the hardest to shift.

pb

[forget]

Impressive response Mr F. I can see why you were paid the big bickies.

Your line below should be framed and hung in the lobby of every flying school, alongside the old poster with the bi-plane stuck in a tree.

The important thing for pilots is to have a theory - any theory - which assists them to do the right things in the sky. If it works for them as an individual then it is fine. Just fine.

John Farley. 2007.

As a simple long time ‘fixer’ I subscribe to Arty’s Lift Pixies – better that than thinking that ground speed, once the wheels are off the deck, has anything at all to do with aerodynamics.

[Art Field]

Is it fair for the general aviator to think, thanks to people like JF, that an aircraft flies because the makers say it will?. How it is flown and what effect that has on how it performs, ah well, that is a totally different matter.

[John Farley]

Deliverence and PTT

Thanks

The submarine thing and those who speak of flying round in circles in IMC miss the point rather badly. I am not stupid and if I fly around in circles I do not expect to notice the IAS fluctuating even if the air mass is moving at quite a lick across the earth.

If on the other hand (and you will have to actually read this and not just not assume I am talking rubbish) you sit in your jumping jet at 40kt IAS facing north and the wind happens to be coming from the north you will find you are stationary over the ground. If you now do a flat pedal turn onto south – which will take 3-5 seconds tops - can you really expect that just as you stop the turn on south that you will still have 40kt IAS? (Remember that would require you to now be hurtling at 80kts over the ground in a southerly direction).

The problem arises (as it did in Edwardian days) with very low IAS conditions combined with high wind strengths and a rapid change of direction.

[Capt Pit Bull]

John,

By 'do a flat pedal turn' I presume you mean no bank, loads of rudder.

i.e. grossly out of balance. i.e. high drag. In a conventional aircraft (one capable of going that slow - lets say a motor glider?) you'd expect to lose a load of airspeed if you did that - regardless of wind or ground speed. Plus the necessary rudder input to achieve that, at low speed, would probably spin you in - again, regardless of wind or ground speed.

But your Harrier is a different beast because its hovering on the output of on of RRs finest pegasi, and spinning round largely because of puffer jets powered by that. I.E. you don't need to worry about trivial details like the wing still producing lift, and you've got at least one set of 'unmodelled' forces acting on you.

But in summary no one is denying that a turning aircraft needs more thrust, but the wind has nothing to do with it (barring gusts or shear). The extra thrust required (or the IAS loss experienced) for a given heading change is not influenced by starting heading.

pb

[Capt Pit Bull]

After a few minutes thought,.....

... basically your Harrier is not constrained by keeping any of the aerodynamic surfaces working at low IAS, so your puffer jets can get the aircraft to achieve grossly high sideslip angles (? any ideas? over 90 degrees I'd have guessed). You're going to spin around and so change heading far more rapidly than changing velocity.

pb

[advocatusDIABOLI]

6 Pages, on one of the simpler 'Flight Questions'. CFS have a lot to answer for! Perhaps any A1 QFIs out there, could give us the definitive, 4 colour, 'perfect' answer........ no, I thought not.

Advo

(PS: I'm only an A2, sorry)

[Wizofoz]

John,

Let's look at it this way....

To maintain a constant hover in your harrier in a head wind, you would have needed to keep a forward TAS equal to the strength of the wind, thus a small reward deflection of your nozzles.

If, whilst keeping that nozzle setting, you had used the wing-tip jets to yaw the aeroplane through 180 deg, it would have kept the same TAS, but now had a ground speed of 2x the wind. Aerodynamically, there would have been no change. As people have said, the aircraft doesn't "Know" there is a wind, it react to the relative airflow.

Similarly, you were mistaken in you previous post regarding the stall turning biplane.

In a stall-turn (I prefer "Hammerhead" as the aircraft doesn't actually stall!), the aircraft is flown aerodynamically vertically, that is to say at an A of A that produces zero lift, pivots with the rudder and flies the same A of A vertically down. In nil wind that puts the aircraft 90deg to the ground up and down.

Your contention is that flown into a wind the aircraft goes from head to tail wind, and therefore looses airspeed. If flown as above, however, the aircraft will drift at the speed of the wind whilst vertical. Coming to pull out, you ALREADY HAVE a ground speed equal to the wind speed, therefore you have no extra acceleration to make up.

AH! you may say, but what if the pilot allows for the wind! Fair point, but in this case you are flying a different man oeuvre. on the way down you would need a negative angle of attach to stop the drift, and this would indeed require a greater distance to pull out. If, however, you flew the same negative angle of attack in nil- wind, you would require exactly the same height to recover as in the case were there is wind, so it's not the presence of wind (in an aerodynamic sense) but the flight path followed that makes the difference.

Hope this makes some sense!!

[Islander2]

A fascinating contribution from JF on experiences, thanks to the Harrier, that are denied to the rest of us (with apologies to helo drivers!).

As a Hawker (Kingston) design engineer from JF's era, I hesitate to take him to task. However, here goes!

The effect he describes is merely another cause of IAS loss in a turn. It has nothing whatsoever to do with the downwind turn argument. Consider the situation 180deg reversed from his example.

Now we are downwind at 40kts airspeed (80 kts groundspeed), and we do a rapid, flat pedal turn into wind. The downwind turn flat-earthers would hold that airspeed increases on turning into wind. Does that happen here? Nope, it decreases ... just the same as in the rapid flat turn downwind! Indeed, thanks to the aeroplane's inertia, with an infinite rate of turn the airspeed in both cases would change initially to minus 40kts!

Edited to add: BTW, with respect to earlier observations by MO, it is this rather than the alleged loss of airspeed in a downwind turn that is analogous to the effects of windshear.

[PPRuNe Pop]

The problem arises (as it did in Edwardian days) with very low IAS conditions combined with high wind strengths and a rapid change of direction.

Quite so IMHO! Plus, I would not have the temerity to question JF.

I was always taught that if you did the above you would would cause the 'inside' wing to stall (lack of upward lift) and the result could be catastrophic, depending how close to the ground you were. I tested this theory later at various heights and in various aircraft in my display days and it was spot on.

As a matter of fact I was following a certain Prince William of Gloucester who had taken off before me in an air race at Tollerton a good few years ago, when he was flying his treasured spam can. He reached the turning point and made a very determined attempt to get round it as quickly as possible. He didn't make it. He simply stalled the inside wing at about 300' and went in.

In short. You must have the right airspeed to execute a turn in the prevailing conditions.