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

[4L3X]

This is a question i have been asked to find out by my 'educated' (polite way of saying 'old') instructor who also happens to be Ex RAF.

Many think you need to include a vast variety of scientific words and avionic lingo, however, he always says the shortest answer is often the simplist - logically! Or so he says! Plus he hates people making up bulls**t for the sake of speaking or pretending to know the answer!

His exact question is and i quote; "In one clear and concise paragraph, explain why we lose airspeed in a turn, and what causes this?"

Please assist, as this question is doing my head in - i kind of know the answer but just not how to phrase it simply, or to his standard.

If you could include some references or sources where you got your answers from i would be most grateful.

Many thanks in advance.

Al

[oncenterline]

As soon as you start a turn you have to pull back on the yoke to maintain your altitude. This increases the drag and therefore you lose speed if you do not increase power.
In shallow turns normally there is no noticeable difference, but with bank angles more than 30 deg you will definitely have to increase pitch and power to maintain altitude and speed.

[747dieseldude]

When in a turn, you have to produce more lift, because a portion of that lift is used to turn you.
More life=more drag
More drag=less speed

[Mike Oxmels]

The above is the most correct answer but an additional factor is that if you are flying into a reasonably brisk wind and then rapidly turn downwind you will tend to temporarily lose airspeed. This is due to the momentum of the aircraft being related to groundspeed whilst the windspeed goes from a positive to a negative value. A large groundspeed acceleration is required to recover the 'equilibrium' airspeed.

Some people will argue that this effect does not exist, saying that the airspeed will not vary (only true in light winds with gentle turns). If this effect did not exist, neither would windshear.

Doing tight low level field circuits in a heavy helo with a strong wind, the effect is very noticeable, requiring significant nose down and power to recover airspeed on turning downwind.

Edited to add: If turning from downwind into wind your airspeed will temporarily increase.

[Art Field]

In a balanced turn the lift generated by the wings is no longer just acting vertically to maintain level flight but is also acting horizontally to make the turn. To maintain level flight the total lift generated by the wing must therefore be increased by increasing the angle of attack resulting in more drag and thus speed loss. Same answer, another way of saying it.

[L Peacock]

Ignoring the indirect aerodynamic effects and just using phyics.
If you're flying at 350kts stabilised, then initiate a turn through 180, you've effectively accelerated in velocity terms by 700kts. It's always going to need thrust above that required for straight & level flight.
It's incidental that for an aeroplane, the direction change (relative acceleration) requires lift which leads to drag.

[Mike Oxmels]

Sorry LP. I disagree with that explanation. In still air (unlike my example) the acceleration is lateral, not in the direction of flight (mechanics of circular motion). Imagine any object rotating on perfect bearings (no friction or drag losses); it will rotate forever - it will not decelerate because of the apparent change of speed you describe.

In still air it's purely about the inclination of the lift vector and the increased drag that results. With wind the momentum effect, I mentioned also comes into play.

[Flame Out]

Don't forget in your own words:) I'm sure your experienced instructor looks on here as well?

[L Peacock]

yeah but no but yeah but no but...

an aeroplane isn't an object rotating on a frictionless bearing, it's a body with mass travelling with a velocity of +350kts which in my example is accelerated to -350kts, deriving this acceleration from the properties of the atmosphere around it.

If it was a vehicle in frictionless space, it would fire a rocket motor, still consuming energy.

However, I've had a bottle and a half of wine now and sense I'm going to lose this one.

[Mike Oxmels]

But that rocket motor would fire sideways to get the centripetal acceleration (plus a couple of puffer jets to establish rotation) and circular motion. You would not need any thrust in the direction of flight.

If your vehicle travelled in 1 dimension only, along a line, coming to a stop and then going back the other way, you would be correct. For circular motion I regret, Sir, that you are not.

[Mike Oxmels]

What the hell am I doing talking physics on Pprune. Time to open a bottle myself!

[L Peacock]

Disagree.
We aren't talking about just rotating an aircraft about its axis without changing the direction of travel. A turn will always reduce the velocity on the original heading and increase velocity on the reciprocal (relatively).
My spacecraft analogy was on the same lines ie don't want to go to the moon now, want to turn and come home. First an RCS burn to rotate, then a main engine burn (don't know the acronymn) to decel/accel.

[BEagle]

"When in a turn, you have to produce more lift, because a portion of that lift is used to turn you."

When in a level turn, you have to produce more lift because only a portion of that lift now opposes the weight of the aircraft - the rest is used to turn you.

More lift=more drag
More drag=less speed

[Wizofoz]

The above is the most correct answer but an additional factor is that if you are flying into a reasonably brisk wind and then rapidly turn downwind you will tend to temporarily lose airspeed. This is due to the momentum of the aircraft being related to groundspeed whilst the windspeed goes from a positive to a negative value. A large groundspeed acceleration is required to recover the 'equilibrium' airspeed.

Some people will argue that this effect does not exist, saying that the airspeed will not vary (only true in light winds with gentle turns). If this effect did not exist, neither would windshear.

Doing tight low level field circuits in a heavy helo with a strong wind, the effect is very noticeable, requiring significant nose down and power to recover airspeed on turning downwind.





Point me at one authorititive text which confirms the "Down Wind Turn".

An aircraft flys with reference to the air around it. The fact that that air may itself be moving is completeley irrelevent.

You make the mistake because you have an erroneous idea as to what momentum is. Monentum is kinetic energy which is mass x velocity.

Ah! you will say, but velocity is relative to the earths surface.

No it isn't.

It is relative to whatever you want it to be relative to. For an aircraft in flight, the only momentum that matters is that relative to the air around it.Does a space ship in inter-steller space have to look back at the earths surface to find out it's velocity?

Look at it another way- the earth is turning at arond 600mph at the equator. Does this mean if an east-bound aircraft turn it starts flying backwards bacause of all the momentum it has due to the earths rotation?

Go flying some time on a windy day. Get some height, and by reference to the ASI alone, try and determine the wind dirsction- I assure you it won't be possible

Wind shear is a different thing. It occurs when the aircraft encounters a change in wind speed.

[BEagle]

Kinetic Energy = 1/2 mass x velocity squared
Momentum = mass x velocity

Otherwise I agree with you, wizofoz - the 'downwind turn' myth is indeed UTTER BOLLEAUX!

[Remove Status]

4L3X I guess that's what you are looking for:-

we lose airspeed in a turn because of DRAG and what causes this is the disturbance of the airflow over the wing.

[L Peacock]

where's Genghis?

[Mike Oxmels]

I see that the 'some people' I referred to have turned up.

Yes the aircraft flies with respect to its airmass, but its momentum is related to groundspeed.

My heavy helo strong wind example:

Airspeed 60kts,
Headwind 30 kts,
Groundspeed 30 kts

Turn downwind:

Airspeed 60kts
Tailwind 30 kts
Groundspeed 90 kts

A GS acceleration of 60kts is required to maintain the steady state, equilibrium airspeed. There will inevitably be a lag (more noticeable, the more aggressive the turn) resulting in a temporarily reduced airspeed.

This effect can be very noticeable in the circumstances I described, as can the opposite, turning into wind. As I acknowledged in gentle winds and gentle turns this effect will not be noticeable.

If my statement is wrong, explain to everyone how rapid changes of windspeed can affect airspeed in windshear.

Beags, Wizowaz? I'm waiting. HAVE A THINK ABOUT THE KINETIC ENERGY OF THE AIRCRAFT INTO WIND AND DOWNWIND.

LP, the speed (not velocity) is constant. No tangential acceleration.

[Mike Oxmels]

Poor fella probably wishes he never asked! 4L3X, just tell your instructor about the greater lift required due to the inclination of the lift vector, and the increased drag that results.

Beags, Wiz, Deliverance: Talk to me about the kinetic energy of the aircraft in my example at 30kts GS and 90kts GS. You've got to input energy if you wish to maintain airspeed. If you don't you'll come out of the turn at a reduced airspeed.

[PPRuNeUser0211]

If you're losing speed in a turn your aircraft isn't generating enough thrust;)
WRT the downwind turn, can see how this applies to rotary folk:

Helicopter in the hover with a 10 kt headwind... IAS 10kts

Turns 180 out, IAS -10 kts

voila a 20 kt decelleration.

However, with the fixed wing case not so sure this applies!

[john_tullamarine]

Getting back to the original question, the following points are relevant -

(a) for a constant power setting at constant level, a heavier aircraft will fly slower than the lighter example

(b) in a balanced turn, the load factor increases in relation to angle of bank

(c) one can view increased load factor as being much the same as increased weight (mass, whatever)

(d) ergo, turn and you slow down

.. for most of us, this is the typical calculation basis for a first approximation effect of turn on performance.

The underlying cause, as stated elsewhere, above, is that we need a tad more lift .. needs a bit more pitch ... which results in a bit more drag ... equals slow down. For the instructor who posed the question, the preferred answer should lie with the increase in load factor.

[Mike Oxmels]

Deliverance, with regard to your example at post 21:

The KE of the ac is 0
The GPE of the ac is 0, considering its initial ht as the datum.

Zoom climb begins:

GPE increases with height.
Airspeed decays, increasingly negative groundspeed. KE increases.
KE if the wind reduced by increased aerodynamic drag in the pullup.

Energy conserved.

Your move.
Please explain why windshear affects airspeed, if the Earth frame of reference is irrelevant. Also consider that a turn in wind is effectively a varying headwind component with respect to the aircraft.

[Mike Oxmels]

PBA and Deliv, you've hit the nail.

Helicopter in the hover with a 10 kt headwind... IAS 10kts

Turns 180 out, IAS -10 kts

voila a 20 kt decelleration.


I suppose the turn in the hover is the ultimate example of an aggressive turn, possibly a Rate 10 turn. A turn in forward flight will be less rapid to varying degrees.

All that is happening is that he is changing his IAS to maintain a fixed ground speed!
The pilot hasn't done anything. The aircraft has experienced a reduced airspeed. The aircraft has a tendancy to vary its airspeed because in varying headwind component, the Earth frame of reference is significant. Positive action from the pilot is required to maintain airspeed. In IMC, as mentioned earlier, the IAS is scanned and the pilot will conciously or subconciously increase the forward thrust slightly to nail the speed. Likewise in an E-3 doing a Rate 1 turn where the wind is 5% of the TAS, the effect will be negligable. In a slow aircraft, turning rapidly in a strong wind you WILL see in airspeed variation and you will have to adjust thrust to control it.

Goodnight

[Arm out the window]

How windshear affects IAS:
Imagine model plane flying steadily with zero groundspeed, 20 kt IAS in front of a fan.
Fan is suddenly turned off.
Model plane is suddenly at zero airspeed, will lose height rapidly and either crash or accelerate to a suitable airspeed to fly away before it hits the floor.

Not the same as the downwind turn in steady strong wind case - as succinctly described above, aircraft flies with reference to the air and, to the pilot in IMC, will feel the same if a constant IAS turn is flown no matter what the steady wind.
In visual low level manoueuvering, the visual slip/skid illusions fool us into thinking the airspeed changes are an effect of the wind.

[Hilife]

Why? Change in Thrust/Drag couple.

Cause? Thrust is constant, but drag increases causing drop in relative air speed.

[Wizofoz]

Mike,

What you're not understanding is that kinetic energy is relative.

I'll give you an example-

My cofee cup is sitting beside me. It's on a table. The table is on the floor which is attached to the Earth. The earth is spinning at 600mph and orbitting the sun at about 3000. The sun is orbiting the centre of the galaxy at about 100 000 mph, and the galaxy is receeding from the centre of the universe at a signifigant percentage of the speed of light.

My coffee cup has enough kinetic energy to destroy a fair sized city- better not drop it!!

Of course it's not a problem because the table and everthing around the cup is moving at the same rate, so RELATIVE to it's surroundings, the cup is stationary.

An aeroplane flys due to interaction with the air around it. Imagine a gold fish in a bowl on the back of a truck. Does the fact that the truck is moving at a constant rate make it hard for the goldfish to swim in the direction the truck is moving? No! he can happily swim in circles to his hearts content, not even aware that the truck is moving. In the same way, a turning aeroplane interacts with the "Bowl" of air around it, completely indedendent of the surface of the earth, moon, sun or horse head nebula!!

If the truck brakes, it's a different story- That's windshear, a sudden chane in velocity of the environment the aircraft is in at the time.

As I said- you're sitting in front of a computer- do a little research and find one authorititive confirmation of the "Down wind turn" and I'll eat my shorts!!!

[Hipper]

Here's a link.

http://www.pilotsweb.com/principle/load.htm

Would a way of looking at loss of lift in a turn be by seeing lift as a vertical component and a product, amongst other things, of horizontal wing surface area? If so, on a turn the horizontal wing surface area is reduced (ie the wing surface that can be 'seen' by a vertical force) and therefore the lift generated is also reduced. It will require an increase in angle of attack or forward speed to keep the amount of lift as it was.

(from an ignoramamus and it may show - I've got my tin hat ready!)

[SAR Bloke]

The KE of the ac is 0
The GPE of the ac is 0, considering its initial ht as the datum.

Zoom climb begins:


Mike,

Could you please explain how a zoom climb begins if the initial KE is 0?

[Mike Oxmels]

Thank you Deliv. No, I am not mental. My comments are based on an A grade in Maths Mechanics A Level, 1st Class Honours in Aero Eng from one of the big 3 universities, RAF wings, the teaching received from a number of OCU/frontline instructors and experience.

Sarbloke. An object with zero groundspeed has zero KE wrt the Earth.

How can a pilot flying on instruments subconciously increase IAS I actually said thrust. Whilst carrying out your selective radial scan, you carry out numerous tiny corrections, and when experienced will do so without a great deal of concious decision making. Under the circumstance I've described the pilot may have nibbled the throttle open without ever having conciously decided "I'm going to open the throttle".
When flying an approach, do you still make big concious power/attitude decisions like a first solo stude or do your inputs 'just happen', subconciously, like when riding a bike?

How windshear affects IAS:
Imagine model plane flying steadily with zero groundspeed, 20 kt IAS in front of a fan.
Fan is suddenly turned off.
Model plane is suddenly at zero airspeed, will lose height rapidly and either crash or accelerate to a suitable airspeed to fly away before it hits the floor.

Not the same as the downwind turn in steady strong wind case - as succinctly described above, aircraft flies with reference to the air Yes correct about windshear. But are you (plural) trying to say that sometimes GS is relevant (above) and sometimes it isn't (turning)?

As I said- you're sitting in front of a computer- do a little research and find one authorititive confirmation of the "Down wind turn" and I'll eat my shorts!!!

To quote John Farley, no less:
On the other hand, if the air is not still, but moving at a steady speed over the ground, the aircraft will suffer a fluctuating ground speed after the “steady ” turn is established at “constant” airspeed. This ground speed (and its related kinetic energy) will be a minimum when flying into wind and a maximum when downwind.

This energy change and the problems it produces (thanks to inertia) is what is at the heart of the so-called downwind turn stall issue. I say so-called because this effect (while always having some transient effect on airspeed, related to the transient change in ground speed) will only cause a STALL in quite EXTREME CIRCUMSTANCES. Normally there will be no more than a minor reduction of height or a slight loss of airspeed (both usually so small that I recommend you don’t even think of wasting your time airborne by trying to measure them in real air, which is anything but steady) (Google 'downwind turn airspeed' and you'll find two ancient cached pprune pages. The above paragraph is from the thread begining 'among light aircraft pilots...')

How would you like your shorts cooked Deliv?

I reiterated that what I have described is a transient effect, negligible in normal circumstances, light winds and gentle turns but it exists.

[RETDPI]

So by MO 's logic, maintaining a constant airspeed and altitude 45 deg banked turn would require continual changes of power throughout each 360 deg turned , these depending on the windspeed. :ok:

[ImageGear]

Constant vector, speed is proportional to thrust and drag.

Change the vector, angle of attack must change, drag must increase so speed declines therefore thrust must increase proportionally to maintain constant speed.

Or so I think, :oh:

Imagegear

[W.R.A.I.T.H]

Looking at the starting post, the instructor in question would probably trash most of you for talking the exact scientific bullsh!te :)

1st Newton's law: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force (http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l1d.html#balanced).

Hence, to bring a craft into a turn from a straight flight requires action of force. The energy to generate that action of force must come from somewhere (energy is conserved), and that's from its kinetic energy of forward motion.

That being said, if you had a ball flying in a vacuum and wanted to change its direction of flight without touching it (external input=force added into system=unacceptable), you would have to slow the ball down. That exactly applies even to electrons orbiting around atomic core and any lift-drag and other bollox is totally irrelevant here.

[Hilife]

Hipper

Loss of lift in a turn ….?

On the contrary, lift increases substantially in a turn as it is this increase in lift that provides the centripetal force to enable the aircraft to change direction (vectored thrust aside).

[Mike Oxmels]

Speaking of scientific bullsh1t WRAITH, ideal circular motion is a constant energy motion. The centripetal force acting on the body acts radially towards the centre of the circle but there is no change of displacement in this direction and no radial velocity. Hence, no energy is expended by the radial force. The tangential speed is constant hence constant energy.

Hilife, I think what Hipper was trying to say, is that with an angle of bank, the vertical component of lift decreases. To maintain level flight, the total lift must be increased, by increasing alpha, to maintain a vertical component of lift equal and opposite to weight. The increase of lift and alpha, increases induced drag, and if thrust is not increased to match drag, airspeed will settle at a reduced value. I think this is the one point that we have all agreed on since post 2.

OUT.

[Genghis Couldn't]

"So by MO 's logic, maintaining a constant airspeed and altitude 45 deg banked turn would require continual changes of power throughout each 360 deg turned , these depending on the windspeed. :ok:"

Yup, and that's what kills his argument. MO can argue all he wants, but if his assumptions are incorrect (i.e. we are talking about a particular reference frame, he is mixing two) then no matter how great his logic and maths, it still isn't right.

It doesn't matter how many degrees you have of what patches you wear - the logical argument based upon correct assumptions is always going to triumph (thank goodness).

[Mike Oxmels]

Genghis, would you care to comment on John Farley's paragraph which I quoted? I believe that he knows what he's talking about more than most.

[BEagle]

'Medium Turns' is usually the first exercise a student QFI learns to teach.

And with it comes the acronym KISS!

None of the grandiose 'energy concept' bolleaux is worth a rat's - it's utter boffinry and irrelevant. Ignore frictionless particles, kinematic viscosity and the time of day on Saturn's moons - and keep it simple!

Aircraft in turn requires more lift because only the vertical component of lift opposes weight. The increase in lift is achieved by increasing the angle of attack; this also increase drag. An increase in thrust is necessary to avoid deceleration.

As for 'downwind turns' and relativistic mechanics or whatever, if you really want to complicate matters and you remember that the angle of attack is the angle between the chord line and the relative airflow, if some external factor such as a vertical gust changes the relative airflow then angle of attack will also change as a result. Mention that during a medium turns brief to a trapper and the pen will start to make notes - followed by "Why's that, sir?".....:uhoh:

The illusory effect of wind on apparent speed is covered during low level intro - but only demonstrated if there is a significant wind during the air exercise.

I found that my Aero Eng degree was worth the square root of sod all when it came to teaching P of F - the CFS way was infinitely simpler and easier for all to understand....:ok:

[SAR Bloke]

Ok Mike,

So if taking your datum where PE and KE is zero, where does the energy come from to zoom climb? In your example both PE and KE increase. The energy to do this must come from somewhere. If not, I'd patent your findings and make Bill Gates look like a pauper.

A turn in the hover is not an aggressive turn as you describe it. It is completely different mode of flight as the pilot is intentionally maintaining groundspeed. Airspeed is irrelevant in this case (subject to aircraft and power limitations). In the downwind turn being discussed the pilot is maintaing airspeed.

[W.R.A.I.T.H]

Speaking of scientific bullsh1t WRAITH, ideal circular motion is a constant energy motion.

It is indeed Mike, where have I suggested otherwise?

No energy is being expended because a balance exists between the central and the centrifugal force (force acts, but no displacement => no work done => no energy consumed), none of which we can dismiss from the consideration. Note that Newton explicitly mentions that unbalanced force is required to change the state. This is where energy is consumed in the ideal case. In reality though, aerodynamic aspects come into play, but as I understand it that's exactly what the instructor didn't want to hear.


EDIT: Although yeah, if you want to get into aerodynamics, Beagle is spot on.

[Mike Oxmels]

WRAITH,
Hence, to bring a craft into a turn from a straight flight requires action of force. The energy to generate that action of force must come from somewhere (energy is conserved), and that's from its kinetic energy of forward motion.


That's where you suggested otherwise. That part of what you said is not correct. A force can exist without consuming energy, for example the centripetal force in circular motion. There is a centripetal acceleration but at constant zero radial velocity and no change of radial displacement.

SAR bloke, read again what I said. Drag doesn't just act on the aircraft, its equal and opposite effect acts on the air. That's why a slipstream exists behind lorries. KE is taken out of the air that is decelerated from '300 kts' to a lower value by drag, in the example.

If you're hovering into a 25 kt wind with, say 8 deg nose down and you yaw the helo down wind maintaining your 8deg nose down, yes eventually you will be flying at 25kts AS, 50kts GS (ignoring stoofing into the ground due to loss of translational lift), but it will take time to reach that equilibrium. During that time you are experiencing a transient reduced airspeed. Same deal to a lesser extent in forward flight.

[Genghis Couldn't]

"Genghis, would you care to comment on John Farley's paragraph which I quoted? I believe that he knows what he's talking about more than most."

Yes, certainly. He is incorrect on this point ;-)

Would you care to comment on the logic of the steady state turn example?

[W.R.A.I.T.H]

I stand corrected.

Then it is the partial loss of upwards lift component as suggested by Beagle. Over.

[Wizofoz]

Mike,
Care to comment on my Coffee cup?

You continue to insist kinetic energy is ONLY relative to the Earths surface.
The Earths surface is a very small piece of the universe where, I believe, universal laws of physics still apply.

And how about my Goldfish? You seem to br selectively ignoring arguments which clearly dis-prove your case.

Don't believe I'M saying this but, yes, Johm Farley is wrong on this occasion.

You were also pretty selective in your googling. I've just done the same and come up with a myriad of articles de-bunking the myth-

Heres a few-

http://www.x-plane.com/myths.html
http://www.aeroexperiments.org/brainteasers.shtml
http://www.djaerotech.com/dj_askjd/dj_questions/downwind.html

Can you also answer me this? If the effect is small in the light winds at the surface, it must be proportionally greater in strong winds. Iv'e been in holding patterns in 120kt winds and, guess what? turning downwind the aircraft did no fall out of the sky.

[rmac]

I am guessing that the instructor is a pprune member and is currently p1ss1ng himself laughing at the storm he has sneakily unleashed ;)

[Raymond Ginardon]

Not going to comment on the logic of the arguments, but for anyone interested in experimental evidence (we had this discussion at work a little while ago)….

FL 250, wind 244/70, 300 KDAS, smooth air, GPS pod, instrumented ac, autopilot (height hold) and autothrottle engaged.

Did a bunch of 720’s at various AoBs and then looked at the traces afterward*. No temporary loss of speed, no increase in fuel flow (thrust) as you went into/down wind.

*And for the ‘concerned’, it was done at ‘no cost’ while I had to hang around for another asset to appear.

Ray ;-)

[Shackman]

I don’t think I’ve ever seen so much horses**t written in a thread before regarding the ‘effects’ of groundspeed. . Your aircraft flies in a (relatively) steady state air mass, so you should always fly with regard to your IAS – Indicated AIR Speed, and it matters not if that air mass is still or moving at 10kts, 50kts or even 200kts. Your VMax remains the same and your stalling speed even more so. If you are above 4000ft or in cloud you wouldn’t even notice changes in ground speed.

Unfortunately as you get lower you start becoming aware of groundspeed, and sadly one of the physiological aspects of flying as you get lower is that your eyes start to look outside more than inside, and by about 500ft or so you subconsciously start flying relative to groundspeed. That is all very well if you are already downwind (in your steady state air mass), because if you turn into wind using your visually assessed groundspeed your IAS will increase. Unfortunately if you turn downwind ……….! Again, no problem if you have a lot of speed on already, but if your IAS is relatively slow – ie you’re looking at something specific on the ground – the principles of flight will win out and stalling speed will catch you very quickly. Now the only way out is to stuff the nose forward (if you can) and open the throttle, and even more unfortunately the terra firma magnet is there waiting for you. If you want a classic case of relatively inexperienced pilot flying groundspeed instead of airspeed look at the police Optica, crash, which unfortunately killed not only the crew but also a very promising aircraft project.

One of the first things we teach during the low flying intro to all military rotary pilots is how easy it is to get suckered into flying groundspeed instead of airspeed, and just how easy it is to be bitten when you’re heavy, low and slow.

As an aside, a few years ago I was diverted to pick up a microlight pilot with a serious back injury who had stalled in from about 200ft. It was a windy day and he was at the top of a ridge line, along with about 30 – 40 other microlights and hang gliders. As we sat waiting for him to be stabilised and stretchered into the aircraft we watched these ac launching off the top of the ridge into the updraughting wind and getting plenty of lift as well. Over 50% of these pilots turned back over the ridge once they were above it and got quite a fright, ‘cos they lost AIRSPEED by maintaining groundspeed, with the ensuing incipient stall. Most recovered, but by the time we lifted we had four more patients for Swindon hospital!

WRT to the original question - go look at your PoF notes re Lift and Weight

[BEagle]

Any reference to helicopter aerodynamics (if that isn't an oxymoron) is, in any case, irrlevant. Everone knows that the only reason they stay airborne is because they are so ugly that the earth repels them.....

Ray,

"FL 250, wind 244/70, 300 KDAS, smooth air, GPS pod, instrumented ac, autopilot (height hold) and autothrottle engaged."

"...no temporary loss of speed"

No $hit, Sherlock!

[Raymond Ginardon]

"...no temporary loss of speed"

No $hit, Sherlock!

Yes - the autothrottle in the aircraft is so amazing that it can hold the EXACT airspeed and never lets it drop by even a knot... :rolleyes:

[Capt Pit Bull]

None of the grandiose 'energy concept' bolleaux is worth a rat's

Beagle

Thing is Beags, since energy is a scalar rather than a vector, and scalars are much easier to work with than vectors, there are a quite a few things in the universe that are a hell of a lot easier to comprehend by looking at the energy changes involved rather than by looking at the forces involved.

Mike

None of which detracts from the fact that while you have a better grasp of some parts of basic Physics than most of the folk here, you have lost the plot. Educational qualifications not withstanding, please examine the possibility that you are muddling your frames of reference. The fact that there are many others on this thread who are revealing varying degrees of Physics non-comprehension does not automatically make you correct on this point. The turning effect you describe does not exist, (and yes, your quote reveals JF to have been having a bad day as well).

pb

[Arty Fufkin]

I have to say that I have never heared such rubbish spoken on the subject of Aerodynamics. The reason for the effect you describe is as follows:

Lift on an aircraft's wing is purely a result of the presence of Lift Pixies. Whilst they are best known for their elevative powers, Lift Pixies also have the notable properties of being invisible to the sober and having an extreme aversion to peace and quiet.

When a pilot begins the take off roll, the throttle is opened and the engine makes a lot of noise. This noise attracts any Lift Pixies in the area and they jump on the wing thus creating lift and raising the aircraft into the air.

When an aircraft enters a turn some of the Lift Pixies lose their footing and fall off the aircraft's wing. This causes a loss of lift and unless the pilot takes immediate action the aircraft will descend. By adding more power, the engine makes more noise and entices the Lift Pixes back onto the wing and lift is restored.

Simple.

[critical winge]

Awesome Arty, well put. I :D you with the most accurate and easy to understand answer so far. Well done, have 5 house points and a free pizz at the next dining in night!

[samuraimatt]

I am sorry I don't agree with you Arty about the use of lift pixies. Lift pixies are really only good for civilian aircraft as they usually require payment in cakes, flowers or nice thoughts. These are usually sourced either from the in-flight catering companies, or provided cost-free by the passengers who fly on civilian carriers.
As you can see this would not work well within the operational military environments that the RAF work in. RAF In-flight baking is not renowned for the "light and fluffy texture" that the lift Pixies demand and the availability of in season flowers might be a problem in desert operations, also nice thoughts may be hard to find during times of active operations.

[Arty Fufkin]

I'm sure we have all noticed a shortfall in aircraft performance when operating in theatres such as Iraq or Afghanistan. Some bloke once tried to explain it all to me by using terms such as ISA Deviation, Density Altitude, Pressure Altitude etc......... amateur!

Don't worry, Lift Pixies can be found all over the world and can co-exist with the most miserable of people as is proved by the commercial sucess of Ryanair.

The reason for the slugish performance of aircraft in operational theatres is as follows :

Lift Pixies don't wear shoes (obviously) hence they easily fall off wings in turns. Your average airframe parked in the sun gets pretty hot so its unsuprising thst the little fellas are reluctant to jump onto the wing's surface. The only way to entice them onboard is more noise hence more power. Either that or put up with a longer take off run.

Hope that helps.

[ProfessionalStudent]

Arty

As you seem to be PPrune's resident Lift Pixie SME, could you explain how they work in helicopeters? How do they ever manage to stay on the fast-whirling blades - do they perhaps wear little velcro booties? Obviously from what you have already said, the traditional teachings of rotary P of F are just pure bunkum...

Yours in anticipation,

ProStu

[RETDPI]

It is an oft quoted "twitcher" observation that many bird species tend to scatter in panic when a "goroundoscope" approaches.
Perhaps they know something............

[W.R.A.I.T.H]

ain't that something about them being ugly, or so i've heard?

[ProfessionalStudent]

ain't that something about them being ugly, or so i've heard?

Surely that's no way to talk about the lift-developing little fellas?:eek:

[Arty Fufkin]

Pro - Stude,

Thank you for your enquiry regarding so called "helicopters." I must admit that I don't agree with them personally, never have!

As for how they manage to thrap themselves into the air, I'm afraid i simply have no idea. It is safe to say that no self respecting Lift Pixie would be seen dead clinging onto one of those ungodly contraptions. I can only imagine that some form of static-charge repulsion is responsible. Either that or the work of the devil.

Hope that helps

Dr A Fufkin

[daze_gone_buy]

I think you will find that the complex aerodynamics involved in rotary flight do not rely on pixie power, but on the sheer will of the occupants for it to remain in the air :rolleyes:

[samuraimatt]

As I said earlier lift pixies are not suited to military operations, however Thrust demons are. They are a nasty bunch who require payment in blood or souls something that most airlines would not be able to supply.

[MightyGem]

the engine makes a lot of noise. This noise attracts any Lift Pixies in the area and they jump on the wing thus creating lift and raising the aircraft into the air.

So how about gliders?

[SALAD DODGER]

Gliders work on the principle that their pilots are so ugly, that they are repelled from the ground...:8

[Shackman]

Thank you Arty Fufkin, I stand totally corrected. I have never heard such an erudite and comprehensive reason for the way things work,

The only item missing is of course the Gremlin, which I assume is 'anti-pixie' and more prevalent in hot, high and/or dusty places (and middle earth) and the reason for so many things falling out the sky.

I'm now wondering how to make my whirly-whirly helicopeter thing more attractive to pixies!

[Dan Winterland]

The helicopter is a triumph of engineering over common sense!

[Spaghetti Monster]

Mike O: Can I humbly suggest you get yourself into a heavy jet and go and fly some orbits at F300 in a nice steady 150 kt wind? If your theory is correct then you should alternate between stalling and dropping sonic booms, as the headwind/tailwind component changes dramatically.

The vagaries of ATC and traffic flow cause me to do this (ie the orbiting thing, not the stalling/sonic boom thing) on an all too regular basis and I can happily report that as long as the wind is steady, so is the airspeed - with basically no change in thrust whatsoever.

[Mr Teatime]

Very interesting debate on the finer points of lift etc. I am afraid I only agree with the latter drift of the thread concerning Lift Pixies. My own experience is confined to the lesser known Thrust demons as found on rotary craft.
They too are atracted by noise and vibration hence the inherent clattery nature of all things rotary. However Thrust demons also like thrills and the ability to show off. The challenge of whirling round and around while holding on and avoiding hitting the fuselage is candy for them. It may be noted that the tips of the roto5 blades move faster through the air hence the bigger stronger thrust demons can be found here. Unfortunately being demons they also like hot sandy places, it is like home. They therefore need more noise vibration etc to entice them to play. Note that the chinook lift is very good and this maybe due to the added danger to a Thrust demon of dodging his mates on the other blades as they barely miss each other. Thrust Demons are, I am told, closely related to Gremlins. As they get alond well this probably explains the higher maintenance and lower serviceability in general of RW ac.

Gliders and lift pixies. I assume some are initially attracted due to the winch noise or fall off the towing ac. Once released the noise goes away and the pixies get bored and go off, hence ac descends.

parachutes and bang seats probably work in a similar way. Initial noise and screams of "oh my god i'm going to die" attract pixies for the initial deceleration and then a gentle fall.

However I am no expert.

[ProfessionalStudent]

I love the way this thread has developed. It's refreshing to find a thread on Prune that hasn't developed into a inter-service/inter-type/international willy waggling contest.

And it's edumacational too:ok:

Though I still like the "so you lift the lever up, science occurs, and the helicopter lifts to the hover" approach to rotary PofF. All this talk of pixies and demons is all a bit A level for me....

[tiger26isfinals]

4L3X...im sure if you asked your instructor nicely he would give you a very pleasent whiteboard brief. Come to think of it, im sure he would :)

[samuraimatt]

The full explanation of Pixies and demons can be found >>>>here (http://www.messybeast.com/dragonqueen/liftdemon.htm)<<<<. A very interesting read.

[Soft Ground]

Turns out that every aircraft will be different. The fast jets hardy ever notice it as they have lots of power, lower drag and go fast. The heavy, draggy, propeller driven, slower transports however are much more aware of it.

In a turn going into the wind the airspeed will gradually increase, but so will the effect of drag, slowing you down. In a turn going with the wind the airspeed will slowly drop, but the reduction in drag will allow the thrust to increase the air speed.

The heavier you are the more inertia you have and the slower you can recover. The faster you turn the less time there is for the air speed to recover.

SG

[PlasticCabDriver]

Soft Ground,

In a turn going into the wind the airspeed will gradually increase

No it doesn't. It will increase if you try to fly (whether you mean to do it or not) a constant groundspeed, but that is because you have made it happen, not simply because you have turned. However,

but so will the effect of drag, slowing you down. is right but not for the reasons you give.

It has all been explained very clearly above:

As you put on bank, the vertical component of lift is reduced, so more angle of attack is needed to generate the extra lift required to maintain altitude. An increase in AoA gives more lift but also more drag, so the ac slows down.

It has nothing to do with into wind/downwind or indeed breaking wind.

This bit:

In a turn going with the wind the airspeed will slowly drop, but the reduction in drag will allow the thrust to increase the air speed is equally as wrong, unless you are trying to fly a constant groundspeed.

Was never implemented though as they couldn't afford the solution!

Perhaps they were simply being polite!

[Spugford]

Soft Ground.

Please put my (slightly fuzzy) head to rest and tell me you don't wear military wings of any sort...

Please.

[Soft Ground]

I am aware that lift is the main cause, but that has already been explained.
SO I did not repeat it.
I was only referring to the effect of turning into a head or tail wind.

Speed will be lost because of the increased drag caused by the higher angle of attack needed to generate the extra lift.
Inertial will try to maintain ground speed, not the pilot. But the drag/thrust balance of the aircraft will try and maintain airspeed.
Simple physics.

In practice the pilot will make small adjustements to thrust to maintain airspeed. You dont need to understand this to fly, but in some circumstances it may help.

SG

[PlasticCabDriver]

I was only referring to the effect of turning into a head or tail wind.


But turning into a head or tail wind has no effect on airspeed, unless you are making some reference to your groundspeed.

When flying on instruments, and you make a turn, a little extra power is needed to keep the airspeed. If you do a 360 degree turn, by your argument, you would be increasing and decreasing pwr all the time as you turn with and against the wind, but that simply doesn't happen. The extra bit of power is added at the beginning to maintain the airspeed, and is left there until the aircraft is rolled out again.

This may not be in accordance with your "simple physics", but it is what actually happens!

[Wizofoz]

Simple physics.


No, it's a complete mis-interpretation of "Simple Physics"

[forget]

...........the engine makes a lot of noise. This noise attracts any Lift Pixies in the area and they jump on the wing thus creating lift and raising the aircraft into the air.

Arty’s got it Spot On. :ok: In fact, with the right conditions and a Pixie filter, you can photograph Lift Pixies.

They don’t like it – but you can; they're only little.

PS. What Arty didn't mention, and perhaps should have done, is thrust reversers.

They're there to blow the Lift Pixies off the wing.

http://i21.photobucket.com/albums/b270/cumpas/Untitled-2-9.jpg

[Arty Fufkin]

And spoilers tend to flip them off over the leading edge. It all seems quite logical to me.

[AC Ovee]

Soft Ground, me'old, wind is the movement of air relative to the ground. If you are not on the ground, or fixed to it, you will not feel the wind. Go up in an air balloon and you will travel with the wind but your scarf will not fly like biggles and your airspeed will be zero.

I believe you are confusing wind shear with the problem. Wind shear occurs when the aircraft encounters 2 or more air masses that are not moving in the same direction or at the same speed; ie turbulence.

Turning into a headwind is an expression to describe an aircraft that turns within an air mass such that the aircraft's new direction is opposite to the direction that the air mass is moving relative to the ground.

Wind is irrelevant in aerodynamics, but is everything in navigation. You only need to ask the lift pixies....

[forget]

Arty's right again - I'd forgotten about the spoilers
:ugh: :ugh:

http://i21.photobucket.com/albums/b270/cumpas/Untitled-4-1.jpg

[airborne_artist]

In practice the pilot will make small adjustements to thrust to maintain airspeed. You dont need to understand this to fly, but in some circumstances it may help.


SG - total nonsense I'm afraid. Once in a mass of air that is (or is not moving) the aircraft has no idea what that air mass is doing relative to the earth's surface.

Do tell us which renowned school of aviation taught you this totally incorrect theory.

[India Four Two]

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:D

[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. :ok:

[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. :ok: 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. :ooh:

[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. :hmm:

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! :uhoh:

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.

[Asking]

Do you require less thrust to fly east than west in still-air conditions on a rotating earth? The centrifugal force of an eastbound aircraft must counter and thus reduce the effect of gravity while spinning 'with' the earth as your rotational velocity with respect to an unaccelerated frame of reference is greater. Less 'gravity-like force'=>less 'weight'=>less lift=>less drag=>less thrust. It must be a minimal effect but it disproves those who suggest flying circles IMC without power changes shows that the airmass is the only valid frame of reference.

I'm assuming they launch space shuttles eastward though I've never flown over florida at the right time to see for myself.

[212man]

One of those subjects that generates real 'black and white' responses.

Regarding the 'earth reference point', consider a 50 kt Northerly wind air mass, with two gliders on reciprocal N/S tracks doing 50 kts KIAS (with lateral separation, before the smart arses say they collide!) As they land in a field they encounter a brick wall at the point of touchdown. The one heading North touches down gently from a hover and the pilot steps out unscathed from his intact glider. The southerly heading one disappears into a pile of glassfibre shards and all they find is the pilot's floppy sunhat.

Relevant? Maybe not, but don't tell me an aircraft's total energy is not related/referenced to the earth.

Following on from JF's theory on theories, I once read an article written by a USN Lt. Cmdr. that explained in great detail how it was impossible for a light SE aircraft to carry out a turnback from 500 ft following an EFATO. It had lots of calculations and diagrams to prove the point. Strangely enough, as a 40 hour Bulldog student, I was able to fly quite nice turnbacks from 450 ft and upwards, so I guess that theory (which is still widely held) was wrong!

[Genghis Couldn't]

212,

I don't think anyone here disputes that (?) - you are having a raging agreement here! BUT what some people are missing is that energy is relative to something - you can measure it against what you want and then argue that black is white.

Your gliders, relative to the air mass, have the same energy. Relative to the ground, they have different energies. It depends what you measure it against.

G

[Spaghetti Monster]

This has me concerned, as I'm due to undertake a train trip soon. If I'm walking towards the back of a moving train, and then turn around and start walking towards the front, will I suddenly fall over?:uhoh:

[TheInquisitor]

Ok, here's my stab at this.

I think MO's theory wrt conservation of energy is, at least partially, correct, but he is misattributing where the energy comes from. Take this example:

You are flying an aircraft at 100KIAS directly into a 100kt headwind. Relative to the ground, you have the same kinetic energy. Therefore, stating that 'your kinetic energy is zero' is in fact correct (in keeping with relativity). An object's kinetic energy will only become relevant if it meets another object with a different level of kinetic energy (imagine two bullets fired from guns next to each other - if they touch, the effects are negligible; they have massive amounts of kinetic energy due to their high speed, but roughly the same amount as each other. If you now fire them directly towards each other, and they make contact, there is a very large energy transfer and the effect IS relevant.)

Now, initate a 180deg turn so you are flying directly 'out' of the wind. In the time it takes to make the turn, the aircraft will accelerate (the quicker the turn, the greater the acceleration) from zero to 200kts groudspeed and both you, and the aircraft, will FEEL the acceleration - you will percieve it as an increase in 'g'. Ordinarily, when an aircraft turns, acceleration, or 'g', is produced and felt. But the energy to create this acceleration comes from aerodynamic sources - the wings are directing some of the lift force towards turning the aircraft, so to maintain constant IAS and height, some extra energy must be added (more thrust) to provide more lift at the same IAS - total energy is conserved.

In the case above, the energy to accelerate the aircraft (relative to the ground) comes NOT from the wings, but from the air (the air 'behind' the aircraft will slow down relative to the ground). Therefore, although you will feel 'g' during the acceleration, the aerodynamic forces produced by the wings will not change - there is no increase in wing loading, despite the 'g' that you are feeling - therefore there will be no effect on IAS. Energy is, once again, conserved.

In JF's hovering Harrier turn, and Prince William's air race crash, there is a difference - they were both DELIBERATELY trying to fly with reference to the GROUND. The fact that PW crashed was nothing to do with IAS loss in a downwind turn, but the fact that he must have overloaded and stalled his wings in a turn, in an attempt to make the 'gate'. Bringing Helos into the argument will always complicate matters, as, when hovering or flying slow and low, they are almost invariably flying with reference to the ground.

Does that make sense? Or am I talking bollox as well? Makes sense in my head.

[SixOfTheBest]

John F,
Being in the hover, into wind (say 10 knots), bang on the markers.
You then decide to turn downwind and, hey presto, you need a bunch of power to remain in the hover because you have just lost 10 knots worth of lift pixies (I think at lower speeds they a called a 'Clatter' of LP's, then a 'Gaggle' until about 600+ knots when they are referred to as a 'Philharmonic'), which is a fact! However, sitting in the hover on the markers, is, by it's very nature, referenced to groundspeed (0) and therefore, we have to make power adjustments to maintain our position on the markers....
Now, if we then take away the earth and we fly 0 IAS and maintain this whilst turning downwind without adjusting power, we will not fall out of the sky......if we then magically make the earth re-appear and look down......we will no longer be on the markers but will, in fact, be moving downwind.....and our net lift-pixies remain the same!
If you agree, then I mis-read your post and apologise! Easy:
Working Groundspeed = Be careful
Working Airspeed = Fill yer boots

[Islander2]

This has me concerned, as I'm due to undertake a train trip soon. If I'm walking towards the back of a moving train, and then turn around and start walking towards the front, will I suddenly fall over?:uhoh:Not necessarily, but according to The Inquisitor you will FEEL the acceleration:

Now, initate a 180deg turn so you are flying directly 'out' of the wind. In the time it takes to make the turn, the aircraft will accelerate (the quicker the turn, the greater the acceleration) from zero to 200kts groudspeed and both you, and the aircraft, will FEEL the acceleration:hmm::hmm:

[Hipper]

I read a crash report that said that the aircraft completed a normal 30 degree banked turn to port, levelled out and flew level for a short time. The port wing then dropped suddenly, came level again and then dropped again hitting the ground. It was concluded that the aircraft was flying below the stall speed for its condition (nearly fully loaded).

What is going on here? Why should only one wing drop? Would the plane have levelled itself or could it be as a result of the pilot's actions?

[advocatusDIABOLI]

Gents,

We're all getting sidelined by physics:

Actually, everyone has a good point, but non yet have covered the 'common ground' Inertia.......

In the hover case, change in direction does indeed change your IAS, as the wind is unable to instantaeously speed you up.... ie headwind becomes tailwind.

In the 'Flying Case', all respondents are correct, but by different levels! In a FJ, it means nothing, in a Tucano it might be apparent in the finals turn, based on your gournd track (as stated). Helos and Hover Jets, I'm less sure, but the point is, that the 'Air' cannot move you straight away, and so there is a 'Relative' change in air motion over your craft. This you see as 'Sheer' or (If you are using a ground reference) IAS loss.

Basically. The 'AIR' has a momentum too, Oh and it's a lot bigger than yours!

Personally, I've only had a scary once with wind sheer, in the Falklands, and that was covered by the 10Kts I always held for 'Granny', Now I know why I did that.

Advo

[Contacttower]

Surely it is simply due to the increased drag that you get when you turn that slows you down, assuming that you didn't loose any airspeed due this your IAS would remain the same no matter what the wind is doing, and anyway surely it is almost impossible to quantify or prove that airspeed is lost in a turn due to the wind, given that you would lose speed in an abrupt turn anyway.

[Wizofoz]

Actually, thinking about it, the "Monentum" and "Kinetic Energy" crowd are correct. The vectors should resolve themselves even to an observer on the ground.

And they DO!

The mistake many people are making (Mike O of the impressive credentials chief amongst them) is to substitute Speed for Velocity. They are two very different things.

Speed is a scalar quantity and as such only has magnitude whilsy velocity is a vector quantity and thus has magnitude AND direction.

Turn an aircraft through 180deg at a constant speed in nil wind and you haven't changed it's speed, but you HAVE changed it's velocity.By how much? Well, do the vector diagram.

To change a vector of, say, 100kts due west to one of 100kts due east you must add a vector of 200kts to the East. You do this by tilting the lift vector, or reversing your harriers nozzels or however. The result is a delta vee of 200 kts.

Now lets introduce a head wind of fiftey knots. Your ground speed is fiftey knots. We introduce the same angular acceleration by tilting our lift vector or whatever in exactley the same way as nil wind.

Now we are heading due east with an airspeed of 100kts and a ground speed of 150kts.

Our velocity with reference to the ground has changed from 50kts to the west to 150kts to the east. A change of? 200kts to the east, exactley the same as in nil wind.

The "down wind turn" is a crock.

[Contacttower]

I don't really understand this negative Airspeed business, if you hovering in a 10 knot headwind, you have a IAS of 10 knots, in order to maintain that you need some power, as you turn around you will gradually start moving relative to the ground, but a given power setting will always (all other things being equal) maintain that 10 Knot IAS, what has negative airspeed got to do with anything?

[Contacttower]

I take it though that the instance of the helicopter only applies to helicopters, in the case of the harrier the thrust nozzles would be moving with the body of the aircraft and therefore would provide thrust in the same direction that the nose is pointing (provided you didn't adjust them) so a flat turn coming out of a headwind would not result in a negative airspeed. Is that correct?

[Islander2]

I don't really understand this negative Airspeed business, if you hovering in a 10 knot headwind, you have a IAS of 10 knots, in order to maintain that you need some power, as you turn around you will gradually start moving relative to the ground, but a given power setting will always (all other things being equal) maintain that 10 Knot IAS, what has negative airspeed got to do with anything?I wonder whether a different scenario entirely will help explain the concept.

Consider a Su-29 in a vertical dive at 250kts IAS which is rotated rapidly in pitch through 180deg to point vertically upwards. Does it retain 250kts IAS? Does it initially have any positive IAS? Nope, it tail slides at negative IAS. A nice airshow party piece!

Conceptually, that scenario is not materially different from a level flight rapid rotation in yaw through 180 degrees, and is independent of wind velocity (as long as it's constant). This is inertia at play (albeit, in the vertical plane, aided by gravity), eventually overcome by thrust.

Any clearer?

[Contacttower]

That sounds slightly different from the helicopter idea, where my understanding is that the body of the helicopter is now moving backwards relative to the rotors (hence negative airspeed). With the Su-29 it tailslides (also I understand negative airspeed) but that is due to its temporary loss of momentum.

[Islander2]

Sorry, but the Su-29 tailslide I described is because of its momentum with respect to the air mass (aided by gravity), not because of a loss of momentum. If you very rapidly yaw a helicopter (or a Harrier) that has some airspeed in level flight, the effect is the same ... not as severe, because you only have inertia, you don't have a force (gravity) attempting to maintain the previous flight path, but you for sure will initially experience a loss of airspeed.

And windspeed relative to the ground is immaterial. The effect is just the same in zero wind as it is in a steady 80kts howling gale.

[Contacttower]

I have to admit that I don't really understand the finer detail of this, but just to be clear, I understand all these effects discussed more recently on the thread; Su-29 etc. are all by virtue of the effects experienced in a turn and nothing to do with this 'downwind turn' rubbish talked about earlier.

[Mike Oxmels]

I'm back, and for those that were wondering, no I was not beaten down by the fierce banter nor did I need to take medication. I've been away on a cse without internet access and so am amazed to see the power of this thread. It is now attracting new posters who clearly have not read from the beginning and hence repeat old ground. This means that the thread has now reached a critical mass and has the capability of living forever, or never dying (depending on your point of view). Congratulations to the original poster.

You are in for a treat as I will contribute later, having clarified my thought processes in happy hour, but for starters, let me just say that if you do not see eye to eye with the likes of JF on this one, perhaps this issue isn't as black and white as you'd like to suggest!

FS Bell, (very young to be an SNCO!) before dismissing opinions contrary to your own as rubbish do a little research and compare John Farley's experience to your own.

As for trotting out my quals, that was merely to demonstrate that I am not actually retarded as suggested by, I believe, Deliverance. Hopefully we can discuss this at a slightly higher level than the 'that's rubbish', 'you're mental', 'your mum's fat' level.

TTFN :ok:

By the way, Wizo, can you tell your mum to give me back my slippers.

[Contacttower]

Oxmels,

To my knowledge I have not suggested that I disagree with anything that John has said, but you have admit, when faced with something said (even from someone of your calibre) that goes against everything you've ever been told one is bound to question it (and when I said rubbish I was merely quoting what an instructor had said to me when I asked him about the 'downwind turn')

[Wizofoz]

By the way, Wizo, can you tell your mum to give me back my slippers.

Err... You'll have to reassemble her ashes first- the two year fight with cancer was not the happiest- a tad uncalled for perhaps?

lf you'd like, however, to point out the errors in any of my posts, please do so.

You might also note that John Farley has said that people more qualified than HIM have told him he's wrong...

[Contacttower]

Could someone please clear up this related issue....

I have read similar threads on this issue from back in 2000 and someone then claimed that at Boscombe Down in the 1960s they attributed the 'downwind turn' effect to the 'fact' that although we assume that the plane flies relative to the air around it, the plane's intertia is relative to the earth mass and therefore looses some when making a downwind turn, explaining the loss of airspeed.

Many people on this thread have denied that the above is true and until today I was inclined to agree with them, but now I'm not so sure.

[Wizofoz]

Remain sure.

Once off the earths surface the only contribution the earth makes is gravity, which is a vertical acceleration.

If not, could someone please tell me at what height this mysterious force cuts out? 100 miles? 1000?

If i'm half way to Mars is my momentum still only relative to the earth?

Don't think so...

[Contacttower]

I am really guessing now but perhaps they believed the relativity of intertia to the earth decreased gradually as you moved away fromt the earth?

Again I am inclined to agree with Wizofoz but when someone who is clearly an experinced helicopter pilot like Mike starts saying otherwise one starts to question one's existence.

[Wizofoz]

F/sgt,

I've flown a glider which had a stalling speed of 32kts in a constant banked turn at an IAS of 40kts in a 35kt wind. The IAS didn't budge. The centre of my circle (and the thermal I was in) just drifted downwind at 35kts, without myself or the glider being aware of it.

Similarly I've been in a holding pattern with 120kts of wind blowing. If the downwind turn existed I'd currently be dead.

[Contacttower]

Agreed. But what Mike seemed to talking about earlier was that in a flat turn (or at least quite abrupt) you do get a 'downwind turn' effect; John F was talking about doing 180s in Harriers and the associated loss of IAS when turning out of the wind.

I think there must be some effect going here, but neither of the two have yet explained why they are putting this down to the wind and not the fact that they are conducting very high drag manoevres which will always loose airspeed.

Of course if you have a 40 knot IAS in a 40 headwind and turn round you will not have an 80 knot groundspeed immediately, because the plane will loose airspeed in the turn, and take time to put it back on. (to use an example used earlier)

What would be nice would be someone who advocates the 'downwind turn' effect proving that airspeed is being lost due to the wind, not the other effects of the turn.

[Islander2]

What would be nice would be someone who advocates the 'downwind turn' effect proving that airspeed is being lost due to the wind, not the other effects of the turn.You'll be on a long wait!

An aeroplane in flight is completely oblivious to a steady wind ... it has not the slightest clue as to whether it's pointing upwind, downwind or sideways.

The pilot, however, isn't oblivious if he can see the ground, and therein lies the real danger of the low-level downwind turn!

[Contacttower]

I thought so.

On a slightly different point though...

Some posters seem to be equating a turn out of a steady wind as being the same as windshear, i.e. the oncoming wind speed is suddenly very different, leading to a loss of airspeed. Would anyone care to clear this one up?

[Islander2]

Well, we're back to Su-29s!

An aeroplane cannot "turn out of a steady wind", assuming 'wind' is being used in its conventional sense of airflow across the earth's surface. Aerodynamically, the airmass is the aeroplane's inertial frame of reference. What that airmass is doing with respect to another frame of reference (the earth) is completely irrelevant. It doesn't know, and doesn't care.

True it does have an "oncoming wind speed"; however in level flight that wind speed is provided by its thrust, and is nothing to do with the movement of the air over the ground. The aeroplane has momentum through the air, which is proportional to the product of its mass and air velocity. Accordingly, if it points its nose very rapidly in a different direction, it initially maintains its original flightpath through the air, resulting in a loss of airspeed (which, in extremis, can even end up negative). It also has momentum across the ground;this is almost certainly a different value but that's because momentum and energy are both inertial frame of reference dependent.

You could say the loss of airspeed is because "the oncoming wind speed is suddenly very different", but you would be referring to "wind" as the speed of the air along the aeroplane's longitudinal axis, nose to tail, not in the conventional sense of the speed of the air over the ground (and the loss of airspeed is the same whether you're turning upwind or downwind).

Windshear, however, relates to a very different inertial frame of reference. Here you would be talking about a sudden change of wind speed (or direction) with respect to the earth's surface.

An aeroplane yawed rapidly anticlockwise does see the same effect on its airspeed as the wind veering sharply due to windshear. But this is not an effect the aeroplane sees in a balanced turn.

Bluntly put, I know, but those that equate normal turning with windshear are simply wrong. An aeroplane in a normal, banked level turn loses airspeed for one reason only - increased drag, as correctly stated by the first two respondents to 4L3X's request for help!

[H Peacock]

"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."

Sorry to pick on you PTT, but I have to disagree. During my time in the world of aviation I have looked quite closely into this 'misunderstanding'. However, I can assure you that if you take a relatively small/light ac and try to fly a low IAS turn in a strong wind it is much harder than in calm coditions.

I managed to find a 130/150kt jet (no turbulence) and fly steep turns at 100knots in a Tincan. Control inputs/power changes needed all the time, but not when the exercise was repeated clear of the jet or at higher IAS.

This was all flown on the dials and not with reference to the gnd.

Furthermore, during many IRTs I have seen others battle with this as they attempt to fly level 180kt steepies!

The proof is in the pudding!!!
:)

[BEagle]

Cobblers!

In a word.

Difficulty in flying low IAS turns in strong winds is all to do with associated gusts and turbulence, nothing to do with the 'downwind' nonsense.

As for 180 KIAS steep turns, difficulty is all to do with gyroscopic instrument turning errors.

[Contacttower]

Ok, but what would Peacock say to Wizofoz's earlier experience that flying in an 120 knot wind in the hold does not effect the airspeed?

[Islander2]

Ok, but what would Peacock say to Wizofoz's earlier experience that flying in an 120 knot wind in the hold does not effect the airspeed?I imagine he'd say that Wizofoz was mistaken, and that the aeroplane undoubtedly experienced changes in airspeed. And Peacock would almost certainly be right ... but I imagine his explanation would be wrong!

A steady wind of 120kts at typical holding altitudes is as big a myth as the downwind turn! See BEagle's post (#145) - the real reason for the small changes in airspeed that Wizofoz failed to observe is the variability of the wind's velocity.

[MightyGem]

Don't just talk about it, get out there and do it. I've just been flying in a 35kt wind. A/c S and L, nicely trimmed for level flight indicating 80kts plus or minus 2 or 3, over the shoreline, wind off the sea, so no turbulence. Trimmed into a 15 deg turn, adjusted power to maintain height. A/c quite happily flew 360 deg turns with airspeed plus or minus 2 or 3 knots, which can be ignored for gusts. :ugh::ugh:

[Contacttower]

I have to say Islander's description of windshear does seem to sort of vindicate what Mike and John have been saying all along, doing low level circuits in a helicopter with abrupt turns seems likely to cause a sort of 'windshear' effect.

Although to me why Mike thinks groundspeed has anything to do with the plane's momentum in terms of the air is still unclear.

[Islander2]

I have to say Islander's description of windshear does seem to sort of vindicate what Mike and John have been saying all along, doing low level circuits in a helicopter with abrupt turns seems likely to cause a sort of 'windshear' effect.Absolutely, it does! But contrary to what Mike stated, in a steady wind that 'windshear' effect is identical whether turning upwind or downwind.

Although to me why Mike thinks groundspeed has anything to do with the plane's momentum in terms of the air is still unclear.'Cos he's a 'downwind turn' flat earther! :rolleyes:

I know I won't be able to persuade him otherwise, but just maybe I can give an illustration that will help others that are less entrenched.

For this, we need to engineer an absolutely steady wind. So we are going to embark on HMS Ark Royal and go boating. But bear with me, some aviating comes later!

Let's clear the ship's hangar of all those nasty whirly and V/STOLy things, and put a proper aeroplane in place. We're going to fly it in the hangar, so it's going to have to be small. A model aeroplane. And perlease, don't throw Reynolds Number at me, that's completely irrelevant for this discussion.

Okay, let's close all the doors to the hangar so it's air tight, and ask the ship's captain to give us 35 kts. To eliminate any confusion, btw, we've chosen a day and location where there's zero real wind ... that took some finding, I can assure you! Inside the hangar we have perfectly still air, yet that air is moving over the earth's surface at 35kts. We've just created the perfect, never seen in practice, absolutely steady 35 kt wind!

Now let's fly our model. As we fly upwind at 35KIAS towards the ships stern, we have a groundspeed (earth reference) of zero. Now let's cross our fingers and risk one of those terrible downwind turn thingies. On completion of the turn our model is now headed towards the ship's bow with a groundspeed (earth reference) of 70kts.

By 'eck, says Mike, look at all that extra momentum (or he may say look at all that extra kinetic energy). That needs an energy input from somewhere, so extra power is required, and until it takes effect airspeed will decrease.

Flying the model, meanwhile, we leave the power unchanged and, aside from a tiny decrease in airspeed in each of the turns, which is promptly restored upon levelling the wings, the airspeed as measured by our super-accurate instrumentation remains absolutely constant regardless of the direction of travel.

Which leaves Mike somewhat bemused!!:confused:

[Systemessage]

The airborne world would be a very bizarre place if it worked the way MO preaches:

Imagine BFM – rather than come in out of sun, you’d probably come in based upon the wind direction! That first, critical, 180 would be interesting as (all else (but the wind!) being equal) you’d get your nose on first - particularly if it was a real stonker of a wind ;-)!

I can hardly believe that there is an argument going on about this – but it’s dead amusing to watch.

And as for ‘The proof is in the pudding’ (H_P)

“Control inputs/power changes needed all the time”

That doesn’t say much!

If you can’t understand the logic, then the last hope is experimental evidence – objective assessment is what is needed, not subjective (“Control inputs/power changes needed”). Go up in an aircraft with suitably sensitive recording equipment and see what really happens.

Going ‘slow’ (for your particular aeroplane) might make the handling a little trickier in turns, but that’s not because of the steady wind!

[H Peacock]

Cobblers!

In a word.

As for 180 KIAS steep turns, difficulty is all to do with gyroscopic instrument turning errors.

Ahh, BEagle; what on earth have you flown that has 'gyroscopic instrument turning errors' in a constant steep turn? Don't know about you but I use just an AI (you can call that an artificial horizon). Are you telling me that the possibility of a slow erection towards a false vertical datum is causing it!!! :)

[Islander2]

Are you telling me that the possibility of a slow erection towards a false vertical datum is causing it!!! :)

Errr ... combined with pendulosity errors, actually yes! Very well documented, and although insignificant at the rates of turn normally associated with instrument flying, problematic for steep turns.

[Arty Fufkin]

I think Beagle may have been refering to Gimble Error. Noticeable in the Tincan and other aircraft with simple Gyro-compasses.

Or did I dream it?

[H Peacock]

I think Beagle may have been refering to Gimble Error. Noticeable in the Tincan and other aircraft with simple Gyro-compasses.

Or did I dream it?

Correct, so if you hold a steady bank angle in a continuous turn you don't ever need to look at the compass/DI/E2 etc! I am not referring to any effects when rolling in or out of the turn.

[Islander2]

Correct, so if you hold a steady bank angle in a continuous turn you don't ever need to look at the compass/DI/E2 etc! I am not referring to any effects when rolling in or out of the turn.And how do you hold a steady bank angle? Or, come to that, how do you hold a steady pitch attitude?

The point about conventional gyroscopic AI turning errors is that the bank and pitch indications on the AI vary throughout the 360deg turn during a constant rate turn. They are not "rolling in or out" errors!

So in a high-rate, high-bank-angle turn, flying to maintain constant pitch attitude and constant bank angle on the AI means the aeroplane is continually varying in pitch and bank throughout the 360 degrees (with maximum error every 180deg). Sorry, but it is those turning errors, combined with wind velocity fluctuations, that cause your IRT candidates problems with airspeed variation, AND CERTAINLY ISN'T ANYTHING TO DO WITH ALLEGED DIFFERENCES BETWEEN TURNING UPWIND AND DOWNWIND!!

[Contacttower]

Going back to the Ark Royal example, if the ship could decelerate instantly to a standstill, the plane wouldn't loose any airspeed would it?

So why then when you experience real windshear the plane does loose airspeed?

[Arty Fufkin]

Islander 2, H Peacock,

Just to expand on my last, as i said, the effect that Beagle was alluding to is Gimble Error. This manifests itself as a cyclic speeding up or slowing down of the rotation of the compass card in an aircraft flying a steep turn. It is caused by the difference in the aircraft's anlge of bank to the orientation of the Gyro platform and if memory serves me, the stock A2 / IRE explanation involves a dinner plate, a whiteboard maker and a swivel chair. You are quite correct to say that it is only really a snag when attempting to roll out on a specifc heading and has no effect on the other parameters of Altitude, Bank Angle and Speed.
Islander 2's comment that pitch and bank indications also cycle during the turn is way off the mark. The closest I've seen to that is the Stby AI on the hawk which suffers from a pitch / roll couple, but thats due to its instalation. With a normal AI, pitch and bank indications are acurate throughout the turn.

Cheers.

[Islander2]

Going back to the Ark Royal example, if the ship could decelerate instantly to a standstill, the plane wouldn't loose any airspeed would it?Try walking along a train carriage, against it's direction of travel (i.e. 'upwind'), at say 5mph ('airspeed'). Now ask the train driver to slam its brakes on. What will your trajectory be? Still think the plane won't lose any airspeed?

Islander 2's comment that pitch and bank indications also cycle during the turn is way off the mark.I suppose the standard instrument texts (try Pallett: Aircraft Instruments) devote several pages to the turning errors in gyroscopic attitude indicators for the hell of it, then! :ugh:

[Contacttower]

Your completely right Islander, I don't know why I thought otherwise.

Thanks for answering all my questions, from the perspective of quite a young aviator, you've made a lot more sense than most on this thread.

[Genghis Couldn't]

"Try walking along a train carriage, against it's direction of travel (i.e. 'upwind'), at say 5mph ('airspeed'). Now ask the train driver to slam its brakes on. What will your trajectory be? Still think the plane won't lose any airspeed?"

You don't have 'airspeed' - your feet are on the floor!?! If you suddenly jump when the brakes are slammed on - what happens then ;-)

The hangar on a ship analogy breaks down when you do the 'sudden stop' thing because the air is 'trapped' (taken along) inside it - you are 'tying' the reference frames together (the air and the container) and this makes the particular analogy invalid.

Can someone please take up an instrumented Tincan in a bit of wind and fly some slow speed turns - please.....???!!

[Contacttower]

And there was me thinking I'd understood things:confused:

As far as the ship idea is concerned this is how I understood it:

When the ship decelerates the air inside it decelerates, therefore the plane looses airspeed, just like it would in windshear (which after all in a odd sort of way is the aircraft's frame of reference decelerating, just like the air in the ship)

I don't really see how tying frames of reference together comes into it.

[H Peacock]

I suppose the standard instrument texts (try Pallett: Aircraft Instruments) devote several pages to the turning errors in gyroscopic attitude indicators for the hell of it, then!

Islander 2. Please tell me more about this interesting phenomenon. How does the vertical gyro know the ac is turning? I stand to be corrected but have never heard of this. As I mentioned before, apart from possibly trying to erect itself to a false datum (at a fantasticaly low rate) an AI does continue to show the pilot the actual attitude!:) :)

[Islander2]

You don't have 'airspeed' - your feet are on the floor!?!Oh? So your sensitive pitot tube based ASI doesn't measure your speed through the carriage? [Of course you do, and it does! Something needs to propel you to give you 'airspeed', and your feet are no different to the engine's thrust in the aeroplane!]

If you suddenly jump when the brakes are slammed on - what happens then ;-)If you think something materially different occurs to the trajectory compared with when you had your feet on the ground, go and try it then you tell me! [When you get out of hospital, you will be able to tell me that you experienced a substantial reduction in airspeed in both cases, so much so that in each case you flew backwards and clobbered your head on the seats!]

The hangar on a ship analogy breaks down when you do the 'sudden stop' thing because the air is 'trapped' (taken along) inside it - you are 'tying' the reference frames together (the air and the container) and this makes the particular analogy invalid.Err, you really don't get this, do you? The two possible inertial reference frames are the ship (i.e the hangar air) and the earth. What has the air versus the container got to do with the price of bread? Oh, and btw, there is no tying together of reference frames in my illustration. [Yes, it is trapped and, as the ship stops, the wind speed (i.e. the speed of the trapped air relative to the earth's surface) thus becomes zero - just the same situation as the wind dying due to windshear! The Ark Royal illustration is exactly analogous to flying in steady wind, and perfectly valid]

Whilst Mike O's grasp of fundamental physics is not what one would expect from someone with his academic engineering attainment, he did wisely and correctly predict that this thread will run, and run, and run, and run ........!!!

I've given my twopence worth, I'll just be repeating myself and getting ratty with people from here onwards. So I'll bail out now. You all take care with those downwind turns! :=

Islander 2. Please tell me more about this interesting phenomenon.
Lazy bu**er, I've given you one reference (and there are plenty of others) - look it up!

[Wizofoz]

A steady wind of 120kts at typical holding altitudes is as big a myth as the downwind turn!

Geezz Islander, and there I was thinking we were on the same side!!

"Typical" holding altitude, possibly- But FL 290 over Bindook during a two hour delay due fog and traffick into Sydney in winter? The core of the jet was at 370 and was 190kts. If you've ever flown in a jetstream (which you wouldn't in an islander!) you'll know that at a constant altitude it is one of the smoothest, most constant winds you will ever encounter.

I'm sure the vaguries of the internet makes things sound a little blunt-but I don't appreciate having it implied that I'm telling porkies!!

[BEagle]

See http://www.allstar.fiu.edu/aero/GSI.htm for a simple explanation of turning errors.

Basically, 'centrifugal forces' cause the vertical gyro to precess towards the inside of the turn; the precession increases as the bank steepens.

[50+Ray]

Thank you all for an hour of amusement while wauting for the dust storm to die down. I have advised the KISS principle many times in the 23 years since I got my A2. Beagle is right, but the Lift Pixie theory gets most marks for humour.

[H Peacock]

Thanks for the link BEagle.

Principal Attitude Indicator Errors

TURN ERROR

During a normal coordinated turn, centrifugal force causes the gyro to precess toward the inside of the turn. This precession increases as the bank steepens; therefore, it is greatest during the actual turn. The error disappears as the aircraft rolls out at the end of a 180 degrees turn at a normal rollout rate.

Therefore, when performing a steep turn, the pilot may use the attitude indicator for rolling in and out of the turn, but should use other instruments (VSI and altimeter) during the turn for specific pitch information.

ACCELERATION ERROR

As the aircraft accelerates (e.g., during takeoff), there is another type of gyro precession which causes the horizon bar to move down, indicating a slight pitch up attitude. Therefore, takeoffs in low visibility require the use of other instruments such as the altimeter to confirm that a positive rate of climb is established immediately after takeoff.

DECELERATION ERROR

Deceleration causes the horizon bar to move up, indicating a false pitch down attitude.

However, the errors above are all to do with the gyro erection mechanism, not the actual spinning rotor. Remember that an AI has vertical gyro axis, and therefore does not not know it is turning. However, the AI needs to know where the true vertical is, and usually uses gravity/pendulum. A 'good quality' AI will ignore any perceived changes to this vertical if beyond a certain angle of bank (7deg-ish) called pitch erection cut-out. If it doesn't ignore it then this false datum will cause the gyro to precess, but only very slowly. Futhermore, if the gyro spin axis has moved to a false daum this error will not 'disappear' (as quoted above) as you roll out - it will either need fast erecting or leaving a while to return to the true vertical

I do have a big humble pie ready to eat, but it will have to wait for another day!!!:)

[Genghis Couldn't]

Islander,

No need to get ratty my dear chap, it's a discussion not an argument (isn't it?!) :-)

(1) Of course I agree that you have speed through the air, but as your feet are on the ground I would call it groundspeed (or 'footspeed' in this case).

(2) Ok, there are two of you walking abeam each other when the train decelereates, one carries on 'feet on floor' walking and the other jumps having been walking at exactly the same pace as the other. Are you telling me that they will remain abeam each other? The bloke with his feet on the floor will experience a force (through his feet) and the bloke (who we'll imagine stays airborne for a bit) who jumps feels no such force and will, relative to the train (reference frame) move. And you're telling me 'I' don't get it! ;-)

(3) I think we might be having a screaming agreement on the ship/hangar thing! I agree with your steady state (35Kt) argument, what I was saying was (bady expressed I admit) that stopping the ship and consequently the air inside isn't representative of environmental reality (because you are 'tying' the air to the ship) but it is physically correct - it's the reality of the analogy that I don't agree with. The model will suddenly change airspeed, as you say. Stopping the ship and the air is more akin to windshear (from the viewpoint of the model). The air versus the container tends to increase the price of bread by the way (historically).

My head hurts now and I think I will join the 'lift pixies' brigade.

[Contacttower]

What does whether you have your feet on the floor matter?

If two people are moving forward against the direction of travel (one on the ground and one in the air) inside the train and the train suddenly stops dead (instant deceleration) both will move backwards at the same rate. They both had the speed of the train, for example 40 knots (regardless of their airspeed) and when the train stops they will for a moment retain that 40 knots and hence fly backwards towards the front of the train. The reason why they might not end up abeam each other is because the on the floor (having fallen over) will stop faster.

[Genghis Couldn't]

Yes, given the conditions you quote, they will. I wasn't talking about an instant deceleration.

If you have a deceleration that is 'train typical' then the chap with his feet on the floor will feel the deceleration through his feet (and decelerate with the train), the chap airborne will not feel the decel (force) through his feet.

If you have two baloons, one is blu taked to the floor, the other is floating above it - the train decelerates, they do not remain abeam each other.

[Contacttower]

Fair enough, but the example of Ark Royal stopping instantly was being given to explain windshear which is often an almost instant deceleration of the wind, i.e. the air which the plane is flying through stops very quickly.

[Genghis Couldn't]

Yes :-)

But my head really hurts now and I am sticking to the pixies.

[Contacttower]

To quote Jeremey Pratt (author of The Private Pilot's Licence Course PPL3 Naviagtion & Meteorology):

The real danger of a strong windshear is the dramatic and rapid effect it can have on a plane's airspeed. Imagine the wind velocity changing so that an aircraft transitions from experiencing a 20 knot headwind component to a 10kts tailwind component in a 100 foot height band. As it descends with an airspeed of 90kts, experiencing a 20kts headwind, its groundspeed is 70kts. As the aircraft passes throught the windshear zone, the headwind of 20kts becomes a tailwind of 10kts. Due to intertia, the aircraft retains its original groundspeed for a short time, so the airspeed drops suddenly to 60kts.

Bearing in mind what has been said on this thread the above doesn't sound completely correct. Surely the momentum of the plane has nothing to do with the groundspeed. (that last sentence has been repeated over and over again by almost everyone)

Pratt also claims that larger planes are more vunerable to windshear than small ones, because they have more momentum. Why on earth is that true?

[Wizofoz]

No, in this case he is quite correct.

As we have been saying, the aircraft flies relative to the air around it. If the air suddenly changes velocity relative to the aircraft (note the ground doesn't come into it!!) you have winshear.

The aircraft does have momentum which is mass times velocity. Velocity is relative to the observer. The velocity that counts in this instance is that relative to the airflow. A larger aircraft has more mass, therefore more momentum.

Ground speed is a useful reference in this case, as the magnitude of the change in the airmasses velocity relative to the aircraft will be the same as it's change realative to the ground.

You could, however, reference the aircrafts speed relative to the moon, sun,or alpha centaurus with the same result, it's just that the ground is nearer and therefore more convinient!!

[Contacttower]

What I was taking issue with was that he seemed to be saying that the aircraft has a certain amount of inertia (taken to mean momentum) because of its ground speed and that the momentum of the aircraft related to its groundspeed. (Which sounded a bit like what Mike Ox was saying right at the start.)

[Wizofoz]

As I said, Momentum is proportional to velocity which is relative to the observer. Yes, an aircraft with more ground-speed has more momentum, because to have more ground-speed it must also have more airspeed. The figures work whatever the frame of reference. If you were in an aircraft going the other way, or in a balloon travelling at the speed of the local wind, the speed changes you would observe in the aircraft encountering windshear would be the same relative to you.Groundspeed is just a convienient reference, no truer than any other.

If you look at an earlier post of mine, you'll see that I tried to explain that the confusion here lies with the difference between speed and velocity. The downwind turn brigade are of the opinion that an aircraft turning at a constant gound speed has a constant velocity. It doesn't. Velocity is a vector meaning it has magnitude and direction. Change direction and you have changed velocity, which is defined as acceleration.

They think that because an aircraft going from up-wind to down-wind changes ground-speed it has to accelerate more in the down-wind case. It doesn't. In both case the acceleration is the same, cause by imparting the same force and with the same result.

Think of it this way- going from 100 kts west to 100 kts east is an acceleration of 200 kts towards the east. Going from 50 kts west (with a 50 kt wind) to 150kts east is ALSO a 200 kt acceleration to the east, so it actually does work reference the ground (or any other frame of reference.)

As a pilot, however, the most convienient frame of reference is the air around you, and that is the only one that the aircraft feels.

[Contacttower]

So big planes are more vunerable to windshear because when they loose airspeed they don't regain it as quickly because of their momentum (mass x airspeed)?

[Wizofoz]

Yes, exactley (Although it's mass x velocity- and that velocity can be mesured relative to any frame of reference)

[Contacttower]

Thanks for clearing that one up :ok:

Before I learnt to fly I was always under the misconception (probably like a lot of people) that heavy jets were somehow less affected by the wind and associated atmospheric phenomena than small ones.

[Talon Driver]

See like dude when you turn like some of the little lift guys fall off the wing and get killed like so you gotta pull back on the stick (a yoke is what transport guys have, pilot guys have a stick) to kinda glue the little buggers on to the wing so you don't leave little dead lift guys all over the place. When you pull back on the stick these lift guys get much bigger (which keeps them on the wing) but they decide to have a party and invite all their drag mates to come along 'cos they think they're so tough. This is a real bummer 'cos the drag guys mess the whole thing up by burning energy which slows ya down.

See it's that simple!

or I guess you could talk Bernoulli, Coefficient of Lift Vs Coefficient of Drag etc etc but I like the first answer better.:\

[Smudger552]

Holy moly...........:zzz:

[Wizofoz]

You are in for a treat as I will contribute later, having clarified my thought processes in happy hour,

In your own time Mike...

[advocatusDIABOLI]

So, if stones sink in a pond but gravy doesn't........ but wood floats, and 'She Floats', She's made out of wood 'Right?'........ So She's a Witch!!!! ...... Burn Her! ... etc etc etc

Advo

PS: This is the funniest thread I've read in ages. Thanks.

[look you]

Wouldn't it simply be easier to make all of your turns while descending, so that you don't have this problem?

[scottyhs]

No shemy, bite the bullet next time mate before you say something

[Jaguar Pilot]

Kinnell 4LSX.

If you don't know the answer to this, stick to driving cars.

[Wizofoz]

Meanwhile we wait with baited breath for Mike O to re-write the laws of physics for us...