The Downwind turn

[The Ant]

To all you instructors out there, does an aircraft lose airspeed when it turns downwind? Aerodynamically it shouldn't, or should it? There are many recorded incidences of aircraft crashing under just such circumstances. Does anybody have an answer?

[Cornish Jack]

I see that you have moved "houses" ! I posted a reply to your previous in Flight Testing but was nadgered (I think) by AOL (see posting Computer/Internet issues).
Sooo... I say again - This issue has been around a LONG time. It was heavily written about in the early sixties in "Air Clues". Lots of highly articulate hypotheses with more lift,drag and thrust arrows than one could shake the proverbial stick at, plus wind vectors ad nauseam. If memory serves me correctly, it was finally nailed by a chap from Boscombe Down who added the magic ingredient INERTIA ! His theory was that inertia relates to the earth mass not to the 'packet of air' in which you are flying. To maintain this inertia, when turning downwind in strong winds, requires a strong nose-down pitch input and this might be somewhat unnatural to a light aircraft pilot who would be likely to be flying to visual, rather than instrument, cues. The original articles would probably have long since disappeared but you never know. :)

[212man]

Likewise, I see you have moved.

It's a fact, though it goes aginst most basic flying training guidance.

Essentially, as has been pointed out, the a/c momentum keeps it at the same relative groundspeed, so that a turn downwind results in a loss of airspeed. Conversely, if you stonk along with a strong tailwind and turn into wind, the airspeed will increase. Try it next time you have a chance, it is very clearly seen in any more than about 25 kts of wind with a light a/c (or any helicopter).

[WileyP]

The inertia issue is quite interesting as well as the "flowing river" discussions I've heard. You know, the one about a fast powerboat doing a 360 turns on a fast flowing river.

Anyway, if an airplane at a true airspeed of 100 knots is flying with a tailwind of 20 knots, it has a groundspeed (inertia) of 120 knots. After a tight 180 degree turn, it will be traveling in the opposite direction at a groundspeed of 80. The total change in groundspeed (inertia) from 120 knots in one direction to 80 knots in the other is 200 knots, right?

It would be the same in still air: 100 knots groundspeed in one direction to 100 knots groundspeed in the other equals 200 knots change.

It would be the same if the airplane started with the 30 knot headwind: 70 knots groundspeed in one direction to 130 knots groundspeed in the other equals 200 knots change.

Now, can someone please explain to me again about this inertia difference turning downwind? http://www.pprune.org/ubb/NonCGI/confused.gif

...and above all,

FLY SAFE!

WileyP

[Checkboard]

Provided the wind is steady, there is absolutley NO loss of airspeed turning downwind and no gain in airspeed turning upwind. (Ignoring the increased drag experienced in all turns, that is.)

There is, however, a strong set of visual illusions (apparent slip/skid) that confuses many.

Inertia is not tied to the mass of the Earth.

[This message has been edited by Checkboard (edited 26 January 2000).]

[212man]

The product of mass and g/s is momentum, not inertia.

If you look at the kinetic energy side of things you will come up with different figures, and after all that is what all the work being done by the engines is to achieve, as well as overcome drag.

The bottom line is that it is a fact. After fatal accident (of a company a/c) in 1992 resulting from a downwind turn at night in 55kts of wind, this very topic was discussed at length. I spoke to very experienced ex ETPS tutor and he confirmed it to be the case. I've witnessed it too.

[OneMileHigh]

I,ve heard quite a lot about this inertia/momentum idea, but for many including myself it cuts no ice. If such were the case we would be seeing these affects at all altitudes. The best way to see the 'downwind turn' effect is in a glider at 5-600 ft. With no noise upfront(or behind for the eze's) and with a visual effect telling you you're way too fast, the natural inkling is to pull back/roll out, hence less speed. Scary but I think true

[foxmoth]

I agree with OMH, If it was correct you would fall out of the sky at 5,000 feet turning downwind in a 60kt wind, and you don't.

[Capt Homesick]

A friend of mine was killed in an Islander a few years back, on base with a very strong crosswind (ie, tailwind for him).
All the ingredients were there- long duty day, late back after an ambulance trip, sub-Arctic weather, and- crucially- high ground under his base leg. He was caught in the illusion (high groundspeed, ground close therefore even higher apparent groundspeed, therefore TAS MUST be high, mustn't it......?).
There is no loss of airspeed in the turn to downwind, other than normal induced drag losses- as Checkboard said. I've tried it in a C152 in a 30 knot wind, and I've tried it in a 146 in a 130 knot wind. Same effect both times- no irspeed change, other than that to be expected in calm air as a consequence of the turn.

[Flangemeister]

Go for it boyz'n'girlz, some good (and fuddled) stuff - although anyone loosing sleep over this lot needs to get out more. For what its worth, here's my (scrambled) ideas...

Believe it or not, the lil' teaser you've got here is (by physical def'n) similar to the issue of windshear on finals which is itself an 'inertia' prob. Bear with me: in a big jet I increment my Vat due a loss in ias I expect due to changing hwc as I descend on the profile close to terra firma. Say on app at 150kias I flop thru an inversion and I go instantly from 50kt hwc to zip. My lumbering tin tube full of in-flight meals and fatties has the inertia (mass) thing so for a few moments has g/s of 100kias with zip hwc. This manifests itself as a redn of ias for a very short period only because the ac is now descending due ma nature and subject to the thrust from the motors (providing a force due system no longer being in equilibrium). All these accelr'ns work to put the ac back to a steady state of equilibrium at the original ias wot was (auto)trimmed prior to the disturbance. Without any messing from me (coz i'm asleep mostly) the ac has a 'dip' in the desc prof. In a little plane the physics are the same but the effects experienced differently due to difference in mass.

Where woz i? O yeah, downwind. Say my wee cessna were able to 'instantly' change direction 90 degs to the downwind bit. For the same reasons inertia/mass prevails and it has a momentary redn in ias before equilibrium is again the status quo.

The rub??? THIS IS ALL TOSH AND RED HERRING. The flight dynamics of ac have far greater effect on your ias than all this turning around in rivers stuff. Fact is that incr in LDD much outweighs circling boat in the decreasing ias stakes. Wot about that moth ridden pitot being blanketed by the door I forgot to shut and granny is dangling out in the flow? Wot about the fact that my clapped out cessna hasnt had the instruments calibrated/leak checked since Wonky Dan the Instrument Man threw them in under a budget? Ever seen an asi needle dancing away in turbulence (vertical gusts) - how about that - downwind is well within the mechanical turb layer?

In short, an amusing topic of conversation in the tea bar but we're in danger of loosing the wood for the trees. Me big dumb-bo so apply basic/sound flying techniques taught me by much wiser man so that don't fall out of sky when turning downwind.

Enjoy!

[212man]

Flange, I agree.

I was going to bring up the windshear analogy myself. Another comparison is a rotating blade, or model a/c on a string, in a wind; constant g/s but changing a/s.

As you say, there are so many other variables that it is pointless getting fixated by it. I have lost a very close friend in a Bulldog, and a colleague in a Super Puma, through accidents through turning downwind at low level, but I would not argue that they were caused by the effect we are discussing. ie They were as a result of the visual illusion that every one refers too.

Anyway, best to use caution what ever the dynamics of the situation.

[Capt Homesick]

Falngemeister, am I missing something? I got so lost in your descriptive colour, I could not figure out whether you think the "downwind turn= loss of airspeed" phenomenom exists or not!

The comparison with windshear is a spurious one- windshear, by definition, is a transient event- a turn in a particular body of air is not. And while a rotating mower blade is not operating at a constant airspeed, it is a steady state problem- we simply treat a rotational speed in the same way as we would treat straight-line speed. I daresay that it is a good analogy for helos, but I know very little about them.

[OneMileHigh]

Hey Flange

For someone who doesn't lose sleep and gets out a lot you sure put a lot of time and effort into your ideas :-)

[Flangemeister]

OnemileH - nice one! I'm just new to all dis whizzo stuff so i think bit over-enthusiastic. Only trying to help tho.

CaptH - sorry if me explainin is poop. Read it over again meself & u rite - it ain't hot.
U rite 2 bout the transient nature of w'shear but don't think it 2 spurious once u bend yer nog around it.

Also u point about turnin being steady state be true in physical world only if no wind. Remem - 'slip' happen to turning plane just like turning prop
due 'mass' (thanks 212 - u gave me that one)
which is a change in momentum; and since when was wind in real world in turb layer steady state?

Yike. Blah caption come on. Thing is, the decr ias theory could be proven to hold soup on paper with rithmatic. In real world tho, many other phenomena are at play which dominate due to the aerodynamics of ac in turn and so mask this.

Experiment for u. 1. Take little cessna & park into 100kt wind. 2. Read asi. 3. Turn little cessna 90deg to wind. 4. Read asi. 5. Compare readings. (6. Retrieve ac from fence). Can do experiment at taxy if want.

Enjoy.

[212man]

Final point.

We all know that drag increases/lift componant dcreases in a turn and requires more power to maintain IAS. However, do we know if some of that power is being used to counter the effect we are referring to? I think not. It's question of conservation of momentum and energy. If you fly into a cliff with 100 kias and 30 kts headwind, you will cause less damage than if you had 30 kts tailwind (the old v srqd bit). If you turn abruptly to avoid the cliff that conservation of energy/momentum will show in the ias.

[Oktas8]

Is it too late to join in? I hope not...

When I was in high school I used to play with windshear effects by launching paper aeroplanes down the length of the bus just as it was about to accelerate or brake. The local mass of air changed speed with the bus - watch those paper planes fall out of the sky!

As to those getting confused by KE and momentum - no. It sounds plausible, but an aircraft does not "know" what its GS or Trk is. It only "knows" airspeed & heading. These change 100% aerodynamically - ie relative to the local airmass. Why should the aircraft attempt to conserve GS when turning downwind, when the equations governing flight make no reference to what might be happening 1000 feet away from the wings?

In general, momentum and kinetic energy theory does not concern itself with velocity relative to any "absolute" - ie groundspeed. The mathematics refer only to changes in velocity relative to the medium in which the object travels: the atmosphere in our case. For this reason, groundspeed is a total irrelevance to discussions of aircraft turning / banking / climbing / descending performance.

On a more empirical note, do orbits in a cloud, with a safety pilot flying & you just gazing intently at the ASI. Will it fluctuate? Only one way to find out... :)

O8

[Diesel8]

IMHO.

I cannot believe this tale is so persistent as to once again prop up. Airplanes do not lose airspeed turning downwind, only ground speed will increase. We are flying in an airmass that is moving, however, airspeed is in relation to the speed of the ac through the air ( read: in steady moving air, regardless of speed of the air, to the ac and airspeed indicator the air is standing still) It might be possible to get airspeed changes turning downwind, but ONLY if the ac could instantaneous make a 180 degree turn, which obviously is not possible. This could be alikened to a rapid change in windspeed, ie a gust of wind, which would change airspeed.

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It's been real, it's been fun, but it has not been real fun


[This message has been edited by Diesel8 (edited 03 February 2000).]

[Pub 45]

Right said Diesel8

Flangemeister's 'experiment' is quite pointless to the question at hand as it would only be a study in windshear. An aircraft, while flying, is part of the air mass. The only time the 'wind' will affect its aerodynamics is if it changes abruptly - and even then, as has been pointed out, only transiently.
The consequences of a windshear would depend on a few factors including its strength and the aircraft's proximity to the ground. Turning at 800-1000 ft AGL on a typical downwind, I think would require quite a monstrous windshear to cause a crash.
Those which have occured must surely have been due the other reasons quoted in this thread.

On the other hand I may just be misinterpreting F'Meister. Can't quite figure out which arguement he's for or against.



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'Watch thine airspeed
Lest the ground arise and smite thee!'

[Capt Homesick]

Diesel8, I'm worried, I'm agreeing with you again.... http://www.pprune.org/ubb/NonCGI/eek.gif
Seriously though, I didn't bother replying to Flangemeister this time, because I figured that if he couldn't be bothered to make his replies intelligible, it was probably a windup.

[StrateandLevel]

Surely the inertia effect is on the pilots balance organs located just behind the ears. They are only designed to operate at about 5 mph. A down wind turn in a strong wind produces a sensory and an optical illusion commonly know as disorientation.

[Checkboard]

Not only does the aircraft fly relative to the airmass, but so do your ears! :) Even this is a "bad" way of expressing what is going on, but will suffice.

Turning in calm air will have exactly the same effect on your balance organs as turning in a steady 60 (or 100, or 200..) knot wind.

[Pub 45]

I agree with you checkboard.

However, I think the optical illusion (that stateandlevel mentions) will still occur due to a change in velocity relative to the ground. This could cause disorientation, because the sensory organs would be indicating something else.

[This message has been edited by Pub 45 (edited 08 February 2000).]

[StrateandLevel]

Because your balance organs act like a pendulum they will detect both the angular acceleration in the turn and the linear acceleration as you turn downwind. In still air they will only detect the angular acceleration.

[Checkboard]

Sorry - still not correct, the whole point is that the acceleration experienced turning from up wind to downwind is exactly the same as the acceleration experienced in still air. For the aircraft, for your body and for your ears.

[StrateandLevel]

If thats the case how does an IN platform measure the change in groundspeed?

[rolling circle]

The original question was - "To all you instructors out there, does an aircraft lose airspeed when it turns downwind?". The answer, as any real instructor knows, is NO! Anyone who thinks different should e-mail me 'cos I've got this great financial opportunity that you will all want to subscribe to!!

Isn't it comforting that there is a never ending supply of suckers??

[Checkboard]

The INS or IRS meaures the change in groundspeed the same way for both situations.

200 knots North, 180° turn, ending up at 200 knots South in calm air, 200 knots TAS the whole time. Change in velocity: 400 knots South.

In a 20 knot Northerly: 180 knots G/S North, 180° turn, ending up at 220 knots G/S South, but still 200 knots TAS the whole way around. Change in velocity: 400 knots South.

The ground speed matters for navigation, but in terms of how it effects the airspeed of the aircraft, or the forces required to move it about, it is about as important as the speed of a car along a freeway below it, or the speed of an aircraft a mile away.

Same forces involved, same accelerations measured by the INS.

[This message has been edited by Checkboard (edited 10 February 2000).]

[StrateandLevel]

Checkboard, you clearly don't understand how an accelerometer works. The IN measures ground speed by integrating the output of three such devices. They consist of a ball and spring. The ball displaces due to the acceleration in exactly the same way as the fluid moves in your balance organs.

As you turn downwind, the fluid moves aft and the pilot percieves a pitch up. The increased groundspeed should induce the optical illusion of pitching down. Does the pilot unwittingly react by continuing the pitch up he is already sensing thereby reducing the airspeed? We know the True airspeed stays constant.

If as you claim everything in the aircraft maintains a constant velocity IN would never work would it ?

[This message has been edited by StrateandLevel (edited 10 February 2000).]

[Checkboard]

I never claimed the aircraft or anything else maintains a constant velocity, only a constant speed.

The "ball & spring" thing is cute.

[BEagle]

This 'Downwind Turn' business is demo'd during 'Low Level Intro' to Bulldog students. It's just illusion leading to insidious pitching attitudes resulting from the perception that the cows should still be going past the window at the same rate!! Other dangers are also highlighted - but there is no aerodynamic reason for loss of speed turning into or out of wind. How does the aircraft know what the wind is doing?? It's a bit like thinking that a Thermos flask somehow knows that it's supposed to keep hot things hot and cold things cold - but not the other way round!!

[StrateandLevel]

I always thought velocity and speed were synonimous.

Of course if something actually deflects then both its ground speed and airspeed change.

Glad you are keeping your eyes on the old cows outside your window Biggles. Saw some nice ones down the pub tonight!!

[4dogs]

strate,

i guess your physics match your spelling...
http://www.pprune.org/ubb/NonCGI/Forum39/HTML/000017.html


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Stay Alive,

[email protected]

[rolling circle]

Ah - that explains it. To believe that the downwind turn affects airspeed, you first have to believe that speed and velocity are synonymous.

Inbound in the hold yesterday at 100 knots IAS in 55 knots of headwind. Turned outbound with no variation of airspeed or pitch attitude. Perhaps the advocates of disaster in the downwind turn would like to explain that away.......

[2R]

Question was answered correctly by rolling circle.And someone else mentioned the illusion thing ,where the ground speed increase's and an inexperienced pilot may slow the airplane down and enter a stall in the next turn as the wing is loaded in the turn,made even worse if the turn is skidding.
Attitude And Awareness will prevent most problems.
If the question was "At what angle of bank does the airspeed change ?' the answer would be different.
Aristotle said "Finding the question is harder than the answer".