Downwind Turns

[THAI TUN]

Aaaargh! Not that old chestnut!

:uhoh:

I'm interested if anybody thinks there are real dangers in turning downwind at low level in windy conditions. I'm familiar with the theory, an aeroplane flies in a parcel of air and it doesn't matter if that parcel of air happens to be moving over the ground, but I still think there are very real dangers.

I think probably the biggest factors are the pilots *perception* of increasing airspeed speed due to the increasing groundspeed leading to the temptation to pitch up or reduce power, and the *perception* of the aircraft slipping as it turns across the wind leading to the temptation to unbalance the aircraft with innapropriate rudder input.

If it is a climbing turn to downwind there will be a tendency to lose airspeed because of the increasing tailwind component due to wind gradient.

(I also have a pet theory developed after spending many airline standby hours standing in a field flying a radio controlled model glider, that a gust is sharper than a lull, therefore:
When flying into wind the relatively sharp gust results in a momentarily increased airspeed, but the gentler lull results in a smaller loss of airspeed.
When flying downwind the relatively gentle lull causes a small gain in airspeed, but the sharper gust causes a bigger loss of airspeed.
It certainly felt like this was the case with the model - and no I wasn't being fooled by the winds effect on the models groundspeed, I was thermal soaring and tended to trim the model for just over min sink speed and then let it look after itself in pitch as much as possible. I freely admit this could be complete bollox, I haven't been able to find any information to support my little theory, and even if it is true it may not be particularly relevant to full size aircraft. Any thoughts?)

I'm especially interested to hear the views of John Farley, or anyone else who has spent a lot of time at high alpha, near the ground, on windy days, and maybe glider pilots are more likely to have noticed these effects? Are there any accident reports or studies that have looked into the effects of turning downwind at low level in strong winds?

THAI TUN

[John Farley]

THAI TUN

Hi.

I have exactly the same view as you with regard to r/c models. I mainly fly mine over the sea and have learned the hard way that when it is gusty (when is it not so far as a model is concerned?) then you really need to watch it with such turns. I agree the balloon when turning into a gusty wind and the sink when turning downwind and a gust hits you up the backside.

At full scale speeds there are plenty of very experienced guys who rubbish the whole idea and back up their views with the standard theories that you have touched on. Rufus Heal wrote a very convincing article in Flyer a while back explaining that when full sized aircraft crashed turning downwind in the circuit it was for other reasons.

I have a very trusted mate whose maths are of the standard to question the detail of Einstein’s theories (I am not jesting and nor are the Unis he works with) and who is also the only bloke to get a Harrier (not Kestrel or P1127) back onto an airfield following a donk failure. He says there is no such thing as the downwind turn problem in a steady wind. But he says gusts are quite a different matter. And as we have said, when is there a wind that has no gusts.

It is never a problem with a jet borne Harrier because the lift forces are being generated by the exhaust gas velocities which are enormous compared to any wind gust speeds and so the distortion of said lift system by any gust is negligible. But be careful of turning your tail to wind as it is like reversing into your own hot air which will loose you thrust at 1000lb per 10deg C rise in the temperature of the air entering the intake.

Cheers

John

[Avi8tor]

Go fly a Chopper and give the downwind turns a try, at low speed. (Hint: pls don’t attempt it at low altitude)

It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant. Ergo, extremely high rates of turn are possible.

The aircraft is not able to accelerate to overcome the loss of headwind component/increase in tailwind component. Think of it in terms on wind shear. Only here the wind is constant, and the aircraft changes velocity.

NO, this is not demonstratable in a B747 at FL350 holding in 100kt winds, but believe me, try it in a Chopper/gyrocopter or microlite with 30kts of headwind and a 30KIAS.

Watched a very spectacular prang of a Christan Husky at Wonderboom, Pretoria, about 15yrs ago, attempting a steep downwind turn. Literally crashed and burned, very sad, pilot died.

[john_tullamarine]

There was a detailed report in the RAeS mag some years ago on this topic ... I found it interesting reading .. shouldn't be too difficult to track down a copy.

[Ivan Urge]

Do not forget that in the early 90's there was a helicopter on a rig to rig flight, of just minutes, that turned downwind at night (I think) after departing a North Sea oil rig. All perished. Verdict was the loss of performance during the turn downwind. Relative speeds are important. It blows its arse off in the North Sea and these guys were probably in visual contact at night (at what may have been 4 pm) with their destination in a helicopter that really performs.

It's a real phenomena. Don't underestimate it in slow flying/underpowered aircraft or in low level/ low visibilty conditions.

[Intruder]

In many years of aircraft carrier operations, where the airport moves as well as the air -- often at speeds well in excess of 30 knots -- I have NEVER heard of anyone running into difficulty in the downwind turn! The main reason is that over open water there are virtually no speed clues, so the turn is made ON INSTRUMENTS.

If it's windy, it's more likely to be gusty. If it's gusty, your stall margin is reduced when your airspeed is instantaneously reduced in a gust (or its let-up). If you don't compensate in a turn by increasing airspeed, that stall margin is reduced even more. If the stall margin goes to zero, so may your altitude.

After 30+ years of light to heavy (Areonca Champ and Schleicher kA-6 to AH-1W to 747-400), zero- to four-engine airplane and helo flying, nobody will convince me the 'downwind turn demon' is anything other than the pilot improperly substituting ground references for instrument references for airspeed and bank angle.

[THAI TUN]

Thanks for the input guys.

Avi8tor, "It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant. Ergo, extremely high rates of turn are possible." I think you've got it wrong there, imagine the parcel of air is the air inside a railway carriage, as the train trundles along at a constant speed you could fly a paper aeroplane around inside it, and it would behave no differently than it would if the train were stationary.

john_tullamarine, I phoned the RAeS library but they didn't have any articles indexed under 'downwind turns', do you have any more details?

Mark

[Avi8tor]

Hmmm…. I am a little shocked that people battle with this basic concept. At no stage did I mention the turn being executed with reference to a ground feature. The result WILL be the same if the said manoeuvre was carried out on instruments. If it has nothing to do with inertia and ground speed, then explain wind shear!?!

The block of air theory is all good and well, till we come to overcoming inertia. If we were flying into a 60kt head wind with 60kt TAS, we would all agree that the groundspeed is zero. If this headwind component drops to zero, the IAS would drop to zero. This is due to inertia.

In wind shear conditions, headwind component turn into tailwind component; the aircraft is not able to accelerate fast enough, with spectacular results. It requires that aircraft accelerate from 0kts groundspeed to 120kts groundspeed(60kts w/v and then 60kts TAS) in a matter of seconds (if the head/tail wind is 60kts). This phenomenon is not in question. If you have any doubts, the FAA has some good reading on the topic.

Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.

QED

[Diesel8]

I believe this is the old wives tale, that does not go away.

WRT the A-320, it may affect ground speed mini, perhaps that is why you are seeing and increase, although, I cannot say what would cause it in a 76.

Spent 12000 hrs in anything from J-3 Cubs to DC-8, never seen anything remotely like it.

Have done slow flight in the J 3 going backwards over the ground, but even wehen I turned, I did not fail out of the sky.
GS and IAS are to entirely different creatures.

The windshear scenario does not apply, windshear is an abrupt change in wind over a very short distance, it would take some interesting (not to say impossible) maneuvering to achieve this change while turning.

As far as the a/c that crashed into the lake, stallspeed increase with angle of bank, so if the a/c was close to stall, it could very well perpetrate an into the drink, because it stalled. Or, as an aircraft is turned, the pitch has to be increased due to loss of vertical lift component, failure to do so, may cause a/c to go swimming. Then again, perhaps he caught the wingtip, or was flying in ground effect.

The only scenario I can imagine, is one that really cannot be considered flying. See if you can guess it:)

[bookworm]

Hmmm…. I am a little shocked that people battle with this basic concept.

Oh, so do I. ;)

Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.

Yes, it does. What you're neglecting is that what you call inertia is actually momentum, which is a vector quantity. That is to say it has a magnitude and a direction.

An aircraft making a 180 degree turn in nil wind also undergoes a change in momentum of 120 knots (times the aircraft mass), from +60 knots to -60 knots. In your example, the aircraft also undergoes a change in momentum of 120 knots (times the aircraft mass), from 0 to -120 knots. From the point of view of the aerodynamics, there's no difference -- same momentum change, same force in both cases.

[THAI TUN]

Avi8tor, sticking with steady winds and ignoring the added complication of gusts or windshear for the moment:

If inertia were groundspeed related then my paper aeroplane flying round in circles in a railway carriage travelling at a constant speed, would be affected by it's groundspeed, but it isn't.

I'm no physicist, but I think momentum/inertia are relative to whatever you choose as a point of reference, and the appropriate point of reference for studying the behaviour of an aircraft flying in a parcel of air, is the parcel of air. The momentum of the aircraft relative to the ground may suddenly become of interest though, if the aircraft contacts the ground.

THAI TUN

[Intruder]

Now, apply the same principle to an aircraft with a headwind component of 60kts with 60kts TAS. Ground speed is zero. The aircraft executes a max rate 180-degree turn. The aircraft now has to achieve a groundspeed of 120kts in the time it takes to complete 180 degrees.

So what have you proven? Only that an airplane's momentum and/or energy vector changes when it turns. We knew that already. It takes forces to accomplash those changes, and they are supplied by the flight controls (to re-orient the airplane), the engine (to pull/push the airplane in the new direction), and the airflow over the wings (to lift the airplane in a different direction).

As others have pointed out, this happens with or without gusts or wind shear. As I pointed out earlier, gusts (or wind shear, which is simply a gust of a particular magnitude) affect the airplane because they are sudden. You are correct that the airplane's inertia causes the airspeed to vary significantly during the gust/shear.

However, a coordinated turn is not a sudden maneuver. A 180 degree turn may take anywhere from 30 seconds to a minute or more. In any steady-state wind, the control and power response is sufficient to accomplish the required change in momentum/energy vector in a controlled manner with respect to the air mass, and with no regard for the ground.

Only if the pilot changes his frame of reference at an inappropriate time (i.e., fixates on the ground) or if the air flow around the airplane changes suddenly and uncontrollably (i.e., a gust or wind shear) is there any "problem" in completing the turn. Again, if power and indicated airspeed are not increased in the turn, stall margin is reduced, and the effect of any gust or wind shear is exacerbated.

[victor two]

Some interesting points being raised here. To share a vivid example of tailwind effects, I fly light aircraft in gusty winds from a local field in Australia most weekends. The field is shared with a variety of homebuilt aircraft and gliders including a guy who flies a gyrocopter. One morning it was blowing a particulary brisk 30 knots straight down the strip when the gyro pilot taxied out, lined up and gave it full power down the strip. The machine became airborne in no time and climbed like a missile to about three hundred feet when ( no doubt inspired by the spritely performance) he cranked it around to fly downwind back down the strip ....when it promptly fell like a stone and smacked back into the ground about three feet from where he started the take off roll. Blades and bits of gyrocopter fly in every direction and he extracts himself with a puzzled look on his face.

Obviously the old gyro is more vulnerable under these conditons but turning downwind at low level in higher than usual winds certainly has it's risks. I can only compare it to ships losing helm ability in a following sea. It's all fluid ain't it.

cheers all

Vic

[zac21]

Hi Fellas,
After near 40 years of flying I still don't know the answer. I do know that after many years of very low level work that when planning an escape route in gusty conditions it will be into wind where possible. Otherwise increase the power and airspeed for the downwind turn.
:confused:

[CBLong]

Further reading for those who remain unconvinced:

See How It Flies (http://www.av8n.com/how/htm/maneuver.html#sec-infamous-downwind-turn)

[Intruder]

Obviously the old gyro is more vulnerable under these conditons but turning downwind at low level in higher than usual winds certainly has it's risks. I can only compare it to ships losing helm ability in a following sea. It's all fluid ain't it.

I can't infer the gyrocopter's vulnerability from your story...

It is very possible that the pilot did the same thing that other pilots have done in the past -- used the ground as the frame of reference for his turn when there was a significant wind. If it was an open gyro with minimal or no instruments, the probability is even higher!

The "following sea" analogy is an interesting one, worthy of some further thought. As a sailor and kayaker as well as a pilot, I've fought following seas in many circumstances, but haven't quite been able to correlate them to flying yet. Maybe in a helo, when the gust factor is higher than the forward airspeed...

[currawong]

Intruder has it.

Fly the air you are in, not the surface you are over.

I do literally hundreds of downwind turns, 100' to 200' often climbing and at MAUW most days.

If you are careful, keep your airspeed, do not load it up in the turn too much, and KNOW THE DIFFERENCE BETWEEN STALL BUFFET AND LOW LEVEL TURBULENCE, you should be OK.

Increase margins for rough air, like over trees, it eats performance.

Put yourself in a position where you are obliged to tighten the turn to avoid an obstacle whilst operating in or near the region of reversed command, then you are going down....

[Ascend Charlie]

Nobody has yet mentioned Kinetic Energy, the energy of movement. Equal to half x Masss x V squared. This is relative to your frame of reference.

With reference to the ground (which is the thing you are most likely to hit), a helo travelling at 30 kt in nil wind has energy related to 30 squared, or 900 somethings of energy. When it is doing 30 kt into a 30 kt headwind, it has zero groundspeed, and zero kinetic energy - it can't hit anything, so it can't be damaged by that energy.

If it is doing 30 knots with a 30 kt downwind, i.e. 60 kt groundspeed, the KE is related to 60 squared, or 3600 somethings of energy - seriously bigger than before, and a lot more energy to dissipate if it wants to avoid hitting something.

So, we watch as the chopper hovers into the 30 kt wind. Zero KE. We watch it start a turn out of the wind, and if it goes slowly, we see it fly crosswind at about 30 kt groundspeed, and eventually downwind at 60 kt groundspeed.

Where did the extra 3600 units of energy come from to do this? Did it extract the energy from the airflow? Did the temperature of the airflow decrease to supply the energy? Did it move slower, having given up some of it's "oomph"? No, and no. It got hotter (from being whacked by the rotor blades) and it moved faster, because the blades ADDED energy to the airflow. However, if it did somehow extract energy from the airflow, how does it "give it back" when it turns back into wind?

From practical experience gained over 35 years and 12,000 hours of flying, mostly helicopters at very low level, it sure as heck seems that the energy has to come from the engine. If you don't have plenty of power, you don't turn downwind. And the IAS drops off the clock, too. So, any turn out of a strong wind is gradual, and accompanied by a power increase, and do it slowly to allow the aircraft time to build up the KE.

Turn back into wind, and lower the power.

Yes, most of the manoeuvering is done with reference to ground features - powerline inspections require that. But once the turn out of wind is commenced, it is not relative to the ground features - we are not trying to follow a road or such, we are just trying to hold height, hold airspeed, and NOT fall from the sky.

When the speed of the air and the speed of the aircraft are similar, the theories of parcels of air don't seem to work. The paper aeroplane doing slow circles in a train moving at any speed, is actually moving backwards relative to the earth. Any helicopter pilot who does that at low level on a regular basis is inviting a call from his maker.

[bookworm]

Where did the extra 3600 units of energy come from to do this? Did it extract the energy from the airflow? Did the temperature of the airflow decrease to supply the energy? Did it move slower, having given up some of it's "oomph"? No, and no. It got hotter (from being whacked by the rotor blades) and it moved faster, because the blades ADDED energy to the airflow. However, if it did somehow extract energy from the airflow, how does it "give it back" when it turns back into wind?

It's a thoughtful point, but...

Whirl a 1 kg stone attached to a string around your head so it has a velocity of, say, 6 m/s. It has a KE of 18 J, constant around the circle. Now take it on to a bus travelling at 6 m/s, and continue to twirl it in the same way. It now goes from a KE of zero when it is moving backwards with respect to the twirler, to a KE of 72 J when it is moving in the same direction as the bus.

Does it behave differently? No it doesn't, and if you don't believe me, try it next time you're on a moving walkway in a deserted airport.

So how does it get and give back that extra energy?

[the coyote]

I can't comment on the math, it is interesting reading but not too practical when your in the saddle and about to turn downwind.

What I believe in and what I used to drum into students is this:

The aircraft doesn't know or care where the ground is or how fast it is going over the ground. All it knows is the air coming at it, and it will perform according to that. Forget the ground, set the power and attitude to give sufficient airspeed and performance and voila!, one shouldn't hit the ground. Scan the bloody instruments!

In my opinion, all turning downwind accidents are in some way related to the pilot letting the ground come back into the basic performace equation.

We are flying AIRcraft not GROUNDcraft eh? :confused:

[Field In Sight]

It's all down to relative movement as far as I am concerned.

The Kinetic Energy used in the examples above is all measured relative to a point on the ground. i.e. if you are not moving relative to the ground the Kinetic Energy is Zero. But the ground is moving at 1000 miles/hour (depending on latitude) around the earth.

The earth is flying round the sun at an ever faster rate.

The sun is moving around the Milky Way Blah Blah Blah. You get the picture.

You should only be concerned about the steady state environment the aircraft is in i.e. the air.

That's my view anyway :D

FIS

[NickLappos]

Bookworm has it right.

I could spout on about Inertial Reference Frames, but let me pose a simple test. In a steady wind of 25 knots or so, trim the aircraft (helo or airplane) into a steady state banked turn, constant power, altitude and airspeed. Watch it for a few circles, and try to see any climbs or descents during its circles.

There are none. None.

The downwind turn is right up there with extra gunmen on the grassy knoll, all myth.

When bad pilots see the groundspeed increase, they pull back on the stick as they turn downwind, they then lose speed and stall, and Darwin gets another point.

[John Farley]

The downwind turn is right up there with extra gunmen on the grassy knoll, all myth.

Hi Nick

Agreed up to a point. The maths/physics is indisputable as you and others have made clear. BUT and it is a big but: Gusts are another matter.

Things change if you are flying low and slow and are hit by a gust up your backside. Gusts are the issue here, not 'steady' winds.

In a strong low level wind there WILL be gusts. How big varies and how bad depends on how slowly your machine normally flies.

When the gust speed reaches say 50% of your normal flying speed then they can be a pretty hairy influence if you take one up the backside following a turn.

Personally I think this is still something for GA pilots to keep in the back of their mind when flying on a very windy (and therefore gusty) day.


JF

[THAI TUN]

John Farley, I agree with you, but it does raise the question:

Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.

The aeroplane flying downwind would only be worse of if a gust of wind is sharper than a lull. I believe this to be the case, but is there any evidence to support it? Any micrometeorologists here?

Mark

[Fred the Red]

Interesting how this always brings up differing opinions from all levels... To me, my simple fizzics say that it shouldn't matter, because my simple brain can cope with the 'parcel of air' theory.

But... there must be something out there that makes us all, including me, wonder. Here's a hypothesis:

Most of the 'downwind turn' bingles that I've heard of have been when someone's turned downwind early after taking off in a strong crosswind. If, when you are turning away from the wind, you are climbing through the height where the wind is increasing as it is no longer being slowed by the ground , then voila, you have a temporary decrease in airspeed. Admittedly, this is a show-off manoeuvre, but it does happen, and it does give the possibility of an illusion that can cause us to doubt the 'don't lose speed/height in a downwind turn' mantra.

If it's not this, then there must be some kind of illusion out there that's keeping us wondering. I used to fly hang gliders, and when showing off, groundspeed could change from 45 to 5 mph (and vice versa) in a trice: you don't go zooming up and down doing that.

[cpt]

I feel you are perfectly right ZAC21, I don't know the answer neither.
I just try not to be "caught back" by my down wash at low speed...every time I went down by surprise, that was the case....better to learn that fast and remember it when flying logging or crop spraying.

[Intruder]

Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.

It's exactly the same.

Once the airplane is in relatively steady state (straight & level, upwind or downwind), few pilots have problems in this context. The cited problems usually occur while in the turn to downwind, with the pilot trying to force the airplane to fly a specific track over the ground instead of a simple climbing, constant-bank, constant- (though slightly increased) airspeed turn in the air. It is the reduced stall margin in the turn (if the airspeed is not increased), combined with the gust factor, that will get some into trouble.

[John Farley]

Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.

THAI TUN

As Intruder says it is the same, providing the shape of the gust is the same. But I never said it was different did I? The thread is about downwind turns isn't it?

[Avi8tor]

I am glad that this issue has caused some debate.

I have read the discussion here gusts and ground features, which is a TOTALLY different issue.

The devil here is in the detail. As for the fact the earth is moving though space at a zillion miles an hour, is all quite true. But so is everything around us. For simplicity, we can look at forces require to change things, in relation to the surface of the earth. I know this is simplistic!!!!

I think Book Worm has Newton I & Newton II a little confused. He mentions the momentum, but the aircraft with 60KIAS in the 60kt headwind has ZERO momentum, it only has inertia.

Ascend Charlie is quite right, the aircraft that turns out of wind has overcome its INERTIA and gain MOMENTUM some how. If this was a Super Cub, at 60KIAS a 60-degree bank turn is totally sustainable. This would equate to a rate 4.5 turn, and a 180-degree turn would take 13 seconds. Not even a Super Cub could gain 120KTS in 13 seconds. That bit of science is easy.

The stone on a string example seems to be a question of conservation angular momentum.

The bit I can't get my head around, is the forces in the turn. The Super Cub at ZERO groundspeed has NO momentum to overcome. Here centrifugal/centripetal forces don't apply. Therefore, with the aircraft banked, the horizontal component of the total reaction is unbalanced!!!

Does anybody have any ideas if the conventional rate/radius maths applies? Conventional logic tells me that with no centrifugal force to overcome, incredibly high rate/low radius turns are possible!!! Also, what effect would these unbalanced forces have on angle of attack?

[bookworm]

I think Book Worm has Newton I & Newton II a little confused. He mentions the momentum, but the aircraft with 60KIAS in the 60kt headwind has ZERO momentum, it only has inertia.


That is entirely the point, Avi8tor. Newton's laws concern velocity, which is a vector quantity. A 180 degree turn requires the same change in momentum, regardless of the steady wind (and in fact, regardless of the frame of reference).

Newton's laws govern changes in velocity and momentum, not changes in speed.

[Intruder]

The bit I can't get my head around, is the forces in the turn. The Super Cub at ZERO groundspeed has NO momentum to overcome. Here centrifugal/centripetal forces don't apply. Therefore, with the aircraft banked, the horizontal component of the total reaction is unbalanced!!!

First, as soon as the airplane starts the turn, groundspeed will change. But that doesn't matter anyhow...

Also, if it has 0 momentum, why does it need power to remain aloft and maintain that "0" momentum? It is because there are external forces acting on it from the airflow.

If you are above a smooth cloud deck and do not have GPS, INS, or DME, you do not know your ground speed, nor does that ground speed affect your flight condition. The airplane also does not know or care about its ground speed -- regardless of cloud cover -- because it can only "see" the air around it. When aloft, the airplane's frame of reference is the packet of air surrounding it -- NOTHING else. All forces on the airplane are with reference to that air packet.

If the earth happens to pull a mountain or building in front of the airplane, that's a whole different story, because the airplane will not be aloft for long...

[overpitched]

I think there are 2 things here

1. Newton didn't lie. I think everyone agrees that if you are doing 60 kias with a 60 knot headwind and that INSTANTLY changes to a 60 knot tailwind, by either turning the wind or turning the aircraft then you have a little problem. Thats the theoretical application.

2. The practical application during most flight is that if the aircraft is flown properly during the turn then it will have time to overcome its inertia during the turn.

You can't argue with Newton but with airmanship you can out manouver him. I think you guys are aguing two sides of the same coin.

[Avi8tor]

I will tackle this all one point at a time:

Bookworm: -
That is entirely the point, Avi8tor. Newton's laws concern velocity, which is a vector quantity. A 180-degree turn requires the same change in momentum, regardless of the steady wind (and in fact, regardless of the frame of reference).
Newton I concerns inertia (state of rest or state of motion) and Newton II covers momentum. Sorry, you are wishing away the physics by saying the momentum is the same regardless of the wind!!

We all agree that the momentum with 60kt headwind/60KIAS is ZERO. If the wind was zero the aircrafts momentum would be the mass X TAS (groundspeed). These are clearly different situations!!!


Intruder: -
First, as soon as the airplane starts the turn, groundspeed will change. But that doesn't matter anyhow...
COURSE IT MATTERS, that the whole crux of the question!!! The Super Cub will have to attain a groundspeed of 85kts by 90-degrees through the turn!! IF the conventional rate/radius maths applies, it would have had to accelerate from 0 to 85kts in 6,5seconds!!!

(Quick explanation for those not in the picture: By 90-degrees through the turn the super cub will have zero headwind component therefore TAS = GS = 60kts. Also it would have had to attain 60kts in the direction of the wind. These two are at vectors are at right angles therefore 60/.707 = 85. If you don’t believe me, draw the picture.)


Also, if it has 0 momentum, why does it need power to remain aloft and maintain that "0" momentum? It is because there are external forces acting on it from the airflow
Totally agreed, all normal thrust/lift/weight/drag remain the same. I think we are in agreement that angle of attack and aerodynamic issues are NOT groundspeed dependant.


The airplane also does not know or care about its ground speed.......
Explain your statement!!! We have CLEALY demonstrated that the Physics is different, and it cannot be wished away!!

If I am wrong, show me the maths!!!

[Bertie Thruster]

With regard to gusts and turbulence, into wind or downwind, what you are discussing can also be experienced in a boat. Try rowing (at the same speed) into the waves or with the waves There shouldn't be any difference in the ride. But there is.

[bookworm]

OK, one more time.

Newton I concerns inertia (state of rest or state of motion) and Newton II covers momentum. Sorry, you are wishing away the physics by saying the momentum is the same regardless of the wind!!

Newton's first law is a special case of the second law, which says that the rate of change of momentum of a body is proportional to the net force on it. No force, no change in momentum.

When you talk, rather vaguely, about a body "overcoming its inertia", you mean changing its momentum, which requires a force, according to Newton's second law.

We all agree that the momentum with 60kt headwind/60KIAS is ZERO. If the wind was zero the aircrafts momentum would be the mass X TAS (groundspeed). These are clearly different situations!!!

In the former case (with the headwind) the change in momentum in a turn downwind is zero to 2 X mass X TAS, i.e. - 2 X mass X TAS.

In the latter case (still air) the change in momentum in a turn similar 180 degree turn is mass X TAS to - mass X TAS, i.e. - 2 X mass X TAS.

What the aircraft has to do to "overcome its inertia" in each case is to change its momentum by 2 X mass X TAS. This requires the same aerodynamic forces.

Physics works in any inertial frame of reference. You can consider the problem in a reference frame moving with a jet on an airway flying overhead if you like. The absolute values of momentum are different but the change in momentum is the same. There is no absolute value of momentum, the only thing relevant to Newtonian physics is its change.

[cwatters]

The bit I can't get my head around, is the forces in the turn. The Super Cub at ZERO groundspeed has NO momentum to overcome. Here centrifugal/centripetal forces don't apply. Therefore, with the aircraft banked, the horizontal component of the total reaction is unbalanced!!!

Only for an instant. An unbalanced force implies an acceleration - so the instant it banks it also begins a turn and the balance is restored.

Remember that acceleration is changing velocity. Velocity has speed and direction components. Therefore even turning at a constant speed implies an acceleration.

[CJ Driver]

Avi8tor - reading between the lines of your Cub example, your point seems to be "don't confuse me with energies, forces or momentum; we all know a Cub hasn't got the horsepower to accelerate that quickly, so it MUST drop out of the sky".

I believe that the answer to your confusion is that, in the ground-based frame of reference that you are considering, the "horsepower" is in the 60 knot wind that's blowing! Remember, your little Cub is blatting away as hard as it can, indicating 60 knots airspeed into wind, even though it is stationary. Pretty much any manouever that stops it pointing into wind will immediately "accelerate" the airframe down-wind, at least from the point of view of a ground based observer. For example, if you close the throttle and pull the nose straight up to the vertical, the subsequent stall and tail-slide will appear - to the ground observer - to be travelling down-wind at the aforementioned 60 knots. For our hapless aerobatic pilot it's going to look pretty bizarre out of the window, and she's probably going to crash, but that's where we came in on downwind turns.;)

So, back to your example, as the Cub pilot starts the 180 degree downwind turn, they progressively stop fighting the wind. As they turn away from the wind, the wind itself contributes the 60 knot component so that, yes, when we are 90 degrees around the turn we will be going sideways at 60 knots over the ground (but perfectly balanced in the air).

This explanation certainly works for me. In fact, now I'm just waiting for a strong steady wind so that I can practice flying backwards :E

[4dogs]

Folks,

A lot of brain cells are being extended to prove a Newtonian result in an earth-referenced scenario.

The "v" in all of the physical equations is TAS and is airmass-referenced! All of your extensive arguments only reinforce the pitfalls and inherent dangers of flying an airmass-referenced machine by reference to the ground.

But if you wish to discuss windshear, then you are dealing with the physics of a machine that is operating in an unstable medium - one that changes far faster than the relatively miniscule changes in lift, drag and thrust can compensate.

Stay Alive,

[Captain Stable]

I've split off the posts on handlaunched model gliders as not relevant to this discussion.

[cowabunga438]

I am flabbergasted that this debate still continues in the new millenium.

The simple fact is that you fly in air. In a steady state wind (whether that wind is zero or 1 million km/hr) turning in any direction does not effect airspeed.

Think of it this way.

You are sitting 100 feet above the earth in a hovering helicopter.

Someone puts some cloud below you, a big cloth below you, stops you from looking down whatever.

Is it possible to determine your groundspeed?

Could you turn your aircraft in such a manner as to be able to determine the wind?

The answer is no.

Now to one HUGE misconception

from avi8tor
>>
It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant.
>>

This is simple hilarious.

For a start let's since this is supposed to be a technical forum let's get the terminology correct.

Inertia
1) Inertia is the property of bodies that have mass to resist acceleration. F=MA is a direct consequence of inertia.

Inertia is not dependent upon anything except your mass. Your ground speed being zero or 1 million km/hr does not effect inertia (in classical physics - let's forget about relativistic effects and relativistic mass).

Maybe avi8tor is thinking momentum - which is dependent upon velocity and mass. Velocity is a relative quantity (i.e. relative to what you are measuring it against). As we all know our airspeed and groundspeed are different.

However even in that case the important quantity is airspeed. Your momentum relative to the air you are flying in is the important quantity.

Now it makes no difference AT ALL to your performance what speed the fluid you are travelling in is doing to some other observer. If the fluid is NOT accellerating (there are no bumps, no turbulence etc) it is in fact impossible to tell what speed that fluid is doing to someother observer.

Remember all motion is relative. Your flying performance is purely a function of your speed in that fluid, the fact that that fluid is moving with respect to something else is completely irrelevant.

Avi8tor, why on earth would you think your inertia or even your motion relative to the ground is more important in flying performance terms than say your motion relative to pluto, or sirius. The air around you is going at about a million miles and hour relative to the sun - why isn't your inertia dependent upon that?

The only time your motion relative to the ground matters in any shape or form is when you want to stop on it, or leave it.

Now two other points.

1)It is rarely a steady wind (no acceleration in any direction) close to the ground.

2)When close to the ground perception often over rules.

>>
COURSE IT MATTERS, that the whole crux of the question!!! The Super Cub will have to attain a groundspeed of 85kts by 90-degrees through the turn!! IF the conventional rate/radius maths applies, it would have had to accelerate from 0 to 85kts in 6,5seconds!!!
>>

It will also have attained a plutospeed of around 55 000km/hr in the same time. My god it must have a big engine.

Groundspeed no more effects your flying performance than does your pluto speed. Why would you think your change in momentum with respect to the ground is any more important than a change in momentum with respect to the sun?

btw a 90 degree turn in 6.5 seconds is a bit quick.

[DJ Mixmaster]

Keep it simple, guys! Of course it has nothing to do with your ground speed. It's the air speed that keeps you in the air, be it true or indicated or whatever...the motion of the aircraft in regards to the surrounding air. Now, I'm pretty shocked to see so many explanations from experienced people not being close to the truth. Try this in a simulator: W/V: 360/100kts. HDG:360. IAS: 100kts. This will give you GS:0kts. Then simply change the heading to 180, as you would in a turn, except that this happens instantly, not letting the aircraft accelerate. Of course it will fall out of the sky, because of negative airspeed.
How about it, Avi8tor? I hope at least you'll agree...:ok:

[Captain Stable]

I have been resisting contributing towards this thread until now.

The problem is not with steady-state wind, as has already been pointed out by John Farley in an excellent post.

The problem is that there is no such thing as a steady wind.

If you are flying into a gusty wind, your groundspeed will not vary directly with the wind gusts because of your momentum. So turning downwind will affect your lift.

This is, basically, the same phenomenon that can trick people flying into a microburst. As you get near, the headwind increases massively and rapidly, but your groundspeed does not change much immediately because you have inertia providing momentum to be lost first. So people see their ASI getting all excited and cut power. Just at that moment, as their airspeed is adjusting itself naturally, the engines reduce thrust, overall energy decreases - and then the headwind drops, reducing airspeed further, and the downdraft hits them. Low engine power, low airspeed, low momentum and massive downdraft and the ground stops their fall.

You cannot consider instantaneous airspeed to be a simple function of grounspeed and windspeed as there are fluctuations due to gusts and the (sometimes) cushioning effect of your momentum. So when you turn downwind, a gust of wind up the chuff will rapidly reduce your airspeed.

The downwind turn phenomenon does exist - no matter how much common sense dictates that it should not.

[cowabunga438]

DJ

I am not really sure what you are saying. In a sim if you did that turn (in a steady state wind) there would be no drop in IAS - I don't know what you mean by an instantaneous turn - they simply don't happen (you would need infinite energy to do that). The turn would proceed as per normal (in fact i have done this in the sim).

Capt Stable.

What you say in regards gusts is true, but it has nothing to do with turning downwind. The same can (and does) happen turning upwind, crosswind or not turning at all.

Gusts and IAS fluctuations is well known.

In regards turning downwind, even in a moderately stable wind there usually will be no change in IAS. The only change in IAS is due to a gust NOT the fact you are turning downwind.

[Field In Sight]

I've changed my mind. I agree with the minority now.

The downwind turn/loss of lift phenomenon does exist in gusty conditions.

But as cowabunga438 has indicated:

The crosswind turn/loss of lift phenomenon in gusty conditions also exists .

The upwind turn/loss of lift phenomenon in gusty conditions also exists.

The little bit crosswind and little bit downwind at the same time turn/loss of lift phenomenon in gusty conditions also exists.


:} :} :}

I think licenses should be endorsed for "Flight Restricted To Calm Wind Conditions" until this topic is fully understood :p

FIS.

[John Farley]

Field in Sight

Just one small point about your brave post (!). If we assume the gusts are mainly, repeat mainly, variations of wind speed rather than variations in wind direction the crosswind stuff hits you abeam and while you may have to work at the ailerons you do not expect to see the ASI going quite so wild. Which leaves gust effects appearing on the ASI during downwind and into wind turns.

If there is anything in my gut feel that while flying my toy R/C aeroplane gusts tend to arrive all of a sudden and fade away more slowly then this would account for why the gust as you turn into wind does not bother people but the one hitting you up the jacksi seems worse and has given rise to so much debate.

I really would like a met man to discuss (factually) the shape of typical speed changes in a gust.

[cowabunga438]

You are also then assuming the wind is on runway heading.

In that case the downwind turn thingo would never happen at Sydney airport.