Right then, it seems like the problem with the downwind turn in strong gusty winds is that you can't judge your airspeed by reference to either your inaccurate ASI or the ground, doesn't it? So what do you do?

From 16 Flight Maneuvers, http://www.av8n.com/how/htm/maneuver...-downwind-turn


"Summary: Changing Headwinds and Tailwinds
For ground-reference maneuvers, a steady wind has a direct effect.
For other maneuvers, a steady wind has no effect on the airplane or on the pilot in the cockpit. However, the maneuvers will appear different to ground-based observers.
In the presence of windshears, you can gain or lose energy due to the albatross effect. In real life, this means for instance that you will get slightly better performance climbing into the wind. This gives you a reason to turn downwind a little later than you otherwise would.
For any maneuver that doesn't depend on a ground reference, a steady wind has no effect on the maneuver. For example, a standard-rate turn to upwind is just the same as a standard-rate turn to downwind. You can't even determine the magnitude or direction of the wind without using a ground reference.
If you want to calculate the energy in the ground-based frame of reference, you must account for the airplane being batted by the wind. "

Nick...

Does that mean that from the cockpit you cannot determine the nett effect or their is no nett effect ??

And obviously nobody has told the atsb that a rotor can't tell where the wind is coming from.

Ambient wind conditions can have significant and differing effects on a tail rotor equipped helicopter's performance. Engine power is delivered to a transmission system, which drives the main and tail rotors The power required to drive the transmission system is determined primarily by the amount of drag being produced by the rotors and the power available is determined by the power output of the engine(s). The difference between the power available and power required is known as the power margin. If the power required to drive transmission exceeds the amount of power available from the engine, then the main and tail rotor speed will decay, or droop. When the speed of the main rotor droops significantly, the main rotor loses lift and the helicopter descends. Wind blowing over a main rotor provides translational lift that can significantly reduce the power required to drive the helicopter's transmission system. Wind may also assist a helicopter to maintain heading, which also reduces the load demand on the transmission and therefore reduces the power required to drive the transmission. Conversely, a wind from an adverse direction may increase the load demand on the transmission and, in turn, the power required from the engine. Therefore, the wind may cause a net effect which, depending on its strength and direction, will reduce or increase the power required for a tail rotor equipped helicopter to maintain flight.

Nick

Excuse my ignorance, but what exactly is the 'albatross effect'?

Cheers

TeeS

Headwind, tailwind, groundspeed......

Example:
Distance 120NM, zero wind, speed 120 ( speedfactor 2, Groundspeed same).
Outbound 120/2 = 60 min.......inbound same = 120 minutes total flight time.

Distance 120NM, 30 headwind (dead on), speed 120, groundspeed 90 ( factor 1.5) = 120/1.5 = 80 minutes outbound.
Inbound same numbers except for tailwind ( groundspeed 150 = factor 2.5 ). 120/2.5 = 48 minutes = total flight time 128 minutes

Where did the 8 minuttes come from ? ;-)

You had a headwind factor effecting you for longer than the tail wind factor. The quickest return round trip is with nil wind, unless you can get a tail wind both ways.:O

rotordk

Same place infinity would come from if you had a 120kt wind - i.e. you would halve the outbound time but never get back!

Cheers

TeeS

Dead on Nigel.....it toke a couple of days to get it myself...thought I could baffle you guys, but I'm way behind :-)

So why doesn't anybody care about circling minimas for helicopters IFR........we are talking JAR law based upon inproper information (laws of physics) according to this discussion so far.
Any thoughts ?

I've just been rereading this thread and scratching my head.

From what a lot of you guys are telling me. Once I leave the ground I can manouver at will, upwind, downwind, crosswind, as slow or as fast as I like and it's not going to affect my aircrafts performance AT ALL ( don't tell me "maybe just a little bit" "hardly so you would notice"... I'm talking science..it either exists or it doesn't) as long as I don't look at the ground because I am part of the air, part of the sky, its all one etc etc...... then

the same people tell me I will however have a problem with wind shear, mountain waves, bumps, eddies, rubber duckies etc....

Now make up your minds. Am I one with the wind or not. You can't have it both ways. If my inertia no longer exists(relative to the Earth) because I'm flying well then it doesn't exist. You can't say .. well it's not there if you turn the aircraft , but if the wind turns you could be in deep do do

And at this point I'm not interested in how the aircraft does or doesn't behave I want to know the scientific law, the rule, the principal, thought out and handed down by some musty old fella with a long white beard in some recent or not so recent time in history.

Anybody help???

overpitched

I reckon forget about inertial reference frames and windshear and its effect and if a 60 KT headwind suddenly turns into a 60 KT tailwind. Any windshear has unpredictable efffects anyway and it sounds as if you are pretty confused by people taking everything to the n'th degree. Stick to the basics mate.

If you turn downwind maintain a safe AIRSPEED (unless you have HOGE power available). EXPECT the ground speed to increase.

Don't use the GROUNDSPEED as your judge of AIRSPEED.

Scan/maintain the attitude that gives you the airspeed you need, set the power you need to hold height, and keep checking the ASI.


The goal here is to not spud in if you turn downwind isn't it? Keep your AIRSPEED on and you won't. Simple.

How about you think it all out and hand us down the guru stuff?

Coyote

By telling me I have to "manage my airspeed" on a downwind turn you are agreeing with me...... ie the helicopter has to accelerate to maintain airspeed. If somebody has a different idea to that I just want to know the scientific principal is all, so I can do a bit of further reading.

Now the reason I think I'm right is that while in flight the rotor itself is doing just what we are discussing. each blade in turn goes upwind crosswind downwind crosswind etc. While you are hovering its all good but as forward airspeed of the aircraft increases the upwind and downwind components get larger creating dissymetry? of lift. Theoretically speaking I can't see why there would be one rule of motion for an aircraft turning downwind but a different one for a rotorblade doing the same.

You are not going to tell me that retreating blade stall is caused by the retreating blade trying to turn downwind with visual reference to the ground are you ???

Charlie S Charlie - typo error on my part - it should be 20 kts IAS, 40 kts GS. I could have just edited the post but I thought I ought to own up!

Coyote - "managing your airspeed" just means maintaining it by maintaining your attitude. As mentioned before, the natural tendency is to use groundspeed as a reference and keep it constant - this will lose you airspeed as you turn downwind.

Overpitched - if you transition forwards, backwards or sideways - at 10 to 15 kts of airflow across the disc the rotor will experience translational lift and an increase in thrust. That is why the rotor doesn't care which way the wind is coming from, it just responds to airflow across it.

Crab

And your point is ??

Great thread.....I think I've got it now:

in an "aha me hearties" tone:

"When low level and it's blowin' strong, turns downwind can go mighty wrong."

Turning downwind
 

Went water-skiing on the river last weekend. Couldn't figure it out, every time the boat turned downstream we sank into the water because the boat slowed down. It all makes sense now, obviously it was inertia.

Fact is we didn't sink during the turn. Boat speed never varied. Never expected it to either. Wasn't that the current was not strong enough as it was kinda quick in a few places.

Well - if we had sank into the water on the downstream turn I suppose we could have blamed inertia, couldn't we?

I've been following the thread trying to work out what's what. Haven't managed it yet - too complex.

However, one thought. In an RFM / POH / whatever, what is performance related to / graphed against ? Airspeed or groundspeed ?

And if you also remember that

Performance = Power + Attitude

does it help ?

Overpitched

Windshear is what it says, the two bits of air moving in different directions at the same time. It's like jumping from the up elevator onto the down elevator - you will land on your ar*e. Nobody on this thread has suggested that turbulence, windshear etc will not affect you.

The question I think is being asked is, 'in a steady, non-turbulent horizontal air flow, will a turn downwind have any effect on the flying characteristics of the helicopter?' - The answer is 'No it won't!'

Cheers

TeeS

Ok TeeS

Tell me why not. When you fly in a constant direction from a parcel of air moving in one direction into a parcel of air travelling in a different direction thats called windshear and it affects you,

but on the other hand when the air moves in a constant direction and the aircraft travels in one direction and then travels in another direction that is called a downwind turn and it DOESN'T affect you....... seriously??? is that what you are telling me ?????

Now picture the rotor blades again. Advancing blade traveling upwind... lots of lift, retreating blade traveling downwind... less lift. You will remember it from basic training , it is one of the aerodynamic limitations of a helicopter. Now why would that happen on a blade(wing) but not the 2 wings attached to a plane. And if it happens to a plane why would it not happen to a helicopter.

River, water skiing, sinking boats, what crap is that???

overpitched,

I think I can explain it...

That's basically right. Except that if you get below 15-20 kts AIRSPEED, you will lose translational lift, and therefore require more power. You could do this by either slowing down, or due to losing a headwind. But it's AIRSPEED we're talking about, remember. If that doesn't change, neither will the performance.

Yes, but for different reasons. If a downdraft in the mountains is dragging the aircraft down, it'll require more power to keep flying. In the event of windshear or sudden changes of direction, the ASI won't be accurate since the pitot tube can't cope (read earlier in thread for details) so you'll need to keep the ATTITUDE of your aircraft in order to maintain you AIRSPEED ...not always easy in the mountains, so you can come to grief. Bumps, eddies, mountain waves...all same thing. But this isn't due to complicated rocket science or aerodynamic principles relating to each blade being different...if it was I'd need a diagram and a lot of thought and probably wouldn't be posting about it in case I got it wrong. :confused: It's simply due to downdrafts dragging you down, updrafts giving you a bit of free lift, and so on.

Well, I just re-read that and I think it's clear. But to summarise...

Performance won't alter unless you go below translational lift airspeed. Up and down drafts may affect your performance, for fairly obvious physical reasons.

Hmmm...if it's not that simple, someone had better tell me.

Overpitched - imagine you are in a steady 5 foot hover with no wind, the hover attitude is set with cyclic, the hover height with lever and the heading with pedals; you are in equilibrium with all forces balanced to keep you in a steady position over the ground.
Now consider 2 different scenarios:

1. A wind of 20 kts suddenly appears from the dead ahead and you, the pilot, do nothing with the controls - eventually the aircraft will be translating across the ground at 20 kts in the same attitude, at the same height and on the same heading. (purists will tell me the disc will flap back and pitch the nose up as the gust hits, and they are right, but I am trying to illustrate a concept).

2. A wind of 20 kts suddenly appears from the dead ahead so you correct the tendency to drift back with forward cyclic, correct the tendency to climb (due to ETL) with lever and compensate for all the yaw changes (ETL on the TR and reduced power required) with pedals. If the wind stays constant, you will be in a state of equilibrium again (still over your position on the ground) but with a different attitude, power setting and pedal position.

In situation 1 you are "going with the flow" as you are when you turn at constant speed, power and attitude in a moving mass of air - ignoring the resulting movement across the ground.

In situation 2 you are maintaining ground position and having to compensate for the changes in airflow to prevent the wind blowing you across the ground. If you now switch the wind on and off, you are in a constant state of change with respect to attitude, power and heading which is the same as encountering turbulence or windshear.

Windshear doesn't always affect you. Never been flying on one of those days when the wind direction varies? Or flown close to hills or buildings, when the wind whips around them? That's windshear. It only affects you when you aren't expecting it. And it only affects your helicopter when it causes your airspeed to reduce below translational lift speed, and you don't realise it and add extra power. The same thing happens in a downwind turn if your airspeed reduces. The two are identical.

There was an accident in the North Sea several years ago, with a Super Puma taking off in a very strong wind, and then turning downwind - the aircraft crashed into the water, and if I'm not mistaken, one of the causes was the turn before the aircraft had sufficient airspeed.
Consider if you are maintaining 60 KIAS directly into a 60 Knot wind - obviously, you have no groundspeed. Now you want to turn to go downwind. Anybody done it? What happens?

Whirley:

You are correct, wind shear will affect airspeed.

And the exact same effect of wind shear will be encountered whether you are turning down wind or up wind or cross wind or any combination thereof.

A suggestion for those of you who think turning downwind is dangerous if you maintain airspeed.....always fly into wind, that way you will get to see more places.

Oh by the way anyone here have any opinions on the flat earth theory ? :ok:

Chuck

Shawn; yes, as I wrote earlier in this thread.

Wessex HC2, to get out of a glen in Scotland, to avoid flying backwards for about 2 miles.

Parameters were as earlier stated; IAS 60kts, gnd sp was about 10-15kts backwards. so w/v about 70-75kts. ht about 150ft.(low cloud)

Straight glen, no bends. It was clear-we had just flown up it (very slowly!)
briefed what to expect visually. Came to relative hover (75kts IAS). Turned very very slowly. Allowed d/w drift to accelerate very very quickly. Power came in to maintain ht as came to free air hover going d/w at 75kt g/s. Then accelerated to get a bit of air speed for comfort.

Wow!! We have really got ourselves wrapped around the axel on this!!

Some points to recap:

1) A steady wind from any direction does not change the aircraft's performance in any way, no matter how you turn.
The aircraft is immersed in a fluid, and as long as its conditions are not changed with respect to that fluid, it will perform identically regardless of its direction in that fluid. The aircraft cannot tell what direction the wind is coming from, let alone change its climb performance as a result.
Shawn, when you told the wive's tale about the takeoff crosswind turn causing a loss of altitude, you fool yourself, and mislead everyone else. Same with the Super Puma accident. If the CAA decided a downwind turn is the cause of that accident, it says how little that particular examiner knows about aircraft performance!

2) If the wind has shear and gusts, this will affect the performance. Gusts and shear are not the same as turning the aircraft, they are examples of how the fluid can impart forces on the aircraft that support it. That is called lift, guys. If the velocity with respect to the fliud changes, the lift changes.

3) There might be way too much talk around here, and too little actual flying! Will someone please just take a flying machine out and turn it while trimmed at a steady altitude and speed?

4) The amount of phoney theoretical energy in "downwind turn is both measureable and large, and will show up if any of us would simply try it. The theoretical altitude loss cannot be hidden in a slow turn, it is not a small amount that gets buried in pilot technique.
In a 30 knot wind, there is a 60 knot speed difference between upwind and downwind(if you turn in a 30 knot wind while going at 120 knots, your groundspeed goes from 90 knots to 150 knots.) The difference in phoney energy, if you misapply Newton, is about 634 feet of altitude loss for an aircraft. Try the turn, trimmed, and see if there is ANY altitude loss or gain.

Nick:

Is it possible that the knowledge of flight is this abysmal among licensed pilots?

Or we just wanking ourselves with arm chair pilots?

Chuck

I really like these debates, they sharpen all of us. I think there is lots of good knowledge, out there, just some shakey Physics!

Maybe wanking affects more than vision??

Sayeth Shawn:Unless I'm mistaken (and that certainly could be the case), my understanding of that crash was this:

Puma, at night/strong wind/low weather, going from one platform/rig to another directly behind. The crew took off, did the 180 successfully, but as they came screaming by the intended platform on the downwind at low altitude, the PF who was obviously looking outside at the rig and not at the ASI, decelerated causing what is believed to be the dreaded vortex ring state at which point they impacted the water.

At least, that's how I remember it.

I think Overpitched gets confused because wind is seldom completely steady like out over water. Gusts/up/downdrafts add to and detract from performance.

Overpitched

O.K. One last go (well maybe not the last!)

Imagine being a passenger in a hot air balloon. The balloon, once it has been launched and has stablised, will be travelling along with the wind. It will be travelling at the same speed and direction as the wind. When you stick your hand out of the basket, you feel no wind, none, yet you are travelling along across the ground.

Now just imagine the balloon has a really really big basket, big enough for you to go for a run round a circular running track. Set off at 20kts, you will feel a 20kt breeze on your face. Do you honestly believe that you will feel a different speed breeze on your face as you turn 'down wind'?

TeeS

TeeS :

Brilliant.

And if the earth were flat the balloon would ride the wind to infinity and beyond.

Nick :

Wanking has only a temporary affect on vision and just for a few seconds, I have been doing it for decades with no change in my vision at all. ( except for a few seconds. ) :ok:

Nick...


So how does disymetry of lift and retreating blade stall fit into all this ?? If a plane can't tell its turning downwind why does the blade have less lift on the retreating(downwind) side ??

Your point about conducting the experiment in flight is valid obviously but there may be reasons why the changes are not detectable to the pilot. I might find myself right where Aristotle did. Good experiments wrong answer.

So if we are not operating according to Newtons first law

I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

I would like to know under which law we are operating just for my own benefit and interest.

TeeS

I understand what you are saying but it's not relevant as the balloon has no airspeed. The balloon and the wind have the same relative motion... unlike a moving helicopter or a plane that are moving relative to the wind not floating along with. Now give your balloon 20 knots of airspeed and run around inside it. Tell me you won't notice a difference running into the wind or downwind.

Whirly...

Thanks for the ideas but I don't think you have it clear in your own head. You seemed to be contradicting yourself( as a lot of people seem to have so far in this discussion ) or was that the other whirly sitting on the other side of the fence.

overpitched:

1) The lift on the blades is equal. That is why you don't roll upside down.

2) Newton had it right, of course! You can't apply his laws selectively. As you have stated, the first law does nicely. In a downwind turn, there is no disturbing force on the aircraft (other than its tilted lift vector). The "wind" exerts no force on the aircraft, and so the aircraft sees no acceleration in any direction. Draw a free body diagram of the aircraft in flight, see the lift, drag, weight, thrust. Where is the wind??

Aristotle tried to answer this and was quite wrong (in fact, Galileo had to refute Aristotle, and was pounded for trying!).

The downwind does not make you fall, and the boat going upstream needs no more power (someone posted that a while back!!)

This is complex stuff, and can't easily be transmitted in printed word. Needs at least 3 beers and 2 fully articulated wrists to explain most of this. Lets get some, OK?

overpitched,

I have it totally clear. Where did I appear to be contradicting myself?

thank you nick, it sure gets frustrating reading some of these posts and makes you wonder what goes on in helicopter theory classes.

there are also a few flying instructors around that have such a distorted view of helicopter aerodynamics and physics that they don't give a lot of the greener pilots a proper perspective of the helicopters true capabilities. this fact is demonstrated in this forum by the majority of aerodynamic questions asked by licenced pilots (?) and a lot of the answers put forward.
:suspect:

hey, i certainly don't mean any disrespect to any one and it does take some experience to understand that a helicopter will do anything you ask of it as long as you keep within the performance limits imposed on it.

basic questions get asked and should be answered basically but most times they get muddied and go off at a tangent distorting the original question making it pretty impossible for the questioner to fathom. it's not rocket science, helicopters work aerodynamically today exactly the same way they did when they were invented.

i will add that when i did my training all those years ago we did practical training and very little theory, what theory i did do was more operational stuff, air law etc. i learnt the theory much later.

i would have thought that by today that the game would have been lifted.

Sorry whirly I wasn't trying to give you a hard time its just that you said......

Windshear doesn't always affect you. Never been flying on one of those days when the wind direction varies? Or flown close to hills or buildings, when the wind whips around them? That's windshear. It only affects you when you aren't expecting it. And it only affects your helicopter when it causes your airspeed to reduce below translational lift speed, and you don't realise it and add extra power. The same thing happens in a downwind turn if your airspeed reduces. The two are identical.

You seem to be saying that the laws of motion only apply to you under certain conditions. Now I agree that you may only be able to feel them under certain conditions but if they apply, they apply


Imabell

What are you saying... Snell didn't teach me proper

Hi Overpitched,

If you are flying in an unaccelerated mass of air (unaccelerated means it is not changing velocity), it doesn't matter what the speed of the air is. The helicopter (or bird, or balloon) will not know or be affected by the speed of the airmass. Airspeed is airspeed - if you fly 50 KT airspeed surrounded by a 50 KT southbound airmass, the helicopter will not handle, respond, or perform any differently than if the airmass was not moving. The only way you would know if the airmass was moving would be by looking at a ground reference (or a GPS). Turning downwind has no meaning under these circumstances - there is no "downwind" in a constantly-moving mass of air unless you choose to reference it to the ground.

Ride in a hot-air balloon and you'll decide that maybe the air isn't moving at all, in fact the Earth is turning and the air is still. IT DOESN"T MATTER, until you are trying to fly consistent GROUNDSPEED instead of airspeed.

Inertia only rears its head if the airmass accelerates (changes speed or direction), i.e., wind shear, thermals, wind gusts, downdrafts, etc. That WILL affect performance no matter what maneuver you may be performing at the time. If you are turning downwind and the wind suddenly increases, the inertia of the helicopter initially resists the acceleration, so there is a brief decrease in airspeed (the lighter the aircraft, the briefer the change). This is a result of a CHANGE in the air's speed - if the air's speed remains constant, their is no such inertial effect.

OK, now on to the theory of quantum strings....

Nick, If nothing else you have made me do some study... Good thing I have a couple of days off.

You said..

The aircraft is immersed in a fluid, and as long as its conditions are not changed with respect to that fluid, it will perform identically regardless of its direction in that fluid.

Now if Velocity is a vector quantity which refers to "the rate at which an object changes its position." The air has one velocity and the aircraft has another eg. air- 50 knots from the north aircraft-100 knots(gps) to the north... differential 150knots relative(airspeed) & if

Acceleration is a vector quantity which is defined as "the rate at which an object changes its velocity." The aircraft turns and tracks south the difference in relative motion is now 50 knots so at least 1 of 2 things must happen, either the airspeed reduces or the aircraft accelerates to 200 knots(GPS).

Back to Newtons first 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.

Therefore if the airspeed stays constant and the thrust stays constant the acceleration of the aircraft would come from an unbalanced force which must be the reduction of Air Resistance Force.

Newtons second law

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.



Air Resistance Force
Fair

The air resistance is a special type of frictional force which acts upon objects as they travel through the air. Like all frictional forces, the force of air resistance always opposes the motion of the object. This force will frequently be neglected due to its negligible magnitude. It is most noticeable for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.


Flingwing If as you say

it doesn't matter what the speed of the air is. The helicopter (or bird, or balloon) will not know or be affected by the speed of the airmass. Airspeed is airspeed - if you fly 50 KT airspeed surrounded by a 50 KT southbound airmass, the helicopter will not handle, respond, or perform any differently than if the airmass was not moving.

Why is inertia only important if the velocity of the air changes... surely if ANYTHING changes you have changed the relative velocity between the aircraft and the air.

you also say

Airspeed is airspeed - if you fly 50 KT airspeed surrounded by a 50 KT southbound airmass, the helicopter will not handle, respond, or perform any differently than if the airmass was not moving.

And you are right... but that is not ALL we are talking about we are talking about going from travelling 50 kt in one direction to travelling 50 kt in the other direction. Assuming that aircraft performance, lift or aerodynamics are not affected by compass heading it makes no difference to the laws of physics whether you encounter a tailwind of 50 kt by turning the aircraft or by turning the wind (windshear) accept for the time spent in the turn.

overpitched,

The place you have to work on in your logic is that you are still feet flat on the ground, looking at the aircraft in wind, and seeing this wind as some kind of force on the aircraft. This is fundamentally flawed. AS the wings and rotors see it, the air is just there, all around, and its relative motion is caused by the engine. The wind is useless as a force generator once the aircraft has lifted off the earth. Please try not to see the wind as pushing on the aircraft, it does not.

Ask this question: If I tried to fly a stringless kite, how might it fly? Unless the string is tied to the earth, the kite simply flutters down, as it has no airspeed.

In nil wind if I was flying thru the sky(space) heading south at 100 knots in a 206 at 1400 kg my momentum would be M X V = 140,000 kt/kg

Add a 50 kt tailwind I'm now flying thru the sky at 150 knots in a 206 at 1400 kg my momentum is 210,000kt/kg

The practical application from the pilots point of view is that in both cases he is doing 100kias. From a physics point of view the momentum of the aircraft would increase turning downwind & decrease when turning upwind and that can only be caused in this case by accelleration. Which means unballanced forces.

If the wind was not a force that acts on an aircraft to either assist it or hinder it then navigation would be a hell of a lot easier.

Nick.. The problem I'm having seeing it from your point of view is you seem to be treating the aircraft as if it has no motion relative to the air.
If you pick a fixed spot in the air say a hot air balloon floating with the wind. wind is 50 knots from the north so the balloon is heading south at 50 knots. Agreed ?

I'm in a 206 heading north but at 100 kias. I decide to fly around the balloon and head back south. To maintain a constant speed relative to the balloon all I have to do is fly in at 100kias and back out at 100kias simple

But what I have done with regard to the laws of motion is fly in at 50 knots accelerate and fly out at 150 knots. As I haven't changed power or attitude the force that provides the acceleration can only have come from the wind.

I was just using a speed reference that we are all familiar with. It wouldn't matter what fixed point anywhere you used in that instance the nett difference in momentum would be 70,000kt/kg and its the nett change in momentum that concerns us here