Downwind turns equal disaster??

[Bomber ARIS]

Having claimed more than a few copters over the years, I felt that this (http://www.pprune.org/forums/showthread.php?s=&threadid=118712) discussion over on the Technical Log forum was worth highlighting , especially to those Rotorheads "a little light in the logbook"

[Rocket Surgeon]

So that´s where Nick Lappos has been hanging out.

Rotorheads not good enough for him anymore?????:(

[NickLappos]

Pprune can't get rid of me that easily! I've been really busy lately, with the competition for the new VH helicopter!


Maybe we should bring a helicopter downwind turn thread here in Rotorheads just to get the juices flowing!

[Chuck Ellsworth]

I should know better, but, I have to comment on this subject.

I've been flying fixed wing for over fifty years and rotarcraft for thirty nine years.

Low level flying has been a great part of my career, fifteen years flying heavy fixed wing water bombers and seven years flying fixed and rotary wing aircraft in aerial application.

I have never experienced the down wind turning problem that some pilots claim can cause the sky to fall in on you, or the ground to rise up and smite thee.

I have however experienced many viscious up, down, roll, and airspeed excursions caused by wind sheer and wind gusts..... especially in the mountains.

In my experience it wouldn't matter a rats ass if you were flying down wind , up wind, x/wind or performing loops, the results remain the same..when the air is unstable.

As to gyroplane's crashing and blaming it on the dreaded down wind turn, may I suggest you examine the possibility that they were already right on the edge of the power curve and or a sudden gust unloaded the rotor resulting in a PPO.

For what it is worth I have a Commercial Gyroplane license as well as helicopter and have delved deeply into the high instances of crashes in gyroplanes......

In no instance can I clearly identify the so called down wind turn causing a loss of lift... there has always been some other factor involved usually gusty wind and quick steep roll into the turn at high power at low airspeed and high pitch angle.

Oh, I find that I can't get the Airbus to stall in any turn no matter how aggressive the control stick movement or what direction the wind is.

There is the answer to down wind turns ..get Airbus to computerize your rotorcraft. :ok:

Chuck

[Heliport]

Good idea, Nick. :ok:

So what do people think of downwind turns in helicopters?

[overpitched]

I know by the tone of that simple question that to answer is to put ones foot in the bear trap... so here goes

The way I see it is if you are doing 60 kias with a 30 knot headwind you are traveling across the ground (or through space) at 30 knots.

If you turn 180 deg (or the wind does) without a change in attitude or power you will still be doing 30 knots across the ground( or through space) but will now have a 30 knot tailwind ie ias = 0

Now I 'm sure you are going to tell me I'm wrong but that is how my helicopter seems to work and I can't remember Newtons laws of motion mentioning anything about wind or height above the ground.

Preparing to duck for cover and burn logbook.

[Bomber ARIS]

overpitched says

If you turn 180 deg (or the wind does) without a change in attitude or power you will still be doing 30 knots across the ground( or through space) but will now have a 30 knot tailwind ie ias = 0
Now I 'm sure you are going to tell me I'm wrong
Bomber says

How were you sure that I was going to tell you that you were wrong? Spooky!! :ooh:

[PPRUNE FAN#1]

Overpithed:The way I see it is if you are doing 60 kias with a 30 knot headwind you are traveling across the ground (or through space) at 30 knots.

If you turn 180 deg (or the wind does) without a change in attitude or power you will still be doing 30 knots across the ground( or through space) but will now have a 30 knot tailwind ie ias = 0Here's a simple test that any real helicopter pilot can do: Go up on a day when there's a brisk wind - say, 2,000 feet and fly directly into that wind. Slow to 40 knots IAS, get it stabilized and in trim. Now bank the ship and start a constant-attitude turn - your choice of direction. Do not mess with the pitch attitude of the ship. Keep it going around and around, as many times as you can stand. You'll notice a couple of things:

1) Your indicated airspeed will never vary. As you turn away from the wind, your IAS will not decrease by the amount of the wind, or vice-versa;

2) If you look out at the horizon, you won't be able to tell when you're turning down- or upwind. Oh, if it's really smooth out and if you're really perceptive, you might notice a tiny little increase in performance when coming around into the wind, and a corresponding tiny little decrease in performance when turning downwind, but the gains and sags are in the order of only a couple of feet of altitude. As you keep going around your overall altitude will remain the same;

3) If you look at the ground, you'll see that the ship will be describing a curly-cue as it translates across the ground in the direction of the wind.

As an experiment, I have done this in aircraft ranging from Cessna 150's to Bell 206's to BO105's. (Of course, in the Cessna the speed was just above stall speed, not 40 knots for you nitpickers out there.) The results are always the same. If you have not done this, please do not make any more theoretical postings about what you think "should" or "will" happen. Only those who have actually gone up and quantified it should post in this thread - not guys who've screwed-up a downwind turn close to the ground and have now erronesously concluded that such turns are dangerous.

[Steve76]

define what is "brisk wind"

[Flingwing207]

If you are flying close to the ground, turn downwind and have an engine failure, disaster is certainly closer than if you have that same engine failure into the wind.

However, the helicopter doesn't know (or care) whether it is flying with 75KT airspeed and 50KT groundspeed or 75KT AS and 100KT GS. Once you are in the mass of air, it doesn't matter at all if the air is moving or which direction it's going, until you have to touch the ground. Turns, climbs, OGE hover - doesn't matter whether the wind is 330/18 or 00000, as long as you aren't trying to hold station to a point on the ground.

For a light helicopter, even a gust of wind doesn't change performance much (as long as you are not attempting to maintain a consistant ground speed), as the aircraft will just be accelerated by the wind. Not so for a loaded 747-400 - the inertia of that aircraft will mean that a wind gust causes a significant change in airspeed and resulting performance.

We're so used to flying helicopters by referring to points on ground that we sometimes forget that the air doesn't care. Swimming across a placid river is no different than swimming across a lake unless you are trying to hit an exact spot on the opposite shore.

[Chuck Ellsworth]

Oh my God :

I can't believe I am still in this conversation...but here goes.

Someone mentioned this conversation should be about " helicopters" O.K. try answering this.

You are at one thousand feet in a helicopter over the ocean the air has no vertical instability and the wind is steady at fifty knots from the north at your altitude.

You see a hot air baloon at your altitude.

You fly a constant angle of bank, constant altitude and power setting turn around and around and around the baloon.

If you only look at the baloon for reference will you be able to tell when you turn down wind?

Chuck

[Whirlybird]

I was going to avoid this conversation because I'm not very good at this aerodynamic stuff. :(

BUT...I've done something very similar to what PF#1 and Chuck are suggesting. A while back, some asshole wanted to put three 300ft wind turbines in a field on a hill above the lovely valley where I live. Our action group against it wanted to prove that they'd be seen from just about everywhere in the valley. Having checked as to safely etc, I offered to circle in a helicopter at 300 ft above that field, so that people could find out if they could see me. There was about a 15 kt wind, and I orbited at 30-40 kts till I got dizzy. When I was into wind we hardly moved over the ground, and when we had a tailwind we practically shot off the hillside, or that's what it felt like. But we didn't fall out of the sky...why should we, since no matter what the groundspeed, the airspeed was still 30-40 kts. The only danger would have been if I'd estimated my speed with reference to the ground, and reduced my airspeed when turning downwind. That's the only problem with downwind turns, in either f/w or helicopter - if you look at the ground and think you're going too fast. At least, that's how I see it...and I'm here to tell the tale, so I guess it was right.

Oh, and we won the battle, and didn't get the wind turbines to spoil the view. :ok:

[overpitched]

PPF

So thatI'm clear in my head...


Are you saying this happens because the aircraft has time to overcome its inertia during the turn ??? My head hurts

[MightyGem]

op, you're flying along, straight and level at 60kts and you want to reduce speed to zero. What do you do? You ease back on the cyclic to adopt a deccelerative attitude, and maintain height with collective as you slow down. If you don't change the aircraft attitude, you will maintain speed. If you enter a turn at 60kts, as long as you maintain that 60kt attitude, then your speed will remain the same, regardless of the windspeed/groundspeed.

[rotordk]

Any comments on circling w/regards to IFR helicopters ?
Waste of fuel I reckon :-)

[NickLappos]

Overpitched,

Your puzzlement is understandable, except that you are automatically assuming that the intertial energy of the aircraft is somehow tied to the earth's surface. Not true, the velocity can be referenced to any coordinate system, as long as you are consistent.

A great read about this is from the mind that first developed the concept, and the mind that Newton relied on to guide him: Galelio.

What Galelio conceived of was "Inertial Reference Frames" that allowed him to understand the behavior of objects. He surmised, and we somehow forgot, that any reference frame that is not accelerating (ie the frame is at constant speed relative to another) is as acceptable. The wind/sky reference is quite acceptable.

[rotordk]

Sorry, forgot to mention IFR circling in accordence with JAR.
Lack of understanding the greater powers of Inertial Reference Frames , I guess

BTW Nick, what's the profile on the S92 w/regards to x-wind ?

edited for spelling

[SawThe Light]

Seems to me that the guys who believe the perils of turning downwind are not caused by visual cues should try doing it on top of a good thick cloud layer.

Methinks there will be a major problem just trying to figure out where "downwind" is.

[Milt]

Its all very simple really

Every take off and landing is a transition from one medium into another.

When you have transitioned into the air medium then ALL flight is with reference to the air you are in.

Granted there remains confusion when you are at the interface of the two mediums and there are cross over effects such as gusts.

As for momentum and inertia. Why has no one considered that the parcel of air we fly in is going around at the equator at about 1200 Kts. But it doesn't matter. The aircraft is aptly named. It flys relative to the air.

[NickLappos]

rotordk,

The S76 family has an approved cross wind envelope of 35 knots at all gross weights. This means it has at least 10% control remaining at that cross wind speed.

[heedm]

overpitched, the answer is yes. The aircraft has time to overcome inertia in the turn.

As with everything in Physics, many assumptions are made based on magnitudes of seperate effects. In the case of a downwind turn, with the rate of turn slow enough, the problems people encounter are due to maintaining ground reference so the inertial effects are assumed to be neglible. If you could turn fast enough you may be able to see inertial effects.

Try this in flight. On a very calm day with a light helicopter, find the max angle of bank that will allow you to maintain an airspeed (~60kts) in level flight. On a very windy day (without gusts) with the same helicopter, same AUW, same DA, set the same angle of bank and see if your airspeed fluctuates. Good luck finding the right conditions, even if you do I won't guarantee you'll see much, but in this instance the inertial effects may become apparent.


Milt, the rotation of the earth does factor into aviation. That is why when flying in the northern hemisphere there is always a small correction to the left whereas in the southern it's to the right. It's called a coriolis force. Again, this is so small that it's ignored in most discussions.

Matthew.

[Bertie Thruster]

Why is it always bumpier flying (at the same IAS) into wind than it is going downwind?

[TeeS]

Hi

I have to sit firmly in the ‘I’m flying in a parcel of air, the ground can’t affect me’ school! Except of course in the case Bertie is asking about.

My feeling, for what it is worth, is that in ‘general turbulence’, it will not make any difference whether you are flying up or down wind. However when the turbulence is caused by standing wave type flow, there is a change in frequency of the turbulence due to Doppler Effect. Flying away from the source of turbulence the frequency will reduce - feels better, towards the source the frequency will increase - feels worse.

TeeS

P.S. Try letting the auto pilot do the turns - take the pilot out of the equation and you will see groundspeed just does not come into it!

[overpitched]

I've been away doing a bit of thinking.

And playing both sides of the street sorry !!

Here is my final take on it.

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.

[[email protected]]

The only reason a turn downwind at low level is dangerous is because many pilots in this situation try to maintain a constant groundspeed ( this is natural because they are looking outside not inside at low level so they don't bump into things).

So from 40 kts IAS into a 20 kt wind, the pilot turns downwind (subconciously adjusting attitude to maintain his 20 kts groundspeed) and suddenly finds himself with 0 kts IAS, no translational lift and a descent because he hasn't compensated with extra power for the loss of ETL. Oooo...low speed, RoD and power applied....we know a song about this don't we children? It goes: vortex ring, vortex ring, vortex ring; vortex ring, vortex ring, vortex ring. (In a football chant sort of way)

This is demonstrated to all Britmil helo pilots to highlight the dangers of manoeuvring at low level in windy conditions and to underline the need to constantly cross refer to instruments.

[Bertie Thruster]

....................and in the mountains on a windy day, or near a decent CB, you can suddenly turn downwind without even changing your heading...............................

[MaxNg]

Crab@svn has the stock answer and explanation nailed.

If you are lucky enough to fly a fully coupled a/c then try this and note autopilot inputs espacially on a windy day

IAS 60kts
Alt hold engaged
Hdg bug (set to North or any other Hdg)

Turn Hdg bug through to north again slowly so as to maintain turn direction (not more that 180 degrees of lead so as not to cause turn reversal).

Any ideas as to what happens next? and why?

Will cyclic move Fwd or Aft and why?
Will Collective rise or fall and if so why?



:D

Oh and secondly

There is no such thing as Downwind to the Rotor!!!!!!


So how can it be Dangerous?

:hmm:

[overpitched]

Maxng

You said....


Oh and secondly

There is no such thing as Downwind to the Rotor!!!!!!



That must really make you wonder why all those airfields have windsocks eh!!

And there certainly is such a thing as inertia.

[NickLappos]

overpitched,

Nobody said there was no such thing as inertia, what we have said is that the direction of the wind is of no significance at all, and when you measure the velocity relative to an intertial reference frame, it is all the same.

The idea that the aircraft depends on the wind relative to the earth is flawed, and that the aircraft, if turned quickly enough, would see some magical downwind loss of performance is quite flawed.

Imagine that you are heading east, and that the earth happens in that instant to be moving toward the Andromeda galaxy at 45,000 mph. Now turn 180 degrees, and just imagine what your "change in momentum" must be relative to Andromeda! This is EXACTLY equal to the false logic about the earth reference relative to the wind's direction.

There is NO downwind effect due to intertia, momentum, Newton's laws or even magical fairies.

Maxng,
You are absolutely right that the rotor sees no downwind. In fact, turning your tail to the wind in a hover will not cost you any performance at all in most helicopters. rotors are not wings, and helos hardly need the wind sock, except when it comes time to stop the darn machine.

[whatsarunway]

Andromeda Galaxy , what heading is that? and will the crp-1 compensate for the solar winds?

[heedm]

For the record, "downwind effect" is about maintaining ground reference when you should be attitude flying.

For the technically pedantic (such as myself) inertia will have small effects. Why keep demanding an inertial reference frame when a helicopter in a turn or flying through windshear is in a non-inertial frame? In a turn you're changing momentum. It takes energy to do that. In calm winds you can establish the tightest turn that the helicopter at max power can maintain for a long period of time. Some energy goes to drag, some goes to lift, some goes to turning the helicopter. When you do this in wind, you change the balance. There is now energy in the air. Like I said previously, it's a small effect. Try the test I outlined and you may be able to see some effect. Or maybe not.

In any case, always fly your turns with reference to your attitude and change your speed with reference to your attitude.

Is anybody else flying a mini Lego helicopter in circles around their keyboard?

Matthew.

[Aser]

Very good one Matthew! :D :O :=
Trying to follow this thread (in english) I've found myself "flying" my hand over the keyboard hahahaha , you guys are going to get me crazy :E

Regards.

[overpitched]

OK Nick. I acknowledge you as an expert so you are going to have to help me through this..... I had the same problem in senior physics when they told me that parallel lines meet and there is more than 360 deg. in a triangle.

I believe you I just want to understand it for myself.

Here's the thing. A couple of years ago I was flying down a gorge. It ran roughly N-S, it was about 1000' deep 1/2 mile wide and about 6 mile long & it wanders its way along. The wind was about 30knots from the southeast.

I was flying a kh4 inside the gorge doing about 55kias when I flew around a left hand bend into an area where the wind was being funnelled and my airspeed dropped to zero on the asi. Roll on throttle, lift collective, cyclic forward and cross fingers. Now the wind turned not me but either way it is the same to me ( I think)

The other problem I have is this

I'm flying north asi says 100 kias, gps says 60 knots now if I turn south but maintain my attitude and power etc initially my gps is going to say something like 60 knots still but the asi is going to say a lot less than 100 kias (isn't it ??) unless the turn is gradual and the airspeed increases whilst turning crosswind etc.

Now I read your last post and I know you said I was caught up in this reference to the earth thing but if I choose any fixed point anywhere surely I have one velocity relative to that point and the wind has a different one. Now if I change my velocity relative to that fixed point but the wind remains the same surely I have changed my velocity relative to the wind as well ?????? and if part of my aircrafts performance is dependant on airspeed then surely I have changed that as well.

Now I'm not saying you can feel this or see it at 20000' but isn't it still happening.

And if none of what I said happens then why do wqe worry about windshear.

Hope you can help Nick because from what I'm reading on this thread I obviously need it.

[Bertie Thruster]

All that fun the fixed wings never have........................

How about into wind along a Scottish glen, 150ft agl, IAS around 60kts, ground speed around 10-15 kts backwards, now try the downwind turn................

[[email protected]]

This is not rocket science chaps - the clue is in the terms "airspeed" (the speed at which you are moving through the air and measured by your Air Speed Indicator) and "groundspeed" (the speed at which you are moving across the ground and measured by doppler/GPS/INS etc).

On a nil-wind day your groundspeed will equal your airspeed whichever direction you fly in (assuming at lowish levels where TAS/IAS differences do not apply).

On a day when the wind is 20 kts from the North, when you are heading North at 60 kts Airspeed you will have 40 Kts groundspeed; when you are heading South at 60 kts airspeed you will have 80 kts groundspeed. In both situations, the power and attitude will be the same as the IAS is the same!

When you are heading East or West with the same 60 kts airspeed and the same 20 kts wind from the North, you will have 60 kts groundspeed.

If you turn from North to South using the same 60 kt attitude and a constant angle of bank, the airspeed will not change but your groundspeed will gradually increase.

Turns at low level are frequently carried out using external visual references because you are close to them and it is unnatural to fly on instruments close to the ground. It is a human failing, not a physics related one, that in such a turn from into-wind to down-wind, a pilot wants to keep his apparent speed (the groundspeed which he is judging visually) the same and so he does not maintain the correct attitude but subconciously adjusts it. He therefore loses airspeed as he turns downwind but only because he has changed the attitude to change the airspeed.

In turbulence or wind-shear, the strength and direction of the wind changes suddenly and the same effects are seen: If you are flying North at 60 kts IAS with a 20 kt wind from the North you will have a groundspeed of 40 kts - if, in an instant, the wind changes to 20 kts from the South you will suddenly have 40 kts IAS and 40 kts GS until the aircraft accelerates due to it's selected 60 kt attitude up to the point where you are at 60 kts IAS with 80 kts GS.

The reason that wind-shear and turbulence are so dangerous, especially at low level, is that the wind can alter constantly in both strength and direction so the pilots IAS is constantly changing and therefore the power required to maintain height is constantly changing. If you are at a low enough IAS (on approach for example) then the IAS fluctuations can cause you to lose ETL (which leads to increased power requirements and heading changes) and even leave you with negative IAS and positive GS.

The solutions are:
a. Never fly in turbulence or windshear (not very pratical)

b. Try to visualise the flow of air through/around the features you are flying over (at least the you can anticipate the turbulence)

and c. Constantly cross refer to instruments (this may save your life)

[Whirlybird]

Am I right in thinking that in the case of sudden changes of wind speed/direction, either due to gusts or windshear or sudden turns, the ASI won't be quite accurate, in some helicopters at least, due to the flow of air over the pitot tube being disturbed, lag in all instruments etc? I seem to remember that's the case, and it makes sense...but I'm not saying it for certain since I'm slightly out of my depth here.

[Chuck Ellsworth]

Yes Whirly you are right.

Wind shear and sudden gusts will be evident in airspeed fluctuation's.

Try throwing your rubber ducky in a fast moving stream and watching it react to sudden changes in the water flow in rapids and sharp changes of the direction of the flow.

Your helicopter like the rubber ducky will react in the same manner in the fluid called air.

Just as the ducky changes velocity, direction and bobs up and down so will your helicopter. If the ducky was powered his / her speed also would change in relation to the changes in the water speed.... and the change would be evident as it reacts to the sudden changes in water flow speed, once established in a new flow speed the indicated speed will recover to the same as before the sudder change in the fluid speed..

Jeesees I,m getting myself confused...but I'm sure you get the drift? get it? drift? rubber ducky down stream? He HE HE

Chuck

[[email protected]]

Whirly, the pitot tube is designed to receive a nice straight, undisturbed flow so the ASI can tell the difference between the static air pressure from the static vents and the static+dynamic pressure from the pitot tube. Only when the airflow is straight down the tube, undisturbed by rotor wash and any other turbulence does it have a chance of being accurate.

Unfortunately the rotor will almost always affect the free stream airflow on it's journey to the pitot tube no matter where you place it (this is why you see test aircraft with pitots mounted on a long boom to keep it clear of the downwash).
The pitot system is great for fixed wing that only need IAS indications at or above flying speed but not so good for helicopters that, ideally, would have IAS information accurate from zero to Vne.

Add the problems of sideslip to the equation and you further degrade the system accuracy - then really worsen it by not allowing the airflow down the tube to settle to one value due to turbulence. The result - not a very accurate way of assessing the airspeed of the helicopter, especially at low speeds.

[Shawn Coyle]

Here's my two (rapidly devaluing US$) cents worth.
If you don't maneuver in the vertical (that is, climb or descend), then what is said may be true. Once you start to maneuver in the vertical, (i.e. climb or descend), then things become a bit different.
Now you have to look at your total energy with respect to the earth, and groundspeed becomes important.
My experience is that if your airspeed is less than 4 times the windspeed (and this is only a rough guess), then the effect of wind is quite pronounced when you maneuver in the vertical.
The folks who see this most are model airplane fliers, and then ultralight pilots. Both operate at very low airspeeds, and any wind effects them dramatically. I've watched Cessna 150s depart from Mojave with a 35 knot wind and when they turn crosswind, they simply stop climbing. I've also had a Dash-8 crew tell me their experience when climbing into a 100 knot headwind at altitude. When ATC gave them a turn out the headwind, they said they literally fell out of the sky.
One last story- when the F-15 Streak Eagle set the altitude record, they did a wifferdill (specific maneuver to get to best climb speed at altitude) to go downwind with a 100 knot jet stream before they started their climb - this gave them more kinetic energy with respect to the earth.
So something gives. Somewhere there is a physicist who could put this to bed nicely. Anyone looked at See How it Flies for more discussion?

[Flying Lawyer]

'See How It Flies', which Shawn mentioned, is a useful reference source for principles of flight. Although it's written for light aircraft pilots, much of the material is of general application.

Link (http://www.av8n.com/how)

[Whirlybird]

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?

[NickLappos]

From 16 Flight Maneuvers, http://www.av8n.com/how/htm/maneuver.html#sec-infamous-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. "

[overpitched]

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.

[TeeS]

Nick

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

Cheers

TeeS

[rotordk]

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 ? ;-)

[Nigel Osborn]

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

[TeeS]

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

[rotordk]

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 ?

[overpitched]

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???

[the coyote]

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?

[overpitched]

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 ???

[[email protected]]

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.

[overpitched]

Crab

And your point is ??

[Bertie Thruster]

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."

[SawThe Light]

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?

[The Nr Fairy]

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 ?

[TeeS]

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

[overpitched]

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???

[Whirlybird]

overpitched,

I think I can explain it...

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


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.


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


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.

[[email protected]]

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.

[Whirlybird]

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.

[Shawn Coyle]

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?

[Chuck Ellsworth]

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

[Bertie Thruster]

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.

[NickLappos]

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.

[Chuck Ellsworth]

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

[NickLappos]

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??

[PPRUNE FAN#1]

Sayeth Shawn: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.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.

[TeeS]

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

[Chuck Ellsworth]

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:

[overpitched]

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.

[NickLappos]

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?

[Whirlybird]

overpitched,

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

[imabell]

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.

[overpitched]

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

[Flingwing207]

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....

[overpitched]

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.

[NickLappos]

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.

[overpitched]

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.

[overpitched]

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

[Woolf]

Hi All,


quote:
--------------------------------------------------------------------------------
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.
--------------------------------------------------------------------------------

As CSC rightly pointed out it always depends on the reference frame, e.g it depends on where you set your zero energy datum. Overpitched, you set yours fixed to the earth and thereby you are right in your calculations and with increased tailwind your energy will increase - with respect to your datum - the earth.

To illustrate, if you where to hit a mountain (which is bound to your reference point - the earth) you are quite right in saying that with a 50kt tailwind your impact will be a lot harder than without because your energy with regards to the reference point was increased.

In all energy systems we can chose the datum as we wish as long as we stick to this particular datum all the way. This is very important and I think the reason why you are a bit confused.

Lets look at speed vectors for a moment and to make it easier only at the speed vector along the aircrafts fligthpath at this moment in time.

I'll take your above example. You are travelling due south (although this has no relevance) at 100kts with nil wind. You are executing a rate one turn for 180 degrees to end up due North. Your speed vector is therefore changing from 100kts facing south to 100 kts facing north.

This means that in the turn, to overcome the forward momentum of the aircraft which obviously wants to keep moving in that direction (see Newton), the turning force (which is the changed Thrust vector at the rotor hub) has to decelerate the aircraft from 100 kts to zero speed (which would be at 90 degrees e.g either east or west depending on which way you go) and back to 100kts facing the other way. Remember I am only looking at the speed vector along the North-South flightpath axis and not at the aircraft airspeed! The deceleration to zero and back to 100kts equals an overall speed and therefore energy change of 200kts.

Now let's look at the 50kts tailwind scenario. Same principle applies. You start a rate one turn and this time (with reference point earth) the speed is 150kts. At 90 degrees (east or west) your speed is now not zero but 50kts going south due to the wind (still only looking at speed along North South axis) so in the first 90 degrees the speed has changed from 150kts to 50kts (100kts difference). The next 90 degrees of the turn changes the speed vector from 50kts south to 50 kts north at the end of the turn facing due north. This again equals a speed change of 100kts. So overall the speed and also energy change has been 200kts.

This shows that for the aircraft the same amount of energy is required to turn 180 degrees regardles of the wind. No extra power necessary at all. The only difference looking from our reference point (the earth) will be the shape and endpoint of the turn as we obviously will end up a lot further downwind in the second scenario.

I have recently tried this going around from a rig here in the north sea with winds of 65 kts and 75kts airspeed without any loss of height. If what you are suggesting would be true, the speed would have changed from 75kts to 10kts going downwind - below TL and the power requirement for that would have been very much higher.

Hope this helps, a drawing would probably clear things up a lot quicker. Maybe someone could do a nice graph?

Woolf

[[email protected]]

Flungdung - I agree, I think we've done this one to death now; unless overpitched is still not convinced!

[Flingwing207]

one more go - I'm pretty sure this won't work either...

Say you are in a C-5A flying along at 350 KT. What is the airspeed INSIDE the aircraft? If you were flying a little R/C helo inside the airplane, would it have a tailwind? Of course not. It would fly exactly the same inside the moving C-5A as it would inside a parked C-5A.

Wow - I don't really even get that one :hmm:

Oh well, I'm done.

[overpitched]

You're right flingwing.. not even close to working. But nice try

Crab. As much fun as this has been I think it was finished a couple of days ago However I would like to say in closing....

I feel my arguements were logical, well researched, well presented and also very convincing and if it was anybody else other than Nick lappos whom everybody knows ....

1. Is a guru &
2. Knows heaps more about this crap than me &
3. Has a cooler job.

Then I feel I would have won many supporters
:p :p :p :p :p :p :p :p :p

And if you really haven't had enough try the plane and the wind


http://www.physicsclassroom.com/Class/vectors/u3l1f.html

[Flingwing207]

...do you notice that in the website you posted, the velocities are always described as "relative to the ground", "velocity of the plane relative to an observer on the ground", "its speed with respect to an observer on the shore", and so on? No mention of velocity with respect to the air rather than airspeed, always referred to an observer "on the ground".

Any bells going off?
Anything?... ...Anybody?... ...Beuler? Ferris Beuler?

[overpitched]

Yeah you're right flingwing, as a matter of fact I took my boat out for a sail this morning and made the silly mistake of turning down wind and down current at the same time and as you pointed out the boat maintained its speed relative to the water and the sails maintained their speed relative to the wind. If only the current and wind were exactly the same I guess the sails would still be on the deck ????

Ever think that the site refers to an observer in a position fixed in space separate to the system you are observing because its relevant !

[Crashondeck]

When you are IMC you do not notice the wind (unless it is gusty or turbulent) - it is only when you can see a fixed point that you notice the wind. The helicopter still flies in IMC / at night and its characteristics dont change the moment you enter a cloud. Therefore using the ground as a reference for aircraft aerodynamics is irrelevant, in the same way that the seabed is irrelevant in ship design.

[The Nr Fairy]

overpitched:

"Ever think that the site refers to an observer in a position fixed in space separate to the system you are observing because its relevant !"

You need to understand that in the circumstances we originally started with - downwidn turns - it IS NOT relevant. Once you've got that hurdle over with, the rest is easy.

[NickLappos]

overpitched,

This thing about reference frames has you boggled, but let me try this:

In this case, there is no absolute energy or momentum. It is all relative to the reference frame you chose. Since you calculate the energy/momentum in one frame, you get confused and wonder where the extra energy/momentum came from when downwind vs upwind. You forget that the wind is a big contributer to the energy/momentum of the situation, perhaps because the air is so clear it seems to not be there.

If you are in a balloon drifting in a 50 knot wind, and you bump into a traffic sign, the energy/momentum is enormous, and you might be killed. If you bump into another balloon, the energy/momentum is small, and you say "Pardon me!". Why? Because these quantities are relative, not absolute, and are dependant on the reference frame you decide to use.

You have decided to stay in the earth reference frame, and calculate all energy/momentum that way. That is fine, just don't mix the numbers to suit your theory (which is that a turn downwind must cause a big performance problem). Where you get an F in physics is that you ignore the enormous energy pouring through your system from the wind.

If we take a 100 foot square of surface, and pour a 30 knot wind through it, we have a constant energy source of about 90 Horsepower, enough to entirely power a small airplane. But you simply ignore this energy/momentum contribution, and have done so for about 10 of your posts! Where does the aircraft get the ""extra" energy/momentum as it turns downwind? From the Wind!

[autosync]

Sorry folks, don't mind me, this is all to technical for my fragile little mind, however way way back Bertie Thruster asked

Why is it always bumpier flying (at the same IAS) into wind than it is going downwind?

My take on that is, compare the movement of air to the movement of the sea, if you ever sail into wind you are going to hit the wave head on and ramp over, but if you are sailing downwind its seems very smooth beacuse the wave is catching up with you, and you are basically surfing down wave,

Won't butt in on the curent debate, I am of the opinion of just be sensible be aware and just fly the damn thing.

[Bertie Thruster]

Crashondeck......I see how you got your name;

"using the ground as a reference for aircraft aerodynamics is irrelevant,"

[CBLong]

I'm sorry, I just can't resist sticking my oar in here.... :)

Overpitched, here's another thought experiment for you... imagine you're flying in calm air (ie no wind), circling round and round at constant IAS, bank, etc etc. On the ground below you, as far as the eye can see in all directions, is the world's biggest conveyor belt, which is currently switched off. An observer is standing on the conveyor belt directly underneath the centre of your circles.

Now, what happens if, just as you're turning through North, we turn on the conveyor belt and it starts moving North at, say 25kt? The ground observer and everything else you can see now appears to be moving North, or (more likely) you will believe yourself to be drifting South. Don't touch the controls! Would you expect to feel anything unusual through the seat of your pants, or see anything unusual on the instruments, as you continue circling?

I hope you'll say "no". Now, why not?

cbl.

[Crashondeck]

From a scientific point of view it doesn't matter what reference system you use, you should always be able to satisfy the laws of physics and get the same answer.

The simplest and most useful reference system in aviation is that parcel of air that an aircraft flies in. You could also use a point on the ground or even infinity as the origin of your reference system to analyse aircraft aerodynamics, but the maths is more complicated.

Take for example the police radar trap. The radar measures the velocity at which you are approaching the gun ie gives you a direct measurement of your speed. However, if you could measure your velocity relative to the centre of the earth and compare it with the velocity of the policeman relative to the same point you will get the same result - a £60 fine and three points!

But Overpitched has decided to use a point on the earth for his (can I assume that you are male?) reference point. Nothing wrong with that, except that you will get different answers because you are looking at momentums and energies associated with the aircraft and ground interacting - something we all try to avoid. I think what is puzzling Overpitched is that there is a change in the magnitude of aircraft momentum between up and downwind without a force to generate that change. That got me puzzled too, but I think I have the answer:

Lets assume an aircraft speed of 100kts, a wind speed of 30 knots and an aircraft weight of 1000kg. The reference point is an observer on the ground.

If the aircraft is flying directly into the wind, the ground speed is 70kts, giving momentum RELATIVE TO the Earth of 70,000kt.kg. In the same way down wind the momentum is 130,000kt.kg. But why the difference - how has the momentum changed by 60,000kt.kg by turning down wind without any extra forces acting on the system? We have to remember that we are using the earth as a reference system so when we apply aerodynamic theory to the aircraft we have to do it relative to the ground.

If we maintain a constant total rotor thrust vector by maintaining power and disk attitude, the only other horizontal force acting on a helicopter is air resistance, which is depends on velocity of the air flowing over and around the airframe. When using the air as a reference system, TAS is used to calculate skin friction and form drag. But we are using the earth as a reference system so we have got to account for the motion of the aircraft reative to the air and the air relative to the ground.

Into wind, the skin friction and form drag (relative to the earth) are calculated from the speed of the aircraft through the air PLUS the speed of the air over the ground (wind). In this case 100 + 30 = 130 kts.

Down wind, the skin friction and form drag (relative to the earth) are calculated from the speed of the aircraft through the air MINUS the speed of the air over the ground (wind). In this case 100 - 30 = 70 kts.

Hence the skin friction RELATIVE TO THE EARTH changes and that accounts for the change in the magnitude of momentum when using the ground as a reference system.

It is interesting in the above example that regardless of the reference system you use, the change in momentum for a 180 degree turn is the same - 200,000kt.kg. That's because in the turn the momentum is changed by the centripetal force acting towards the centre of the turn. This centripetal force is generated by the horizontal component of total rotor thrust and is therefore independent of wind or ground.

It makes sense to me, but I am still going to duck for incoming! :rolleyes:

[[email protected]]

Overpitched - does an aeroplane wing stall at a specific groundspeed? No but it does at a specific airspeed. If an aircraft flys past you (you are on the ground not moving) at an indicated 60 kts airspeed into a 20 kts head wind, it will fly past you at 40 kts GS. If it turns round and flys past with a 20 kt tailwind (still with 60 kts IAS) it will fly past you at 80 kts GS. As far as you are concerned the plane has twice the speed and therefore momentum because you are stationary on the ground. As far as the pilot of the plane is concerned he is flying at 60 kts airspeed both ways and the plane behaves in exactly the same manner (same power setting, attitude etc). His reference is airspeed (which is moving) and yours is a fixed point on the ground, which is not.

[CBLong]

Crashondeck,

Your last paragraph is the correct and complete solution to the mystery - I'd delete the rest if I were you... :)

In your 30kt wind example, you have missed the fact that momentum is a vector quantity. The change in momentum is from 70,000kt.kg in one direction to 130,000kt.kg in the opposite direction. The change is therefore:

70,000 - (-130,000) = 200,000

not 60,000 as you stated. If you change the example to a 30kt tailwind, you get a change from 130,000 to -70,000:

130,000 - (-70,000) = 200,000

As you stated in your last paragraph, the change in momentum is completely independent of the reference frame - you could be moving with the wind, fixed on the ground, or watching from a passing train - the change in the aircraft's momentum will always be 200,000kt.kg, and is the result of the same forces (tilted lift vector) acting in the same directions over the same periods of time, regardless of what the air is doing relative to the ground.

A bit of algebra may help further: stick with a 1000kg aircraft flying at 100kt IAS in a headwind of W knots:

Starting momentum = 1000 x (100 - W) = 100,000 - 1000W
After the 180 turn = 1000 x (-100 - W) = -100,000 - 1000W

The change is therefore:

(100,000 - 1000W) - (-100,000 - 1000W)

The two 1000Ws cancel out, leaving just

100,000 - (-100,000) or 200,000!

There is no "W" left in the answer, which is the key - the wind has no effect.

cbl.

[[email protected]]

CBL, but by the time you've worked all that out, you have hit the ground because you didn't maintain the airspeed as you turned downwind!

[SawThe Light]

O.K. Enough already.

We went flying yesterday and I forgot to take my calculator along. We were flying into a reasonable head wind and I was too scared to turn as I couldn't figure out if it was going to be safe to do so. Ended up coming back by train.

Can we just get on with something not so scary so those of us that are not mathematicians can get back into the air without being petrified with the apparent new physics laws.

STL

[Shawn Coyle]

So, let me get this right.
Nearly everyone is saying that if you stay in the same airmass, and it happens to be moving across the earth, that you won't see any acceleration or deceleration as you do a level turn.
So, your inertial navigation system wouldn't register any change then.
Better tell this to all the people using inertial navigation systems...

[NickLappos]

Oh Shawn! Of course your inertial will show the acceleration, it has a little window, and the guy inside peeks out to see.

[Ascend Charlie]

Inertial navigation? Too right!

The last lot of navigators I saw in the air force were pretty much inert.:zzz:

[Gibbo]

Velocity is speed in a specified direction; Acceleration is a change in velocity. Simple? :confused:

MY OPINION - Turning downwind at low level can be a risk when the pilot's perception of a dramatic increase in GROUNDSPEED is compensated by reducing AIRSPEED, which leads to descent, which may be further complicated by raising the nose to arrest the descent; which may result in a major reduction of airspeed if aviation rule No. 1 is breached.

(n.b. Aviation Rule No. 1 - whilst in flight thou shalt not make inadvertant contact with the ground) :ok:

[overpitched]

Nick..

I have always maintained that the wind provides the force that provides the acceleration in a downwind turn. I thought it was you that said the wind had no effect.

Now if the force of the wind provides the acceleration during a downwind turn is it not possible that given the right(wrong) circumstances eg low airspeed steep turn, that the acceleration is not quick enough and you loose airspeed.

I know a lot of people here keep talking about loosing speed low due to ground references but who has ever been taught to fly ground references. Surely in visual flight we fly attitude.

And if you maintain attitude and don't increase power and we know from conservation of momentum theory that it is not just the force applied but how long it is applied, so in a quick turn the force of the wind doesn't have a lot of time to act and this may contribute to the problem ????

[Crashondeck]

CBL - thanks for the comments. I realise that Momentum is a vector quantity by virtue of its velocity component and I should have mentioned that I was comparing magnitudes rather than the whole vector quantity. I was meerly trying to demonstate that when using the ground as a reference system, things get more complicated and more difficult to visualise.

Overpitched - Ask yourself this - What is wind? It is simply the movement of air relative to the ground. No ground = no wind. The helicopter does not know whether there is ground there or not but it still flies. So how can wind affect the aerodynamics of a helicopter.

[[email protected]]

This is starting to hurt!

The wind accelerates you as you turn downwind - but only with reference to the ground.

The wind does not accelerate you as you turn downwind - with reference to the wind.


Now no-one has mentioned coriolis force/effect yet where the turning helicopter would be deflected to the right in the Northern Hemisphere, or the red or blue doppler shift as you accelerate or decelerate relative to an observer at the edge of the universe (is he coming towarsd you or going away)?

[Whirlybird]

overpitched,

Are you playing devil's advocate? I can't believe you still don't understand this. Are you just giving everyone a hard time for the sake of it? ;)

[overpitched]

Whirly. I can't believe that you would think such a thing. ;) ;) ;)

And besides I know you guys are right because I did a simple experiment.

Tonight when I was at the gym I had a run on the treadmill and while I was running a long at about 7km/h I realised it was exactly like flying into wind. As long as I didn't look at the ground my speed was 7 km/h relative to the treadmill.

And because I know you guys are right I knew it was safe midstride to turn around and run with the treadmill. Yep you guys were right, I never ended up on my arse or nothin.

You guys should try it too. :D :D :D

[CBLong]

Have I been had?? :)

I used to read a motorbiking newsgroup years ago where the old hands would always wind up newbies by claiming that it's impossible to pull a wheelie on a shaft-driven bike... is the downwind turn the same thing for prooners?

:)

[TeeS]

Well done overpitched.

I knew you would get it in the end. A word of caution, treadmills at the gym tend to be quite short so turning 'downwind' can result in you falling off the end (a bit like turning downwind into a mountain - your frame of reference changes very suddenly!)

Try it instead on an airport walkway or similar, you will see that once again it is quite easy to turn round and run in the opposite direction. Just beware that there is a tendency that as you do turn round, the apparent change in speed of the advertising hoardings gives you the impression that you are speeding up. It is not unknown that people trying this, subconsciously slow down and end up stopping on the walkway (a bit like losing translational lift when turning downwind at low level!)

Cheers

TeeS ;)

Edited to sort my thoughts out (doh!!!)

[Bertie Thruster]

Unlike on gym treadmills, I tend to walk on airport walkways WITH the mat movement. So turning round would tend to make the hoardings slow down or even go backwards, rather than faster as TeeS has suggested.

Can you imagine it?!........."What's that tw*t doing on the walkway??"

"He's a Rotorhead Ppruner doing downwind turns"

[TeeS]

Come on Bertie, give us a break, I didn't say which direction you had to start off in, just suggested overpitched try his experiment on a larger playing field.

Anyway, I always enjoyed going up the down escalator, it's a bit like trying to make your point on this thread!!

Cheers

TeeS

[Whirlybird]

OK, overpitched. So, you made some of us who normally just fly and happily turn downwind without thinking about it give some very serious thought to airspeed, groundspeed, wind, Newton, forces, vectors, and all those good things we generally don't want to bother with. No harm done. But me, I think I'll just go back to flying. And if I ever get any students who insist on arguning about downwind turns...I'll send them in your direction. Purely becasue I know you'll enjoy it of course. ;)

[headsethair]

How many helicopter pilots does it take to fly downwind ?

1,000,000.

1 to fly the helicopter and 999,999 to stand upwind and talk bollox.

Thereby neutralising the effects of the wind.