Downwind turns equal disaster??
 

Having claimed more than a few copters over the years, I felt that this discussion over on the Technical Log forum was worth highlighting , especially to those Rotorheads "a little light in the logbook"

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

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

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!

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

Good idea, Nick. :ok:

So what do people think of downwind turns in helicopters?

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.

overpitched says

Bomber says

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

Overpithed:Here'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.

define what is "brisk wind"

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.

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

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:

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

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.

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

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.

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

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.

Back to Basics
 

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.

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.

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.

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

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!

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.

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.

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

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:

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.

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.

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

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.

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.

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.

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................

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)

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.

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

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.

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?

'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