Helicopter pitch change when orbiting around a fixed point??

[Alchef]

Today I was given a lecture about flying a recce of a confined landing site or flying an escape out of a steep sided mountain valley, starting at an into wind position and turning down wind..

I was told that in order to maintain airspeed whilst turning down wind, the pilot must change the aircraft's attitude. I have observed this previously and figured that it is some sort of optical illusion given that you are flying in relation to the ground, and not in relation to the air. For example a rate one turn flown at 60kts on the ADI has no pitch change, it just looks like an egg over the ground if there is any wind. He went on to say that this is only because we are operating at low airspeeds (40kts). I would say "Low airspeed relative to the wind."

The lecturer went on to say that when flying this circular pattern, if no pitch change was made, then the aircraft would run out of airspeed as it went down wind.

An example was given of a 40kt pattern with a 30kt wind. He quoted 1/2x4500kgx(70^2-10^2) as the energy change that the aircraft requires to make it round the turn to downwind without losing airspeed. He is working on ground speed, which surely has no effect on us as long as we are still flying? The only problem if we don't pitch change and use lever is that we will lose position around the confined area, or hit the mountain side.

So...

What are people's thoughts on this? Can you please say let me know if you are a test pilot, engineer, aerodyanamics expert or pilot.

Many thanks..

al

[Hughes500]

As Nick Lappos once said the disc does not care where the wind is coming from. Once above translational lift the wind will only affect your ground speed. If anything if you are trying to do a recce at say 40 kts groundspeed in a 30 kt wind then when you turn downwind you will have to reduce power to an airspeed of 10 kts, now this may result in a loss of translational lift which would mean you having to use more power than at 40 kts ( this is dependent on lots of factors) You may also have to use some power due to poor handling at slow speed downwind ( wind trying to weathercock you round)
Are you sure you understood what was being told ?

[L2driver]

I think Nick has been misinterpreted.
Don't forget forget that aircraft have mass - a lot of it. If you are turning downwind, the mass needs to be accelerated in relation to Mother Earth's gravity to keep the same IAS, and that takes energy - engine power or altitude. If the mass is not accelerated, you lose IAS and therefore usable energy. This applies to all aircraft, not only helicopters. Have a look at the speed vs power required curves you find in all Flight Manuals, from small helicopters to powerful fighter airplanes to big airliners.
Below Vy (best rate of climb speed, where you find the best lift over drag ratio), the power required increases with decreasing airspeed. Translational lift plays a minor role in this picture. In other words, if speed is reduced below Vy you need more power to stay airborne. Many helicopter pilots doing photo and similar missions have learned this the scary way.

Please read this report from the UK AAIB. This Super Puma, which is a fairly powerful machine, was turned downwind at a sustained low energy state with fatal results.

http://www.aaib.gov.uk/cms_resources/2-1993%20G-TIGH.pdf

I have made it my rule when flying offshore during high wind conditions, never to turn downwind at speeds less than Vy + 10, and then keep a close eye on the airspeed indicator during the turn and never let the IAS decay.

And yes, I am a pilot, test pilot and an accident investigator too.

:)

[Three Blades]

We have had this discussion before but I still find it interesting.
When I look at it from a Newtonian mechanics point of view I see that there is little argument for power change (as per the first post). However there are plenty of pilots with many more hours than myself who state that this is not the case.

As such I have this question:
If you execute the turn with a lot of pedal input, does this mean that the helicopter rotates within the airflow without accelerating through the circular turn and hence will require power for the downwind leg ? The alternative of a 'pedal free' turn means that (in theory) the helicopter accelerates around the turn and hence maintains the same IAS.

I have trouble with the accelerating mass theory relative to the Earth as mass is not related to gravity and this acceleration would still be required in space.

Clearly we have to assume flight OGE in all cases here as otherwise there is obviously a GS factor to be taken into account.
Also we cannot assume that you start from S&L flight but are in a constant turn as you will clearly need to raise power to initiate the turn.

I am looking forward to hearing the universally accepted argument to this one.

TB
(low time, engineering trained, pilot)

[L2driver]

We have had this discussion before but I still find it interesting.
When I look at it from a Newtonian mechanics point of view I see that there is little argument for power change (as per the first post). However there are plenty of pilots with many more hours than myself who state that this is not the case.


No need for power change? Wow, these plenty of pilots may not have noticed the altitude and/or airspeed they lost or most likely they did not notice the subtle collective increase they unconsciously did.


I have trouble with the accelerating mass theory relative to the Earth as mass is not related to gravity and this acceleration would still be required in space


Sorry - Mass is a term used in the JAR-OPS/EU-OPS these days. Please substitute mass with weight which is relative to gravity for people to understand.

In space?? There is no aerodynamic lift, no wings/rotors required, no downwind turns, but still a requirement for additional energy if you change the flight path vector.

As such I have this question:
If you execute the turn with a lot of pedal input, does this mean that the helicopter rotates within the airflow without accelerating through the circular turn and hence will require power for the downwind leg ? The alternative of a 'pedal free' turn means that (in theory) the helicopter accelerates around the turn and hence maintains the same IAS.


This is interesting; First of all. if you do the turn with a lot of pedal input, you are probably flying uncoordinated which at some forward speed will require additional energy (power).

There is no difference from the earlier scenario if you are going into a downwind flight situation, the aircraft still needs to be accelerated using engine power or altitude. Nothing is free.
:)

[Three Blades]

L2 Driver, do your comments apply to a constant circular turn (ie constant attitude, IAs etc) or a constant orbit relative to the fixed point ?
If the latter then I fully agree that power changes will be needed. But I am still struggling to see how they are needed for a constant rate turn (except the initiation of the turn).

So, going back to the first post, should we take the phrase "whilst turning down wind" to mean that a change in turn rate or similar is taking place or that the helicopter is just in a given orientation as part of a constant turn ?

In practice, if you are doing an orbit about a LZ then I assume that this will involve constant changes in power as it is not a constant rate turn unless there is no wind.

[B540]

Mr. L2 has it right. I'm a Test Pilot but more importantly I've spent a fair amount of time in the mountains on the "seismic trail". What this instructor was probably talking about is making a lz recon while keeping the aircraft somewhere close to the bucket speed (thats how I do them). Lets say it's 45 KIAS. While traveling into a 20 knot wind, the power and attitude required will be the same regardless of the wind (the disk don't know) however your groundspeed will be much lower. As you turn downwind the IAS will drop rapidly (this is readily observable and in fact is a technique for determining wind direction and velocity). The reason it will drop is because the mass/inertia of a system (the aircraft) is not a function of aerodynamics. If you were to maintain the exact power/attitude the aircraft would eventually work its way back up towards the original IAS (a slight increase in power would be required to accomplish the acceleration) but the important aspect in this discussion is that it would take a considerable amount of time, which if you are at relatively low altitude in the mountains, you don't have. This is why you want to make that power/attitude change as you begin the downwind turn, that way you can keep the KIAS up close to that reference airspeed, and keep the aircraft from dropping out from under you. This is why I like to use
bucket speed, it gives you the most excess power available for the maneuvering necessary for low altitude/high DA operations.

[L2driver]

L2 Driver, do your comments apply to a constant circular turn (ie constant attitude, IAs etc) or a constant orbit relative to the fixed point ?
If the latter then I fully agree that power changes will be needed. But I am still struggling to see how they are needed for a constant rate turn (except the initiation of the turn).


I see where you are going. You are right in that you have to increase power to make up for lost ground speed when flying into the wind and decrease power flying with tailwind to make that circle circular. That is however not what I am talking about. I am talking about the need to release stored energy (engine power or altitude) to keep your IAS in relation to the air around you to keep you flying safely.

If you are flying a constant f. ex 45 degree banked turn without any fixed ground references, the the energy requirement will change constantly during the turn. They will increase turning downwind and decrease turning upwind. This is of course only valid during accelerations (Acceleration= velocity vector change, not only speed change). This is due to the fact that we live on Earth, and all accelerations need energy.

I have experienced this a lot of times flying compass swings on the AS332L2, where you are required to keep a constant bank of 30 degrees and thereafter 45 degrees in both directions with a constant airspeed. I have done this at 50 kts winds and I can assure you that even when using autopilot, the required power, manually or automatically, changes a lot during the turn.

So if you want to keep a very constant IAS and complely level altitude, you need to vary your engine power. Simple as that.

:)

[Alchef]

Thank you for all your input this afternoon!

Here's some more thoughts/questions...

When I turn back into wind, will I need to reduce power and raise the nose?

The aircraft does not know what the wind speed is, so why would it's flying characteristics change? It surely has to be something to do with flying relative to a point close on the ground, unlike to a distant horizon that you use for basic training?

Is this also true of a fixed wing aircraft doing a recce of a point over ground?

What about a sailplane? He can't just increase power when turning downwind.. and I don't think they would descend faster just as they turn downwind and conversely climb into wind? If they did, again this would surely be an optical illusion.

[L2driver]

I enjoy this:)
To start with the last: sailplanes are no different from other aircraft, they normally don't have an engine, so their energy management is using altitude. If you want to go fast, lower your nose. Want to slow down? Get the nose up. I'll let you in on a secret: I have flown them too. Of course
they have speed-brakes, but that is for bad planners and landings in tight spots.

Back to the post:
When I turn back into wind, will I need to reduce power and raise the nose?

That will depend on what you want to do. If you are in the traffic pattern, get your airspeed right. If not, take your time.

The aircraft does not know what the wind speed is, so why would it's flying characteristics change? It surely has to be something to do with flying relative to a point close on the ground, unlike to a distant horizon that you use for basic training?


The aircraft does not care about wind speed or ground speed, all it knows is Indicated Air Speed, IAS (and of course for you nerds CAS and TAS).
The flying characteristics changes only with air speed, not wind or groundspeed.


Is this also true of a fixed wing aircraft doing a recce of a point over ground?


Helicopter, fixedwing, spaceship- same thing (I think)

:)

[Alchef]

When I turn back into wind, will I need to reduce power and raise the nose?

I want to maintain 40kts IAS whilst flying around a fixed point on the ground, or turn back into a tight valley to try again at making an approach that I've just overshot...

[MightyGem]

Sorry, the only reason that you are gaining/loosing airspeed at low level is the influence of the ground being close, and your are sub consiously adjusting for groundspeed.

Climb away from the ground, say to 2000', in strong wind, say 30kts, and assuming that you can, set the trim to a constant speed, say 70-80 kts. Trim in 5 or 10 degrees of bank. Adjust the power to maintain height. Let go of the controls. The aircraft will fly around in circles at your set speed, plus or minus a knot or two for gusts.

[L2driver]

BS, this has nothing to do with altitude. where did you go to school??. Give your opinions to the families of the victims of the Cormorant Super Puma accident.

Yeah - trim it up, use IAS hold plus ALT hold and then look at the FLI or whatever instruments you use for power management during the turn. The power changes as soon as you have accelerations which is about immediately. The problem with most know-it-alls is that they don't know how to fly accurately.For C***** S**** use the FDR or FDM if you don't believe.

[puntosaurus]

Steady on chaps. Not to comment on the merits of your argument L2 (although I remain to be convinced), your accident report does not back up your thesis.
Climbing to a height of 250 feet and whilst turning downwind, the handling pilot, who was also the commander reduced power and raised the nose of the helicopter such that the airspeed reduced to zero and a rate of descent built up.
In other words it was pilot action which caused this crash, not inaction.

[puntosaurus]

And having thought about it, I'm happy to challenge your physics.

At a constant rate of turn at a constant altitude, there is a constant acceleration towards the the axis of the turn, and a constant acceleration upwards to match gravity (whose vector is hardly likely to change over this scale). There is no energy change and no requirement for power change.

Now as three blades said, though L2 egregiously misquoted him, many pilots DO record a change in power requirement when turning downwind. In that case the physics must be a little more complex than L2 implies.

[B540]

If you want to maintain the exact airspeed, then yes, you will want to decelerate. The reason few people teach techniques for the 'into the wind' turn is that it is not nearly as critical or time sensitive. You don't want top get too involved in the "math" of why you're doing things until you have a good understanding of the techniques. The primary concern when flying in high DA, precipitous terrain, at high GW is that you maximize performance and efficiency (for the customer, that means flight time) while keeping the aircraft from hitting things (like trees, rocks, etc.)
One of the common mistakes is making a downwind turn at relatively low speed and having the aircraft build a high rate of descent as the airspeed drops and power requirement go up, without the altitude necessary or excess power available to arrest the rate or "fly out". It happens very quickly in the high wind conditions found in mountanous areas, and the aircraft is usually being flown at the limits of performance because of environmental conditions/customer requirements. In the Rocky Mountain region, the average wind speed on many days could easily equal 50-60% of your desired manuevering speed. You have to account for that by adding a little cushion (V+10,20,whatever) and by anticipating the changes in power required accurately and quickly. The other critical requirement is that you be able to read the terrain and its effect on the wind. In many areas the terrain actually dictates wind direction, velocity as much or more than the free stream/prevailing weather. One technique that I have taught is to try to visualize what effect the terrain features would have on a flowing mass of water. The atmosphere acts in exactly the same fashion.

[L2driver]

My friend
At a constant rate of turn at a constant altitude, there is a constant acceleration towards the the axis of the turn, and a constant acceleration upwards to match gravity (whose vector is hardly likely to change over this scale). There is no energy change and no requirement for power change.


A constant turn is acceleration. You are changing your velocity vector. I have flown fighters for many years and I can tell you that a 4G turn requires a lot more power than just cruising along at 40 000 feet

Anyway, regarding the AAIB report:

b. Causes

The following causes were identified:

(i)The handling pilot's failure to recognize the rapidly changing relationship between airspeed and groundspeed which is a fundamental problem associated with turning downwind in significant wind speeds

(ii)The Commander, who was the handling pilot at the time, shortly after takeoff inadvertantly allowed the airspeed and then the height to decrease whilst turning away from a strong gusting wind

(iii)Despite the application of maximum power, the helicopter was incapable of arresting its established descent within the height available. Incipient Vortex Ring state and down draughts may have contributed to this problem, as may the height of the wave crests.

Can we now agree that the turn downwind was a factor?

[puntosaurus]

Er.... QED. Nothing wrong with your last post or the report, but the points in my previous post stand.

[L2driver]

Great. Let's go fly one day followed by a beer. You pay of course:)

[puntosaurus]

I try and steer clear of trolls in my drinking time.

[lelebebbel]

an aircraft flies at a constant power setting, straight and level in calm wind, airspeed = groundspeed = lets say 100kts.
Suddenly, the wind pick ups to a 40kt headwind.

Does the airspeed increase momentarily? Of course it does. The inertia of the aircraft will keep it moving at 100kts groundspeed and a higher airspeed will be observed for a moment (how high depends on how quickly the wind picked up), until the aircraft has slowed down to 60kts groundspeed, 100kts airspeed again.

For the same reason, you WILL see a momentarily increase of airspeed when turning into the wind and a decrease when turning downwind. It's not just perception, it works whether you look out the window or not.

If the inertia of the aircraft did not make a difference, why would the airspeed indicator jump all over the place when flying through turbulence?

[AnFI]

Terrifying that self declared 'test pilots' don't seem to understand the basics of their craft.

They have been able to ply their trade with out understanding:

Relative velocity.
Conservation of enery.

L2Driver and B540 and lelebbel urgently need to rethink their understanding of the dynamics of flight.

A circle flown in the air at a constant airspeed (80kts) , above cloud with say a 70kt wind, is just a constant speed circle in the air!

Unless the aircraft can 'see' the ground it doesn't know it has to get faster or slower over the ground - it does not need 'extra energy' on the 'downwind leg' it's airspeed is unchanged. Its speed is unchanged just a constant 80kts.

The kinetic energy is Relative to the medium against which the force will be applied ! (amazingly) So the 'useful' kinetic energy to a pilot is a function of Airspeed NOT groundspeed.

You can't use the 70kts plus 80kts - 150kts of ground referenced kinetic energy (in the above example) to do a bigger pullup than the 80-70 = 10 kts ground speed kinetic energy when flying at 80 kts in the 'other' direction. It is 80kts both ways.. same pullup - same flight characteristics (just different ground result - if you can detect the ground position)


MIGHTY GEM said it more elegantly - but nobody listened !:D

Now pilots (who don't understand that) might get confused -

[Nigel Osborn]

If you are walking along a pavement at say 1mph, your legs will move at a certain speed to achieve 1mph. Now if that is a moving pavement at say 4mph & your legs still move at 1mph, then you are going at 5mph one way or 3mph the other way, depending in which direction you are facing. The speed of the pavement does not effect the speed of your legs & I think the same could be applied to flying by replacing the pavement with air.:confused:

[AnFI]

Nigel ...

they've got that - I think.

but they're trying to say that you'll have to put in some more effort to do 1 mph to 'accelerate' in the 5mph direction and will need less effort when turning to walk at 1mph in the 3mph direction on the moving walkway....

that is the dangerous idea - eh?

[B540]

actually if you're walking along at 1 mph on a 5 mph moving sidewalk and then step off or come to the end of that 5 mph sidewalk those 1 mph legs had better get stepping or you get a good example of what inertia is. Imagine an aircraft at a 5ft stationary hover, with a 25kt wind. The wind suddenly stops. What happens to the aircraft. Does it remain at 25 KIAS? Does it instantly accelerate to 25 KIAS? What it does is hit the ground, because power applied is less than power required. With enough altitude it would accelerate to 25 kts but at a 5ft hover, the time required is more than the time available. Low level maneuvering at relatively low airspeeds with wind speed or gusts that are significant require the techniques mentioned if you want to use the aircraft more than once. That much I'm sure of. Enjoyed the discussion, you guys stay safe.

[AnFI]

B540
Yup that's true - but a different thing. If the 'wind' changes then you recieve or lose energy. but if you run circles inside a cruise ship - you cant tell which way the ship is travelling - no kinetic energy difference (relative to the ship)

Surprisingly
Autorotation downwind is, in energy terms, more useful - since the wind sheer 'adds energy' - from the reduced tail wind (equivalent to increased headwind)


WHO WAS GIVING THE LECTURE referred to AT THE BEGGINING OF THE POST - was it military?

[L2driver]

Terrifying that self declared 'test pilots' don't seem to understand the basics of their craft.


Actually the Air Force and then my company declared me a 'test pilot'. I have the papers to prove it.

You can't use the 70kts plus 80kts - 150kts of ground referenced kinetic energy (in the above example) to do a bigger pullup than the 80-70 = 10 kts ground speed kinetic energy when flying at 80 kts in the 'other' direction. It is 80kts both ways.. same pullup - same flight characteristics (just different ground result - if you can detect the ground position)


Just for the fun of it: If you have two people throwing a ball of the same weight. One is throwing it from a standstill, the other one is throwing it from a truck travelling at 60mph. Which ball flies the longest distance?

Please read the G-TIGH accident report again. In appendix F, FDR readout, you can see that the IAS went from ca 73 KIAS to about zero at the same time the helicopter turned out of the wind into a downwind heading. Now, why did the speed drop?? because of low altitude and below clouds? It happened because the helicopter had mass and it tends to stay at the same GS. So going from a headwind condition to a downwind one, they lost IAS. When IAS drops below Vy, which in a S.Puma is about 70 KIAS the power required will increase. During this accident the power stayed at 14-15 degrees of collective pitch, which normally will keep the S. Puma flying nicely, but not at very low airspeeds.
This is called being behind the power curve and it applies to all aircraft. Less speed = more power required. If you are in this state, and speed is decreased even very little, the aircraft will not revert to the speed it had before unless you add power. It will decelerate, this has killed people numerous times.

I have the utmost respect for the UK AAIB, and they got it right.

(i)The handling pilot's failure to recognize the rapidly changing relationship between airspeed and groundspeed which is a fundamental problem associated with turning downwind in significant wind speeds


That is all from me on this, careful with that turn after takeoff:)

[Three Blades]

How about this scenario.

I am in a closed stationary train and flying a model helicopter around a smooth circle. Nothing complex yet and the power is constant.

I stop the flying. The train accelerates to 100mph. Once it is steady at this speed in a straight line I start to fly my helicopter again.

Surely the power and smoothness of orbit in these two cases is the same despite the helicpter flying downwind (or even backwards) relative to the Earth.

Is this not akin to flying compass swings in a strong wind that L2 describes.

Why does one exhibit varying power and the other not ?

As I said initially, I am not saying that anybody is wrong here, I am just trying to understand where the difference comes in.

Thanks
TB

[Hughes500]

I have to say that when I teach 30 degree turns the following happens everytimein a 300. Set 20" mp,60 kts, roll into the turn, by about 45 degrees into turn nose will start to drop. Add .5" mp the nose will come back to 60 kt attitude and the ac will go round and round in circles at a constant height and speed all day. Tried this in loads of different conditions always the same so seems to show that wind does not make a difference

[L2driver]

Can't help it...

Train scenario: As long as the helicopter model is kept inside the cabin -yes the model helicopter will behave exactly the same stationary or at speed as it is not exposed to the relative wind on the outside. Same as a fly in an airliner at Mach 0.82. There is no acceleration except that the model or fly do in relation to the environment. Same as us all living on Earth.

Hughes 300 scenario:
Yes you are correct as long you are on the "right side" of the power curve. The aircraft will correct itself back to the initial speed due to a "good" L/D ratio. Try this at 30 kts with 20 kts wind, and I think you will find it different

What is dangerous here is that some people don't understand the relationship between GS and IAS and energy required. Most posters have elected to ignore the dynamics of slow speed flight, i.e. flight at speeds below best L/D. At higher speeds, the aircraft takes care of itself. At lower speeds, it does not unless you are on 4 axis autopilot.

Energy in aircraft is either altitude or engine power (and you might add your current speed, but that is temporary). If you are out of one of these , be very careful. If you are out of two or more, you should not be there in the first place.:)

[HuntandFish]

I only fly model Helis so risk the wrath of proper pilots by posting here so apologies in advance .

I am talking about decent sized IC engined models with a full set of controls inc a govenor to keep constant head speed .
I often do walking speed manouvers out of ground effect but low enough to have the ground as a reference . In a flat calm pitch change is only really needed to maintain constant height if airspeed changes (no wind so airspeed =ground speed )
A breeze particularly if gusty needs contant and quick attention to pitch to maintain height and groundspeed particularly with slow turns .

[L2driver]

No need to apologize,- same dynamics.
Your model would climb and descend according to the wind because of the before mentioned power required curve.

However, if you made your model take off into a 15 kts wind, and then made an immediate 180 degree turn, I think you will find out what I mean.:)

[Nf stable]

HuntandFish, once again you are referencing to the ground.

L2driver, I'm trying to keep an open mind either way here, but I think you conceded to the opposition here

Train scenario: As long as the helicopter model is kept inside the cabin -yes the model helicopter will behave exactly the same stationary or at speed as it is not exposed to the relative wind on the outside. Same as a fly in an airliner at Mach 0.82

Surely if we take the parcel of air inside the train to be a free stream air flowing over the earth, then this is exactly how we fly everyday.
I also feel you mised a point made by the AAIB:
(ii)The Commander, who was the handling pilot at the time, shortly after takeoff inadvertantly allowed the airspeed and then the height to decrease whilst turning away from a strong gusting wind
If the wind is gusting, and most wind is to some degree, then as we turn into/out of it, we will experience a change in airpeed, but no more than we would experience anyway in level flight. The only difference I can see is that we find it harder to judge/cope with due to the constantly changing relative direction.

[HuntandFish]

L2driver
If I take of with 15kt head wind and hold constant postion hover I need to drop the nose a bit as I have a 15kt forward airspeed .
Turn 180 and quick corrections neccessary to hold as I have changed airspeed tp-15kts .
Is that what you mean .

[Three Blades]

Why is the model heli in the train or the bug in the aircraft any different to a real heli that is well OGE doing a constant 60 degree bank turn (and hence spiralling down wind when looked at relative to the ground) ? None of them are 'feeling' the ground in any different way as they are all moving in an orderly manner relative to the airmass.

[L2driver]

Not really
Unless you let the airspeed build up nicely and make a gentle turn out of the wind with sufficient power on, I think that you will find the entire exercise very expensive.:)

[chopjock]

This "model" scenario is all related to the pilot on the ground. Obviously with the pilot on the ground, the pilot will use a given power for flight into wind and will most likely notice the model fall out of translational lift on the downwind turn, so will instinctively add power to prevent this. So therefore the model pilot will be constantly modifying power to maintain level flight with reference to the ground, because that is the only reference he has.
The full size helicopter pilot flying a constant airspeed turn without reference to the ground will not require a power change just because he is going downwind, there is no (relative) downwind.:)

[L2driver]

I am sorry that I have to remind you all, but this thread started with: Helicopter pitch change when orbiting around a fixed point??

And to Chopjock: why don't you show some respect to people that were killed in the G-TIGH accident. According to you the AAIB is completely wrong.
Read the report for God's sake:ugh: And the previous posts too!:ugh:

I am seriously worried about the lack of basic physics knowledge and airmanship a lot of posters show on this forum. We are supposed to be the Professional Pilots Rumour Network, not the idiot commentators we find in the media elsewhere.

[L2driver]

NF stable
You are quite correct in most of what you say.
However:

The Commander, who was the handling pilot at the time, shortly after takeoff inadvertantly allowed the airspeed and then the height to decrease whilst turning away from a strong gusting wind


When the handling pilot allowed the airspeed and then the height to decrease whilst turning away from a strong gusting wind it means he did not increase the power to a sufficient level.
And this is the crux of the matter gusts or not, they did not have (use) the energy they needed to keep flying.:)

[Three Blades]

L2, when you do compass swings, are these orbits relative to a fixed point or just constant rate turns ? I understood from your earlier post that they were the latter and that you said that these required constant changes in power.
Have I misunderstood ?

If they are constant rate turns then please can you explain why they need power changes as I am still stuck on this point as I see it as the same as the model in the train as there is no reference to the fixed ground.

Thanks

[L2driver]

They are constant bank angle turns. The requirement in addition making the bank constant is also to keep the IAS constant at 100 KIAS.
You just fly figure 8's and maintain these conditions until the aircraft system tell you it has the data it needs. Very much like advanced compass system in boats these days.

And again, the L2 has mass, that needs to be accelerated to keep the same IAS going into downwind. See post 27 in this thread

[Rotorbee]

That is one of the big mysteries of flying and has confused quite a lot of people.
Let's take a trusty old C150 and practice turns around a point as required for a checkride. Normaly there is some wind therefore I have to correct for it every moment of the 360° turn. I start into the wind and do my first 90°. The wind is pushing me toward the point on the ground and I have to BANK a bit less than in a no wind condition ... and so on all the way round. Now, what does change when I fly like this? Airspeed? No, because I don't change the power setting and I do not climb or decend (apart from a bit less speed while having a higher bank angle, but that does not really matter). Groundspeed changes? Yes. Do I care? No. Groundspeed has nothing to do with the aerodynamics of my plane or helicopter.
When I do a rec for a landing spot, I should do a circle around it. Is there any difference then in a plank? No. I just bank more or less depending where the wind is pushing me, but I do not change airspeed if I do not climb or descend or change the power setting. Therefore you can not crash due to loss of airspeed while doing normal circles around a point, because the airspeed does not change when you do it correctly, only the bank angle.
Now, if you fly on a extremly windy day, you probably have to hurry turning while going downwind to keep the point on the ground in sight and bank angles will come close to aerobatic limits, but airspeed would still not change a bit.
... an now something completely different ... hopefully.

[chopjock]

I am sorry that I have to remind you all, but this thread started with: Helicopter pitch change when orbiting around a fixed point??

And to Chopjock: why don't you show some respect to people that were killed in the G-TIGH accident. According to you the AAIB is completely wrong.
Read the report for God's sake:ugh: And the previous posts too!:ugh:L2 driver, Calm down matey. I was discussing on topic, not the G-TIGH accident.

[MightyGem]

Sorry, L2. Your theoretical knowledge is far superior to mine(as is most peoples, it has to be said :().

Anyway, I've just landed. We've got a bit of a breeze up here tonight, and at 1000' it's 30 knots. You're right, of course, altitude has nothing to do with it: it just gets you out of the turbulence zone, and as we are on the coast, 1000' is fine.

So, S&L, 65 kts, 1000'. Trim into a 10 deg banked turn. Now, as you know, the nose is going to climb or drop slightly, depending which way the turn is. Trim adjusted slightly for that, and hands off.
Yeah - trim it up, use IAS hold plus ALT hold
No IAS or ALT holds, just the trim, and around she goes. Speed stays the same plus or minus 2 or 3 knots. Did the same at 80kts with 20 deg of bank. Nothing different.

Now, I spend alot of my time going around in circles, so this is not a strange occurrence.

[MightyGem]

Also, I was a bit puzzled about your comment on gliders:
Of course
they have speed-brakes, but that is for bad planners and landings in tight spots.

As a glider pilot you would know that they are not speed brakes, but are used to control rate of descent and are used in all landings, whether a tight spot or not, and not as a result of bad planning.

[puntosaurus]

L2 I'll give you the benefit of the doubt. And again, the L2 has mass, that needs to be accelerated to keep the same IAS going into downwind. See post 27 in this thread
You keep making good points that everyone accepts about ground speed illusions, being on the wrong side of the power curve, and the energy impact of windshear (gusts). But then you exhibit this confusion about acceleration.

This acceleration that you feel the helicopter has to undergo when turning downwind is relative to what ? Early on you seemed to think it was relative to 'mother gravity', and hopefully I skewered that when I said that the gravity vector is not going to change over these distances, it'll still be straight down. Now from the quote above you seem to imply that the acceleration has to be relative to the air, but why ? Especially if you accept the model helicopter in the train scenario.

Your airmanship views concerning the downwind turn are smack on the button, but your physics is a bit rusty.

[n5296s]

I've been resisting this one, but my strength ran out... actually I think both sides are right, subject to the appropriate context.

Of course the aircraft does have inertia relative to the ground. So if the wind changes instanteously, there will be a change in IAS for the time it takes the (new) force of the wind to effect an acceleration. It would take more tedious math than I can be bothered with to work out how long it would take, but based on what it's like hovering or doing pedal turns in a gusty wind, I'd say not long - a second or two max.

In a steady wind and a normal banked turn (heli or fixed), the time it takes to change direction is significantly greater than this wind acceleration time, so it has no noticeable effect. Otoh the visual effect of the changing ground speed can be very striking, especially at lower speeds typical of a heli. The first time I flew a heli in the pattern with a 30 knot wind at 1000', I found it very noticeable and it took a conscious effort to focus on the airspeed indicator and not the ground. (Much less of an issue in even a small plane at 80 knots than in a heli at 60). That is what (so they say) causes problems for low-time fixed wing pilots - they see the ground suddenly rushing by and they reduce power or pull on the yoke, and stall.

Gusts are a different matter. A sudden gust or wind-shear can turn 60 knots into nothing in a fraction of a second. Airliners have fallen out of the sky because of this on several occasions (higher absolute speeds of course, but 727s don't hover very well anyway) - although their extra mass and much smaller ratio of effective frontal area to mass probably makes it a lot worse for them than an R22 or a 152.

If you were OGE at 1000' and had a sudden 60 knot wind change (i.e. 30 knots N turns to 30 knots S), I guess you'd feel it but you'd have time to add collective and unless you were watching the altimeter like a hawk it probably wouldn't be big shock. Same thing at 30 feet or 3 feet and it could indeed spoil your whole day. (I confess I didn't read the G-TIGH report so I don't know what their altitiude was).

My 2c, off to bed now...

n5296s

[RVDT]

be Dragons!! (http://www.gbfs.co.za/images/Dragons.pdf)

[farsouth]

RVDT -thanks for that link. The article it leads to is in my view a very clear and correct description of the whole issue.

I have spent the last couple of days trying to frame my response to L2's mish-mash of truths, half-truths, and downright misunderstanding of the situation described, but the above article answers all the issues.

I particularly take issue with his interpretation of the TIGH accident report, and as for

"I am seriously worried about the lack of basic physics knowledge a lot of posters show on this forum. We are supposed to be the Professional Pilots Rumour Network, not the idiot commentators we find in the media elsewhere."

All I can think is pot....kettle......black........

(Please note I have deleted airmanship from the above quote, as I am not questioning that, only the physics aspect)

[MightyGem]

I particularly take issue with his interpretation of the TIGH accident report
Para 2.4.1 on page 47 of the report states that the accident was probably caused by the pilot maintaining a constant groundspeed as he turned downwind. I don't suppose it helped that the "ground" was moving as well. :(

[pilotmike]

Oh, the irony, as L2driver smugly declares:I am seriously worried about the lack of basic physics knowledge and airmanship a lot of posters show on this forum. We are supposed to be the Professional Pilots Rumour Network, not the idiot commentators after such mumbo jumbo nonsense as:
Sorry - Mass is a term used in the JAR-OPS/EU-OPS these days. Please substitute mass with weight which is relative to gravity for people to understand....

sailplanes are no different from other aircraft... I'll let you in on a secret: I have flown them too. Of course they have speed-brakes, but that is for bad planners and landings in tight spots....

With such awful knowledge of basic physics and sailplane operation, his mocking tauntwhere did you go to school??. of another contributor is laughable.

His claim to be "a pilot, test pilot and an accident investigator too." is worrying. "Actually the Air Force and then my company declared me a 'test pilot'. I have the papers to prove it."!!!!:eek:

More worrying is the way he uses this forum to belittle others whose knowledge of the physics of flight is far greater than his.

But most worrying of all is that other more impressionable readers might well fall for his arguments, and believe the same nonsense that he clearly believes and spouts as fact.

As for expecting others to buy him a beer for the quality of his teaching -pah! It really is about time that 'idiot commentators' and their bunkum physics misunderstandings are 'brought down to earth with a bump'.:=

Never mind L2driver - I'm sure the big expensive Breitling still impresses everyone even if your physics doesn't.

[Arm out the window]

Good one, n5296s. I think you've explained it nicely.
Steadily moving airmass which the aircraft turns with respect to, no appreciable change in IAS in the turn.
Rapid changes (ie gusts, shear), or pilot reacting to slip/skid illusions or trying to maintain position with respect to a ground feature, then we get IAS changes.

[Two's in]

dY IAS when referenced to GS always has a value based on mass - rate of change is not instantaneous.
dt

[imabell]

l2driver, self praise is never a good recommendation. you prove that adage many times in your posts.

having a beer with you even if you paid, i would imagine, would be one of the most boring things a person could do. let me know what pub you drink at so i can avoid it at any cost.

pompous, arrogant and demeaning to your peers probably made you quite the outcast at school and a target for practical jokes.

i can't agree with you even if you are right and i don't think you are.

post some where else please mr "test pilot" and leave us to our ignorance.

gg

[RVDT]

AS 332 or Connaught?

L2 Driver - http://farm4.static.flickr.com/3201/2888421224_337c51088f_m.jpg

[jimjim1]

For some further reading on the effect of wind on instantaneous airspeed in the case of a turning aircraft (the aircraft having inertial mass - 'inertial' for emphasis, mass would suffice of course) have a look at Dynamic Soaring.

This is a technique of imparting energy to an aircraft (or bird) by utilising the differing wind speed in different envionments and by manoeuvering the aircraft between the environments. Apparently this is the method that albatrosses use to cover 1000s of miles across the oceans without significant effort. In the latter case the variation of wind speed in the troughs and crests of waves is utilised.

The speed record for radio control model gliders using this technique near mountain ridges is apparently 392mph, so the effect can be seen to be not small. This is *sustained* flight - as long as the wind blows. Yes really!!!!! From an on-line video of 370mph I estimated that the model was pulling 35g - I am not 100% on the integrity of the video or my calculations.

There seems some interest in Dynamic Soaring within the full sized glider community.

Dynamic Soaring links
YouTube - Great sounds of formation DSing at Jones Pass (http://www.youtube.com/watch?v=x9HMI5SThBU)
YouTube - New Dynamic Soaring World Record -- 392mph (http://www.youtube.com/watch?v=WaQB16ZaNI4)
Dynamic soaring - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Dynamic_soaring)
ESA Experimental Soaring Assn (http://www.esoaring.com/)
http://www.esoaring.com/albatros_presentation_esa.pdf
YouTube - Dynamic Soaring (DS) on 50ft Dam in Tennessee... (http://www.youtube.com/watch?v=GWGK-EeKHWY&feature=related)
-- dam in Tennessee
-- DS at 1:51-2:35 and 4:42-5:10 the rest is slope soaring I guess.

The last link is from an on board video showing DS utilising the different wind speeds above and in the lee of a dam.

I'll redo the links in a mangled form if they disappear.

Little explanation -ette, sort of thinking out loud.
It might perhaps be thought of as picking up energy from the wind at high altitude and then recovering position in the lower wind speed at low altitude in the lee of a wave or hill. Rinse and repeat. [I just made this para up so it might be crap since I do not really follow what is going on here.] Quite how this turns into 392mph I have no idea. Ah yes, the airspeed of the aircraft downwind (300mph in a 30mph wind) is not relevant the aircraft *still* gets yet more energy out of the wind. I'm still not sure I follow myself.

[InTgreen]

Here's a different idea to explain this then..... At normal cruise speeds, the effect that we are discussing is minimal, due to the relative speed of 100 kts cruise vs 20-30kt wind. Now lets decel the helo down to 40kts for the reccie as in the original question, and a 30kt HEAD wind. (Example a Frog Helicopter with head turning Clockwise when viewed from above)

Advancing side of the disc is at a speed of Rotational velocity + 30kts (max at the 9 o'clock position), causing FlapBack (Disc High) in the 12 o'clock, and the Inflow axis will cause a 2 o'clock disc high position. If the wind is turbulent or the turn is tight (low A/s therefore tight Radius of Turn) As the helo is turned at a low speed, the Inflow Axis will be close to the 11-5 o'clock position across the disc = half past 2o'clock disc high, therefore initially the helo will be rotating the fuselage into the disc high region - meaning to stop the helo decelerating, the cyclic needs to be tilted forward - just as in overcoming flapback. If this does not happen quickly enough, the helo will decelerate - in the example towards 30 kt and the danger of Vortex ring...........

Now, did anyone follow that???

As a continuation, Why does a helicopter in the hover next to a hill experiencing updrafting air, not get into Vortex Ring?? Hover = Low airspeed, Updrafting air = Rate of Descent Flow, = Hover = Power applied.

[RVDT]

Why does a helicopter in the hover next to a hill experiencing updrafting air, not get into Vortex Ring??

Oh yes they do! Go fly a Llama.

[InTgreen]

Bugger. That would be distressing! Guess my Sea King has a little more flex in the aerodynamics! First time anyone has probably said THAT!

[Nf stable]

Advancing side of the disc is at a speed of Rotational velocity + 30kts

Not unless you're hovering, it would be Vrot + 40 by the simple PofF explanation. The rest I'm afraid I could not make head nor tail of, sorry.:confused:

[InTgreen]

Granted, the forward airspeed should have been taken into account as well, however, with those speeds taken into account for the advancing and retreating sides of the disc, its just exasserbates the overall issue. I think that as the fuselage turns thorugh 180 degrees, its the inflow roll that causes the disc from the pilots view point to flap up, leading to a nose up attitude to develop unless the pilot does stuff. Hence lose speed/need more power in the turn to maintain the original into wind speed.....

The above theory works for a helo with a clockwise head (when viewed from above) turning at low speed to the right - still working the idea otherwise..:{

[[email protected]]

Alchef - your starting post was referring to a lecture about valley flying and recces of Confined LS and specifically about starting from intowind and then turning to escape downwind.

There has followed lengthy discussion about constant speed, constant AoB turns (not the same thing at all) and some unhelpful mudslinging regarding physics.

I think the message your lecturer was trying to get across is that at low level, when using primarily external references to fly by, it is too easy to try and maintain constant groundspeed rather than constant airspeed. The result of turning downwind and subconciously maintaining the same groundspeed will always be a reduction in IAS, usually accompanied by an increase in power required to maintain altitude if you are the lower end of your speed range (as you often are when recceing or mountain flying).

All aircraft require more power when rolling into a turn compared to level flight at the same IAS so if you initiate the turn without adding power and you let the speed wash off (usually by letting the nose come up) you are quickly into a potentially dangerous situation, sinking towards the ground with reducing airspeed which could, if left unchecked, require more power than you have available to correct, especially at high DA.

At this stage all the theoretic arguments about frames of reference, inertia etc are not much use.

In a nutshell, if you want to make a safe turn downwind, especially in the mountains, think push with your right hand (you won't but it stops you accidentally pulling - and pull with your left hand so you add some power.

[puntosaurus]

Ah, the voice of reason.

Oh No ! Hang on a minute. If you look back at Alchef's post it appears he already understands the airmanship aspects of the downwind turn, so doesn't need to be taught how to suck eggs.

He did however question the dodgy physics, and thanks to good old PPRUNE he got a full answer.

[pilotmike]

puntosaurus is completely correct. Alchef simply disbelieved the incorrect explaination that was given to him by his instructor and needed clarification. And now he is wiser.

It was unfortunate that L2driver, who clearly suffers from the same misunderstandings, wanted to correct us all when he was wrong.:eek:

Any mud slung was simply being 'heartily reciprocated' to L2driver after he picked the fight by slinging it at those who refused to believe his (mistaken) world view.:ok:

[RVDT]

In a nutshell, if you want to make a safe turn downwind, especially in the mountains, think push with your right hand (you won't but it stops you accidentally pulling - and pull with your left hand so you add some power.

I'm up for it, a "nutshell" rating, when do I start? :ouch:

[puntosaurus]

Well it appears you start by pulling hard with your right hand .... Or was it left ?

[topendtorque]

Having not read or studied too hard the thread philsophy and at risk of plagiaizing my own garbage opinion, it may be prudent to point out that each and every action is supposed to have an equal and opposite reaction.

pulling, pushing,
left hand; right hand:
dammit, I'll leave it there,
bin a long day.
tet

[L2driver]

Why don't you discuss the matter at hand instead of using a lot of space attacking me as a person? Maybe I know something you don't.

I am not belittling anybody at all, even if you think so. Not even you.

For some reason, it seems that having a little bit of formal competence is a major minus (sic) to some people. (and that means you)

Your post is exactly why many people stay away from expressing their thought, ref the threads on the Air France accident.

And I don't flash a big Breitling, it is in my safe. I even keep my Test pilot and USC Safety Certificates in a drawer. I do not need to brag about this:)

[L2driver]

OK, last one from me on this one (maybe);
Scenario: you are sitting on a deck offshore North Sea, nose into the wind, wind is 270/60. You lift off in a heavy L2 using 6 on the FLI (collective pitch angle on a scale from 0 to 10, etc etc) to stay in an unstable turbulent hover. You apply 6,5, make the TDP (take off decision point) at 20 feet, get out of the turbulence, and then as you reach 65 kts IAS make an immediate 45 degree bank turn to 090 degrees. (180 degree turn)
You do not touch the collective after setting the 6,5.

The 64 000 dollar question is: what happens??

I can tell you the answer, but I guess you know already, don't you PM. If you do not, you should not be in this business:)

And to everybody else on this thread: don't over-over-theorize this, it is not that complicated.

[Wee Willy Winky]

Maybe I know something you don't.

I am not belittling anybody at all, even if you think so. Not even you.


You have already judged other people's "lack of basic physics knowledge" from your own misconceptions, and then called those who correct your misunderstandings "idiot commentators".

From the evidence available it would seem that once again you are mistaken.

Amongst others, farsouth offered excellent commentary withAll I can think is pot....kettle.....black.......

[L2driver]

Why don't you just challenge my previous post message instead as going after me, nice person as I am??

That is a major problem with many of the posts here.

Stick your head up, and I will surely cut it off, even if somebody has knowledge that would be useful for others.
The Danish Law of Jante springs to mind:

The ten rules are

Don't think that you are special.
Don't think that you are of the same standing as us.
Don't think that you are smarter than us.
Don't fancy yourself as being better than us.
Don't think that you know more than us.
Don't think that you are more important than us.
Don't think that you are good at anything.
Don't laugh at us.
Don't think that anyone of us cares about you.
Don't think that you can teach us anything.And this is how some forums here operate

THE BALL LADIES & GENTLEMEN - NOT THE PERSON!!!!

And what is your opinion of my last post before this? (No. 69 for those of us with failing memory), Afterall that is what it was all about. Look at the post and then the immediate reply from someone called www - not much about the same thing, was there??

And to my aviation friends: I never used the term "idiot commentators" about one of us. It was meant for some fractions of the press. I sincerely hope you are not a member of that community.

[Nf stable]

L2, this seems to be an easy one, please correct me if I'm wrong. In a hover in a 60kt wind we're flying straight and level. Ok, we adjust IAS by 5kt, but in any EC I've flown, .5 FLI will not support a 45 degree turn (I will say now that I do not fly L2), and certainly not if we are still accelerating, so surely we descend, regardless of all previous arguments.

[chopjock]

Scenario: you are sitting on a deck offshore North Sea, nose into the wind, wind is 270/60. You lift off in a heavy L2 using 6 on the FLI (collective pitch angle on a scale from 0 to 10, etc etc) to stay in an unstable turbulent hover. You apply 6,5, make the TDP (take off decision point) at 20 feet, get out of the turbulence, and then as you reach 65 kts IAS make an immediate 45 degree bank turn to 090 degrees. (180 degree turn)
You do not touch the collective after setting the 6,5.



All right, I will play,
L2, your scenario would be un realistic because in the real world it would be un wise to perform a 45 degree turn downwind with out first obtaining a positive rate of climb. :rolleyes:

[L2driver]

NF
You are right; 0.5 will most likely not be sufficient for a 45 degree bank turn in even calm wind. So let us increase the power a bit more to keep us flying at that bank.

But the question is: going from a GS of 5 kts (65KIAS minus 60kts wind) to a new GS of 60kts+ in a matter of seconds, does that require any additional energy? If so, where does it come from?
If you don't go to that new GS means that you run out of IAS, and then how about power requirements?
I think the answer to the first question is yes and I will suggest it must come from our engines as long as our flight path vector (velocity and direction) is changing.
I put forward that if we do not supply that energy, we will end up very wet quite soon after takeoff.

To Chopjock: yes you are right too, but I was not discussing airmanship, only trying to visualize a theoretical scenario.:)

[ldo]

The short answer is that it is counterintuitive (as the discussion so far has proven), but if you integrate the acceleration component in the East-West direction that you are subjecting the helicopter to in the turn you will find out that indeed you will reach a GS of 125KTS by the time you will turn downwind.

Where is the energy coming from? It is the excess energy over straight and level light that you need to supply in order to make a 45 degree bank turn.

Without resorting to the explicit kinematic calculations (which I will be glad to post if requested) consider the following: in a 45 degree banked turn, you are accelerating the helicopter 90 degrees off the direction of travel at 1g (I am not talking about how many g's you are pulling, just what is the sideways acceleration component).

As you start the turn there is no acceleration component in the E-W direction. As you are crossing the midpoint of the turn, you are subjected to a 1g accelleration in the E-W direction, and finally at the end of the turn you are subjected to no acceleration component in the E-W direction.

If you integrate the E-W acceleration component across the turn, you will find that the resulting change in the E-W velocity component will be equal to 2 x 65 KTS = 130 KTS. This means you will go from a groundspeed of 5 KTS to a groundspeed of 125 KTS in the opposite direction, without requiring any excess energy over the one required for a standard turn.

References
A Note on Steep Turns and Wind (http://www.huygens.org/sape/pilotage/Hudson/steepturns.html)
Kinematics - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Kinematics)

[NickLappos]

The absurdity of the argument that turning downwind makes aircraft descend never ceases to amaze me! The physics of the situation earns this topic onto my list of the Myths of Aviation, perhaps as #1.

Were Galileo with us, he would explain what an "Intertial Reference Frame" is, so that even accident investigators would know not to blame the dreaded Downwind Turn when simple pilot error will suffice, aided by the decieving visual cues during the maneuver.

For clarity: Turning Downwind takes no more power and no more energy than turning Upwind, unless you are flying purely by ground reference, and attempting to hold a fixed ground radius turn.

If the ground speed change needed more energy (as is said by the budding Newtons who espouse to the theory) then I have a bigger "headwind" for us to buck:

The Earth rotates at a speed of about 1500 KM/Hr toward the east at the equator, about 1000KM/Hr at temperate latitudes. As you sit at your computer, know that the monitor, keyboard and desk move about 277 meters per second toward the east. If you rise from your chair and walk westward at 1 KM/Hr, and then turn eastward, you must accelerate yourself to catch up with that rotation (according to the pprune Newtons).

But you don't fall to your knees, and you don't make holes in the wall as it hits you at 277 M/sec. Why? Because all of the things in your room are immersed in this motion field, and all of the things are synchronized to that velocity. That is the definition of an Inertial Reference Frame. THERE IS NO CHANGE IN ENERGY STATE RELATIVE TO THE OBJECTS IN THE INERTIAL REFERENCE FRAME, EVEN IF AN OBSERVER HOVERING INDEPENDENT OF THE EARTH'S VELOCITY FIELD WATCHES YOU ALL ROAR BY AT 277 M/SEC!

Here is a thought experiment for the "down-winders" to try when they get all wrapped up in energy computations relative to the earth below an aircraft in flight. Stand on an airport conveyor belt, the kind that carry you from one terminal to the next. As you translate at 1 M/sec across the airport, watch the ground go by and calculate the energy in your usual way. Now take your briefcase and start to swing it back and forth going parallel with the moving conveyor's motion.

Do you feel the tremendous force needed to swing it forward, relative to the slight force needed to swing it backward? Do you? No! You feel no difference in force from the bag even though you are propelling the bag forward at perhaps 2 M/sec and then backward at perhaps 0 M/sec relative to the airport floor, walls and building (while you are symetrically propelling the bag to plus and minus 1 meter per second relative to the conveyor belt, and relative to your arm and your body.)

A flying aircraft in a steady wind is immersed in an Inertial Reference Frame, and no enegry calculations are needed to compare the aircraft to the upwind ground speed, nor to the speed of the earth's rotation, nor to the motion of the Earth as it swings around the center of the Milky Way Galaxy (42,000 miles per second for the downwinders to place in their equations!)

This great mystery will be back in about a year (the life-cycle of a pprune Myth). I hope I will be around to re-post this at that time!

[jellycopter]

Nick

you state:
Do you feel the tremendous force needed to swing it forward, relative to the slight force needed to swing it backward? Do you? No! You feel no difference in force from the bag even though you are propelling the bag forward at perhaps 2 M/sec and then backward at perhaps 0 M/sec relative to the airport floor, walls and building (while you are symetrically propelling the bag to plus and minus 1 meter per second relative to the conveyor belt, and relative to your arm and your body.)

However, if you were able to measure the forces required to swing the briefcase, you would indeed detect an increase in force to propel it at 2M/sec and a reduction in force to propel it 'backwards' at 0 M/sec due to the differecnce in air resistance (drag).

Now, move this argument to an L2 Puma suspended by a long rope on a crane moving along at 60kts. The Puma is pulled, like a pendulem, first in one direction, then the other, to the same angular deflection and then dropped from either end. Will it acheive 120kts G/S in one direction and 0 kts G/S in the other direction as it passes the vertical? Or will the air resistance, that's 'outside' the reference frame have any effect?

JJ

[L2driver]

I go back to the UK AAIB G-TIGH report. I am not sure how may have read this report, but please consider this from the report:

From the Analysis part of the report:

2.4.1 Wind Limits.
The wind at no time precluded flight, but it did provide particular problems. The prime one, which the commander appeared not to have considered was the effect of turning downwind whilst flying solely by external references, a potentially hazardous manoeuvre that is emphasised in basic flying training.

Although the helicopter can hover at zero airspeed, it requires much more power to do so than it does to maintain forward level flight with forward velocity. A power setting required to maintain forward level flight will, therefore, be inadequate to maintain hovering flight and any loss of airspeed will cause the helicopter to descend if not checked. Such a reduction in airspeed is the likely outcome of turning away from a substantial headwind whilst visually maintaining a constant groundspeed.
This appears to be exactly what occurred as 'GH' lifted from Cormorant 'A' and immediately entered a 180° climbing turn: whilst the perceived ground speed remained fairly constant, the airspeed decreased to zero and an inevitable descent ensued.


From causes:

b. Causes

The following causes were identified:

(i)The handling pilot's failure to recognize the rapidly changing relationship between airspeed and groundspeed which is a fundamental problem associated with turning downwind in significant wind speeds

(ii)The Commander, who was the handling pilot at the time, shortly after takeoff inadvertantly allowed the airspeed and then the height to decrease whilst turning away from a strong gusting wind

(iii)Despite the application of maximum power, the helicopter was incapable of arresting its established descent within the height available. Incipient Vortex Ring state and down draughts may have contributed to this problem, as may the height of the wave crests.

[212man]

L2, you are really digging a hole for yourself that I fear will be impossible to extricate yourself from! Nothing in the paragraphs you quote supports you theories, and does not attempt to. It simply expands on the old age problem associated with turning downwind at low level and low airspeed, whilst either intentionally relying on visual references, or being otherwise influenced by them.

In a nutshell, the AAIB are saying that the pilot instinctively decelerated as a result of the strong visual stimula presented by the platform 'flashing past' and his airspeed therefore decayed, requiring more power to stay level- because of the drag curve, not kinetic energy. This power application was applied too late to arrest the resulting rate of descent, and they hit the water.

I knew both the pilots and am quite well acquainted with the circumstances of that accident. Nothing in the report supports anything you are quoting, and I'd be greatly obliged if you'd refrain from using it in your vain quest to bamboozle those who may not understand physics as well as others (and, frankly, don't need to!)

I also lost a very good friend in a fixed wing accident (Bulldog), which was almost certainly due to inappropriate use of rudder to correct a perceived skid, whilst turning at low level in a strong wind. The problems of the downwind turn are many and numerous, and have been knwon about since the begining of powered flight. They certainly have nothing to do with energy associated with groundspeed.

Listen to Nick - he really does know what he's talking about (even if you disbelieve me)

[the beater]

Can someone please explain to me just what relevance the last two posts have towards the original question?
I thought that it involved maintaining a constant AIR SPEED.:ugh:

The above refers to posts 77 & 78

[Nf stable]

Or will the air resistance, that's 'outside' the reference frame have any effect?

jellycopter, is Nick not saying that the air IS the reference frame?

[the beater]

RTFQ, ATFQ
Would be surprised if L2driver was capable of driving a mk II cortina!

[jellycopter]

Nf

In the downwind turn case he is, but not in his breifcase scenario.

JJ

[NickLappos]

jellycopter is right, on the conveyor belt, there is a small but real difference due to the fact that the air is in the terminal/floor/walls frame, and so airspeed does change. That is a fine point, and small inconsistency in my "thought experiment." Good catch.

In the airborne turn, the speed is always constant, as is the power and the altitude. In fact, one can trim the helicopter in a strong wind to a constant bank, and turn all day long without adjusting pitch or power in any way. The only way you can tell which way the wind is blowing is by reference to some nav instrument tied to the ground - there is no change to the aircraft's power attitude or altitude due to the wind.

BUT one point must be made. If the pilot is making a constant radius turn about a ground reference point (like circling in a tight downwind/base turn about a rig on short final approach) then the turn from upwind to cross wind to downwind would require a vast increase in bank angle to keep the radius of the turn constant as the groundspeed increased. In other words, the pilot must tighten up the turn to make the radius constant as the ground speed increases, and this increased bank needs lots of power, which a loaded helo will not have. At constant bank angle/load factor, the radius of turn gets bigger with the square of the velocity, so a 60 knot (airspeed) helo with a 30 knot wind will need THREE times the load factor to make the radius upwind (30 knot groundspeed) match the radius downwind (90 knot groundspeed). Close to the ground, the helo cannot give up altitude to fuel the load factor, so the speed is sacrificed with back stick, but it is a tricky maneuver, often done with an offshore poor horizon and weak altitude cues from a moving sea. The result can be a botched turn, big descent, and oops, sea contact.

The cure? Make the approach a bigger one, extend and make gentle turns, pay off radius for load factor so that it takes 2 min longer but you end up with a longer, more controlled final approach into the wind from a healthy distance (1/2 mile?)

Don't be fooled by the fact that you can do a sweet pylon turn in a VFR practice environment - at sea, with a poor horizon, squirming sea and few altitude cues, in dusk or misty conditions, the sudden realization that you have built a 2.5 G. 2000 fpm descent while at 60 degrees of bank and a full load WILL lead to a very tight situation - sometimes a nasty splash.

This has LITTLE to do with 1/2MVsquared, and all that downwind energy stuff. It has to do with the load factor/radius of turn/ground speed relationship that is described here. Note that for a 30 mph turn of 250 feet, it takes 15 degrees of bank. For that same 250 foot radius at 90 mph, it takes about 2.5 G (beyond the performance limits of most loaded helicopters). Disregard the modeler's comments about scale speed - the chart is useful anyway.

http://www.scaleaero.com/figure2.gif

[pilotmike]

Such a reduction in airspeed is the likely outcome of turning away from a substantial headwind whilst visually maintaining a constant groundspeed.

You're just not getting it, are you L2driver? You quoted this bit from the acccident report believing that it supports your erroneous argument that a turn downwind in a constant wind somehow causes an aircraft to mysteriously lose airspeed, due to gravity, inertial frames of reference or some other such bogus fictitious nonsense.

You really are completely muddled in your thinking, and the scary thing is that you are trying to teach others who might be fooled into believing your nonsense.

212man politely points out that you are digging yourself deeper and deeper. My advice would be to listen carefully him, and stop trying to impress us all with your flying theories and certificates.

People have so far been rather patient and polite in telling you that you have it wrong, but now you really are making yourself look silly. The outpouring of ridicule is beginning:

Would be surprised if L2driver was capable of driving a mk II cortina!

Ha ha! But steady on old chap - do I really need to remind you of his braggingI don't flash a big Breitling, it is in my safe. I even keep my Test pilot and USC Safety Certificates in a drawer. I do not need to brag about this

L2driver, you have clearly misunderstood some elementary principles of flight from the beginning.

To have you trying to teach us that somehow WE'RE all wrong because we disagree with you was irritating.

Being chastised for our "lack of basic physics knowledge" was particularly hard to take, coming as it did from you.

When you mockingly called us the "idiot commentators" for disagreeing with you, you were really starting to annoy, and it made you look very stupid.

And yet still you're here trying to give everyone a lesson in downwind turns, making yourself look more stupid by the minute.

Your continued lessons on flying, given your obvious lack of basic knowledge are very hard to take, and are becoming rather tiresome. Listen to Nick Lappos, and learn from the excellent article whose link was posted earlier. There are none so blind as those who refuse to listen!

Remember, 2 ears, 1 mouth. Use them in that proportion. Stop with trying to impress us all and giving us all flying lessons. Start to listen and learn from those who actually know what they are talking about.

I would strongly recommend taking Wee Willy Winky's advice, which neatly translates to:

You're trying to teach dolphins to swim.

EITHER LEARN OR LEAVE, OR THE THIRD OPTION: PREPARE TO BE FLOGGED!

[rotornut01]

L2driver, there are a lot of knowledgeable people on this forum, most of them it seems, also have a lot of patience.

For each of the replies that you see addressed to you on this topic there are probably another 50 people sitting there shaking their head at your comments and your obvious lack of understanding. Until now I was one of them.

I would advise that you get some one on one tuition (from an engineer or scientist) on the principles involved in this discussion and eat some humble pie.

Oh and if I were you, I would send those certificates back to where you got them from and explain that you are not worthy. Not because of the mistakes that you have made in your explainations per se, but because you just continue to spout the same s$#t and don't open your mind up to the fact that you might just be wrong.

[212man]

the beater,
sorry if my post resulted in your headbanging. In my defence, I'd say I was trying to add weight to the argument that the position L2 is adopting is nonsense, in an attempt to prevent more gullible people from believing him - and the instructor that was lecturing the originator of this thread!

Can someone please explain to me just what relevance the last two posts have towards the original question?
I thought that it involved maintaining a constant AIR SPEED.

The above refers to posts 77 & 78

RTFQ, ATFQ

Ok....

What are people's thoughts on this?

My thoughts are that you are correct with your stance and should politely ignore your instructor's lecture. He is using misunderstood school boy physics to confuse himself, you and many others. :ok:

[the beater]

Oh dear! Talk about how to lose friends and alienate people!
You see, 212man, I actually agree with you. The problem was that as I wrote my reply, you posted yours, so my reply mentioning the previous two posts didn't make sense. Hence the editing of my post.
I hope that no-one posts whilst I'm writing this, or we could be locked in a loop forever!:ok:

[RVDT]

You couldn't make this up!

I posted a link that explained the issues for all to understand. Many thanks to the chap in SA.

Nick Lappos also had to wade in and explain it in very clear terms as he is so good at.

For all you Naysayers and Hellfire and Brimstoners - Wikipedia is your best friend - read - weep!

Moderator - please shoot this thing. It is in the public domain and it is embarrasssing!

It is difficult to soar like an eagle when you are surrounded by fr@#kin TURKEYS!

Rant over because I think I just noticed that the sky is falling! (with hopefully subsequent result)

[2FLY]

Having finally found time to read this thread (I've been meaning to ever since it popped up) I thought I'd add my 10 cents....

The perils of the low IAS, downwind maneuvers are well documented and hopefully all pilots flying out there are aware of the implications, if not perhaps a refresher flight with a instructor (and sufficient height agl) would be a wise investment.

VRS in a hover in a updraught, possible but unlikely as your power in use is probably considerably lower than in a normal hover (your in a updraught!), however if you start to sink a bit and decide to pull pitch......:\

To answer the initial post: if flying a constant radius turn around a ground feature at a constant IAS on a nil wind day there will be no requirement for pitch change. Throw in some wind but fly at a constant IAS and pitch change will be needed as the angle of bank required to maintain the 'K' radius of the turn changes, otherwise the a/c will climb and descend as the associated load factor changes.

A question I have always pondered is where in the orbit is the demand for Bank Angle (and thus pitch/power) greatest??

Conventional Principle of Flight would mandate this be in the downwind position due to the squared function of velocity in the Load Factor equation as discussed by Nick Lappos in post #84. However when flying a constant radius turn at say 60 kts IAS in a 20 kts wind the greatest angle of bank appears to be somewhat after the downwind position during the turn to crosswind? is this another illusion or due to the combination of high groundspeed (admittedly decreasing at this point in the turn but still higher than the IAS) and the drift caused by the (increasing) cross wind component...??

Any takers?

[RVDT]

A question I have always pondered is where in the orbit is the demand for Bank Angle (and thus pitch/power) greatest??

Constant radius/airspeed turn around a point on the ground = where the G/S is the highest i.e. downwind.

[[email protected]]

2fly - I am with you on this one and I believe it to be crosswind (after the turn downwind) since you have maximum drift and must therefore have max AoB to negate it.

[Alchef]

Thank you all for your time and effort in answering my question. I can see this is a hot topic, and didn't mean for everyone to fall out with each other!

As for InTGreen's comments on the flapback effects, titled "Principles of flight", are you suggesting that this only happens on the first turn downwind, or as I continue turning would I get more and more nose down pitch, even as I come back into wind?

This would put the aircraft out of limits (15 degrees nose down) very quickly.

From my limited experience, this is not the case.

[[email protected]]

Alchef - your original question revisited an old chestnut that has been debated at length in many crewrooms and several tiomes on pprune in different forums.

Hopefully, Nick has put to bed once and for all the myth of needing more power in a constant AoB /IAS turn as you go downwind.:ok:

The potential pitfalls of turning downwind and maintaining constant groundspeed have been extensively covered and this is the area that catches out the unwary helicopter pilot, especially in the mountains when visual refernces can already be confusing due to the terrain.

I have taught lots of mountain flying and although I took flak for my suggestion about pushing then pulling - it works and has helped lots of students of all levels to improve their mountain techniques.

[rotornut01]

To attain a constant radius turn in wind (at a constant airspeed) the most bank angle is required in the middle of the downwind leg, that is when the wind is directly behind as this is when the ground speed is the greatest and you therefore need the greatest accelerating force towards the centre of the circle to attain the same radius.

If the maximum banking is timed incorrectly, by that I mean banking a little too late in the turn, it appears as though the greatest bank angle is required in the crosswind. This is however only a consequence of not banking enough during the downwind leg.

This is something that definately works in practice as the theory suggests it should.

[puntosaurus]

That graph of Nick's above is quite clever, and you can use it to prove this point. If you hold a pencil against a fixed radious of turn and then tilt it to simulate different groundspeeds, you can see the angle of bank changing in the turn, and the angle of bank is indeed greatest when the groundspeed is highest.

A few people have dabbled on this thread with trying to look at secondary aerodynamic issues when rolling into a turn. I'm quite interested in that, although I suspect the answers are complex and maybe type specific also. I have a firm belief that the R44 pitches up on rolling into the turn, and then pitches down as the turn develops. In all my experiments I've tried as hard as possible to move the cyclic purely laterally, but of course I can't rule out user error !

Anyone care to confirm, counter, or attempt to explain ?