Helicopter pitch change when orbiting around a fixed point??

[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.

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.

[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?

[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]

Pilotmike

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 with

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

[L2driver]

WWW and PILOTMIKE

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

1. Don't think that you are special.
2. Don't think that you are of the same standing as us.
3. Don't think that you are smarter than us.
4. Don't fancy yourself as being better than us.
5. Don't think that you know more than us.
6. Don't think that you are more important than us.
7. Don't think that you are good at anything.
8. Don't laugh at us.
9. Don't think that anyone of us cares about you.
10. 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.

[L2driver]

NF and Chopjock

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]

Why do you accelerate to a GS of 125 without "additional" energy?

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
Kinematics - Wikipedia, the free encyclopedia

[NickLappos]

Downwind Turns Need NO More Power

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.

The above refers to posts 77 & 78