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...3%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.