Downwind turns equal disaster??
 

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

Chuck

Good idea, Nick. :ok:

So what do people think of downwind turns in helicopters?

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

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

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

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

Preparing to duck for cover and burn logbook.

overpitched says

Bomber says

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

Overpithed:Here's a simple test that any real helicopter pilot can do: Go up on a day when there's a brisk wind - say, 2,000 feet and fly directly into that wind. Slow to 40 knots IAS, get it stabilized and in trim. Now bank the ship and start a constant-attitude turn - your choice of direction. Do not mess with the pitch attitude of the ship. Keep it going around and around, as many times as you can stand. You'll notice a couple of things:

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

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

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

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

define what is "brisk wind"

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

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

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

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

Oh my God :

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

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

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

You see a hot air baloon at your altitude.

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

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

Chuck

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

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

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

PPF

So thatI'm clear in my head...


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

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

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

Overpitched,

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

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

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

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

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

edited for spelling

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

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

Back to Basics
 

Its all very simple really

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

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

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

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

rotordk,

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