Rotor Lift with the wind and into the wind.

[dragonflyerthom]

Now I know this has been covered before, but I was flying the other day when I began to sink. I had to increase my throttle to arrest the altitude loss. I was low at the time. My flight speed with the wind according to the ASI was 70 mph. In order to gain altitude I turned into the wind and began to gain altitude. Now all of you who say it shouldn't matter as long as the air speed is constant I can now say bunk. It did matter which direction I was going.

[Vance]

I can usually tell which way the wind blows from the ground speed my GPS shows compared to my indicated airspeed.

When I fly to Lompoc from Santa Maria I often have a 15 mile per hour tail wind. My GPS shows 65 miles per hour when I am flying 50 miles per hour indicated air speed. If I am flying at 800 feet (500 feet agl) I get lift and sink along the way. On my return, headed into the prevailing wind, I usually get lift and sink in the same places. It does not appear to matter which way I am going as far as throttle setting to maintain altitude.

It doesn’t seem like bunk to me.

I have found it is not good to attempt to debunk a prevailing myth with a single example of particular phenomena.

I am higher at the end of the runway if I take off into the wind as opposed to a down wind take off. My rate of climb seems about the same but it takes me longer to get to the end of the runway so I am higher.

When I fly over my secret valleys I always get sink in the middle if the winds are up no matter which way I am going in relation to the wind. I suppose if I turned around in the middle I would see I had lift by turning into the wind.

I suspect that makes me part of the bunk.

Thank you, Vance

[Udi]

You flew into, and out of, descending air Thom. That's all. Not rocket science...

I used to fly hang gliders and with these light and slow aircraft you can really map rising and falling air currents. Except for really special circumstances at VERY low altitude, it makes no difference whether you are flying into or with the wind. Air currents move together with the wind, as are you.

[Harry_S.]

Except for really special circumstances at VERY low altitude, it makes no difference whether you are flying into or with the wind. Air currents move together with the wind, as are you.



Special circumstances are maybe T/O and Landings?

I have a mile and a half, hard surface N/S runway and a 3600' E/W grass strip at my home base. Any wind, around the compass, is no concern when using the N/S for T/O or Landing. I ALWAYS T/O using the N/S and land with the E/W...easier on the tires and the machine. But, say with a STRONG East wind, I will land on the 09 strip and not the 27 because I would maybe be into the trees at the end of 27, that is, with my normal approach.

In demonstrating this to newbie pilots, using the N/S runway, they were amazed at how much runway was used up at idle power, a strong tailwind and only about 10' up, vs landing into the wind.

Consistent winds at the lower "gyro altitudes" are no problem; the wind shifts, gusts, wind shear and the hill/mountain roils will add a little diversity to the normal S/L flying. ;)


Cheers :)

[Udi]

If we could only spread some chaff that would make the air currents visible...

Pilot reports indicate that there may be low level currents that create upwind/downwind dissymmetry in aircraft that are sensitive to vertical gradient (PPC's, gyros) - i.e. aircraft that have their lifting surface significantly higher than the aircraft center of drag and their pitot tube. HOWEVER - I don't believe these may exist at an altitude higher than 50 ft or so.

[birdy]

I have found it is not good to attempt to debunk a prevailing myth with a single example of particular phenomena.
VERY good advice Vance.

You flew into, and out of, descending air Thom. That's all. Not rocket science...
Thats it.
Simple init?

If we could only spread some chaff that would make the air currents visible...
Yup, if most pilots could actualy 'see' wot they were flyn in, they wouldnt.

Airflow is all very simple and logical, if you account for all the forces and energies that make it move.
BUT, coz we cant see it, it seems to be mysterious.

And you never stop learning.
This last coupla days iv been [tryn] to chase more moos. The weather has been shocking, never getn above 50f all day [ 7-8 hours], with a stiff 30knt breeze gustn to 50, over country a sparrow would find difficult to land on, and mountains all round. [ not my happiest days]. :(
But, even in these conditions, most airflow is predictable, coz you can see how the air will flow round the rock faces and where monster thermals will build, so i had no supprises, cept one.
I was puttn DW with bout 35 on the ASI and 55 on the gps, lookn streight down through the scrub, wen i suddenly swaped ends.
???????
The mind was kinda raceing, coz i couldnt figure wot could happen next.
Wot did i just fly into?
Wen will i get out of it?
Wots go'n to happen wen i get to the other side of it?
I just sat there, bout 100' up, pointn back the way i came, and for sum reason, i was traveln back the way i came.
IOW, the air i was in simply swaped directions, with out any alt change, and i was go'n with it.
I was well away from any mechanical turbulance horisontaly and virticaly, and it wasnt a wirly wind, [they only wip you round 90*, not 180*].
It was like a horisontal shear, without alt change.

Even more interesting was the exit.
I simply, gradualy, turned anatha 180* and followed the airflow out, without any input from me ???????????????????

Never hit one of them before, and not real keen on meetn the next one either.

Like i said, you never stop learn'n, but im haven trouble pictureing wot can cause airflow to change 180* inside itself.

[Resasi]

Birdy in thousands of hours of completely predictable flying we sometimes get that occasional moment that defies all we ever understood.

All we can do is keep on trucking just hoping to hell we never meet that particular phenomenon again.

[Udi]

...I was puttn DW with bout 35 on the ASI and 55 on the gps, lookn streight down through the scrub, wen i suddenly swaped ends.
???????...
I think the real question Birdy is what you had for lunch that day. Too many mushrooms perhaps? :whip:

On a more serious note - to swap ends like that you must have been hit by a wind sheer of at least 50 mph from the back.

[fiveboy]

Birdy

I just soiled myself reading your post. In many ways I think of you as the canary in our mineshaft.

Yikes.

fiveboy

[birdy]

On a more serious note - to swap ends like that you must have been hit by a wind sheer of at least 50 mph from the back.
I think your bout rite Udi.
But that was only wen the machine did the 180, coz after it turned round, the wind was still behind me, going against the general flow , at the same speed.
Luckily, i was rideing a bit of an updraft from the small cliff face i was following, on low power, so the disc was pretty flat.

[Walter]

Birdy,
IMO what might have happened is that you got into a newly forming thermal, just at the moment it leaves ground. The direction of turn should have been to the left, if I am not wrong (like the dust devils, which are in principle the same phenomenon). The rising air draws air into and beneath it al a low altitude, which enters spiralling due to the coriolois force.
Would this be a possibility?

Walter

[mark treidel]

Walter, if your hypothesis is in fact true, his turn should have been to the right since he is in the southern hemisphere. Am I wrong? I'm so confused.....

[All_In]

Right or left confused some so it is counter clockwise in the northern hemisphere, and clockwise in the southern, so yes! Or did I reverse them, no that's right.

But the explanation is a good one, had and experience in a parasail that I think he just described perfectly.

[birdy]

Weneven you fly into spiraling air [ wirlywinds], you only rotate at most, 90*, not 180.

[Doug Riley]

Coriolus effect (it isn't really a force, just an effect of inertia) makes continent-sized air masses follow curvy tracks. It doesn't affect anything as small as a dust devil (or a toilet bowl).

[Udi]

You are referring to Earth Coriolis effect, Doug. Coriolis effects can be very meaningful at small scale too, if your point of (rotating) reference is adequately small. One example of this are Coriolis force mass flow meters - commonly used in the chemical industry. These flow meters measure the mass flow of gasses and liquids, using the Coriolis principle. They are very expensive, but very precise.

A dust devil spins in the direction of the wind currents that started it. It can turn right or left regardless of which hemisphere you are in. The reason dust devils spin is because hot rising air (thermal) is creating an area of localized low pressure. The low pressure is pulling air from the surroundings, into the column of rising air. Due to inertia, the air going into the spinning column will spin faster and faster, just like an ice skater who is puling her arms and legs in is spinning faster and faster.

[Dean_Dolph]

......One example of this are Coriolis force mass flow meters - commonly used in the chemical industry. These flow meters measure the mass flow of gasses and liquids, using the Coriolis principle. They are very expensive, but very precise.....Slight hijack here but while at Shell R&D, I bought the 2nd Micro-Motion flow meter sold in Houston; Rice U. got the 1st. It was/is a neat meter and I felt like we were on the cutting edge when we bought it. Micro-Motion had the patent on this type of meter.

[ckurz7000]

Coriolus effect (it isn't really a force, just an effect of inertia) makes continent-sized air masses follow curvy tracks. It doesn't affect anything as small as a dust devil (or a toilet bowl).

Right. On a slightly off-topic note: someone told me that it also makes for unevenly worn rails on train tracks that run N-S. Kind of hard to believe, though, assuming there's about the same number of trains going N as there are going S.

-- Chris.

[Bcoz]

Right. On a slightly off-topic note: someone told me that it also makes for unevenly worn rails on train tracks that run N-S. Kind of hard to believe, though, assuming there's about the same number of trains going N as there are going S.-- Chris.

To continue in off-topic, for my first post here:
I have been working for a train company, and I confirm that this effect actually exist. The right rail is most worn than the left one. And for your objection, generaly there is two tracks, allowing trains to cross without incidents !

[atkinson]

To continue in off-topic, ... The right rail is most worn than the left one...

Um, if you turn 180 and examine the other right rail, is it also the most worn?

[Resasi]

if you turn 180 then it would be the left rail surely? On the other set of track that is

[Bcoz]

if you turn 180 then it would be the left rail surely? On the other set of track that is

No

Um, if you turn 180 and examine the other right rail, is it also the most worn?

Yes.

In north hemisphere, when you move in any direction (exept exactly from E to W or W to E) the Coriolis effect produce a force to the right.
In fact, Coriolis effect is very easy to understand (if not, it's probably an effect of my bad english):
When you move from South to North, you become closer to the axis of rotation of the Earth (on the equator, you are at 6350 km of the axis; at 45° of north latitude, you are at 4500 km of the axis). As you are spinning with the Earth from West to East, like a skatter squeezing up his arms, the conservation of kinetic momentum make you accelerate in the direction of your rotating movement, so to the East (so to the right of your movement from S to N).
When you go from N to S, you move away from the axis of the Earth, so you deccelerate, so you are subject to a force to the West, so to the right of your movement.

I suppose you know that it is for the same reasons than the teeter axis of our rotors is a few centimeters above the plan of rotation of the rotorhead, avoiding this way the need of a trainee axis on the base of the pales (sorry, I'm not sure to use the right words in english).

[Udi]

...I suppose you know that it is for the same reasons than the teeter axis of our rotors is a few centimeters above the plan of rotation of the rotorhead, avoiding this way the need of a trainee axis on the base of the pales (sorry, I'm not sure to use the right words in english).
You have lost me there, Bruno. The way I understand it, the teeter axis should be in line with the imaginary line connecting the CGs of both rotor blades in flight. In other words, the whole rotor CG, in flight, should pass right through the teeter bolt.

[dragonflyerthom]

Hi Udi

Glad to have met you at Mentone. I know that the rotor on the RAF is tilted 3 degrees to the left. I have heard that this is to compensate for the unequal lift and the extra load with two people.

[All_In]

Hi Thom!!!!
That's all; just following the thread and wanted to say I enjoyed meeting you and your wife.

OK back on topic.

[C. Beaty]

If you stand at the center and walk toward the rim of a rotating carousel, your body acquires kinetic energy and exerts a torque against the carousel that opposes rotation.

Same with a train traveling north or south.

The effect is greatest near the geographical poles and minimal when crossing the equator.

[C. Beaty]

You have lost me there, Bruno. The way I understand it, the teeter axis should be in line with the imaginary line connecting the CGs of both rotor blades in flight. In other words, the whole rotor CG, in flight, should pass right through the teeter bolt.Viewed from the rotorhead axis, Udi, there is an imaginary Coriolis force in a rotor without undersling.

As the blade of a coned rotor flaps upward, its CG moves nearer the rotational axis; the CG of a downward flapping blade moves away from the center of rotation.

But like a rock being twirled on a string, a hinged blade rotates about its own axis; not caring where the rotorhead axis is pointing.

A seesaw rotor without undersling would produce some nasty stick shake. Because of coning, as the rotorhead was tilted, the blade would have to be swept through an arc. This would occur 2/revolution in forward flight. The total CG of such a rotor is forced to rotate in a 2/rev circle.

[Udi]

Hi Udi

Glad to have met you at Mentone. I know that the rotor on the RAF is tilted 3 degrees to the left. I have heard that this is to compensate for the unequal lift and the extra load with two people.
Hi Thom - It was great meeting you and your wife as well. I believe the RAF rotor is tilted 3 deg. to compensate for prop turning torque. The teetering rotor is compensating for lift dissymmetry all by itself (unless you have too much friction in your teetering bolt).

John - is was great meeting you too. I only spent a couple of hours with you but I could feel that energy bubbling inside you. I bet you are hard to keep with (especially when you are "all-in") ;)

[All_In]

Udi I really enjoyed talking to you. I wish to come visit and just sit and listen and suck out all the knowledge you casually toss out to we mere mortals.

You all were so cool and friendly Mentone was truly GREAT!!!! I can't wait to see all you guys and the rest of you rotorheads there next year!!!
OK back on topic, sorry but new friends and all I could not help myself saying hi!!!

[Udi]

Chuck, I don't see what that has to do with Coriolis. The way I visualise this, the CG of a teetering rotor hinged without undersling moves back and forth over the axis of rotation twice/rev. When the blades are at 90/270, the total CG is right over the axis of rotation and when at 0/180 the total CG is back of the axis of rotation due to flapping. We are using undersling to hinge the rotor at, or close to, the total CG so there is no shifting of mass when the rotor is flapping.

I can see the Coriolis effect in a fully articulated rotor where the blades flap independently, not in a teetering rotor. I must be missing something...

[Bcoz]

You have lost me there, Bruno. The way I understand it, the teeter axis should be in line with the imaginary line connecting the CGs of both rotor blades in flight. In other words, the whole rotor CG, in flight, should pass right through the teeter bolt.

Udi,
C.Beaty explain in post #27, better than I can, what I wanted to say.
It's too the reason why three blades rotors must have a trainee axis, as it's not possible to align the four points on a same plan.

Bruno

[C. Beaty]

One picture is worth one thousand words, Udi.

Viewed from the rotorhead axis, the upflapping blade’s CG moves nearer the axis of rotation and must speed up if momentum is to be conserved. The downflapping blade’s CG has moved farther away and must slow down if momentum is to remain constant. That’s Coriolis for you.

Viewed from the rotor plane axis, nothing peculiar is happening except the total CG is forced to rotate in a 2/rev circle when there’s no undersling.

[All_In]

Chuck "when there’s no undersling" I did a search of undersling to see how it's applies but I can't find a definition. Where can I find what this is, sorry for ignorance to interrupt, but maybe someone else doesn't know either and I was following along so well until now.

[Udi]

Ok, I got the Coriolis effect Chuck. The Coriolis effect would induce in-plane moments if the rotor was not underslung. That, I suppose, is a good reason of the drag hinges in an articulated rotor.

I still don't think the Coriolis effect is the main reason for the undersling. The way I see it, the main purpose of the undersling is to minimize the 2/rev shift of rotor mass around the axis of rotation that is a result of flapping - not Coriolis effect. And... it seems to me that the total CG would draw a line, not a circle. Think about it - when the rotor is at 90/270 the total CG is right above the (non underslung) rotor axis.

[C. Beaty]

Udi, the figure on the left represents a seesaw rotor with CG above the teeter bolt. 180º of rotation later, it is in the same position. At 90º and 270º, the middle ball is aligned with the rotorhead axis. How does it get from one position to another? By following a circular path. Think about where it might be at 45º and quadrants thereafter.
***********
All, the second figure ought to explain undersling to you. The object is to get the rotor’s CG in alignment with the teeter bolt to minimize vibration.

[All_In]

Ohhh! Thanks not what I was thinking at all!!

[Bcoz]

... I suppose, is a good reason of the drag hinges in an articulated rotor.
Drag hinges is what I improperly named trainee axis.

I still don't think the Coriolis effect is the main reason for the undersling...the main purpose of the undersling is to minimize the 2/rev shift of rotor mass around the axis of rotation that is a result of flapping - not Coriolis effect...
It is the same phenomen, different analysis depending of the referentiel you use. Coriolis effect is an easy way to understand what happens when you are in the wrong referentiel, it's not real forces, the only physical law involved is inertia.
If you continue your analysis, 90/270, 180/0,... you will see that the move of CG is a circle.

Bruno

[C. Beaty]

Bruno, you’re a bright* guy. If I tried to explain this in French, I’d look like a total idiot.

*means very intelligent

[Resasi]

Thank you for your explanations to us Mr Beatty, probably done many times but you seem patient enough to do them again to help us understand better.