Question on Retreating Blade Stall

[Guest pokey]

nothing to post

Edited August 5, 2014 by pokey

[helonorth]

Helonorth, look at Chris's diagrams showing angles of attack around the disc. You might notice that they are different all around, the AoA is NOT the same on the advancing and retreating sides. The swash plate is continually changing the pitch through every cycle of rotation (Hey! THAT might be why it is called CYCLIC!)

Mr Hunt,

I understand that. I'm saying the pitch angles are the same on the retreating and advancing sides but the AOA's are different due to the changes in the resultant relative wind. In other words: AOA's are different due to flapping (flapping, NOT feathering) to compensate for the difference in velocity. And of course you would also have different AOA's at the front and aft portion of the disk in forward flight. I don't think you have any understanding of what blade flap is.

If you want to show me "the books" where you get you opinions (until then, they are just opinions) I certainly would love to see them. Until then, not buying it. Maybe in the southern hemisphere they have their own rules of physics that are different than ours?

Edited August 5, 2014 by helonorth

[Nearly Retired]

I'm with Eric here. I kind of laugh at pilots who insist that a rotor in forward flight will tend to flap *up* on the advancing side...in spite of hard evidence that the blade does just the opposite. Is this where the Robbie term "flapback" comes from?

 

Of course the advancing side does not flap upward. It flaps downward to produce that downward-tilted disk that allows us to fly forward.

 

In forward flight, if the cyclic were held in the exact center we would have severe dissymmetry of lift as the pitch angles of the blades would be the same all the way 'round the mast. Phase lag still comes into play, and so the Bell or Robinson helicopter would pitch up and roll left as the blades flap per our textbook definition.

 

But that's not what happens. With forward cyclic we REDUCE the pitch of the blade on the advancing side and INCREASE it on the retreating side. This causes (at least) two things to happen: 1) Phase lag will make the advancing blade flap down in front; and 2) the increased angle of attack of the retreating blade will move it closer and closer to stall as the speed increases.

 

One cannot talk about flapping and feathering separately.

[helonorth]

I would say it feathers downward on the advancing side yet flaps up due to the increase in velocity. The reverse on the opposite side. How else can you explain overcoming dissymmetry of lift? If you guys want to get together and write a book on helicopter aerodynamics and have it accepted by your aerodynamic engineer peers, that would be great. Until then, I will believe the information that we currently have available.

[Guest pokey]

Juan de la Cierva,,,,,,,,,,,,,,,,,,,,go read up on him

 

 

discussion over on my part

[Guest pokey]

 

Of course the advancing side does not flap upward. It flaps downward to produce that downward-tilted disk that allows us to fly forward.

 

 

OK i lied,, i was not done with this yet,,, well i was? but? nearly retired? i would like to to hear this

the advancing blade flaps up to compensate lift on the "other side" did Juan get it wrong??

 

i guess the auto gyro never flew??

 

it only flaps down if you have a delta 3 configuration

 

and even then? it is not called "flapping"

 

it is called "cyclic feathering" as when the blade wants to flap up? the cyclic geometry changes the pitch and "pulls" it back down

 

sheesh ! and i thought i was done with this,, and helonorth? chris? you 2 have this discussion pretty well nailed as to "right on"

 

and furthermore, as i have state previously... i know of NO helicopters that use a delta 3 hinge in their main rotor system

Edited August 5, 2014 by pokey

[Nearly Retired]

Well Pokey, cyclic feathering causes...what?

 

I think you would have to be a very unobservant helicopter pilot to say (or even think) that the advancing blade flaps "up" in forward flight.

[Guest pokey]

Well Pokey, cyclic feathering causes...what?

 

I think you would have to be a very unobservant helicopter pilot to say (or even think) that the advancing blade flaps "up" in forward flight.

 

 

amazing! isnt it? the the forward airspeed , along with the flapping hinge, allows the helicopter to fly

 

and we are NOT talking about the "observant " helicopter pilot. we were talking of helicopter aerodynamics

 

any helicopter pilot knows that the disc is lower in front,, that is why you are looking at the ground on take off

Edited August 5, 2014 by pokey

[Pohi]

Wow. I could have sworn that the rotor disk tilts forward because the pilot made it go forward with the cyclic.

 

I wonder why, when sitting on the ground, if a pilot moves the cyclic the rotor disk moves (tilts) in the same direction. Since there is no advancing blade or retreating blade due to lack of relative wind, how is it flapping?

[Guest pokey]

Well Pokey, cyclic feathering causes...what?

 

I think you would have to be a very unobservant helicopter pilot to say (or even think) that the advancing blade flaps "up" in forward flight.

 

cyclic feathering causes forward flight, , but without blade flapping? you would roll over on your side

 

most likely on your left side, if your rotor is spinning counterclockwise (as viewed from above)

Edited August 5, 2014 by pokey

[Pohi]

I'm not sure if you guys are trolling or not, but well done either way. You made me look in a few books and try to figure out if my memory had completely failed me and disagreed with all aerodynamic references I have available

[Nearly Retired]

Pohi:

 

Since there is no advancing blade or retreating blade due to lack of relative wind, how is it flapping?

 

Seriously, Pohi?

[Guest pokey]

Pohi:

 

 

Seriously, Pohi?

in a still wind hover, there is NO need for blade flapping

[Eric Hunt]

 

 

I would say it feathers downward on the advancing side yet flaps up due to the increase in velocity. The reverse on the opposite side. How else can you explain overcoming dissymmetry of lift?

You still believe that as the advancing blade moves downwards to reach the front of the disc (which is tilted forward to cause forward thrust and make it fly) but it is really flapping upwards?

 

This dissymmetry of lift on which you are all hanging your hats, is a phenomenon which would cause some problems to a helicopter trying to get into forward flight. It would cause flapback, and in fact it does!!

 

Have a look at how you transition to forward flight from the hover.

 

You introduce some forward cyclic, the disc tilts forward, the aircraft starts to move. As soon as that happens, the aircraft tries to nose up and stop moving forward. This is flapback, caused by dissymmetry of lift.

 

THE PILOT ADDS MORE FORWARD CYCLIC TO OVERCOME FLAPBACK.

 

So, once you use cyclic, you are forcing the blade to go where YOU want it, and not where good ol' Juan allowed it to go. Remember that the autogyro was called that name because the rotor disc was in autorotation all the time. It was flapped back, creating lift, but also creating lots of drag (lift vector pointed backwards) and the engine with propellor was dragging it forwards through the air. Whole different ball of wax.

 

You don't need a delta 3 hinge on a main blade, and I wonder where you got that weird idea from.

Edited August 6, 2014 by Eric Hunt

[Eric Hunt]

helonorth says:

 

 

I would say it feathers downward on the advancing side yet flaps up due to the increase in velocity. The reverse on the opposite side. How else can you explain overcoming dissymmetry of lift?

You might say it, but it is awfully wrong.

 

Look at Chris's copies of your own books, Dissymmetry of Lift para 1-105, and then Feathering, 1-110. I don't have to write my own aerodynamics book, these kind people already did that for me.

[iChris]

Pokey, the cyclic pits pitch on the blade, the blade's lift changes, and the lift causes the blade to rise or fall - flapping. Result - the cyclic controls the blade's flapping. So does the collective.

 

If you couldn't control the flapping with cyclic, the machine would not fly in a predictable manner.

 

That's correct. Cyclic pitch inputs (feathering) allow us to trim the helicopter in forward flight and control flapping in order to produce the required control moments for maneuvering.

 

The pilot has the ability to control flapping.

 

Flapping is sometimes a by-product or consequence of another action. I think that's what some people are missing.

 

 

'>http://youtu.be/Mx7vLB3KP1w

[Guest pokey]

 

That's correct. Cyclic pitch inputs (feathering) allow us to trim the helicopter in forward flight and control flapping in order to produce the required control moments for maneuvering.

 

The pilot has the ability to control flapping.

 

Flapping is sometimes a by-product or consequence of another action. I think that's what some people are missing.

 

 

 

 

Well of korse ! cyclic allows the helicopter to fly not only forward, but sideways and backwards too. Trim is relieving the stick force.

 

How would the pilot control just flapping? I know in my 300, if i want to make the blades really slap (flap) i descend with collective and pull back on the cyclic at just the right power setting, but? this is only useful for waking up your neighbors on an early sunday morning.

 

flapping is always the by-product of another action, but the reason the flapping hing was invented? again,,, read your history on the auto gyro

[Eric Hunt]

What point are you trying to make here, Pokey? You keep referring back to the autogyro, a different animal from a helicopter.

 

You keep referring to Juan and the flapping hinges, as if they are the ant's pants. Look at a BO 105 or a BK117 - they don't have flapping hinges - the head is totally rigid. The flapping forces are absorbed by the blade flexing. The AS350 and similar Eurocopter rotor heads are made of flexible composites and don't have flapping hinges (or delta 3 hinges either). Juan doesn't get a look in here at all.

 

read Chris's post. Look at the video. Then tell me again that the blade, while moving down, is flapping up.

[Guest pokey]

What point are you trying to make here, Pokey? You keep referring back to the autogyro, a different animal from a helicopter.

 

You keep referring to Juan and the flapping hinges, as if they are the ant's pants. Look at a BO 105 or a BK117 - they don't have flapping hinges - the head is totally rigid. The flapping forces are absorbed by the blade flexing. The AS350 and similar Eurocopter rotor heads are made of flexible composites and don't have flapping hinges (or delta 3 hinges either). Juan doesn't get a look in here at all.

 

read Chris's post. Look at the video. Then tell me again that the blade, while moving down, is flapping up.

 

early helicopters, (sikorsky),, why do you think he quit and then went off to build airplanes?---because he knew not of the flapping hinge, the auto gyro and its discovery of the flapping hinge made forward flight possible. When that hinge was incorporated into the helicopter? WOW ! it works now !

 

they did not have elastomerics, nor fiberglass blades back in those days, as like they do in modern helicopters,

 

a little bit of history goes a long, looong.. long long way

 

and that huey in the video? the disc is "tilting" via cyclic input, flapping will not occur until some wind or forward flight is included.

 

and BTW? 2 bladed bell products do not have a flapping hinge, its called a teetering hinge.

 

lets take it a bit farther, underslung rotor, fully articulated, and of korse ! the good ole robbie 'tri-hinge"

 

anyone care to elaborate ?

Edited August 6, 2014 by pokey

[Nearly Retired]

I think ol' Pokay is cornfuzling flapping with coning.

[Pohi]

This just might be me, but if I have to choose to believe either

 

1) every textbook or reference I have seen

2) some random person on a forum

 

It's not a tough choice.

 

Yeah, at one time they said the world was flat, but eventually the textbooks came around. Until the experts agree with you guys imho, you are just horribly confused and are trying to make aerodynamics work how you want them to rather than how they actually do work.

 

Post a few more times about how all the books are wrong, who knows... it might come true.

[Guest pokey]

I think ol' Pokay is cornfuzling flapping with coning.

 

 

no ole pokey is not, coning is when ya pull pitch and the blades "cone" up

 

tell me how coning has anything to do with flapping?

Edited August 6, 2014 by pokey

[eagle5]

This just might be me, but if I have to choose to believe either

1) every textbook or reference I have seen

2) some random person on a forum

It's not a tough choice.

Yeah, at one time they said the world was flat, but eventually the textbooks came around. Until the experts agree with you guys imho, you are just horribly confused and are trying to make aerodynamics work how you want them to rather than how they actually do work.

Post a few more times about how all the books are wrong, who knows... it might come true.

Textbooks shmeshbooks, what does Wikipedia say!

[HighCountry]

no ole pokey is not, coning is when ya pull pitch and the blades "cone" up

 

tell me how coning has anything to do with flapping?

Nothing, that was his point. What does the aerodynamic trait of blade flap have to do with the acoustical trait of blade "slap"?

[Guest pokey]

What does the aerodynamic trait of blade flap have to do with the acoustical trait of blade "slap"?

 

i have no idea, but i do know i can make the blades make a helluva noise if i want to.

 

maybe you know?