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Helicopter Aerodynamics/Speed Limit Question


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Hi guys, I have a question that one of you probably knows the answer to...

 

I of course know the issue that once you hit a certain speed at a given configuration, the bottom rotor disk on the right half of the disk is raised upwards due to the advancing blade creating more lift. Also, the top rotor disk does the opposite...the right half of the disk droops due to the retreating blade effect.

 

My question is this, I know that the blades will eventually hit each other if you go past a certain speed limit. But for a conventional helicopter with one rotor disk, does some equivalent speed limit exists? In other words, is there a point at which the retreating blade stalls and the advancing blade has such an increase in lift causing an uncontrollable dyssemtry of lift that the blades break off from the disk?

 

I am wondering if this strict speed limit issue only exist in co-axial helos and that conventional helos would just stall to the left before preventing structural damage.

 

I hope anyone who flies helicopters understands what I mean and I'm not sure if I explained it good enough... I hope so :)

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

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My understanding is that the first problem a helo will run into as the speed of the helo increases, that the advancing blade will start breaking the sound barrier and as such make tiny sonic-booms that will reduce stability, and as such, no one really knows what awful thing would happen if the blades went even faster.

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One way to make a real simple gedanken-experiment to find the problem single-rotor designs have that the coax avoids is to make the following assumptions:

 

Helicopter moves at 200km/h

Rotor rotates at 200km/h (at the tip)

 

This means that at the advancing side, the rotor tips would be doing 400km/h (200+200), on the retreating side they're doing 0 (200-200). Obviously, that helicopter doesn't fly and indeed will never actually get close to that state while still being controllable. Co-axials (assuming that they're also contra-rotating... :P ) avoids this problem, but of course have different ones.

 

It is my understanding that the british Lynx avoids aspects of these problems with it's special rotor tips - someone that knows more about that might be able to shed some light on it? That would be very interesting.

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gedanken-experiment = mental-experiment?

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"Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance." (Dr. A. R. Dykes - British Institution of Structural Engineers, 1976)

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Well, I've got a master's degree in helicopter aerodynamics although I haven't used any of that knowledge outside of Black Shark in 8 years, but I'll give it a shot anyway. :)

 

The Lynx uses the BERP tip, which was intended to weaken the vortex generated at the tip, as this increases stability, reduces noise, and improves efficiency. If the BERP tip helps achieve higher forward speeds, I would guess it's because the rotor itself is being made more efficient, thus there is excess power left over for further speed increase.

 

The question of single-rotor helicopters' speed limits I think has already been answered: the problem is twofold: advancing blade reaching transonic speeds and retreating blade soon being consumed by a region of zero or even negative-velocity regions, which destroys the lift it produces. Thus you get very unstable lift on the advancing side and minimal or no lift on the retreating side, and obviously that's not too conducive to staying in the air. :)

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gedanken-experiment = mental-experiment?

 

Yes :smilewink:

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Thanks all for your responses and it clears some things up for me but brings me another question:

 

I understand that you'll have the retreating rotor region in stall or even negative relative velocity at a certain speed limit, but does this cause structural failure due to the lift differential, or would the the helicopter just roll to the left (assuming the left half of the rotor is the retreating half) and allow for recovery (assuming enough altitude), must like an aircraft recovers from a stall?

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

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Well, in a normal helicopter you would probably have to do some rather drastic stuff to reach this state of flight. Getting to the point where only half your rotation gives any lift at all is unusual.

 

However, cyclic trim can ease the effects as you approach the regime where this becomes a factor through allowing cyclic to balance the alpha and (thereby) lift

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Single rotor helos do indeed have a tendency to roll to the retreating side (US helos generally to the left, others to the right) as forward airspeed increases. The phenomenon is known as Dissymetry of Rotor Lift.

 

It is noticeable in the normal flight regime, i.e. at cruise airspeeds.

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The way I understand, the speed where retrieving blade stall occurs is refer to as VNE (Speed never exceed) The effects on the helicopter is dependent of the designed. AFAIK, you could just roll (left or right) and pitch (up or down)or like in KA-50, rotors could strike each other. You could also hit part of the tail boom or fuselage due to Gyroscopic Precession (lost of lift on the right or left side will affect the rotor until it travels 90 degrees, more or less) The effects would be most likely define in each individual helicopter manual.


Edited by mvsgas

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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Simple mathematics:

Advancing blade travels at, say, 600mph + helicopters forward speed

Retreating blade travels at 600mph - helicopters forward speed

 

The retreating side will stall at certain speeds, while the advancing side will overspeed. The point at which the retreating blade stalls would be the aircrafts limited maximum forward speed, since it causes the aircraft to become unbalanced, and the CG of the aircraft will shift to that side.

 

The blades reaching supersonic speeds lose efficiency in creating lift. Bernoulli's principle states that with an increase in pressure at subsonic speeds, velocity decreases, and vice versa with speed and pressure. At supersonic speeds, the exact opposite happens. Youll lose lift because pressure increases above the rotor disk. Its negative pressure that keeps aircraft flying.

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Vne on a helicopter is usually the forward speed at which the advancing blades are near supersonic. In most cases, this will happen well before the retreating blades stall. Rotor blades on a helicopter are not at all designed to go supersonic, they would either come apart or be seriously damaged if they did, so the fact that won't generate nearly as much lift is a secondary concern. Fully-articulated rotor systems are really good at compensating for dissymmetry of lift, so retreating blades stall is even less of a concern. At any rate, the aircraft's Vne will either reflect the speed at which the advancing side approaches supersonic (usually the case) or the retreating side approaches stall speed.

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I was quoting Faa rotocraft hand book

 

You can avoid retreating blade stall by not exceeding

the never-exceed speed. This speed is designated VNE

and is usually indicated on a placard and marked on the

airspeed indicator by a red line.

 

http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

 

Page 3-7

 

page 11-7

Retreating blade stall is a major factor in limiting a

helicopter’s top forward speed (VNE)...


Edited by mvsgas

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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Just spent some time grilling a few pilots and RBS is indeed the primary factor in determining Vne. Compressibility stalling is very rare, since in order to get to high enough airspeeds to induce it requires a high level of thrust which will put you into RBS faster.


Edited by AlphaOneSix
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Thanks all for your responses and it clears some things up for me but brings me another question:

 

I understand that you'll have the retreating rotor region in stall or even negative relative velocity at a certain speed limit, but does this cause structural failure due to the lift differential, or would the the helicopter just roll to the left (assuming the left half of the rotor is the retreating half) and allow for recovery (assuming enough altitude), must like an aircraft recovers from a stall?

 

These days the blades are made such that they don't fail when you hit RBS conditions - bear in mind that when the blade stalls, it generates no more lift so in effect it's under less stress in that portion than normal operation.

 

The "normal" pilot explanation also says that with RBS you don't get a roll to the retreating side necessarily - you'll in fact get a pitch up. The explanation is because of the (in FAA parlance) Gyroscopic precession which means that while the stall is on the retreating side, the max loss of lift point is 90 degrees later in the rotation plane. In effect the stall is felt on the disk as a loss of lift at the aft of the disk coupled with lift being produced at the front - a net pitch up of the disk.

 

The generally accepted recovery procedure is to lower collective (reduce blade pitch from stall angle) and reduce forward cyclic (slowing the forward speed).

 

Colloquially most people say that the helicopter will also roll but the reasons for that are unclear - could be something as simple as Center of Gravity position, the sudden loss of airspeed and associated reversal of inflow roll, or other things.

 

There's an "instructor story" of a guy arriving in LA from San Francisco complaining that the aircraft (MD500 in the version I was told) had a fault since it kept porpoising all the way - which was subsequently found to be RBS because the guy was exceeding Vne. Sounds unlikely, but who knows!

 

Either way, it is a recoverable event but as Alpha says above, the Vne can be lower for other reasons before RBS becomes a factor (type I fly for example, had a Vne reduction recently because the horizontal stabiliser can depart the aircraft given certain conditions at high speed).

 

r.

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Well, sens we are in the subject about helicopter aerodynamic:

What are the benefits or the "Hingeless Rotor" or "Semi-Rigid Rotor" as oppose to the fully articulated rotor? Is it just simplicity of maintenance or are there any aerodynamic benefits as well?

 

For example

DI53G4.jpg

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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Well, sens we are in the subject about helicopter aerodynamic:

What are the benefits or the "Hingeless Rotor" or "Semi-Rigid Rotor" as oppose to the fully articulated rotor? Is it just simplicity of maintenance or are there any aerodynamic benefits as well?

 

For example

 

Until recently, fully-articulated rotor systems did require a great deal more maintenance than semi-rigid rotor systems. This is due to the fact that there are a number of hinges involved that all require lubrication and inspection, and eventually wear down, as well as the requirement for a damping mechanism in the lead/lag hinge, which also requires more maintenance. Newer systems utilize elastomeric bearings and rubber dampers which remove most of the maintenance requirement, but they are still a bit more complicated than a semi-rigid system.

 

In order to have a fully-articulated rotor system, you need to have at least three rotor blades, so any helicopter with only two rotor blades cannot be fully-articulated, and will instead be semi-rigid. Semi-rigid rotor systems "teeter" (like a see-saw) about the rotor mast in order to provide a flapping motion for the blades. Since there are only two blades, one can flap up while the other flaps down, and vice-versa. Since there is no hinge point between the two blades, they cannot lead and lag. You can actually have two, three, or even four rotor blades in a semi-rigid system (Lynx is a great example of a four-bladed semi-rigid rotor system).

 

The Bell 412 rotor system, which you have pictured, is able to flap, lead, and lag on an individual blade basis, but only to a small amount. This is due to the installation of an elastomeric bearing at the pivot point of the blade. So it would not be classified as a semi-rigid system, but calling it fully-articulated would be a bit of a stretch. It is a hingeless system in that it doesn't utilize hinges for the flapping and lead/lag joints, but relies instead on flexible beams and elastomeric bearings. This makes it very maintenance-friendly, but also very expensive.

 

Hingeless and fully-articulated rotors are both able to move in all three directions (flap, lead/lag, and feather), but hingeless systems typically do not have the same range of movement as a fully-articulated system. Hingeless systems are also sometimes called "rigid" systems since they have no hinges, but in practice they act more like fully-articulated systems.

 

Semi-rigid rotors can feather and flap, although the flapping up of one rotor necessitates the flapping down of the opposite rotor, they cannot flap independently. Also, semi-rigid rotors cannot lead or lag at all. This typically causes more vibration than you might see in a fully-articulated system.

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Well, sens we are in the subject about helicopter aerodynamic:

What are the benefits or the "Hingeless Rotor" or "Semi-Rigid Rotor" as oppose to the fully articulated rotor? Is it just simplicity of maintenance or are there any aerodynamic benefits as well?

 

For example

DI53G4.jpg

 

There is a table that defines the differences between the types- I think it's in the FAA syllabus notes somewhere.

 

It came down to the Flapping, Lead/Lag and Feathering (pitch changing) and whether each blade is hinged for those motions....

 

 

Fully Articulated - Each balde can Lead/Lag, Flap and Feather independent of the others.

Semi-Rigid (Robinson/Bell Types) - Each blade can flap and feather about it's own hinges.

Rigid - Each blade can Feather independently (Military types)

 

The differences can come down to cost. Fully articulated use a hinge system and so are cheapest to produce since the blade material is "simple" as it doesn't have to absorb much of the aero forces - the hinge does the work.

 

Whereas a Fully Rigid system - the blades have to absorb the forces of flapping and lead lag which therefore requires some very strong but flexible composite material (there's a vid around somewhere of a blade as it goes round - it's in slow-mo but the motion is scary/fascinating depending on your point of view!).

 

The other trade off is performance. Fully articulated are generally considered less responsive than Semi Rigid which are in turn less responsive than fully rigid. Bear in mind with helicopters that "less responsive" to "more responsive" might only be half a second, but still....

 

Finally from the flight side, Fully rigid are often said to provide a "stiffer" ride to the cabin compared to a fully articulated system where the hinges are able to absorb more of the aero forces.

 

Typically Semi-Rigids (a'la Robbies and some Bells) present a mast bumping problem - not ideal for military flying (or civilian for that matter). Fully articulated rotors can get ground resonance. Not sure about fully rigid in terms of their potential pitfalls although for the military storage is one since you can't take off a couple of blades to store the things on a ship for example - the whole head has to come off.

 

 

Anyway, as you can see the whole problem is a big tradeoff of performance, cost, maintenance and ease of flying.

 

The picture you showed looks like a Eurocopter Starflex system - that system is interesting in that it meets the definition of Fully Articulated (each blade can flap, feather and lead lag independently) but as you can see from the picture - there are no actual hinges. The starflex uses elastomeric bearings to allow blade movement in all axis and an elastomeric block for dampening, making it simple in maintenance and design, cheap to maintain (not a great deal to break on the rotor head) and offering performance closer to that of a rigid system.

 

 

EDIT : Found the rotor blade vid I mentioned. Think it was a german helicopter manufacturer (which dates it!) -

 

r.


Edited by ryuzu
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Well guys, as always, thank you for the information. :thumbup:

I understood the difference of the rotors and how the work, I was wandering more about the benefits or each system and from what you guys are saying; maintainability is the main benefit from hingeless rotor, with some improvement in response time... correct?

 

The reason I was asking, is because I see more and more new helicopters using, what appears to me, to be this hingeless rotors.

 

Looks like the AH-1Z, UH-1Y, many Eurocopters(EC135, AS 350B2/EC130, EC145) and some Bell (412, 417), BO-105, etc, are using this rotor systems.

 

Thanks for the video ryuzu, that is very cool.

 

I saw this one looking and trying to understand the hingeless rotor.

If the real thing flexes as much as this computer animation, that is crazy the amount of forse this composites can absorb.

 

By the way, the rotor I posted was label as an AS-350 rotor, not sure how accurate that was.

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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These days the blades are made such that they don't fail when you hit RBS conditions - bear in mind that when the blade stalls, it generates no more lift so in effect it's under less stress in that portion than normal operation.

 

The "normal" pilot explanation also says that with RBS you don't get a roll to the retreating side necessarily - you'll in fact get a pitch up. The explanation is because of the (in FAA parlance) Gyroscopic precession which means that while the stall is on the retreating side, the max loss of lift point is 90 degrees later in the rotation plane. In effect the stall is felt on the disk as a loss of lift at the aft of the disk coupled with lift being produced at the front - a net pitch up of the disk.

 

The generally accepted recovery procedure is to lower collective (reduce blade pitch from stall angle) and reduce forward cyclic (slowing the forward speed).

 

Colloquially most people say that the helicopter will also roll but the reasons for that are unclear - could be something as simple as Center of Gravity position, the sudden loss of airspeed and associated reversal of inflow roll, or other things.

 

There's an "instructor story" of a guy arriving in LA from San Francisco complaining that the aircraft (MD500 in the version I was told) had a fault since it kept porpoising all the way - which was subsequently found to be RBS because the guy was exceeding Vne. Sounds unlikely, but who knows!

 

Either way, it is a recoverable event but as Alpha says above, the Vne can be lower for other reasons before RBS becomes a factor (type I fly for example, had a Vne reduction recently because the horizontal stabiliser can depart the aircraft given certain conditions at high speed).

 

r.

 

ryuzu you nailed what I was asking. Most people were just repeating what others had originally said.

 

Thanks for figuring out what I was looking for and thanks for the help. Your explanation makes perfect sense.

"Witness mere F-14s taking off from adjacent flight decks, gracefully canting left and right, afterburners flaming, and there’s something that sweeps you away—or at least it does me. And no amount of knowledge of the potential abuses of carrier task forces can affect the depth of that feeling. It simply speaks to another part of me. It doesn’t want recriminations or politics. It just wants to fly.”

― Carl Sagan

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