Confusion

[FlyGirl86]

There has been some debate as to exactly what windmill brake state is. Everyone I ask pretty much gives me their own definitions of it and I am completely confused as to what is ACTUALLY is. My instructor gave me the answer he thinks is best, but he is not doing my checkride. Whoever is doing my check ride might have another idea. So I was just looking for some opions, definitions, explanations? Thanks guys!

[tvman345]

I have only heard mention of WBS in passing. It's my understanding that it is one of those deep aerodynamic concepts involved in Autorotation. Perhaps someone else on this board far more intelligent than me can explain it.

That said, any DE that nails you to the wall for not knowing the specifics of WBS is totally out of line IMHO. Go to another DE. They should be far more concerned that you can select an appropriate landing spot, perform a successful auto, and not ball up the helicopter in the process. But that's just my opinion.

 

Good luck.

[Jeff]

I have only heard mention of WBS in passing. It's my understanding that it is one of those deep aerodynamic concepts involved in Autorotation. Perhaps someone else on this board far more intelligent than me can explain it.

That said, any DE that nails you to the wall for not knowing the specifics of WBS is totally out of line IMHO. Go to another DE. They should be far more concerned that you can select an appropriate landing spot, perform a successful auto, and not ball up the helicopter in the process. But that's just my opinion.

 

Good luck.

 

I agree. Don't worry about it for your checkride. If it's not in the Rotorcraft Flying Handbook or the Pilot's Handbook of Aeronautical Knowledge, then most likely you won't be required to know it.

 

Jeff

[coanda]

This is a very technical study on blade geometry. Your eyes will probably be crossed by the time you get to it but they do talk about windmill brake state. Hope it helps and your brain stays in its solid form!!!

 

http://www.ae.ic.ac.uk/research/rotorcraft/pubs/ERF30.pdf

 

* WBS is first mentioned in the last paragraph on 11-5 and continues from there.

* OK I read a little further and got completely lost so sorry if this confuses you more. I still couldn't find a solid definition.

Edited January 13, 2007 by coanda

[Linc]

Windmill brake state is the state of the rotor in greater than autorotative descent rates.

 

From Army FM 1-203 (Oct 88):

Windmill brake state

 

At very high rates of descent, the airflow is almost entirely up through the rotor system. The rotor system is acting similar to a windmill. It is extracting more energy from the air than is required for flight. It is not a normal operating state for the rotor system; some energy must be extracted to prevent a rotor overspeed. This can usually be accomplished by increasing collective pitch, which adds more drag to the system.

I agree with everyone else. Don't worry about it for your checkride.

[Goldy]

Fly- you probably wont be asked, but here is my version in a nutshell.

 

During a normal auto, most of the blade is stalled, right? Except for the tip portions of the rotor, known as the "driven" region of the blade. This driven region drives the blade and keeps your rotor rpm up (as you descend), and overcomes the rest of the stalled blade, which is mostly drag. In doing so, the tips of the blade are producing downward thrust ( which slows your rate of descent), the center of the blade is stalled so air just flows up and past it..

 

When you are in the windmill state, you have now lost the lift being produced by these tip portions of the rotor...they turn to mush, so there is no longer any lift being produced by the tips, so all airflow is upwards.

 

WHEW !

[Eric Hunt]

Goldy said:

"During a normal auto, most of the blade is stalled, right? "

 

WRONG

 

The inner 25% is stalled, and producing only drag.

The next 55% is producing more and more lift, and is driving the blade forward, as the total reaction moves from pointing forward to pointing vertically.

The last 20% is producing lift, but the total reaction is tilted backwards and trying to slow the blade down, plus it is coping with the tip vortex.

 

It produces lift by extracting energy from the airflow coming from below, not by pushing the air back downwards.

 

Then Goldy says:

"When you are in the windmill state, you have now lost the lift being produced by these tip portions of the rotor...they turn to mush, so there is no longer any lift being produced by the tips, so all airflow is upwards."

 

Goldy, if there ain't no lift, nothing is making your blades turn, they will clap together above your head, and you are in accelerating freefall, headed for the dirt at 10,000 feet per minute plus.

 

In an auto, you are usually going down at 1800 fpm in a B 206 and a bit higher in bigger, draggier machines.

 

But as everybody has said, don't sweat the examiner asking about Windmill Brake State. If he does, tell him it is near Washington State, and you don't intend to go there.

[nsdqjr]

Fly- you probably wont be asked, but here is my version in a nutshell.

 

During a normal auto, most of the blade is stalled, right? Except for the tip portions of the rotor, known as the "driven" region of the blade. This driven region drives the blade and keeps your rotor rpm up (as you descend), and overcomes the rest of the stalled blade, which is mostly drag. In doing so, the tips of the blade are producing downward thrust ( which slows your rate of descent), the center of the blade is stalled so air just flows up and past it..

 

When you are in the windmill state, you have now lost the lift being produced by these tip portions of the rotor...they turn to mush, so there is no longer any lift being produced by the tips, so all airflow is upwards.

 

WHEW !

 

This explantion is incorrect. The blades aren't stalled at all, they produce lift regardless of the direction of airflow.

 

The Windmill Brake State occurs when almost the entire rotor system is acting as the Driving Region and accelerating the blades.

 

When you talk about the Windmill Brake State, or the Steady State, you want to think in terms of Total Aerodynamic Force, not in terms of lift. There are a two main things you want to think about in terms of what affects the TAF: Blade Speed (not RPM), and the amount of Upward Inflow.

 

Blade Speed. This is pretty simple. The faster the blade spins, the more drag is has. Because of this increase in drag the TAF points up and behind the airfoil. Basically it is producing lift and trying to slow down, this is why the Driven Region is on the outer (fastest) portion of the blade. As you move inward on the blades the velocity decreases, the drag does also, and the TAF moves further forward until it is pointing up and to the front of the airfoil. This is the Driving Region. There is a point of equilibrium between the Driving and Driven region when the TAF basically points straight up, producing lift but neither attempting to speed up OR slow down.

 

Upward Inflow. I can't show you lift vectors on here so I'll try to make this as simple as possible. The greater the Upward Inflow, the further forward the TAF gets. The Windmill Brake State exists during the Entry and the Flare of the Auto because of this. The sudden lowering of the collective and the aft cyclic in the entry cause a great amount of Upward Inflow to occur. Once the helicopter is stablized it transitions to the Steady State where the rotor system divides itself into the three regions. At the bottom, the flare causes an increase in Upward Inflow again and the rotor system enters the Windmill Brake State again. In the Windmill Brake State almost the entire rotor system is acting as the Driven region. This, combined with the Coriolis Effect due to Coning, is what help us build the Rotor RPM during the flare. It's very pronounced in a Low Inertia Rotor System.

 

Whew, good stuff!! I made a pretty good Power Point of this if anyone is interested, just message me. Hope this helps!

Edited January 14, 2007 by nsdqjr

[Witch]

... nothing is making your blades turn, they will clap together above your head...

 

Maybe notso in a Robbie. Check this out.

 

When performing a preflight next time, try pushing the blade up as far as you can. Can't go up too far? There's a stop there.

 

The Chief CFI asked me what happens when you put a forklift under each blade and go up on the forks. I said they'll go up until they hit the stop and the whole bird will lift off the ground. He said that they'll go all the way up. This confused me because I have lifted the blade up until it hits something and stops. After some arguing and a lot of name-calling and expletives, he admitted to me that there is a stop on the hub.

 

Where am I going with this? Haven't a clue.

 

Later

[nsdqjr]

Maybe notso in a Robbie. Check this out.

 

When performing a preflight next time, try pushing the blade up as far as you can. Can't go up too far? There's a stop there.

 

The Chief CFI asked me what happens when you put a forklift under each blade and go up on the forks. I said they'll go up until they hit the stop and the whole bird will lift off the ground. He said that they'll go all the way up. This confused me because I have lifted the blade up until it hits something and stops. After some arguing and a lot of name-calling and expletives, he admitted to me that there is a stop on the hub.

 

Where am I going with this? Haven't a clue.

 

Later

 

Pushing the blades up until they hit the droop stop is a FAR different thing!! If you did like you said and lifted each blade with a forkilft they'd eventually snap off. The blades are nowhere near strong enough to support the helicopter until you add the component of Centrifugal Force. Imagine spinning a yo-yo on a string above your head. The string is rigid and will support the weight of the yo-yo because of centrifugal force. Same thing in the blades. Low Rotor RPM=blades folding up and the end of your flight.

Edited January 14, 2007 by nsdqjr

[Bristol]

Then again, The Mr. Fusion Home Energy Reactor RHC Rotor Blade Add-On Kit w/ one point twenty-one jigowatts of Posterior Cortical Atrophy (see figure 1), will take care of all that, including spar seperation at speeds in excess of warp seven. Just make sure that your defibulator is fully charged before installing the vector.

In tensor representation a vector can be expressed as the sum of the products of each of its components times the basis vector belonging to that component in two ways (repeated indices are assumed to sum)

and entering Windmill brake state will occur almost immediatly.

 

Hope that helps......

 

 

 

FIGURE 1

[Goldy]

Then again, The Mr. Fusion Home Energy Reactor RHC Rotor Blade Add-On Kit w/ one point twenty-one jigowatts of Posterior Cortical Atrophy (see figure 1), will take care of all that, including spar seperation at speeds in excess of warp seven. Just make sure that your defibulator is fully charged before installing the vector.

In tensor representation a vector can be expressed as the sum of the products of each of its components times the basis vector belonging to that component in two ways (repeated indices are assumed to sum)

and entering Windmill brake state will occur almost immediatly.

 

Hope that helps......

 

FIGURE 1

 

Damn- I knew I shouldnt have tried to answer that one in a shortened version of rotor dynamics !

Depending on who you ask, the driven region of the blade can be as small as 25%..oh well..

[flingwing206]

After a quick read through this thread, just to reiterate two points:

 

as nsdqjr said, windmill brake state is (at its logical extreme) when all the lift being created in the rotor is going toward driving the rotor. If you take the "brake" out of the phrase, it's a little easier to understand.

 

To expand (and perhaps confuse, but here goes) - we control the RPM of the rotor (given a constant airspeed and pitch attitude of the aircraft) by changing the AOA of the blades. This in turn re-orients the combined lift/drag components. The more you raise the collective, the more the net of these combined vectors "lean back", slowing the rotor. If the collective is lowered these vectors tilt "forward", speeding the rotor. (I have left out the discussion of what happens to total lift.) The reality is that helicopters don't have the blade-pitch range to get into the state described in the RFH.

 

Second point:

 

If you were to try to pick up your R22 by the blade tips when the rotor was not spinning, the tips would rise until the coning hinges reached the limits of their travel, at which point the blades would start to bend. This, however, has nothing to do with the windmill state - centrifugal force in a spinning keeps those blades near-horizontal and very rigid, and the coning hinges never get near their upper stops.

 

And a total aside for all Robinson pilots - I hope you've gotten the very recent service letter which instructs a daily blade-tip/leading edge inspection for ALL Robinson blades (R22, R44, R44-II). While I'm sure you have a look down the blade while you preflight the rotorhead, this is a get-the-stepladder, foot-by-foot kind of inspection. On the good side, it is a daily, not a per-flight inspection.

[Eric Hunt]

Flingy said:

 

"The more you raise the collective, the more these combined vectors "lean back", slowing the rotor. "

 

This is actually because the various regions on the rotor move outboard. Pull more pitch, and the stalled region gets bigger by moving further out, causing more drag. The driving region moves further out, and the lift-producing region on the outboard section which is producing drag will decrease as it falls off the end. The overall effect is a bit less lift and more drag, so you will lose RRPM - but used diligently, it is the way to control auto RRPM.

[FlyGirl86]

Whoa-Thanks for all the help. I have to admit some of the responses did confuse me a little bit the first few times i read them, but after going through all of them, i think i can peice together an answer for my oral if I am asked. Thanks again!!!

[flying high]

Whoa-Thanks for all the help. I have to admit some of the responses did confuse me a little bit the first few times i read them, but after going through all of them, i think i can peice together an answer for my oral if I am asked. Thanks again!!!

i wouldnt sweat it it more than likely wont come up. It didnt come up once during any of my rides

[Witch]

What's a windmill brake effect again?

[Linc]

What's a windmill brake effect again?

It's when you're driving through the countryside and you brake to see the windmill farms.

[FlyGirl86]

It's when you're driving through the countryside and you brake to see the windmill farms.

 

 

You crazy, crazy rotor heads. Crack me up.

[RockyMountainPilot]

Maybe notso in a Robbie. Check this out.

 

When performing a preflight next time, try pushing the blade up as far as you can. Can't go up too far? There's a stop there.

 

The Chief CFI asked me what happens when you put a forklift under each blade and go up on the forks. I said they'll go up until they hit the stop and the whole bird will lift off the ground. He said that they'll go all the way up. This confused me because I have lifted the blade up until it hits something and stops. After some arguing and a lot of name-calling and expletives, he admitted to me that there is a stop on the hub.

 

Where am I going with this? Haven't a clue.

 

Later

 

When you lift one blade and you hit a stop you are hitting the tetering stop. Lift it up more and you are now moving the blade on the coning hinge. The blade can cone up a great deal at the coning hinge, but you are actually lifting the blade and they are pretty heavy. Next time you do this, left up real hard to see it move about the coning hinge.

[mountainchopper]

Actually the blades don't weigh a thing on a robbie, you can easily hold one up with only one arm outstretched.

 

secondly, it is my understanding that the driving region of the blades is producing lift, it is just that the total lift vector is also providing thrust as well as lift for the blade. The equilibrium regions are producing lift, but not thrust nor drag, and the driven region is producing the most lift due to a greater velocity, but consequentially is creating induced drag as well.

 

My ppl only students will learn enought to keep them from getting dead, as well as being able to pass their checkrides. If i know a student is theoretically going for their cfi, then they get the full monty first time around and if i can incorporate some ifr stuff in there as well, then I will. just my way of doing things.

[RockyMountainPilot]

Actually the blades don't weigh a thing on a robbie, you can easily hold one up with only one arm outstretched.

 

When you push up on one blade causing the other to drop, you are not lifting the blade, you are forcing the blades to teeter. When you lift the blade off the droop stops, they are quite heavy if you are pushing up at the root from the frame steps. If you lift them up from the blade end, they are pretty light, but you will also need to lift it quite high.

[coanda]

When you push up on one blade causing the other to drop, you are not lifting the blade, you are forcing the blades to teeter. When you lift the blade off the droop stops, they are quite heavy if you are pushing up at the root from the frame steps. If you lift them up from the blade end, they are pretty light, but you will also need to lift it quite high.

I would think you would be bending the wings after they hit the stops thats why they feel heavy. I don't know. I do know they are light when off of the helicopter.

[RockyMountainPilot]

I would think you would be bending the wings after they hit the stops thats why they feel heavy. I don't know. I do know they are light when off of the helicopter.

 

The Robinsons have coning hinges. They can hinge up about 70 degrees if I remember correctly. When you lift one blade and the other one descents until it hits the stop and you continue to lift, the bade you are lifting will come off its droop stop and you can lift it as high as you can push it. Obviously they are not that light if you haven't discovered this yet.

 

I highly recommend you attend the Robinson Safety Course. This is explained in a detail.

[coanda]

When you lift one blade and the other one descents until it hits the stop and you continue to lift, the bade you are lifting will come off its droop stop and you can lift it as high as you can push it. Obviously they are not that light if you haven't discovered this yet.

 

I highly recommend you attend the Robinson Safety Course.

Right, so what your saying is that once I push past the opposing wings stop, the wing I'm pushing will continue to travel upward and eventually fold onto the other wing?!?!?! All I meant was that the travel of the wing eventually stops and then you are bending the wing.

 

You can't lift one of the wings when its detatched? Wheaties bud.

After a few long months I will be in Torrance...then I'll be as smart as you.