New member

[Chang739]

Hi guys,

Recently joined this forum and thought I would say hi. I haven't done the 'forum' thing in a long while but, after discovering that there is a 'safe space' for rotorheads, I had to join. Being the only rotorhead in the Australia reddit discord channel gets hard when no one can offer you any advice.

Looks like there's already a lot of good info here. I'm about halfway through a CPL(H) and figured now is a good time to do my theory exams (flying around solo with no knowledge of Air Law seemed a bit iffy). MET finished and AERO is this Saturday so I guess my first question here is: Any pearls of wisdom for AERO? I've read both the 'Principals of Helicopter Flight' by W.J.Wagtendonk and the AFT material, so I am reasonably confident...

In terms of media, I maintain a Youtube channel with various videos of my hobbies but I have put together a mostly complete playlist of my flight lessons so far. My original intent was to re-watch the lessons and pick up all the tid-bits of info I missed at the time, but it has turned into a way to share the info/learnings with other student pilots or enthusiasts. I'm sure the footage doesn't compare to the flying some of you have probably done, but its been pretty interesting for me being new! Hopefully someone finds the info useful: https://www.youtube.com/channel/UCqXYc_ ... subscriber

Cheers,

Chang

[Jabberwocky]

Welcome! Coming from reddit you’ll know how the internet and it’s forums operate so shouldn’t get too upset with how things roll from time to time.

As far as Aero, understand induced flow and it’s affects and you should get a good grip and what’s going on. And get your exams sorted sooner than later and you’ll enjoy your lessons more. M
Good luck

[Eric Hunt]

What a great way to revise the lessons! And pass it on to the wannabes as well.

Waaaaay back in history, we used to carry a little cassette recorder (modern technology then!) with a little mike we put inside the earcup of our headsets. The recorder had a vox activation, so it didn't record the silences, just the really important bits of wisdom that the golden-haired instructor put out..

A few years later we put a lipstick camera on the tail fin, but it still needed cables running along the tailboom, through the door, and under the seat to the big VCR. Problems securing the camera, the cables, getting the cables through the door and under the seat without getting severed by the sharp edges.

Now with GoPros so prolific, it is simple to record the lot. Well done.

[Chang739]

Yeah sure, I don't have any issues with the vector diagram or aerodynamic/gyroscopic effects.

Erik that sounds like a tangled up tail rotor waiting to happen!

Some sections of the 'Principals of Helicopter Flight' book covered topics that I didn't absorb in too much detail as they didn't seem to apply to the AERO exam. Am I incorrect? Do I need to study these for AERO?

- Sling loading (talked about types of slings, where you should attach them, etc)
- Weight and balance
- Helicopter design/builds (components such as the Bell stabilizer bars)
- Performance charts

I understand that these are important and will likely be required for other exams, but time is of the essence; the exam is this weekend!

Some of the phenomenon I would like some clarification on as well:

Transverse Flow results from tilting the disk forwards and having a reduction in rotor thrust at the rear of the disk due to higher induced flow/increased induced flow angle, which causes a reduction in angle of attack. Due to procession, this is felt as a roll to the right (in counter-clockwise rotors). Why is this a bigger concern at lower airspeed? I would have thought this would be a bigger issue at high airspeed due to disk tilt and high airspeed?

- Retreating Blade Stall results from a high TAS which reduces or reverses the airflow over the retreating blade. The blade root starts to stall while the outer blade operates at a high angle of attack and low lift/drag coefficient.This tilts the disk rearwards and slows the helicopter; the text states that this causes a left roll? I'm not sure what force is at play here that would cause a left roll.



Cheers for the help!

[Eric Hunt]

Some comments below:


Erik that sounds like a tangled up tail rotor waiting to happen!

The cables were wrapped around the boom and gaffer-taped to the boom, so no chance of coming loose.

Some sections of the 'Principals of Helicopter Flight' book covered topics that I didn't absorb in too much detail as they didn't seem to apply to the AERO exam. Am I incorrect? Do I need to study these for AERO?

- Sling loading (talked about types of slings, where you should attach them, etc)
- Weight and balance
- Helicopter design/builds (components such as the Bell stabilizer bars)
- Performance charts

These bits will be in the Performance exam.

I understand that these are important and will likely be required for other exams, but time is of the essence; the exam is this weekend!

Some of the phenomenon I would like some clarification on as well:

Transverse Flow results from tilting the disk forwards and having a reduction in rotor thrust at the rear of the disk due to higher induced flow/increased induced flow angle, which causes a reduction in angle of attack. Due to procession, this is felt as a roll to the right (in counter-clockwise rotors). Why is this a bigger concern at lower airspeed? I would have thought this would be a bigger issue at high airspeed due to disk tilt and high airspeed?

At higher airspeed, there is less difference between the front and the back, as the airflow has less time to be affected and turn into induced flow. I will get to "precession" later.

- Retreating Blade Stall results from a high TAS which reduces or reverses the airflow over the retreating blade. The blade root starts to stall while the outer blade operates at a high angle of attack and low lift/drag coefficient.This tilts the disk rearwards and slows the helicopter; the text states that this causes a left roll? I'm not sure what force is at play here that would cause a left roll.

The serious stall starts at the tip and works its way inwards. Loss of lift from a long way out causes a large moment, wanting to roll the aircraft left, but with phase lag, the loss of lift isn't "felt" until a bit further around the disc, causing a nose pitch up as well. So, retreating blade stall is characterised by vibration, then a pitch up and roll left. Lower collective, slow down.

Precession does not actually happen on a helicopter. The delay between the control input being fed in and the final position of the blade is called "phase lag" but to make it easier to understand, they describe it as "It is similar to a gyroscope." It isn't always 90 degrees, it is 72 degrees on an R22 for example. But "precession" is a simple way to get around it. The swash plate starts to put the pitch on the blade from the over-the-nose position, and reaches its max input at the 90 degree left position; the blade develops more lift from the nose, which STARTS to accelerate it upwards. But it is rotating at the same time, so by the time it reaches its highest point of travel, it is about 90 degrees further around the orbit from the maximum pitch position, it is now over the tailboom, and the swashplate is already telling it to reverse its movement and go back down. Minimum pitch input is at the 90 right, and minimum position is over the nose, and around it goes again.

Simples! Sktcchh!

[Chang739]

Precession does not actually happen on a helicopter. The delay between the control input being fed in and the final position of the blade is called "phase lag" but to make it easier to understand, they describe it as "It is similar to a gyroscope."

Whoa you've just blown my mind on something I thought was a solid truth. There it is right there, I've never noticed it before but the R22 pitch links aren't 90 degrees ahead of the blade. Some further reading tells me that the rotor wants to behave like a gyro, and it does in a vacuum, but the reality is its behavior is a combination of aerodynamic and gyroscopic forces. I would love a source on the equations used to determine the phase lag angle, as it seems different helicopter designs have different angles (some even as low as 45 degrees ahead).

The serious stall starts at the tip and works its way inwards. Loss of lift from a long way out causes a large moment, wanting to roll the aircraft left, but with phase lag, the loss of lift isn't "felt" until a bit further around the disc, causing a nose pitch up as well. So, retreating blade stall is characterised by vibration, then a pitch up and roll left. Lower collective, slow down.

Given the new knowledge above, then it would make sense that there is a left roll during RBS as the forces are now unbalanced. ie. The deepest stall, hence the highest drop in lift, would be 90 degrees to the left (0 deg being front) which would cause the disk to drop down (in a R22) 90+72 = 162 degrees in the orbit, or slightly left of rear, causing a pitch up and left roll. This would imply that helicopters will smaller phase lag angles would have a more violent left roll during RBS? I might be barking up the wrong tree here.

Thanks for the wisdom

[jimiemick]

Welcome to the forum Chang.


Just sub'd to the youtube channel.

[huey]

Welcome to the forum.

Just viewed your channel, very good. I think I recognise a few things on your vids lol.

I don’t think you’ll have too much trouble in the exam, you’re pretty switched on. Remember RTFQ!

Huey

[Eric Hunt]

When you get rotor heads with 4 or more blades, it starts getting tricky feeding the advance angle in with the mechanical linkages. The BO105, which you have noticed, has a lead angle of 45 degrees, and the swash plate tilts around 45 degrees ahead too, making the 90 degree lead.

Big birds like the 7-bladed CH53 are a triumph of science and technology over common sense.

Don't waste any brain cells looking for equations. Use them for something useful, like absorbing beer.

[Helical]

Good to see you're still at it Chang, met you about a year ago at one of the Corsaire BBQ's. I'm currently ticking off the theory subjects, but before the flying.

[Chang739]

Thanks for all the help guys, ended up being a really easy exam and walked out after 30 minutes.

Two down... only 5 more to go.

I'm the same Helical; no more flying until I've finished exams! Good luck with them.