ATPL H Sample Q....

[NEWBOY123]

Hi I have a sample Q in a set of Rotor Craft Principles of flight notes, just would be interested on any comments......

Here it is....

When moving into forward flight, translational lift is produced by:

a - Increasing the mass of airflow through the rotor
b- Imparting a large acceleration to a small mass of air
c - Increasing the angle of attack on the blade by forward movement of
cyclic pitch

Cheers :ok:

[the beater]

I'd probably go for:
d- None of the above.

[RVDT]

a - Increasing the mass of airflow through the rotor....................................................... .................................................because the rotor moves fast enough horizontally through the air to no longer have the loss of lift incurred from reingestion of the tip vortices which then effectively makes the disk larger.


Feel free to contribute here - Wikipedia - Definition of Translational Lift (http://en.wikipedia.org/wiki/Translational_lift) :rolleyes:

[[email protected]]

It is c. In a free air hover the air is coming straight down through the rotor giving a high induced flow and requiring a high pitch angle to give sufficient AoA to produce enough thrust to balance the helicopter's mass.

Tilt the disc and move into forward flight and the air is now coming at the disc from an angle, reducing the induced flow, increasing the AoA and increasing rotor thrust.

Any more?

[RVDT]

So crab if I push the cyclic forward I instantaneously get translational lift? -

c - Increasing the angle of attack on the blade by forward movement of cyclic pitch


Really?

[KrisRamJ]

I'd say c too, but the answer is a bit vague, sure it should be "increasing the angle of attack on the bladeS" rather than "blade"

I guess it wouldn't be as confusing then, though.

[Cron]

Cab/Kris, this scenario is something I struggled with whilst doing PPL(H) and still struggle with. I would very much appreciate a more detailed explanation as my small Birmigham brain just cannot see how a blade (hover) at say 6 (a figure just for argument) degrees aoa to the relative airflow then get tilted via disc tilt and ends up with a greater aoa to a more horizontal relative airflow.

Please unconfuse me.

Regards

Cron

[parabellum]

I found this book essential reading when I did my CPL(H):
The Helicopter: History, Piloting and How It Flies (http://www.abebooks.com/servlet/BookDetailsPL?bi=1153299699&searchurl=fromanz%3Dfromanz%26tn%3Dthe%2Bhelicopter%2Band%2B how%2Bit%2Bfly%26x%3D21%26y%3D10) by John Fay,
ISBN # 0715372491.

It is obtainable from www.abebooks.com (http://www.abebooks.com)

There are 27 secondhand copies available in the UK right now. Hope this helps.

[KrisRamJ]

Hey Cron

OK, I'll give it a go. First off, this is translational lift, not effective translational lift, so we're talking about performance increases as a result of some wind going over the rotor system as opposed to the surge you feel at 16-24kts. Think of a zero wind hover compared to a hover in a 10kt wind; you will use less power in the 10kt hover even though you're still well below ETL.

The reason we get the performance increase is because there has been a change in the angle the air enters the rotor system. It's no longer being drawn exclusively from above, it's now getting some 'help' from wind which is coming into the rotor system in the same plane as the rotor system.

When you draw this out as a vector diagram you can now split the airflow through the rotor system into two components. Given the same downwash (i.e. the same mass flow rate of air), what you find is that the helicopter's forward movement has increased the component of airflow in the plane of the rotor system and that has led to a reduction in the required airflow perpendicular to the rotor system. The word we use to describe airflow perpendicular to the rotor system is 'induced flow'.

Bear with me - that's the tricky step out of the way, it's all downhill from here!

We know that induced flow is a bad thing because higher induced flow means lower angle of attack for the blades. Do you remember the vector diagram with rotational relative wind, induced flow, resultant relative wind, chord line, pitch angle and angle of attack? - v important that one, memorise if you don't know it - just keep drawing it out until you have it. You can use it again and again to describe IGE/OGE, inflow roll, retreating blade stall etc.

So reducing induced flow is officially "a good thing" because it leads to an increased angle of attack for the blades. :ok: In the question it talks about forward movement of cyclic leading to increased angle of attack. That's why I favoured answer c.

I think what this question is driving at is that translational lift is more to do with the increased 'bite' that the blades get on the air due to reduced induced flow than due to increased mass flow rate.

At least that's my take on it, not sure if that helps. I had to teach aerodynamics as part of working a CFI but I seldom got into this much depth apart from with my very best students.

;)

Kris

[RVDT]

Picture (http://www.cavalrypilot.com/aerodynamics/translational.html) = 1000 words

[Bertie Thruster]

....so what exactly is ".....forward movement of cyclic pitch" ??

[[email protected]]

RVDT - your first comment is obtuse and pointless, your link to the pictures however will be much more helpful to Newboy.

To get into forward flight you have to move the cyclic forward to tilt the disc which then moves the aircraft forwards thus altering the angle of the airflow and increasing the AoA. And no, it's not instantaneous!

Cron - it is the Relative Airflow that changes - draw the vector diagram for the hover and then reduce the amount of induced flow (this as Kris says is because you now have a horizontal component of airflow into the rotor as opposed to purely vertical). You will note an increase in AoA which increases the Lift vector (and the drag) but with a net increase in the Total Reaction.

Translational Lift and Effective Translational Lift are just differences in terminology for the same thing.

[Cron]

Thanks Kris. thanks Crab.

Regards

Cron

[RVDT]

crab,

Personally I think it is the other way around, as this is a rotorheads forum I made some assumptions of peoples prior knowledge, but I digress. Induced flow is actually only a tiny proportion of why the rotor is less efficient below "translational lift". Think of where in the disk the conditions for this could occur. Most of it is turning too slow and the most efficient part i.e the tips is engaged in a massive battle with tip vortices of their own and the garbage from the previous blade. All these theories assume ideal conditions and were conjured up many years ago. To say that the increase in lift is all due to the reduction in "induced flow" is not correct.

a) is the correct answer. (as in all exams more than likely it is the "least wrong) Increasing the mass flow as you no longer have the mass that is trying to go through more than once!

Nick Lappos sez: (http://yarchive.net/air/translational_lift.html)
:ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh::ugh: :ugh::ugh::ugh:

[Cron]

Bugger, now I'm more confused.

Regards

Cron

[TalkSpike]

"Transition is the change of state from hover to movement in a horizontal direction, or a change from horizontal flight back to the hover. As a helicopters forward airspeed increases a horizontal airflow is created across the disc. This horizontal airflow reduces the amount of induced flow through the disc. The resulting increased angle of attack and reduced rotor drag means the disc becomes more efficient and less power is required. This effect is known as translational lift."

Right out of my ATPL notes.

Less induced flow = greater AoA for same pitch setting

Has to be answer C!

To get from the hover to forward motion you have to push forward on the cyclic, this itself does not increase the AoA but the reduced IF. Read the answer again.

Totally ignore the movement of the disc when thinking about these questions just concentrate on the vector diagram, it makes more sense that way.

TalkSpike.

[BusinessMan]

Newboy123, have you got the actual answer from anywhere? Or option d) for that matter? I'm guessing maybe you're studying for the exams and hence would be given the answers after being told to have a go at the sample Qns? I'm curious now :ok:

[RVDT]

Gawd!

The looneys are in charge of the asylum!

[NEWBOY123]

Hi,

The answer stated is A!

The explaination based on what I have read and have been told so far, I would have to go with Talkspike. To me if there is an increased mass of air moving through the rotors there must be more work being done, hence more engine power required??? But hey what to I know, interested to read other people’s ideas as you can never have a wide enough view point in life, thanks to everyone that has posted.

What do you all think to this one!:ok:

Flapback:

A – Is prevented by cyclical feathering
B- Is prevented by collective pitch change
C- Is prevented by movement of the cyclic pitch control

To me the term “prevented” is misleading should it not be “overcome”???

Cheers All:O

[Whirlygig]

Hey Newboy, from which book/CD are you getting these questions? :eek: :confused:Have they been translated from the original Esperanto? :} They are both badly worded; yes, I would agree that "overcome" is a better word than "prevent". :ok: I'll go for C this time!

Cheers

Whirls

[NEWBOY123]

Hi Whirlygig,

You are correct! They come from a set of Rotary principal of flight notes provided to me from a well-known ATPL ground school…

[Whirlygig]

You mean Bristol? And are these progress tests? Anyway, it'll prepare you well for the real ones as they are just as badly worded!

Cheers

Whirls

[NEWBOY123]

lol na not Bristol..... why are they as bad!

[Whirlygig]

I don't know as I didn't go to Bristol; it was only because I didn't recognise the questions!

Cheers

Whirls

[[email protected]]

RVDT - sorry chap, I'll take the hit on that one:ok:

If that is the standard of question writing the CAA approve then it is a wonder anyone gets a licence at all. As you say, a is the least wrong but it almost implies that adding collective pitch (thereby increasing mass flow throught the rotor) would cause translational lift - it's crazy.

Newboy - follow the link to Nick Lappos' answer, it might make it all clearer.

[RVDT]

Crab,

With regard to increasing the mass flow by increasing the collective pitch, you probably could achieve vertical translational lift. The problem would be how much power you have available and whether you could actually get a clean vertical flow into the disk. Of course you would have the drag of the fuselage working against you. It probably does happen but is hidden by other forces acting against you. Think picking yourself up by your shoelaces as we do now put pulling a bit harder!