aerodynamic question

[91205]

During balanced level flight, the pitch angle of the main rotor blade:

a - remains constant on both sides
b - increases on the advancing side, decreases on the retreating side
c - varies according to the circumference disc, decreases in the front, increaes in the back
d - decreases on the advancing side, increases on the retreating side

Can anyone here explain the answer?

[thecontroller]

this is another awful JAA ATPL PoF question

anwer is (d)

in level flight the disc is tilted forwards to 12 o'clock position, hence the pitch angle of the advancing blade must be decreasing to achieve this (precession)

[MightyGem]

Reason for angle of attack changing is below. Note: IF = induced flow, AoA = angle of attack.

[jab]

VTO
Afraid I dont agree. Pitch angle is changed by both the cyclic and collective. In a steady hover with no wind (A) would be correct. In level flight the pitch angle must change to avoid dissymetry of lift, reducing the pitch angle (and Angle of Attack) on the advancing blade and increasing on the retreating side, therefore answer (D).

I see others got in before me.

[thecontroller]

this is not an inflow roll question, it concerns different PITCH angles, not angles of attack

trust me, the answer is (d). i looked it up on the online question bank

[thecontroller]

errr...pitch angle does not change to overcome disymmetry of lift. the blades flap and this changes the direction of the relative airflow, which changes the AOA, which solves the problem.

PITCH angle is the angle between the chord line and the plane of rotation. flapping does not change this.

[jab]

Thats "Flapping to Equality". The Pitch Angle must change to keep advancing and retreating sides producing the same amount of lift.

[skadi]

Originally Posted by Vertical T/O

B and D refer to the angle of attack depending on which way the rotor turns.

It doesn't matter wether the rotor turns clock- or anticlockwise, the advancing blade during normal cruise is the one with lower pitch angle, on some helicopters its on the right side, on others its on the left...

skadi

[HillerBee]

The pitch angle is changed by the cyclic and collective as already mentioned. In forward flight the pitch angle is different (more) on the retreating side and less on the advancing side, this is called cyclic feathering. The angle of attack changes through flapping as well. Flapping alone wouldn't be enough to solve dissymetry of lift.

But take a look at the swashplate and blade angles if you move the cyclic forward. (on the ground obviously, engine off)

[NickLappos]

Hillerbee and skadi are quite almost right. We just have to adjust for gamma (rigging adjustment for "precession").

The "pitch angle" is the measure of what the controls do to the blade angle. It is geometric, and is measured by a protractor.

The helo, in balanced flight at forward speed, must have more AoA on the downwind blade than the upwind blade, due to dysymmetry of lift, since the downwind blade has less airspeed, it must have more AoA to create equal lift. This means the pilot has "balanced" the condition by applying cyclic to create this difference in AoA.

To confuse things further, the rigging must be about 90 degrees off (for "gyroscopic precession" let us call it), so the pilot places the cyclic to the right as he flys faster (US/UK convention) and this makes the geometric pitch of the blade increase in the forward section of the disk, and decrease in the aft (90 degrees before the desired blade motion).

Thus, the answer is "pitch angle Increases in the front, decreases in the back" which is an answer not given!

[HillerBee]

Nick is absolutely right, in my hurry I forgot the precession. That's why the pitch horn is at a 90º offset.

[thecontroller]

having flown in both systems, the JAA system isnt 'better' - it simply results in weeding out more people who cant afford the time/effort/money to study for the exams

i believe ex-mil people can automatically get a CPL if they have 500 hours on type, and an ATPL if they have a certain number of "command" hours. i dont really know the details. i also believe if they have a certain number of hours there is a "military bridging" groundschool option which is fewer than 14 exams

but yes, the LPC is a rip off.

[thecontroller]

for a UK person....

in the UK: work visa: free. licence: expensive

in the US: work visa: expensive or non-existant. licence: cheap

[HillerBee]

The question could have been an FAA question as well. Aerodynamics are the same everywhere. I have done both systems and I don't see a lot of difference (aerodynamics), maybe because I just want to know what's behind it and don't just study to get the exam.

In general JAA is more compicated, but why do we have to talk about that all the time? We are not going to change anything about it.

[Impress to inflate]

The answer is D. The advancing blade would generate more lift due to 1/2 Ro V squared lift formula if the pitch was not reduced and vis-versa for the retreating blade.

[arismount]

After reading about these tests you guys have to take

After reading about these tests you guys have to take, I'm happier than ever to be a Yank. Really, seriously, what does it matter about the angle of the dangle? I'm not a mechanic or an engineer. I pull up, it goes up, I push forward, it goes forward. I do that a few times and I get paid. End of story. Anyway, best of luck on passing these tests but if I were you guys and had to do that I'd just put in a little extra tuition and become an engineer instead of a pilot. You'd probably live longer, make more dough, and be happier. Cheers from Yank land.

[thecontroller]

we love it. it makes us all feel superior. and gives us something to moan about on this forum.

[ichris]

FAA test question

Originally Posted by HillerBee

The question could have been an FAA question as well. Aerodynamics are the same everywhere. I have done both systems and I don't see a lot of difference (aerodynamics), maybe because I just want to know what's behind it and don't just study to get the exam.
In general JAA is more compicated, but why do we have to talk about that all the time? We are not going to change anything about it.

Here is a like question that was on the FAA CPL test.

During forward cruising flight at constant airspeed and altitude, the individual rotor blades, when compared to each other, are operating at:

a) unequal airspeed, equal angles of attack, and unequal lift moment

b) unequal airspeed, unequal angles of attack, and equal lift moment

c) constant airspeeds, unequal angles of attack, and unequal lift moment

[dragman]

91205

Hope you're still following this!! I had a better explanation to your answer, but then my computer crashed. So, I'll just put in my ten cents and say the answer is (D). It is known as flapping to equality, but the term is mis-leading as the blades do not actually flap, they "move down" on the advancing side through a reduction in pitch angle, and "move up" on retreating side through an increase. This serves to restore changes in angle of attack due to an airflow over the disc during level flight.

[Gordy]

Originally Posted by arismount

Really, seriously, what does it matter about the angle of the dangle? I'm not a mechanic or an engineer. I pull up, it goes up, I push forward, it goes forward. I do that a few times and I get paid. End of story.

I'm with you---although you do over-complicate it---I always remember this--
Lift left hand----trees and cars get smaller
lower left hand----trees and cars get bigger
How easy is that?