Precession

[flyingscotty]

Can anyone clarify the flight profile when Gyroscopic Precession occurs? Having a few beers with a couple of Canadian friends over the weekend, this topic came about.
My understanding has been that it happens during Inflow Role only, although one of my friends commented that it also occurs during Flapback.............

If i am wrong there are quite a few beers i will owe, and can these two down it!!!!:mad::mad:

[newfieboy]

Mmm, not sure about the gyroscopic thingy........:{But yep, if there was an olympic event for beer drinking, the Canucks would sure be on the podium......:D And they would be gold medal for the rye drinking event......I know from first hand experience many many times.......:D :ok:

[Ascend Charlie]

Hoo boy, you have said the "G-P" words.

Whether it is really precession, or whether G-P is a simple way of explaining the delay between the point when one blade gets a boost (or loss) in lift, and the point where it has reached its maximum movement (up or down) as a result of it, the answer is this:

It happens any time there is a difference in lift between the blades.

It might come from the first moment of moving forward (resulting in flapjack) or when the change in inflow as a result of rotor tilt (inflow roll) but it is still the same stuff.

The first one that happens is flapjack, even from a 1-knot difference in relative wind onto the blade. Inflow roll happens a bit later.

And by the way, "G-P" also "happens" when you move the controls to tilt the disc, and the swash plate uses its advance angle to make up for phase lag.

[Arm out the window]

There's a bit of a divide between the American and UK/Aussie schools of helicopter aerodynamics teaching, from what I've seen.

Simplistically speaking:

The yanks (or some, at least) favour the use of gyroscopic precession to explain why a control input has its effect 90 degrees later in the rotor blades' direction of rotation - ie a force applied to a spinning gyro will cause a tilt 90 deg. later.

Thus, for a forward cyclic input on a helicopter in the hover, the pitch angle on the blade is at a minumum directly out to the right hand side (for a US-direction of rotation blade), but the disc will be lowest at the front. Explanation - gyroscopic precession.

The poms say, though, imagine that same cyclic input, where the pitch is lowest out to the right.

Say the disc started out level, in the hover. A forward cyclic input will reduce the pitch on the blade on the right hand side of the disc in this fashion, considering the blade sweeping around from back to front:
Just slightly round from the back, there'll be a small pitch reduction, getting bigger so pitch angle is lowest when the blade's directly out to the right, then coming back from that low setting until it's back to neutral at the front (and will then get relatively bigger in a similar way on the left hand side).

However, the key point is that whenever the pitch is lower than the original disc-level hover setting, the blade will be flapping down, slowly at first, reaching a maximum rate out to the right, and reducing to nearly zero approaching the front - which is where the blade reaches its lowest point and starts going up again, of course.

This explanation is my favoured one because it makes aerodynamic sense to me. Blades on a rotor head aren't the same as a gyro because they're not rigidly in plane all the time, but flap up and down (and lead and lag) to the extent of the flexibility of the materials and the hinges.

They are big and heavy though, and spin bloody fast, so I'm not saying gyroscopic forces don't exist, just that I find the aerodynamic explanation of how they move to be more compelling and believable.

Flapback:
Disc starts out level, increasing airspeed causes lift increase on advancing blade, maximum at position directly out to the right, vice versa on retreating blade.

Point of max lift increase is where blade will be flapping up fastest, progressively reducing around the advancing side until we reach the front of the disc, where it has reached its maximum amplitude of flapping up and starts to go down again.

Inflow roll:
As helicopter moves forward, air at rear of disc has spent more time over it and has relatively higher downward velocity than that at the front.

Therefore there is a greater lift reduction at the rear of the disc due to this greater inflow, so the disc should be flapping down at the highest rate at the rear, continuing to do so until it reaches its lowest point half-way round on the advancing side. This results in a roll towards the advancing side.

NB: Disclaimer ... all off the top of my head, having not thought about it for a good few years, so I'm not saying this is all kosher!

[Arm out the window]

Bloody hell, deja vu!

On the other thread that just appeared (helpful moderator, perhaps), there's all manner of discussion about gyros and rotor blades, but there's a post by me about the same length as the one I just wrote here, saying basically the same thing, in 2001 though...

Lucky I didn't contradict myself, as has been know to happen.:)

[Senior Pilot]

AOTW,

I didn't want to merge the threads, but for reference there is an old thread Helicopter dynamics: Gyroscopic Precession (http://www.pprune.org/rotorheads/19678-helicopter-dynamics-gyroscopic-precession.html) that I have bumped to give some more background to this discussion.

Especially read the posts 15, (http://www.pprune.org/201191-post15.html) 25 (http://www.pprune.org/201201-post25.html) and 34 (http://www.pprune.org/201210-post34.html) by Nick Lappos which comprehensively debunk the Gyroscopic Precession myth :ok:

[Arm out the window]

Thanks, SP.

This gyroscopic vs aerodynamic thing is all well and good, but the big question is if flyingscotty will have to buy the beer or not, and if not, will he buy us all some instead?!

[topendtorque]

AOTW has the answer, what is the line that Scotty is pushing? and is he a Scotsman by nature with short arms? his reputation and stature now hangs in the balance of the spinning top and the thirsty ozzies..

cheers tet

[Thomas coupling]

Flyingscotty, it is important not to differentiate here:

Having a beer CAN occur during flapback. It doesn't necessarily have to wait for inflow role.

The important thing here is to make sure the beer isn't spilt during GP or the guy next to you removed by 90 degrees and in a different room to you may get soaked:D

Enjoy!

[SimFlightTest]

Instead of looking at the rotor disk as a whole, break the problem down into the individual rotor blades.

If we move our frame of reference from the rotor hub to the CG of the rotor blade, the blade is observed to be rotating about its CG just like a top or gyroscope. The aerodynamic forces as well as the forces imparted to the blade by the blade grip can all be transfered to the blade CG as simply one force and one moment acting at the CG. This is exactly the classical case of a force/moment applied to a gyroscope, and hence the blade responds to the force/moment EXACTLY 90 degrees out of phase with regards to where the force/moment was applied. That's Newton at work... there's no getting around it or arguing it away.

What confuses the matter is when the system is viewed from the rotor hub. It then becomes an articulated gyroscope with all the quirky behavior that such an object entails (such as responding at a phase angle other than 90 degrees)... but in the end it's still a gyroscope.

In my area of expertise (simulation), we live or die by being able to accurately predict the behavior of complex mechanical systems... one such system being a rotor head and its associated rotor blades. When you break down the equations of motion for such a system, they become exactly the same equations used to predict the motion of a spinning top.

[flyingscotty]

Thank you to everyone who has responded and with the merged threads which i will look at later, when i have a wee bit more time.

I think my friends & i could probably say we are both correct.............. so what the hell i will open my floorboards ( a true & canny Scot) and go to the pub prepared, making sure i will not be flying for a day or so afterwards!!!!:ok: