aerodynamic theory question
 

forgive my ignorance...

is the tip path plane the same thing as the plane of rotation?

tip path plane is generally described as being parallel to the plane of rotation
mk10

Tip path plane
 

Agree with mk10, with the following reservation

in order to strictly define a plane, the tip would have to follow a strikt circular path. For an number of reasons this is not so

- global higher orders of the aerodynamics resulting in higher order loadings. In the case of a R44 in forward flight there seems to be at high speed a clear second order as showed by the coning angle which is not constant during rotation (even assuming fully ridgid blades)
- vibrations of the blade resulting in bending of a 'higher order' because of the higher order excitations (cfr the rather extreme flexing of the rotor blade film that was mentioned even recently on this forum
- during transients (ie sudden control inputs), before the rotor takes a steady regime

In my simulator the tip path is defined as the average tip path, that is taking higher orders out.

Example : look at the dotted red line :

http://www.portmyfolio.com/prive/hel...h%20delta3.jpg


d3

Ah! Perfectly clear now!!!:}

...0.7 degrees forward down...

Is this something to do with those lost 18 degrees of blade advance that the late Lu Zuckermann used to worry us silly about? :uhoh:

:E

delta3,
You ask too many deep questions!
It is my belief that the mythical "tip path plane" is a construct that helps explain things, but is not a physical reality. Perhaps we could define the "tip path plane" as the average resultant path of a set of uniform blades, infinite in number.
The actual tip path is a mutating lumpy pulsating thing (look again at that famous film clip, rotor.avi for the reality).
An analogy might be "mean sea level" which is a plane, yet the sea is a wavy dynamic entity.

Tip path
 

Nick,

I fully agree, somewhere 'theory' and 'practise/practical' have to meet.
I'll be most of the time at one end of the spectrum, I am afraid.

A real specialist is one who can explain things to others, as you clearly do.
I am still in the discovery phase.

d3

is there any danger of getting my original question answered with a simple answer?!

thecontroller,
You want a plane and simple answer?

This is not meant to be glib, but, 'the plane of rotation of what?'
At the risk of getting toooo complicated, here are four planes (or axes) to choose from. http://www.unicopter.com/Devil.gif :O


Dave

Well, actually, the above posters have got it all wrong.
The tip path plane is actually something quite different, and if you'll allow me I'll just draw a diagram which will set it all straight immediately!


AAAAHHH - JUST KIDDING!!

Thank Heaven!!!

:D

It gets mildly worse - sorry. I'm reasonably convinced (working in the dark as engineers often do :} ) that in the R22 Tip Path Plane does not quite follow the Control Plane (ie swash plate). There seems to be slight overshoot to transient inputs, which results in the pilot needing to learn to moderate hover cyclic inputs.

No data, but reasonable conjecture - i'll get me coat...

Mart

jeezy creezy
controller...the simplest answer to your question is no.
For the ppl or cpl theory, the tip path plane is (as asserted by MK10) parallel to the plane of rotation and is the plane through which the tip of the blade travels.
The distance of the tip path plane from the plane of rotation will be determined by disc loading and centrifugal/centripetal (don't even get them started on that one) force, increasing or decreasing the coning angle.

thecontroller,

There is a lot of incorrect information posted on the Internet. Two incorrect posting can be found on this thread.

Um... lifting.... was wrong when he spoke about the mouse and confusion.
Pornography is the biggest industry on the net. The mouse reduces the confusion.

Pear was also wrong.
Centrifugal and centripetal forces are not relevant. This is because one cancels out the other.


The only correct posting is this one.

Anonymous http://www.unicopter.com/Spy.gif

Controller

My understanding is this:

The TPP as a few people have written is just a contruct to "help" sort POF questions out. If the helicopter is sitting on the ground in nil wind at operational RRPM with no pitch applied the TTP and plane of rotation are one and the same thing.

If the blades are under load and begin to cone, no wind, cyclic neutral, same thing.

However, if there is any difference in the relative velocities of the blades and they begin to flap or teeter, or if a cyclic input is made, the path described by each blade will no longer be uniform : consider flapping to equality of angle of attack or rotor thrust for example. Also refer to Hooks Joint Effect. The TPP is then used as an "average" of the orbit described by the aerofoils.

TT

At the risk of having my head (now peering over the parapet) shot off and without the benefit of my dusty PofF books to hand to confirm this I'm going out on a bit of a limb from memory....

Surely, the POR is the plane described 90 deg perpendicular to the AXIS of rotation (i.e. a vertical axis extending through the mast).

The TPP is the plane described by the rotor tips - nothing "Construct" about it TorqueTalk (lets discount the effects of tracking / out of track blades)

With cyclic input applied (and the effects of Hookes Joint, Flapping etc), the the TPP can describe a plane which is NOT parallel to the POR.

It is for this reason that the two descriptions (POR and TPP) exist otherwise they would be described by the same definition ... mange tout, non ?

... he ducks for cover awaiting a triade of "durr!" statements
FO

Since we are getting into the details, I've got a question toooo.

Why is the action of teetering hinges and flapping hinges referred to as a Hookes joint (Universal & Cardan joint)? These rotorheads have basically one hinge, whereas the Hookes joint has two hinges, which are located at 90-deg to each other.

Why aren't these hinges referred to as a Knuckle joint?

Dave

Correct.

The reason the tip path plane is described as a construct is that there is usually not a single plane in which the path of the tips lies. The "higher harmonics" that have been mentioned earlier in this thread explain this phenomenon.

For example a second order harmonic can result in the tips being higher at the fore and aft positions than at the left and right positions, hence no single plane will pass through the tips at all locations.

The TPP "construct" basically involves finding the plane from which the average displacement of the tips is zero - i.e. the blades go above the plane as much as they go below the plane.

It is not just cyclic input that can cause the tip path plane to differ from the plane of rotation, translational velocity can cause it to do the same thing (this is flapback).

The third axis - that Dave mentioned - is the control axis/plane. With zero translational velocity, this corresponds to the TPP, and differs from the axis/plane of rotation as determined by cyclic input. With translational velocity, the TPP will tilt back away from thr direction of motion.

So in forward flight, your control axis/plane will be tilted forward (forward stick trim at speed), but flapback will cause the TPP to be tilted back towards the plane of rotation.

Daniel

Here you can actually SEE the "plane" :}
http://www.griffin-helicopters.co.uk/vids/ROTOR.avi