View Full Version : aerodynamic theory question
thecontroller
25th Oct 2006, 09:32
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
delta3
25th Oct 2006, 11:31
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/heli/Flap%20with%20delta3.jpg
d3
Helipolarbear
25th Oct 2006, 11:45
Ah! Perfectly clear now!!!:}
delta3
25th Oct 2006, 12:04
...0.7 degrees forward down...
ShyTorque
25th Oct 2006, 12:58
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
NickLappos
25th Oct 2006, 13:08
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.
delta3
25th Oct 2006, 15:27
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
thecontroller
25th Oct 2006, 16:19
is there any danger of getting my original question answered with a simple answer?!
Dave_Jackson
25th Oct 2006, 18:13
thecontroller,
is the tip path plane the same thing as the plane of rotation? is there any danger of getting my original question answered with a simple answer?!
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/B329.html#Axes_Systems_Rotor) http://www.unicopter.com/Devil.gif :O
Dave
Arm out the window
26th Oct 2006, 03:08
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!!
NickLappos
26th Oct 2006, 03:13
Thank Heaven!!!
:D
Graviman
26th Oct 2006, 04:08
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...
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.
Dave_Jackson
27th Oct 2006, 17:59
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
Torquetalk
28th Oct 2006, 17:55
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
FlightOops
28th Oct 2006, 20:13
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
Dave_Jackson
28th Oct 2006, 20:41
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
Deemar
29th Oct 2006, 05:24
Surely, the POR is the plane described 90 deg perpendicular to the AXIS of rotation (i.e. a vertical axis extending through the mast).
Correct.
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)
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.
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 ?
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
FlightOops
30th Oct 2006, 08:24
Thank you Aser/Griffin for the video ... I had seen something similar which I was led to believe was a Huey blade and was staggered by the amount of "movement / flexing" the blade experiences in flight...
However, I fear there is a danger of over-egging the pudding here in that The Controller was looking for a simple answer to their question.
The simple answer is drawn by simple PofF vector diagrams showing a difference between the POR and TPP.
http://static.flickr.com/111/283396025_258b02c497_o.jpg
Link fixed.
H.
delta3
30th Oct 2006, 11:37
FlightOops
Thinking back at Nick's analogy of defining MSL by filtering out the ocean waves, the tip path is obtained by filtering out higher order bending and flapping harmonics.
So, one way to obtain the simple and clear picture, you refer to, is to restrict the flapping to the first order harmonic behaviour (that is those sin-waves that have the same frequency as the MR).
So the tip plane is defined by the first order flapping harmonic.....
did it again.... cheers...
d3
jessie13
31st Oct 2006, 00:07
From a mechanics point of view- the tip path plane is the path that an individual blade travels around the plane of rotation. I've tracked and balanced many rotors over the years and can quite happily say that the POR and the TPP are rarely the same. Hence when a blade is out of track, it's because it is not flying in the same TPP. When often adjust blades out of track to reduce vibrations and depending on what vibe equipment you use, the plane of rotation generally comes from a master blade on the head. On some aircraft, you adjust the blades from the master and on others, from a median. I hope that makes sense.