Is a 'zero' degree advance angle possible?

[Irlandés]

I was having a conversation with some colleagues recently on advance angles and someone asserted that it would be impossible to have an advance of angle of zero degrees and as such asserted that there were no production helicopters in the world that had a zero degree advance angle (or no advance angle to be semantically correct ;) ). I (possibly in my youthful ignorance) can see no reason why this setup is mechanically impossible as long as the relevant control design ensures that forward cyclic translates to forward disc movement. I can understand that it may not be a very efficient way of doing things but is it true that no helicopters have ever used this arrangement??

Enlightening comments very welcome! :)

P.S. From looking at the Robbo head, it looks like its advance angle is somewhere between 70 and 80 degrees (rough guess). Anyone know the exact figure? Thanks!

Irlandés

[sprocket]

I think the R22 has 72 deg. With an 18 deg. delta three phase angle.
(That is if I have understood your terminology correctly)

[Dave Jackson]

Irlandés,

Yes, a phase angle of 0-degree is theoretically possible, and it should prove to be extremely efficient due to faster control response and minimal cross-coupling.

Rotors that are articulated at the center of the mast and teetering rotors (without delta3 - such as the Robinson's have) have a 90-degree phase lag. So called rigid rotors and rotors with offset hinges have a phase lag of less than 90-degrees.

The Sikorsky ~ XH-59A ABC (http://www.unicopter.com/0891.html) had a flight adjustable swashplate phase angle with a range from 0 to 70-degrees.

The UniCopter that I am developing has a phase angle of 0-degrees (I.e. no phase lag).

More information on Phase Angle (http://www.unicopter.com/0998.html)

[Irlandés]

Sprocket,

thanks for that. Yup! I think we're talking about the same thing.

Dave Jackson,

I must admit on first reading your post to not having a clue what you were talking about (through my own ignorance, not yours, I hasten to add!). Upon connecting to your link, I realised that I had walked into the mouth of the veritible Lion's Den :eek: and after stumbling around ineptly for an hour or so like a blind man in the dark, I can honestly say that I have come away a slightly more enlightened man. On re-reading your post however I have realised the value (for the umpteenth time) of clear and concise terminology. I mentioned in my original question 'advance angle' which according to Wagtendonk's book is defined as "...the angular difference between the attachment point of the pitch link on the swashplate and the blade to which it refers." So for a given helicopter helicopter I assume that the advance angle is a fixed mechanical angle (for example 72 degrees on the R22 according to Sprocket). You refer in your post to 'phase angle' and on your web-page it states at the end that "The phase angle of a specific helicopter can vary depending on the forward velocity and other aerodynamic events at a specific time." So I can only assume that phase angle and advance angle are totally different concepts and that there has therefore been a slight misunderstanding between my question and your well-intentioned response. :) Either that or this is even more confusing than I thought! ;) You have nevertheless opened up whole new horizons to me on the issue. Thanks! :D

Irlandés

[Dave Jackson]

Irlandés

It is a fairly complex subject. There have been numerous threads on it, in this forum, over the past year. A search and reading of these threads may help clarify the subject.


"You refer in your post to 'phase angle' and on your web-page it states at the end that "The phase angle of a specific helicopter can vary depending on the forward velocity and other aerodynamic events at a specific time." So I can only assume that phase angle and advance angle are totally different concepts...."

Fortunately or unfortunately, they are the same concept. :eek: The note on the web page will be clarified. What should have been said is;

A helicopter's optimal phase lag angle will change as its flight conditions change. Because it is difficult to change the physical phase lag angle on the helicopter, the designer selects an angle that is approximately the mid-point of the desired range. On the Robinson, this angle is 72-degrees.

This is the same situation as the undersling dimension on hinge of a teetering rotor. The optimal dimension will vary with flight conditions (greater payload = greater coning angle = desire for greater undersling) but the designer must obviously select a single fixed value.

Hope this makes sense.

Dave J.

[Nick Lappos]

Irlandes,

The lead angle is a natural thing, governed by the physics of how the blade flaps, and we rig the controls to make the pilot's job easier by assuring that the nose follows the cyclic (within a few degrees). The lead angle (we call it gamma) is driven by the flapping phase of the rotor to the cyclic pitch input, a fast acting blade needs less lead angle than a slow acting one. A blade has to respond to the aerodynamic change we make (the mechanical angle change is very fast). The response time for the blade to rise is a factor of the flapping inertia (weight, hinge location, location of the blade's CG, rotor rpm) and the forces that damp its rise (aerodynamic resistance, mechanical damping). If the blade is heavy and wide, it will take a long time to rise, if it is light and thin, it will jump up more quickly.

We can also mess up the swashplate part of the rigging angle if we change the angle that the blade flaps through, for example if we place the pitch change rod ahead or behind the flap hinge so that the blade flaps up and either moves forward or backward relative to the rotation. This changes the need for mechanical gamma by exactly the angle of the pitch rod to the flap bearing (we call that angle delta-3). This correction confuses some folks and makes for very long posts.

Suffice it to say, the gamma we rig is always our best attempt to make the cyclic behave properly.

[Irlandés]

Thanks Dave/Nick,

I've been trawling your previous posts on the subject and I'm beginning to see the light! :) It seems I have to start unlearning some previous 'taken for granted' concepts. Is this the path to Nirvana? :D

Irlandés

[ClearBlueWater]

This is the path to certified insanity.

[Irlandés]

Certified insanity?? Isn't that a prerequisite for obtaining a helicopter pilot's Class 1 medical?? Ha ha! :D