Traverse flow, coning and co

[Pogue]

The quiz question is what is this something?

 

The way the helicopter is rigged. Transverse flow effect becomes a non factor as airspeed increases, so the right movement of the cyclic in just removing the input required to counteract the right roll while the effect from coning is not yet significant enough to require noticable input to counteract it.

[HumblePilot]

Hello Eric

 

I just love those boobs bouncing up and down:) .... can't get my eyes of them

Thanks for the picky picky ... You see ... even as a helicopter pro you can learn new things (and I am very grateful for every new thing I learn)

 

In the mean time I've just received a mail from KAMAN and Robinson concerning my question. I'll get back to you all when I've gone through their comments.... but I suspect that the last replies of flyby_heli and Pogue might be going in the correct direction

Edited January 4, 2007 by HumblePilot

[HumblePilot]

Hello Folks

 

The Robinson Helicopter plant sent me a link to the following article in which Frank Robinson himself explains the rigging provisions on the R22 for the "Wee-Wa" and Coning effect. FRobinson_Rigging.rtf

 

I found however on Unicopter.com a very interesting passage ...

 

"Para 6-12 b. Compensating for Transverse Flow Effect A left cyclic input decreases the pitch angle and angle of attack of the blade over the nose while increasing the pitch angle and angle of attack of the blade over the tail. These changes to blade angles of attack cause changes to lift. As the pilot senses the right tilt of the rotor, he must apply left cyclic to prevent the right tilt of the rotor as a result of transverse flow effect. At higher airspeeds, lift differential between the fore and aft portions of the disk begins to decrease. The cyclic stick must be moved back to the right at higher cruise speeds."

 

So I guess the scenario is:

 

Hover to Point A: cyclic goes left to compensate increasing transverse flow effect

Point A to Point C: cyclic goes to the right due to decreasing transverse flow effect

Point C to Point B: cyclic goes left to compensate increasing coning effect

 

 

As stated by F. Robinson provisions may taken the manufacturers to counteract these effects. I guess that the rotor system itself (semi-rigid, articulated, rigid) will strongly determine how the helicopter effectively reacts to these effect.

 

Many thanks you all (Linc, Pogue, Flyby_heli, Eric, Hal) ...

[Linc]

Pardon my stubbornness, but what causes the coning in the rear half of the disk and makes it less efficient? From what I've seen, the coning on the advancing half is due to increased efficiency and the need to decrease the lift being produced so as to not overpower the retreating side and flip the aircraft over (left roll). Maybe that's why I can't wrap my head around Prouty's explanation?

 

I have to throw in another comment, and I'll probably bungle the description of it, but once I read where Nick Lappos mentioned something about the phase lag angle (commonly referred to as gyroscopic precession) decreasing (less than 90 degrees) as the airspeed increased. I could be wrong about how I remembered it, probably am wrong in my understanding of it, but throwing that out for conversation/rebuttal as well. Anyways, it seems to me that it would explain, in addition to rolling moments caused by flapping, as to why I have to have increasing forward right cyclic as airspeed increases.

 

That's it for me for now. My flight got cancelled. I'm going home.

[Eric Hunt]

Look at the blades, not at the virtual disc / tip path plane. In forward flight, the back blade is tilted up from the hub more than the front blade.

 

The back blade is less efficient because the airflow is being presented to it from above, as induced airflow. AoA is reduced, less lift.

 

The front blade has the airflow more from the side, almost along the span, with less downwash / induced flow for the front part, until the airflow is sucked down towards the rear.

 

And Linc is right when he talks of the phase angles changing with speed. The gyroscopic effect is just a layman's way of trying to understand how a phase shift occurs. The phase lag is NOT always 90 degrees, it varies with the aircraft type and rigging, and I believe the R22 phase lag is 72 degrees. Lu Zuckerman was always debating with Nick Lappos about the missing 18 degrees.

[Linc]

Thanks, Eric!

[HumblePilot]

And Linc is right when he talks of the phase angles changing with speed. The gyroscopic effect is just a layman's way of trying to understand how a phase shift occurs. The phase lag is NOT always 90 degrees, it varies with the aircraft type and rigging, and I believe the R22 phase lag is 72 degrees. Lu Zuckerman was always debating with Nick Lappos about the missing 18 degrees.

 

That's what is decribed in the article FRobinson_Rigging.rtf.

This seems to be an interesting subject in itself. I'll open a new topic addressing this subject.

[HumblePilot]

Additional information:

 

I forwarded the following question to Robinson helicopters:

Do you have by any chance a plot of the cyclic displacement for the R22 and R44 v$ airspeed?

 

Patrick Cox (RHC Tech support) answered

 

That is proprietary, so I am unable to disclose it. However, I can say it is close to a straight line due to the previously discussed less-than-90-degree pitch input; Frank did this specifically to make the flight controls "honest".

[Linc]

Look at the blades, not at the virtual disc / tip path plane. In forward flight, the back blade is tilted up from the hub more than the front blade.

 

The back blade is less efficient because the airflow is being presented to it from above, as induced airflow. AoA is reduced, less lift.

When I have flown wingman, I noticed that at cruise airspeeds all blades on the other aircraft appear to have the inflow coming from above them. Is this unique to my aircraft or do other people who get to see aircraft at cruise from the "outside" see it as well?

[Eric Hunt]

Nothing unique about the 206 family, Linc.

 

But the back section has more vertical inflow than the rest of the disc.

[HumblePilot]

The relative airflow changes in the rear area of the disc as the coning angle gets greater. Airflow is still coming from above.

 

[Linc]

Yeah, I was looking at an academic aerodynamics book with Amazon's "Search Inside" feature (I ain't paying that much for a book), and they were talking about inflow during forward flight, and as the aircraft speed increased, the differential across the disk decreased. There was a lot of discussion as to theory and calculation and which theories accounted for what actually happened in experiments and practical application. Anyways, the conclusion was that there wasn't an increase as the aircraft's airspeed increased. The diagram you're using to depict greater coning in the rear half is not actual aircraft attitude at higher airspeeds and doesn't accurately depict rotor position. I don't doubt that there is coning in the system, but this diagram does nothing to prove that greater coning or inflow exists in the rear half than the front half.

 

See, I told you I was stubborn.

[HumblePilot]

Hello Linc

 

Being stubborn can be good sometimes. In academic books you will often find stuff that has been taken from a previous book, just because the author did not take account (or did not know) of new research results or just found it easy to copy what was accepted by the mainstream community. And that is how theories get passed on and on although they might be obsolete or even wrong (It took mankind a lot of time to believe the earth was a globe and not flat disc).

 

That is why it is important to keep the discussion going, to keep an open mind and take the effort to look at things from another perspective.

 

Could you please check out the Topic "a closer look a gyroscopic precession".

[Eric Hunt]

I am having trouble with Prouty's diagram, Humble.

 

The blade at the front is shown as being in autorotation, with the airflow coming up from beneath. That isn't right.

 

The airflow ahead of and above the helicopter can be depicted as a series of horizontal streamlines, parallel to flight direction. As the helo gets closer, the streamlines start to turn downwards towards the disc. The front of the disc gets airflow that is still almost horizontal, but the downwards turn of the streamlines means that the back of the front blade, and in particular, the whole of the back blade, get an increasing amount of downwards flow.

 

As speed increases, the streamlines have less time to be deflected downwards, so the effect diminishes with speed.

 

But Prouty's front blade should be showing airflow from the top, not the bottom. In powered flight, the blade will only produce lift by sucking air from the top and pushing it down. In autorotation, it receives air from the bottom and extracts energy from it to keep rotating and produce lift.

[HumblePilot]

Hallo Eric

Your are now addressing the front part of the rotor disc.

 

 

I also asked myself the same question. R.W. Prouty wrote an article about this in "even more aerodynamic" ("Rotor upwash", Chap 9). He relates this effect to a so called horseshoe Vortex system..... In my opinion you are flying into the upstream of the Vortex created at the tip of front rotor blade or something similar to fix wing aerodynamic is happening (Downwash created by a wing is preceeded by upflow in front).

 

 

Figure 9-2 show measured up/downwash at two different airspeeds.

 

 

I hope these figures will help to understand this effect. I guess standard CPL/PPL books discard this theme for reason of simplicity (... absolutely understandable) and just depict airflow for powered flight from above along the entire disc.

Edited January 7, 2007 by HumblePilot

[Linc]

Again, the reference I was looking at before did happen to mention that the exceptions to the inflow proportion models was the edge of the rotor blades and the no-lift areas.