Which way does the rotor flow.

[Jean Claude]

1. Please note the downwash (DW) is only a transient feature. If you look at the lower right side of the picture you will see that the streamlines are almost exactly the same as the lower left side. Hence Lift has been produced but the effect is short lived and so the streamlines have returned to normal. This suggests that the picture in Post #3 is not entirely accurate.

The photos you show are for a section between two partitions. Ttherefore, they are representative of an infini aspect ratio, for which the DW is nul. This is not the case of a disk, for which aspect ratio is 1.27, So, DW is extended numerous diameter in rear as in my picture.

 

[Vance]

I failed to ask the question correctly.

I failed to ask the question correctly.

Vance asked a very simple question that has been answered with 7 pages of stuff that

is way over my head,I believe I have understood about less than 10% of it,so having

said that what is the answer. Which way does the airflow ?





Best regards,

I failed to avoid a semantics discussion Eddie.

Don Hillberg had the best suggestion but I am not sufficiently motivated at this time to conduct a smoke test.

Jean Claude answered the question in post three.

David gave me a new way to be confused on a semantics level.

Chuck gave me some insight on an unrelated subject.

I feel confused on a higher level.

I am grateful for the help.

It is still hard for me to visualize what is happening around the airfoil in the driven part of the rotor blade.

 

[Arfur Bryant]

The photos you show are for a section between two partitions. Ttherefore, they are representative of an infini aspect ratio, for which the DW is nul. This is not the case of a disk, for which aspect ratio is 1.27, So, DW is extended numerous diameter in rear as in my picture.

JC, of course I know they represent a section but I still contend you cannot treat the disc as a solid object for the purposes of a downwash discussion. The disc a is a rotating series of high aspect ratio airfoils passing in quick succession. At the tips there will be a vortex (circular) but there will be little downwash in consideration of the blade as a whole. High AR reduces induced drag and effective downwash. It is too simplistic to consider a gyroplane disc as a solid object and then to state that airflow is 'up'. I agree the airflow is 'up' but it has to pass through the disc! If you insist the disc is solid, then you will have your low aspect ratio. Unfortunately this does not represent what is a complex aerodynamic situation.

Regards,

 

[kolibri282]

The pictures of 3D CFD calculations of actual rotor flow fields to me indicate that airfoil flow is way to crude an approximation. There is nothing anywhere near the helical vortex wake of a rotor in the flow field of an airfoil. Also there is a flow right across the rotor disk, which again does not happen with an airfoil. I would therefore like to propose that we discuss flow patterns connected with rotary wing aircraft.
(Images from:
http://cr4.globalspec.com/thread/40666/Re-aircraft-design
http://www.aero.polimi.it/~quaranta/research.html)

 

[quadrirotor]

A full scale, in flight, visualization of the rotorwake for a tip-jet helicopter:


https://www.youtube.com/watch?v=Mh-tRy2bE14

 

[kolibri282]

Great video, André, and great experimental work! To me the flow patterns we see look so similar to the ones computed by 3D CFD that my conclusion would be: we don't have to burn an ounce of fuel today, just juggle a couple billion bytes about to get the answer.

 

[Vance]

I feel a rotor in auto rotation may have a different flow pattern than an engine powered rotor.

 

[kolibri282]

I am aware that I am walking on thin ice but I think that the differences are small. In both cases the flow pattern is brought about by the circulation about the airfoil section of the blade. Now with a gyro both the driving and the driven regions of the blade need some amount of circulation to act their respective part and so a vortex sheet forms behind the autorotating rotor blade just the same way it does for a powered one. Therefore the difference would rather be visible in a very detailed small scale analysis of the distribution of the circulation along the blade span than in the flow pattern at large. My two cents.

 

[Jean Claude]

of course I know they represent a section

The key words was "between two partitions": This 2 D flow is not representative of downwash in a open airstream, unless an infini Aspect Ratio. Downwash angle to rear: 2 Cl / pi. AR
I do not see that I treat the disc as an impervious surface: My sketch shows a flow through.

 

[birdy]

Roughly speaking;
A gyro and a heli
Same weight
Same speed
Gyro rotor cruises with disc tilted back @ 9 degrees
Heli cruises at same speed, with the disc tilted forward @ 9 degrees
Same inflow velocity, at same angle but opposit side.
So the gaps between blades will not be that much different.
Reduce the disc AOA on the gyro, you get wop wop. ( blades strikeing eachothers wake.)
Reduce the disc AOA on the heli, same same.
With the only difference being the gyro blade flys up to hit the wake, the heli blade flys down to hit the wake.

Sont rocket science.

 

[Smack]

This brings up another question

This brings up another question

Wow, interesting sentence, Birdy.
You are saying that at some AoA/airspeed combination, the blades will hit each other's wake and make that 'wop wop' sound.
Does that sound ALSO indicate a drop in blade efficiency?
If so, that would detract a bit from the efficiency gained by partially powering the rotor (which would allow a lower AoA), right?
At the extreme, if you had enough rotor power to fly at an AoA of 0 degrees and ... 100 mph airspeed, the blades would definitely be flying in each other's wake.
Maybe this scenario could happen with the Sikorsky X2, Airbus X3, AH-56 Cheyenne, or other compound helicopters.
What would happen to efficiency/sound/lift?

Great discussion.
Brian

 

[Lee Scatt]

How about a ball bearing in a glass tube held vertically, if you supply enough air, you can "float" the ball without any air being directed downwards.

 

[Doug Riley]

Lee, what matters in the creation of lift and drag is a change in the air's VELOCITY. If the ball slows some of the air down in its upward path, that's a change in velocity.

 

[Arfur Bryant]

Jean Claude: The key words was "between two partitions": This 2 D flow is not representative of downwash in a open airstream, unless an infini Aspect Ratio. Downwash angle to rear: 2 Cl / pi. AR
I do not see that I treat the disc as an impervious surface: My sketch shows a flow through.

Yes, your sketch shows a flow through but it displays a straight disc which is affecting airflow as if it is a single unit. In my opinion, the sketch is wrong. A gyro disc does not act that way.
Each gyro blade has a high aspect ratio (probably around 15:1?). Whilst not infinity, this is relatively high and therefore any effective downwash will be relatively small, as the spanwise flow is relatively less. This means the following blade is not likely to interfere with the wake of the preceding blade under normal conditions.
Your sketch shows a stream flow which, again in my opinion, is unrepresentative of reality. This difference of opinion will only be resolved by an experimental demonstration of a gyroplane in a wind tunnel. If that were to be done, I suspect the experimental stream flow will show a turbulent area above (upper surface of the blades) and behind the disc with little downwash behind the aircraft and certainly no affect on the free stream away from the disc - as in the downward-bending stream flow in your sketch.

I repeat that the aerodynamics of a gyro rotor are more complex than your sketch shows. You may be right but I am yet to be convinced.

All the best,

 

[Jean Claude]

I suspect the experimental stream flow will show a turbulent area above (upper surface of the blades) and behind the disc with little downwash behind the aircraft and certainly no affect on the free stream away from the disc - as in the downward-bending stream flow in your sketch.

The photo in my post # 16 show the downwash effect away from the disc and above , as in my sketch. Aerodynamic laws are compliant to this.

 

[Jean Claude]

I am aware that I am walking on thin ice but I think that the differences are small. In both cases the flow pattern is brought about by the circulation about the airfoil section of the blade. Now with a gyro both the driving and the driven regions of the blade need some amount of circulation to act their respective part and so a vortex sheet forms behind the autorotating rotor blade just the same way it does for a powered one. Therefore the difference would rather be visible in a very detailed small scale analysis of the distribution of the circulation along the blade span than in the flow pattern at large. My two cents.

Juergen, Here the rotor wake calculated relatively to the disk, according Naca report 1319. I added the horizontal flow for obtain downwash compatible with an upward thrust

 

[Sky Scooter]

Good work Jeane now tell that euro dick why he cant unload or better still why he shouldn't promote unloading rotors for the love or experience of it, especially to in experienced pilots who know no better.That last phrase is inadequate to say the least just does my head in.

 

[kolibri282]

Very nice visualization, JC! I have rotated both parts of the image, such that the blue arrows are horizontally. It is easier now to see that in the longitudinal plane the flow patterns are indeed very similar (of course for the case of autorotation the flow is from below the disk). The down wash angle for the autorotation case is probably less for the same aircraft because the rotor does not have to produce the thrust that drives the aircraft. As I pointed out before the vortex sheet pattern as seen from above would also be very similar.

 

[Vance]

Those are very interesting flow patterns!

Those are very interesting flow patterns!

Juergen, Here the rotor wake calculated relatively to the disk, according Naca report 1319. I added the horizontal flow for obtain downwash compatible with an upward thrust

Thank you Jean Claude.

That is very instructive and gets to the heart of what I was asking about.

I don’t fly with the rotor 30 degrees back in any of the gyroplanes I fly or even reach it when I flair for landing.

I feel that it is usually less than ten degrees back.

How would that lesser angle affect the flow patterns?

Thank you.

 

[XXavier]

I wish to thank JC for his excellent posts, specially for his last one in this thread.

Concerning what Vance says, I believe that the picture shows the case of a relatively large angle just for the sake of an easier visualization. Things would be, I think, quite similar with, say, 10º of disk AoA, but the downward component of the wake would be smaller, and more dificult to see...