Hub-bar research project

[asra]

G"day All, Please refer to the attached document for information regarding a research project I"m undertaking.

[asra]

http://www.youtube.com/watch?v=Ug6W7_tafncHello Steve, I guess this is a bit further down the track, but I am very interested to find out exactly what gyro rotors are doing in flight. Watching this video of a helicopter blade it is hard to tell what the tip is doing as the camera appears to be mounted on the blade root. Although the tips may be turning on a flat plane I will describe it as it appears in the video. At 90 degrees the blade is straight as though it has even lift along the length of the blade. At 0 degrees the tip curves down slightly. At 270 degrees the tip curves up when the lift on the retreating blade is near the tip. Between 270 and 180 the tip dips very sharply to its lowest point at 180 degrees. At about 225 degrees the tip appears to flip down very quickly and this is the exact point that the two per rev stick shake was coming from when I had extra pitch put on my blades. I found that by going above or below a certain teeter height the shake changed and by setting the undersling at the point where it changed virtualy eliminated the stick shake. The problem with it being simply teetering of the hub bar and having the teeter height at centre of mass of the coned rotor was in the way the stick shake changed. With low teeter height the shake was a very strong two per rev coming from the rear left. When the teeter height was high rather than being the same shake from the opposite direction, it was a weak two per rev when kept small but quickly turned to a one per rev if allowed to increase and that would keep increasing. Having the exact teeter height only became critical with extra pitch and more cone angle. In looking at that video it would seem that even if the tips remain on a perfect plane the hub bar would do a lot of teetering. It would be interesting to see what happens to a gyro rotor in flight, both composite and aluminium as they appear to have very different characteristics. Also as on a gyro the rear rotor is low and the front rotor is high, how they would compare to a two blade helicopter. On a scale drawing of my gyro with the rotor disc at 9 degrees and a cone angle of 3 degrees the tip of the rotor would pass through the prop wash.

[asra]

G"day PeterRotor behaviour in flight is definitely a freaky phenomenon.

[asra]

Steve, I built a model rotor and hub to see what the rotors are doing in flight. I exaggerated the cone angle and teeter or blow back angle. This also exaggerates the tip movements.With mine I set the cone angle to 45 degrees, teeter or blowback to about 20 degrees. I clamped the hub section to a shed post and used the floor as the tip path plane and turned the rotor by the tips.Well worth doing.

[asra]

G"day Peter, I"d be interested to hear about your observations with your model.

[GraemeMonro]

plenty of factors which could see the experiment yield useless data, or fail altogether, but also plenty of potential to achieve something ground-breaking - fingers crossed......Fingers crossed...Your one of the lucky ones Steve, there are some amoung us that can"t do this.Graeme.

[Waddles]

right graheme,but i"ve got other things to cross instead!!!waddles.

[asra]

Cheers Graeme, Regardless of the outcome of my particular experiement, the upside is that the seed has now been planted here at the univerisity for this area of research - i.e. gyroplane rotor dynamics and loads.

[asra]

Steve, it is a very complex manouver for the rotor to turn one revolution and difficult to describe.From the teeter height to the blade tips it is easy to see and describe, but what is happening below the teeter bolt apart from turning a two rev circle for every one rev of the rotor is different.I have noticed it is difficult to eliminate two per rev shake in rotors with high cone angle when in forward flight. This is why I exaggerated both cone and blowback angle.With no blowback angle as in a vertical decent all parts of the rotor turned at a constant velocity.With approx 20 degrees blowback angle there was a very noticable difference in tip speed.Starting at the centre front of the disc (0 degrees) the retreating blade tip constantly increases velocity untill it reaches the rear centre of the rotor disc. From the rear centre (180 degrees) the tip is constantly decreasing velocity untill it reaches 0 degrees. When the blade tips are at 90 - 270 degrees they are traveling at exactly the same speed.When they are at 0 - 180 degrees is the point of most difference in tip velocity. At 0 degrees the tip is at its slowest speed. At 180 degrees tip speed is fastest.While this is happening the sections of blade level with the teeter bolt maintain a constant velocity.I chalked the tip path on the concrete floor as that was the tip path plane. When I set the blades at 90 - 270 degrees, to complete a half revolution the tip traveling the rear section had to cover 2/3 the circumferance while the tip covering the front section of the only traveled 1/3 of the circumferance.Although with the model the angles are exaggerated and this exaggerates what the tips are doing it would seem that on a teetering rotor in forward flight the tip at the rear section of the disc must always have a higher velocity than the tip in the front section of the disc. The other thing I noticed is that although the tip path is a circle, the centre of lift will be changing in that circle twice per rev. When the rotor is in the for- aft position (0-180 degrees) the center of lift is on the centre of the tip path circle. When the rotor is broadside (90-270 degrees) the centre of lift is on the hub axis.The rotor tips lifted off the floor twice per revolution the highest point at 90-180 degrees so rather than spinning on a flat plane it is a circle with two waves per revolution.Peter.