Rotor Blade Question

[Ross85]

What is the purpose of the swept back or the bent over tips of the rotor blades on some of the newer helicopters? Thanks

[flingwing206]

Reduced noise and drag.

[Eric Hunt]

The drag is reduced by delaying the onset of the critical mach number drag rise - same as a swept-wing fighter goes faster than a straight-wing machine, for the same power. The critical mach number is dependent on the flow from front to back of the airfoil section, and when it is swept back, you introduce the cosine of the angle of sweepback, giving you some more breathing space.

 

But the stresses produced on the blade are increased with sweepback, and it needs modern metals and blade grips to cope with it. That's why older and cheaper machines don't have it, technology and cost couldn't overcome the stresses efficiently.

 

Noise is reduced by changing the tip vortex production.

 

Latest research is into airfoils that change shape when an electrical current passes through it, up to 50 times a second. This can help with retreating blade stall too.

[Ross85]

Very good explanation Erik thanks, also another little question about design on a helicopter, Why does the blackhawk and ch-53 have a tilted tail rotor?

Edited December 15, 2005 by Ross85

[PhotoFlyer]

Correct me if I am wrong...

 

Tilted tail rotor provides some vertical thrust. Helps prevent a tail low attitude.

[crashresidue]

As I understand it - it is an engineering fix for a major c/g problem in the disign.

 

The vertical lift off the t/r keeps the *ss-end from dragging the rest of the a/c down for a winie roast.

[Eric Hunt]

It wasn't a flaw in the design, it was there from the start.

 

If you suspend something which is long from one point in its centre, the cg is fairly critical if you want it to stay horizontal. But if you add another suspension point at the end, the cg becomes more manageable, and can have a larger envelope of movement. The Chinook is the extreme example.

 

So, the T/R is tilted to provide some lift. It makes for some interesting mixing units, to compensate for pedal inputs, to avoid big pitching movements.

[Rob Lyman]

It wasn't a flaw in the design, it was there from the start.

 

If you suspend something which is long from one point in its centre, the cg is fairly critical if you want it to stay horizontal. But if you add another suspension point at the end, the cg becomes more manageable, and can have a larger envelope of movement. The Chinook is the extreme example.

 

So, the T/R is tilted to provide some lift. It makes for some interesting mixing units, to compensate for pedal inputs, to avoid big pitching movements.

Actually, the tail rotor is tilted not only to provide lift (as in the CH-53) but also as an attempt to give the helo a fly home capability in the event of loss of tail rotor drive. The idea was that with a large vertical stabilizer and autorotating the tail rotor the aircraft could fly in level flight (with a great deal of yaw) at some airspeed with a freewheeling tailrotor. The explaination they gave us in the SH-60B FRS was that the hovering characteristics were unacceptable, so they "chipped away" at the vertical stabilizer/tail pylon until hovering work load became acceptible. The end result was an aircraft that did not have a fly-home capability.

 

The UH-60/SH-60/HH-60/S-70 actually has a great deal of forthought in the design, right down to the built in left and forward tilt of the main transmission and the moveable electric/hydraulic horizontal stabilator which is mixed in with the flight controls.

[Phil77]

If you guys don't mind I have another question:

 

I have seen one of the countless apache videos and at one time it looked like the apache has two seperate Tailrotors... as I looked up some pictures of a parked one I could see that basicly the four blades are divided in pairs of two blades. Each pair seems to have its own teetering hinges or at least pitch links but both share one driveshaft?!?

 

Does somebody know what is the purpose or goal of the separation?

 

Appreciate your comments! Cheers, Philip

 

 

PS.: If somebody has some more detailed information about the "Bearingless Main Rotor system" (BMR) of the EC 135 e.g. I would like to know more how the Lead/Lag, Drag and Flapping is accomplished (through bending I know - but more detailed please)

Edited December 16, 2005 by Phil77

[flingwing206]

Eurocopter doesn't have a lot of info on this one on their website, I've never seen the EC135 rotorhead up close, but it doesn't look like a 4-blade derivitave of the Spheriflex (definitely not a Starflex), so this pic is as good as it gets from me right now!

 

The EC 135 was based on the Bo-108 prototype - I believe the Bo-108 rotor was based on the Bo-105, but I'm not sure...

Edited December 16, 2005 by flingwing206

[nbit]

If you guys don't mind I have another question:

 

I have seen one of the countless apache videos and at one time it looked like the apache has two seperate Tailrotors... as I looked up some pictures of a parked one I could see that basicly the four blades are divided in pairs of two blades. Each pair seems to have its own teetering hinges or at least pitch links but both share one driveshaft?!?

 

Does somebody know what is the purpose or goal of the separation?

 

Appreciate your comments! Cheers, Philip

PS.: If somebody has some more detailed information about the "Bearingless Main Rotor system" (BMR) of the EC 135 e.g. I would like to know more how the Lead/Lag, Drag and Flapping is accomplished (through bending I know - but more detailed please)

I believe that the stacked tail rotor design of the AH-64 is a derivative of an option for some of the MD500E/530F models called the Quiet Knight or QTR(quiet tail rotor). This option was developed with police departments in mind to reduce noise over urban areas. It was also a stacked set of two pairs of blades. By increasing the number of blades, the overall thrust can be increased, and therefore the RPM and the tip speed reduced to reduce the noise signature when providing the same thrust to counter torque. As I recall, the Quiet Knight option had the tail rotor geared to turn a lower speed also. The stacking of the blades and the offset of them at the odd angle is done so that the induced flow from one set of blades interacts with the other set optimally to produce greater tail thrust. On the Fenestron, the blades are offset at odd angles to reduce harmonic vibrations and therefore noise as well. This is also probably an added benefit to the tail design on the AH-64 .

[Phil77]

Thanks guys!

 

(sorry my reply took so long - I've been away)

 

Fling:

Exactly that lack of information I discovered too, but the hint to look after the BO... models is good, I'll try this one.

 

nbit:

Sounds reasonable to me - the induced flow is a good (and probably THE) explanation... I'm going to ask a guy I know next time I meet him - he's a former army aviator and knows a lot about army helicopters - and get back to you to tell you his opinion.

 

Merry xmas and a happy new year to all of you!!!