So what are the numbers?

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Wonko the Sane
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So what are the numbers?

Unread post by Wonko the Sane » Thu Feb 14, 2019 3:19 pm

Hi Rotory wing pilots

Please accept my advance apologies for not being the prerequisite “real” Helicopter pilot.

I was looking at this picture and the thought came to me that I haven’t seen it mentioned anywhere (that I could find). I even asked cage...
Look at the angle of the receding blade (right hand one looking at the picture, for non-technical persons) It seems high. (Thanks Splatter)
IMG_1109.jpg
IMG_1112.jpg
Being a helicopter “thumb” pilot (performance and scale), part of the mechanical set-up, is the cyclic and collective pitches.
Cyclic pitch is set up to 6 degrees and collective pitch at +12 and -12 degrees for 3D machines and +12 and -2 degrees on scale birds.
0 deg.JPG
6 deg.JPG
12 deg.jpg
The -12 degrees pitch is what gives a model helicopter the ability to HOVER inverted.

Helicopters like the Lynx and BO 105 that perform aerial aerobatics do so using momentum and speed to loop and roll. They can not HOVER inverted.

The 6 degrees cyclic and 12 degrees pitch is also the main reason for boom strikes on models as it is more than possible to perform a manoeuvre where the angle of the blade is equal to the full 12 degrees pitch plus the 6 degrees cyclic, totaling 18 degrees. (Usually pyro flip/funnel combinations where pitch and Aileron are at max) The resultant angle will cause the blade to stall and move in the grip, just enough to impact the boom.

Also to be noted that 3D blade profiles are Symmetrical and Scale blades are not...
easa-part66-module-82-aerodynamics-7-638.jpg
On my scale machines I use negative pitch to compensate for wind and turbulence only.

So the question is... what are the numbers for real helicopters w.r.t Cyclic and collective pitch?

W
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Last edited by Wonko the Sane on Thu Feb 14, 2019 9:14 pm, edited 1 time in total.
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Re: So what are the numbers?

Unread post by hugo_visser » Thu Feb 14, 2019 4:54 pm

Learnt something, thank you Andre.
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Re: So what are the numbers?

Unread post by csparksfly » Thu Feb 14, 2019 5:37 pm

Well done, made me look! :wink:


According to the Robinson R44 maintenance manual
R44-MM-BladeAngles.png
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Re: So what are the numbers?

Unread post by csparksfly » Thu Feb 14, 2019 6:01 pm

Robinson R22 maintenance manual
Robinson R22 Beta II

Collective pitch: 11.0°/12.0°

Cyclic Fwd: 10.5°/11.0°
Aft: 8.5°/9.0°
Left: 9.0°/9.5°
Right: 5.5°/6.0°

Tail Rotor full right pedal: 9.6°/10.6°
Tail Rotor Full left pedal: 19.0°/19.5°
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Re: So what are the numbers?

Unread post by csparksfly » Thu Feb 14, 2019 6:04 pm

..then again "some" might say that the Robinson's aren't real helicopters :lol:
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Re: So what are the numbers?

Unread post by Chopper One » Thu Feb 14, 2019 6:39 pm

In forward flight the rotor disk is tilted forward, so the blade is at it's lowest position in front of the helicopter from there the pitch angle and subsequent angle of attack increases cyclically to fly up to the highest point at the rear of the helicopter so the retreating blade has a high angle of attack and the advancing blade conversely a low angle of attack.

I think the wide chord width on the Huey's main rotor blade accentuate this.
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Re: So what are the numbers?

Unread post by Wonko the Sane » Thu Feb 14, 2019 7:03 pm

Chopper One wrote:
Thu Feb 14, 2019 6:39 pm
In forward flight the rotor disk is tilted forward, so the blade is at it's lowest position in front of the helicopter from there the pitch angle and subsequent angle of attack increases cyclically to fly up to the highest point at the rear of the helicopter so the retreating blade has a high angle of attack and the advancing blade conversely a low angle of attack.

I think the wide chord width on the Huey's main rotor blade accentuate this.
True, but this one is landing so probably pulling some collective to decelerate as well...


W
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Re: So what are the numbers?

Unread post by Induna » Thu Feb 14, 2019 8:23 pm

interesting topic. looking at the windsock it looks like a fresh crosswind there (actually strong), so probably needs higher angle of attack on retreating blade to compensate?

Normally in forward flight the retreating blade will be at a higher AoA to compensate for asymmetrical lift. In this case it might just be accentuated due to the crosswind as well.. (assuming the heli still has forward speed in the picture)
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Re: So what are the numbers?

Unread post by hugo_visser » Fri Feb 15, 2019 4:08 am

I think 90 deg before because of gyroscopic forces.
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Re: So what are the numbers?

Unread post by ray fox » Fri Feb 15, 2019 6:54 am

In forward flight the advancing blade is flying faster and the retreating blade slower, to compensate for lift the pitch between the blades will be different
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Re: So what are the numbers?

Unread post by Chopper One » Fri Feb 15, 2019 7:51 am

Wonko,

I have a video of a Huey that was re positioned to the Heliport when it opened. Even from above one can see the pronounced retreating blade. I will try to post it. Another reason could b that the top of the blade has got very pronouncing colours, while the bottom of the blade sort of get lost in the background. Anyway interesting thread.
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Re: So what are the numbers?

Unread post by Iceberg » Fri Feb 15, 2019 7:58 am

These blade pitches sure are complicated - and have to be in balance to secure stable flight.
So one can imagine what can happen if something goes wrong with one of the pitches in a certain sector of the blade sweep.

I think I'll stick to fixed wing. :)
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Re: So what are the numbers?

Unread post by Bell 407 » Fri Feb 15, 2019 8:18 am

Wonko,

Here is a run of 3 pictures of a B222 with big flappy blades passing by at speed. You can see the very pronounced difference in AoA between the advancing and retreating blades.

Image

Image

Image
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Re: So what are the numbers?

Unread post by Wonko the Sane » Fri Feb 15, 2019 8:22 am

Bell 407 wrote:
Fri Feb 15, 2019 8:18 am
Wonko,

Here is a run of 3 pictures of a B222 with big flappy blades passing by at speed. You can see the very pronounced difference in AoA between the advancing and retreating blades.

Image

Image

Image
Nice shots Bell 407!

W
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Re: So what are the numbers?

Unread post by Wonko the Sane » Fri Feb 15, 2019 8:23 am

I will make a video this weekend on how I perceive it...


W
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