Angled rotor tilt axis

[PTKay]

I tried to understand, why the tail rotors of big helis
have the teetering axis of the dual blades angled (not rectangular)
to the blade ?

And this is both on US and Russian helis.

See examples.

PTKay

[quadrirotor]

That's to manage the CG...those helicos are in fact tandem rotors helicos!!!...yes some helicopters are tail heavy because of the army requirements!!!! The Navy wants to store as many helicos as possible in the less space possible!!!! after being folded many times, they must have a given size!!!!!

[PTKay]

I think, Andre, you didn't get my point. (my poor English).

On the drawing below you see the red lines:

they are angled, one is blades axis, the other tilt axis.

In a normal gyro rotorhead they are rectangular (green line).

So why it is different here ?

PTKay

[quadrirotor]

Sorry PTkay, the angle between each rotor is intended to suppress the resonant vibrations, which could built up between main rotor and tail rotor and frame and the capitain and so on...(this helps also to fold the helico...)
sorry not an answer neither!!!!
the angle between rotor head and axis of gimbal is for giving to the rotor a "DELTA 3 EFFECT" which tends to keep the rotor in the intended plan of rotation...If the rotor does not stay in his plan of rotation the pitch of one blade is lessened (and the other augmented...) to get back the rotor in his plan of rotation. :)

[Victor Duarte]

hi paul, hi andre,

there are 2 techs in these tail-rotors :

- first is they use non symetrically spread blades tail rotors to avoid eigenmodes, rotation having a "break" thos reduces the risk, i m quite shure of that, perhaps some one could confirm

- the second is the "offset" (or spin) of the teetering axis : simple its to keep the blades automatically in the vertical plan.
i my french i remember they call it "K link" (dont know the english word), its a system that works like that :

imagine the axis is at 90 °, normal, if the blade lifts or thrusts to the side, the pitch will still the same, so, the blade stills follow the same path..

in the pic you see, look, if the blade goes lateral, you ll see the pitch decreases and, so, will keep the blade in the same vertical path..

it is also used in quite all teetering helicos (bell etc) to lower advancing blade and keep the rotor in the expected disk.. it is done by positionnning the pitch control forward and after the flapping axis (you see it draws an oblique axis too)

if you want ill do a sketch for you, just ask, hope it helped

best regards

victor

[C. Beaty]

That's called Δ3 angle, at least in the English speaking world, PTKay.

The purpose is to reduce cyclic flapping.

With a skewed flap hinge, the pitch of a blade is changed as it flaps, i.e., flap/pitch coupling.

[PTKay]

I thought, while tilting, the angle of attack of the blades is changed.

Thanks for the anwsers, Chuck, Victor and Andre.

But then, why don't we use it for gyros ?

The two double blades (also angled to each other) is another question.

PTKay

[Victor Duarte]

paul , i tried to reply but beaty and andré did it faster :D, see previous

ok it s delta "delta 3" it sounds more "action" ;)

[PTKay]

Also this tail rotor (2-blades) is angled.
(Bell AH-1W Super Cobra)

PTKay

[PTKay]

Victor, do I understand you correctly:

also the main teetering rotors have it, the Δ3 angle ?

Not on the Robinson, to my knowledge. So only on big ones ?

And again, why not on gyros ?

PTKay

[quadrirotor]

The Delta 3 effect can be obtained with the linkage also...All depends on your needs to limit flapping...

[Victor Duarte]

paul, lots of helicos have that purpose but it is not done like on your pics, by skewing the axis, it is done by positionning the pitch control correctly ..

ok try this :D : put your arm on the table, let s say the elbow if the flapping hinge and your hand is the blade and can rotate about your wrist, the trailing edge being oriented to your face ok? ... now imagine (with the other hand, you can control the pitch angle on your inch, you ll see, if the blade tries to go up, the control stays at the same position and lowers the pitch angle, so the blade will come down to its position...

why not used in gyro? dont have a good answer , perhaps for simplicity ?, it s a good question for who knows better gyro than me...

regards

victor

[C. Beaty]

PTKay said:

"But then, why don't we use it for gyros ?

The two double blades (also angled to each other) is another question."

The second part of your question was correctly answered by both Victor and André but things may get garbled between French, English and Polish.

The blades are angled to each other so as not to produce evenly spaced impulses and therefore diminish the likelihood of exciting resonances.

Pitch/flap coupling (Δ3) in effect imposes an aerodynamic spring between airframe and rotor, increasing the frequency of the rotor above its rotational rate. Phase shift between force/displacement becomes less than 90º, requiring a compensating adjustment of the swashplate angle. Works OK under constant conditions but a change of airspeed will cause some pitch /roll coupling in the cyclic control system.

Delta 3 coupling also decreases the damping supplied by the rotor. Normally, a disturbance that displaces the airframe is resisted because of rotor lag. Delta 3 reduces rotor lag and therefore, damping.

The A&S-18A used a large amount of pitch/flap coupling, a Δ3 angle that looks to be 45º.

This was to automatically lower blade pitch to autorotational pitch following a jump. Pitch/flap coupling also produces pitch/cone coupling; as the rotor slows following a jump, the coning angle increases and collective pitch is automatically reduced. I'm told the 18-A is a squirrel to fly.

The Robinson R-22 also has considerable Δ3 coupling, looks to be 20º or so. I've read a lot of BS about yeeyaw (tilt rate effect) but I suspect the real reason was to suppress flapping and reduce the likelihood of rotor/tail boom collisions.

Most helicopters use Δ3 coupling sparingly if at all; the B-47, Victor, has everything at 90º.

[Victor Duarte]

"the B-47, Victor, has everything at 90º"

well i should pay my turn of beer perhaps :o i quoted the wrong manufacturer, i cant be forgiven for that, as i could examine very closely b-47 during 2 years and may have reminded that... beer, scotch? maybe a french wine? ;)

sorry for my bad english, i must straighten it.. and thanks fo the good explanation in the right words.

victor

[C. Beaty]

No problem, Victor.

Your English is infinitely better than my French.

During the time I spent in France, the only restaurant menu item I knew the words for was a ham and cheese sandwich (jambon et fromage), so that's all I ate.

If I needed a liter of oil for my VW Beetle, I'd have to lift the engine cover and point to the oil spout. For me, it is impossible to pronounce "huile" in a way that is comprehensible to a Frenchman.

[PTKay]

Chuck, thanks for the explanation.

Actually I once had an Idea to use a tail rotor head from a bigger heli
for a main rotor head for gyro with collective pitch control for jump starts.

They are available, they are strong and the pitch control through the
middle of the shaft is may favorite (see Mi 28 Havoc rotor -1st pic.)

The cyclic could remain normal (gimbal), my only daubt was skewed teetering.

Now I know more, but the idea (above) is probly anyway not practicable....

PTKay

[Helidev II]

Leaving the delta 3 for now as its been pretty much adequately explained, the reason for the non-normal placement of the blades is more of a geometric one, as opposed to an aero one. Its just simpler to rig the control rods.
Think about it this way, if the blades were 90deg to each other where would you run the linkages?
OK you could make up some funky U bends and the like, however its much simpler to rig it in a way that you can have straight linkages to the pitch horns.
Its in Ray Prouty's book so it must be true.

[quadrirotor]

Sometimes Ray Prouty says ununderstandable things!!!!

[PTKay]

Found a delta 3 drawing for a 3 balade rotor on the XV-1 revival document.

BTW: Excelent document, worth reading

http://rotorcraft.arc.nasa.gov/publications/files/NASA-CR-2003-212799.pdf

I hope some decision making people have read it carefully
and will try to work for an alternative to the most complicated
and uneffective VTOL: the (in)famous V-22 Osprey. :)

[PTKay]

If I can correctly interpret this CarterCopter rotorhead pic,
it has also a slight delta3 on the gimbal ?

[Victor Duarte]

if we draw lines, it apprears to have a little, yes, but there is a perspective effect..
i m not shure, better have a top view

[PTKay]

I have no better pic...

[Victor Duarte]

well..i cant say if theres a d3 in the teetering part , i really wonder why they may have a built-in d3 if they can have the same effet just by adjust the placement of the pitch horn point (like you see in the xv1 rotor drawing).

the problem, from what i know is that a built-in d3 causes the blade to advance a little, while a 90 ° teeter axis+d3 by pitch rods allows the blade to go up on the same vertical plane > no advance, no additionnal parasite effort on mast..

built in d3 may work well for tail rotors whitch are not really sensible to advancing-retreating blade problem. i consider that as a "easy" way to simplify
see the B47 tail rotor : http://www.kiwiaircraftimages.com/pages/a01b4710.html
it has built-in d3 effect, not the main rotor.