Author Topic: tailrotor question!  (Read 6812 times)

Offline Mike

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tailrotor question!
« on: October 23, 2009, 12:38:44 AM »
Ok, here is another one.

My fuel truck driver asked me this one (he is also a fixed wing pilot)

A Helicopter main rotor blade has a twist in in just like a propeller which makes sense since the outside travels faster through the air than the inside of the blade.

Why does a tailrotor blade not have a twist in it?

I never thought about it and walked the ramp looking at a Hughes 300, S-76, AStar, Jet Ranger, Huey, and a Bell 47.
The 47 had a slight twist in it but none of the other aircraft do...

Any thoughts?


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Offline mnicprs

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Re: tailrotor question!
« Reply #1 on: October 23, 2009, 02:10:26 AM »
main rotors and propellers are the primary source of power or lift, hence the need for optimized efficiency. not that a tail rotor isn't a primary control, it is, perhaps it doesn't require the same engineering? 

Offline The_Muppet

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Re: tailrotor question!
« Reply #2 on: October 23, 2009, 03:16:13 AM »
My guess...

My understanding of blade pitch washout along the main blades is that it is to equalize to some extent the lift created along the length of the blade so as to prevent too much bending force from being applied to the blade - perhaps with the shorter length of the TR blades and the higher rpm (more centrifugal force to stiffen them), they are able to ignore (well... engineer around) the potential for bending.

If that's not it, then my next guess is...

...  ???


(I hope this isn't a checkride question next time!)

Offline Ragwing

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Re: tailrotor question!
« Reply #3 on: October 23, 2009, 03:17:44 AM »
A propeller is a bit of a complex design.
The propeller blade on a fixed wing or helicopter is actually a wing.

When spinning, the outer diameter of a propeller has a lot more linear velocity than close to the hub.
A wing meets the air at an angle called the angle of attack. The slower the speed, the steeper the angle of attack must be to generate lift.
Therefore, the shape of the propeller's airfoil (cross section) must change from the center to the tips.

The tail rotor does not have the loading or required efficiency that an aircraft propeller or helicopter rotor requires.
It also needs to change from a positive advance per revolution to a zero advance (or even negative advance in some designs.
Therefore, it would have to have a design where the twist changes during use.

The inexpensive flat tail rotor will be with us for some time.

Offline Mike

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Re: tailrotor question!
« Reply #4 on: October 23, 2009, 04:13:28 AM »
Wow, a lot of that stuff makes sense. Especially Ragwing's post.
And no Muppet, it won't be a checkride question. At least not until I am completely sure of the exact reason.
(and keep in mind, you may have to do you Bell checkride with me this time  ;) )

----------

But Rag!!!
Were you seroius with this remark?!
The inexpensive flat tail rotor will be with us for some time.
::rofl:: ::rofl:: ::rofl::

have you bought one lately ??

(don't have the T/R numbers in my head but I do remember seeing a B206 blade (one!) for 41k last week)


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Offline G-man

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Re: tailrotor question!
« Reply #5 on: October 23, 2009, 03:56:49 PM »
Ragwing got it---has to do with tip speed and stall. I will also try to explain...though prolly not as technical as him

Lift formula is CL X half roe X V squared X S. (sorry---don't know how to put the symbol things).

CL is the coefficient of lift, or the blades ability to produce lift at a given angle of attack. The higher the angle of attack at a given velocity, the closer you are to stall. At the balde tip, the velocity is higher than the root. Therefore, in order to increase the VNE of a helicopter, (factor of retreating blade stall), one must reduce the blade stall near the tip---this is done by having a negative twist on the main blade, thereby reducing the angle of attack at the tip and evening out the overall lift along the length of the blade.

TR blades are too short for it to really matter....

NOW for the bonus question----explain the BERP. . . . (Hint--think Lynx).
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Offline Ragwing

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Re: tailrotor question!
« Reply #6 on: October 24, 2009, 02:55:33 PM »
But Rag!!!
Were you seroius with this remark?!
The inexpensive flat tail rotor will be with us for some time.
::rofl:: ::rofl:: ::rofl::

have you bought one lately ??

(don't have the T/R numbers in my head but I do remember seeing a B206 blade (one!) for 41k last week)
41K is one big ouch.  No, I did not know it went for that much.   ::sweat::

Prices of recreational equipment is bad enough.  Cost of aviation equipment is down right scary.
I still need to replace the Emergency Locator Transmitters (ELTs)...   ::silly::

Offline Chopper Doc

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Re: tailrotor question!
« Reply #7 on: October 26, 2009, 01:04:03 AM »
Interesting discussion, folks.

Another point we haven't covered is that the change in pitch towards the end of the airfoil, called washout, is also used to reduce the angle of attack in the outboard portion of a fixed wing in order to create a portion of the wing less-likely to stall.  This feature helps to resist the likelyhood of stall-spin entry.

Rotorcraft use a simple, straight wing section on the tail rotor as it's purpose is to counter torque, and as we vary the torque applied to the airframe we have to vary the thrust created by our anti-torque rotor without changing the rotational velocity of the wing.  This we do by changing the pitch of the blades collectively over a range from slightly negative to very positive, so a simple airfoil is both cheaper and more predictable in performance.

In the case of the main rotor blade, G-Man points out that we can increase VNE using washout to decrease angle of attack at the tip, but we also use the negative angle of attack that results when the collective is bottomed to provide the driving zone during autorotative flight.

The BERP tip is the rotorcraft world's answer to the swept wing on transonic fixed wing aircraft, allowing higher advancing blade tip speed without the undesirable compressibility effects on a straight airfoil.  The tip shape also has other beneficial effects in delaying retreating blade stall, which also helps to increase the VNE: at 228mph the BERP-equipped Lynx is the fastest rotorcraft on the planet.
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Offline G-man

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Re: tailrotor question!
« Reply #8 on: October 26, 2009, 03:46:35 PM »
but we also use the negative angle of attack that results when the collective is bottomed to provide the driving zone during autorotative flight.

And also to hold a helicopter on the deck of a ship while it is tied down in rough seas. I "think" the lynx and SeaKing have a negative pitch option---but don't quote me on that one.
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Offline Chopper Doc

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Re: tailrotor question!
« Reply #9 on: November 04, 2009, 11:31:02 PM »
All helicopters have the requirement for the M/R baldes to have a net negative pitch; without this, we don't get an autorotation without power, we get the blades folded/greased anvil approach.

Another reason why helicopters need more than just determination to fly.
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Offline Chopper Doc

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Re: tailrotor question!
« Reply #10 on: November 09, 2009, 05:37:28 PM »
When you want to plant a helicopter on a moving vessel, use a beartrap.  Since the early '60s the Canadian Navy has been "beartrapping" helos to the deck in order to make landings/takeoffs safer at sea.  This avoids the requirement to have a bunch of crewmen on the deck to secure the aircraft at touchdown, and makes handling the machine safer and easier.

Using negative pitch to "hold" a machine to the deck seems a bit dangerous - I'd be concerned about the loss of rotor blade clearance over the ship-crews heads once the blades coned downwards.  Could cause a few blade-strike inspections.

http://www.helicoptersmagazine.com/content/view/167/61/ 
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Offline Canberra Man

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Re: tailrotor question!
« Reply #11 on: March 07, 2010, 12:46:12 PM »
The tail rotor at rest returns a to a flat pitch if the rudder bar is central.  When the main rotor is turning the fuselage obeys the rule, - to every movemnet there is an equal and opposet reaction.  If the pilot wanted he could use the tail rotor to pur the fuselage at a right angle to the line of flight.

Ken

Offline Chopper Doc

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Re: tailrotor question!
« Reply #12 on: March 29, 2010, 04:20:40 AM »
Canberra Man: I think if you go look you'll find that the t/r blades at pedals-neutral will have some positive pitch.  This is due to the torque forces that have to be resisted at 100% NR even at flat pitch - remember that it typically takes 16-24% Tq to just spin the m/r at 100%, and we don't want to use all our power pedal on an icy pad, do we?

In an auto the offside pedal is depressed (whether you pilots remember it or not) to unload the t/r - just think of what you do every time you go downhill with a load under the machine.  It only makes sence that 16-24% of full pitch is required with the pedals neutral.

As far as putting the tail at right angles to flight, pilots will usually not use the tail rotor as a "curb-feeler".  Also, there's LTE to consider when trying to make your aircraft fly without putting the pointy end first.
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