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Just for Flying High and Joker


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Ok.... ya all what a "hearty" discussion.

 

 

Do tail rotors, in american helicopters, PUSH or PULL the tail rotor back in lign with the nose of the helicopter.

 

Now I have asked this ? to 200 hr pilots and 10,000 hr pilots. Of course, the answer always varies.

 

It seems to be about 1/2 and 1/2 Some say it PUSHES and other say it PULLS...

 

 

We all know the down wash is blowing out to the left, so it's PULLING air from the right side, eh?

 

So is this thing that we call a TR pushing or pulling?

 

If you know the answer....... PROVE IT.

 

 

Now this should be good.............................................. reply on fellas

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Guest pokey

well, i see this is a "trick" question <_< , so when all else fails? apply logic :unsure: it all depends on which pedal ya push :rolleyes:

 

now lets see, IF you were to remove the T/R assembly & hover the helicopter w/ an "accomplice" standing in place of the missing T/R, and assuming that the rotor is spinning CCW (viewed from above):

 

IF your accomplice were standing on the right side of the ship ( as pilots perspective) he/she would be pulling

 

Standing on other side of the tailboom, he/she would be pushing

 

 

DISCLAIMER: do NOT try this at home, & IF ya do? i am NOT responsible for the outcome B)

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Guest pokey

In addition to what i said in previous post, THAT is the force that the T/R blades are dealing with, HOW they do it is relatively easy to prove too. It all goes back to the very basics of aerodynamics and good ole Bernoulli, (remember him)? He's the "differential pressure" guy from long ago, who said that the faster the air moves---the lower the pressure, the reason why an airplane flies & why an airfoil works. The tricky part to understand tho is: IS the top of the wing trying to be sucked (pulled) up to the lower pressure? OR is the bottom due to the higher pressure being pushed up? AND since pressure flows from high to low? I would place my $$ on pull. Altho that ideal principle is based on an assymetrical airfoil, when angle of attack is introduced to a symmetrical airfoil? I would bet my $$ that there is some "push" involved too.

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You're looking at two things at once trying to get one answer. Dependant on which side the t/r is mounted and in which direction the nose is moving, you will have four possible answers.

 

Let's assume we are talking about a H300 or R22 in powered flight:

 

Obviously torque rotates the fuselage to the right so the t/r counteracts that. Being that the t/r is mounted on the left side of the tailboom, and the boom wants to move left you can safely say that the t/r pushes. If the t/r were mounted on the other side of the tail boom, like in some Huey models, then it would be pulling.

 

If were were discussing powered off flight and no resultent torque and now you wanted the nose to go right then the opposite would be true. The standard H300 or R22 would now be pulling a tailboom to the left.

 

The other thing you are debating is which is the cause for the thrust, Bernoulli or Newton. Most would agree that Bernoulli is doing most of the work and a simple example is the fact that the main rotor blades still spin and produce lift while in autorotation, despite the drag.

 

I just got called for a flight, though, so I'll leave it at that for now.

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The tail rotor produces thrust by accelerating a mass of air around it.

 

There is the "barn door" effect - the air strikes the broad side of the rotor blade and is deflected - the blade (and tailboom) is deflected in the opposited direction - so that's "push".

 

There is the induced flow effect - pushing this air away creates a low-pressure area, inducing air to move into it - that's "pull".

 

So the answer is... ...yes! ;)

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Saw this last night, went to bed, and everyone's answered it already!

 

The answer depends on which relationships we are talking about as CoG states. Just to expand...

 

Tailboom / Tailrotor relationship - then the answer of course depends on which side the tailrotor is mounted.

 

Tailrotor / Air relationship #1 - Bernoulli level - I would say an airfoil 'pulls' into the low pressure air above. Thats because the shape, angle and speed of the airfoil are all designed to cause Bernoullis principal i.e. to create that 'low' pressure above the airfoil.

 

Tailrotor / Air relationship #2 - Newton level - The action / reaction force here is definitely a push. If you stuck your hand out of the car window and angled it a bit it tries to go up. Well this is mostly due to the deflection of the air on your hand. I would call this a push.

 

As CoG stated, Bernoulli works more than Newton when it comes to airfoils. While both are present, I would say that generally a propellor (TR) 'pulls' its way through the air.

 

 

Joker

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Guest pokey
I would say that generally a propellor (TR) 'pulls' its way through the air.

Joker

 

I agree Joker, altho this concept is really tuff to comprehend. I have been trying to think up a simple analogy to clear it up. This may do it?

 

Picture a tank with a piston inside, (obviously the piston is being "pushed" by some mechanical means)compressing the air to 50psi. Atmospheric pressure = 14.7. When the valve is opened, Is the air pushed out of the tank by the 50 ( & the piston)? OR pulled out by the 14.7?

 

Now, lets raise the atmospheric pressure to 50 also, what is the result? no airflow, & all we have eliminated is the "pull" side, the piston is still "pushing"

 

 

ANd yes Fling, the propeller/ tail rotor produces thrust by accelerating a mass of air around (thru)it. ANd it is Newtons 3rd law (action/reaction) of this rearward acceleration that "pulls" it forward, but i think the brunt of this discussion is "what surface" ( front OR back ) of the propeller is this mysterious aerodynamic forceworking? Interestingly enough too? The "face" of a propeller is on the back !

 

ANd i am sure we have ALL seen & tried this experiment. Get a strip of lite-weight paper a few inches long by an inch or so wide, put it to your lower lip & blow over the top of it-- the strip is being sucked/pulled into the hi velocity, low pressure airstream,,,,

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Thought you all would like this one...

 

You asked for a hearty discussion.... and it's something different then the SSH BS.

 

 

 

So, I would and have been teaching my students that it PULLS.

 

If if you just to remove the TR, on a counter clock wise rotor head, the tail would swing clock wise right.

 

So the TR would have to PULL the tail back...

 

 

I dont think it matters, which side the tail rotor is on.... If your main rotors turn counter clockwise, then your tail rotor would HAVE TO PULL the tail back the other way.

 

In any case........ No matter what your thought is.. Push or Pull.

 

It is not easily taught WHY to a student...

 

It just is what it is... with out getting all to technical and scientific for them to comprehend.

 

 

To be 100% honest.... I could care a less if it pulls or pushes and I really doubt any of you do. All that I care about is that it CONTINUES to turn and has plenty of thrust, while I am strapped to the machine.

 

Thanks for all of your replys and ya all fly safe...

 

Mark

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Picture a tank with a piston inside, (obviously the piston is being "pushed" by some mechanical means)compressing the air to 50psi. Atmospheric pressure = 14.7. When the valve is opened, Is the air pushed out of the tank by the 50 ( & the piston)? OR pulled out by the 14.7?

 

Now, lets raise the atmospheric pressure to 50 also, what is the result? no airflow, & all we have eliminated is the "pull" side, the piston is still "pushing"

Er - both? There is a difference in pressure between the inside and the outside - at heart, pressure can be defined by the total "impact pressure" of the air molecules hitting the walls of the tank - so there are more molecules hitting on the inside than the outside. Now you open the valve. The molecules are still hitting all the closed walls, but they are escaping unhindered out of the valve (nothing to hit). We talk about air wanting to move from high to low pressure, but remember air is trying to go everywhere at once, there's just other air in the way. Low pressure is just less air (in the same volume of space), so you could easily say that just like your piston analogy, air is pushed from high to low pressure. Or pulled. Or both. It doesn't matter, call it what you want.
ANd yes Fling, the propeller/ tail rotor produces thrust by accelerating a mass of air around (thru)it. ANd it is Newtons 3rd law (action/reaction) of this rearward acceleration that "pulls" it forward, but i think the brunt of this discussion is "what surface" ( front OR back ) of the propeller is this mysterious aerodynamic forceworking?
Again, both - since the whole purpose of an airfoil is to redirect moving air with as little drag as possible. About Burnoulli and the wing getting "sucked" upward - if you take a symmetrical airfoil and give it a zero AOA into moving air, are both sides feeling this "suction"? Would the airfoil explode like an overinflated balloon if the airflow was fast enough? Burnoulli causes the airflow to change direction, the mass of the air being redirected causes a reaction, we call that lift. Impact on the bottom of the wing redirects (a lot of) air, we call that lift.
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Guest pokey
About Burnoulli and the wing getting "sucked" upward - if you take a symmetrical airfoil and give it a zero AOA into moving air, are both sides feeling this "suction"? Would the airfoil explode like an overinflated balloon if the airflow was fast enough?

 

That is "intersting concept" Fling, but no, it would not explode, because what "sucks" the wing is the differential pressure.

 

What WOULD be an "interesting" experiment tho, would be to make an assymetrical airfoil with uper & lower surfaces made from a balloon, put in wind tunnel w/ zero AOA & "watch" the upper & lower surfaces, then increase angle.

 

I dug out my notes from A&P school because i remembered this interesting Vne requirement for fabric covered wings: "Anti-tear tape is used on aircraft where the Vne is 250 mph or greater, recommended under re-inforcing tape on the UPPER surface of the wings tip to tip, AND on the bottom surface of that part of the wing which is in the propeller slipstream"

(This anit-tear tape is used to reinforce the rib stitching holding the fabric on the ribs.)

 

 

MAYBE it would explode Fling !!! NO of course not, but this is a mind boggling concept huh?

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I think this is actually a very simple concept, being made difficult by a CFI. If you look at the whole system instead of breaking it down it is too complex. A tail rotor does, in fact, push when mounted on the left side. Just like if you are moving a shopping cart you can either push or pull it dependant on which side you are on. There is also very little supporting argument that Newton works harder then Bernoulli. It, for some reason is hard to get your head around, but the lift on an airfoil comes mostly from the top, ie suction. Several texts suppport this and are easy reads. (Wagdendonk and Coyle and the FAA.) If it were not so, blade stall would be at a much higher AoA.

 

Would you be having the same question about a main rotor pushing or pulling? Probably not, and that is because of the mounting. If a main rotor were mounted underneath the helicopter (aside from making it hard to get in and out of) it would be pushing the helicopter, but it wouldn't change the fact that the lift comes from the upper camber.

 

As for the pressure inside an airfoil, it can cause the blade to explode or delaminate. That is why there are holes in some of them.

 

Now if someone could explain to me why the FAA thinks that Magnus effect deserves first mention in total lift?

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Get a strip of lite-weight paper a few inches long by an inch or so wide, put it to your lower lip & blow over the top of it-- the strip is being sucked/pulled into the hi velocity, low pressure airstream,,,,
Actually, the paper is being pushed by the air pressure underneath. You have lowered the balancing pressure above the paper.
That is "intersting concept" Fling, but no, it would not explode, because what "sucks" the wing is the differential pressure.
The reason the wing goes "up", is because of the pressure underneath and the fact that the wing is causing a mass of air to go "down". Air molecules can't pull, they can only push.
MAYBE it would explode Fling !!! NO of course not, but this is a mind boggling concept huh?
My mind remains unboggled. Take an electric fan with stamped metal blades - not a hint of Burnoulli there, but it still moves air. Balsa airplanes without any camber or airfoil to their wings fly just fine. Streamlining and Burnoulli greatly reduces drag, but it still remains that if air isn't being redirected, thrust isn't happening. Your tail rotor certainly creates a low-pressure area on the upstream side, but the tail rotor creates thrust by accellerating air, and the relatively higher pressure downstream, not because the low-pressure side is somehow "pulling" on the rotor blades. It's still PRESSURE, it's just an unbalanced pressure. That low pressure area upstream induces more air in from the surrounding higher-pressure areas, so that the air pump that our T/R is can grab it and fling it hard, creating thrust.
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I think this is actually a very simple concept, being made difficult by a CFI. If you look at the whole system instead of breaking it down it is too complex. A tail rotor does, in fact, push when mounted on the left side. Just like if you are moving a shopping cart you can either push or pull it dependant on which side you are on. There is also very little supporting argument that Newton works harder then Bernoulli. It, for some reason is hard to get your head around, but the lift on an airfoil comes mostly from the top, ie suction. Several texts suppport this and are easy reads. (Wagdendonk and Coyle and the FAA.) If it were not so, blade stall would be at a much higher AoA.

 

Would you be having the same question about a main rotor pushing or pulling? Probably not, and that is because of the mounting. If a main rotor were mounted underneath the helicopter (aside from making it hard to get in and out of) it would be pushing the helicopter, but it wouldn't change the fact that the lift comes from the upper camber.

 

As for the pressure inside an airfoil, it can cause the blade to explode or delaminate. That is why there are holes in some of them.

 

Now if someone could explain to me why the FAA thinks that Magnus effect deserves first mention in total lift?

 

I agree. As much fun as it is to make this so complex. I like simple answers and I really like to use comon terms. So... I'll say this:

(1) Imagine an airplane at the end of your tail. With the prop on the right side, air moving left, over the the plane.

(2)Now imagine if the prop is on your left, air moving away from the plane.

 

A fixed wing pilot would call the first a puller prop and the second a pusher prop.

 

I rest my case

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I agree. As much fun as it is to make this so complex. I like simple answers and I really like to use comon terms. So... I'll say this:

(1) Imagine an airplane at the end of your tail. With the prop on the right side, air moving left, over the the plane.

(2)Now imagine if the prop is on your left, air moving away from the plane.

The PROP wouldn't know the difference.
A fixed wing pilot would call the first a puller prop and the second a pusher prop.

 

I rest my case

Yep, one pushes the plane and the other pulls the plane, but they both act exactly the same to the air. The props at either end of an Adam A500 or a Cessna 332 do exactly the same thing as the props on either wing of a Beech Baron. They move air back, the airplane moves forward.

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I think everyone's seriously overcomplicating the question. It's doing both.

It's pulling the air in from the inflow side and pushing it out through the outflow side.

It's not a question of one or the other. It's a zero sum cycle. In & Out.

Take a box fan and completely cover one side...it stops moving air regarless of which side you cover

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Seems like a lot of people are still thinking that the airfoil itself may have something to do with the push pull argument. Name an airfoil application (Wing, Rotor, Prop); none of them are being pulled into the low-pressure air. It is the more highly pressurized air seeking a low-pressure area that pushes an airfoil in the direction of the less pressurized air. You might be able to prove this to your self by considering a perfect vacuum in a container. All gas molecules have been evacuated from this space. You have atmospheric pressure outside the container and it can press on it for a grand total of ~14.7 PSI. Even given an infinitely powerful vacuum pump you will never be able to increase the pressure being exerted on the container beyond this difference in pressures (0 and Atmospheric, and it doesn't go lower than 0 :) ) If you were to pump a gas into the container you could easily exceed atmospheric pressure 1000 times. If a Vacuum were sucking/pulling in on the container you should be able to exert whatever pressure you wanted on it by simply pulling more vacuum. It is of course not the vacuum on the inside pulling on the container but the pressure outside pushing.

 

Or in other words: Gases and liquids can't pull. (Okay - air may as well not be able to pull and liquids can but this is a function of molecular interactions and for most any engineering project its fair to say that Air/Water can't pull.)

 

As far as the argument goes - the only question is: what direction is the force in and what side of the boom is it mounted on. Imagine the rotor was attached to the boom with a piece of rope, when the rotor is operating would the rope go slack (Push) or would it become taught (Pull).

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For the SH-60B, the manuals say it is a "tractor type" tail rotor. I guess that means it pulls since it is on the right side of the tail. That's my simple answer. If the tail rotor applies a force away from the tail, it is pushing. If it applies a force toward the tail, it is pulling. As somone already mentioned, it is like a shopping cart. You are either pushing or pulling depending on where and in what direction you are applying the force.

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