Helicopter torque reaction?

[cbennett]

Im sorry in advanced if this has been previously been covered, but iv searched the net and this forum and didn't find what i am looking for. At work im currently studying the helicopter theory of flight and we've come onto the topic of torque reaction. Now i know that it involves Newton's law stating that everything has to have an equal and opposite reaction opposing it, but what i do not understand is how the force is physically created in the opposite direction to the spinning rotor blades. At first it seemed logical for the body of the aircraft to want to spin in the same direction to the rotor, not against it but taking Newtons law into consideration it makes sense to spin in the opposite direction to the rotor blade. Im imagining that it must be the force of the fixed main rotor gear box acting on the mechanism to turn the the blades? Im not to sure however. Any input helping shed light on the matter would be much appreciated.

chris

[[email protected]]

Get one person to stand on a childs roundabout and another to try to push him around whilst standing on a skate board. Equal and opposite reaction will mean that the roundabout moves one way and the chap on the skateboard moves the other.

The roundabout represents the gears of the MRGearbox attached to the main rotor head and the guy on the skate board is the engine bolted to the fuselagetrying to drive the gearbox gears. The engine can't move but is attached to the fuselage that can and so it spins in the opposite direction to the rotor.

A simplistic explanation but I hope it helps.

[Um... lifting...]

Action-reaction
The relative rates of spin (in relation to the earth) will be inversely proportional to their respective moments of inertia (the rotor system and its turning bits) and the fuselage. But that's in theory... in reality, the relative rates will differ from those theoretical values due to aerodynamic forces.

crab-
I'm sure that works, but I think I'd prefer to watch you do it rather than try it myself.

[jeepys]

If I smacked you in the face then you would return by smacking me in the face but in the opposite direction to which I whacked you.

Does it get more complicated than this?

If so then I better tell my instructor.

[wallsend]

If the the last defender before the goalie stands with his back to the wind in the northern hemisphere, then the torque reaction acts to the right. Unless it's a Dauphin then he'll be out leg before wicket. Or is it something to do with ice skaters' legs? :confused:

[AnFI]

Um .... Lifting

Physics check ?

Rates dependant on inertia? (accn perhaps ... but rate) are you sure?:=

[rotordude]

Chris, I´ll try to keep this simple with a more practical explanation.

First of all, torque is produced by the engine. Torque becomes apparent when the engine is trying to overcome the resistance force(s) acting on the "output end". In general - The higher the resistance, the greater the torque.

Lets use a powerful cordless drill as an example. If you drill into a hard material like concrete or steel, you might have to hold the cordless drill with both hands in order to prevent it from rotating in the opposite direction (in relation to the drill bit). But drilling into styrofoam or wood can easily be controlled by using only one hand.

When the rotorblades on a helicopter are turning, they experience a resistance force called drag. The drag changes for a number of reasons (angle of attack, pitch angle, density of the air, etc.). More drag - the engine has to work harder in order to keep the RPM constant - and a higher torque is the result.

Hope this helps!

rotordude

[Um... lifting...]

AnFI-
Pretty sure... though the experiment would only work with no atmosphere and/or a frictionless surface, and then it wouldn't work anyway. Acceleration, definitely. Mass is not a geometric in either kinetic energy or acceleration equations, so I believe that your rotational speeds would indeed be and remain opposite in direction but different in angular velocity to a fixed reference (whatever the hell that is, as one physics prof used to say), assuming that the rotor system and the fuselage have differing moments of inertia about the mast (which they ought to). It only makes sense, really... if you consider changing the arm on one of your two rotating masses during steady state (and thus the moment of inertia), it won't be steady state anymore, it'll accelerate one way or t'other.
I can't actually get worked up enough to blow the dust off a textbook (if I even have one that would answer that) however, so I have no intention of doing so... I'm not real sure it's critical we know for the purposes of the original poster's question, anyway.
Once had these completely excellent toys (well, they were toys) known as 'Wizzzer's (I bet they're still hiding in Mom's basement, I think between little brother and self we may have had as many as four... though one suspects at least one was a casualty of some gravitational experiments). They were fine tops with a high rotational moment of inertia and a rubbery tip to impart all sorts of spinning moment by rolling it obliquely on a surface (Mom's walnut dining table... bad choice... basement floor with hard composition tiles... good choice, as I recall...) all I know about rotational energy comes courtesy of the Mattel Toy Company and hours on the hard tile flooring... that university level physics, engineering school and military flight training wasn't worth a tinker's d*mn by comparison.
Apparently they are no longer available except as technology inside of other toys such as toy automobiles. A crying d*mn shame. I'd go right out and buy four more.
http://www.inthe80s.com/toys/images/user-image-1228515931.jpg

[jimjim1]

[In the time I have been writing this there have been a number of new posts, seems like the hour for the DIYers:)]

The critical thing here is to notice what the engine does.
The engine applies a force *between* the rotor blades and
the helicopter body. The engine tries to turn the blades in
one direction and tries to turn the body in the opposite
direction. Without something to resist the body turning,
it will do so. Generally this is not what is wanted:-)

Note that this is no different from any other rotating
machine. Your washing machine tries to turn the drum in
one direction and the earth in the other direction. Because
the earth is so large you do not notice the effect on it
*however* the effect is still there. The difference with the
helicopter is that it is not attached to the earth and the
effect on the body of the engine's efforts are significant.


The best 'normal life' example I can think of is
an electric drill. Like a helicopter it is a box
with an engine and an output shaft to which a rotational
force is applied. It works by creating a force *between*
the body and the output shaft.

In this analogy the place of the rotor blades is taken by
the drill bit.

The key thing is once again to notice that there are two
force outputs. Equal and opposite. One tries to turn the
drill bit in one direction and the other tries to turn
the body in the opposite direction.

What the drill motor does is to apply a torque
*between* the drill chuck and the drill body.
There is *no* favouritism. The torque applied
to the drill bit is *exactly* opposed by the
opposite torque applied to the drill body.
If you were to imagine restraining the drill bit
in a vise and turning on the motor the body of the
drill would rotate in the opposite direction
to the normal direction of rotation of the
bit. In normal use the operator must resist
the movement of the drill body to allow
the torque to be transmitted to the drill bit.

If you are studying this seriously you might be able to
get a drill and try it out for yourself. Don't blame me
if you lose any limbs, eyes or other parts of your body.
Power tools are dangerous. Get someone familiar with
them to make sure that you don't hurt yourself.

[Um... lifting...]

jimjim1-
I used precisely this argument to justify in my own mind the purchase of a hammer drill. It worked!:ok:

[Farmer 1]

I like the analogy with the electric drill. When you switch on the drill, it kicks in an anticlockwise direction. Perfect example of Newton's third law of motion.

Trouble is, if you put the drill in reverse, when you switch it on, it still kicks in an anticlockwise direction.

[ShyTorque]

Trouble is, if you put the drill in reverse, when you switch it on, it still kicks in an anticlockwise direction.

That's because the electric motor (which produces the torque) still turns in the same direction and the "kick" comes from that.

[Farmer 1]

Yes, I know, but I just thought I'd mention it.

[cbennett]

Thanks for all of the reply's, i definitely understand the topic more now its been put into other situations like the drill for example. It all seems clear now :) .

Many thanks

[jimjim1]

Quote:
Trouble is, if you put the drill in reverse, when you switch it on, it still kicks in an anticlockwise direction.
That's because the electric motor (which produces the torque) still turns in the same direction and the "kick" comes from that.

I have written this not to argue with the quoted posters but to address the original poster's question.

Thing is that the "kick" you are describing is NOT so strongly related to the helicopter situation since it is due to the changing speed as the motor starts. In other words it is an effect of acceleration and of rotational *inertia*. If you put the drill in reverse and actually *drill* as described, then as expected the effects are indeed reversed. I am not a helicopter pilot but I understand that the helicopter rotors essentially run at constant speed in flight (well the one I flew for 30 mins did - R22) and that inertial effects are negligible (and certainly small compared to the aerodynamic torque forces) as far as the original posters question is concerned.

Of course to drill in reverse you will need a left handed drill bit.


I carefully tried to avoid any inertial arguments in my earlier post so as to present the simplest possible case for the benefit of the questionner.:)




Just to mention again, messing about with modern power tools can be damaging to your health.

[ShyTorque]

Just to mention again, messing about with modern power tools can be damaging to your health.

And helicopters, too.

There's a fundamental difference between an electric motor being started (in a drill or elsewhere) then allowed to run "free".

A helicopter's rotor blades are constantly being pushed against the considerable resistance of the air. The electric drill being used to actually drill a hole in a material, which will cause a resistance to the drill bit therefore makes a better comparison.

[Farmer 1]

jimjim1,

I could not disagree with you less.

[ShyTorque]

Just to mention again, messing about with modern power tools can be damaging to your health.

And helicopters, too.

There's a fundamental difference between an electric motor being started (in a drill or elsewhere) then allowed to run "free".

A helicopter's rotor blades are constantly being pushed against the considerable resistance of the air.

The electric drill being used to actually drill a hole in a material, which will cause a resistance to the drill bit, therefore makes a better comparison.

[blackhand]

As a pedant I would like to point out that the counter rotation of the helicopter is "evidence" that the equal and opposite force exists. It is not the force its self.

This force is not related to the plane of rotation of the engine - think S58 and Hughes 500 where the engine is mounted at an angle to the horizontal.

Tech teachers use the reaction of a fire hose as a demonstration of the force.

Cheers
Blackhand

[jimjim1]

The force is obvious!

As a pedant I would like to point out that the counter rotation of the helicopter is "evidence" that the equal and opposite force exists. It is not the force its self.


I don't see the need for this "evidence".

If you start at the beginning, and consider the function of the engine, then it is clear that the engine produces a force *between*, let me repeat that, *between*, the rotors and the rest of the machine.

No evidence is required. It is the case by definition. Forces *always* have to be between two thingies. No thingies, no force. Thingies may be resistancies (e.g rotor blades, propellors, brakes), or [inertial] masses or some combination of the two. Note - inertial added for emphasis, 'cos there is no other kind of mass. Well not that I know of:-)

There is of course a case whereby a force can be generated by expelling a reaction mass, as in a rocket, from the rotor tip. No current helicopters, however, operate in this way. (Check out Fairey Rotodyne for a reaction helicopter, note the lack of tail rotor, or for the pedants:) other opposing force generators.)

I am sorry if I appear unduly pedantic, however this is fundamental stuff and I have the idea that I have a good understanding of it. Sadly I can't tell you how to develop such an understanding other than to keep asking questions.

PS. I am making all this up as I go along, but I think that I am doing OK. Let me know if you disagree.

[blackhand]

I don't see the need for this "evidence".


jimjim1

Force as in F= ma

Although you may prefer einstein physics were it is all relative, and in fact mass equals energy which not only confused catholics but made fat people happy.

But getting back to Mr. Newton.

The force exists because it exists. Much like gravity and dare I add stupidity. Stupidity being the equal and opposite of intelligence.

Cheers
Blackhand