Assume you are in a conventional helicopter (one main rotor with an anti torque tail rotor) in unaccelerated forward flight(ie. level, constant airspeed, zero turn rate, and in trim) with zero wind.

Does the helicopter fly straight through the air or at an angle(ie. with a crab angle)?  
If it's with a crab angle is it to the left or right?  
Explain your answer.

If in trim, the longitudinal axis will be parallel to the direction of flight. Pom-pom aligned with the windshield post.

Incorrect.

This sounds like a transverse-flow effect. You would need to apply left stick to counter it. So I would say the craft would be going to the right. Assuming American Rotors.

At speeds above translational lift, the pedals are used to compensate for torque to put the helicopter in longitudinal trim so that coordinated fight can be maintained. The cyclic control is used to change heading by making a turn to the desired direction.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch03.pdf

Please explain your definition of "Trim". What is your frame of reference? Do you fly, maintain, build helicopters?

The amont of pedal input required to attain trimmed flight varies with aircraft load, airspeed, angle of vertical stabilizer (if a stabilizer is used).

Correct.  However, level trimmed flight does not necessarily mean that an aircraft is going straight through the air.

Trimmed flight means Unaccelerated flight.  Ie. Net forces acting on the aircraft are zero.  Lift equals weight, drag equals thrust, and left/right side forces are equal.  Frame of reference is the inertial reference frame.  What I do with helicopters does not have any impact on the answer!

Tail rotor thrust is the cause of a helicopter not flying straight when in trimmed flight....  you're very close!  
Hint: how do you counteract tail rotor thrust in trimmed level flight?  Answer that and you're the winner!

At some significant peril to my ego and reputation, or what's left of them, I'll play

You've also got forces from the vertical stabilizer, any horizontal stabilizers, and mast tilt. However, now that you've given the big hint, to get you the answer you want I'm going to say there is just a little bit of left cyclic in there, still. Nevertheless, does that just have the ship in a slight left roll, or does it also cause a slight left yaw, or both?

It's not the answer I want, it's the correct answer!

in order to be steady-state the torque from the main rotor Must be cancelled out.  This is done by the tail rotor(and vertical fin/pylon) by producing thrust directed sideways.  For trimmed flight this sideways thrust must also be cancelled out.  You can't cancel the tail rotor thrust by tilting the main rotor because the aircraft would then be banked (and thus turning and not steady state).

In forward flight the tail rotor thrust is opposed by the fuselage not flying straight through the air.  Ie. There is a small angle between the axis of the helicopter and the track of the helicopter.  The airflow at this small angle creates sideways force on the fuselage that opposes the tail rotor, allowing the helicopter fuselage to be generally level in trimmed flight.  The pilot doesn't know or think about this in flight, he simply moves the controls as needed to fly a straight line with the aircraft in trim, the result being a small side slip that few, if any, pilots ever notice.

In real life the fuselage is not completely level - things such as the tail rotor position(high on the tail or low on the tail) and type of rotor head(the rotor disc on an articulated head can input a moment onto the rotor mast and impact the fuselage attitude) make a difference.  Gross weight also greatly impacts the fuselage attitude in cruise flight(higher weight means more power which requires more tail rotor thrust, so more sideslip for trimmed flight).

A typical "inherent side slip" angle is 2-3 degrees.  Large helicopters at high gross weights may operate at higher sideslips.

If you look at helicopters and pay attention to where air sensors are(pitot/static ports, wind sensors, ice detectors, etc) located, you will see that they are commonly on the right side of US helicopters(CCW fromabove, right side advancing, etc) and on the left side of copters with backwards rotor systems.  They are placed on the side that leads into the wind in cruise flight to get clean airflow.

I guess the point of this question was to point out that helicopters are heavily asymmetric and do not fly straight through the air.

It does because the terms used in the RC world don't necessarily cross into the "Full Scale" world.
So, my query, do you have experience in the helicopter industry?

Typical trim indicator found on small and medium size helicopters.

Attached File

You won't be able to notice a couple degrees on a piece of string moving in the wind, but if you were to fly a helicopter in forward flight such that a string is perfectly aligned with the axis of the aircraft then the aircraft would be slightly out of trim.

A string on a helicopter would be more accurately called a "airflow aligned with the aircraft" indicator.  But new helicopter pilots are frequently dozens of degrees out of trim, so a string approximation of trim is adequate for the intended purpose.

What about Chinooks?

Dozens of degrees? How many dozens?

Common sport, when are you going to tell us about "your" helicopter experiences? How about a picture of what you fly?
Surely you have a couple "No shit, there I was" stories.

Tandem and coaxial helicopters have their own quirks, but none as interesting as tail rotors.  
Tandem and coaxial are also almost exclusively military aircraft and much less familiar to civilian pilots.

I'm just really bored working in a shithole country, figured I'd help everyone learn something!

What about this?
This monster is anything but boring.

Attached File


K-Max. Note the trim tabs on the rotor blades. Look into the control rigging for it. The original Kaman had a radial engine.

Unfortunately I have not been able to fly a Kaman yet - I assume they have semi rigid rotor hubs(ie. teetering)?  
The flight controls are not unique - other aircraft have had flap controlled rotors but Kaman is the most well known of them.  
A bell-hiller mixer is similar - the flight controls operate the paddles, and the aero forces from the paddles pitch the main rotor blades.  Commonly described as "flying the paddles".

Aero-wise the only downside I see is that the rotors are not horizontal - a portion of power is wasted on the horizontal component of thrust each rotor produces.  
There are many advantages that very likely more than make up for this tiny fraction of wasted power though, most significantly:
The narrow fuselage has a very small shadowed area (the horizontal silhouette of the fuselage that blocks rotor downwash)
The rotor system has all the aero advantages of a coaxial rotor (except for the wasted horizontal thrust)

It's designed to lift, and it is a great design for that.  
I think a coaxial would be easier to design from scratch, but the Kaman design has been around a long time and is established.
The USAF had a Kaman in the 60s-70s timeframe, I believe it was only used for CSAR for a short time period?

Then how do you explain a centered yaw string? And yes, I can see a couple of degrees.

It flies straight ahead because a slight, almost imperceptible, tilt of the rotor to cancel out the tail rotor thrust.

I was wrong, and Morgan321 had it right all along!

Except for the ones used in the logging industry and as air taxis in the NY/NJ metro area.

No fucking clue! I have the autopilot on, I’m reading the paper and jamming some tunes through my Bluetooth DC headsets.

 Ohhh this is a rotorhead conversation!

Since it appears you don't believe anything I say and you continue to drag my quizzes off topic:
I'm not sure what the point of the quote above is but since you seem to like the faa chopper handbook,  if you had read chapter two you would've found the below tidbit:
"In forward  ight, the tail rotor continues to push to the right, and the helicopter makes a small angle with the wind when the rotors are level and the slip ball is in the middle. This is called inherent sideslip. ".

Additionally, you can read about inherent sideslip in any textbook covering helicopter aerodynamics, design, testing, etc.  R&W even printed an article about it.

Stop being intellectually lazy and at least make an attempt to learn things.  If you don't want to then at least stop making noise that distracts from others being able to learn something.

I think the question on a lotta folks' mind is what makes you an expert on all things rotorcraft?

I enjoy learning about all things aviation- and even have a bit of rotor, glider and balloon time- but I also like to know that the person I'm learning from is an expert in the field- or at least a highly skilled practitioner of it.

Actually that would be several pages UP as it is found in Chapter 2, Page 2-14 of the FAA Helicopter Flying Handbook.

And after reading I stand corrected and am willing to humbly concede

I never claimed to be an expert on anything.  I could be a homeless bum under a bridge or the smartest guy in the world - neither would make my answer more or less correct.  You can learn from anybody - a simple internet search for "inherent sideslip" would've given you all the information there is and confirmed what I said.

I can easily prove everything I say is true but that's not why I'm here.  I'm trying to help inquisitive people learn something on their own, I don't even spoon feed answers to my kids!  Anybody can be told something and regurgitate it, really smart people can figure something out on their own.

I'm doing an experiment here, play along and try to teach yourself something new!

Thanks, but I'm out.

Rotor head jargon is some weird shit.

Tailrotors are for the weak.

I speak Chinook.

IMG_0003-Log Turn by FredMan, on Flickr

IMG_0007-Liftoff by FredMan, on Flickr