Tandem rotor aerodynamic nuances

[cw4ogden]

@NineLine

Apologize for the ping, and my longwinded nature, but as opposed to making a bunch of bug reports I would like to combine several topics into a more wish-list like thread.  Things it would be awesome if the DCS FM happened to mimic.  A to be passed along to whomever kind of thing.



Rotary wing aerodynamics, being akin to voodoo is a not well understood science.  Tandem rotor aerodynamics is arguably much less understood, often even by those who've learned to harness it.  

This is meant as a collection of little understood phenomenon attributable to the tandem rotor configuration.  Not an insult to anyone's intelligence, or assertion that these are the 100% factually accurate causes.  These were the best explanations why the CH-47 did what it did.  It would be nice if the flight model had some of the quirks the real aircraft does, and for that to be possible a thorough understanding of how tandem rotor differs from a conventional helicopter is necessary.

Most if not all of these happen AFCS on or off, the only difference being who is counting the phenomenon, the pilot, the computer, or a blend of both?  I can speak intelligently on the underlying aerodynamics, but your contemporary SMEs will have to speak on how the digital AFCS handles them from a pilot's perspective.

The discussions will focus on AFCS off flight, and leave how the DAFCS handles, to your more qualified individuals.  I hope only to fill in any gaps in institutional knowledge.

 

Difference in angular tilt of the fore and aft Rotor heads.

As mentioned in my other thread, the difference between the angular tilt of the fore and aft head causes any movement of the thrust to manifest in an immediate and linear pitch change.  This linear change in pitch can be exasperated, as can any transient in a negatively stable system.  Said another way, while the cause of this phenomenon is linear, if not reacted too quickly it can exasperate itself.  

One of the more noticeable regimes of flight this rears it's head is the roll on landing.  AFCS off roll on landings often require the pilot to need forward cyclic during the flare portion when thrust is reduced.  This is counter intuitive to most pilot's muscle memory, and is the primary hurdle to pulling off a decent AFCS off roll on.

Another is doing precision hovering with significant changes in altitude, i.e. taking slack during a sling load or any vertical climb / descent such as masking.  The chinook wants to drift forward when you pull thrust, and drift aft when you reduce thrust.  It just does. The extent to which the newer AFCS compensates is what I can't speak to.

Vortex Ring State / Mushing

The CH-47 is one of a handful of aircraft that lightly loaded has enough power to pull itself out of VRS, theoretically.  It will get into VRS, but it is not particularly prone to it like one might think.  The dual rotor system tends to give the pilot a little more warning than in a single rotor helicopter, as the induced flow moves progressively from back to front, it engulfs the rear rotor first (assuming decelerating flight).   The preferred method of recovery is lateral cyclic, as forward cyclic can aggravate the problem due to differential collective pitch increasing angle of incidence of the aft rotor system.

In 15 years of flying chinooks I never encountered VRS, unless I was showing it to a student.  Nor do I know of a single VRS accident in the community.

A much less well known pitfall is the phenomenon akin to mushing where the AFT rotor head can achieve a Vortex Ring state during rapid decelerations.  This manifests in the an abrupt nose up and transition to a sinking attitude of flight which if not recovered from quickly, for will result in a hard landing at the least.  Of these, there have been a handful or more.  This is the exact same phenomenon that is responsible for V-22 osprey crash(es).  It happens to them in tight descending turns due to their horizontal tandem configuration.


Transition to forward flight / Transverse flow effect

When picturing how a Chinook reacts to transverse flow effect, it's useful to use a simplified understanding of fore and aft head, acting in unison and ignore the effects on each individual rotor system.  Looking at it as an oversimplified combined rotor system, Transverse flow effect manifest to the pilot as a slight nose up while transitioning forward through ETL.  The AFT rotor system ingests "dirty" air first resulting in a few degrees of pitch up, resulting from the tail wagging the dog, so to speak.  

The individual rotor systems are affected, but due to their counter rotation nature, the force created is a twisting, torsion force absorbed by the fuselage.  The pilot feels the back end sag accelerating through ETL, and the reverse when slowing to a hover.

Retreating Blade Stall / Blade compressibility 

The forward speed of a CH-47 is limited by one of two phenomenon, retreating blade stall, or the advancing blade approaching supersonic flight regimes, referred to for physics reasons as blade compressibility.  If you again look at the CH-47s two rotor systems as a single system you will be able to predict the results.  Retreating blade stall tends to be somewhat self correcting as it induces the aft head to lose lift first.  Blade compressibility tends to self aggravate for the opposite reason.  Either may or may not be accompanied by rolling transients.

 

Dissymmetry of lift / LCTs

Due to the tandem rotor having no inherent ability to cyclic feather, a system is needed to compensate for dissymmetry of lift.  This system is of course the longitudinal cyclic trim system.  Up until a certain airspeed, cyclic feathering is unnecessary.  The system can compensate via blade flapping alone.  But the direct result of blade flapping is rotor blowback, and beyond a certain point, you begin to put excess strain on the rotor heads and may encounter droop stop pounding in flight, or worse.

The other often overlooked role of the LCTs is the level the 47's fuselage at a hover.  This was a necessary change with the addition of the triple hook cargo system.  The fuselage needed to be level for ground crews to reach fore and aft hooks.  Prior iterations of the 47 hovered significantly nose higher, (I'm told).

Weathervane effect a.k.a. the CH-47 Rodeo

While I never experienced this personally, a CH-47 AFCS off will do a 180 degree turn, in cruise flight if you let the yaw get away from you.  This is just a simple combination of whether vane effect and the subsequent induced yaw inertia.  If you let the nose get between what I'd estimate is somewhere between 20 to 30 degrees, you will be going 100 knots in reverse before you know it. 


*** For anyone active in the CH-47 community - important *** 

Beyond the scope of a DCS discussion but for any active 47 people this is a phenomenon you need to be aware of.  Any excess rearward flight, or significant tailwinds at a hover can trip your DECU temp mismatch logic causing you to suddenly be in dual reversionary FADEC mode. 

While this may sound fairly benign, it is a nasty little demon of the 714 engines that can kill you.  If you haven't heard ever been made aware of this, you really need to know about and understand it.  As the reversionary can't sense rate of thrust movement, it is designed to over/under program fuel, to mitigate rotor droop, or overspeeds during thrust changes, while operating in reversionary mode. 

If this happens, to you, you will get a master caution and dual FADEC failure indications, caused by the temp probe in the rear of the engine disagreeing with the one further inside.  If you happen to be making a power change, this can very easily look like a dual engine runaway, or a dual engine power loss depending on which way the thrust was being moved. 

It can trick the pilot / crew into thinking they have an engine high side, and make the problem worse by pulling more power.  Or if encountered when reducing thrust, be tricked into thinking both engines suddenly rolled to idle and auto rotate with two perfectly good engines.

This phenomenon, has Induced at least one unnecessary autorotation, a crew who had this happen and believed both engines had rolled to idle.   And I had the reverse, the high side experience once, and thought my engines were going to spin the blades completely off as the overspeed fuel control held the rotor RPM at 114, overriding the FADEC hell bent on killing us all. 



I'll leave it at this for now.  If this is well received I can probably drudge up more.


A pilot can often operate fine at an application level of knowledge.  Meaning, if I know the nose pitches up when X happens, I don't necessarily need to know why.  But with understanding comes the possibility to achieve the next step in learning correlation, or spontaneous learning.  When you learn something new, on your own, because of the various things you took the time to "understand".  It's when you have that epiphany moment and it all makes sense.  A pilot who rests firmly in camp understanding, or even better to correlation, can anticipate.  A pilot who only makes it to the application level, can only react.

Edited August 12, 2024 by cw4ogden

[Vee.A]

Some things I've noticed is a lack of buffeting when crossing translational lift and during VRS. Also a lack of the distinctive blade flap sound when maneuvering, which most other DCS helicopters have. As someone who knows what you're talking about, what would be realistic?

[cw4ogden]

@Vee.A

Best I recall you largely don't hear it up front barring a maneuver like a 60 degree sustained turn.  And even then it's going to be a slight change in noise up front.  My opinion of the sound is it's actually really good.  I'll give a listen, but barring the APU being a little quiet, currently no avionics cooling fan, and no power transfer sounds, what's in place sound wise sounds really good to me.  It sounds like being in a chinook

Edited August 13, 2024 by cw4ogden

[McRuffen]

As the Chinook uses differential collective for pitch changes, do you experience a yaw tendency when pitching due to differential torque as well? So moving the stick forward to pitch down, the torque is reduced in the front rotor and increased in the rear rotor, causing a left yaw tendency and vice versa?

How noticeable is this pitch -> yaw coupling (if any), and is it compensated for by manually by the pilot, by mechanically adding a slight sideways cyclic (ie. yaw) input to longitudinal stick inputs, or by the AFCS?

[47_Driver]

@cw4ogden I will tell you right now as an operational CH-47F pilot, your depth of knowledge on these phenomena is greater than about 95% of the Chinook community these days. This was an incredible read and I greatly appreciate it. Will be mentioned at our next WO meeting at work. Dual FADEC PRI failures are one of the scariest things we can experience in this aircraft in my opinion. I've been exposed to it one time, thankfully at a hover (when I probably shouldn't have been). Can't imagine some of the things you have seen in this bird. Really appreciate you sharing your knowledge with us.

 

@McRuffen no yaw tendency that I'm aware of even AFCS off, but this could be because the yaw is generally just all over the place when the AFCS off because of the Chinook's inherent lack of directional stability (it's just a giant school bus, it's not really all too aerodynamic). As OP mentioned above, the yaw can completely spin the helicopter around VERY quickly when the AFCS is off if you let it get away from you. When the AFCS is off you will notice large pitch transients as power is increased or decreased, due to the difference in tilt on the Fore and Aft head. Increase power and the nose drops, decrease power and the nose climbs.

[cw4ogden]

@Brickle Thank you for the kind words.  If you were involved in the testing for DCS I'm pretty impressed with the product so far.  

Edited August 15, 2024 by cw4ogden

[cw4ogden]

11 hours ago, McRuffen said:

As the Chinook uses differential collective for pitch changes, do you experience a yaw tendency when pitching due to differential torque as well? So moving the stick forward to pitch down, the torque is reduced in the front rotor and increased in the rear rotor, causing a left yaw tendency and vice versa?

How noticeable is this pitch -> yaw coupling (if any), and is it compensated for by manually by the pilot, by mechanically adding a slight sideways cyclic (ie. yaw) input to longitudinal stick inputs, or by the AFCS?

Most rotor related phenomenon that produce yaw tend to get cancelled out because of the opposing rotation of the two rotors.  If one head wants to yaw left, the other wants to yaw right and the forces work to "bend" the helicopter, which thankfully bends very little.

[cw4ogden]

12 hours ago, McRuffen said:

As the Chinook uses differential collective for pitch changes, do you experience a yaw tendency when pitching due to differential torque as well? So moving the stick forward to pitch down, the torque is reduced in the front rotor and increased in the rear rotor, causing a left yaw tendency and vice versa?

How noticeable is this pitch -> yaw coupling (if any), and is it compensated for by manually by the pilot, by mechanically adding a slight sideways cyclic (ie. yaw) input to longitudinal stick inputs, or by the AFCS?

Reading deeper into your question, I see my answer falls short of answering your example.

In pitching forward, I.e. Reducing pitch / torque on the forward head, and increasing it on the aft head, it does seem like it should induce a yaw.   The effects in your example would be additive, and not cancel each other out was my mistake.
 

But as Brickle said, it doesn’t manifest to the pilot.  I can hypothesize why it might work that way, but the fact is there’s little to no yaw / pitch coupling.   And at the moment I don’t have a good explanation why.  
 

My guess would be the forces are there, but where a single rotor helo is the pivot point and is only twisting a small moment, I.e. the tail rotor section, in your example the aft rotor is trying to turn a huge moment, the fuselage and entire front rotor head.  So it becomes un-noticeable.  Best guess. 

Edited August 15, 2024 by cw4ogden

[Yurgon]

On 8/12/2024 at 6:10 PM, cw4ogden said:

This is meant as a collection of little understood phenomenon attributable to the tandem rotor configuration.

Awesome read, thanks for sharing that with us!

If you have more to share, I'm sure we'll all be soaking it up like a sponge.

[McRuffen]

On 8/15/2024 at 8:48 PM, cw4ogden said:

But as Brickle said, it doesn’t manifest to the pilot.  I can hypothesize why it might work that way, but the fact is there’s little to no yaw / pitch coupling.   And at the moment I don’t have a good explanation why. 

Thanks for the insight! Having experience flying FPV quadcopters, I know that creating a yaw moment due to differential thrust (eg. torque) takes a lot more thrust differential than creating pitch/roll - you can hear the motors changing their tune much more pronounced when yawing.

I've also noticed with the Chinook in DCS that with the rotors stationary, changing thrust lever (collective on both rotors) causes much more movement in the blades than pitch forward/aft (differential collective). So the fact that even a fast pitch acceleration requres a relatively small change in both rotors' collective (and therefore a small-ish amount of torque differential produced) and the fact that this torque differential would have the longest axis and therefore the largest moment of inertia to work on, makes the effect small. Maybe the different angles of the rotor discs mean that you get gyro effects somewhat resisting the yaw as well, as you can never yaw in the exact rotational plane of both rotors. Using pedals and differential lateral cyclic on the two rotors produce a much more powerful force for yaw control, so maybe the differential torque from pitch changes is there but negligible compared to the other forces at work.

Just pure speculation here, but I find things like this very interesting.

Edited August 17, 2024 by McRuffen