Helicopter Pendulum effect

[Shalomar]

This thread mainly concerns the Huey but I could not find a forum for it.

 

The Huey seems to model pendulum effect halfway. That is, once displaced from a hover it models the effect of wanting to stay displaced. However, once settled into a stable hover it should also tend to want to stay in a stable hover.

 

My point of reference is the Dodosim 206 for FSX. It was the first addon for FSX to have an externally generated flight model which overcame almost all FSX limitations. FSX itself does not model pendulum effect or vector ring state at *all*, but the Dodosim Bell 206 *Does* both very well.

 

The Bell 206 is a lighter heli with a lightweight rotor system, and it has much more tendency to want to stay hovering once established. The Huey with its heavy blade system should be even more stable in a hover **Once established**.

 

Any opinions???

 

Donald

[msalama]

Any opinions???

 

None, except noticing that yours is an opinion too, meaning the Dodosim FM should be proven 100% correct first before making any comparisons.

[Anatoli-Kagari9]

Thanks for your post Shalomar - this is the kind of things that really interest me in ab flight simulator - flight dynamics modeling :-)

 

I don't think I fully understood what you're saying, but I'lltry to think about it latter and post back.

 

I had been away from DCS for a while, and even gave away all of my modules, but bought back the uh-1h :-)

[BitMaster]

A Helicopter is an "inherit instable" device, technically seen referring to its stableness, or lack there of.

 

Whatever the pendulum effect does, it is way too little to overcome the main characteristica of helicopters, that is being non-predictable where it will drift off to ( inherit ).

 

The pendulum is also constantly being "disturbed", I am not aware of stable helicopters by default, for a second yes, but the second there after is unknown.

 

Bit

 

Goes for all helis without exception

[Flagrum]

A Helicopter is an "inherit instable" device, technically seen referring to its stableness, or lack there of.

 

Whatever the pendulum effect does, it is way too little to overcome the main characteristica of helicopters, that is being non-predictable where it will drift off to ( inherit ).

 

The pendulum is also constantly being "disturbed", I am not aware of stable helicopters by default, for a second yes, but the second there after is unknown.

 

Bit

 

Goes for all helis without exception

About "constantly being disturbed" ... I have a question here. My first thought was, when I read OP's question, a flying helo does not resemble a pendulum as there is no free swinging part (I assume, the idea here is, the airframe swings under the rotor, right?) due to the fact that constantly forces are exchanged between rotor and the airframe by the swash plate.

 

Is that correct ... and is that what you mean with "disturbing"?

[msalama]

Flagrum, what he probably meant was just that all non-autostabilized helicopters are inherently unstable, and thus can't hover without constant control input. Or in other words, that autostab system is there for a reason ;)

[Shalomar]

The pendulum effect... once a helicopter has all that mass level under its rotor disk it tends to want to keep its fuselage under the rotor disk and takes work to get out and then back in. The DCS Huey seems more like it only models the difficulty of getting into a hover and no pendulum effect once established in the hover.

 

I guess you never heard of dodosim but if you go to http://www.hovercontrol.com you will find their forum. They do extensive work in commercial flight sim and their flight model for the Bell 206 has been widely regarded as the gold standard in accuracy since it was released. It has been used extensively by real world student pilots and yes, it is accurate...

 

Donald

[Flagrum]

Flagrum, what he probably meant was just that all non-autostabilized helicopters are inherently unstable, and thus can't hover without constant control input. Or in other words, that autostab system is there for a reason

Yes, I understood that. My question was targetted at what the reason for that instability is, even in hover.

 

The pendulum effect... once a helicopter has all that mass level under its rotor disk it tends to want to keep its fuselage under the rotor disk and takes work to get out and then back in. The DCS Huey seems more like it only models the difficulty of getting into a hover and no pendulum effect once established in the hover.

 

I guess you never heard of dodosim but if you go to www.hovercontrol.com you will find their forum. They do extensive work in commercial flight sim and their flight model for the Bell 206 has been widely regarded as the gold standard in accuracy since it was released. It has been used extensively by real world student pilots and yes, it is accurate...

 

Donald

I admit, I still have to wrap my head around this idea ...

 

Atm I am struggeling to understand why anything in a helo should be stable when in hover. Even if the helo just hangs under the rotor - for the whole system to be stable, it would require that the rotor itself is stable in the first place. But why should it? If the thrust vector is not exactly pointing opposite to the gravitiy force, i.e. not 100.00 % above the COG, the whole thing will move. What would prevent it from doing that?

 

But I will check your link out, thanks.

[msalama]

and yes, it is accurate...

 

That may well be, but all you've said so far is still too generic and/or vague to be accepted as proof. And the UH-1H is a different beast anyway, isn't it?

 

Now it's entirely possible that you're right, so I've no beef with your doubts per se. It's just that hunches are hunches, not evidence.

 

Oh and BTW, there're some RL Huey pilots on this board - and they have, at least to my knowledge, all said that our Huey is very accurate ;) But by all means dig deeper into this if you want, and please present your findings here as well if you bump into something tangibly fishy!

 

PS. And maybe the Dodosim 206 flies on rails, OTOH? Heard that accusation about FSxxx models many a time.

Edited December 28, 2014 by msalama

[msalama]

the rotor itself is stable in the first place

 

Yes. The individual blades should be exact copies for this to happen, having precisely the same mass, form and dimensions - and exact copies they are not. Now, throw in weather phenomena such as windage, turbulence, etc. and what do you get? A non-autohovering-by-itself copter most likely...

[Flagrum]

Yes. The individual blades should be exact copies for this to happen, having precisely the same mass, form and dimensions - and exact copies they are not. Now, throw in weather phenomena such as windage, turbulence, etc. and what do you get? A non-autohovering-by-itself copter most likely...

The rotor blades as such I am not even talking about. But you would at least have to get the rotor(disc) exactly level - no pitch, no bank. If one side is only 1 cm lower than the opposite side, the axis is already off the COG and would produce some movement into one direction. How would anyone be able to get into a 100% perfect hover in the first place?

 

Maybe the fuselage hanging beneath the rotor causes some sort of dampening effect - due to it's inertia? Maybe the fuselage lags behind when the rotor starts to move and then swings under it INTO the same direction as the rotor moves ("pendulum"), overshooting and thus pitching/banking the rotor disc back into the opposite direction ... and thus "stabilizing" the whole system?

 

Dunno ... don't even know if the fuselage can really swing feely here - which leads me back to my question I asked BitMaster - if there aren't always forces present between rotor disc and fuselage as they are connected by the swash plate.

Edited December 28, 2014 by Flagrum

[CASoldier2014]

One does not simply compare DCS and FSX flight models.. even if they're external..

[BitMaster]

It's instability has a number of reasons, some of them make it unpredictable what will happen next, some are predictable. The sum of all disturbances makes it difficult.

 

The main instability comes from either main blade pushing against the wind and inducing a torque, the rest that follows is a derivate of that force.

 

The pendulum effect for sure is there, as with any top-wing AC or opposite, a low-wing fighter being more agile.

 

 

I think we are talking about two classes and amounts of forces involved, the pendulum effect makes it harder to get going in either direction and such soothes the heli but it also makes it slower in reaction to keep hover control if the pendulum swings the wrong way to what the pilot wants I guess.

 

The base line is, a heli aint easy to fly and wont fly by itself

 

Bit

[Snottus]

My experience rl flying an R22, R44 and AS350 says that there is no such thing as a stable hover, at least not without aid from some sort of system, such as in the Back Shark.

 

Holding a hover without any aid, such as in the Huey, requires constant effort by the pilot. The slightest breath of air will send you careening off unless you counter it.

 

I haven't yet tried Dodosim, but I've flown Dreamfoil's R22 a lot of hours, and it is very similar to the real thing.

 

One of my biggest gripes with sims so far, is their inability to accurately implement weather and its physical effects on flying. Try holding an R22 in a stable hover with gusty down-/crosswind...

[zaelu]

IIRC Huey from DCS does not have the heavy metal rotor but the light composite one... so it's not really heavy to be stable by inertia.

 

I also think Huey will not have tendency to stay in the hover by itself... you put some trim in it just to make it easier to control.

 

I imagine a coaxial counter rotating rotor and a heavy low round fuselage would be a bit more stable in hover... but Huey? It doesn't look to me able of that.

[Yurgon]

The Huey seems to model pendulum effect halfway. That is, once displaced from a hover it models the effect of wanting to stay displaced. However, once settled into a stable hover it should also tend to want to stay in a stable hover.

 

I'm no RL pilot, so take all I say with a grain of salt. However, this is the first time ever I hear about a helicopter's inherent hover stability. Everything I've ever read, heard and seen about helicopters suggests otherwise -- i.e., a helicopter is not stable in a hover and requires constant inputs by the pilot or a flight control system.

 

I guess you never heard of dodosim but if you go to http://www.hovercontrol.com you will find their forum.

 

This one?

 

A search for "pendulum" in that forum gives me 4 hits, none of them seems related to hover behavior.

 

Can you give us something more specific? For instance, is the DodoSim 206 manual available for download so that we can actually take a look at the source?

 

Also, the first hits for "helicopter pendulum effect" and "bell 206 pendulum effect" currently lead to your (merged?) posts right here in the ED forum.

 

Given the circumstances, my opinion is that you probably misunderstood something or I misunderstood your posts. You should really get into more detail and supply us with links and references to what this effect does, to which helicopters it applies (by which I mean "applies", not "should apply because it's heavier") and how it manifests itself in terms of pilot workload when hovering.

 

Holding a hover without any aid, such as in the Huey, requires constant effort by the pilot. The slightest breath of air will send you careening off unless you counter it.

 

Thanks for your input, much appreciated!

[Flagrum]

I did now a bit research as well and could not find much in regards to the term at hand either. The best, or most interesting I found, was from some helo training material. It explained, that (depending on the rotor type, ofc) the fuselage does not directly react to attitude changes of the rotor disc when maneuvering. Instead, like a pendulum, it reacts to accelleration/movement when the rotordisc, due to the attitude change, starts to move. The result is, that the fuselage reacts way slower to control input than the rotor itself - and thus makes keeping i.e. a hover more difficult for the pilot due to the delayed reactions of the helo.

 

Makes totally sense to me, but it is more of contrary to what the OP was suggesting ...

[b00ce]

Here's what the army says about it.

 

FUSELAGE HOVERING ATTITUDE

 

Single-Rotor Helicopter

1-53. The design of most fully articulated rotor systems includes an offset between the main rotor mast and

blade attachment point. Centrifugal force acting on the offset tends to hold the mast perpendicular to

the tip-path plane (figure 1-34, page 1-23). When the rotor disk is tilted left to counteract the

translating tendency, the fuselage follows the main rotor mast and hangs slightly low on the left side.

 

1-54. A fuselage suspended under a semirigid rotor system remains level laterally unless the load is

unbalanced or the tail rotor gearbox is lower than the main rotor (figure 1-35, page 1-23). The fuselage

remains level because there is no offset between the rotor mast and the point where the rotor system is

attached to the mast (trunnion bearings). Because trunnion bearings are centered on the mast, the mast

does not tend to follow the tilt of the rotor disk during hover. In addition, the mast does not tend to

remain perpendicular to the tip-path plane as it does with a fully articulated rotor system. Instead, the

mast tends to hang vertically under the trunnion bearings, even when the rotor disk is tilted left to compensate for translating tendency (figure 1-35, B, page 1-23). Because the mast remains vertical,

the fuselage hangs level laterally unless other forces affect it.

 

1-55. When there is forward tilt of the mast, the tail rotor gearbox is probably lower than the main rotor.

Main rotor thrust above tail rotor thrust to the right causes the fuselage to tilt laterally left (figure 1-36,

page 1-24). Although main rotor thrust to the left is equal to tail rotor thrust to the right, it acts at a

greater distance from the CG, creating a greater turning moment on the fuselage. This is more

pronounced in helicopters with semirigid rotor systems than those with fully articulated rotor systems.

Tail rotor thrust acting at the plane of rotation of the main rotor would not change the attitude of the fuselage. The main rotor mast in semirigid and fully articulated rotor systems may be designed with a

forward tilt relative to the fuselage. During forward flight, forward tilt provides a level longitudinal

fuselage attitude, resulting in reduced parasite drag; during hover, it results in a tail-low fuselage

attitude.

 

(Tandem rotor crap)

PENDULAR ACTION

1-57. The fuselage of the helicopter has considerable mass and is suspended from a single point (single

rotor helicopters). It is free to oscillate laterally or longitudinally like a pendulum. Normally, the

fuselage follows rules governing pendulums, balance, and inertia. Rotor systems, however, follow

rules governing aerodynamics, dynamics, and gyroscopes. These two unrelated systems have been

designed to work well together, in spite of apparent conflict. Other factors, such as overcontrolling,

cyclic-control response, and shift of attitude, affect the relationship of the rotor system and fuselage.

 

(Overcontrolling)

 

Cyclic Control Response

1-59. The rotor response to cyclic control input on a single-rotor helicopter has no lag. Rotor blades

respond instantly to the slightest touch of cyclic control. The fuselage response to lateral cyclic is

noticeably different from the response to fore and aft cyclic applications. Normally, considerably more

fore and aft cyclic movement is required to achieve the same fuselage response as achieved from an

equal amount of lateral cyclic. This is not a lag in rotor response; rather as figure 1-37 shows, it is due

to more fuselage inertia around the lateral axis than around the longitudinal axis. For single-rotor

helicopters, the normal corrective device for the lateral axis is the addition of a synchronized elevator

or stabilator attached to the tail boom. This device produces lift forces keeping the fuselage of the

helicopter in proper alignment with the rotor at normal flight airspeed. This alignment helps reduce

blade flapping and extends the allowable CG range of the helicopter; however, it is ineffective at slow

airspeeds.

Edited December 29, 2014 by b00ce

[Rongor]

Forgive me, I didn't read the whole thread, only the OP.

 

Let me tell you:

 

What you experienced in MSFS helos is far from realistic. Don't take it as a measurement.

 

What you experience with DCS helos and the Huey in DCS is generally as real as possible. I never had a feel so near the reality like in DCS. MSFS is pure fantasy in comparison.

 

The "pendulum" is experienced by many pilot students within their first steps in learning to fly a helicopter, it simply is the lack of feel for amount of needed control inputs and sense. It will vanish with enough training.

 

Modern helicopters dampen these effects by SAS and other stabilization features. Not so the Huey.

 

Train this, you will get better after a dozen hours, the pendulum will vanish.

[Flagrum]

Forgive me, I didn't read the whole thread, only the OP.

 

Let me tell you:

 

What you experienced in MSFS helos is far from realistic. Don't take it as a measurement.

 

What you experience with DCS helos and the Huey in DCS is generally as real as possible. I never had a feel so near the reality like in DCS. MSFS is pure fantasy in comparison.

 

The "pendulum" is experienced by many pilot students within their first steps in learning to fly a helicopter, it simply is the lack of feel for amount of needed control inputs and sense. It will vanish with enough training.

 

Modern helicopters dampen these effects by SAS and other stabilization features. Not so the Huey.

 

Train this, you will get better after a dozen hours, the pendulum will vanish.

He is not talking about pilot induced oscillations, he is talking about the physical effect that the fuselage can swing under the rotor.

 

But it seems, according to the postings in this thread, that this effect does not cause some sort of stabilization while in hover, as the OP suggests.

[HuggyBear]

The result of the helicopter fuselage hanging beneath the rotor creates 'some' stability in the roll/pitch axes, but not in drift.

 

'Some' stability means that it is less prone to departure, not that it will return to a stable position.

 

Dodosim's 206 FM is very good but far from perfect, so too is DCS's UH-1H FM, some X-Plane payware helo FMs are of a similar quality.

 

None are perfect, but they're the best we have. :)

 

- Bear

[Anatoli-Kagari9]

Very old topic, but the other day someone was commenting on the tendency to return to level flight when perturbed by cyclic roll input and return o fthe stick to neutral that can be experienced in the DCS UH-1H as opposed to the RL couterpart.

Since I've never flown any sort of heli IRL, even less a UH-1H I can't but try to look at the physics / maths of the flight model and try to find out if it look plausible or not, but the user commenting on it did have experience in helis, and he pointed out that the real UH-1H would not return that easily to level flight when cyclic roll is imput.

Interested in finding out more about this...

 

[Calabrone]

Anatoli I fly with the Huey, but I have never had a similar problem, could you explain better what you mean ? Thank you.

[fapador]

@ShalomarThe DCS Huey FM is not perfect. People who say it is, dont know what they are talking about... Look here for my report/post where  few weeks ago I had two real life pilots have a go on it and what they had to say . 

Also this FM effect you mention has been discussed before years ago  by @Hueyman pilot irl  along with some other inaccuracies of the Huey module here:

 

look at the bottom of the post for Flight dynamics comment.

 

 

 

 

No matter of this post, no changes will be implemented by the developers,  expect nothing. Even with data proof shown to ED nothing has improved all these years.

Edited February 3, 2022 by fapador

[Anatoli-Kagari9]

Thanks for you posts @fapador and @Calabrone,

regarding what I was referring to and Calabrone asked to be more explicit, it's the fact that when flying staright & level, if I give the heli a roll input, and then retrn the stick to neutral, it tends to rapidly return to level flight as if it was a fixed wing aircraft with huge dihedral effect.

It's mentioned in the links fapador offers above, among other problems...