Static / Dynamic rollover definitives required...

[TheFlyingSquirrel]

Yep - this old chestnut again. I'm looking for a simple and plain definitive description of both conditions from our rotary experts please. I've been though all the old threads, text books and google searches, but am still unsatisfied with what I've come up with. Over to you...

( merry x-mas by the way kids ! )

[helopat]

I'm no academic, but since its just come up in another thread, here's my take on static and dynamic rollover.

Static rollover, simply, is the angle at which the helicopter will 'fall over' if you lift it up by the skid or wheel (ie. lift the left skid of the parked aircraft until it passes the point of balance and tips over)...the angle at which its c of g passes outside the skid and it tips is the static rollover angle.

Dynamic rollover isn't really as simple as an angle...it has to do with a RATE of roll which exceeds the control authority (ie. land on with some sideways drift and the skid or wheel catches/touches the ground...if you're moving slowly, you may have the control authority to counter the rolling moment in which case you don't roll over and mess up the paint job...if you're moving more quickly and do the same thing the rolling moment may be too much and you won't have the control authority to counter the rolling moment in which case you get to EXPERIENCE dynamic rollover).

In my military rotary experience the aircraft has a defined static rollover angle (defined in the manual) and a sloping ground limit (also in the manual)...the sloping ground limits (I presume) are determined by the manufacturer AND the military test pilots who accept the aircraft into service and serve to keep me from accidentally exceeding any control limits whilst doing sloping ground landings (usually well within the aircrafts and my ability to handle with a big, fluffy safety margin).

Does this help at all?

All the best in 07.

HP

[Arm out the window]

I've always thought of them like this:
Static rollover - as helopat says, reaching or exceeding the angle at which the helicopter will topple over rather than drop back onto the skids, ie when a line straight down from the CG goes outside the skid base.
Dynamic rollover - imagine you're taking off, one skid catches on an obstruction and you start to roll.
Say for the sake of argument that full left cyclic can tilt your thrust vector 15degrees to the left of the mast. If the fuselage reaches 15 degrees right tilt with a skid caught, full left cyclic will only point the rotor thrust straight up, so it's not going to help you.
Any more fuselage tilt will turn any rotor thrust into a pro-rolling force, and the only option open to you is to dump the collective to get rid of the force, although it may well be too late by then.
Apparently dynamic rollover can happen very quickly because the rotor thrust is actually dragging you around the pivot point.

Just read the initial post again - I see you were asking for experts...definitely not one, but hope that helped anyway.

[Whirlybird]

I don't claim to be an aerodynamics expert either, but this is how I understand it...and explain it to students....

Static Rollover The displacement of the helicopter's C of G causes it to roll over. This will only happen on a very steep slope, estimated for some helicopters to be around 40 degrees! It can happen without any power applied, since it only depends on the displacement of the C of G.

Dynamic Rollover Can occur on very shallow slopes, or even due to mishandling on flat ground. The crucial difference between this and static rollover is that it occurs due to power being applied. When a helicopter is sitting on the ground and power is applied, the Total Rotor Thrust has a horizontal component, which acts about the point of ground contact of the skids. This component is normally very small, and is opposed by the weight of the helicopter. But if one skid lifts off the ground first, the skid in contact with the ground now acts as a pivot point about which the horizontal component of TRT acts. This component increases as more power is applied, causing the helicopter to roll about the skid that is in contact with the ground. Beyond a certain critical angle it is impossible to stop the helicopter turning over, and this angle of bank is quite small, far less than that required for static rollover.

I hope this helps. I hope it's right...but I'm sure the experts will tell me if it's not. I sometimes think...one rotary aerodynamics question, two pilots, three opinions!

[Graviman]

Whirly, is dynamic rollover more likely when taking off than landing? From your explanation it sounds like it occurs if pilot does not go through the "light on the skids" and "balance the aircraft" actions before pulling collective, rather than just a landing with too much sideways speed.

I guess another way of looking at it is that it is any situation where the skid can present a sideways force high enough to start a roll can become dynamics rollover. Basically the aircraft gets into a situation where if it has enough momentum to pass the static rollover angle so you can't stop it. If it just tilts up, but not quite enough to keep going it goes down as a note in the logbook. :oh: The rotor following the aircraft as it rolls will just help it gain the momentum to roll over.

Mart

[Whirlybird]

Whirly, is dynamic rollover more likely when taking off than landing?

As I understand it, it IS more likely when taking off, because you have more power applied. However, it will also depend on how much you mishandle it when you're landing, won't it? ;)

[GoodGrief]

Download the Rotorcraft Flying Handbook from the FAA.
http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf
Open the pdf, goto page 104, or search for dynamic rollover.

[topendtorque]

I am no great expert either but I would agree that static is merely the tipping past the C of G of the article be it a helicopter or a square box until it yields to the common gravity. I would also suggest that the two threads regarding the subject be merged as B Sousa has raised an important issue in the B206 slope landing thread.

However the word Dynamic I have had problems with, I think the use of the mathematical ‘Kinematics’ would more aptly describe some of the forces and accelerations and resultant actions that occur.

As far as (normal) students are concerned then the word “dynamic” is OK as it is a simple word of few syllables and should be imparted to them with profound and frightening facial expression on the central SYLLABLE, which should concur at a point in time with their receptive mute being broken open to the fact that the instructor is uttering a word. If that word is followed by full effect fright, simultaneously with the word ROLLOVER then hopefully they should have forever understood that this is a situation which is “not good” and should be avoided at all costs.

Whether or not they understand any of the kinematics behind it.

To my mind in simple state (as always) I would think that a dynamic rollover is one which is accelerated faster than a normal static roll because of an outside force that contributes to the rate of roll.
A sort of triggered roll such as when a car sliding sideways flick rolls because it has contacted a curb. Whirlybird nailed it the best I think, and in the way it was described to me - way back when.

Interesting to note the ‘Handbook’ ref from “GG” which differs from mine, revised in ’78, which doesn’t even mention it but I am positive my old first (now lost) edition bought in 1973 did for sure.

Of course with slope landings, don’t forget the old slide down the hill trick even if you can put the a/c down at or beyond published limits.

Now to the issue that B Sousa has raised that I would comment on when contemplating slope landings at or above the limits or at least ~ full cyclic deflection.

He mentioned mast bumping. Now I am (and I think he would be) most averse to this phenomena having read in various places that it is also “not good”.

What I suggest all pilots do if test flying an a/c immediately after major maintenance, or for that matter any daily insp, is to check the cyclic position relative to the swash plate position prior to start up. The swash plate should be at right angle to the cyclic, both fore and aft and laterally, and or if the cyclic is at right angles to the floor of the a/c then the swash plate should be at right angles to the mast.

If flying an aircraft with teetering or Articulated head only (I.E. not a ridged head where the following would be prohibited) then after start and warm up, at flight RPM, CAREFULLY and SLOWLY check the fully displaced cyclic position in each quadrant. Vibrations, hopefully not mast bumping vibrations, should be equal in all areas. If they are worse in one corner then shut down, inform the engineer why you shut down and ask him or her to check the nominal rigging of the flight controls.
The difference can be almost non discernable by eye but most obvious with the vibes.

The FAA AC 90-87 1/27/1986 gives good descriptions of what to watch for with regard to static and dynamic rollovers except for the definition - to my mind - of the word ‘Dynamic.’
Cheers TET

[sunnywa]

Static Rollover is simply the tilting of the aircraft such that the CoG moves past the line of contact (ie skids or wheels) and thus will fall over. Happens to cars and other vehicles as well.
Dynamic is the rate of movement of the CoG and the aircraft's control force being unable to stop it before it reaches the critical angle, thus rolling on its side. Can happen on flat ground with a skid being caught and pilot applying full power to 'yank' it off the ground. Has happened on ships decks when one chain left on and aircraft tries to pull mother into orbit (normally get wet trying this one). Can happen on landing if having excessive sideways vector and catching skid on something.
It can also be may worse by the tail rotor force acting in the way you are trying to roll it. Funnily enough, most cases I know of are the result of poor sloping ground technique and rolling the aircraft upright too quickly and rolling the aircraft into the upslope.

Recovery action (noting a nice steady lift off should stop it from occurring) is to first lower the collective as this reduces the size of the rolling moment. Don't throw it to the floor otherwise the aircraft may roll over the other way due bouncing and then the static rollover might occur. Using the cyclic alone will not save you as the rate of roll is beyond the cyclic in the first place.

ToT, I wouldn't recommend moving the cyclic around on the stops as it may damage the mast stops/droop stops. At least I wouldn't be game as they make a hell of a chatter.

Its sunny in WA:ok:

[NickLappos]

sunnywa is right on. Some amplification:

The typical dynamic rollover is much much easier to experience in a teetering rotor helo, because the lateral cyclic has very little power at low collective. Recall that the control in a teetering head is almost purely done by tilting the thrust vector, no thrust, no control. At very low collective, the rotor is producing only 10 to 20% of the thrust that it has at a hover, so the cyclic is particularly ineffective.

In a rigid or articulated rotor, there is appreciable control even with no lift (typically at least 1/2 to 2/3 of maximum) so that it is more difficult to get a rollover, and easier to recover.

Since all helos of the 1st generation were teetering heads, the books and courses (and the rules of thumb) do not differentiate and so all helos are tarred with the same brush. The universal probability of dynamic rollover is perhaps one of the "myths" that perpetuate our world.

http://webpages.charter.net/nlappos/control.jpg

[Matthew Parsons]

My definitions:

Static rollover is when center of mass is not over the footprint.
Dynamic rollover is whenever other forces are involved.


I like Nick's use of "typical dynamic rollover", presuming it referes to dynamic rollover on a slope when you aren't at a static rollover condition. I think the universal probability he referred to considers only the "typical".

Rigid or articulated heads are just as susceptible to stuck skids/wheels, strong wind gusts, ground contact during lateral translation, etc. Of course, with cyclic control power always available they are more able to recover.

Matthew.

[topendtorque]

ToT, I wouldn't recommend moving the cyclic around on the stops as it may damage the mast stops/droop stops. At least I wouldn't be game as they make a hell of a chatter.

Its sunny in WA:ok:

i should have clarified that better, yes when / if they do clatter - as you say - don't persist back off. The idea is to have it rigged right so that it doesn't damage anything.
The point is that to help you in a slope landing you may need full cylic control, if you haven't got it because the A/C isn't rigged right then it's not the engineers fault for the poor rigging, it's yours for not checking it.

of course there are many other applications of flight where incorrect rigging only becomes apparant when it becomes critical; rolling too quickly towards an off centre load when the disc is wrongly rigged toward the load is one of the more spectacular. Pretty f*n dynamic that i'll say. Recovery, is usually bottom pedal - then aft cyclic. It can be top pedal, collective down a bit then aft cyclic. these things (secondary effect of controls) should be checked and learnt at ab-initio stage.

another is when say a machine is loaded well toward the forward CofG and the fuel burns off out of the tanks which are behind the CofG.
tet

[breakscrew]

When airborne, a helicopter can cope with large cyclic control inputs; however, on the ground, rather serious problems can easily occur from slight control inputs that cause small angles of tilt on the main rotor disc. What is also rather dangerous is that the human physiology does not easily pick up small motion cues, and an unaccelerated angular motion disturbance can go unnoticed for quite some time until it has reached the point where it is potentially perilous. Because movement is often picked up through visual cues before the other senses, if you are operating at night and possibly using optical night vision devices, much of the normal movement prompts are degraded. If you are distracted or not fully concentrating when you lift off, you can easily be caught out.

Most American designed single rotor helicopters have their rotor-blades turning in an anti-clockwise direction when viewed from above, and these helicopters tend to hover with the left landing gear low. This is caused by the main rotor being tilted to the left to counteract the rightward thrust being generated by the tail rotor. The degree of fuselage tilt depends on the hinge offset in the main rotor hub and the vertical position of the tail rotor in relation to the main rotor, as well as the aircraft’s centre of gravity. When the helicopter is in the process of lifting off the ground and into the hover, the total rotor thrust is increased. This also increases the tail rotor thrust, amplifying the turning moment. In our case, the rotor flapping will roll the helicopter to the left if no cyclic input is made to counter this effect. The more you raise the collective lever, the more you will increase the rolling moment. Normally, the pilot subconsciously corrects for the roll, centring the rotor disc so that the aircraft rises vertically away from the surface, albeit with the fuselage hanging left side low. However, some pilots may inadvertently input too much right cyclic on the ground in order to correct for this tendency, setting up a rolling moment in the opposite direction. If the helicopter is allowed to continue to roll on its landing gear, the stabilizing moment caused by the centre of gravity, which is normally acting down through the fuselage between the undercarriage, goes rapidly to zero as soon as the centre of gravity moves over that wheel or skid.

Of course, increasing the all up mass will increase the overall torque required, consequently increasing the tail rotor thrust and thereby increasing the propensity for the aircraft to roll. If the centre of gravity is allowed to get close to the lateral limit, you can clearly exacerbate the rolling moment. A high centre of gravity makes the helicopter less stable in this condition.

As pitch is increased, the induced flow from the main rotors is dispersed outwards when it comes into contact with the surface. Should there be a cross wind, the vortex created from the into-wind blade can cause the downwash to recirculate, reducing the lift in that part of the disc. This loss of lift requires an increase in power to the main rotor, which also increases the tail rotor thrust, amplifying the existing problem.

Onboard ship, the movement of the vessel or the disturbed air flowing across the deck can exacerbate the conditions required to produce dynamic rollover. Lifting off from sloping ground is also problematical. In this situation, the rolling moment of the fuselage has already begun as the aircraft lifts the down-slope gear from the ground. If the movement is sharp or poorly controlled, the pilot is already inducing the danger but if, as a result of this mishandling, the fuel in the tank sloshes across from one side of the aircraft to the other, the lateral centre of gravity can shift markedly and aggravate the roll. Naturally, an aircraft with a narrow track landing gear is much more prone to dynamic rollover than one with a widely spaced undercarriage. Furthermore, if the pilot inadvertently tilts the rotor prior to lift off against firm oleos or a springy undercarriage, he is setting up an adverse condition from the moment he unsticks the helicopter from the ground.

As every student knows, the classic situation that causes rollover is when one wheel or skid is either trapped or stuck to the ground by mud, ice, tie-downs or anything else which can manage to lash one side of the undercarriage to the surface. In this case, the helicopter pivots around the fixed anchor point in the manner of a hinge until the rotor blades come into contact with the surface.

An unfortunate instinctual action by most pilots when they perceive the roll starting is to raise the collective faster in an attempt to climb into the hover. However, all they achieve is to accelerate the rollover, since they are increasing the total rotor thrust that is causing the problem in the first place. Applying opposite cyclic may also seem like a good idea, but the slowing and reversing of the rolling motion takes time, and the speed that the rollover occurs after onset is faster than most pilots can react. The crucial time between a safe take off and rollover can be measured in milliseconds, and once it starts, the process tends to become inevitable. That said, the only sensible corrective action is to lower the collective lever as soon as possible when the condition is first perceived.

Dynamic rollover can catch any helicopter pilot out. If you are about to get airborne in a narrow track, wheeled helicopter with firm oleos, which has a high centre of gravity, and you are taking off from sloping ground with a crosswind, at night using optical devices, please concentrate very hard on your actions.

[topendtorque]

All good stuff screw job but I really needed a break from yours so while there I penned another in furtherance to the sunnywa comment here which I have been chewing over all day


Funnily enough, most cases I know of are the result of poor sloping ground technique and rolling the aircraft upright too quickly and rolling the aircraft into the upslope.


Now this is a bit of confusion.

That is what I would call finger trouble.

We all know that grace and the fall from it in these situations of close ground proximity can be damm quick.

However if the machine was dynamically rolling down slope (for reasons well enough defined why by AOTW, Whirly and Nick) and if hero has been able to roll upslope then it must follow that the low skid / wheel was free and the roll was simply a recovery from an unusual attitude.????? I.E. A Controlled CFIT.

One could easily be in a situation where a dynamic roll initiated after the landing was secure.

Say you landed with a load (just thought of a very funny joke about that, some other time) and you were sitting there at flat pitch with the disc conveniently parallel with the slope so as the ground crew have clearance to unload you, then as the load disappears a different C of G takes over and away you go.

This is another answer for Gravimans Q re the more from either landing or take–off.
tet

[noblades]

Sounds complicated.

Lets keep it really short. You're looking at helicopter looking really sad and sorry for itself lying on its side. were the blades turning at the time??
No = static rollover.
Yes = Dynamic rollover.
New heli = Lotto rollover :8

How's that for short and sweet :ok:

[Whirlybird]

topendtorque,

I'm not exactly sure that I've understood either what you quoted from sunnywa's post or what you said yourself. However, I think you're querying how it would be possible to roll upslope due to dynamic rollover. In fact, on sloping ground this is exactly what is likely to happen. Think about it...as you're trying to take off, the downslope skid comes off the ground first. Therefore your pivot point is the upslope skid. If you raise the collective too quickly, before you've centralised the cyclic enough, you will get dynamic rollover, about the upslope skid.

If I've misunderstood you, ignore the above.

[topendtorque]

Hi whirlybird, no probs
I would say that a dynamic roll to the crashed position is caused by an outside force which creates a rolling moment that cannot be controlled by the pilot with cyclic and collective.

You may have an artifical but balanced rolling force by say, positively holding one skid into a ledge, if the pilot further rolls the aircraft into the ground uphill with a cyclic input that could have controlled the rolling moment had it been applied the other way, then I would say that is finger trouble. BIG FINGER TROUBLE.

That is where I disagree with what I though sunnywa was saying, i could easy be mistaken.

Mind you most people (especially those that own/insure A/C) would also comment that allowing oneself to be trapped into an uncontrollable dynamic roll is also, major finger trouble.

Happy new year and BTW someone had better roll out the welcome mat for the vanquished elleven, 'tis only a game after all.
Cheers tet

[Deemar]

I thought the defining aspect of a dynamic rollover was that you reached a position where there is not enough cyclic control to allow you to recover from the situation - no matter how quickly or far you push the cyclic away from the roll, you will not be able to stop the rolling motion.

The key part to understanding this is to think about the way the thrust of the rotor is acting when you lean the helicopter over. Most of the thrust will be acting upwards, but some will be acting sideways. If this is combined with the force on the skids from the ground, which is acting in the oppositte direction trying to keep the helicopter from sliding, then a rolling moment will be generated that tries to tip the helicopter over. Note that although a rock or a tie down chain will assist this process, it can happen without these.

Now beyond a certain point, there is not enough cyclic control to counteract this rolling moment. As Nick mentioned, because an articulated head provides positive cyclic control even in a zero lift situation it is less suceptible to this effect (you don't run out of control as quickly).

Note in particular, that the two instictive reactions that a pilot might have - use the cyclic to correct the roll, or pull collective to lift off - are not useful in this situation. The cyclic input can't correct the situation, and because the rotor's lift is what is causing the problem in the first place, pulling collective only makes it worse.

If you really want to get a good "feel" for the effect, buy one of the cheap fixed pitch micro electric RC helicopters. You can get it tipping over and play around with the dynamic rollover effect. Also, the blades don't tend to break, and they aren't anywhere near as expensive to replace as real ones.

Daniel

[Shawn Coyle]

I think you'll find that dynamic rollover almost universally happens to teetering rotor helicopters.
The few occasions it's happened to articulated or other, non-teetering rotor helicopters, it's been with something snagging the undercarriage. The movie of the Ch-46 on the aft deck of the ship comes to mind for that.
For the teetering rotor helicopters, it also appears to happen mostly when lifting off - and when something has snagged the skids.
The unfortunate thing in a teetering rotor helicopter with dynamic rollover on liftoff is that nothing you do with the cyclic is going to help you - there is only one solution if a roll rate develops that you don't like, and that's to bottom the collective.
The reason the lateral cyclic doesn't help you in this situation is that tilting the thrust vector to the side only slightly changes the vertical component of the thrust vector, which is the main thing rotating you around the skid stuck on the ground. There is no rolling moment to the airframe from the rotorhead to stop the roll with a teetering rotor head.
Unfortunately, not well described in any book (including my current one).

[FH1100 Pilot]

Since all helos of the 1st generation were teetering heads, the books and courses (and the rules of thumb) do not differentiate and so all helos are tarred with the same brush. The universal probability of dynamic rollover is perhaps one of the "myths" that perpetuate our world.Whoa. Can't let that one go. Man, Igor is gonna be ticked off when he hears that you uttered such blasphemy, Nick. Especially coming from you! Maybe you should have said, "With the exception of Sikorsky's R-4, since most training helos of the 1st generation..."

Although any helicopter can snag a skid/wheel and over she goes!, it must be said that yes, DR generally happens to birds with teetering systems with low control power. And here, I stand with Whirly: It's the tail rotor thrust that gets you, "pushing" the ship over. The hapless pilot gets the yaw trim setting wrong while lifting off...or maybe because one of the skids has sunk into the dirt a bit and he thinks everything's cool because he doesn't feel any unintended yaw when getting light-on-the-skids (which he misinterprets), then O-S-G's!

When I first got to the GOM, I used to worry about this when taking off from wooden deck helipads: Is one of my skids stuck between the boards? Then I learned to simply not land parallel to the boards. D'oh!

Which brings up a question: Is there a "common" side that helicopters dynamically rollover to? Because of my believe that the TR is the culprit, my guess would be that the ship would rollover to the right more than the left.

Any info on that, Shawn? Is there a trend?