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Why is Rolling G dangerous?

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Why is Rolling G dangerous?

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Old 6th May 2005, 12:55
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Why is Rolling G dangerous?

In another lifetime I flew the Australian Wirraway trainer, which looked like a Harvard. I recall it could take a very high G loading.
On our Pilot's Course, we were taught how to carry out flick rolls by simply yanking hard back on the stick at cruise speed of 120 knots and around we would go. The stress on the Wirraway must have been significant but there was no prohibition on the manoeuvre.

Later on we were told about the dangers of applying "rolling G' where normal G limits were lowered. In fact we lost a Wirraway during an air to ground gunnery exercise where one wing folded when the pilot pulled out of the dive and banked around the same time.

Can someone please explain the dynamics of rolling G and why the manoeuvre can be dangerous. During flight simulator training for unusual attitude recoveries in the 737 simulator, we often see pilots hauling back the stick during a dive recovery before they have first levelled the wings.
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Old 6th May 2005, 13:50
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Centaurus

Consider the case where you are doing a straight pull out close to or at the limiting g for the aircraft. That specified limiting g will have a margin of structural strength for safety. Then you apply aileron to roll. Immediately you begin to overload the wing with the extra lift produced by the down going aileron. In extreme cases the up going wing will overload to the structural limit and fail.

It's called a rolling pull out. Beware and avoid like the plague..
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Old 6th May 2005, 16:36
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Do a Hover - it avoids G
 
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During flight simulator training for unusual attitude recoveries in the 737 simulator, we often see pilots hauling back the stick during a dive recovery before they have first levelled the wings.
Good grief!
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Old 6th May 2005, 19:19
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JF “Good Grief”; yes my first reaction, but on reflection we should not be too concerned. In my experience few civil aircraft are flown at or even near their limiting ‘g’. The current techniques for upset recovery focus on the avoidance of rudder to pick up a wing, thus the aileron input. This also dispels the myth that rudder was the only way to pick up a wing after a stall.

Milit makes a valid theoretical point, but I understand that Va provides some load protection irrespective of the applied ‘g’. I recall the need to fly a most unpleasant flight test of recovering from a nose low rolling upset at Md/Vd.

The stick force per ‘g’ characteristics for most civil aircraft is such that pilots are very reluctant to apply the maximum load unless they are very frightened. In such situations they are most likely to bend the aircraft anyway. Another aspect is that few civil aircraft have an accelerometer.

All of the above is probably a good vindication of the Airbus philosophy of a fully protected aircraft.
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Old 7th May 2005, 13:06
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Alf5071h (A bit of an eyeful that username -any chance of shortening it to just Alf?)
____________________________________________________ The current techniques for upset recovery focus on the avoidance of rudder to pick up a wing, thus the aileron input. This also dispels the myth that rudder was the only way to pick up a wing after a stall.
___________________________________________________

I don't recall having seen or read of using rudder to "pick up a wing" during UA recovery. In the GA world of instructing the term "pick up the wing with rudder" where a wing drop had occurred at the point of stall, was a common misconception. It really meant that further yaw from the wing drop at the stall should be prevented by appropriate rudder while of course the wings are levelled by use of aileron.

Reagarding the "Good grief" gasps of disbelief, it is quite common for pilots undergoing type rating training in a simulator on their first jet transport eg B737, to have never seen a severe unusual attitude until having it demonstrated by the simulator instructor.

Although UA training is normally undertaken during CPL or training for an instrument rating in general aviation light aircraft, clearly there is a limit in terms of commonsense flight safety how far the instructor should go in a real aeroplane. In any case, that sort of real aircraft training is done in VMC and although theoretically "under the hood", often it is all too easy to peek outside. Unless as part of aerobatics, UA recovery training is therefore limited in it's application

In a flight simulator where UA recovery practice is done in virtual night IMC, and until they get the hang of it, frequently pilots will lose many thousands of feet and build up huge knots while attempting recovery. Unless the recovery is from an extreme nose high slow speed attitude, rudder should never come into it - and then only to get the nose down to the horizon.
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Old 8th May 2005, 00:57
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Hey Folks: I hope you don't mind me jumping in here on this but I do have an opinion on this issue.

The problem with using flight simulators to “teach” unusual attitude recoveries, is that simulators are merely computers, doing what they’ve been programmed to do. The thing that makes a B737 simulator fly like a B737, and a B747 simulator fly like a B747, and not vice versa, is the programming of the appropriate simulator computers with data obtained from the B737 or B747, respectively. The data that is available is typically just a bit more than the maneuvers performed for and during aircraft certification. AND, the big factor is that ALL of this gathered data is within, and just barely beyond, the normal operational envelope of the aircraft. Yes, there are some additional data that are acquired that are only for flight simulation purposes, but its not a lot and it certainly doesn’t go beyond the flight test validated envelope.

When someone takes a simulator outside of the flight test validated envelope, while the simulator may still roll or loop, there is absolutely no guarantee that the airplane will do anything like what the simulator does. Some serious queries have been made, on more than one occasion, to the experts in the field, regarding the ability to get the data and the cost, the reliability, the repeatability of that data. Interestingly, they ALL, everyone of them, has said the following. The ability to get the data would be limited. It would be very costly. It would likely be dangerous in more than just some cases. The accuracy of the data could be easily challenged (this is mostly because repeating the maneuver several times is likely to yield wildly different results). We discussed the possibility of extrapolating the data… Certainly this could be done, but to what accuracy? Where would the data curve go if it were to be extrapolated beyond the flight-tested results? More to the left, more to the right, would the curve wrap tighter or relax? Extrapolation would be directly dependent on the suppositions used. What suppositions would we depend on? To whom could we turn? What pilots have experience in these uncharted areas in the airplane we’re attempting to simulate? Who would have the experience to make such suppositions?

Please understand; I am NOT advocating not using simulators to address the recovery from unusual attitudes. To the contrary! What I am saying is that how we use these very valuable training, testing, and checking tools is critical and we must prevent our dependence on them beyond their capabilities. For example, showing a pilot a 120 degree bank and 10 degree nose down, and then explaining a technique to roll the airplane all the way around – to “take advantage of the rolling energy” already established – (yes, I’ve heard just such stories!) would be ill advised, at the very best. This is true if for no other reason than the fact that no one knows how the airplane would react if such a stunt were attempted. However, showing a pilot what 120 degrees of bank and 10 degrees nose down attitude looks like outside the front window in comparison with the flight instruments is not only possible – it is quite valuable. We’re not talking about how to recover, we’re looking at what information is available to the crew and how they can best use that information. The discussion could go on from there about the shortest distance to roll to get the airplane back to a “wings level” condition. You could discuss whether or not you should pull on the controls during the roll? You could discuss what kind of “g’s” are likely to be generated in doing so? How much aileron should you use in this circumstance? I don’t know. And neither do you. However, you could discuss what to do about “the ball?” Is it important to know what to do with it? Or is it irrelevant?

Simulators are really very good tools. Lets use them to the very limits of their ability and not one bit more than that.

Thanks for the opportunity to "sound off."

AirRabbit
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Old 9th May 2005, 14:39
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AirRabbit - while I agree that sims are only as accurate as their programmed responses, I do find unusual attitude recoveries a good thing to do. I was lucky, I had aerobatic and very comprehensive UA training. Most of my contemporaries do not. In seeing a recovery from an unusual attitude in a sim, while not perhaps 100% accurate, is still valuable.

Seeing a UA and then discussing it is very different to seeing it and recovering from it. When I tried it in a 757 sim, I was very surprised to see how hard I pulled to get out of a dive. I was also surprised to see what my follow pilot did. Now that we have had a chance to actually DO it, while not 100% accurate it gave us some tactile feedback which you never get from discussion. It also highlighted the fact that light aircrat are a bit different - rudder mainly, but also in response. The basics are the same, the response is different. JF's response of "good grief" is alone a good enough reason to actually try it, while bearing in mind the limitations of the machine.

Try and spin a sim and whatever it does, the aeroplane will be different - even if it is an all singing all dancing development sim.
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Old 9th May 2005, 19:42
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Centaurus, re rudder to "pick up a wing" during UA recovery, you appear to operate in a sheltered operation, possibly due to some good basic training in your part of the world. I note that many myths of aviation have been dispelled via the FSA safety magazine May-June 2002, use of rudder in the stall being one. However elsewhere in the industry, certainly pre the AA A300-600 accident, there are many inappropriate upset recovery techniques. In addition, not every one gets in-flight upset recovery training as you describe.
Simulators are valuable for teaching the techniques for upset recovery; not always being realistic for ‘g’, disorientation, or body sensations; a real aircraft is always best for these.

AirRabbit, given that the majority of upsets occur within the normal flight tested envelope (including stalling), then most simulators are of some, if limited value for upset recovery training; at least to demonstrate what the aircraft feels like. Not necessarily to determine the quickest or best way of flying the recovery, but how the controls / trim change with rapid speed change.

Many simulators do include extrapolations of the aircraft’s performance in pitch and roll; even the largest of airliners may have been demonstrated to 120 deg roll and probably +60 to -45 deg in pitch, thus extrapolation is quite easy; it is the non linear aerodynamics post stall or at excessive speed that could be unrealistic.
Pilots would have greater benefit from upset training that teaches awareness of the situations that lead to an upset, the onset cues, and how to recovery from the initial stages of the upset. The most likely human aspect affecting a line pilot who has entered an upset is that he/she forgets – the mind is focussed on ‘what the …’ attempting to understand the situation. Even when the upset is understood it is most unlikely that the pilot will recall any discussion on amount of aileron, or roll and pull for certain circumstances; the most valuable training is to embed the basics of recovery into subconscious action (over train the basics) and the control of surprise/fear. Upsets are fortunately rare events and would not normally require such attention if crews flew the aircraft correctly during controlled flight, thus training in these aspects may be of even greater value, keep it simple.
For a more scientific approach to the above see ‘Human Factors of Upset Recovery Training’, Capt Janeen Kochan & Mike Moskal, Calspan Research, FSF EASS Warsaw Mar 2005.
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Old 10th May 2005, 02:43
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This thread seems to have lost its way somewhat.

Why is rolling G dangerous was the original question?

I believe tha answer comes from the fact that aircraft are designed to have certain margins for structural strength, with that design come certain "assumptions" about how the pilots will make inputs - in general this includes a certain amount of symmetry, or single axis inputs particularly at the extremes of Gz. If the designer chappie had to account for maximum roll rates being applied at maximum pulling g then the fuselage would have to be extremely strong to cater for the torsional stresses developed between wing and tail. Basically to stop the tail from falling off!!

As an example an F-15 was quite happy at 9g in a straight pull, but a twitch of roll would trigger an "over g" warning.

There are a couple of solutions: make the fuselage stronger = excess weight for the whole life of the aircraft to cater for a small region. Tell the pilot not to pull and roll = fine in the "good old days" but subject to human failings. The ideal solution, is to incorporate appropriate scheduling into the flight control system either by limiting the roll rate at high g or reducing the demanded g when a large or abrupt roll input is made.


Makes any sense?

T
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Old 10th May 2005, 04:06
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Interesting article on this subject at Avweb.
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Old 10th May 2005, 18:48
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Rolling G

Rolling G in most cases is not dangerous. However, the torque exerted on the attachment points and other structural appendages when the G force is nearing an operational limit can cause great damage. For those that have not flown formation or watched a stress video of a tail structure during high rolling moments you will just have to believe what I am saying. First I will start off by saying that most older aircraft have controls that will allow you to overstress the aircraft once you are flying at or above the manouevering speed for a particular configuration. Most new large aircraft have load alleviating functions that reduce inputs to control surfaces as airspeed increases. Second, each aircraft has design limits that are usually published for symetrical loads and for most high performance aircraft they are also published for unsymetrical or rolling G. An example is 7.33G symetrical and 5.85G unsymetrical. Large transport category aircraft have design limits of about 3G symetrical and about 1.5-2G unsymetrical. However, in most cases this is never even close to the typical loads encountered. As well, these same aircraft have stress limits that change depending on the fuel load and cargo C of G relating to airspeed. One thing to note, is that the structural limit for large aircraft is not far beyond the design limit and you will bend it or break it if you go beyond the design limit. One of the previous writers noted the extra moment applied to wing structures during rolling manouevers during a pull-up. This is real and can be as much as 1.5 times the load as a symetrical application of elevator. Another item to consider is coupling. In a T-tail application the force exerted on the elevator in symetrical G is in line with the vertical stab; however, add a rolling moment and you have the vertical stab flexing perpendicular to its attachment while it is under load longditudinally. Think of your T-tail looking like a banana. If you were to also add rudder inputs, you could easily exceed the structural limit and cause the tail separate from the fuselage.

Now in certification testing, aircraft are typically required to have a full rudder input and then back to neutral as part of the certification process. No other inputs to any controls are made! It is well known in the test community that extra rudder or control inputs can create stresses that aircraft were not designed for. The same applies to elevator and aileron.

Following up the comment on upset recovery; the most simple procedure is CENTRALIZE ALL CONTROLS, ANALYSE YOUR SPACIAL POSITION then: Nose Up; Roll to the nearest horizon with power (usually full power) with 0-1G then as airspeed increases to flyable pull out of any ensuing nose down attitude: Nose low; Roll to the nearest horizon with normal loading reducing power as required, then pull out of the dive. Do this for all aircraft types unless your aircraft manual says it is OK to use rudder to pick up a wing... Why do I say this, because it will ensure the quickest recovery and will prevent rolling G overstress incidents.

Who am I... a previous military fast jet instructor and test pilot for both fast jet and transport aircraft.
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Old 11th May 2005, 02:34
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Twoicebergs You are a pro.

Presume that your reference to 7.33 g relates to the F-111 (or what else) which has a design ultimate of 11 g. Where did the 5.85 g come from particularly as the F-111 case is complicated by wing sweep, aileron substitution with spoilers and roll power from the tail.

But if we now start to consider structural loads caused by asymmetric pull outs with all of the variables introduced by the F-111 I suppose we have to placard the worst case and leave it at that unless there are operational advantages in having several worst cases.

Erudite comments?
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Old 18th May 2005, 22:05
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I don't want to worry you guys but the best guess about the K7 glider that crashed at Strubby last year was that the roll (and reversal) initiated hit the wing's primary natural frequency and caused the failure of the internal structure. This is a known phenomenon that has occured on powered aircraft before and that can occur below Vmanoeuvre (in fact can happen below Vanything)

Fly carefully

Last edited by Incipient Sinner; 19th May 2005 at 06:38.
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Old 19th May 2005, 03:15
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That reminds me - there's a very detailed analysis of a relevant PA-28R wing failure online in the UK.

The AAIB report.

Last edited by djpil; 1st Jun 2005 at 04:08.
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Old 20th May 2005, 11:18
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This thread just helps to strengthen my opinion that all pilots should undertake Unusual Attitude Recovery training with a qualified aerobatics instructor, in a suitable aircraft.

All aircraft are capable of extreme attitudes in all three axis, and all pilots should learn how to best maximise their chances of a safe recovery.

And no, I do not own an aerobatics school.
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