Rolling 'G'
 

For the aerodynamicists

I'm looking into UPRT training for large jet aircraft, specifically Boeing.

AFM limitations provide values for + and - G but no 'rolling G' (combined roll and pitch), and UPRT recovery generally separates manoeuvers by pitch and roll (not combined). During nose low recovery, the nose is often raised (unintentionally) by inexperienced trainees as the lift vector becomes more vertical, whilst they are rolling wings level, and thus inducing greater G.

Am I guessing correctly that the + and - G limits are fixed whether in a turn, rolling or wings level?

Thanks in advance..

It was my understanding that rolling G was limited to two thirds of the normal G limitation of the aircraft (so a 2.5G limit is reduced to 1.65 whilst rolling) - although I’m not 100% sure that applies to transport category aircraft, but it does apply to FAR 23 certified aircraft.

Certainly the G limit whilst rolling is NOT the same as the G limit in wings level flight.

In the UPRT simulator sessions that we do, there is a focus on rolling G and in fact the UPRT functions of the simulator give cautions to the instructor if the pilot conducts a recovery that triggers rolling G over a pre determined threshold.

Very much appreciate the insight Col. Klink...

Great gen...

Doesn't matter, people are gonna roll while pulling out no matter what. If that test pilot crew in the 717 video couldn't get it right when it counts, what hope is there for the average schmuck?

Another thread on the subject with test pilot comments

https://www.pprune.org/flight-testin...dangerous.html

Some flight manuals specifically state two thirds rolling 'g'.

§ 23.349 - Rolling conditions.
(b) (b) The loads resulting from the aileron deflections and speeds specified in § 23.455, in combination with an airplane load factor of at least two thirds of the positive maneuvering load factor used for design.

You can roll and pull. You just have a limit. T-38 max weight, straight pull - 6g, asymmetric pull - 4.4g.

Thank you all - greatly appreciated...

Sure, that's true. But what do you think is a pilot's capacity to respect this limit, when doing
- something extremely startling and disorienting
- for the first time in their life
- there is no instrument in the cockpit showing this information
- as the airspeed is rapidly shooting toward the limit, forcing you to pull high G to level off
- the airplane is trimmed for a big pull without the pilot doing anything
- and there is an overwhelming instinct to pull also
​​​​​​

Vessbot, jimtx, et al

You have to consider stick force per g in the human actions.
Commercial aircraft have much higher control forces than most military aircraft.
Then additively the ergonomics of the pitch combined with the rolling stick force

It's common to see a reduction in actual flyjng skill in test pilots.

On the other hand your poor average schmuck has been trained to roll and then pull. I think they'd do just fine.

Chesty, #10

Spherical Objects

also

Many accidents show that what we think will happen is not what happens in real, surprising conditions.

-------

Megan et al, the FAR reference # 5 was for a part 23 aircraft; the op question relates to 'large jet aircraft'.

However Part 25 is similar

§ 25.349 Rolling conditions.
The airplane must be designed for loads resulting from the rolling conditions specified in paragraphs (a) and (b) of this section. Unbalanced aerodynamic moments about the center of gravity must be reacted in a rational or conservative manner, considering the principal masses furnishing the reacting inertia forces.
(a) Maneuvering. The following conditions, speeds, and aileron deflections (except as the deflections may be limited by pilot effort) must be considered in combination with an airplane load factor of zero and of two-thirds of the positive maneuvering factor used in design. In determining the required aileron deflections, the torsional flexibility of the wing must be considered in accordance with § 25.301(b):

Question:- is the design maneuvering factor greater than the AFM limit?

3721
Not my words but the words of someone who, for the best part of the last 40 years, has been a military and civilian experimental and developmental test pilot and test pilot tutor/instructor who has flown more types than you or I ever will, probably combined,

Yes, by (unsurprisingly) fifty percent for the transport category.

I would have thought that the fifty percent is the difference between the design loads and the ultimate load ( unless it is stated that the particular load is actually the ultimate load).

Clandestino, zzuf, thanks,

Thus if the rolling 'g' limit is determined by the pilot's limiting stick force, or that the structure does not break (although it might twist) within the AFM 'g' limits, as determined by flight test, this implies that if you can do it (rolling g) then it's OK ?

From experience in a manual control commercial aircraft, the combined pitch and roll control forces at VMO / MM0 are very high, much higher than pilots might expect. Are simulators representative ?

For those who have been above VMO, the forces become extremely high which may require both pilots input for recovery.

This discussion should also raise the question of how such situations might be encountered, particularly if the aircraft started from a trimmed condition; and although some procedures (generically) require a check or reset of trim position to a normal range, trim should not be used to recover the aircraft because of the risk of exceeding the normal g limit.

Not my experience as a certification test pilot who flew with experimental TP's from: Aerospatiale, CASA, Boeing, Airbus, Dassault, GAF, Grumman, Hawkers, BAE, Shorts, RAAF, Cessna, Canadair, DHC, Swearingen, Beechcraft, Fokker, Scottish Aviation (as it was) probably a few other that I don't recall atm.
Generally, the majority of test points are easy to fly but some can be extremely demanding and require a work up and a few attempts to obtain acceptable data.

PEI3721
You are quoting a structural design standard, not a flight handling standard, and seem to be missing the intent of the standards, and probably misinterpreting what the requirements are.

zzuf, yes missing, misinterpreting - most probable.

The quest was for a practical answer - if rolling 'g' is not limited by the AFM, then do crew have to be concerned by a combined pull / roll during an upset recovery.

My conclusion is no, and that in some circumstances the control input is difficult.
Furthermore, with the surprise in such situations then the likelihood of the crew recalling an obscure limit, if it exists, is remote.

No the limit is shown by compliance with your quoted FAR ie 2/3 of the G loading used in structural design. The aileron angle will be maximum permitted or that limited by the pilot ability to apply the aileron eg pilot strength.
To find out what is required to be demonstrated in flight read the flight handling and performance parts of FAR 25. To get some idea of how to do, read the Advisory Circular - Flight Test Guide FAR25.
There are plenty of ways to break aircraft that are not covered by limitation in the AFM. What is the maximum rate of descent for landing, what is the maximum sideslip angle?
Also remember that once the aircraft is certificated there is no need to repeat any of the certification procedures.
A classic is training sections requiring a demonstration of VMC, in flight. That is a really good way to set up a loss of control situation.

zzuf,
It's along time ago, age, and feeble mind. (been there, done that, can't remember)
For simplicity, a practical situation, pilots perspective: aircraft 2.5'g' AFM limit, with roll and pitch upset.
What 'g' should the crew observe during a combined roll and pitch recovery; noting post # 2 Colonel Klink.

Roll and pitch combined upsets require special attention.

The loads to speeds graphs appearing in the aircraft flight envelope charts are for a single input deflection.

Engineers consider each axis separately in designing for the air loads accompanying a full control at any flight speed up to Maneuverin Speed (Va) in aircrafts flight envelopes.

Inputs on more than one axis simultaneously need to be taken into account separately.

Two factors need to be kept in mind during a banking pull:
The pull to counter the centrifugal forces and the increased apparent weight of the bank , are two forces adding to each other.

If the ball is kept in the middle, loads will be symmetrical both the wings and fuselage will feel the same acceleration levels.

The next factor to consider is when rudder is simultaneously applied to the pull wether in a bank or not this increases the load on the rudder’s side wing.
Inputs in more than one axis, like inside rudder while pulling then unloading (Aerobatics pilots know this maneuver as a "Flick" or "Snap Roll) will load inside wing approx 30% more than the G meter will display on fuselage panel G meter.

"Design loads" are another story they are structural "G" margins accounted for in certification as safety buffers expressed in percentage:

In "Normal" category, G loads are factored at 3,8G's + 50% margin; Structural failure should not occur before 3,8G's + 50% = (5,7 G)
In "Aerobatic" category positive G loads are min 6 G's + 100% = (12 G)

Rolling pull out can be dangerous. One example of this was at the Werribee (Victoria) bombing range in late 1950's. The aircraft was a RAAF Wirraway doing practice dive bombing. The briefed session was a 45 degrees angle of dive using partial power. Release the 25 lb bomb at 1000 ft followed by a straight ahead pull up to 1000 ft then join the circuit for another bomb run.

On this occasion the pilot decided he could bomb more accurately by steepening the dive to 60 degrees and with closed throttle to keep the speed in check. Witnesses saw the aircraft pull up at well below 1000 ft and immediately roll into steep climbing left turn to join the downwind leg of the circuit. One wing broke away and the aircraft crashed killing the pilot. The cause of the accident was found to be asymetric G forces on the wings.
The offcial accidenr report in part stated:
During a recovery from a dive bombing pass over the Werribee Air Weapons Range 1204hrs 01/04/58 the aircraft was overstressed resulting in the catastrophic failure of the port wing which then caused the aircraft to crash. Pilot was Warrant Officer E F Dillon was killed instantly. .

This is the first time I’ve come across this.

Are you suggesting in the example of a 10 degree nose pitch down upset with significant bank on, that if I roll at the same as pulling up to wings level and an attitude of (around) 0 degrees, and as long as this is in balanced flight, that the G loading on both wings is symmetrical?

Because my understanding is that it is not - and if the upset happened to be a 45 degree bank to the right with significant nose pitch down, that rolling and pulling at the same time towards wings level would result in the right hand wing experiencing a far greater load factor than that of the left hand wing. Hence the need to roll to approximately wings level prior to applying any substantial G loading in the subsequent recovery.

Happy to be corrected though if I have any of that wrong!

I think that the statement is essentially correct.
I flew the A4 Skyhawk and we did our share of ai to mus. The Squadron AEO became concerned at the number of worn rivets, predominantly on the RH wings. He had a long discussion with the Squadron Senior Pilot, who agreed that the pilots were pulling and rolling at 4+ g during the dive recovery.
The upshot was the Squadron AWI rebriefed us on dive bombing recovery: Straight pull at 4 g to 20 degrees nose up, release the g then roll to desired bank, then reapply the g. Voila the rivets miraculously stopped wearing out their rivet holes and brown streaks on the wing surface thinned out.
Much, much later, my airline began UPRT training on the guidance of senior Standards Captains. When I piped up and mentioned rolling g, I was laughed at, but I stuck to my guns and said that the limit is 2.5 *.66 or you will have airframe damage.the Standards Captains did at least go away and do their research. They changed their briefs to include mention of rolling g.
But the overstressed pilot staring at a window of brown and green is not going to think about rolling limits until much later.

Interesting the right wings damage; my first paraglider turned right continuously - thought it was me being cack handed - the lines showed no differences so it was a deformed canopy. Discussed it with my VC10 instructor who after retiring worked with the paragliding world champion who stated it was over stressing pulling out of a spiral decent; surprisingly occurred with right handed but also left handed pilots with all wings turning right after being over stressed.

Thank you for your remark,
I can relate my observation to stalling which is function of G load. Ther may be other factors
In a stall while establishing an angle of bank first, then pulling to the break, if ball is in the midle the aircraft will mush
in case the ball is not in the middle one wing will fall and it's the one where rudder is applied. if rudder is not countered it will start to roll.

This if pilot first puts aircraft in a bank, neutralses the bank and then pulls,
Need to check for behavior if one continues to bank while pulling.

We have aerobatic aircraft with recording G meter, and can give it a try will be happy to come back with my findings.

If there is a test pilot somewhere who can help, will be highly appreciated,

The spiral descent as its referred to here is a killer as speed goes up exponentially (Conratry to a spin when its low and stable) and stall speed increases, This leads to an accelerated stall where stall speed is function of the square root of the G load

Some excellent points…..

While rolling G limits should be mentioned during upset training the training itself should emphasize no pull while rolling to wings level. Any G you load the wing to slows the roll capability and accelerates you toward the place you don't want to go to.

A very useful and valuable discussion for those who might read it. As a now pilot theory instructor, I try to push the takeaway to be along the lines of "if you are pulling hard while rolling, pull a bit more gently". Most pilots have no objective way to measure what loads they might be applying, so a generic approach might be the next best way to reduce the chance of their killing themselves ?

JT Good advice
It should not be lost on people that for the majority of the airborne time, a jet transport does not have the manoeuvre margin to get even close to the 2/3 limit load.
Also, the design limit rolling input could be seen as fairly aggressive: at Va - full roll control, at Vc - the same roll rate at achieved at Va, at Vd - one third the roll rate achieved at Va.
Interestingly, during maximum rate turn manoeuvres fighter pilots, to avoid reducing the G loading, change bank angle to correct pitch attitude and airspeed speed error. The roll inputs, in these cases are relatively small.

While I have the feeling you're on the right way, I'm not sure I follow you completely here so let me try to clarify: to get certified, transport category aeroplanes have to withstand 3.75G positive without structural damage. Some terms & conditions also apply, the ımportant for us it is straight pull, no rolling. What you and Megan have posted is the requirement there would be no naughty surprises if the roll is ıntroduced at two third of that, which is (not at all coincidentally) 2.5G, the AFM limit. So while you are right that properly designed aeroplane should withstand roll abuse at its positive G limit, making advantage of it makes no sense operationally as one has no idea how much G one is pulling without G-meter and it can get you quickly into the uncharted envelope territories, not to mention it's harder to maintain spatial orientation while fighting the aeroplane in two axes simultaneously. Thence the folks who believe that 0.666 factor applies to the AFM limit are actually advocating the right stuff for the wrong reason.

Hi Clandestino, the problem with your argument is that structural damage, to an airframe, is permitted at loadings between the limit load and the ultimate load. The damage is not permitted to be catastrophic until ultimate load.
FAR 25.349 is clear that the 2/3 factor is applied to the design manoeuvre load.
Attempts, by pilots, to develop aircraft handling procedures by reference structural design standards can be fraught, particularly if you don't understand the intent of the of the standard and the history of its development.
As I wrote before I would encourage study of AC25-7D, especially
CHAPTER 5. FLIGHT: CONTROLLABILITY AND MANEUVERABILITY

In my extensive collection of books I have got one, "Stalls Spins and Safety" by Sammy Mason, which adresses this issue in one of its paragrahs.
Now I have moved abroad for pension and have most of my stuff in boxes due to lack of space. Just in case somebody has the book i'd appreciate to let us know.

When I have a moment I will go in my garage and see if I can find it.
It adresses both climbing turns and descending turns if my memory is correct.

One point though, during my flying career (As a hobby, it was not my profession) I have owned many aircrafts, all aerobatic, and I have tried to correlate aerodynamics effects experienced in flight with the fundamental of physics being fond of the subject.

Subject that even with the correlation between theory and practice can still leave many questions unanswered to me. That is why also above I mentionned if there would be a test pilot somewhere who reads this, to get involved. Cheers to everybody, am happy to find out there are many passionate and knowledgeable pilots !!!!

I disagree. I think the wing with the down going aileron (and consequently generating more lift) will fail before the other wing in a rolling pull-out, even if the ball is central.

JT, zzuf, Clan, et al, interesting views.

zzuf, 'Limit is shown by …'; actually its demonstrated for certification (playing with words - the manufacturer shows that the requirement is met).
However, the lowly flight-crew have no indication, no show and tell, no instrument or tech description; thus take it easy as JT #29.

zzuf, Back to #20, or without load indication, not knowing doesn't matter because you can't get there #30 ?

Clandestino, we are facing the same direction.
However, is your application the 0.666 factor to the AFM 'normal g' limit a precaution because the pilot has no indication (good practical advice), or a real limit to be respected ?

Three Wire, re A4; as I recall a high 'g' pull up with asymmetric slats (one hangs up - not enough grease) resulted in significant rolling 'g'. There was a video of a TA4J shedding a large drop tank.

It's imperative that one must roll the wings level then pull out of the dive...
below may help a little. Just change Va to Vp it's confusing but where I got Vp from is Aerodynamics for Naval Aviators and he calls Vp maneuver speed...I keep forgetting to write the correction so I'll do it now.

https://www.pprune.org/3704889-post16.html

If you wish to become pedantic about compliance with certification standards, perhaps this will help:

25.1 Applicability.

(a) This part prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for transport category airplanes.

(b) Each person who applies under Part 21 for such a certificate or change must show compliance with the applicable requirements in this part.

There are plenty of times where "acceptable means of compliance" can mean other than demonstration.

Your comment on "not knowing because you can't get there" was done to death before any certification authority accepted the manoeuvre limiting features of most FBW aircraft. I suggest some research into the development of the current FBW standards.

Your suggestion that 2/3 factor could even be considered to be due to what instrumentation is fitted does not stand much scrutiny for what is a structural design standard.
Strictly speaking, there is no limit in the AFM this is a structural design standard for which compliance has been SHOWN during certifcation. There are plenty of structural design standards not mentioned in the AFM.

The suggestion that transport aircraft should be fitted with accelerometers has been discussed many times as are "alpha" indicators. Demonstrably, the certification authorities have not been convinced.

PEI ,You are correct, but we learnt to sense and give a little rudder input to force the slat to release.
The problem for us in academic bombing was that the circuit was right handed, hence the brown lines from fuel seeping from worn rivets. There was an approved fix; a "rubber bomb" introduced to the wing via inspection panels that reinforced the tank lining.

where I got Vp from is Aerodynamics for Naval Aviators and he calls Vp maneuver speed

An old, but good, book referring to old references. What is Va was Vp in CAR3 at p3.184. If you can't be fussed digging CAR3 out, I put a bit of a story on Bob Tait's website at Bob Tait's Aviation Theory School - Maneuvering Speed with weight - Bob Tait's Aviation Theory School Forums Hopefully I didn't waffle on too much into nonsense stuff in the thread

Thanks a million JT! And for a very very simple easy to read, easy to understand well written treatise on the topic!