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Normal G limits vs Rolling G limits ?

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Normal G limits vs Rolling G limits ?

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Old 22nd May 2009, 05:55
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Normal G limits vs Rolling G limits ?

How much lower are rolling G limits vs normal G limits ?

When I've flown aerobatic aircraft I couldn't find anything in the flight manuals specifying a value

I've heard in some military aircraft (F-16?) normal G limit is +9G but rolling G limit is something like +5G

Can anyone elaborate ?
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Old 22nd May 2009, 08:42
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I have never seen a defined limit as there are too many variables (how effective are ailerons etc, so it must be type-specific) however the general guide is 2/3 max g limits to my knowledge. However I write only as a light aircraft type so I wait with interest for more informed replies.
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Old 22nd May 2009, 11:39
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It's not just a function of aileron effectiveness - it's a function also of the roll/yaw inertia coupling on a given aircraft, the amount of sideslip that may build as beta and alpha are exchanged, the degree to which, say, fin strength margins are in excess of the average, and many other factors.

The amount of time you extend the roll also matters - again, it gives longer for the sideslip to build. The Hawk, IIRC, has a whole family of limitations as a function of total bank rolled and stores configuration - some are quite restrictive.
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Old 22nd May 2009, 16:44
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Based on my personal experience, my opinion is that the limitation is due quite only to the fact that the down-going aileron ( that of the up-going wing) will cause an added load on an oalready g-loaded wing. In other words, if you make a 6-g pull-up, together with a co-ordinated roll to right (one where no side-slip is induced), the left wing ( already loaded with 6-g from the pull-uo) receives an added equivalent load of 3-g's due to the rapidly down-going left aileron, creating an equivalent load of 9-g's.
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Old 23rd May 2009, 00:26
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general guide is 2/3 max g limits
which is the FAR 23 requirement. The only AFM where I've seen it is the Decathlon which also gives guidance on how much aileron can be applied above Va.
I believe that the CAP 10 manual also has some good guidance.
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Old 23rd May 2009, 15:38
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ISTR pages of very comlicated limitations for the F-4 with its many and varied combinations of pods, bombs, launcher rails, fuel tanks etc. That said the 2/3rds reduction in 'g' limit when rolling seems a good broad brush compromise.
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Old 23rd May 2009, 16:24
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...'twas thought to be rolling 'g' that broke up the 'white' Vulcan many years ago - covered here on PPRuNe. AoA on downgoing wing increases thus increasing the lift loading on it, and obviously rate of roll affects that.

My mate D120A knows a lot about the theory.
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Old 24th May 2009, 07:06
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I have some examples:

In the T-38:

Code:
Fuel               G's 
         Symmetrical    Unsymmetrical 3000 lbs      -2.5 to +6.0     0 to +4.3 2000 lbs      -2.7 to +6.6     0 to +4.7 1000 lbs      -2.9 to +7.2     o to +5.1
How much roll you can have is not defined, but full deflection is only allowed in 1 G flight. As far as I can remember if you had any roll rate, you had to take the asymmetrical limits.

Then the Alpha Jet

Code:
a/c weight               G's 
         Symmetrical    Unsymmetrical 5300 kg      -3.6 to +6.8         -1.5 to +4.6 4500 kg      -3.6 to +8.0         -1.5 to +4.6 3500 kg      -3.6 to +8.0         -1.5 to +4.6
Here the asymmetrical flight is defined as full aileron deflection.

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Old 25th May 2009, 13:27
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Thanks for the responses

I'd hate to have to work through the equations to work these rolling limits out.
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Old 26th May 2009, 23:53
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When I think of this particular hazard to airframe integrity. I see the ailerons as imparting a twisting force through the wings and main spar which I imagine must then fail for a lower g loading than when the ailerons are neutral.
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Old 27th May 2009, 02:00
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Except that for various aircraft, the critical case isn't the wing - it's the rear fuselage or the fin, and the alpha (required to create the 'g') becomes beta as the aircraft rolls not about it's roll axis but about either the flight path axis or some inertia-coupled axis.

Most Hawk rolling 'g' limits (for example) are either the fin coming off (beta=high fin sideload), or an underwing store flying off the pylon (beta = high sideload, plus various nasty inertial effects).... (with the appropriate safety factors, so the bits don't actually break at the quoted 'g' and roll conditions, of course)
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Old 27th May 2009, 12:48
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dh90, not quite the way I understand it. Failure will occur at the same g loading, it's just that assuming you are pulling max g, then any aileron input will increase the loading on the upgoing wing above the max g limit (and reduce it on the other wing) so the upgoing wing may fail. Very simply of course - as has been mentioned there are all sorts of other considerations. The reduced rolling g limit should mean that at that limit (say 4g for normal 6g aircraft), you should be able to apply full aileron without exceeding 6 g on the upgoing wing. However aileron effectiveness, aspect ratio, external stores etc. etc. mean that 2/3 max g can only be a guide.
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Old 29th May 2009, 04:46
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Extract from the T-28B/C manual.

ACCELERATION LIMITATIONS
The maximum permissible accelerations for flight in smooth air, at weights of 8050 pounds or less, are shown in figure 1-27. Accelerations at which buffeting is encountered shall not be exceeded. When flying in conditions of moderate turbulence, it is essential that accelerations, because of deliberate maneuvers, be limited to 4 gs, at a gross weight of 8050 pounds, in order to minimise the possibility of overstress as a result of the combined effects of gusts and maneuvering loads. As gross weights are increased above 8050 pounds, the permissible accelerations decrease. To determine the maximum permissible accelerations at gross weights in excess of 8050 pounds, multiply the accelerations shown in figure 1-27 for smooth air or that given for moderate turbulence by the ratio of 8050 pounds to the new gross weight.

On aircraft equipped with strakes, the maximum permissible acceleration is 5.7 gs when carrying stores. However, when stores are carried in turbulent air, the maximum permissible acceleration is 3.7 gs. This limit is necessary so that the 5.7 g limit will not be exceeded when severe gusts are encountered.

Because rolling pullouts impose additional stress, maximum permissible acceleration is two-thirds the maximum permissible acceleration for a normal pullout.

At indicated airspeeds between 170 and 185 knots, critical combinations of acceleration and buffet can cause overstress. In the clean configuration, the maximum permissible acceleration for flights in smooth air (dynamic stall line) is 5 gs at 170 knots IAS and 6 gs at 210 knots IAS, varying linearly from 170 to 210 knots as shown in figure 1-27. Stick back pressure should be relaxed at the onset of buffet in the 170 to 210 knot speed range.

When carrying stores, the maximum permissible acceleration is 6.0 gs (5.7 gs on T-28C). Maximum permissible accelerations for modified T-28C aircraft are shown in figure 1-28. However, when stores are carried in turbulent air, the maximum permissible acceleration is reduced by 2 gs. This limit is necessary so that the maximum g limit will not be exceeded when severe gusts are encountered.

ARMAMENT (MODIFIED T-28C)

1. On modified T-28C aircraft with six store stations, figure 1-28 establishes maximum permissible accelerations for symmetrically loaded store configurations at 2500 feet or less at a maximum speed of 295 knots in smooth air. However, when stores are carried in turbulent air, reduce the maximum acceleration shown in figure 1-28 by 2 gs.

2. The maximum permissible acceleration for store loadings heavier than 100 pounds asymmetrically is 3.2 gs at 9500 pounds gross weight or greater and 3.7 at less than 9500 pounds gross weight.

3. Rolling pullouts for all configurations may be performed at two-thirds the symmetrical limits. Lateral stick deflections are limited to half throw when stores over 250 pounds are carried at stations 2 and 5.

4. High sink speed landings are prohibited for all store configurations other than 150 pounds maximum at store stations 3 and 4.


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Old 2nd Jun 2009, 10:32
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Hi DB6, the aspect of this that interests me is what happens when rolling G is applied to an airframe at speeds above Va. In other words what happens if a pilot accidentally throws a full aileron deflection and simultaneously pulls a lot of G whilst his airspeed is substantially above Va? ...Surely an airframe can break for twisting forces as well as G?
I am thinking this two thirds G rule that some guys are coming up with surely cannot safely mitigate the additional torsional stresses applied to an aircraft if it is manoeuvred too hard above its advertised Va.
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Old 3rd Jun 2009, 01:07
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dh90pilot, Va is the maximum speed at which full control deflection is permitted. So to answer your question
In other words what happens if a pilot accidentally throws a full aileron deflection and simultaneously pulls a lot of G whilst his airspeed is substantially above Va?
the result would be tears all round for the next of kin, as likely as not.
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Old 3rd Jun 2009, 03:02
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The military Bulldog had a rolling G limit quite a bit lower than the "normal G" limit, explained to me as the upgoing wing "seeing" more G than the meter would indicate.
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Old 7th Jun 2009, 02:13
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Just to be clear, in more than thirty years flying I have personally never used a large aileron deflection above Va. In fact it is a sore point with me, because in the past I have witnessed guys sitting to my left do it. ...In the worst case I was covering the transmit switch on my controls when the guy next to me threw the aileron control to the right so hard I felt it hit the stops and at the time we were 114+ knots faster than our Va. ...I could feel the stress in the airframe and I really did think we were going to break up.
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Old 2nd Jul 2009, 11:09
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Dh 110 Crash Farnborough Airshow (late 50ies?)

Crash at airshow attributed to rolling g, which torques wings front to rear, and in this case caused spar failure.
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Old 3rd Jul 2009, 15:02
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I was involved in an accident investigation where an aircraft had lost half of its port wing.

The conclusion was that the motion of switching from dive to climb and left roll to right roll was contributary to the wing breaking.

It turned out that the forces generated in the manoeuvre were not enough to break the wing but the instantaneous force imparted on the wing from the motion of the aileron combined with the manoeuvre was the additional force to push the wing past it's limit.
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Old 4th Jul 2009, 05:04
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DH110

Quite right, dbboy, about the DH110 at the 1952 SBAC display. The down-going wing produces a higher angle of attack and hence incremental lift somewhere near the quarter-chord point, whereas the deflected aileron produces a bigger down-force nearer the trailing edge. The resulting nose-up moment twists the wing, and might well break it. In addition, of course, an insufficiently stiff wing may lead to an aeroelastic divergence and hence certain wing failure.

Interesting that the DH110 had carried out an identical display all week, and had not come to grief during that rolling-out, pulling-up manoeuvre. However, on the final day the reserve aircraft was used and this lacked an experimental wing fence that was fitted to the first. The wing fence, although being tried for aerodynamic reasons, had the additional structural effect of stiffening the wing in torsion. Its absence on the aircraft used on that final day was enough to make a tragic difference to the outcome of the manoeuvre.
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