No abrupt manuevers with high G-loading

[CJones]

A few years ago at a formation flying clinic, one of the speakers was discussing why not to do abrupt maneuvers (namely sharp turns) when in a high-G situation. He was essentially addressing the "low-pass, pull up, turn hard" that people seem to like to do.

The rule of thumb I came away with was "Don't turn hard when the wings are 'loaded'".

The other night, I was trying to think of how the math behind the 'rule' worked in a way that I could explain it to someone else (possibly future students, etc.), but I couldn't get the numbers straight in my head.

Anyone here want to take a stab at explaining the math behind the "Don't turn sharp/hard when the wings are under high-G loading situation"? All I can remember is that it seemed to have to do with the different type of force placed on the wing (?twisting?) when doing a hard turn and how that increases the load on the wing exponentially.

The person that explained this at the clinic used to visit this board from time-to-time, but I haven't seen him here in quite a while.

 

[tonycondon]

chris to me it seems that if you are pulling G's (high AoA) and then do a quick turn you risk stalling the wing that should be going up in the turn, thanks to a higher AoA on that wing thanks to a quick aileron deflection.

 

[t0r0nad0]

I have a couple of quick guesses, but not much time to type them, so I'll be concise:

1) When you bank, you decrease the lift of one wing while increasing the lift of the other. This causes an uneven G-Loading which can torque the airframe.

2) When in a bank, you now have centrifugal force to contend with as well as the G-load from the sudden pull-up, now you are increasing your G-Load and could be over-stressing the airframe.

I could be way off, but these are the first things that came to my mind.

 

[CJones]

chris to me it seems that if you are pulling G's (high AoA) and then do a quick turn you risk stalling the wing that should be going up in the turn, thanks to a higher AoA on that wing thanks to a quick aileron deflection.

True. That makes sense.

IIRC, the point the speaker was making was that sharp turns would exceed the G limit of the wings faster than a normal pull-up or other 'equal' application of G-loading to both wings.

 

[jesse]

Banking does have some value.

I do tend to do a lot of low passes and I do them pretty low and fast. I *do not* do them high G, primarily, because I don't see the point. Making it high G doesn't make it cooler.

That said, I do almost knife-edge at the end of the low-pass. I smoothly transition from the pass into a very steep climb. Eventually you reach a point where that climb will become a stall which I'd rather avoid. A really easy way to avoid the stall is just to bank the airplane to near knife-edge which will then cause the nose to drop naturally, gently, no aggressive shove-over required. My crop-dusting dad taught me this as the best way to get out of an aggressively nose-high attitude without stressing anything or stalling. It seems to work well.

 

[tonycondon]

True. That makes sense.

IIRC, the point the speaker was making was that sharp turns would exceed the G limit of the wings faster than a normal pull-up or other 'equal' application of G-loading to both wings.

well you also increase G loading with bank angle. 60 degrees of bank doubles your G. plus manuevering speed is based only on elevator deflection, not full and abrupt control movement in any/all axes.

 

[CJones]

Banking does have some value.

I do tend to do a lot of low passes and I do them pretty low and fast. I *do not* do them high G, primarily, because I don't see the point. Making it high G doesn't make it cooler.

That said, I do almost knife-edge at the end of the low-pass. I smoothly transition from the pass into a very steep climb. Eventually you reach a point where that climb will become a stall which I'd rather avoid. A really easy way to avoid the stall is just to bank the airplane to near knife-edge which will then cause the nose to drop naturally, gently, no aggressive shove-over required. My crop-dusting dad taught me this as the best way to get out of an aggressively nose-high attitude without stressing anything or stalling. It seems to work well.

That's how I've been doing my runway inspection passes as well, although I do pull a couple G's at the immediate pull-up (although not enough to get into an accelerated stall). Once I am in an established no-G climb, I will roll into a sharp turn with no problems.

The issue addressed by the speaker was combining the pull up with the sharp turn at the same time.

 

[CJones]

I have a couple of quick guesses, but not much time to type them, so I'll be concise:

1) When you bank, you decrease the lift of one wing while increasing the lift of the other. This causes an uneven G-Loading which can torque the airframe.

2) When in a bank, you now have centrifugal force to contend with as well as the G-load from the sudden pull-up, now you are increasing your G-Load and could be over-stressing the airframe.

I could be way off, but these are the first things that came to my mind.

I think you are really close. I'm just trying to remember the math behind it. There was some sort of formula presented that showed how fast you can overload the airframe at given G factors combined with roll rate. Or something like that.. It's been at least 3-4 years, so I've forgotten the details.

 

[gismo]

Actually, as long as your roll rate is constant (and the plane is coordinated) there's no additional lift being generated by the rising wing, it's just operating at a different angle of incidence than the descending wing. OTOH if the roll rate is increasing (as it would have to be while initiating the roll) or decreasing then the lift on one wing is more than the lift on the other. This is because lift that's not balanced by weight (or weight times g loading to be more accurate) must result in acceleration, not just motion. The same situation applies to lift in a constant rate climb where the lift must equal weight or the airplane would accelerate upwards with increasing vertical speed (ignoring the thrust vector).

In addition, the certification requirements dictate that when flying below the maneuvering speed (Va), full deflection of the controls cannot cause a permanent deformation of any aircraft structures. AFaIK this is true whether the plane is ballistic at 0 g or on the verge of a Va stall, but I could be wrong about that. It has been discovered that rapid alternating full deflection of a control (e.g. an Airbus rudder) can lead to structural failure at or below Va in an airplane that meets the current certification rules.

So it would be possible to increase the load on one wing by rapidly rolling into or out of a turn and if the load was already at the maximum allowed and the airspeed was high enough (i.e. above Va) to allow excessive forces to be developed, the combination of high balanced loading and a rapid increase (or decrease) in roll rate could cause permanent damage.

BTW, the roll rate or aileron deflection isn't directly the issue here, just the rate at which the roll rate changes. Normally a fixed aileron deflection produces a constant roll rate due to the roll's effect on each wing's AoA, so what should matter most is how rapidly you move the stick (not how far).

I have a couple of quick guesses, but not much time to type them, so I'll be concise:

1) When you bank, you decrease the lift of one wing while increasing the lift of the other. This causes an uneven G-Loading which can torque the airframe.

2) When in a bank, you now have centrifugal force to contend with as well as the G-load from the sudden pull-up, now you are increasing your G-Load and could be over-stressing the airframe.

I could be way off, but these are the first things that came to my mind.

 

[Capt. Geoffrey Thorpe]

well you also increase G loading with bank angle. 60 degrees of bank doubles your G. plus manuevering speed is based only on elevator deflection, not full and abrupt control movement in any/all axes.

Bank angle has no effect whatsoever on G loading. Pulling on the yoke increases G loading - not banking.

So at 60 degrees of bank, if you manage to pull just enough to maintain a level turn you will double the G. But if you don't pull that much you won't double the G, or if you pull more, you will more than double the G. If you don't pull at all, the G loading won't increase at all no matter what the bank angle (but the nose will drop) .

 

[tonycondon]

Bank angle has no effect whatsoever on G loading. Pulling on the yoke increases G loading - not banking.

So at 60 degrees of bank, if you manage to pull just enough to maintain a level turn you will double the G. But if you don't pull that much you won't double the G, or if you pull more, you will more than double the G. If you don't pull at all, the G loading won't increase at all no matter what the bank angle (but the nose will drop) .

yea. ive been staring at a computer for too long. starting to go brain dead.

 

[Capt. Geoffrey Thorpe]

yea. ive been staring at a computer for too long. starting to go brain dead.

It doesn't help that 90% or more of the training stuff tries to tell you that increasing bank increases g / stall speed. And, last time I looked, the first question in the FAAs list of questions was something like "which maneuver increases the stall speed" with the 'right' answer being "Bank". After a while you start to think that it might be real...

 

[Doggtyred]

2) When in a bank, you now have centrifugal force to contend with as well as the G-load from the sudden pull-up, now you are increasing your G-Load and could be over-stressing the airframe.

I concur with this summary..

The several T34 accidents that occurred over the past decade occurred in air combat vendor aircraft that were involved in yank-bank scenarios that exceeded limits.. repeatedly.

A good way to stay out of trouble is to do what the military jet jocks do..
bank, stop bank, then yank, then stop yanking.. dont do both at the same time.. That doesnt mean you cant yank while wings are not level, it just means dont ADD bank while yanking or ADD yank while adding bank..

 

[kyleb]

The issue is assymetric g's which reduce effective load limits. Let's say you're flying an RV-6 which is rated to 6 G's. You're pulling 6 g's and decide to add roll to the equation.

To roll, you have to generate more lift with one wing than the other. So the aileron on that side goes down by enough to create the additional lift required to generate the roll you desire. That additional lift is additive to the 6 g's you're already pulling. And that additional lift is created by the aileron and is (almost) entirely transmitted to the aft spar. Which is now bearing its design loading of 6 g's plus whatever additional load the aileron imposes, taking it to beyond its design limit.

This is why the operating limitations on military aircraft include a section on rolling (or asymmetric) g's. Typically, the g limit in rolling maneuvers is ~1/3 lower than in symmetric maneuvers. Extend that to my RV, and I have an airframe that is good for about 4 g's during a rolling maneuver.

FYI, Douglas B-26's were notorious for failing rear spars during rolling pull-up's...

 

[CJones]

The issue is assymetric g's which reduce effective load limits. Let's say you're flying an RV-6 which is rated to 6 G's. You're pulling 6 g's and decide to add roll to the equation.

To roll, you have to generate more lift with one wing than the other. So the aileron on that side goes down by enough to create the additional lift required to generate the roll you desire. That additional lift is additive to the 6 g's you're already pulling. And that additional lift is created by the aileron and is (almost) entirely transmitted to the aft spar. Which is now bearing its design loading of 6 g's plus whatever additional load the aileron imposes, taking it to beyond its design limit.

This is why the operating limitations on military aircraft include a section on rolling (or asymmetric) g's. Typically, the g limit in rolling maneuvers is ~1/3 lower than in symmetric maneuvers. Extend that to my RV, and I have an airframe that is good for about 4 g's during a rolling maneuver.

FYI, Douglas B-26's were notorious for failing rear spars during rolling pull-up's...

I think you just hit the nail on the head.

This was at an RV Formation Flying Clinic where I heard this for the first time, by the way. You know... Those hot-shot RV guys and all..

 

[poadeleted20]

Kyle's got it -- rolling pulls run the g-load higher on the "up" wing than the g-meter in the cockpit says you're pulling, and that can lead to structural failure if the g-meter says you're already at or near the limit. This was really pounded into our heads when we learned dive bombing.

 

[gismo]

Kyle's got it -- rolling pulls run the g-load higher on the "up" wing than the g-meter in the cockpit says you're pulling, and that can lead to structural failure if the g-meter says you're already at or near the limit. This was really pounded into our heads when we learned dive bombing.

Once again, I believe it's not the rolling that produces higher loads, it's the rate at which the roll changes. This might be a rather subtle difference as you must change the rate of roll (at least twice) to enter a bank but it seem to me that it's something any pilot concerned about combining banking with high Gs.

Consider an airplane with symmetrical wings rolling rapidly while on a zero G ballistic parabola. Which wing is generating more force at the wing root?

 

[Henning]

Pull a high g load and you risk an accelerated stall, if you add a steep turn with a bit of skid in it and guess what, you just did a snap roll. That's how you do them, yank and stomp simultaniously.