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In an accident where a flight instructor performed multiple aerobatic manoeuvres in an uncertified aircraft, the load on the airframe caused it to break apart.

I know in this accident the instructor was performing extreme techniques that involved maximum elevator inputs as well as rolling, so my question is - if you simply perform a full 360 degree aileron roll to the left or right, how much G-force does that exert on the airframe?

I ask, because you could theoretically perform this in any light aircraft, so I wonder what prevents so many capable machines (i.e. most ultralights) from being aerobatic certified.

Cloud
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    Do you mean an Aileron roll or a Barrel roll because the answer will be very different WRT forces exerted (I know your title said Barrel, but then your body just says "360 degress roll"). – Jamiec Jan 03 '24 at 12:34
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    The answer is: It depends completely on how many Gs you pull. And that has little to do with the geometry of the maneuver itself. You can do a very low G barrell roll, (done slowly and carefully), and you can do a high G snap roll, that can overstress the airframe. The reason that barrell rolls are of concern is that unless they are done carefully, during the nose low descending portion of the maneuver, it is easy to allow the pitch attitude to get too low, and airspeed too high, where you are more likely to overstress the airframe pulling out of the dive. – Charles Bretana Jan 03 '24 at 13:12
  • @Jamiec After reading this: https://aviation.stackexchange.com/questions/26173/what-is-the-difference-between-a-barrel-roll-and-an-aileron-roll?rq=1 I mean aileron – Cloud Jan 03 '24 at 14:33
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    Obligatory low-g roll video: https://www.youtube.com/watch?v=V9pvG_ZSnCc – Anonymous Physicist Jan 04 '24 at 02:38
  • The other obligatory roll video: https://www.youtube.com/watch?v=Ra_khhzuFlE – Mark Jan 04 '24 at 03:51

3 Answers3

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simply perform a 360 degree roll

It's not so simple. In the process of rolling the aircraft from wings level 360 degrees to wings level with only aileron inputs a lot of lift is lost, resulting in the aircraft tending to dive.

Once the aircraft exceeds its manuvering speed, pulling out of the dive too aggressively can cause structural failure of the aircraft.

To give go some idea of how much trouble you're in if you wind up too fast in a 20 degree dive: Full thrust of a Cessna 172 at 100 knots, maybe 400 lbs. Gravitational thrusting force: sine 20 degrees × 2400 lbs = around 800 lbs. You pull the throttle back but it's too late, the plane is accelerating, making it even harder to pull out of the dive.

2 choices: break the plane or hit terrain. Simply too dangerous.

The diving issue is exacerbated by forward set weight with tail downtrim, which increases the tendency to nose down while sinking. While a pure longitudinal roll puts very little G stress on the airframe or pilot, rudder and elevator inputs must be added to help keep the nose from dropping. A more rearward CG placement also lessens the tendency to dive by reducing the longitudinal stability of the aircraft.

Proper pilot training and knowledge of the aircraft "safe" envelope is essential to safely performing even slow rolls.

Jpe61
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Robert DiGiovanni
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  • Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed. – Ralph J Jan 08 '24 at 01:20
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If executed PROPERLY, both an aileron roll and a barrel roll should each be a 1g maneuver throughout. The key there is to do it properly throughout. Get an entry attitude wrong, apply elevator pressure incorrectly or end up in a dive attitude close to the ground and it’s easy to rush and load the airplane up, combining bending and torsional loads on the airframe at dangerous levels.

A snap roll, or a flick roll, is a different kind of animal. This maneuver requires an abrupt pull up combined with a hard yaw input, in order to stall one of the wings. Do this kind of a maneuver in an airplane not approved for them, combined with speeds well above Va, and you’re at a high risk of structural damage or failure.

As the narrator in the video points out, he had exceeded the aircraft maneuvering speed Va in the last maneuver. Combine that with a hard pull, and you’re very easily could exceed the structural load limitations of the airplane.

It’s just another case of an idiot flight instructor who thought he knew more than he did and ventured out into dangerous waters. He became another tragic case of people learning the hard way why you don’t break the law and regulations. The laws of physics caught up with him faster than the FAA could and three people lost their lives as a result.

Romeo_4808N
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  • That’s a slow roll, not a barrel roll or aileron roll. They’re flown differently. – Romeo_4808N Jan 03 '24 at 17:08
  • See https://aviation.stackexchange.com/questions/47455/can-a-barrel-roll-really-be-executed-at-a-constant-1g – sdenham Jan 03 '24 at 20:04
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    @Romeo, You can't do a Barrell roll without exceeding 1G. The last quarter of the maneuver is rolling out of 90 degrees of bank and raising the nose from a nose low attitude, (a dive), back to wings level flight. The plane will not pull out of the dive unless you put more than 1 G on it. – Charles Bretana Jan 03 '24 at 20:31
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    "If executed PROPERLY, both an aileron roll and a barrel roll should each be a 1g maneuver throughout." -- that can't be true. If so, they would each follow the same trajectory. The truth is, people's definitions of these rolls vary widely. Esp for barrel roll but also for aileron roll. (E.g., is heading constant? Or not? Is ball centered? Or not?) But I don't believe that any semblance of either one could be executed holding exactly 1 G. Maybe after an initial pullup and excluding a final pullout, it might be possible, depending on the exact definition at play. – quiet flyer Jan 05 '24 at 17:05
  • Not necessarily true. For instance, inverted at 1G, the airplane is not tracking the same vector as it was in level flight at one G. – Romeo_4808N Jan 05 '24 at 19:01
  • @Romeo, yes, you are correct, *Any* G applied will cause the aircraft flight path to deviate from the purely ballistic (zero-G) flight path, in the direction or orientation of the lift vector, (determined by the bank angle). The higher the G load, the greater the deviation. – Charles Bretana Jan 06 '24 at 14:03
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An Extra 300 has a wingspan of 7.39 m and a maximum roll rate of 400 degrees per second. The centripetal acceleration is $r\omega^2$. I do not know exactly where the axis of rotation is, but the maximum centripetal acceleration is around $3.7\times7^2=181$ m/s$^2$ or around 18 g near the wing tips. There would also be vertical acceleration.

Yes, in an aerobatic airplane at maximum roll rate, the wing tips of the airplane experience very high acceleration. The parts near the axis of rotation, which is most of the parts, experience less acceleration.

As the other answers mention, any maneuver can break a plane if you do it very badly. An aerobatic wing is very strong against tension.

Anonymous Physicist
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  • Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed. – Ralph J Jan 08 '24 at 01:22