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Safety, by type


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It is largely semantics. Acceleration is what does the damage, but in a crash situation the accelerations are likely to be approximately proportional to the kinetic energy.

 

Ai, there's the rub The acceleration (in this case a negative number or deceleration---Delta V) is not directly proportional to the kinetic energy in that the more of the KI the more deceleration must happen over a constant time frame. The other and equally important factor is the time frame in which the KI must be disipated to zero. A relatively small KI when forced to zero over a very short time will yield damaging results, whereas a huge KI forced to zero over a long period of time will yield nothing but heat. Said differently, the same KI will have different results when forced to zero depending on the length of time it has to go to zero.

 

Take your car at 50 mph, drive it into a concrete wall or take the same car, at the same speed and apply the brakes until stoppage. The KI is essentially the same in each situation, however the delta V is radically different.In the case of the concrete the KI is dissipated by some heat, some metal bending (heat) and some bone breakage(heat). In the gentle braking scenario, the KI is dissipated into heat, mainly by the brakes with some possible straining of the passengers on the seat belts and the physiology necessary to resist that with it's attendant production of heat as well as other factors. We may see this phenomenon in softening of gear legs if they are not shielded properly.

 

To see why this is true without doing the maths, consider car braking distances. Drivers should know that braking power is limited to a fixed deacceleration.

No, Braking power is definitely not limited to a fixed deceleration. It may have a fixed maximum. The deceleration of braking is, to the point of maximum, determined by the nut behind the wheel. You can stop gently, with only smiles on the faces of your pax, or you can slam on the brakes and suffer the comments of your pax as well as possible injuries caused by our friend delta V.

 

 

If you double your speed your braking distance quadruples. To stop within the same distance, you have to quadruple your deacceleration also.

 

I can see an argument that if you're moving faster you're likely to have a longer distance to deaccelerate, provided you don't hit a cliff. For example, if you fire two bullets into water, a high velocity round will penetrate deeper. If you land and hit a hedge, perhaps you'll penetrate further through and the acceleration will not be quite proportional to the square of the velocity.

 

Essentially my point, only if the braking force is constant (we won't consider brake fading here)

 

On the other hand, if you land with a high sink rate or nosedive into the ground, most of the deacceleration will be provided by the landing-gear and aircraft structure, rather than the ground. There's a fixed amount of structure, whatever the change in velocity.

 

Yea, However in a three dimensional world you must consider deceleration in three dimensions. Vertical deceleration is as important as horizontal. ie crushed spines, etc. With the vertical deceleration, the landing gear and aircraft structure merely serve to decrease the delta V by their abosorbtive qualities. Put a similar landing gear setup on the nose of the aircraft, with the same (identical)mechanical properties and attachments, if you hit something with the same KI, the resultant effect to the fuselage on the horizontal axis will be the same. (of course when flying, those bull horns sticking out into the breeze might be a detraction.)

 

 

It's also absolutely clear that if you have a given amount of kinetic energy and you reduce this to zero by stopping, all that energy has to go somewhere. Conservation of energy is a fundemental law of nature. Ideally it will all be dissipated by the airframe, hedge and by digging up earth or heating water during a water landing. In the worst possible scenario it all gets dissipated by mashing the pilot to a pulp, leaving the rest of the world intact.

 

Yea, and in a normal landing it is transformed harmlessly to heat by the brakes. (of course there is wear of the pads and the disks)

 

 

This isn't an entirely theoretical problem - early protective paragliding harnesses were rigid carbon fiber, and could survive just about any paragliding accident unscathed. They actually increased pilot injuries though.

 

Again exactly my point. Greater delta V

I Canardly contain myself!

Rich :D

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Ai, there's the rub The acceleration (in this case a negative number or deceleration---Delta V) is not directly proportional to the kinetic energy in that the more of the KI the more deceleration must happen over a constant time frame.

First of all, delta V is the change in velocity, which is not the same as acceleration. Acceleration is dV/dT. If you are travelling at 20mph and reduce speed to 19mph your delta V is the same as if you decrease velocity from 10 to 9mph. The time taken is the same - assuming constant deacceleration - but the distance is doubled. If you limit the distance (as in a crash situation) then the time is not constant.

 

Perhaps my braking example was confusing. I was referring to emergency stopping distances, where the acceleration is (approximately) equal at all speeds and limited to a fixed proportion of the acceleration due to gravity. If you have a coefficient of friction of 1, the largest deacceleration you can maintain without skidding is 10m/s/s. Typically cars seem to manage 0.5-0.8 of this value. There are caveats (sports cars can produce downthrust at high speed) but at lower speeds it's basically correct.

 

I obviously wasn't claiming that a car couldn't brake gently, or that it's harmful to land an aircraft at high speed even though it can dissipate the energy in the brakes. We're talking about crashes and accidents here.

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First of all, delta V is the change in velocity, which is not the same as acceleration. Acceleration is dV/dT. If you are travelling at 20mph and reduce speed to 19mph your delta V is the same as if you decrease velocity from 10 to 9mph. The time taken is the same - assuming constant deacceleration - but the distance is doubled. If you limit the distance (as in a crash situation) then the time is not constant.

 

Perhaps my braking example was confusing. I was referring to emergency stopping distances, where the acceleration is (approximately) equal at all speeds and limited to a fixed proportion of the acceleration due to gravity. If you have a coefficient of friction of 1, the largest deacceleration you can maintain without skidding is 10m/s/s. Typically cars seem to manage 0.5-0.8 of this value. There are caveats (sports cars can produce downthrust at high speed) but at lower speeds it's basically correct.

 

I obviously wasn't claiming that a car couldn't brake gently, or that it's harmful to land an aircraft at high speed even though it can dissipate the energy in the brakes. We're talking about crashes and accidents here.

You, of course are right about delta V. It seems like we are really talking about the same thing. It was a pleasure sparring with you. Perhaps others benefited from our clarifications.

I Canardly contain myself!

Rich :D

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You, of course are right about delta V. It seems like we are really talking about the same thing. It was a pleasure sparring with you. Perhaps others benefited from our clarifications.

If this jest is over, I am ready to get out my slide rule (used to be pretty fast on the draw) or my chess set and challenge any of you to a duel !!!

WTJohnson

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If this jest is over, I am ready to get out my slide rule (used to be pretty fast on the draw) or my chess set and challenge any of you to a duel !!!

WTJohnson

 

If you get out your slide rule I may have to start cursoring

I Canardly contain myself!

Rich :D

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