Ravin Aircraft down in Camperdown - 15 March 2014

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Re: Aircraft down in Camperdown.

Unread post by jimdavis » Tue Oct 13, 2015 5:04 pm

Volo wrote:From what I have read and understood about this aircraft , I would put my money on over control / PIO .
Very easy to do at or near VNE in turbulence and even if an experienced hand is sitting in the other seat , his / her intervention would be too late .
Yep, Volo, my thoughts exactly. Apparently the aircraft is very sensitive in pitch, and very slippery - so it would be easy to let the airspeed run away.

@Mike, I am sure we are on the same page - but Vne is actually a function of TAS not IAS (strangely) but I am sure you know that.

I seem to remember reading that this aircraft has an extraordinarily high Vne - can't remember the figure. Wasn't there also discussion about some repair done to one of the flying control surfaces? That could certainly initiate flutter - then the rest is history. (sorry, I am too lazy to dig back through 25 pages to find out what was said).

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Re: Aircraft down in Camperdown.

Unread post by Big Will » Sat Nov 07, 2015 12:11 pm

I have seen reference made to Va and how it might change with regards to loaded weight.
From the perspective of the stress in the wing spar (assuming a steady load), it makes no difference. A 1000kg plane (gross) will generate 3800kg of lift just as it is about to stall at Va. The stress in the wing is largely due to the amount of lift produced as well as the wing torsion at that speed. All components of mass in the plane will experience 3800/1000 = 3.8g acceleration.

If you now fly at a reduced load, say 800kg, at the same speed, Va, and pull up to the point of stall, the wings will still produce 3800kg of lift. The stress in the wing spar will be the same (same bending moment, same torsion). However, the acceleration experienced by the airframe and its occupants is now 3800/800 = 4.75g. The bolts or rivets of any part of the airframe that is not directly supported by the air load itself will experience increased stress. So if something like the battery installation (or any other item of mass) was only designed for 3.8g acceleration, it will fail. If the speed is reduced to a "new" Va that results in a stall at 3.8 x 800 = 3040kg total wingload, it will simply mean that the wings now sit with additional reserve margin of safety while everything else experiences the full 3.8g at the point of stall.

The same goes for gust loads at cruise speed, where a lightly loaded plane results in higher acceleration for a given amount of lift on the wings. It is a very specific worst case scenario and would have been given due consideration in design calculations and testing. It does however highlight the possibility of other failure sequences beyond the obvious. For example, a damaged aileron mass balance arm might fail due to high gust induced acceleration even though the wing itself is not being stressed beyond design limits. The resulting flutter WILL load the wing to the point of failure.
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Re: Aircraft down in Camperdown.

Unread post by Volo » Sat Nov 07, 2015 12:50 pm

I can't follow your logic Big Will . The G that you would experience is not related to the the stall - only the speed of change of direction of the Aircraft coupled with its speed . If you pull the stick in to your gut just above the stall you might register 1.5 G . If you pull the stick into your gut at VNE - bye bye wings ( on some aircraft ) .
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Re: Aircraft down in Camperdown.

Unread post by Volo » Sat Nov 07, 2015 1:26 pm

And the more heavily loaded you are on the non lifting parts ( fuselage )the quicker your wings will depart .
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by A Corbett » Tue Nov 10, 2015 8:26 pm

Is there an airfield in Camperdown? Is it still active?
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Dobbs » Tue Nov 10, 2015 11:15 pm

The main Cato Ridge airfield closed some years ago, but yes, there are still a number of active airfields in the Cato / Camperdown area.
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Re: Aircraft down in Camperdown.

Unread post by Iceberg » Wed Nov 11, 2015 10:31 am

Big Will wrote:I have seen reference made to Va and how it might change with regards to loaded weight.
From the perspective of the stress in the wing spar (assuming a steady load), it makes no difference. A 1000kg plane (gross) will generate 3800kg of lift just as it is about to stall at Va. The stress in the wing is largely due to the amount of lift produced as well as the wing torsion at that speed. All components of mass in the plane will experience 3800/1000 = 3.8g acceleration.

If you now fly at a reduced load, say 800kg, at the same speed, Va, and pull up to the point of stall, the wings will still produce 3800kg of lift. The stress in the wing spar will be the same (same bending moment, same torsion). However, the acceleration experienced by the airframe and its occupants is now 3800/800 = 4.75g. The bolts or rivets of any part of the airframe that is not directly supported by the air load itself will experience increased stress. So if something like the battery installation (or any other item of mass) was only designed for 3.8g acceleration, it will fail. If the speed is reduced to a "new" Va that results in a stall at 3.8 x 800 = 3040kg total wingload, it will simply mean that the wings now sit with additional reserve margin of safety while everything else experiences the full 3.8g at the point of stall.

The same goes for gust loads at cruise speed, where a lightly loaded plane results in higher acceleration for a given amount of lift on the wings. It is a very specific worst case scenario and would have been given due consideration in design calculations and testing. It does however highlight the possibility of other failure sequences beyond the obvious. For example, a damaged aileron mass balance arm might fail due to high gust induced acceleration even though the wing itself is not being stressed beyond design limits. The resulting flutter WILL load the wing to the point of failure.
I also don't follow the g loading argument. If the 1000kg aircraft flies level at Va (for that weight), the lift generated is 1000kg. It experiences 1g.
So if I yank back fully on the controls, I am guaranteed not to exceed 3.8g before the aircraft stalls.
The heavier the aircraft, the slower it will react at a given speed when pulling back the yoke - that is why Va is largest at MTOW.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by StressMerchant » Wed Nov 11, 2015 10:47 am

I also don't follow the g loading argument. If the 1000kg aircraft flies level at Va (for that weight), the lift generated is 1000kg. It experiences 1g.
So if I yank back fully on the controls, I am guaranteed not to exceed 3.8g before the aircraft stalls.
Be careful with that. It depends very much which Va you are talking about.

To quote the FAA on the design Va:
a. The design maneuvering speed is a value chosen by the applicant. It may not be less than Vs√ n and need not be greater than Vc, but it could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.

b. VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits, nor should it be interpreted as a gust penetration speed. Only if VA = Vs √n will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.

c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507. VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits.

FAA Advisory Circular AC23-19A
Problem is that there was a second Manoeuvre Speed mentioned in the standards - now known as Vo.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Iceberg » Wed Nov 11, 2015 10:57 am

Thanks StressMerchant. The devil is in the details it seems...

At the design limit (3.8g), the material and other safety factors apply as well (adding up to about 1.5), so actual failure should only occur at around 5.7g + ?
That is how I understand it. Anyone care to elaborate?
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by StressMerchant » Wed Nov 11, 2015 11:15 am

It's not purely G. There is also the effect of torque (twist) on the wing, which can cause failures at lesser G loadings.

Also bear in mind that at lighter weights, the aircraft could exceed the stated G limits without overloading the wings, but it could experience structural failures elsewhere in its structure. This should be accounted for in the design, but I have seen STC analyses where this has been missed.
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by PRDT » Wed Aug 16, 2017 2:46 pm

3 and a half years later and still no CAA accident report?
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by happyskipper » Wed Aug 16, 2017 11:02 pm

All I can say is that it was a tragic and very distressing crash - whatever the cause [which we will probably never know - considering our Accident Investigation Team's results, lately].

I had the privilege of doing some flying from that field a while back, and chatted to those eye witnesses. All agree that it is a mystery as to why the wings came off - and that it was the most horrific thing to witness....

Stay safe out there guys and gals, and fly within the aircraft's, and your, limitations......
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by lodge » Thu Aug 17, 2017 12:50 am

3+ years for the report is a long time !
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Calle_Hedberg » Sun Aug 20, 2017 9:02 pm

Iceberg wrote:Thanks StressMerchant. The devil is in the details it seems...

At the design limit (3.8g), the material and other safety factors apply as well (adding up to about 1.5), so actual failure should only occur at around 5.7g + ?
That is how I understand it. Anyone care to elaborate?
Iceberg,

I never saw your post back in 2015... Just note that for composite aircraft, the safety factor required is 2.0, not 1.5. The CSIR testing rig broke at around 9G when they did the original testing of the Ravin 500 wing, but the similar Ravin 700 wing was tested to over 10G last year, I think.

As I said before, taking the Ravin 500 into the territory just below the Vne is not the issue - I've done it several times - but you need a very light touch on the controls easing it out of the dive. I believe that's where this flight went haywire - the pilot, who had jumped into the aircraft for the first time 20 minutes earlier, had no experience with the sensitive aircraft controls at high speeds.

Regards
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Re: Ravin Aircraft down in Camperdown - 15 March 2014

Unread post by Iceberg » Mon Aug 21, 2017 8:37 am

Calle_Hedberg wrote:
Iceberg wrote:Thanks StressMerchant. The devil is in the details it seems...

At the design limit (3.8g), the material and other safety factors apply as well (adding up to about 1.5), so actual failure should only occur at around 5.7g + ?
That is how I understand it. Anyone care to elaborate?
Iceberg,

I never saw your post back in 2015... Just note that for composite aircraft, the safety factor required is 2.0, not 1.5. The CSIR testing rig broke at around 9G when they did the original testing of the Ravin 500 wing, but the similar Ravin 700 wing was tested to over 10G last year, I think.

As I said before, taking the Ravin 500 into the territory just below the Vne is not the issue - I've done it several times - but you need a very light touch on the controls easing it out of the dive. I believe that's where this flight went haywire - the pilot, who had jumped into the aircraft for the first time 20 minutes earlier, had no experience with the sensitive aircraft controls at high speeds.

Regards
Calle
Metals like steel have a very predictable stress-strain curve - hence a combined safety factor of 1.5 can be used.
With composites the stress-strain curve is dependant also on the quality of manufacturing which necessitates the higher factor of 2.
I suppose with large aircraft and very precise QC this can be reduced.

Flutter can however occur at any speed if the surfaces resonate with the aerodynamic forces present at that speed. This can be (and often is) in a torsional mode as Stressmechant mentioned. This can lead to failure even though the load factor is just 1.0.
So during testing/certification it must be determined that this will not occur below the dive speed Vd which is 10% higher than Vne.
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