Light aircraft down Pretoria, ZS-UKU KR2

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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Lazy duck » Sat Dec 18, 2021 11:39 pm

The main spar failed by photos posted. A number of incidents could have contributed. Hard landings, flutter , a few gs in flight, turbulence. Holes in the cheese lined up. All those exceedance inspections on all private aircraft should be carried out as per manufacturers manual on every 100 hour . They might not be reported. Even every 50 hours in the case of experimental.
Experimental aeries might not be as specific so some criteria may be missed.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Mon May 16, 2022 12:42 pm

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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Volo » Tue May 17, 2022 9:08 am

Really good report - says it all . Not the first such failure in KR2s.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Hop Harrigan » Tue May 17, 2022 10:47 am

Pls can someone explain to me how the residual moment on the aileron works...
I see from he photo of the aileron that the moment appears to be being measured from the hinge line to the end of the aileron. I would have thought that the moment would be measured from the hinge line to, either the CG of the aileron or to the Centre of Pressure of the aileron?
What am I not understanding?
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by StressMerchant » Tue May 17, 2022 1:49 pm

Pls can someone explain to me how the residual moment on the aileron works...
I see from he photo of the aileron that the moment appears to be being measured from the hinge line to the end of the aileron. I would have thought that the moment would be measured from the hinge line to, either the CG of the aileron or to the Centre of Pressure of the aileron?
Unfortunately I'm not somewhere I can create a sketch for you, this is always easier to explain with a visual aid. Hope my explanation works...

The key here is the "moment", ie the force multiplied by the distance. Moment = force x distance. (FWIW there is a slight error in the report - the units should be kg.mm, not kg/mm. Purists might also object to using mass instead of weight (force), but under 1G static loading the outcome is not affected)

If the CG is on the hinge line, the distance is zero, and hence the moment is zero. Moment = force x distance = force x 0 = 0.

If the CG is not on the hinge line, then it will cause a moment around the hinge. Let's call in M1= F1 x d1, where F1 is the force (weight) and d1 is the distance between hinge line and CG. Let's assume it causes the aileron to hang trailing edge down (most common condition). In order to balance this horizontally, you'd have to counter it with an equal moment in the other direction. You can apply the balancing force anywhere, but it's convenient to do it somewhere where you can easily measure the distance, eg the trailing edge.

So to balance the moment, you apply a force up at the trailing edge, which you measure with a scale. You then know that the moment you have applied to counter the imbalance is M2 = F2 x D2. M2 is the reading on the scale, D2 is the distance from the trailing edge to the hinge. And for balance, M2 = M1.

The significance in this case is that the designer specified that the ailerons should be fully balanced (also known as zero balanced). M1 should be zero, M2 should be zero. The investigators have shown in their little test that the aileron was not zero balanced. The effect of the actual amount of imbalance is difficult to determine, but they have shown that it was not balanced, ie out of spec.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by davelo » Tue May 17, 2022 2:05 pm

Hi Hop
I reckon the aileron was supported at the hinge line and, with it in a horizontal position, the scale was used to determine the residual "weight" at the trailing edge (which should be zero if it were perfectly balanced and naturally in a horizontal position with no force applied to it). The measured 0.415 kg is multiplied by the 0.145m distance from the pivot line, then should be multiplied by accel due to gravity G (9.81 m/s^2) to determine the residual moment in Nm. I get 0.59 Nm. Remember a moment is actually a force x distance, and F = ma (Newton), where a = G in this instance.

To balance the aileron you would then need a mass of M kg with its CG at a distance D metres on the other side (front) of the hinge line such that (M x 9.81) x D = 0.59 Nm, since F = Ma. At least that's the way I see it.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by davelo » Tue May 17, 2022 2:07 pm

Apologies StressMerchant, I think we were typing at the same time, I didn't see your reply. I think we're essentially saying the same thing though, no?
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by jimdavis » Tue May 17, 2022 2:33 pm

StressMerchant wrote:
Tue May 17, 2022 1:49 pm
The key here is the "moment", ie the force multiplied by the distance. Moment = force x distance. (FWIW there is a slight error in the report - the units should be kg.mm, not kg/mm. Purists might also object to using mass instead of weight (force), but under 1G static loading the outcome is not affected)

If the CG is on the hinge line, the distance is zero, and hence the moment is zero. Moment = force x distance = force x 0 = 0.

If the CG is not on the hinge line, then it will cause a moment around the hinge. Let's call in M1= F1 x d1, where F1 is the force (weight) and d1 is the distance between hinge line and CG. Let's assume it causes the aileron to hang trailing edge down (most common condition). In order to balance this horizontally, you'd have to counter it with an equal moment in the other direction. You can apply the balancing force anywhere, but it's convenient to do it somewhere where you can easily measure the distance, eg the trailing edge.

So to balance the moment, you apply a force up at the trailing edge, which you measure with a scale. You then know that the moment you have applied to counter the imbalance is M2 = F2 x D2. M2 is the reading on the scale, D2 is the distance from the trailing edge to the hinge. And for balance, M2 = M1.

The significance in this case is that the designer specified that the ailerons should be fully balanced (also known as zero balanced). M1 should be zero, M2 should be zero. The investigators have shown in their little test that the aileron was not zero balanced. The effect of the actual amount of imbalance is difficult to determine, but they have shown that it was not balanced, ie out of spec.
Stressors, a question if I may: If the imbalance is exactly equal on both ailerons, and they are connected by rods which effectively have no play, would each act as an effective counterbalance for the other? I suspect this might be a theory thing that for some reason doesn't work so well in practice.

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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Tue May 17, 2022 3:15 pm

jimdavis wrote:
Tue May 17, 2022 2:33 pm
StressMerchant wrote:
Tue May 17, 2022 1:49 pm
The key here is the "moment", ie the force multiplied by the distance. Moment = force x distance. (FWIW there is a slight error in the report - the units should be kg.mm, not kg/mm. Purists might also object to using mass instead of weight (force), but under 1G static loading the outcome is not affected)

If the CG is on the hinge line, the distance is zero, and hence the moment is zero. Moment = force x distance = force x 0 = 0.

If the CG is not on the hinge line, then it will cause a moment around the hinge. Let's call in M1= F1 x d1, where F1 is the force (weight) and d1 is the distance between hinge line and CG. Let's assume it causes the aileron to hang trailing edge down (most common condition). In order to balance this horizontally, you'd have to counter it with an equal moment in the other direction. You can apply the balancing force anywhere, but it's convenient to do it somewhere where you can easily measure the distance, eg the trailing edge.

So to balance the moment, you apply a force up at the trailing edge, which you measure with a scale. You then know that the moment you have applied to counter the imbalance is M2 = F2 x D2. M2 is the reading on the scale, D2 is the distance from the trailing edge to the hinge. And for balance, M2 = M1.

The significance in this case is that the designer specified that the ailerons should be fully balanced (also known as zero balanced). M1 should be zero, M2 should be zero. The investigators have shown in their little test that the aileron was not zero balanced. The effect of the actual amount of imbalance is difficult to determine, but they have shown that it was not balanced, ie out of spec.
Stressors, a question if I may: If the imbalance is exactly equal on both ailerons, and they are connected by rods which effectively have no play, would each act as an effective counterbalance for the other? I suspect this might be a theory thing that for some reason doesn't work so well in practice.

Jim
No, because an upset in roll has the reverse effect on each aileron and the one would aggravate the other rather than restore the upset.
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Jan » Tue May 17, 2022 5:33 pm

Thank y you for the explanations.

I have a question re figure 17: How does the bell crank or horn attach to the aileron? It seem so be just bonded in/on? I cannot see screws or other hardware in the figure.

I other words - if that was more sturdy would it maybe have survived the flutter?
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by mnmodels » Tue May 17, 2022 5:49 pm

Jan my understanding and experience with flutter is that nothing will last in a flutter. You need to stop it fast and the problem is that once you notice it - it is already in an advance stage of oscillation....

It will bent steel and shear composite like a toothpick...if the horn did not fail - the next in line would have...
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by jimdavis » Tue May 17, 2022 6:21 pm

There will be an article on flutter, and the surprising number of South African accidents caused by it, in SA Flyer at the end of this month (May 2022).
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by KEO » Tue May 17, 2022 7:40 pm

A short while after this accident, and because of it, I re-tested my aileron mass balance. It was out on the RHS due to a slightly thicker layer of paint that was applied after the initial balancing, which was done prior to the final top coat layer. I have friese type ailerons with the mass balance in the leading edge, so the arm is very short. I don't have an external weight like that on Russel's KR1 (see video posted above). I fibre glassed in place onto the forward portion of the friese a few automotive wheel balance weights (the flat rectangular type with adhesive on the back, used on mag wheels I think). All of my control surfaces are 100% mass balanced.

I also never go anywhere near VNe, after all, I am a member of the LCC. :D
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Wingnutter » Wed May 18, 2022 5:20 am

Volo wrote:
Tue May 17, 2022 9:08 am
Really good report - says it all . Not the first such failure in KR2s.
I agree, a well written report.

However since the cause of the accident was concluded to be flutter, it would have been good to see a more detailed account of how they arrived at this conclusion eg. analysis of the failure modes of the spar, aileron control connections, torsional cracking in wing skins etc.

“Post- accident visual examination of the right-wing spars was an indication of structural overload in both a positive and negative direction.” How was this indicated - the report doesn’t elaborate or provide evidence?
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Re: Light aircraft down Pretoria, ZS-UKU KR2

Unread post by Russell Phillips » Wed May 18, 2022 11:55 am

The top and bottom spar caps look identical at the failure point. Since they are also of identical size it makes sense that the loading was "symmetrical" (equal pos and negative g). Had it been a simple overload in one direction the cap that failed in compression would have looked different to the cap that failed in tension.
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