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big saab gripen crash

Old 09-20-2016, 08:14 AM
  #51  
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Originally Posted by Halcyon66 View Post
Bottom of the page shows the rudder.

http://www.rc-network.de/forum/showt...sse-2016/page8

Regards,
It would appear not to have a Glass Cloth finish, can anyone confirm?

Last edited by JP-1; 09-20-2016 at 11:27 PM.
Old 09-20-2016, 11:40 AM
  #52  
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Good pics. Interesting, all balsa, ..well, it can be used to make a jet if done like this:

http://bvmjets.com/JetKits/s_bandit/balsa_wing.jpg
Old 09-20-2016, 08:06 PM
  #53  
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Originally Posted by pkoury View Post
Seems to me the weight limit might be a contributing cause of the crash. The bigger models built to attend the event may sacrifice proper construction in order to meet the weight limits. Of course even a properly designed and constructed aircraft an be overstressed by pilot error. Remember the Airbus that flew into wake turbulence and lost its vertical stabilizer when the pilot danced on the rudder and overstressed the structure.

Yeap. I would tend to agree. It seems some of these super large models are very light based on there flight characteristics. Something has to be sacrificed to achieve these super low weights. Equipment (engine, fuel, servos, etc) are a fixed requirement/weight so theres not much that can be done with those. That only leaves aircraft structural design and materials, both of which can cause problems when done badly.
Old 09-20-2016, 09:08 PM
  #54  
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Originally Posted by Halcyon66 View Post
Bottom of the page shows the rudder.

http://www.rc-network.de/forum/showt...sse-2016/page8

Regards,
Well, definitely doesn't seem to be an assembly issue while at the field since it looks like the vertical stab was fixed into place and not removable. Maybe it just didn't have quite enough structure in that area and it stressed just a little too hard. Shame.
Old 09-20-2016, 11:24 PM
  #55  
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There is a detailed report in JetPower 2015/5 of the build. Its not clear who did the design calculations.
They do talk of building in a freezing garage and glue not drying!
Anyway, its made National news here, even the girl in my local small town coffee shop has seen the video and knows of it...not good for the hobby.
Old 09-20-2016, 11:50 PM
  #56  
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Sad that we only talk about this crash. There where a lot of crashes during Jetpower 2016 where the cause of the crash is not directly know but what I have seen there where also a lot of material fatique. In general we have a sport where plane will crash in many ways.
With this type of planes more distamce is needed but prefer a big plane crash than a small (high speed like opus with 5kg thrust turbine) plane because with the big plane we have some time.
Old 09-21-2016, 02:31 AM
  #57  
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Originally Posted by MNModels View Post
Exactly!!! Been at Jetpower 8 times. Two years ago pilots were not allowed to fly over 25kg. Not sure if this jet was under 25Kg. Doubt it, but maybe wrong.

In my opinion yet again Jetpower organisers bending rules as they please. If somebody got killed who would be to blame? What is the German rules on this?

Rgds
I have a Tomahawk L39 that has been through the LMA scheme in the UK the model is 25kg with out fuel this was not allowed to fly so i dont think the rules have been bent.

Pete
Old 09-21-2016, 04:22 PM
  #58  
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The difference between structural integrity, full size one, some 10 000 kilograms, 1/2 size toy model around 100 kilos. No wonder it fell appart as it did.
Old 09-21-2016, 06:17 PM
  #59  
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Originally Posted by invertmast View Post
Yeap. I would tend to agree. It seems some of these super large models are very light based on there flight characteristics. Something has to be sacrificed to achieve these super low weights. Equipment (engine, fuel, servos, etc) are a fixed requirement/weight so theres not much that can be done with those. That only leaves aircraft structural design and materials, both of which can cause problems when done badly.
+1
Old 09-21-2016, 09:35 PM
  #60  
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Originally Posted by Joseph Frost View Post
The difference between structural integrity, full size one, some 10 000 kilograms, 1/2 size toy model around 100 kilos. No wonder it fell appart as it did.
Unfortunately it doesn't quite work that way comparing scale weights... in a real aircraft you have thousands of pounds of avionics, cooling, hydraulic, life support, and other systems that are not required in our models. Once you start dropping that weight, you can start dropping the supporting structures, such as spars and landing gear. It's amazing how fast you can drop weight off of the structure when you don't have to support all of the equipment.
Old 09-21-2016, 10:09 PM
  #61  
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Originally Posted by ChicoFlyer View Post
Unfortunately it doesn't quite work that way comparing scale weights... in a real aircraft you have thousands of pounds of avionics, cooling, hydraulic, life support, and other systems that are not required in our models. Once you start dropping that weight, you can start dropping the supporting structures, such as spars and landing gear. It's amazing how fast you can drop weight off of the structure when you don't have to support all of the equipment.
Also, by reducing the size of the structure by a factor two, its volume reduces by a factor 4. The weight of the airframe is a function of its volume and density of material used...
Old 09-21-2016, 10:14 PM
  #62  
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Originally Posted by RayStruik View Post
Sad that we only talk about this crash. There where a lot of crashes during Jetpower 2016 where the cause of the crash is not directly know but what I have seen there where also a lot of material fatique. In general we have a sport where plane will crash in many ways.
With this type of planes more distamce is needed but prefer a big plane crash than a small (high speed like opus with 5kg thrust turbine) plane because with the big plane we have some time.

So share with us. Where are the report, pictures, videos, threads?
Old 09-21-2016, 10:36 PM
  #63  
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Yes Guys, I agree the scale doesn't always work like that but the point I'm making is, half scale model in size, and 100 to one in weight, is a big difference! I would expect model of that caliber to weight at least 300 kg to withstand the structural stresses just to do basic aerobatic maneuvers. Unfortunately extra strength comes with extra weight.
BTW, I don't think there was anything wrong with the model if it was flown in the appropriate manner, aerobatics certainly not!
Old 09-21-2016, 11:54 PM
  #64  
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In this case, what happened was the best result...Imagine a huge out of control airframe with no fin. What happened dissipated what would have been a huge mass in one place.
Old 09-22-2016, 02:44 AM
  #65  
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In R/C Model World, June to September 2009, I wrote a series of articles on "Scale Science", titled Scale Speed, Scale Weight, Scale Power and Scale Spreadsheet. That last one included a spreadsheet which could, for a while, be downloaded from Traplet Publications website.

Scale Science

The science argument goes like this. We use the same lift coefficient as the full size, so the speed and weight are directly related by the common equation which is used to define lift coefficient Cl.
W=L=1/2.rho.V[SUP]2[/SUP].S.Cl
If the speed V of a 1/6 scale model is to be 1/6 that of the full size (Linear Scale) then the modelís weight has to be full size weight divided by 6x6x6x6 or 1296. However models donít fly straight for long, we are constantly turning and flying manoeuvres. If the size of the turns and loops of a 1/6 model is to be 1/6 that of the full size (Dynamic Scale) then the modelís weight has to be full size weight divided by 6x6x6 or 216. We are talking two vastly different aircraft. The light one flies scale linear speed but it turns and loops impossibly tightly and looks generally jerky. The heavy one flies nice scale manoeuvres but lands like a runaway train. We need to choose a compromise in between these two extremes for realistic scale flight.
I noted in the articles that the Skygate Hawks built down to 20kg for the Jet Masters looked floaty, and would look more convincing at 24 kg

The 20 kg Hawks had a weight index (Wix for short) of about 4. In other words their weight was scaled down by the scale factor to the power 4, which results in Linear Scale flight, but floaty, jerky manoeuvres.

Now look at a 1:2 size Gripen, an aircraft whose weight is, say 18800 lb, thrust (dry) 54000 N

For a light floaty model (Linear Scale) it should weigh 18800 divided by 2^4 or 2x2x2x2 giving 1175 lb or 534 kg

For Dynamic Scale a model should weigh 18800 / 2^3 or 2x2x2 giving 2350 lb or 1068 kg

Those are the lower and upper limits for any semblance of scale realism. At the lower weight of 534 kg my spreadsheet suggests a scale thrust requirement of 4155 N thrust for scale performance.

This model under discussion was a tissue paper model, so far from scale science that it is not surprising that it blew apart. Only the model was damaged, so no harm done.

I am sure it is quite possible to build a vast scale-shaped model that will fly unrealistically but still hold together, but WHY?

Last edited by alasdair; 09-22-2016 at 02:50 AM.
Old 09-22-2016, 04:20 AM
  #66  
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Hi all,
Big jets should avoid knife edge flights ( that`s where big rudder deflections and therefore huge yaw forces build up )
I have talked to several full size fighter pilots from our airforce ( F5E and F/A18 guys ) - they would never ever try to execute knife edge manouvers
and maintain level flight !
The full size airframe would not hold up either, like that Saab`s one !
In fact - I do believe that sustained knife edge flight does NOT exist in the full size world !
It has just become a ( questionable ? ) habit during the last years in the modelling world !
see you
Hans

Last edited by hofer; 09-22-2016 at 04:22 AM.
Old 09-22-2016, 04:26 AM
  #67  
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1 second youtube search:

https://www.youtube.com/watch?v=a47-yXqq1mk

Are you sure hans?
Old 09-22-2016, 04:30 AM
  #68  
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Originally Posted by ChicoFlyer View Post
Unfortunately it doesn't quite work that way comparing scale weights... in a real aircraft you have thousands of pounds of avionics, cooling, hydraulic, life support, and other systems that are not required in our models. Once you start dropping that weight, you can start dropping the supporting structures, such as spars and landing gear. It's amazing how fast you can drop weight off of the structure when you don't have to support all of the equipment.
True but keep in mind you still have significant air loads. Scaling air loads is a complicated equation. In modeling, it is typically done by trial and error. 90% of the kits we fly are overbuilt....taking a weight penalty, but increasing the margin of safety <-- we should always err to this side.
Designing a light and strong airframe with optimized load paths is possible.....but at a cost premium.
Old 09-22-2016, 04:40 AM
  #69  
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Excellent post, Alisdair, and I entirely agree.
Frankly, trying to build a machine of this size and weight, 200 pounds, AND powering it with almost 200 pounds of trust AND expecting it to withstand harsh manoevering, 3-4 g turns ( judging by the bank angle in the videos) and knife edge flight, is totally unrealistic. I am surprised it stayed together as long as it did.

However, the operation of the model showed a lack of consideration for the model' s fragility and light weight structure ( having seen some pictures of the wreckage it's hard to believe the poor materials being used in some areas)

In the video one can see the considerable side slip angle being generated as he rolls into knife edge. ( is this really something a Gripen does ? ) This must mean he used a considerable rudder angle leading to fin overstress and failure

Years ago I flew the Boeing 757and 767 aircraft which had huge fins and powerful rudders to cope with engine failure at low speeds. Large rudder input at higher speeds (actually never needed) could have led to fin over stress and failure like the AA Airbus A300 at JFK some years ago. ( That was caused by mishandling in wake turbulence )

To to overcome this problem Boeing ( I believe the 77 and 78 are the same, perhaps the 74 too) uses a rudder ratio system which automatically reduced rudder angle at speed above 240 knots.

I Have used a similar system on my Hawks, obtained by enabling flight modes to limit rudder travel with flaps UP to just enough to achieve a reasonable slow and 4 point roll. I am convinced that this saved my AW Hawk which suffered fin structural failure , but not separation. My SG Hawk also has this system to reduce the chance of fin failure.

I have just just reread the SG Hawk manual, only one large rudder travel is mentioned despite this model having fun structural problems in its early days.

Other guys may wish to use this system, it works, and is dead easy to implement. It might just save a model.

Perhaps, to, it should not be forgotten just how much damage COULD have been caused by this machine. Two weeks ago, at a glider meeting, an out of control sailplane was heading at high speed for the launch area, but eventually crashed about 75 meters away. the size of the impact crater was alarming to say the least and that was just a glider.

Every crash is worth discussing in an objective manner, there is always something to be learnt, and this crash is another reason why I have been urging UK jet fliers to maintain a 50 meter separation for all flying.

David.
Old 09-22-2016, 05:19 AM
  #70  
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David,
I read your article in R/C Jet International the other day. I understood that it was your Skygate Hawk that had a cracked fin? Did I misread? Was it an Airworld model?

When I read about your cracked fin I wondered about the dummy trim tab. Was it free to move? If so it could have contributed to the flutter. Any flexibility anywhere can start or promote flutter.
Stiffness is your friend, and carbon fibre is great for adding stiffness.

I could not tell from the video whether the Gripen's fin failed form excessive sideload of if it fluttered.
Anyone notice flutter?

From still photos elsewhere it looks as though the Gripen's fin was well bolted down using a substantial sleeved tube held in two small plywood formers mounted in the bit of structure atop the fuselage. The tube obviously cannot go right down through the fuselage, so it is held in a mounting with limited depth and limited width.
Loadpath planning come to mind. The fin sideforce is easily carried by the sleeved tube, down into the narrow and shallow part under the fin, and then it has nowhere to go, so it broke out.
I hope Saab got it right.
Where do you react the root bending moment of a fin if there is no structure below it?

Alasdair

PS Many years ago I also flew the Boeing 747/757/767 among others. I remember being told that the footrests (rudder pedals) were not to be moved except for engine failure or kicking off drift on landing. And then, do so gently.

Last edited by alasdair; 09-22-2016 at 05:48 AM.
Old 09-22-2016, 05:27 AM
  #71  
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Sorry the jet came apart, but the good news is, it got all over Facebook and the news. Seems that is often the goal anyway. It was freaky big. Perhaps that's the only reason it got a Jet Power flight slot. Seems we're into freaky big these days.
I remember fondly the days Ali would demo Boomerangs for Alan. Jets the average Joe could identify with. Now -a-days its' about freaky big, or freaky expensive. But, that's what gets on Facebook man...
Sorry, I had nothing constructive to say.
Old 09-22-2016, 06:41 AM
  #72  
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Originally Posted by alasdair View Post
David,
I read your article in R/C Jet International the other day. I understood that it was your Skygate Hawk that had a cracked fin? Did I misread? Was it an Airworld model?

When I read about your cracked fin I wondered about the dummy trim tab. Was it free to move? If so it could have contributed to the flutter. Any flexibility anywhere can start or promote flutter.
Stiffness is your friend, and carbon fibre is great for adding stiffness.

I could not tell from the video whether the Gripen's fin failed form excessive sideload of if it fluttered.
Anyone notice flutter?

From still photos elsewhere it looks as though the Gripen's fin was well bolted down using a substantial sleeved tube held in two small plywood formers mounted in the bit of structure atop the fuselage. The tube obviously cannot go right down through the fuselage, so it is held in a mounting with limited depth and limited width.
Loadpath planning come to mind. The fin sideforce is easily carried by the sleeved tube, down into the narrow and shallow part under the fin, and then it has nowhere to go, so it broke out.
I hope Saab got it right.
Where do you react the root bending moment of a fin if there is no structure below it?

Alasdair

PS Many years ago I also flew the Boeing 747/757/767 among others. I remember being told that the footrests (rudder pedals) were not to be moved except for engine failure or kicking off drift on landing. And then, do so gently.
Totally agree !

In fact Alasdair, I have two AirWorld Hawks, the first suffered a broken fin former, after an aerobatic competition flight in Australia. I was indeed fortunate not to lose it as did Mick Burrel with his AW Hawk. It, and my other AirWorld Hawk, now have considerably strengthened and stiffened fins, so far, 200 flights, so good, both have rudder ratio working.

I Also so have a SkyGate Hawk and because of the AW fin problems, and having seen the original SG Hawk land with a loose fin in Hungary, I was extra careful with its fin, very considerably strengthening and stiffening the fin mount, Extending and strengthening its load path. It showed no sign of damage after the fin failure. The fin has been repaired with considerable carbon plate and cloth.

I suspect the scale trim tab MAY have worked loose and caused flutter, the new one is very carefully secured. I did not notice any flutter on the Gripen videos I Have seen.

Its ready to go again and will fly next week, fin and rudder behaviour will be filmed by the onboard Mycro Cam.

David.
Old 09-22-2016, 09:29 AM
  #73  
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There is a point where you can't avoid flex and you need to work with it and incorporate it into the design. Not sure we know how or if we are using the right techniques, materials, glues etc....

So, I wonder, is there a point, where it is not about making planes stronger, but just having to radically change construction techniques to incorporate the expected flexibility of the airplane? (I personally have no clue )
Old 09-22-2016, 12:16 PM
  #74  
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Originally Posted by Tuggs View Post
1 second youtube search:

https://www.youtube.com/watch?v=a47-yXqq1mk

Are you sure hans?

Hi Tuggs,
Yes and yes - even though your vid is not too bad, I still believe that sustained knife edge flight ( if aerodynamically possible at all ! ) is dangerous to any full scale ship !
see you
Hans
Old 09-22-2016, 01:43 PM
  #75  
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Excellent post alasdair, good to see some real numbers that make sense.

Regards,






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