big saab gripen crash
#76
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> Where do you react the root bending moment of a fin if there is no structure below it?
well put, the vertical fin and fuselage mount in a lot of ways deserves special consideration as a type of cantilevered structure when loaded as it is in knife edge flight.
well put, the vertical fin and fuselage mount in a lot of ways deserves special consideration as a type of cantilevered structure when loaded as it is in knife edge flight.
#77
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Bob
#78
Join Date: Nov 2010
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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 model’s weight has to be full size weight divided by 6x6x6x6 or 1296. However models don’t 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 model’s 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?
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 model’s weight has to be full size weight divided by 6x6x6x6 or 1296. However models don’t 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 model’s 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?
#79
Surely the maths are very simple indeed, a few multiplications and a division.
However, the simple equation given by Alisdair may not be familiar to some guys. It is a basic equation, one of the very first to be learnt on any aerodynamics course. It defines aerodynamic force for lift and drag, so:
W=L=1/2 rho, v2.S.Cl.
Where :
W is weight.
L is Lift ( lift = weight in level flight)
Rho is air density.
V is velocity.
S is area.
Cl is lift coefficient. ( function of wing section )
So knowing these values lift can calculated.
To calculate drag, substitute Cd (drag coefficient) for Cl to get drag.
I recall that in my RAF time we lost two Gnats with fin failure, it's not an unknown problem, so let's hope the new Scale. Composites Gnat is strongly constructed and flown with consideration of the very tall fin. (Although I am sure John will ! )
Frankly I see absolutely no reason why the fin on the SAAB or any othe model could not be made removable, all it needs is sound design, after all many wings , much bigger than the Gripen,s fin are removable.
David
However, the simple equation given by Alisdair may not be familiar to some guys. It is a basic equation, one of the very first to be learnt on any aerodynamics course. It defines aerodynamic force for lift and drag, so:
W=L=1/2 rho, v2.S.Cl.
Where :
W is weight.
L is Lift ( lift = weight in level flight)
Rho is air density.
V is velocity.
S is area.
Cl is lift coefficient. ( function of wing section )
So knowing these values lift can calculated.
To calculate drag, substitute Cd (drag coefficient) for Cl to get drag.
I recall that in my RAF time we lost two Gnats with fin failure, it's not an unknown problem, so let's hope the new Scale. Composites Gnat is strongly constructed and flown with consideration of the very tall fin. (Although I am sure John will ! )
Frankly I see absolutely no reason why the fin on the SAAB or any othe model could not be made removable, all it needs is sound design, after all many wings , much bigger than the Gripen,s fin are removable.
David
#80
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Surely the maths are very simple indeed, a few multiplications and a division.
However, the simple equation given by Alisdair may not be familiar to some guys. It is a basic equation, one of the very first to be learnt on any aerodynamics course. It defines aerodynamic force for lift and drag, so:
W=L=1/2 rho, v2.S.Cl.
Where :
W is weight.
L is Lift ( lift = weight in level flight)
Rho is air density.
V is velocity.
S is area.
Cl is lift coefficient. ( function of wing section )
So knowing these values lift can calculated.
To calculate drag, substitute Cd (drag coefficient) for Cl to get drag.
I recall that in my RAF time we lost two Gnats with fin failure, it's not an unknown problem, so let's hope the new Scale. Composites Gnat is strongly constructed and flown with consideration of the very tall fin. (Although I am sure John will ! )
Frankly I see absolutely no reason why the fin on the SAAB or any othe model could not be made removable, all it needs is sound design, after all many wings , much bigger than the Gripen,s fin are removable.
David
However, the simple equation given by Alisdair may not be familiar to some guys. It is a basic equation, one of the very first to be learnt on any aerodynamics course. It defines aerodynamic force for lift and drag, so:
W=L=1/2 rho, v2.S.Cl.
Where :
W is weight.
L is Lift ( lift = weight in level flight)
Rho is air density.
V is velocity.
S is area.
Cl is lift coefficient. ( function of wing section )
So knowing these values lift can calculated.
To calculate drag, substitute Cd (drag coefficient) for Cl to get drag.
I recall that in my RAF time we lost two Gnats with fin failure, it's not an unknown problem, so let's hope the new Scale. Composites Gnat is strongly constructed and flown with consideration of the very tall fin. (Although I am sure John will ! )
Frankly I see absolutely no reason why the fin on the SAAB or any othe model could not be made removable, all it needs is sound design, after all many wings , much bigger than the Gripen,s fin are removable.
David
#83
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It was worse than anything built by FEJ. I can say for sure that no flutter was visible or audible, because I was watching it. There was a crack as he went knife edge, and then it was over in a second. The snowstorm of balsa floating down told its own tale. I am unsure of the point of building a model of a mach 2 jet fighter that flies like a thermal soarer. Why?
This was not the worst incident at the show IMO. As we arrived early Friday we were watching a sports jet fly, when it lost half of its wing. With amazing control, the piiot somehow managed to complete 3/4 of a circuit and "land" the model on the runway. There was nothing left of the airframe, so I could not actually identify the model type. Whilst the pilot could get full marks for skill, he scores zero for intelligence. He had acres of grass to safely dump the model on, and had a softer landing. But personally I would have grounded the guy because he was flying a barely controlled, high speed model, at one point directly towards the crowd. No imagination! Clearly the safety of the public and other pilots was less important than this guys ego.
John
This was not the worst incident at the show IMO. As we arrived early Friday we were watching a sports jet fly, when it lost half of its wing. With amazing control, the piiot somehow managed to complete 3/4 of a circuit and "land" the model on the runway. There was nothing left of the airframe, so I could not actually identify the model type. Whilst the pilot could get full marks for skill, he scores zero for intelligence. He had acres of grass to safely dump the model on, and had a softer landing. But personally I would have grounded the guy because he was flying a barely controlled, high speed model, at one point directly towards the crowd. No imagination! Clearly the safety of the public and other pilots was less important than this guys ego.
John
#84
Even though I brought my slowflyer this year I never fly towards the crowd. Things can fail at any time at any speed.
Last edited by Henke Torphammar; 09-24-2016 at 07:20 AM.