Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

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chetak
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Re: Aerospace Engineering - Aerodynamics, Stuctures, Avionics, Flight Dynamics - Technical Discussions

Post by chetak »

JayS wrote:
chetak wrote:
very obviously, your job and thought keeps ground bound in more ways than one :)
He he.

Here is the Crash investigation report from FAA for this incidence. Please be a good lad and find me a mention of blade out and pilots' heads getting "pulverized" by them. Clearly I am too dumb to find it (and I don't have good Israeli friends to point it out either :wink: ) and all I can see is that the pilots were very much alive and talking to the ATC much after the engines shearing off from the wings and the recovered engines showed no signed of blade out or any significant internal damage.

http://lessonslearned.faa.gov/ChinaAirl ... report.pdf
It happens in war stories. :)
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Re: Aerospace Engineering - Aerodynamics, Stuctures, Avionics, Flight Dynamics - Technical Discussions

Post by JayS »

chetak wrote:
It happens in war stories. :)
Very conveniently. Luckily the industry doesn't work based on fantasies in "war stories".

Anyways no more from me on this. Not contributing anything of value.
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Re: Aerospace Engineering - Aerodynamics, Stuctures, Avionics, Flight Dynamics - Technical Discussions

Post by Zynda »

JayS wrote: But our folks don't do Fatigue analysis at all for verification analysis. And I am talking about composite wing spar not some chota mota component. Static analysis done and reserve factor calculated. If its above specified number, it should be OK in fatigue. Crack prop doesn't even come into picture. Just some factors to take care of it. I was surprised to see the simplicity in verification analysis for primary structure.
I hope this provides some reasoning behind the absence of fatigue analysis for parts/components made from composites.

Let me start out by saying that I DO NOT have extensive working knowledge on Composites F&DT. I have done some composites static justification but almost no F&DT. So all my current knowledge about Composites F&DT is acquired through literature/material review rather than hands-on experience.

Usually composites are not fatigue sensitive for in-plane loading (apparently they are fatigue sensitive for out-of-plane loads). Also composites exhibit good fatigue characteristics in tension dominated loading unlike metals. However, composites do not favour so well when the loading is compression dominated again unlike metallic structures (the reverse happens in metallic structures).

What happens is that at ultimate load, the strain for composites structures with notches (holes etc.) and the corresponding stress level at this strain will be lower than Fatigue endurance limit for UNNOTCHED laminate. That means, if there is no damage, then the chances of cracks appearing is very less and if there is a damage present, then the chances of that damage propagating or growing is very less (the latter is considered in DT domain).

Note that for composites, the equivalent of a crack is delamination. AFAIK, delamination and its behaviour due to compression is more serious in composite structures (the delaminated area is not supported to adjacent plies due to matrix crack and hence that region can act like an unsupported column and can buckle easily).

Also current static justification for composites includes laminates with BVID which needs to be shown OK for greater than UL. So I think based on the above, a laminated part which holds good in static analysis is usually not investigated (at least analytically) for Fatigue behaviour. Also AFAIK, some static analysis is done of a composite structure containing small delaminated region (usually represented as a circular hole/damage) and I guess buckling checks are performed there.

Also, per Airbus methodology currently there are not satisfactory analytical methods for Composites F&DT justification. The compliance has to be shown via tests (definitely at full-scale level and perhaps at one level lower as well).
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Re: Aerospace Engineering - Aerodynamics, Stuctures, Avionics, Flight Dynamics - Technical Discussions

Post by SBajwa »

Not sure where to post it
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Re: Aerospace Engineering - Aerodynamics, Stuctures, Avionics, Flight Dynamics - Technical Discussions

Post by Zynda »

Thought this article is more apt here. Posting in full as article requires subscription (I will remove it if admins deem necessary). A longish article...

BLADE, Europe’s Laminar Testbed, To Fly in September
In one of the airport’s hangars, A340-300 MSN001 has been heavily modified to become the Breakthrough Laminar Aircraft Demonstrator for Europe (BLADE), and it will soon be ready to make its first flight in this configuration. Airbus expects the aircraft will be able to start its flight-test campaign in the second half of September. “We want to show that laminar flow can work,” Airbus Senior Vice President for Research and Technology Axel Flaig says.

SMOOTH FLYING: EUROPE’S LAMINAR FLIGHT DEMO
>>First Airbus A340 has been modified with laminar-flow outer wing sections
>>GKN-made starboard section has separate leading edge and upper cover
>>Saab-made port section has one-piece composite leading edge and upper cover
>>On a full wing, natural laminar flow could reduce fuel burn by 5%, says Airbus

Smooth laminar flow, as opposed to turbulent airflow, over the wings could reduce skin-friction drag by 50%, Airbus believes. That would translate into a 5% fuel-burn reduction on a nominal 800-nm narrowbody mission and possibly even more on long-range flights. “The next big step in aerodynamics is to work on friction drag,” Flaig says.

BLADE is part of the €1.6 billion ($1.9 billion) European Clean Sky 1 aeronautical research program that includes project areas such as eco-design, systems, green operations, sustainable and green engines, green rotorcraft as well as smart fixed-wing aircraft. Manufacturers Airbus, Dassault, Saab, Safran, Aernnova, GKN Aerospace and Romaero are involved in the effort as is the Catalonia-based technology center, Eurecate. Also included are research institutions Incas, DLR, DNW, NLR, BIAS, Onera and Itainnova. The complex network of partners, coupled with technical difficulties beyond expectation, have contributed to several delays. The aircraft was scheduled to fly three years ago.{I would love to see some Indian Engineering Services company like Quest be a part of the efforts like the above apart from just chasing OEMs for some chunk of outsourcing work. They gotta show initiatives to move up the value chain.}

The transformation of the A340 centered on major changes to the wing: The outer sections have been cut off between ribs 27 and 28 and replaced by newly designed, reduced-sweep laminar wing panels. At a length of around 10 m (32 ft.), each panel represents about two-thirds the size of a wing that would be used in a short-/medium-haul aircraft. The two laminar sections are distinctly different. GKN Aerospace designed the starboard wing, which has a metallic leading edge and separate composite upper cover. The port wing was designed by Saab and features an integrated upper cover and leading edge made of composites.

Comparing the two designs could yield crucial insights into what types of flow disturbance—from tolerance variations during manufacture to insect contamination in flight—are acceptable for real-life operations using natural laminar-flow wings.

BLADE retains its old inboard wings until just past the outer engines. Alternatively, an entire wing would have had to be developed for a smaller aircraft than the A340. However, Flaig explains that would have been a lot more expensive because of the research and development work required. Also, in flight tests it is beneficial to be able to rely on a well-known design. “The internal wing can compensate,” BLADE project leader Daniel Kierbel says. “We will go to the limits and beyond and will break laminarity on purpose.”

Compared to a traditional design on today’s aircraft, the laminar wing is much thinner at the leading edge, and engineers avoid disturbances by designing to keep the airflow as clean and friction as low as possible. “The idea is to have no fasteners, for a continuous surface,” Kierbel says. The Saab and GKN wing designs also do not have slats, although a Krueger flap will be tested to see what protection against leading-edge contamination it can provide. In real life, of course, wings will never be entirely clean: Dirt, insects, small damage—even vibration induced by the engines—influence the airflow, and the wing bends as a whole and locally.

Compared to the 30-deg. sweep of the legacy A340 wing, the laminar outboard wing is swept much less, at 20 deg., because wind-tunnel testing has shown that to be the limit for laminar flow. A future narrowbody aircraft using laminar wings would be limited to about the same sweep angle. The BLADE wings are designed for Mach 0.75 cruise speed, slower than the Mach 0.78 typical for narrowbody aircraft and much less than the Mach 0.85 or so usual for long-haul operations. Natural laminar flow is therefore only a serious option for short-to-medium-haul flying, Flaig believes. A significant reduction in speed would “not be acceptable for airlines,” he says. Hybrid laminar flow, where suction or blowing is used to maintain smooth airflow and is technically more challenging to achieve, would be the solution.

Kierbel expects BLADE to accumulate about 150 flight hours in up to 60 sorties, depending on test results. The aircraft will make its first flight from Tarbes, about 150 km (93 mi.) southwest of Airbus’s home base, Toulouse, from which it will fly the balance of its missions—mainly in the Mediterranean region and in oceanic airspace. While Tarbes Airport’s 3,200-m runway is adequate for the tests, the proximity of the Pyrenees imposes performance restrictions. As even the legacy wing slats will not be used on BLADE for takeoff and landing, airspeed increases in both phases of flight.

Upon completion of initial performance evaluation and flight-envelope expansion, tests will be made in many different aircraft configurations. Initially, BLADE will fly at a 165-ton takeoff weight, but that will rise to 206 tons over time. Engineers also will test the effect of surface imperfections by adding stickers and other devices to disturb the airflow to define future tolerances.

“Observing the boundary layer in flight is challenging,” says Flaig. According to Kierbel, 2,000 parameters will be captured in the campaign using 2,700 sensors, among them 1,200 pressure sensors to measure lift. “It is the most advanced installation we have ever done at Airbus,” Kierbel says. Infrared cameras are installed in three pods—one mounted on the vertical tailplane and two at the wingtips. These will image the differences in wing-surface temperature between laminar and turbulent flow. The wingtip pods were initially designed to ensure that laminar flow extends as far outboard as possible and does not become turbulent too soon. Having the volume to install cameras was a nice side-effect that makes measurements of temperature differences a lot easier, he says.

Flaig says that, depending on timing, an A320neo successor could have a laminar wing. Designing such a wing for a new aircraft that would enter service around 2030 could be feasible, he says. However, design is not the only challenge. Production is key, he notes. The industry is still struggling to produce composite wings at the rates needed, even though they are currently only used on the Boeing 787 and A350 programs, which have much lower output than narrowbody aircraft.

A composite wing that is also laminar would require transformative improvements in wing production not only for volume, but also for assembly precision and surface quality. “We do not have the solution yet for high-volume production,” Flaig concedes.
Image

Image
JayS
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Re: Aerospace Engineering - Aerodynamics, Stuctures, Avionics, Flight Dynamics - Technical Discussions

Post by JayS »

Zynda wrote:.{I would love to see some Indian Engineering Services company like Quest be a part of the efforts like the above apart from just chasing OEMs for some chunk of outsourcing work. They gotta show initiatives to move up the value chain.}
Surely you must be aware that non-EU persons or companies cannot work on EU funded projects. Ditto for US funded projects. Only hope for Indian companies is if GOI funds some research project.

Service companies are also behind screwdrivergiri or shall I see "Keyboardgiri", SW equivalent of screwdrivergiri. Why would they pay for skilled engineers required for such projects when they can earn more money using half-baked engineers at 1/3rd the cost using Keyboardgiri..? Haan ji? Makes no business sense. As such there is no real position for specialised design oriented out sourcing house as it is there in manufacturing. Too little to gain for too much hassle.
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Post by Zynda »

I came across this interesting fact about Honda Jet from this article.

HondaJet HA-420

Posting relevant excerpt:
Like the engines, Honda developed its own airfoil for the HJ to maximize performance and minimize risk of losing control.
What is involved in developing a custom airfoil section vs using existing ones from NACA or similar databases? I assume that even with standard airfoil, the wing has to be analyzed for performance in various flight envelopes. So exactly what benefit does an existing standard airfoil bring over a custom one from a design POV?

JayS, if you know please share with us :)
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Post by JayS »

Zynda wrote:I came across this interesting fact about Honda Jet from this article.

HondaJet HA-420

Posting relevant excerpt:
Like the engines, Honda developed its own airfoil for the HJ to maximize performance and minimize risk of losing control.
What is involved in developing a custom airfoil section vs using existing ones from NACA or similar databases? I assume that even with standard airfoil, the wing has to be analyzed for performance in various flight envelopes. So exactly what benefit does an existing standard airfoil bring over a custom one from a design POV?

JayS, if you know please share with us :)
No one uses standard aerofoils anymore. I don't think in last 4 decades any OEM would have used any standard aerofoil on any aircraft (maybe only those cheap GA aircrafts). Now it has become trivial to design your own aerofoil. Of coarse for aircraft it still involves huge amount of CFD and some experimental work, but practically anyone can develop own aerofoil for all and sundry applications. As a matter of fact I am going to make a new 3D vane with tailored airfoil sections tomorrow. Easy peasy. But even tailored airfoil is not a free form. You assume a certain mathematical form to start with which gives you enough control on all the features of your interest and start tweaking them numerically to get a optimised shape. As of now, machine optimisation has not been as good as human capability in practical terms but nothing is stopping you to make a program to completely automate the optimisation, apart from availability of computational resources and funding to sit down and make such code.

NACA series aerofoils are from the era when there was no CFD. Experiments were the only option. NACA and other such research institutes (NAL also have their own series) would run great many experimental runs and come up with generally good aerofoil families, so anyone can use them without having to do own experimental work, which would be prohibitively expensive for all but big OEMs. Think of it as government subsidising research. Of coarse there is no 'one fits all' airfoil in any family. So you choose a family of airfoil e.g. NACA 4 series or NACA 5 or NACA 6. What this series gives is a systematic way to find a relatively good airfoil in a limited design space rather than shooting in dark in infinitely large design space. So tailoring is needed even with these airfoils but the work is much less with far less parameters to deal with. Think of standard airfoils as buffet meal of large number of courses. Just eat whichever you like but there may not be all your favorite dishes available. But its relatively cheap and gets you good satisfaction. While tailor making your own profile is like having your own personal chef. Ask for anything and he will make it for you. Full satisfaction but little costly.
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Post by Zynda »

Thx...got better understanding now :)
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by Zynda »

The Sikorsky S-97/SB-1 Defiant is an interesting helicopter. The arrangement of co-axial main rotors with a rear push propeller is quite interesting as well.

I was going through the presentation/video of the SB-1 video posted by brar_w on the International Aerospace thread and a few things stood out. One was the advanced rigid co-axial main rotor system. Got me thinking...what does rigid main rotor system mean.

Image

I may have an answer but I am not 100% sure. In conventional helicopters, the blade disc pivots about the main rotor shaft to create forward motion as well as banking abilities.

Perhaps in the S-97/SB-1, the function of the main rotors is purely to generate lift for the chopper. The thrust required for forward (& backward) motion is provided by the rear propeller. That is why these choppers are able to achieve speeds in excess of 450 Kph.

Also, them choppers have active rudders and elevators. So the banking & yawing motions are provided by elevators combination & rudders respectively just like in a fixed-wing aircraft.

A long time ago, in one of my peecha KBs, during a routine conversation about helicopters, my superior stated a fact. Neither I nor he is a CFD/aerodynamics expert...so he could have been totally wrong but at that time it made sense to me.

In a conventional chopper main rotor blades, along the length of the blade, a point which lies nearer to the fixed end/root of the blade rotates slowly compared to a point which lies towards the tip of the blade. He said that in current conventional choppers, the relative velocity of the air near the tip of the blade goes supersonic as the forward velocity of the chopper increases and this puts a limitation on max speed (at around 350 Kph) of a conventional chopper. Since the blade disc in a conventional chopper is "flexible" (not rigid), perhaps this supersonic airflow creates structural stability issues (?). Probably I am not stating the physics in a correct manner. Anyways, I wonder how Sikorsky/Boeing was able to overcome the above problem, if true, in S-97/SB-1 given that it will cruise at 450 Kph! Perhaps the rotors being rigid somehow mitigates the limitation? Anyways, kudos to Sikorsky/Boeing on this engineering feat!!

Would be interested to learn more.

Anyways, like IR mentioned, it would be worthwhile for an Indian private entity to explore JV/co-development opportunities with Kamov for similar products. Issue is that Russians (or any other advanced tech countries) don't treat us as equal partners and want to use the JV as trust fund rather than true collab of engineering.
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Post by deejay »

There has been an incident in the development trials I think. It was reported a few pages ago. These are concepts now and we will see a few nasty incidents but the idea is exciting and will push the speeds possible with helicopters forward.
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Post by Zynda »

Yes Deejay Sir. There was a hard landing with one of the S-97 prototypes. No one was injured but I think the chopper had to be taken off the flights for extensive repair (?).
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by JayS »

Since this topic is OT in LCA thread, I thought this would be a better place to discuss and we can explore some generic technical details.
Cain Marko wrote:^This is from wiki...
https://en.m.wikipedia.org/wiki/Infrare ... ng_systems
Imaging systems Edit
Modern heat-seeking missiles utilise imaging infrared (IIR), where the IR/UV sensor is a focal plane array which is able to produce an image in infra-red, much like the CCD in a digital camera. This requires much more signal processing but can be much more accurate and harder to fool with decoys. In addition to being more flare-resistant, newer seekers are also less likely to be fooled into locking onto the sun, another common trick for avoiding heat-seeking missiles. By using the advanced image processing techniques, the target shape can be used to find its most vulnerable part toward which the missile is then steered.[45] All western Short-range air-to-air missile such as AIM-9X Sidewinder and ASRAAM, Chinese PL-10 SRAAM and Israeli Python-5 use imaging infrared seekers, while Russian R-73 still uses infrared seeker.

Countermeasures

There are two primary ways to defeat IR seekers, using flares or IR jammers.

Flares
Early seekers did not image the target, and anything within their FOV would create an output. A flare released by the target causes a second signal to appear within the FOV, producing a second angle output, and the chance that the seeker will begin to aim at the flare instead. Against early spin-scan seekers this was extremely effective because the signal from the target was minimized through the midcourse, so even a dim signal from the flare would be seen and tracked. Of course if this happens, the flare now disappears from view and the aircraft becomes visible again. However, if the aircraft moves out of the FOV during this time, which happens rapidly, the missile can no longer reacquire the target.

One solution to the flare problem is to use a dual-frequency seeker. Early seekers used a single detector that was sensitive to very hot portions of the aircraft and to the jet exhaust, making them suitable for tail-chase scenarios. To allow the missile to track from any angle, new detectors were added that were much more sensitive and in other frequencies as well. This presented a way to distinguish flares; the two seekers saw different locations for the target aircraft - the aircraft itself as opposed to its exhaust - but a flare appeared at the same point at both frequencies. These could then be eliminated.

More complex systems were used with digital processing, especially crossed-array and rosette seekers. These had such extremely narrow instantaneous fields of view (IFOV) that they could be processed to produce an image, in the same fashion as a desktop scanner. By remembering the location of the target from scan to scan, objects moving at high speeds relative to the target could be eliminated. This is known as cinematic filtering.[46] The same process is used by imaging systems, which image directly instead of scanning, and have the further capability of eliminating small targets by measuring their angular size directly.
My rather layman understanding is that older missiles seekers sought the heat being generated by the fighter exhaust and so could be spoofed by flares if the fighter simultaneously moved out of the seekers fov

With iir systems the seeker is looking for an image generated by the source instead of just the heat. So if the flare does not look like the generated image the missile won't be spoofed.

But of course such missiles are hardly common and I don't think the tspaf has any in it's inventory.
Thanks for posting. That's what I was pointing to. Interesting to know some more technical details.

To be sure, the IIR sensor depends on an Image generated by the heat i.e. IR spectrum. I imagine earlier IR sensors would get a spike in current in circuit (or similar such indication) if it sees hotspot in its POV and they would follow the direction to maximize the current. While in IIR, the onboard computer can take the image in IR specturm, run some intelligent algorithm on it and choose appropriate spot in the space to target to, which need not be the hottest part or even anywhere near it (searching by shape for example). I have known a bit of this stuff from the work I did for DACS for ABM system. The KV of ABM also depends on such image processing algorithms. Arguably its little easier to differentiate re-entering BM terminal stage against the dead cold space above the atmosphere. But ICBMs now have/can have decoys. So the KV needs to differentiate between actual bomb and decoys. This area sees some great image processing research. Most of the systems I have seen have resolution of the order of 100x100 pixels. I am sure there are videos on internet showing how the seeking happens in IIR. I remember to have seen a couple of them some time back.
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Post by JayS »

shiv wrote:
Cain Marko wrote:
My rather layman understanding is that older missiles seekers sought the heat being generated by the fighter exhaust and so could be spoofed by flares if the fighter simultaneously moved out of the seekers fov

With iir systems the seeker is looking for an image generated by the source instead of just the heat. So if the flare does not look like the generated image the missile won't be spoofed.

But of course such missiles are hardly common and I don't think the tspaf has any in it's inventory.
The only way a missile can be programmed to look for an aircraft's IR signature is to look for much cooler spots other than the exhaust itself. If a bunch of very hot spots simply obscured the seeker's view of the cooler spots it could provide a window for the plane to turn away while the seeker is still searching for confirmatory cooler areas that conform to the general shape of an aircraft. At least that is my understanding...

Then again a missile launch warning would be useful..
That makes sense. Basically hot spots would obscure picture just like over-exposure picture from DSLR. With such kind of IR image. The algorithm would find it difficult to find a shape. Though its a dynamic situation and the detection would be based on continuous stream of images, it would still be relatively easier to filter out point hotspots by looking at their motion between consecutive frames. But of coarse this is additional work and would increase time to detection perhaps and might as well through the sensor out of lock if the missile fired from very close distance and time to target is only a few seconds. If Flares give even slightest chance of escape, its well worth it.
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Post by Zynda »

Hi JayS,
I was wondering why ADA were not able to figure out the flutter issues during simulation/analysis studies? I am sure you are aware that solvers like Nastran offer an Aeroelastic package as well.

This is from one of their brochures:
Capabilities:

> Subsonic & Supersonic Aerodynamics
> Static Aeroelastic Trim Analysis
> Proven Aerodynamic Theories & Methods
> Aeroelastic Stability Derivatives
> Aeroservoelastic Analysis
> Hinge Moment Calc
> Proven Flutter Analysis Methods
> Transient Aeroelastic Response
> Freq Domain Aeroelastic Response
> Discrete & Harmonic Gust Analysis
> Design Sensitivity & Optimization
> Unsteady Supersonic Aerodynamics - ZONA51[./quote]
Flutter Analysis
Flutter is a dynamic instability of an elastic structure subjected to aerodynamic forces. Structures are carefully designed to avoid this phenomena.

MSC Nastran allows you to perform modal flutter analysis for subsonic and supersonic unsteady aeroelastic scenarios. Methods for predicting flutter in damped, linear structures include: the K, PK, KE, PKS, PKNL, and PKNLS-method. After analysis, the flutter frequencies and damping are obtained as functions of velocity and the relative modal amplitudes found, and important critical flutter speeds and divergence speeds may be determined via V-g and V-f plots. The resulting data can directly be used in the certification of flight vehicles under the FAA and JAA requirements.
I have no idea about the limitations of the solver & the complexity it can solve, but it seems like a good tool in aiding studies. One thing I noticed is that solver does not include non-linear effects. Again, I have no idea if a non-linear situation arises during flutter. Also, the way it is outlined above, MSC are positioning the solver for commercial aerospace applications rather than Military.

I don't know what was needed by ADA from Rafael wrt Python-5 data to be able to simulate flutter response...
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Post by JayS »

Zynda wrote:Hi JayS,
I was wondering why ADA were not able to figure out the flutter issues during simulation/analysis studies? I am sure you are aware that solvers like Nastran offer an Aeroelastic package as well.

This is from one of their brochures:
Capabilities:

> Subsonic & Supersonic Aerodynamics
> Static Aeroelastic Trim Analysis
> Proven Aerodynamic Theories & Methods
> Aeroelastic Stability Derivatives
> Aeroservoelastic Analysis
> Hinge Moment Calc
> Proven Flutter Analysis Methods
> Transient Aeroelastic Response
> Freq Domain Aeroelastic Response
> Discrete & Harmonic Gust Analysis
> Design Sensitivity & Optimization
> Unsteady Supersonic Aerodynamics - ZONA51[./quote]
I have no idea about the limitations of the solver & the complexity it can solve, but it seems like a good tool in aiding studies. One thing I noticed is that solver does not include non-linear effects. Again, I have no idea if a non-linear situation arises during flutter. Also, the way it is outlined above, MSC are positioning the solver for commercial aerospace applications rather than Military.

I don't know what was needed by ADA from Rafael wrt Python-5 data to be able to simulate flutter response...
Brochures and marketing pitch show very rosy pictures. When you actually start using it in real life applications you realize the short-comings. I am sure I don't have to tell you this. :)

I do not have practical experience of store-separation or aero-elasticity simulations. Never got to work on it. So I am not aware of nitty-gritty. Re why ADA didn't see the flutter issue in simulations - it could be as simple a reason as 'they were never looking for it as it was not at all expected'. Simulation methodology is set according to what you want to see/capture. But it could be very well a case of inability of CFD to capture the physics. Or it could be so that the responsibility of aero integration rested with the Missile OEM and not ADA. Israelies might have refused to share accurate model with proper mass distribution which would enable hi-fidelity aero-elastic simulation. Or its not practically possible to simulate this particular situation with existing technology in reasonable resources/time. The point is its rather useless to speculate on such things where we do no have any data/info whatsoever. I can think of many possible reasons, but there is no way to verify which one of them might be more likely.

Re non-linearity, the physics is non-linear, but the degree of non-linearity and coupling of aero and structures vary from case to case. Some of it can be simulated with reasonable accuracy, using linear solvers. The way aero and structures solvers are coupled matter. Whether you solve aero solution only once and port the forces for structural deflection in structural solver, or you do the aero force transfer at each time step, can affect accuracy of results if the aero and structural physics is highly coupled. Also if composite is involved in structural elements then whole lot of more complexities come into picture. We can talk about it endlessly. There are again very many factors and its not possible to make generalized statements.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by Zynda »

ramana wrote:Rorark gives vibration freq for a triangular beam. What are fin dimensions?
Say 8 inches at the base and 5 inches at apex. 0.25 inch uniform thickness. Material Aluminium.
I have a copy of Roark and it does not cover any vibrations at all. However, other books do cover.

Assuming that fin acts like a cantilever beam, the equation for nat freq is:

f = 3.52*SQRT({EI}/{ml^4}),
where f = Nat freq
m = mass/length

The issue with applying the above equation to a fin is that quantity Area Moment of Inertia (I) varies continuously along the length of the beam.
The above equation is for a beam with uniform cross-section.

I am not a dynamics person but I will try to find out if there is an Nat freq equation for a beam with varying C/S along its span. Even a trapezoid one should do since we can approximate the fin (triangle) to a trapezoid.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by ramana »

Do you have access to ANSYS or NASTRAN? You can model it and tell us the natural frequency.

Use triangular beam element just one large one and fixed cantilever model.

I will also look.

Kirchoff (yes the great Kirchoff of circuits fame) once calculated for a triangular beam but with different orientation.
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Post by Zynda »

I was gonna suggest FE/Numerical method to solve issues with continuously varying structures :) Anyways, yes, I have access to one of the above mentioned programs. Will try to do that over the weekend if I find some time.
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Post by Zynda »

OK...got some time at work to do the above. Took base width of 10 inches and height of fin as 6 in. Uniform thickness of 0.04". Material Aluminum alloy.

Dimensions were gauged roughly based on this image:
Image

Wiki says P-5 dia as 6.13".

First mode of Nat Freq is around 60 Hz from FE solution.

Just FYI, the fin I considered was 2nd one (control surface) from the front of the missile.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by Vayutuvan »

Zynda, what FE package are you using? If you access to either of the two you mentioned, you probably can do lot more than just finding out the lowest mode of one fin. With a few assumptions about the material used for the missile body, you should be able to model the whole missile including all the fins in the front and back and the missile itself. Use 8 noded shells for the fins with six DOFs per node, and automesh the body with ten noded tets or four node tets (or if you have node locked license use mapped mesh with 8-noded hexes) with three DOFs per node. Tie everything using multi point constraints (as the meshes between shell elements of the fins and the solid elements of the body may not be confirming). Model everything in Cartesian coordinates. Finding 50-100 low modes can be done in a matter of tens of minutes on a problem with a couple of million global DOFs even on a reasonably equipped 4 or 8 GB RAM laptop. Use the iterative solver if you are getting say the lowest 20 modes or you are RAM limited.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by JayS »

shiv wrote:
JayS wrote:

Bolded part doesn't make sense to me. One can always fly an aircraft at such combination of speed and AoA that it will maintain steady constant altitude, constant Mach flight, at low altitudes we are talking about here. If you wanna fly faster, reduce AoA. If you want to go slower, increase AoA and still produce same lift, enough to just balance the weight.
You have not understood what I am asking. It is so difficult in this medium. Let me explain.

If you take the same aircraft, same weight, same power setting and simply increase the wing area (for example by extending flaps as is done for take off) - the lift increases. The act of deploying flaps decreases wing loading by increasing wing area for the same weight

In other words - at a given airspeed the same aircraft with will generate more lift if the wing area is higher. That "more lift" will try to take the plane to a higher altitude than if that extra wing area were unavailable. Suppose you cannot retract those flaps and the wing area remains higher than a sister plane without extended flaps then the only way you can force that aircraft with lower wing loading (extended flaps) to fly low is to:
1. Slow down thereby reducing lift
or
2. Force nose down (decrease AoA) by using some aerodynamic surface

Case 1: If the plane is made to slow down it will be flying slower than its sister plane with flaps retracted at the same altitude

Case 2: If the plane is forced to fly low there is increased drag because a force is being applied to counteract the extra lift. That will burn more fuel

In the case of identical sister planes the plane with greater wing area (deployed flaps) will burn more fuel (to keep nose down and fly as fast as sister plane at low altitude) or fly more slowly to maintain the same altitude as its counterpart without extended flaps.

Kindly do not tell me that the Jag and Tejas are not sister planes with the same specs. I know that. And that is what makes the answer more difficult without data.

The questions that arise from here are as follows and I would like hard data if possible
  • 1. Can the Tejas fly nap of earth at the same speeds that Jaguar achieves
    2. Would the fuel consumption of the Tejas, and hence its endurance be reduced if it was flown nap of earth like Jaguar


These are the critical facts in comparing performance of the Tejas versus Jaguar in a role that the Jaguar was designed for. It's not about whether the Tejas can do it or not.

Without hard data on this everything is guesswork. That is what I said in my reply to Sachin, whose question was the very question you said must not be asked.
Since it becomes rathe academic, I think its more suitable to have further discussion here. So x-posting.
Simplified scenarios like this don't help much in problem at hand. Extending flaps, actually make it as different wing and we no longer are comparing apple to apple. But still lets go ahead.
Lets neglect case 1, since its irrelevant. We want to maintain speed. But just to note, lightly loaded wing would be more efficient in cruise, albeit at lower speed, akin to case 1 and perhaps would out-range highly loaded wing, but would take more time. Moving on.

Case 2 - Lets say we make your modified jet fly at same speed as earlier unmodified jet was flying. Assuming no change in weight, same lift needs to be generated to maintain altitude.

Same speed, same altitude >> same dynamic pressure (q). But now we have higher wing area (S).

L = q * S * C_L

Obviously now we need smaller C_L for same L. If you consider typical drag polar:

Image

If you reduce C_L, C_D also goes down. Now whether total drag D, reduced or remains same depends on what was your initial point was on the drag polar. But its reasonable to assume initial cruise point was at max L/D (where the tangent from origin touches the drag polar) to have best efficiency. From there C_D reduces quite a bit, if you reduce C_L. And increase in S, would more likely not compensate for it. Even in worse case, overcompensation would be marginal. So worst case scenario, Case 2 has marginal higher drag, more likely, it has marginally lower drag.

D = q * S * C_D.

Note that I am not trying to make it sound like LCA will be more efficient at low level flight than Jag. IMO, its reasonable to assume that it will be less efficient. But IMO, the difference should be small and LCA should be able to maintain the required speed. Yes, that would increase fuel consumption somewhat and reduce the range of LCA. But even if LCA was equally efficient aerodynamically as Jag is, it still was never going as far as Jag due to lesser fuel fraction, as Indranil mentioned. However when we have practical situations such as flying over Tibet, LCA might out-reach Jag in low flying "effectively", as it could TO from High altitude strips, near to border, while Jag will have to TO from farther airstrips situated in plains, negating its longer range.

But I agree to you in principle when you say, we are basically comparing two different planes optimized for two different jobs. And one need not be overzealous in supporting LCA, that he undermines strengths of other planes. IAF will and should use Jags until they have them. In future, they can very well replace Jags with LCA and come up with tactics which are superior to existing tactics + Jag combination. But more likely it will be AMCA.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by JayS »

Interesting stats from a recent news:

2017 is termed as the Safest year ever for Aviation Travel. Not a single jet accident or not a single live lost. Though some serious accidents happened e.g. Fan module loss from French A380. But overall pretty impressive feat. As of today the chances of a Jet involving in an accident is 1 is 16 Million.

A note - those who are familiar with aero design process would like to note this number - 1 in 16 Million. The current design standard for designed condition is the worst condition expected to be seen once in a life time of any given jet of that type is taken as design case. Its considered to have probability of 1 is a million i.e. probability of 1e-6. So the number is 16 times more conservative.

The ultimate load case which is 1.5x the design load case it expected to occur once for the entire fleet life. Its taken as 1 in a billion i.e. probability of 1e-09.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by Akshay Kapoor »

This and the Tactics threads are good examples of the kind of thread we need on BR. Serious and focussed and technical discussion on a specific area of engineering here and serious and focussed technical discussions on tactics there. Gives a much higher info quotient than the kichdi and rants we see sometimes. Good posts and posters get overwhelmed by low quality posts.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by Zynda »

For people interested in Pure Aerodynamics and connecting the mathematics of it to physical form, I guess this book would be helpful.

Understanding Aerodynamics: Arguing from the Real Physics: Doug McLean

Personally, I found this book a little heavy on Maths, which should be expected since Fluid flow is a vector quantity & dynamics of in involves lot of Vector Calculus, which I don't understand fully :)

The above book was referred by a friend who works in Boeing. Apparently, the author is a retired Boeing Technical Fellow and is highly regarded inside Boeing for his knowledge of Aerodynamics.

The author has a few videos on Youtube where he covers some of the topics from his book in some detail...
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by Zynda »

I have a question about practical interpretation of Angle of Attack (AoA) of an aircraft.

If I refer to any aerodynamics book, AoA is defined as the angle between the chord line of the airfoil (Wing) section and the relative wind flow direction as shown below.

Image

My understanding is that relative wind direction is usually parallel to the direction of velocity vector or movement of the plane. A couple of images to illustrate...

Image

Image

With reference to the last image, is the dashed line parallel (or aligned) with the chord line of the wing section? And the line above it represents the axis (or line) paralle to the fuselage length or longitudinal axis like shown below?

Image

If I could state the image in a different way, if the aircraft is executing a loop, at any point on the loop curve, one can draw a tangent. Does this tangent represent the velocity vector (wind flow direction) and the angle between this tangent and the airplane longitudinal axes represents the AoA?

Finally for a high alpha pass like this, the aircraft movement is parallel to the ground (velocity vector) but the plane axis is inclined probably nearing the critical alpha?

Image

I hope some one like Deejay who has practical experience with flying machines along with folks with good engineering/pratical knowledge can shed light on the above.
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Re: Aerospace Engineering - Aerodynamics, Structures, Avionics, Flight Dynamics - Technical Discussions

Post by Shubham »

Zynda wrote:I have a question about practical interpretation of Angle of Attack (AoA) of an aircraft.

If I refer to any aerodynamics book, AoA is defined as the angle between the chord line of the airfoil (Wing) section and the relative wind flow direction as shown below.

Image

My understanding is that relative wind direction is usually parallel to the direction of velocity vector or movement of the plane. A couple of images to illustrate...

Image

Image

With reference to the last image, is the dashed line parallel (or aligned) with the chord line of the wing section? And the line above it represents the axis (or line) paralle to the fuselage length or longitudinal axis like shown below?
The dashed line may or may not be parallel to the chord line of the wing. If it is parallel then angle of attack is zero.
Image

If I could state the image in a different way, if the aircraft is executing a loop, at any point on the loop curve, one can draw a tangent. Does this tangent represent the velocity vector (wind flow direction) and the angle between this tangent and the airplane longitudinal axes represents the AoA?
First of all don't mix longitudinal axis of aircraft with wing chord line. They both are different , the difference being rigging(incidence angle ) + wash in/out.

For AoA just need to find that sitting on the wing looking in the direction of wing chord from where the air is hitting you. For this drawing tangent in case of loop will give wrong result. Your understanding would be correct if you are able to realize that say in case of nil winds and an aircraft doing vertical climb with the nose pointing vertical upwards, then the AoA is zero and lift produced is zero (since aircraft has no horizontal motion wrt an observer on ground.)
Finally for a high alpha pass like this, the aircraft movement is parallel to the ground (velocity vector) but the plane axis is inclined probably nearing the critical alpha?

Image
As per me in the above image the aircrafts are actually at very low angle of attack ! Because the exhaust is aligned with aircraft longitudinal axis. If the aircrafts were at high AoA then these exhausts would have been horizontal or parallel to the ground

I hope some one like Deejay who has practical experience with flying machines along with folks with good engineering/pratical knowledge can shed light on the above.
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