Bharat Rakshak

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Hi Vina/Shiv Did you guys look at the link that I posted that talks about the F-8 FBW aircraft. It talks about the commands being generated by the FBW computer at the rate of 40 per second. Vina From the plots shown Do you agree then that the elevator changes position at a rate greater than 2hz? Tha...

Hey Indranil Just ask your friend what does he think about updating the elevons twice/thrice a second. I believe that you know him well and trust his judgment on matters related to control. Maybe he could then inform all of us through you what he thinks about this rate. I guess you would be more inc...

Indranil What your friend says is right, even if the sensors update at N hz which is greater than the controller update rate n hz(as a rule of thumb this update frequency is taken as large as possible with the minimum recommended at 10 times the closed loop bandwidth of the control system) this does...

Hi all I could not complete the earlier post as I had to run off to school for some work. A bit of background on the plots presented above, the red line is from the Iron bird simulation that was done at ADE while the black line is from actual flight test. The last two plots show the elevon deflectio...

Ok I was somehow able to get the image scanned into photo bucket here it is http://i813.photobucket.com/albums/zz54/rajeevcm/LCA.jpg Source:"DESIGN, DEVELOPMENT AND FLIGHT TESTING OF CONTROL LAWS FOR THE INDIAN LIGHT COMBAT AIRCRAFT" by Shyam Chetty, Flight Mechanics and Control Division, ...

Can anybody help me to post a scanned copy of some LCA simulation and flight test results. Searched online could not find how to do it

Indranil, I am not sure how much you follow flight control systems. But if you look into conference and journal papers most of them use control laws of the form Delta_e = r - Kx and this command delta_e is given to the elevator actuator each time this law gets executed in the computer. Most current ...

Vina One more post and that's it, when I say the update rate of 20Hz is required implies that the control surface deflections are going to be very small even in the range of a degree, because the error between desired and actual is going to be small and hence the corresponding deflection is going to...

Vina/Shiv and Others I am not trying to prove that Air Marshal Rajkumar is wrong. I have a feeling that he intentionally did not give the correct update rate that is all. I am not going to belittle anybody as I have insisted in all my posts about the air marshal that he knows what is right but does ...

Shiv Isn't it true that in FBW the control surfaces are commanded by the Flight control Computer so if the elevon actuator is effected only at 2 hz, even if the pilot moves the stick at a greater rate he will not get those deflections at a higher rate? Am I right in this? In modern aircraft, where c...

Maybe I need to get clarifications on some points 1) According to an earlier post "The control surfaces move a couple of times per second" What I get from this is that the flight computer commands the elevon actuator every 0.5 seconds or say 0.6 seconds I am assuming that this does not mea...

Vina The bicycle example is really good, the reason that most humans can ride a bicycle is due to the fact the time to double is really large and a human can perceive this divergence and apply correction before this divergence becomes too large. In the LCA the time double is relatively short as comp...

Shiv Lets look at what I said. In the scenario that I presented the aircraft is sitting at the gate, there are no aerodynamic forces acting on the control surfaces, the only motion that the ailerons go through are due to the commands from the actuator and if they move at a faster rate than 2 hz this...

Vina They need to be 0.5 seconds apart since the elevator can move only twice a second. According to the earlier post "The surfaces move about a couple of times per second" . This implies that the elevator can attain only two discrete positions in a second. I am of the opinion that that is...

Some more info on the updation rate of the elevator actuator. if you refer to this document "APPLICATION OF MULTIVARIABLE -CONTROL THEORY TO AIRCRAFT CONTROL LAWS" page 116 shows a graph of the commands to the actuator in simulation, but this is a high fidelity simulation as this report wa...

Vina But according to the previous post the Elevator moves only a couple of times per second. So using that analogy between time t=0 and t=1, the elevator moves at say t=0.5 seconds and t=1 second and retains the deflection commanded at t=0.5 seconds between t=0.5 and 1 second (using a zero order ho...

Shiv Lets look at this upward gust. The upward gust causes an instantaneous change in AOA since the AOA by definition is given by AOA=atan(U/W), where U is the speed of air along the body x-axis and W is the air speed along the body z-axis. At trim U and W are constant, now as the aircraft enters an...

Indranil You are right the model I suggested results in small deflections of the control surfaces which are not discernible to the naked eye, but this keeps the aircraft from entering into oscillations in pitch . I am not sure what you mean by running out of CPU clock cycles, if you are sampling the...

I worked on a GA aircraft where due to some computer limitations we could update the control surfaces only at a max of 15hz. This caused the plane to get into oscillations in pitch. Let me explain why I feel that the elevator changes only 2 times per second may cause instability Current digital cont...

Hey Shiv I do not want to anger anybody. I have had issues when using FBW with control surface at updation rates below 20Hz on a general aviation aircraft. Maybe ACMR means that the bandwidth on the elevator actuator is 2Hz and not that the surfaces move twice every second. On the power requirement,...

The following is from aviation week where they talk of the response time of the Dassault 7X and the Rafale and it is around 12.5ms Compared with the Mirage 2000 or Rafale, the Falcon 7X has inherent aerodynamic stability and is not designed to be flown above Mach 1. Dassault determined it only neede...

Well, I knew this would cause questions, that is the reason that I thought a lot about posting this. My guess is that Air Marshal Philip Rajkumar did not give the correct update rate of the control surfaces in his book for whatever reason. I am sure that the update rate on the surfaces will be close...

Instability : May I ask how will you quantify relaxed instability? The rate at which the amplitude of oscillations would increase along any axis! FBW would control that. I dont know of any open source where this critical data for this FBW dampening/relaxing is published for any of the fighters. I w...