mardlamock

"very powerful" Is not useful data when seeing wether it could be possible or not. The control surfaces can only provide so much lift at a given airspeed, therefore can only keep an aircraft level within a certain range of locations for the CG. If the movements are faster than the autopilot's reactions, then the aircraft will pitch up as I said before. And you have also got to take into account that if its an airbus, the control surface's AoA will never exceed what is allowed by the structure, making it even harder for the autopilot to remain under control of the aircraft.

You dont seem to be getting it, the control surfaces can only go so far, if the CG can change significantly what the control surfaces do wont really matter, what will matter will be the attitude the plane was in before the change. Give me a minute to draw it

I know my static stability, what you are saying is right though, if the AoA was negative then it would just pitch down very violently, but that would only be the case if the plane was accelerating downwards. The plane was IIRC descending at a constant speed, meaning that it had a positive AoA at the time, therefore making it possible to pitch up only using CG changes. The control surfaces wouldnt be generating enough of a moment to counteract that generated by the wings, so long as the CG was behind a certain location, which is what i am trying to figure out. Ill draw a picture to better illustrate what i am saying

Do you know of any good sites to get the technical info? Such as usual position of the CoG, with PAX, where you start losing authority, etc.

The flight computer cant change where the CG is, and if it is too far out then it doesnt have much control authority.. The real question is wether or not you can change the location of the CG such that the control surfaces cant control pitch.

I know, what I am talking about is starting a very steep climb going along the mountain slope just as the plane is about to hit it. If they were going at that speed and dropped to say 140 knots they would have climbed like 300 meters. The pilot could roll the plane for sure, but lets assume he doesnt because he is just waiting for it to crash. What do you think?

Not necessarily, couldnt it be thatthe plane was long enough and the passengers fat enough so as to completly shift the CG such that it is always behind the CL, no matter what the pilot did? And also, couldnt they have all waited until the last moment and then pitched up with the slope of the mountain, reducing the airspeed and hitting the ground at lower speed? Sort of like what this guy does

THANK YOU! Im not so crazy after all! You just made me happy, I feel smart now. I ll see if I can do it on excel, still struggling with matlab

Hello, I was just wondering if there is such a thing as getting the approximate solution to a differential equation with a formula (say a polynomial) by using interpolation on an already existing numerical solution (through RK). Is there? Im doing something of the sort but would like to know if anyone else has done it before. Thanks!

Without power they would move towards the nose and allow the plane to glide. The question is wether they could have more authority than the control surfaces.

The germanwings crash got me thinking, and im just wondering what you guys think. Could the passengers in a commercial aircraft actively control the aircrafts pitch by moving towards the aft end and therefore changing the CG? I think so, but maybe you can get some numbers on how plausible this would be.

The asteroid belt, mars colonies will get some supplies from them, and the earth will get other resources.

I highly disagree with pretty much everything you said. First off, if you have a cost per kg of $1000 it is a lot cheaper to send a bunch of satellites to handle communications than to build a tower on the ground, and controling satelites the satellites is as easy if you have the right software. You ve got a lot more coverage with satellites and you can provide your services internationally (cuz you rotatin), if you have a large enough constellation you can totally get constant coverage on a certain location. And the market can grow, a lot, we are seeing more and more requests for a faster mobile internet with greater coverage, satellites could do that. As for the asteroid mining part: 1-You dont need to send all the equipment and fuel to the asteroid, if you made your moves right you could use the resources found there and not have to send all the fuel, and if you develop space manufacturing processes, you dont need to send ''all'' the equipment. 2-Sending it back piece by piece could actually be rentable, it currently costs 40k/kg to return mass from orbit using the dragon spacecraft, that could be cut to 10k/kg if we assume 1k/kg to launch, the current price of gold is 37k/kg, you ve already got a return percentage of 300% (of course that is not counting all other costs, but you can totally do it IMHO,and the price of gold is prolly gonna go up in the upcoming recession). 3-So what if you crash the market price? It will never go below the operational costs, because you wouldnt sell it otherwise, just as there is almost no scarcity of bread there is also no scarcity of bread producers. The increased supply in those metals would also open up room for highly efficient and cheaper electronics, which could even be reflected in a surge of demand.

If you got a propellant mass ratio of 4:1 you could almost cross the karman line, you'd have to launch from around 30k meters for it to be even remotely possible though. A high powered rocket without much added stuff (a light parachute and camera) could do it. this rocket would probably make it into space.

That first video was really cool!

But would actually help the aerospace industry as a whole by reducing the costs and allowing for a more competitive environment with lower barriers of entry. Two things would happen overtime, american engineering would reduce its cost because companies would only hire those that werent that expensive, and second, the price of foreign engineernig would go up due to an increase in demand. After some time you´d reach an equilibrium point where the previously inflated engineering costs in the US would go down to their actual market levels and foreign engineering costs would go up due to an increase in demand.

I agree with everything you said, I believe that the industry is being held down by the lack of access to cheap foreign labour and exchange of ideas. Imagine how low development, construction and refurbishment costs could go down if the US launch market had access to foreign engineering (say indian, we re talking about their salaries being almost one tenth of their american counterparts while still keeping the technical expertise).

So basically what you are proposing is getting rid of the changing area term on the drag equations and replace it with a simple Cd or Cl that changes as a function of AoA? I understood the first derivation and how it may be useful for a plane (math sense) estimation of the Cd or Cl, but I dont see how it may apply to a more complex shape such as a large tube with a cone on top (aka. rocket). Check this out and tell me what you think, I think it goes along the lines of what you are saying http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml.

I think you might if what you mean is that I shouldnt use a projected area for lift, that I should keep it constant. As for the drag, it will be area calculated as a function of the angle of attack, im pretty sure of that. Here on wikipedia basically says that the area used in the lift equation is held constant, so yeah I might have ....ed up there. http://en.wikipedia.org/wiki/Drag_(physics)#Drag_at_high_velocity. As for the translational motion, I really hadnt pictured it like that, but the way you put it makes me rethink it. I just intuitively though that it would not produce any translation. I will probably have to change the equations to account for the added area from the airframe as a function of the AoA, whilst removing just that from the lift equation. I ve been comparing the model as it is right now to OpenRocket and it seems to be holding up pretty well, around 3% error in height and 5% in velocity, which is not that bad comparing all the mistakes I have to fix and the fact that that is still using euler's method. Anyways, thanks for the comment dude, you cleared up a few misconceptions.

Because doing an actual analysis on the Cd is a lot harder and would probably require CFD to do properly, whereas thin airfoil theory is decent enough for Cl. The Cd also changes with respect to the Mach number, but that should also be done with CFD. How would there be translational acceleration when the center of drag is offset from the CoM?

The drag coefficient is always constant, but the reference area should change as a function of the angle of attack. I was going to include that in the model, but the problem is that if the center of the area (or the center of drag if you want to call it that way) is misalligned from the center of mass, then you wouldnt be getting translational acceleration, but rather angular acceleration. And that just seemed wrong to me. On the other hand, the Coefficient of lift varies as a function of the angle of attack, according to thin airfoil theory, you can approximate it linearly by saying it is equal to 0,0111*AoA. Thats what i did.

He told me he used information he found on regular aerodynamics books. What are you building your model for? I have to say, as hard as it may be, thinking about what goes into the behaviour of a rocket or any object in the atmosphere is extremly fun. And its not that hard even, it just takes (like everything) a bit of time to work things out, I mean, three1415 and I are still in high school but we seem to be doing pretty well.

Oh I see now, looks cool! Maybe I will try it out, im feeling particularly lazy today so dont count on seeing it today, but maybe further down the week I will compare it to Rk. Im testing the old euler method against some flight data from http://www.altduino.de/ this guy, Its holding up pretty well, but I still want to do the orientation and stability analysis. What did you compare the different approximation methods for? I remember you saying something about having a model for drag as well, would you mind explaining what you used it for? Anyways, thanks man, ill update you when I get Rk working and compare it to Euler Cromer.

You have confirmed my suspicions, the only problem is that I dont think adding an energy constraint is possible, maybe making it so that it doesnt increase after MECO could solve it, but if it is harmonic then I am still going to get oscillations. I need some sort of damping but I dont know what force might act as such. Ideas?

Yeah thats a typo, same with the projected area. Funny thing you say that, I had just done that sign change in excel when I was getting positive drag all the time.