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Relaxed static stability F-16 v SU-27 CP aft CG gyro design

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Relaxed static stability F-16 v SU-27 CP aft CG gyro design

Old 05-29-2021, 11:53 AM
  #1  
amckevitz
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Default Relaxed static stability F-16 v SU-27 CP aft CG gyro design

I’ve heard that the F-16 utilizes fly by wire in a far aft CG position and this planes stability is referred to as having “relaxed static stability”

CG is center of gravity
CP is center of pressure

CG is still forward of CP on the F-16. CG is way back there, granted that.

now I was watching the 2 plus hour documentary on the su-27 titled something like the greatest fighter in the world and they discuss that their designers moved the CG aft of the CP!

is this true?

it makes sense too, that CP moves and CG does not, that supersonic flight transitions CP further aft and becomes more inherently stable.

see 1h 08min 30sec Here:

U tube extension /watch?v=KFkVDZWwK1w

(I’m too new to post links according to forum rules)


Looks like the su-27 has negatively static stability. They claim that CP is forward of CG which is akin to controlling a dart thrown at a dartboard backwards! (Tail first).


is it unreasonable to design and fly an rc plane like the full scale F-16 or su-27 using a gyro?



Andy


Old 05-30-2021, 08:28 PM
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otrcman
 
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"is it unreasonable to design and fly an rc plane like the full scale F-16 or su-27 using a gyro?"
Andy[/QUOTE]

In a word, yes. It would be unreasonable. To do what the F-16 and su-27 do goes far beyond a simple model airplane gyro. It's way too deep to get into here, but suffice to say that the full-scale manufacturers employ a very high level of sophistication in understanding the behavior of their craft and in mechanizing the control systems.



Old 05-31-2021, 02:16 PM
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amckevitz
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Well I’m game. Surely somebody has done it already. A reason for my interest building in this area is that if we narrow down to pitch control only, and consider that the 1984 X-29 sample rate for the canard was 40hz or so, then we can do this in a model today. I still think that it carries the title for most difficult or unstable pitch control aircraft to date. Today, our refresh rates are higher. High level functions can come later through use of a pitot tube and flight computer that is now well over 10 years old (RVOSD brand for example).
Old 06-08-2021, 12:16 PM
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BMatthews
 
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The folks that fly the flat foam flippy flyer models typically run them with the CG a little behind the Neutral Point. It helps them for doing hovering maneuvers.

As for doing this on a model I think it is possible. But I'd go with one of the better controllers out there that come with the accelerometers as used in quads to detect the unwanted excursions. Also remember that the pitching moment on a small compact model is going to be a lot less than a full size X29. So you will likely want a more frequent response than 40 samples per second. I'd also suggest that the pitch control servos be faster than normal speed and in fact you may need to upgrade to S bus options since the normal pulse rate to servos using the usual 3 wire connections is only 20 to 25 pulses per second. So even if your controller samples any possible pitch excursion at 100 samples per second it will only get a chance to update and send a signal change to the servo once ever roughly 1/20'th of a second. From the little I've seen about S bus though it gets around that limit. You'd want to confirm this to be the case.

The other more normal option is simply fly with your CG at or just a whisker ahead of the NP.

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