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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
AndrewM wrote ...
I think you are still failing to understand Steve's comment.
Have you tried drawing a force diagram? I am interested to see if you can locate the restoring moment you assume exists.
I realize i maynot have a full grasp on the statics or dynamics at hand, but lengthening the vertical moment arm did have a profound efffect on pitch and stability... it not perfect and 3 inches, but its flyable and manageble..
ill will draw a free body diagram as soon as i figure it out, but i want some evaluation from others on the situation.
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Patrick wrote ...
AndrewM wrote ...
I think you are still failing to understand Steve's comment.
Have you tried drawing a force diagram? I am interested to see if you can locate the restoring moment you assume exists.
I realize i maynot have a full grasp on the statics or dynamics at hand, but lengthening the vertical moment arm did have a profound efffect on pitch and stability... it not perfect and 3 inches, but its flyable and manageble..
ill will draw a free body diagram as soon as i figure it out, but i want some evaluation from others on the situation.
im totally exhausted, ready for bed...
Well I'll just give you the answer - increasing the arm length increased your moment of inertia (by a factor of 4) so your instability just progressed 4x slower. But there is no possible "pendulum" effect in your system.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
AndrewM wrote ...
Well I'll just give you the answer - increasing the arm length increased your moment of inertia (by a factor of 4) so your instability just progressed 4x slower. But there is no possible "pendulum" effect in your system.
ok, but that gives my gyros and PID loops time to put the commands on the servos, and gives the mechanical train time to alter the forces, right?
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Absolutely, lots of the flying toys you see now are statically unstable. Active control makes short work of the issue provided you have enough inertia to give it time to react.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Ok so i was wrong about the reasoning (the pendulum) but i was right that increasing the distance bewtween the CL and CG that the time of instability is longer? right?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...
Well, congrats on getting it to fly!
yeah and as unstable as it is, we only broke one of the cheap servos... so now its will soon be time for the Savox servos and good 11" props! i just need to make it more stable.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Note attaching a balanced wooden stick on a hinge to the airframe with string will provide a "safe" platform for calibration (or finding cg). Without this simple rig people tend to chew through more props etc.
Out of all the sensors, you will likely find a good 3-axis compass provides cleaner data. However, large metal beams/cars will cause interference (most GPS units also suffer precision loss in urban settings).
Registered Member #49
Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362
Patrick wrote ...
Ok so i was wrong about the reasoning (the pendulum) but i was right that increasing the distance bewtween the CL and CG that the time of instability is longer? right?
Yep.
Carbon_Rod wrote ...
Note attaching a balanced wooden stick on a hinge to the airframe with string will provide a "safe" platform for calibration (or finding cg). Without this simple rig people tend to chew through more props etc.
This idea also reminded me - you might consider attaching a number of 4' long wooden sticks to your models primary axes. These will greatly increase your moments of inertia while adding fairly little mass. It will be much easier to fly and tune.
An example is these popular coaxial helicopters:
There is, in fact, nothing inside that fail boom; its just a thin plastic shell. However its not purely decorative; without that boom it is completely unflyable as the gyro and control loop cannot damp the oscillations.
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CSU Chico Tilt Rotor Flying Machine, (Airframe and Electronics, 2 of 3).
