Center of Mass

Aussie_Paul

A reforming stirrer!!!!!
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Since 1982 Gyro 5000+ mostly instructing, and approx. 200 fixed wing in the late 1960s.
We have been having some great discussions on Norm’s forum, re calculation of the CoM or VCoG.<br><br>As an older but not so bolder gyro pilot, I have a test that I perform to see how far out the thrust line and VCoM are. This is certainly not for someone who has not been flying for a lot of years and fully understands the CLT principles.<br><br>I take a machine out for a test flight and fly it at the trim speed, and if needed I adjust the trim speed until it is representative of the particular gyro type, lets say 50 mph. At this point I prefer the machine not to be stab equipped.<br><br>I let the stick go and slowly add full power. If the nose goes down and the machine increases speed, then the thrust line is above the VCoG. If the nose goes up and speed decreases then the thrust line is below the VCoG.<br><br>If the nose goes up slightly, the speed remains constant, and the aircraft climbs then we are talking CLT.<br><br>I then add the h/stab and set it so the response is still the same. The h/stab will then help to iron out the up and down bumps nicely. Also the faster the machine goes the more the parasitic drag of the everything hanging below the thrust line will be forcing the nose down. As this happens the h/stab begins to have a negative AoA and so opposes the nose down pitching moment of the parasitic drag.<br><br>The problem with this system is that you have to have a lot of experience to perform these tests safely. As the developer of Firebird, I am fortunate to have gained this experience over the years in sorting out the Raf stability problems, both pitch and yaw.<br><br>So far my experience has been proven by the CoM pic method. This expertise did not happen over night, but is the fruit of slowly working through these items with the help of Chuck B and others.<br><br>I hope that this report makes sense and is beneficial to others.<br><br>Aussie Paul.<br>
 

Echo

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Nov 3, 2003
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Singleton NSW Australia
Re: Center of Mass

Paul,<br>         If in your test you trim for a speed,fly hands off, apply full power & the gyro climbs but you have to hold the stick forward to stop the nose from rising too much - then what needs to be done to the machine? If anything?<br>   Cheers Eric<br><br> PS Ive asked before, & I may have missed it , but how do you post a pic?
 

Echo

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Re: Center of Mass

I read your post again & guess the thrust must be below the VCoG , as you have to hold the stick forward to maintain climb speed. The gyro will fly along happily hands off using small power changes to change ALT., but thats what happens under full power.<br> Eric
 

Echo

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Singleton NSW Australia
Re: Center of Mass

The gyro doesnt do much at first if you chop the power , ( I mean instantly)  I just let the nose lower itself slowly to maintain airspeed, then during the glide I have to maintain a fair amount of back pressure on the stick.<br>   Does that make sense?<br>  Eric :-/
 

mceagle

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Re: Center of Mass

Eric, <br>Initially it sounds like the thrustline to C of G offset is too far the other way. (ie. the C of M is too high) but considering that the Gyro stays fairly level on sudden power off, then it must be fairly close.<br>Maybe you have to use forward stick pressure on full power climbout because your engine is developing good horsepower.  
 

Aussie_Paul

A reforming stirrer!!!!!
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Ballarat Australia
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Since 1982 Gyro 5000+ mostly instructing, and approx. 200 fixed wing in the late 1960s.
Re: Center of Mass

My choice in order is CLT, then thrust beneath the VCoG.<br><br>A thrust line higher than the VCoG in excess of 5" on an open frame should have an effective airfoil h/stab.<br><br><br>With a cab or pod should not be flown without doubling the h/stab forces. Oh s**t, get my act together, they should not be flown at all.<br><br>Aussie Paul.
 

Udi

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Oct 30, 2003
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Beautiful CO
Re: Center of Mass

Eric,<br><br>It sounds like your center of mass is a bit too high.  Can you lower the center of mass a bit by changing the location of the battery or any other moveable object?  You may also change the angle of the engine so that the engine thrust line passes closer the CG.<br><br>Another option is that your horizontal stabilizer is generating a nose-up moment.  If your h stab has a negative angle of incidence with relation to the prop wash than it will generate a nose up moment as a function of power.<br><br>And, as Paul said, this configuration (nose up with power) is much more desirable than the opposite.  Your gyroplane exhibits stable flight characteristics and that's good.  Some fine-tuning can make it perfect.<br><br>Udi<br>
 

Echo

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Joined
Nov 3, 2003
Messages
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Singleton NSW Australia
Re: Center of Mass

Thanks guys for the input. Adjusting the HS is going to be my first part of tuning. There are a couple of things that I can shift to change the CoM.<br>   Finishing off my trainer has taken up all my spare time....not a lot of flying.<br> Cheers Eric
 
G

gummdropper

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Re:Center of Mass

I feel like your Center of Mass post is spot on. Thanks for the post. It has helped me a lot!
 
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