PDA

View Full Version : Center of Vertical Gravity

Sea&sky
03-16-2006, 08:37 AM
I was wondering how one would determine where the center of thrust should be located with respect to gyros?

Also, could someone define hang testing?

Doug Riley
03-16-2006, 10:11 AM
The line of prop thrust (not to be confused with the ROTOR thrust line or RTV) should pass close to the aircraft's CG. Plus or minus two inches is an often-cited number. This minimizes pitching moments caused by the engine. Pitching moments (from misaligned prop thrust or any other cause) can lead to catastrophic instability unless a horizontal stabilizer with adequate power is fitted.

The basic hang test simply involves hanging the aircraft from its teeter hinge point, suspended from a fixed object high enough that all wheels are off the ground. This test establishes the position of the line connecting the teeter point and the CG. The angle that this line makes with the airframe determines (1) the angle at which the airframe will fly at cruise and (2) whether the control head will be properly centered between its pitch stops in cruise.

A double hang test takes the process one step further. Its purpose is to find the exact location of the CG in both the vertical axis and the longitudinal axis. It requires suspension from first the teeter hinge and then some other point on the frame. The point where plumb lines from each suspension point cross is the CG.

I wrote an article for the PRA magazine detailing the double-hang procedure. They'd gladly sell you a back issue for a small price, should you want all the gory details.

Sea&sky
03-16-2006, 10:24 AM
Thank you,
Is the hang test done with the pilot sitting in the seat, or empty?

And yes, Iwould be intersested in that back issue.

C. Beaty
03-16-2006, 10:35 AM
Here’s a sketch that expands on what Doug said above.

The Center of Mass of a perfect bagel would be at the exact center of the hole, but few things made by Man are perfect.

Say your bagel is lopsided and you need to know its C of M.

Attach a thread to one edge and let it hang while taking its picture. The C of M would lie somewhere along the line of the thread.

Attach the thread to some other edge of the bagel and take its picture again. I show the thread attached at points 90º apart but that need not be so; however, the nearer to 90º, the smaller the potential error.

Superimpose the two photographs and eureka!, the crossing point of the projected lines of the threads is the Center of Mass.

Doug Riley
03-16-2006, 11:01 AM
Sea:

Yes, you're trying to duplicate flight conditions, so you sit in the seat. The really fussy people will do it with the fuel tank empty and again with it full.

Get hold of PRA and ask how much for that back issue. Tell 'em Riley sent ya. Website at www.pra.org.

Sea&sky
03-16-2006, 11:48 AM
Thank you!!!

PW_Plack
03-16-2006, 02:21 PM
Sea,

Paul Bruty documented a method to do this with a standard hang test to get one line, and balancing the machine on the main gear to get the second line, here (http://www.rotaryforum.com/forum/showthread.php?t=982). While it looks like there was a slight error transfering the line to the second photo in his post, the test should be easy to do.

Sea&sky
03-16-2006, 06:23 PM
As per Paul's example, it makes sense.
Although the machine in this case is not actually balanced on the main gear. (The balance point would be slightly forward.)
It simply uses the main gear to get an intersecting line in order to determine CG. Is that correct?

PW_Plack
03-17-2006, 01:32 AM
Sea,

The angle at which the gyro is tilted while resting on the mains is not arbitrary. It's the angle at which the nose and the tail both become "weightless."

All pusher gyros of which I'm aware have the main gear axle behind the CG with fuel and pilot aboard, and so it is with the machine Paul tested. This allows us to taxi with the nosewheel on the ground. To make it balance on the mains, you'll need to raise the main gear on blocks and tip the machine back until it just balances, meaning you could let go of the keel at both nose and tail ends, and it would just hang there.

As with the plumb line through the teeter bolt in the hang test, in this test, a vertical plumb line passing through the main gear axle will also intersect the machine's CG. It will do so at an angle different enough to produce an easily seen point of intersection with the one from the hang test. That intersection is the machine's CG.

Remember that there are no separate "vertical CG" or "longitudinal CG." The CG is a single point, and we're concerned here with its relationship to the thrust line, usually considered to be the same line as the prop shaft. In most fixed-wing aircraft, the datum is treated as a line or plane to simplify calculations of balance, because the CG's vertical location is never an issue, but the actual CG is a point.

There is also a "lateral" location by which we could describe the CG, but most aircraft are so close to symmetrical left-to-right that the lateral location of the CG can be assumed to be in the center when the aircraft is viewed from the front or rear.

Sea&sky
03-17-2006, 04:53 AM
Oh, I get now...
Thanks!!!