Design position of CG

[ckurz7000]

Friends,

in another forum a question cropped up regarding the position of the CG. Someone maintained that it is a design requirement that the CG be directly underneath the teeter bolt when the gyro is in a horizontal position.

I understand that the CG will probably not be far off this position but can't see why it would be a design requirement. After all, the actual position of the CG is determined by the number and weight of occupants as well as the amount of fuel on board, etc. So wherever you design it to be, it won't be there in all likelihood when you fly in reality.

-- Chris.

[Jim]

The CG should NOT be directly underneath the teeter bolt! --- See other forums where the 11 degree hang-angle is discussed.

Jim

[Jazzenjohn]

Correct me if I'm wrong but, I would suspect that it would be difficult to have the cg anywhere but exactly under the teeterbolt when hanging or decending w/o power. When in flight it should be moved forward by the thrust of the engine.

[Passin' Thru]

Originally Posted by Jazzenjohn

Correct me if I'm wrong but, I would suspect that it would be difficult to have the cg anywhere but exactly under the teeterbolt when hanging or decending w/o power. When in flight it should be moved forward by the thrust of the engine.

John, you are correct except for one little detail! He said; " a design requirement that the CG be directly underneath the teeter bolt when the gyro is in a horizontal position."
That just ain't right!

[Fl90]

Hi Chris, the CG will be directly under the teeter bolt when the gyro is hung from it. The starting position is 10 degrees(+or- a degree) nose down, with the gyro ready for flight. Ready for flight includes fluids, any equipment, and pilot installed. I say starting position as the efficiency of the rotor, disk loading, pod, stabilizer, and any other thing attached will have a bearing on the flight angle. From there you can adjust for level flight at cruise. The biggest thing to watch for is control range, make sure you are not flying extreme nose high or low before getting out of ground effect.

If there are any faults with this post, please feel free to correct or add to the info.

Phil.

[Jim]

Unfortunately I do not have the time to go in to detail. About the hang test: Tim Blackwell and Al Hammer wrote as follows:
the hang test is to make sure that with your weight in the seat and full fuel .... that you will get a 10 degree nose down angle. this is to make sure that "in flight" the rotor system will be balancing at the proper angle of attack on the rotor system. This gives you the proper amount of travel in the stick for all directions of movement, and enough room for spring tension adjustment for trim.

I am doing the same on my tractor gyro right now. I have to find the best location for the rotor head mounting based on the proper "down hang" angle.

I am using a set of cheek plates with multiple hole locations. I am matching my weight with sand bags (same for the fuel) and moving the hang point until I get the proper hang angle ( 8-10 degrees nose down ) In actual flight the nose down angle will actually only be about 3 degrees, and I will adjust my H-stab to create that angle of attack in flight.

Hope this helps, and I hope others chime in here if I have this wrong !
1. __________________
Tim Blackwell

The hang test takes advantage of two facts: One is that the rotor thrust line is always perpendicular to the rotor. If you center the controls during the hang test, the rotorhead should be near level to the ground; this way , you know that the imaginary rotor thrust line is a vertical line passing through the center of the rotorhead.

The second fact is that , when hanging, the cg will always hang exactly below the teeter bolt, and so will fall on the imaginary line I just described. Why is this useful? Because, in flight, the rotor thrust line will also pass through, or near to the cg, so the hang test simulates an in-flight condition. It makes the assumption that you have CLT thrust, otherwise there is no assurance that the rotor thrust line will be where it is during the hang test simulation.

However, in flight, the rotor is not level to the ground- it is angled back somewhere around 11 degrees. Your mast will then be about 11 degrees further back, in flight, from where it is in the hang test. It may not be exactly vertical in the test, since the cg may not be exactly on the mast. If it hangs at minus 2(tilted "forward"), then in flight it should be at plus 9. If the keel to mast angle is 9 degrees, then the keel will be level in cruise in this particular case.
You want the controls to be about centered when the rotor thrust line is passing through the cg in flight. This gives you plenty of forward and back stick travel before hitting the stops, so that there will not be a danger of running out of stick when you flare to land or when you need to get the nose down.
The same principles apply, no matter what mods you've made to the gyro. Just remember to measure the hang angle at the mast, and figure that in flight it will be about 10 degrees further back. From that, you know about where the keel will be sitting.
If the design is not CLT, then the rotor thrust line may not pass all that close to the cg, and the hang test will not insure that the fore and aft travel is exactly centered, but still it will be reasonably close.
To find out what the prop thrust offset is, relative to the cg, you would neeed to do a "double" hang test. That involves hanging from two different points, taking photos, and drawing lines from each attach point straight down, and then seeing where the lines intersect.
1. __________________
Al Hammer

[ckurz7000]

Thanks guys for the helpful replies. I'm also on the German gyro forum and there someone persistently claims that a gyro needs to be designed so that the CG is exactly underneath the teeter bolt when sitting on the ground. I could not for the life of me think of a reason why this needs to be just so and a couple of reasons why this wouldn't make sense.

Thanks for your posts, -- Chris.