Reason for the Haing test

[Bob]

This is no doubt going to be an odd post to some of ya out there but haing in there...
...
Recently in doing another haing test to my Highly modified Bumble Bee I started thinking what is the reason for the haing test in the first place ?
Sense my Gyro is now far from a Bumble bee now i heasitate to even call it a Modified Bumble Bee.... its more as Sonny j says a "Kelly Kopter" hehehehe
... the major thing is that it is a tail dragger now not a tricycle landing gear setup.... and there is NO doubt what so ever that has messed with the ballance of the aircraft...
... but what exactly is the ballance of the aircraft in the haing test ? it used to set at 4-5 degrees in a laden haing test... Sonny j convienced me that wasn't enough nose down ... so alot more fiddleing arround and I got it to 9-10 degrees nose down in the haing test....
... but what exactly determans how far down the nose should be in a haing test anyway ? the amount of thrust expected from the prop ? the landing gear configuration ? the movement of the rotorhead ? or the paint color on the frame ????
hehehehe
sense this gyro is no longer stock ( I strove to make it safer on the ground! ) I realy can't go with the stock haing test settings and be confident it is right .

any help would be greatly appreaciated !
thank ya !

Bob.....

[mceagle]

Good one Bob - hehehehe

[gyroblackwell]

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 !

[Al_Hammer]

I think Tim said it very well. He beat me to it.

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.

[mcbirdman]

Hi Al - Can you edit your post to fix a mistake? Please name it Hang test info so that there is a better chance it will show up in a search for this information and not get lost. That would be too bad..... because this is too good...... jtm

[Al_Hammer]

Ok, James, thanks. I put Hang Test Info in the title. Mistake fixed.

[mcbirdman]

Thanks Al - I knew you would "fix" it

[Bob]

hehe ! Mceagle that was not ment as a joke, but a serious question !
<grin>... NO REALY ! HEHEHEHEH !
anyway thanks for the info .
it is as I suspected, just wanted to be sure...
I have hurd of many diferent haing test settings as well, some say 10 degrees some say 15 degrees nose down ... from what I gather it depends on the rotor head angle rotor head travle for and aft .

I have my gyro sitting at 9 to 10 degrees nose down now in the haing test , but I was wondering what it..."SHOULD be"....
but I am getting the impression that depends entirely on the machine.
its not "one setting fits all" so I will re-read your Posts Al and see if I can understand what I need to know to determan the nose down atitude... thank you very much for your responce !
......
However I have never hurd of a double haing test before ... and that sounds very interesting indeed ! How can I do one ... are there certain points that I must attach to ? like the teter bolt and Prop hub perhaps ?... or maybe the engine mounting plate ?
the tall pine trees I have here make the haing test fairly simple ,
and my digie camara will do fine for the photos....
Reason tells me that following the line stright down that holds the teter bolt to the ground ( which may or may not be at 90 degrees)
and another line from the rear at a steeper angle ... should intersect with the other line and pin point the center of gravity of the machine
I never thought of that before ! thank you ! thats bound to help alot
but thinking about it generates more questions !
I can't see how it would pin point the cog because the 2nd line can be placed anywhere.... say on the rear wheel for instance ....
perhaps if each tie point had the same amount of weight on them , but that would take 2 scales ....hummmm can you explain how a double haing test is done please ? thank you very much !
....
And I am happy to see another tail dragger Gyro out there !
I made my temp cheek plates out of 1/2" plywood like alot of folks recomend .... used 3 bolts to hold them on and after about 4 attempts
I finally nailed the 9.5 degrees mark ... then I cut my cheekplates out of 1/8" alum plate and bolted her all up and hung her again , just to be sure ... and it was the same !
.... I am at the Crow-Hopping stage in my training ... and I am trying to get this bird to react corectly.... so far it looks good ... I am waiting for better weather before I try again... its too windy now.
But these Questions keep nagging me and i won't be happy till I understand it all ! because its what ya Don't know that can hurt ya !

Thanks guys !

Bob.....

[Al_Hammer]

Bob,

Here are a couple of photos:

http://www.rotaryforum.com/forum/att...achmentid=6655
http://www.rotaryforum.com/forum/att...achmentid=6657

The tackle is used to set up the second hang point. Remember, you still have to be able to sit in it!
When the gyro hangs from this point, or from any point- the cg is going to be somewhere on a line pointing straight down towards the center of the earth.

For this test, it doesn't matter where the rotor head is pointing. You get photos and draw the lines. Using a computer graphics program you can overlay the images and find the intersection point which is the cg location.

The vertical location of the cg is affected by the mass of the rotor, so a weight that weighs the same as the rotor should be placed on the rotor head for the double hang test. (This is not true for the single hang test.)
It is easier to simply calculate how much the weight of the blades and hub bar will move the cg up.
Assuming you did the double hang test with rotorhead on, but not blades, and the total weight of the gyro in that config is Wt_1.
Multiply ( distance from cg to head) X (weight of blades/Wt_1)
That gives you the distance the cg will move up.

Then you can draw a line where the prop thrust is and find what the offset is.

[Bob]

OOoooh thank you, thank you, thank you !
My gyro is definately a HTL machine then for sure !

Many thanks Al You get a cookie ! <grin>

...now where did I leave that Pic at ....

C ya !

Bob....

[Bob]

Al...
I have another question for ya .
if the cg (center of gravity) in my haing test is in a direct line from the attachment point ( teter-bolt ) down a imagonary line to the center of the earth.... and the thrust vector line ( I asume a stright line from the center of the prop foward 90 degrees to the plane of the prop) then where the 2 lines meet is the center of gravity of the machine...
if i understand it right..... then where is the center of mass located?
I have gone to great lengths to keep all the weight as low as possable
for max Pengulim effect ( sorry for the spelling errors! ) ...so I know I have changed the center of Mass by quite a bit....
I know ideaily the center of mas should be at the thrust line if it were the machine would be a CTL machine... but its not ! the machine is a HTL machine or NCTL at best , I have tried hard to make it a Near centerline thrust machine but do not know how far off I am !
IS there a way to findout where it sits in this respect , besides calculateing every piece of weight and distance from the centerline and all that ? heheheh
though its not nessarry to know this stuff to fly it, it may help me in preventing a PPO of which I am very leary of
when I calculated the area of the HS for this beast I asumed 6" diferance between the thrustline and the center of mass...
but as we all know Assumption is the mother of .... well you know , its not good ! <GRIN>
for all I know... with that heavy wood tail section on it it may well be under the engine ... not infront of it !
and I am wondering how much that effects the safty of the craft !

thanks again for the help AL !

and GyroMaxwell .... get that thing hung and take another pic or two of it ! <Grin> ....can't wait to see a Pic of your creation in flight !
go for it man ! I'm Rootin fer ya !

thanks all !

Bob.....

[PW_Plack]

Bob, the thrustline has nothing to do with determining the vertical center of gravity. And the terms "center of gravity" and "center of mass" mean the same thing. You need first to determine the vertical center of mass, then find out where the thrustline intersects it, to find out how far off you are from CLT.

You can do lots of math starting with the weights of individual components and their relative distance from a datum line, or you can just take a completed machine and do a double hang test with photos. It's described in detail in this thread from April 2004: Double Hang Test
Note: The pilot must be seated in position to get meaningful results. The picture in which the gyro is balanced on the main gear requires some helping hands to be sure it's balanced on the main gear.

[Dean_Dolph]

It seems that the double hang test has become the standard for determining he vertical Cg. Was there something wrong with C. Beaty's spreadsheet that used scale weights and angle of keel tilt for input?

I don't recall the particulars but it sure sounded like a much simpler method. Granted, it did require more, and accurate, equipment but double hanging?

[Doug Riley]

Dean, I think that eliminating the math and the weights actually simplifies the process. In addition, the double setup helps you visualize what you're accomplishing and appeals to the many picture-thinkers among us. Some people did try Chuck's method, went awry and got impossible results.

Since you're going to do a single hang anyway, the double isn't much extra work.

[Dean_Dolph]

Since I haven't used either method I can't offer a valid opinion.

But, it seems to me that the double hang test and picture taking can also go awry or may be not! It just seems that the precision isn't there since it looks like it relys on the quality of the picture taking and then the interpretation of where the lines are and where they intersect. The location of the prop thrust line to determine the relationship to the Cg is one that is really suspect in my mind. I also suspect that I've forgotten that there is a method to conduct the double hang test without taking a picture.

I guess I'll have to admit that I just like the elegance of the spreadsheet method better even if it is more tedious.