weight and balance

Here we have Dennis scratch building, Eric building to plans, Vance with a ready build with weight and balance and limits available and test flown.

Each is a somewhat different situation. Dennis is going to have to do quite a bit in determining his CG and where his safe limits are. Eric hopefully will have the information provided with the plans and provided he does not deviate much will have been completed towards the problem. Vance had to know about the theory of, and how to work the weight and balance sheets provided and remain within the limits that had already been found out. He is of course making major changes now and these will all have to be recalculated, the machine test flown and new limits established.

To my mind rotor aerodynamics are not as simple as fixed wing aerodynamics making it a somewhat more variable and complex problem, but then again I am a beginner in this field, definitely no expert, and may be making more of it that there really is.

Vance sounds as though you and Ed are really enjoying the trip. Hope you have a great time when you get there.
 
Long story short the only design criteria I used for CG was the hang test.

This is how I did mine also. I used wooden parts for mockups and experimented with the mast/rotorhead angle until I got between 9 and 13 degrees nose down. I used the top longeron as "level".

I've made several changes to my fuel tank location and a few other things along the way and I always check the hang angle afterwards.
 
Single hang or double hang?

My understanding is single hang and keel angle determine control movement is sufficient.

Double hang establishes CG and is not concerned with keel angle.
 
Thank you guys

Thank you guys

Thanks to Vance for taking the time away from your trip to help me understand the process, Thank you Resasi for the good words and encouragement, Thank you Chuter for that very informative video it is a great tool for visualizing what is happening while we're enjoying the ride.
 
Single hang or double hang?

My understanding is single hang and keel angle determine control movement is sufficient.

Double hang establishes CG and is not concerned with keel angle.

That’s what I thought too. So I am a bit confused now. This is my understanding of the CG or hang angle. The nose is supposed to hang down about 10o because the rotor flies with a tilt back angle of 10o so the keel is near level in cruise flight and the stick is centered. If the gyro was setup with a 0o hang angle then you would run out of forward stick as the gyro would nose up excessively. Same with going say 20o hang angle the gyro would require excessive back stick to keep the nose up.

The double is only for determining the vertical CG to see where the thrust line is.

I think most rotors fly with a 10o angle to the relative wind. I remember reading about the rafs I presume with out horizontal stabs had their hang angle less than 10o I think around 5 to 7o to accommodate for the nose down pitching caused by the high thrust line. So they were setup tail heavy to offset the HTL.

I guess with a horizontal stab on a gyro the stab can be adjusted to account for modest thrust line offsets.

Please correct me if I am wrong. We have to do a hang test on my gyro and I want to know its right!
 
Sort of drifting a little away, but still to do with CG and hang tests, here are a couple of earlier threads that dealt with the subject and some thoughts from Chuck Beaty

http://www.rotaryforum.com/forum/showthread.php?t=21665&highlight=Hang+testing

http://www.rotaryforum.com/forum/showthread.php?t=12759&highlight=Hang+testing

http://www.rotaryforum.com/forum/showthread.php?t=14109&highlight=Hang+testing



Some brief notes I have assembled for my son and I so far, for when we come to do it.

With mast and keel 90 deg to each other the hang test should result in torque bar between 0-2 deg. nose down. Between 9-11 deg nose down on the keel and engine mount( these on our are parallel) and the mast 79-81 degrees forward with the rotor head shimmed to the mid position and the control stick centered. Pilot in the seat dressed for flight. Do this for all conditions three times from empty tanks half tanks and full tanks to get an idea of how it shifts during flight. For the hang test. Leave the rotor blades off.

With a camera on a stand take a pic as it hangs with pilot sitting on it. The camera should be at the engine's height.

Then .....at the same camera height relative to the gyro, put the main wheels on some stands that will allow us to tilt the gyro back until the gyro will balance on the main wheels with the pilot in the seat (ie not tilting back or forward) with pilot and rotor blades installed. A good way will be to have tie-downs at both tail and nose but loose enough to allow us to determine the exact balance point so gyro will fall either way if disturbed but the ties will stop the ensuing fall. Take a picture of that with a vertical reference ie a piece of string with a weight hanging at the end (a plumb bob) somewhere in the picture. From these 2 pictures the vertical CG can be calculated/seen.

To get the weight use 3 scales, one in each wheel, or more likely 1 scale and two blocks equal to the scale's height under the wheels so that the gyro will be exactly at the same position for the 3 measurements and not tilting from one side to the other each time you move the scale. Add the 3 numbers for the total weight.
There is also an Excel spread sheet that can be used to calculate the vert CG from these reading, I still have to do some practicing with this and make sure that I have the correct chart with the correct formulas locked in. Udi posted one of Chuck Beaty's and I have it saved somewhere.

Hopefully we will then have adequate control movement and enough information to find where our CG lies and where the thrust line and RTV pass in relation to it.

Any misconceptions/inaccuracies/comments on any of this please do so as we are hoping to do it soon and I don't want to screw it up.

A question here. Approximately what degree of tilt should one have on the rotor head with the control rods fitted. I have seen it expressed as approx 1.5'' shorter on one rod but was wondering if a rotor head tilt angle is a better way to go? Or, does one not build that in, simply cope with it on lift-off?
 
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I am deliberately posting this thread separately from the above as I found it rather instructive on how easily it can be for some of us to get the procedure misunderstood. I know that I have misunderstood lots of stuff and probably will do so in the future.

Also how people will persist and still help even when it gets a bit contentious.

Start from post number 90 in the thread.

http://www.rotaryforum.com/forum/showthread.php?t=12238&highlight=Hang+testing&page=6

It does highlight the point Vance made about people (possibly like me) trying to help but perhaps unintentionally misleading.
 
Resasi, I think you have the hang and balance test right.
The 1.5" difference on the control rods sounds a bit much. I do not have my machine here to check but I think I ended up with about .75" difference in control rod lengths.
 
Resasi- I had some double hang tests going at Bensen Days a few years ago. We had a nice high hoist point..and plenty of room.

This test finds the CG of the aircraft. You first hang from the teeter bolt with the pilot in it...wheels off the ground....and use a vertical laser trace to easily mark one vertical line. Line up the vertical laser trace with the center of the teeter bolt...

Tape some posterboard on the craft near the frame and mark this laser trace.

Have the laser trace perpendicular to the aircrafts side...so as to minimize any parallax error. In other words...if the beam is at a 45 degree angle....the laser mark on the posterboard will be off an ever increasing amount that it is away from the vertical line going through the center of the aircraft.

Now put the rotorblade on.....and the trick here is to hang the whole machine and pilot from a distance away from the center of the rotorhub. Any distance away will create a new hang line that will intersect the other line already drawn. The further your 2nd hangpoint is away from the centerline of the hub...the less acute the intersecting angles are...and the easier it is to mark exactly the CG of the ship.

Now...many gyros do not have a handy place to for this 2nd hangtest to attach to....I mean you can hang it from the nose on the frame...but then you would be laying in your back......gas running out...and your hang point would have to be so high to keep your tail wheel off the ground. You then may have to hang off your hubbar away from the center of it to get a good angle. You have to determine a good safe place to do this. But any distance will get a converging line that intersects the first hangtest line...and that will be your CG of the ship. Just draw a line down the centerline of your prop axis.....and mark this line on the posterboard. If this line is below the CG...then the amount it is below is the amount of low thrustline you have. If its above the CG...then the amount it is above is the amount of high thrustline you have.

If it intersects the CG......presto! you have centerline thrust at that amount of fuel you are hang testing with...and the pilot also. Change pilots or fuel amount...and this will change.

Stan
 
Stan,

As the second hang point is so difficult could you not put the mainwheels on blocks, then balance the gyro on the mainwheels with the laser running through the axle centre ?

I think that should work.
 
Peter thanks for that feedback, useful to know that it will probably not be that much difference. I guess the aurora bearing fitting will have adjustment that will cope with the rod length difference.

Stan thank you for coming up with the details of the double hang. I was aware that you had done them at that B days but Stu and I missed it, actually we were still just getting a feel for gyros and giving him his first ride with Dave Searce.

Yes the laser and piece of board on the mast that you mark with a pen is a very convenient way of doing it. I may just have the plum bob line as well if it is easy/convenient as a 'belt and braces' but the laser does sound as though it would be easier.

How big was your posterboard and where on the craft did you fasten it. Presumably somewhere that was going to have the 4/5 lines hopefully converge. ie the 2/3 CG line( two hangs 1 balance) the thrust line and the RTV line.

Just thinking aloud and wondering now. We know the CG is going to change with a burn off of fuel. Assuming the pilot weight is going be reasonably constant, and the CG will move up as fuel is burnt off, where would the best average CG postion stability wise/ burn off wise to start with? Tanks full, half full or nearing empty. ie does one want max stabilty at max weight, near empty tanks and lightest or midweight. Tricky because one doesn't really know when you are likely to want greatest stability and hopefully the shift won't be that great with ten galls.

Yup Karl the balancing on the mains is one way of getting the second line, but the double hang from the end of the hub bar would also be good.
Guess we might do the fuel bit with water to avoid spilling fuel everywhere, will have to calculate the difference in weight though with gas at 6lbs a gall and water at 8.33.
 
Peter thanks for that feedback, useful to know that it will probably not be that much difference. I guess the aurora bearing fitting will have adjustment that will cope with the rod length difference.


Just thinking aloud and wondering now. We know the CG is going to change with a burn off of fuel. Assuming the pilot weight is going be reasonably constant, and the CG will move up as fuel is burnt off, where would the best average CG postion stability wise/ burn off wise to start with? Tanks full, half full or nearing empty. ie does one want max stabilty at max weight, near empty tanks and lightest or midweight. Tricky because one doesn't really know when you are likely to want greatest stability and hopefully the shift won't be that great with ten galls..



Hello Leigh,

I would like to remind everyone here that it is possible to adjust the spherical rod ends to the point that there is not enough engagement of the threads in the control rods. The minimum engagement for the 5/16 rode ends that most people use is in my opinion 3/8 of an inch. It is easy to forget this when making adjustments in the field and there have been some accidents because of this.

I feel it is important to check that the rod ends don’t run out of misalignment capability at the ends of the control travel. I have seen more than one gyroplane where the rod ends would come to the end of their capability to deal with angles and put the control mechanism in bending. I have seen this lead to catastrophic failure of the controls in other applications. I have seen an RAF with failed control rod ends and in my opinion it was caused by this. I feel it is important to make sure that the rod ends are adjusted to give maximum misalignment as the lock nuts are tightened down.

I also like to see a large fender washer that will retain the control mechanism it the spherical rod end fails. I bend them into a cone shape with a ball peen hammer over a socket.

We performed the hang test on the Predator with the tanks half full and then calculated the center of gravity change that the fuel burn would make.

We performed the hang test on her with just me in the front seat and then with both me Terry on board.

We could then calculate the difference a heavier passenger would make.

Because Mark had checked the stability at different loadings we had an acceptable center of gravity range and we wanted to make sure we stayed inside that.

In my opinion the answer to your question is to understand the change and during your flight testing see how the change in the CG from the fuel burn affects how the gyroplane flies and adjust as appropriate.

I like the way the Predator flys best solo with a light fuel load. With that combination she hung a little less than seven degrees nose down. She is stable at all loadings and I would not want to offer an opinion of what is the most stable.

She hung a little more than 9 degrees nose down with Terry in the back and full tanks and/or a heavier passenger will point the nose down a little more. The side tanks are 30 inches long so in the hang test the fuel flows to the front and throws things off a little, this always happens in flight in a steep descent.

Our test pilot was lighter than me which would have had the hang angle closer to 6 degrees nose down.

I have checked the Predator in flight and she appears to fly level.

With most loadings she appears to me to have a slightly low thrust line by perhaps an inch and a half below the vertical center of gravity.

The horizontal stabilizer appears to be set a zero degrees when the aircraft is level. I feel it is important to remember that level is a subjective opinion unless you are given a specific way to measure level and it is defined as such.

Throwing around these numbers sounds very precise, in my opinion it is easy to be off by as much as two degrees based on the subjective nature of the measurements.

Thank you, Vance
 
Karl- Yes, balancing on the mains was done at Bensen Days and worked well fo the Rnd vertical line. I set the plane of the laser parallel to the axle and right on its center. The laser trace was marked on the posterboard where it intersected the first line. That of course is the CG.. Hang the gyro from any point and you will have all your lines go through this point. Balancing on the mains is sometimes easier than trying to find a 2nd place to hang. Resasi- The posterboard should be as close to the lateral centerline of the gyro as possible. The laser plane should be straight out close to 3 or 9 o'clock as possible to minimize paralax rror. There will be no error if the laser plane is perpendicular to the longitudinal axis, and very little error even if you are off 10 degrees. Stan
 
Thanks Stan.

Vance good point. Am breaking the rods down for more strength in the system.

So two per side via the 'flapper arm'. We will also have little Bowden cables if the connection bolt fails/nut falls off + the coned washers.

Not our mast, our flapper will be attached with a bracket.
 

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That looks good Leigh, especially the washers!

In an ideal world the flapper arm would have a wider bearing to deal with the twist imposed by the opposite side control tubes.

Or have a sandwich on the control rods so they can keep from twisting the flapper arm.

Third choice would be to use a T and put them on the same side.

I am sure they will be fine as they are, you will just want to keep on top of wear at the root.

The Predator has some arms something like that but the spherical rod ends are butted together and they are constantly loosing up.

Thank you, Vance
 
Those are on Roy's Hornet in the hanger he lets us share. He has a folding mast.

There are a number of configurations, but here is one we might copy for ours.
 

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Hello Leigh,

At the risk of seeming critical of designs that are giving satisfactory service I will try to describe the challenge I am having with the Predator and what I imagine to be best practice with a link.

Any time the forces are offset there is a twisting imparted when force is applied, more offset more twisting.

On the Predator this twisting is managed both with the hinge itself and the joint is captured between two tabs that can be pulled together with the quarter inch bolt used as the hinge pin.

The twist wears the link pivot and eventually I will need to bush the hinge. It has a steel bush in it now.

This introduces slop into the linkage that exacerbates the stick shake when I am not holding the stick.

In my opinion this link would work better if the spherical rod ends were not twisting the link because of the offset. It is easy to put the links in alignment either by making a T on the lever and placing one spherical rod end above the other or using the link to slightly change the ratio by placing one spherical rod end closer to the pivot than the other.

The quality of the hinge pivot is still important to keep slop out of the linkage and wider is better.

The Predators links are u shaped so they capture the spherical rod ends against each other with a single bolt going through both spherical rod ends.

Most people fly less than fifty hours per year so this is not a problem for several years. It is just something to keep a maintenance eye on.

Thank you, Vance
 
Leigh - Vance is correct….post #37 is not a good way to make a swivel arm bellcrank. A small amount of wear in that design will allow a large amount of play in your control system.

IMHO a better way would be using two base bars, one on the front and back of the mast, and a bushed "T" arm with clevis at the other end, that allowed the rod-ends to be attached in alignment, one above the other. This "T" arm assembly could be made from welded 4130 and still be light. This way would allow no twist because the rod-ends are in alignment.
 
Vance and Edwin, thank you for pointing that out to me.

I am mechanically immature so I do appreciate the direction from you both.

It would seem then that Roy's design for the swivel arm bellcrank would be a better one. (must stop calling it a flapper arm)
 

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