Determining CG for a Gyroglider

PW_Plack

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I've acquired a gyroglider that suffered a blade strike, and will be setting about repairing/rebuilding it. It won't be finished this season, but I'm hopeful for next year. It will need a new mast, cheek plates, rotorhead, hub and blades. Most of the rest is intact, and just needs clean-up and inspection.

How does one calculate the correct rotorhead location to make cheekplates on a glider? I know the basics involved in the calculations, but with the people in the seats representing the largest mass, it becomes necessary to estimate the CG of the people themselves while seated, legs extended.

Is a hang test with test plates (testing multiple sets of holes) the only practical way to do this?

Is it necessary to assign some weight to the cable?

All suggestions appreciated. This is a one-off custom gyroglider, so standard Bensen dimensions may not be too useful.
 

okikuma

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Hi Paul,

It this perhaps the folding "Tow & Stow" gyro-glider that turned over at El Mirage a couple of years ago?

Thinking out loud, using the Bensen Gyro-Glider as an example it stands to reason that with the cyclic in the nominal neutral position, the rotor disk lift vector (nine degrees rearward from the vertical) should pass through the vertical CG point. In my mind's eye, that point would be just about at the same location of one's umbilical (belly button) when sitting in the seat.

Chuck Beatty & Craig Wall, please chime in with your knowledge and experience.

Wayne
 
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Stan V

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Paul, I just had a thought, sense your CG will depend on the weight of the student/pilot, an adjustable mast may be ideal. The RAF, with its folding mast provides a 4 position choice in its fore and aft position. Sense you have to replace the mast anyway you may want give this a try!
 

C. Beaty

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The primary reason for checking dangle angle is to ensure control centering.

Follow the Bensen handbook or if not available, use a scale drawing and project a 10º line from the center of the passengers upward. The teeter bolt should be along that line. (80º relative to the keel)

It’s probably easier to make up dummy cheek plates from 3/8” plywood and find the location experimentally.

Don’t forget the rotorhead to tow boom guywire; it is essential in a towed glider.
 

Doug Riley

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After doing a silly-looking experiment rocking in a plastic bucket seat on the floor, I estimated a person seated in the usual cockpit configuration (leaning back 10 deg. and legs up a bit) has a "personal CG" about the height of his/her navel and a couple inches forward of it. Just about where you might comfortably hold a joystick, in fact.

People vary in shape, so this is just a guide.
 

MadMuz

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From memory, gliders don't need the nose down so far because the vehicle is constantly keeping the nose lower... you generally don't need much stick forward in a glider, to keep the tension on the rope.... last thing you want is to overtake the tow vehicle, or even get slack in the rope.... I would go for the dummy hang plates like Chuck said, making a set of alloy plates with slots costs a bit more but you can keep them and use them on other machines in the future.... handy to have... +1 on the wire, and a releasable tow hook is good too, if something happens, you can release from the tow rope quickly :yo:

gliders are fun.... you will have a blast:D
 

phantom

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If you are flying a gyro glider correctly the tow line will be quite slack in level flight, it is a common mistake to fly gliders with the rotor tilted to far back and this habit will come back to haunt you when you start to fly with an engine,it will cause you to take off behind the power curve.

Norm
 

CLS447

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I asked this on Larry Boyer's Junk Yard Dog thread with no response. I have copied & pasted to this thread......

Hey everyone, Larry B. & myself went together to the airport last night. I got to fly my gyro with only a sweatshirt. The wind was blowing but very smoothly & I enjoyed my flight very much. A nice break from caring for my mother full time. Thanks Larry !

Anyway Larry flew the Junk yard dog, after I retuned he was examining his machine & I asked what was wrong.......He lost his electric trim & said on takeoff he needed 2 hands to hold back pressure. He wants to re-hang test it.

Now I know that the hang test is to make sure that you have enough stick travel front to rear, right ? But what is the proper way to determine the hang angles ?
And why does he need so much trim spring pressure ?

Can changing the hang angle lighten the pressures ?

Thanks..... I hope that Larry gets involved in this conversation......LARRY ??????
__________________
 

C. Beaty

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Stick pressure has nothing to do with dangle angle.

It is determined by offset between spindle bolt and pitch pivot of torque bar as well as by cyclic flapping angle of rotor blades.

Rotor blades with nosedown pitching moment will have high stick pressure. Such blades suppress cyclic flapping and move rotor thrust line rearward relative to spindle axis.

Rotor blades with high nosedown pitching moment can run away in a high speed dive.
 

CLS447

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Thanks Chuck ! That's what I told him. We are still going to do a hang test but what angle should we be hanging at ?

There is a pic of his machine under Single Place designs......Junk Yard Dog photos. He is flying 27' Dragon wings & he has alot of spring on them.
 

C. Beaty

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DWs* don’t have a nosedown pitching moment. A heavy gyro with Bensen’s standard 1” offset between spindle bolt and pitch pivot needs a lot of spring tension for trim; especially if the rotorhead has tall towers for extra undersling.

The mast angle itself isn’t what’s important; with stick in neutral, rotor head angle should range between straight up and 3º nose down.

The easiest place to measure this is with either a bubble protractor or smart level on the torque bar.

*A few of the first DWs did in fact have a nosedown pitching moment but I doubt if any of them are left. The very first DWs did not have a trailing edge reflex; the “ducktail.”
 
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LARRYEBOYER

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Thank you Mr. Beaty. When I fly the JYD, The stick is pretty well centered with adequate room for hard flair. It's just in cruise, it needs a lot of spring pressure to hold nose up and releave back stick pressure. Chis and I did a hang test before1st flight and it fell with in 4degrees forward mast.I added the electric actuator to relieve back stick pressures. I guess I need heavier spring to counter stick pressure. Thanks for straightening me out.
 

helcoro

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I am confused, when i hang my gyro with stick neutral level, gyro atitude must be 9 or 11...13 degree nose dawn???
 

Vance

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I am confused, when i hang my gyro with stick neutral level, gyro atitude must be 9 or 11...13 degree nose dawn???
It depends on what model gyro glider you have and how you are defining and measuring nose down.

Hopefully you have some documentation to go with your aircraft that will explain the hang test procedure and what you are looking for.

In my opinion the main reason for a hang test is to make certain you don’t run out of control movement to accomplish the tasks of flying.

A gyroplane typically flies with the blades angled back around seven to nine degrees.

You need to have enough back stick to flare for landing.
 

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Doug Riley

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Vance is correct about the primary reason for placing the CG in the proper location on a gyro. Additional reasons: (1) to make sure that the wheels leave the ground, and touch the ground, in the proper sequence when landing and taking off, and (2) to allow the aircraft to fly in an appropriate stance so that the pilot has the sense that he/she is "level" when cruising.

Whether the hang angle should be 9 degrees, 11 degrees, 13 degrees or another number depends greatly on the part of the aircraft from which it is measured. For example, the keel of a Gyrobee should be level with the horizon in cruise. The angle that the keel makes with the ground during a hang test should be about 11.5 degrees nose-down. In contrast, the mast of a Bensen is angled aft 9 degrees. The angle of the Bensen's mast during a hang test should be 2.5 degrees forward of vertical (top farther toward the nose of the gyro than the bottom).

The hang angle should be the same for a given gyro with given rotorblades, whether the gyro is a glider or a powered gyroplane. Normally, the rotorhead must be moved aft to preserve the hang angle when adding the weight of a pusher powerplant, however.

If a powered gyro cruises in a nose-low stance, with the hang angle set the same as for a gyroglider, this often indicates that the propeller thrustline is higher than the aircraft's CG -- a potentially dangerous condition unless it is addressed with an adequate horizontal stabilizer. Once an adequate horizontal stabilizer is installed, a gyro with a high thrustline should cruise level when set to the usual hang specifications.

Do not try to "correct" a nose-low cruising stance by merely lessening the nose-down hang angle. Instead, deal with the underlying problem that is causing the nose-low stance.
 
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