Hang Test

worn

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My air command says 0-4 degrees forward on hang test. RAF say's 5 and a half to 8 and a half single and up to 10 and a half double. How does the manufacturer determine what the hang test degrees should be and why are they different?
 
The main hang test will set your cyclic in the center of stroke so you have enough for and aft stick control during powered flight and dead stick landings.
Your torque tube should be at center when hung.
I put shims between my torque tube and stops to maintain neutral during the hang test.
Should also have a half a tank of fuel.
Most machines have a 2.5* sweet spot.
Most gyros fly with a 9* to the horizon disc, so a heavier hang would seem to have less back stick and a lighter hang will most likely not have enough forward stick
A tandem would stay pretty close to a single seat hang with two people because the center of gravity stays pretty much the same.
A side by side the pitch and the roll both change with a passenger and are usually trimmed in flight.
The geometry of the gimbal, and the cyclic control also plays a role.
10* sure seems like a lot for a heavy machine.
 
hang_test2.png

In a hang test, Warrren, the aircraft always assumes a position such that a straight line down from the rotor head (where the rope or chain, from which the aircraft hangs is attached) will pass through the center of gravity (CoG) of the aircraft. In the left picture I assume that the CoG is exactly below the rotor head(which is not true for a real aircraft). In this example, the keel will be parallel to the ground in a hang test and the angle is 0°. When you make a hang test with a pilot/aircraft combination the CoG will move forward, because the pilot sits in front of the old CoG. Now in a hang test the keel assumes a position of about 30° nose down (right picture), because this is the position where again a straight line down from the attachment point passes through the center of gravity. One important point to take away from this example is, that you have to place the proper pilot in the right seat (for a two seater). If you make your hang test with "The Mountain" from Game of Thrones as your pilot you will probably get the wrong result. The hang angle specified by the designer basically is a means for you to easily check whether your CoG position is correct.
 
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For some reason my last entry in the thread was not registered as a new answer, so this is a test whether the counter is working properly now.
 
worn;n1138979 said:
How does the manufacturer determine what the hang test degrees should be and why are they different?

Flight testing and personal taste.

Part of your responsibility as pilot in command is to determine if the center of gravity is within the safe range; particularly with a two seater.

This is affected by your weight, the passenger weight and the fuel on board.

The safe CG range is determined by flight testing.

In The Predator (the tandem gyroplane I fly and train in) the fuel and the person in the back seat are slightly ahead of the center of gravity so for someone close to my limit (around 260 pounds) I run a lighter fuel load. With full tanks I have 132 pounds of fuel on board.

The CG changes slightly as the fuel burns and I can feel it in the response to my control inputs.

I also have to be careful to not exceed my maximum takeoff weight.

I use a datum line and add the weights multiplied by moment arms to determine if the center of gravity is within range.

Calculating center of gravity will be on your knowledge test and you are expected to know the safe range either by calculations or hang angles.
 
The hang-test procedure serves two purposes. The first, as Jake said, is to center the controls at cruise. The second is to allow the fuselage to ride at its designed stance (neither nose-up nor nose-down) at cruise.

The second goal is deeply affected by prop thrustline position relative to the aircraft's CG. The higher the thrustline is above the CG, the LESS nose-down the ideal hang angle will be. So...

The good old "2.5 deg. nose down on the mast " is a Bensen spec. It assumes (1) that the mast is raked 9 degrees and (2) that the gyro has centerline thrust (CLT - - prop thrustline passes through CG).

If you build a gyro with, e.g., a prop thrustline six inches above CG, but use the Bensen spec, you"ll have a gyro that rides badly nose-down at high airspeeds and throttle settings. I know this because I flew one of these -- a no-stab Air Command lowrider -- for years. At 80, throttle hammered, it was like riding in a wheelbarrow waiting to be dumped forward.

The current CLT Air Command can use the Bensen spec.

The exact spec number depends, of course, on which part of the gyro you're measuring. If your gyro has a 9 degree aft mast rake, but you measure the hang angle on the keel, you'll need to add the mast rake angle to the spec to arrive at a keel-based hang spec. In the case of a Bensen, this brings you to 11.5 deg. nose-down on the keel. Example: the Gyrobee has zero mast rake. Its hang spec, translated from Bensen, would be 11.5 deg., measured either on the mast or the keel.

The stock RAF has a very high thrustline. IIR, its 5 degree spec is to be measured on the keel. This would be the equivalent of backing the Bensen spec from 11.5 deg. on the keel down to only 5 deg. This allows the gyro to ride more level despite the HTL.

If a gyro is set up using Bensen specs, but then flies unexpectedly nose-down at fast cruise, it likely has undiagnosed HTL. That situation should be corrected for safety's sake. Rig an adequate H-stab or move weights up relative to the prop's center.
 
Equally important is that the stick be more or less centered during normal cruise and under no condition of flight should it be against a stop. An exception was Bensen's wood rotorblades with excess trailing edge reflex which served as a speed limiter.

The pitching moment of the rotor blades also has a strong effect on stick position; blades that have a nose up pitching characteristic will require that the stick be well forward in the upper speed range; Bensen wood blades for instance, had excess trailing edge reflex that caused the stick to reach its forward stop at an airspeed of 60-70 mph. Blades that have a nose down pitching characteristic require a more rearward stick position.

Negative pitching moment is dangerous if excessive; it can result in a situation where recovery from a high speed dive is impossible.

Competent rotorcraft designers strive for zero pitching moment of rotorblades.
 
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Chuck I still don't know why my sportcoptor blades on my RAF had to have a spring that had 30-40 foot pounds of pressure

on the rear of the rotorhead to fly level.

When I first flew the sportcoptor blades I almost crashed because of the enormous amount of back pressure required .

Is that also the reason you have to have the sportcoptor air trim to overcome that tremendous pressure.
 
I discussed the matter over here.
In brief, the RAF rotorhead and the SC rotorhead have different pitch offsets, that's why.


When I first flew the sportcoptor blades I almost crashed because of the enormous amount of back pressure required .

IIRC, you performed no taxi tests first.
Instead, you simply added power for a takeoff roll and discovered the nose-down forces then.
 
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Gyrobee has zero mast rake. Its hang spec, translated from Bensen, would be 11.5 deg., measured either on the mast or the keel.

Doug this is from an old thread but I am confused about the reference to the Bensen spec.
Since the gyro bee has a vertical mast with the rotorhead tilted back 10 deg I would think one would want the mast to hang verticle when the controls (rotorhead) are centered with the pilot and half tank of fuel. Of course this does not take into account the effect of rotor drag or the thrust line of the prop. But I would think this would be a starting point. Would you comment on this?
 
The place to start when thinking about this is the angle that rotor disk will make with the horizon in level flight. Then work backwards to set the angles of the airframe components. This disk flight angle is determined by the aerodynamics of the rotor. It's sometimes called the disk angle of attack, or disk AOA.

Small gyros fly with a disk AOA of roughly 10-12 degrees at typical cruising speeds. Of this 10-12, about 2 deg, is so-called "blowback," an angle that the disk makes TO THE SPINDLE. The disk leans back a couple degrees relative to the spindle. Therefore, the typical SPINDLE angle to the horizon is more like 8-10 degrees. We'd like our controls to be in the middle of their travel when the spindle is tipped back 8-10 degrees. The Gyrobee head is mounted at a 10 deg. angle to the airframe to achieve this.

The purpose of rotor thrust is to oppose gravity. Gravity pulls straight down, of course. But rotor thrust doesn't pull straight up, it pulls at an angle, as we've said. In steady flight, the gyro will necessarily position itself in space so that this angled rotor thrust pulls right through the gyro's CG (if the rotor thrust pulled NOT through the CG, the gyro frame would rotate due to the off-center pull). For the frame to fly level, the CG must lie right on the 11-degree line from the teeter bolt down to the keel.

In a hang test, we suspend the gyro so that this line is vertical (gravity takes care of that). The mast clearly won't be vertical, and the keel won't be horizontal, when we do this. They will be angled at 11 degrees to the horizontal.

Note: the Gyrobee's zero-degree mast rake is structurally inefficient; Bensen did it right by lining up the mast approximately with the rotor thrust line. The zero rake better accommodates the tall Rotax 2-stroke engines, though -- when they are mounted plugs-up, as they should be. Air Command preserved the structurally superior Bensen raked mast by hanging the Rotax plugs-down. I still have scars on my starter-pulling hand from the blisters I got trying to start an upside-down Rotax. And I'm probably headed straight to hell for the lingo I employed in those situations.
 
Here's a sketch of level flight. When you hang-test the gyro, the line between the CG and the teeter bolt will necessarily be vertical. During the hang, the mast will "lean back" and the keel will point nose-down. To see this, just rotate the whole picture counter-clockwise by 11 deg.
 

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