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[jm-urbani]

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[AirCommandPilot]

While hanging from the rotor head you would draw a line straight down from the center of the teeter bolt using something like a plumb-bob, or an auto leveling laser turned vertically. While balanced on the wheels, you would draw line from the center of the wheel axle straight up. If you could balance the machine on it's nose, the line would still intersect the first line in the same spot.

 

[C. Beaty]

For the second angle, wheel balancing, the rotor must be installed.

Another way of obtaining the second angle is to make up a harness of rope with one end attached to the rotorhead and the other end attacked to the most forward end of the keel with the hang line attached to the center of the harness. Then draw a straight line from the hang line.

The pilot must be seated on the gyro for all of the hang tests.

It’s easy if you have a laser level to project the continuation of the hang line.

 

[Mac]

No Title

Hello Chuck, Am I correct on the thinking of Rotor Thrust Vector (RTV)?

RTV should be ahead of Center of Gravity (CG).
When the gyro is flown at top speed and the control stick is full forward, RTV should pass through or slightly ahead of CG.

The red line passing through the rotor illustrates RTV with stick centered.

 

[Uncle Willie]

You can do it without hanging it at all.
Run a weight and balance on a set of scales.
Then block up the nose wheel and do another weight and balance.
The height of the CG can be computed from the amount the CG line moves.







​​

 

[Doug Riley]

When discussing hang angles and CG locations, it is simplest to avoid thinking about rotor drag. The rotor produces ONE force on the airframe (rotor thrust). "Drag" is one of two vector components of rotor thrust -- IOW, drag is an invented idea that is handy for some purposes, but unhelpful here.

The rotor's thrust force pulls upward-and-back on the teeter bolt at an angle of 10-12 degrees aft of vertical (relative to the horizon) in cruise. The frame will orient itself so that the rotor thrust passes ahead of, or behind, the CG, depending on what other forces are present. As JM said, it's desirable to have the rotor's thrust line pass a little behind the CG. This will happen if either (1) the propeller's thrustline is a little below the CG, or (2) the horizontal stabilizer is adjusted to create a small download on the tail.

 

[C. Beaty]

Here’s a sketch of rotor and propeller thrust vectors and their relationship to the aircraft CG.

The primary influence of the rotor thrust vector’s relationship to the CG is the location of the propeller thrust line with respect to CG. If the propeller thrust line passes above the CG, the resulting offset generates a nosedown rotational moment about the CG which must be counteracted by the rotor thrust vector passing forward of the CG. This is a situation equivalent to a tail heavy automobile which “oversteers” so that when negotiating a curve, the rear end swings out, tightening the curve.

Horizontal tail surfaces can provide compensation for errors of propeller thrust line location over the airspeed range where they are effective.

Uncompensated propeller torque produces a rolling moment of the airframe that must be balanced by the passage of the rotor thrust vector to one side of the CG. Upon entering turbulence at constant power setting, the airframe rolls in response to gusts. The correct solution here is either contra rotating propellers or aerodynamic vanes in the propeller slipstream.

 

[Jean Claude]

When you built your own gyro, you have certainly weighed each element and you also know their position relative to the propeller plate.
This spreadsheet then gives the position of the center of gravity with a good precision thanks to the statistical dispersion of the uncertainties.
It easy know the position variations according the weight of the driver or the fuel
Fill only the blue boxes

 

[Resasi]

The rotor's thrust force pulls upward-and-back on the teeter bolt at an angle of 10-12 degrees aft of vertical (relative to the horizon) in cruise.

Doug it was a while back but I do seem to remember when setting up the cheek plates that we arranged a keel angle of minus 11 degrees when hanging from the teeter bolt. This seemed to give us a pretty level flight attitude. Was this a correct way of doing things? I also can’t remember if we did this pilot in seat or not?

Here’s a double hang done at Wauchula during Bensen Days.

 

[j bird]

Ernie told me 8* to !2* on the Dominator keel.

 

[Resasi]

With reference to my earlier question on setting the keel angle, it seems this would be with pilot and full fuel.

Corrections please if this is incorrect .

 

[mark treidel]

If memory serves, you want pilot plus 1/2 full fuel tank.
That was how I previously did all my single seaters.
Seems prudent to do the same on the Tandem because the back seat (with or without) weight is
almost neutral (within the CG) relative to hang angle from the teeter bolt.

The reason for only 1/2 tank was explained like this.....Lets assume the static hang angle was established at 9°.
The difference in hang angle with a full tank compared to an empty tank is 4°. If you start out a flight with a full tank
and fly until the tank is 1/2 full, then you come down 2° to CLT. If you fly to empty (theoretically) you go 2° negative
below CLT. That being said, if one static balances with a full tank and you start a flight with a full tank, you fly 4°
offset instead of 2°. In other words, balancing initially with 1/2 tank keeps the aircraft closer to CLT at all times during
the flight resulting in less need for increased back stick pressures and/or trim settings.

 

[Resasi]

Thanks very much Mark, answered the question very nicely and made perfect sense to me.

 

[Rowdyflyer1903!]

For the second angle, wheel balancing, the rotor must be installed.

Another way of obtaining the second angle is to make up a harness of rope with one end attached to the rotorhead and the other end attacked to the most forward end of the keel with the hang line attached to the center of the harness. Then draw a straight line from the hang line.

The pilot must be seated on the gyro for all of the hang tests.

It’s easy if you have a laser level to project the continuation of the hang line.

In flight the rotor is lifting so it’s weight is separated from the fuselage. If this logic is correct, the the weight of the rotor system from the point at which the rotor lift is applied, in relation to the weight of the fuselage, is zero. Why, then, must the rotor be installed during the second part of the hang test?

 

[Vance]

In flight the rotor is lifting so it’s weight is separated from the fuselage. If this logic is correct, the the weight of the rotor system from the point at which the rotor lift is applied, in relation to the weight of the fuselage, is zero. Why, then, must the rotor be installed during the second part of the hang test?

In my opinion the rotor is a part of the mass of the aircraft and affects the center of gravity.

 

[Rowdyflyer1903!]

In my opinion the rotor is a part of the mass of the aircraft and affects the center of gravity.

I have to disagree. If you were car engine to find the cg and the only way to do that was to suspend it by a chain hoist from two different points, you would not include the weight of the chain during one measure and not in the other. Another example would be,as in a post several years ago, about the thrust angle. This post gave the example of hanging the fuselage from a tall bridge. Again you would not use the weight of the chain/rope to determine the cg.

One person said one of the reasons we do determine the cg is to center the stick. Now having said that, it does seem to be very important to find the cg to be above or there about the thrust line.

Lets chat about another subject which seems to be sort of ignored and its about the total mass, and specifically parasitic drag. The parasitic drag and the induced drag of the rotor systems seems to be very variable until you get the bird in the air and at its cruising speed and beyond. At that point the battle seems to be at a stalemate.

Common thought says the induced drag will diminish and the parasitic drag will increase with speed. The rotor system will gain efficiency and the fuselage will gain resistance to the wind.

All of these forces are balance around the CG? Or the thrust line. Seems to me the battle of the resistance game travels up and down the mast or fuselage as speed increases or decreases.

All of this play havoc with the stick pressures and trim. Not to mention when the thrust line ceases to be or the influence of the rotor lift/thrust line varies itself in turbulence.

So what really counts? Finding the CG vs thrust line on the ground?

You are correct and it is a part of the mass of the aircraft. If you want to find the cg of the entire aircraft, the leave the rotor on during both phases of the hang test.

In the air, the rotors are flying on their own with a burden of the fuselage independently dangling below. Maybe if it were a rigid rotor system and the rocking of the fuselage affected the rotor or vice versa in some major way might I consider the rotor to be a factor in determining the cg of the fuselage in the air.

I must say I do love our conversations.

 

[Mac]

Wings provide lift much the same as a rotor. Do you figure CG with the wings on or off on a Cessna 140.
The first attachment point is the rotorhead. Adding the rotor would not change the hang angle.

 

[Rowdyflyer1903!]

Of course not. The fuselage is not independent of any motion from that of the wing. Forget the weightless rotor cause they fly discussion. My real question about this was the statement that the hang test from the rotor mast was done with the mast off. Then the test while balancing on the mains was done with the rotor on. It didn’t make much sense if one wanted an accurate reading. This rotor off rotor on didn’t come from me. This came from on of our long standing members. In my case, I have 29’ Skywheels on a 9’ mast and that’s a major moment arm and in my thinking a source of error.

 

[Mac]

The rotor is not mounted because you are adjusting your cheek plate mounting location first - the rotor mounted would be a big inconvenience. The vertical axis will pass through the teeter bolt with the rotor mounted or not.

Think about doing a zoom and at the top of the zoom, you might have zero airspeed and zero lift from the rotor. Does the rotor have weight now.
A heavy rotor raises your CG.

 

[Vance]

I have to disagree. If you were car engine to find the cg and the only way to do that was to suspend it by a chain hoist from two different points, you would not include the weight of the chain during one measure and not in the other. Another example would be,as in a post several years ago, about the thrust angle. This post gave the example of hanging the fuselage from a tall bridge. Again you would not use the weight of the chain/rope to determine the cg.

One person said one of the reasons we do determine the cg is to center the stick. Now having said that, it does seem to be very important to find the cg to be above or there about the thrust line.

Lets chat about another subject which seems to be sort of ignored and its about the total mass, and specifically parasitic drag. The parasitic drag and the induced drag of the rotor systems seems to be very variable until you get the bird in the air and at its cruising speed and beyond. At that point the battle seems to be at a stalemate.

Common thought says the induced drag will diminish and the parasitic drag will increase with speed. The rotor system will gain efficiency and the fuselage will gain resistance to the wind.

All of these forces are balance around the CG? Or the thrust line. Seems to me the battle of the resistance game travels up and down the mast or fuselage as speed increases or decreases.

All of this play havoc with the stick pressures and trim. Not to mention when the thrust line ceases to be or the influence of the rotor lift/thrust line varies itself in turbulence.

So what really counts? Finding the CG vs thrust line on the ground?

You are correct and it is a part of the mass of the aircraft. If you want to find the cg of the entire aircraft, the leave the rotor on during both phases of the hang test.

In the air, the rotors are flying on their own with a burden of the fuselage independently dangling below. Maybe if it were a rigid rotor system and the rocking of the fuselage affected the rotor or vice versa in some major way might I consider the rotor to be a factor in determining the cg of the fuselage in the air.

I must say I do love our conversations.
Of course not. The fuselage is not independent of any motion from that of the wing. Forget the weightless rotor cause they fly discussion. My real question about this was the statement that the hang test from the rotor mast was done with the mast off. Then the test while balancing on the mains was done with the rotor on. It didn’t make much sense if one wanted an accurate reading. This rotor off rotor on didn’t come from me. This came from on of our long standing members. In my case, I have 29’ Skywheels on a 9’ mast and that’s a major moment arm and in my thinking a source of error.

In my opinion one sort of hang test is used to make certain the controls are centered and is unrelated to finding the center of gravity. This is typically referred to as the hang test.

In my opinion if you hang anything from any single place the line will pass through the center of gravity on the way to the ground.

A typical rotor system moves the rotor head through around sixteen degrees.

The goal of the single hang test is to get close to zero forward and sixteen degrees back starting at the approximate angle the fuselage will fly.

If you start with a gyroplane that flies it is not so important to understand the exact reason she flies unless she is doing something you don’t want. The double hang test is important to get the rigging right.

By rigging I mean getting the rotor head in the correct position for the center of gravity and the controls adjusted so that you have a full range of control at the flight attitude.

The flight attitude will change slightly with speed changes for a combination of reasons.

A double hang test is used to determine the center of gravity and is typically referred to as a double hang test.

In my opinion if you hang something twice from two different places where the two lines intersect is the location of the center of gravity.

It is my understanding an object in space rotates around the center of gravity.

If the propeller thrust line doesn’t pass near the center of gravity there will be a power-pitch coupling. This can be managed to some degree over the typical useful speed range with sufficient horizontal stabilizer volume.

For example a Magni flies just fine with a very high thrust line in relation to the center of gravity because it has considerable horizontal stabilizer volume. I have not flown a Magni that didn’t have power-pitch coupling.

I feel the reason you are getting confused is a semantics issue rather than a physics issue.

The laws of motion are pretty straight forward.

The center of pressure is important for a different reason and not related to either hang test.

I feel most of this is explained well in the Rotorcraft Flying Handbook.

I feel there are places in the Rotorcraft Flying Handbook that are not technically correct in an effort to keep it simple and easy to understand.

If you make reference to a chapter and page in the Rotorcraft Flying Handbook I will be better able to explain what appears as divergent to you.

http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/faa-h-8083-21.pdf

If you must reference something someone posted please use their words so people may understand your question. A paraphrase may be misleading.

For example I am not familiar with any hang test done without the mast.

There is also the possibility that the post is simply wrong no matter how many years of experience the poster has or if the post has my name on it.

I strive to be consistent and use appropriate language. I do not always succeed.

I feel a good gyroplane flight instructor will be able to explain and demonstrate these relationships to you.

If they only have experience with a single brand they may have a better grasp of procedure and if you are flying a different brand the procedure may not be applicable to your particular aircraft.