Tandem double hang test


Junior Member
Hi All,
I am preparing for my double hang test to be done in a month or 2, I 'll see if my calculations are ok or not hahaha
I understood that by hanging anything by two different points at 2 different angles from the CoG one could draw 2 lines and that the CoG is situated on where the lines are crossing
as for the first hanging easy , I remove the rotor and I hang it by the head or the mast..
as for the second I did not get how could hanging the gyro by an other point but with the main wheels resting on a stand could make it possible to determine the second line ?
help (((-:
jean mi


Just a fledgeling
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

Gold Supporter
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.


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.


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.





Junior Member
Hi lads’,

thank you for your explanations, I'll try to make a video or pictures for the community
i need to get used to do this test coz I have made it possible to raise the engine up to 110 mm in few minutes, pilot and passenger seats can be shifted horizontally in a few minutes Without affecting the controls
for the moment the centre of thrust is 110 mm under the Center of gravity .
the goal is to verify with the help of hang tests and then fine tune with fly tests

I am not chuck of course
first of all the centre of gravity is where you decide it to be, it is fixed of course
horizontally you have to place it ahead of the rotor head horizontal position in order to create a pitch down force necessary to counteract the rotor drag in order to fly at an horizontal attitude
the Rotor incidence ( and hence the RTV) will change according to many parameters, Speed as you were saying but also the placement of your propeller centre of thrust in regard to the centre of gravity
if your prop thrust line is above the centre of gravity it will generate a pitch down moment that you will counter act ( to fly level) by pulling the stick hence you RTV will pass ahead of the centre of gravity and your giro will be unstable
if your prop thrust line is under your centre of gravity it will generate a nose up moment that you will counter act ( to fly level) by pushing the stick which will make your RTV pass behind of the centre of gravity making your giro stable
things must be analysed dynamically which is very difficult ( at least for me)

Uncle Willy

your input is really interesting for me,
in a practical way your method is easier then the hanging system, it would be interesting to compare the 2 in terms of accuracy given the fact that the angle difference btw the 2 line is maybe less in your method

Doug Riley

Platinum Member
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

Gold Supporter
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.


Junior Member
Doug Riley;n1142448 said:
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.
true but if we go in this direction, in the attempt of explaining all of this to newbies, we should talk about all the moment forces that are acting in different directions ending up in the final attitude of the gyro,

Rotor Drag expression I used wass just an "image" to try to explain why we locate the Cog ahead of the rotor head to get a hanging pitch down attitude that will be compensated in flight by a force acting in the opposite direction to make the gyro fly on an horizontal attitude.

but you are absolutely right is is not scientifically correct

C. Beaty;n1142454 said:
Horizontal tail surfaces can provide compensation for errors of propeller thrust line location over the airspeed range where they are effective.
yes but horizontal surfaces used to compensate an incorrect propellers/cog design are producing a Drag we can avoid by using a correctly designed gyro plane

Jean Claude

Junior Member
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



Junior Member
I have made and used my own spreadsheet but your's seems to be really nice, too late coz my bird is already on it's wheels .. next time I'll try yours

it is wise to have used the propeller centre of rotation as a reference, I set my reference on the ground and ahead of the gyro drawing, it does not change anything to the final result but your method directly expresses the gap btw the prop thrust vertical position and the centre og gravity vertical position which is perfect

good idea