# Gyroplane Thrustlines vs. Center of Gravity

#### Jean Claude

##### Junior Member
Tilt from 45° left to 45° right in one second is possible with a gyroplane. This is 90°/sec. But if you prefer 30°/s, then three times less than 7° will suffice to forestall gyroscopic lag.
Short mast decreases the roll inertia, but inertia does not work when the roll rate is constant.
Whatever the length of the mast, roll continuous rate is the same when the displacement of the stick is identica.
Damping is another matter.

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#### C. Beaty

##### Gold Supporter
Here’s what Mr. Gessow and Mr. Myers have to say about this subject (Aerodynamics of the Helicopter):

“Control Sensitivity
The combination of control power and damping in roll (or pitch)
together determine an important ﬂying-qualities characteristic. This
characteristic is called control sensitivity and may be deﬁned as the
maximum rate of roll (or pitch) achieved by a unit displacement of the
controls. Control sensitivity may be deﬁned in three alternate ways as
follows:

Physically, the manner in which control power and damping deter-
mine control sensitivity may be understood from the following argu-
ment. If the control stick is displaced (laterally, for example) and held,
the helicopter will initially accelerate angularly at a constant rate that
is inversely proportional to the moment of inertia of the helicopter
about its longitudinal axis. [This result follows from Newton’s law
M = I([email protected]/dt).] As the angular velocity builds up, the opposing damping-
in-roll moment increases in proportion until an angular velocity is
reached at which the damping moment is equal to the control moment.
The helicopter is therefore stabilized at that angular velocity, because
the resultant moment on the helicopter is zero. It is thus apparent that
if the rotor damping is large with respect to the control power, then the
maximum rate of roll reached by the helicopter by a given stick displace-
ment would be small, inasmuch as a sufﬁciently large damping moment
would be produced at a small rolling velocity to balance the control
moment. Alternately, it is clear that if rotor damping is small with
respect to the control power, then the maximum angular velocity
attained by a given stick displacement would be large. -
A Helicopters with conventional control systems are subject to high
control sensitivity. In fact, according to reference V146 (Appendix IIA)',
the maximum rate of roll achieved by a small, two-place helicopter may
be as great as those of some modern ﬁghter airplanes at the speeds for
their maximum rates of roll. This is true not because of high control
power, but rather because of low damping, which, for the helicopter, is
a fraction of that for airplanes. This same_ reference goes on to state that
high control sensitivity can lead to overcontrolling, which in turn
results in a short-period, pilot-induced lateral oscillation. '
It is worth while to point out which of the physical characteristics of
the helicopter can be varied so as to reduce excessive control sensitivity.
The height of the rotor and the offset of the ﬂapping hinges do not affect control sensitivity because they change control power and rotor damping in proportion. Design factors and devices which increase rotor
damping without affecting control power result in reduced control
sensitivity. Thus, large helicopters operating at low rotor speed,
helicopters with tip-jet units, and helicopters with devices such as the
Bell Stabilizer Bar and the Hiller Control Rotor will have more desirable
values of control sensitivity.
Although rotor height and ﬁapping—hinge oﬂset do not affect control
sensitivity, the ratio of these values to the fuselage moment of inertia
will determine to a large extent the time necessary to reach the maximum angular velocity.”

#### Jean Claude

##### Junior Member
"Although rotor height do not affect control sensitivity, the ratio of these values to the fuselage moment of inertia will determine to a large extent the time necessary to reach the maximum angular velocity.”

With approximate Dominator data (Roll inertia: 1900 lbs.sq ft, rotor thrust: 550 lbs, teeter bolt 4.4 ft above GC), then the maximum roll rate (90°/s) is reached in 0.4 s, i.e only 20° later. After this period, the height of the mast is without effect on agility

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#### birdy

##### Newbie
Jean, nun of wot i do is constant, cept the constant changeing.
Wen i refer to control rates, im thinkn of the time it takes the machine to change from level to 90* bank, not how fast it can do a 360.

CB, can you enliten me on wot he,s refern to with the " offset flappn hinge on choppys".

.4 of a second can feel like half an hour in close proximity at speed Jean.

#### Jean Claude

##### Junior Member
Yes Birdy, by "agility" I also mean the speed to change the tilt. Not the speed to change course. Inertia of our gyroplane airframe plays only a small role.

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#### birdy

##### Newbie
We agree then Jean.
Its just the level of importance of high rate we wont agree on.

Just a note to anyone thinkn of tryn out their machines rate capabilities, theres more to it than just heavn from one side to the other.
Approach with caution, and wen you feel it getn giggly, youv reached your limit, till you understand wots maken the stick giggle.

#### C. Beaty

##### Gold Supporter
CB, can you enliten me on wot he,s refern to with the " offset flappn hinge on choppys".
Birdy, rotary wing aircraft with central flap hinges are controlled by thrust vector orientation; rotor thrust vector is displaced about the CG to produce a control moment. No thrust; no control even though the rotor may still respond to cyclic pitch input.

Offset flap hinges produce a control moment at zero rotor thrust as a result of the centrifugal pitching couple; the “T” bar effect.

A rotor with offset flap hinges produces greater control power for a given stick displacement than a rotor with central flap hinges, But control sensitivity is unchanged with flap hinge offset because control power and damping both increase at the same rate.

Damping results from rotor lag. At a constant tilt rate, the moment resulting from rotor lag equals control moment and the rotor thrust vector passes through the CG.

#### birdy

##### Newbie
Sorry mate, thats not realy wot i asked.
I can visualise centeral flap hinges, but not offset ones.
Do you have a pic of one i can see, please?

Thanx mate.

#### birdy

##### Newbie
And im assuming central flap hinges is like the gyro teeter hinge?

#### birdy

##### Newbie
.???
Isnt the robinson head both?
It has a teetering hub ( central), with flap hinges( offset).
Mind you, iv never figured why it has both.

#### C. Beaty

##### Gold Supporter
.???
Isnt the robinson head both?
It has a teetering hub ( central), with flap hinges( offset).
Mind you, iv never figured why it has both.
No! The Robinson has a central flap hinge, the teeter hinge and a pair of coning hinges. It flaps about the teeter hinge and cones about the coning hinges.

The coning hinges relieve bending stress on the hub. All teetering rotors ought to have coning hinges to avoid hub cracks.

#### Jean Claude

##### Junior Member
It has a teetering hub ( central), with flap hinges( offset). Mind you, iv never figured why it has both.
On helicopter, coning changes with the load since rrpm is constant. Why no only coning hinge for the flapping ? Because then this require two drag hinges more, and yet two drag damper. Despite that, ground resonance is possible.
Not with the teeter hinge.

#### birdy

##### Newbie
Its all so logical wen you ask the rite people.

#### rtfm

##### Gold Member
Hi,
I am steadily reading through this uber-thread, but so far I haven't come across any consideration of partially powered rotors (PPR) with respect to RTV. I know that PPR decreases the angle at which the rotor flies, and therefore the RTV. So, if one were trying to design a gyro which employed PPR, this would impact either where one erected the mast (horizontally) or the angle of the mast. Would this be accurate? And anyway - how would one determine the appropriate horizontal placement of the mast anyway?

Regards,
Duncan

#### Vance

##### Gyroplane CFI
Most people I know want the rotor thrust vector to pass through the center of gravity at a typical airspeed with the fuselage level.

The Predator has the RTV passing through the center of gravity with seven degrees nose down when hanging from the teeter bolt.

With the cyclic all the way forward and the fuselage level the rotor head is at about a two degree angle back.

In my opinion a partially powered rotor won’t change the location of the rotor head much.

It is easy enough to move with different cheek plates if your guess is incorrect.

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#### rtfm

##### Gold Member
Hi Vance,
Thank you. This is the sort of in-a-nutshell type of info I was looking for.

Duncan

#### Jean - Claude

##### Senior Member
No Title

Duncan, it all depends on "partial" powered.

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