Why do gyros use tilt head rather than swashplate

In message , lid writes
You lost me there. I was talking about the use of a tilt rotor (with a
fixed pitch) on a helicopter. Other than losing auto-rotation, what is the
downside? I know losing auto-rotation IS a big deal. I saw a coaxial kit
that uses a tiltrotor (no swashplate, no collective...no auto-rotation) But
they have two engines. Seems like dropping all the extra hardware for pitch
control (they use engine speed to control lift) would be a good thing.


Look, you can alter the lift force of a rotor by two methods. Either you
keep the same rotor rpm, and alter the pitch angle of the blades, OR you
keep the blade angle the same, and alter the rotor RPM.

Or, of course, a combination of the two.

The problem is that, without collective pitch control, you can only vary
the lift force by increasing or decreasing rotor rpm, ie by altering
engine power.

This is very slow to act, due to rotor inertia (flywheel effect) and
means that control is extremely sluggish and imprecise compared to
collective pitch control, where the rotor speed is constant, but the
attack angle of the blades is changed instantly.

You do NOT need a collective-pitch control to change from powered flight
to autorotation - you could just set the blades to autorotation angle to
start with, and then vary power to increase lift - then, if the engine
quit, the freewheel device in the drive chain would let the blades
outspeed the engine, and you'd enter autorotation automatically.

The confusion you are having is, I think, because you (and several
others) think that the blades have to be at positive pitch for powered
flight, and altered to negative pitch (nose-down to the plane of the
rotor disk) for autos. THIS IS INCORRECT.

Blades autorotate perfectly well with a small positive pitch angle -
normally about 1-3 degrees (depends on the airfoil used)



--
Rod Buck

Rod Buck wrote:

In message , lid writes
You lost me there. I was talking about the use of a tilt rotor (with a
fixed pitch) on a helicopter. Other than losing auto-rotation, what is the
downside? I know losing auto-rotation IS a big deal. I saw a coaxial kit
that uses a tiltrotor (no swashplate, no collective...no auto-rotation) But
they have two engines. Seems like dropping all the extra hardware for pitch
control (they use engine speed to control lift) would be a good thing.

Look, you can alter the lift force of a rotor by two methods. Either you
keep the same rotor rpm, and alter the pitch angle of the blades, OR you
keep the blade angle the same, and alter the rotor RPM.

Or, of course, a combination of the two.

The problem is that, without collective pitch control, you can only vary
the lift force by increasing or decreasing rotor rpm, ie by altering
engine power.

This is very slow to act, due to rotor inertia (flywheel effect) and
means that control is extremely sluggish and imprecise compared to
collective pitch control, where the rotor speed is constant, but the
attack angle of the blades is changed instantly.

You do NOT need a collective-pitch control to change from powered flight
to autorotation - you could just set the blades to autorotation angle to
start with, and then vary power to increase lift - then, if the engine
quit, the freewheel device in the drive chain would let the blades
outspeed the engine, and you'd enter autorotation automatically.

The confusion you are having is, I think, because you (and several
others) think that the blades have to be at positive pitch for powered
flight, and altered to negative pitch (nose-down to the plane of the
rotor disk) for autos. THIS IS INCORRECT.

Blades autorotate perfectly well with a small positive pitch angle -
normally about 1-3 degrees (depends on the airfoil used)

--
Rod Buck


Well...

You also change the flight path. The blades need to see an
updraught to absorb power to spin, to generate lift somehow.

The change in direction of the airflow between a nose down
attitude under power in a helo and a nose up, direction slightly
down for autorotation makes the mechanical aoa at the hub
different from the aoa of the airflow on the rotor.

I cannot claim any authority for this beyond personal
experimentation, and my conjecture, apparantly widely debated if
not misunderstood, seems in at least a common sense way to be
true.

A small toy illustrates a point which becomes intuitive. A stick
twirled between the palms with rotor blades attached ascends
until the energy stored on the rotor is consumed. The toy begins
to descend rapidly. The rotor reverses and spins up. The descent
slows dramatically.

The apparent wind on the blades must be at a + aoa to
autorotate, even if the aoa at the reversed hub wrt the shaft may
be -, and the airflow presents a greater + to the reversed blade
with as much - mechanical aoa as it was + rotating in the
ascending phase, spinning it up by energising it, and with
increase in apparent airspeed of the rotor the aoa becomes less +
in regard to the flightpath of the blade arounds the hub, while
being - to the airflow around the craft. Autorotation is a
delicate balance, yes? One must fly the collective carefully to
do autos. It is a skill I cannot claim.

The airflow changes from downward to upward on the rotor blades
of the toy. If the hub rotation does not reverse, the +
mechanical aoa of a powered nose up gyro becomes - wrt the
airflow in the retreating blade, the flight path becomes
downward, wrt apparent airflow to the craft now nose down wrt
horizon, but still + in one sense, and - in another. In
autorotation, part of the rotor's flightpath must be - aoa wrt
the aparrent airflow on the blade.

Watch the toy fly again. Meditate some more. Grok the universe.
Be one with gravity.

Is it possible to explain this in some other way? I can't seem to
express it well. Perhaps a cartoon video is available somewhere?

This is possibly a flawed intuit, and I cannot yet grasp it all.
I am not a rotorhead, but have watched many autorotations from
the tower at a helo flight school, complete with commentary from
those "3 dimensional thinkers" that direct the traffic. As a
control tower radio tech, I have had some "unofficial" flight
training, er, perhaps I should say unofficial experience in fixed
wing and rotor, and understand aerodynamics to an extent in model
design and flight.

If I claim to be a little perlexed, at least you must respect my
honesty. In autorotation flight, part of the rotor disk must be
in - air aoa, yes or no?

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copyright legislation. Permission to reproduce it is
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On Mon, 04 Aug 2003 02:36:31 GMT, Terry Spragg
wrote:

If I claim to be a little perlexed, at least you must respect my
honesty. In autorotation flight, part of the rotor disk must be
in - air aoa, yes or no?

G'day Terry,

I am going to read your post in full again offline, but let me answer
this bit quickly.

There is no need for any part of the aerofoil to be in -ve AoA. It is
not blown around like a toy fan. Have a look at this page
http://www.copters.com/aero/autorotation.html

See in figure 2-89 that because of the upward inflow the lift vector is
tilted forward? The portion of lift in the forward direction acts to
pull the blade forward (or around).

Hope this helps. I'll reread your post and send some more later, if
required.

Cheers,


Phil
--
Pfft...english! Who needs that? I'm never going to England.
Homer J. Simpson

There is no need for any part of the aerofoil to be in -ve AoA. It is
not blown around like a toy fan. Have a look at this page
http://www.copters.com/aero/autorotation.html

Hi Phil,

Great link! Turns out I understood this stuff better than I thought I did.
I've read plenty about the physics, just never seen it laid out this well in
pictures. Thanks for posting it.

Fly Safe,
Steve R.

On 04 Aug 2003 22:41:43 GMT, (Rhodesst) wrote:

There is no need for any part of the aerofoil to be in -ve AoA. It is
not blown around like a toy fan. Have a look at this page
http://www.copters.com/aero/autorotation.html

Hi Phil,

Great link! Turns out I understood this stuff better than I thought I did.
I've read plenty about the physics, just never seen it laid out this well in
pictures. Thanks for posting it.

No probs, Steve.

Google is a wonderful tool. I typed in "autorotation" and that was one
of the first ones listed. :-)

Phil
--
I am so smart...S.M.R.T.
Homer J. Simpson

On Mon, 04 Aug 2003 02:36:31 GMT, Terry Spragg
wrote:

Well...

G'day Terry,

Did the page I pointed you to help at all?

You also change the flight path.

Yeah.

The blades need to see an updraught

Yep.

to absorb power to spin, to generate lift somehow.

Not sure what this means?

The change in direction of the airflow between a nose down
attitude under power in a helo and a nose up, direction slightly
down for autorotation makes the mechanical aoa at the hub
different from the aoa of the airflow on the rotor.

Not sure what you mean by mechanical AoA at the hub, but the AoA of the
blades increases (more +ve), for the same pitch angle, with an upward
flowing airstream.

I cannot claim any authority for this beyond personal
experimentation, and my conjecture, apparantly widely debated if
not misunderstood, seems in at least a common sense way to be
true.

A small toy illustrates a point which becomes intuitive. A stick
twirled between the palms with rotor blades attached ascends
until the energy stored on the rotor is consumed. The toy begins
to descend rapidly. The rotor reverses and spins up. The descent
slows dramatically.

ISTR from childhood playing with this type of device, but I don't
remember them reversing direction of rotation during flight. They would
descend as rpm decreased. Are you just doing a thought experiment and
extending the flight?

The apparent wind on the blades must be at a + aoa to
autorotate,

Yeah.

even if the aoa at the reversed hub wrt the shaft may
be -,

Do you mean the *pitch* is -ve because the rotor is now turning in
reverse?

and the airflow presents a greater + to the reversed blade
with as much - mechanical aoa as it was + rotating in the
ascending phase, spinning it up by energising it, and with
increase in apparent airspeed of the rotor the aoa becomes less +
in regard to the flightpath of the blade arounds the hub, while
being - to the airflow around the craft.

No. I lost it completely there. Are you saying that although the toy,
now spinning in reverse, seems to have a -ve pitch, it actually has a
+ve AoA because it is descending?

Autorotation is a delicate balance, yes?
One must fly the collective carefully to do autos.

As I understand it (and I never got that far in my (limited) experience
in helicopters), it's a matter of engine failure...collective to minimum
ASAP...leave it there until the correct height above the ground...pull
collective to flare. It's not a matter of adjusting the collective
during the auto. I stand ready to be corrected by those in the know.

As for gyroplanes, they will happily autorotate all day without any
means to vary the pitch collectively at all.

It is a skill I cannot claim.

The airflow changes from downward to upward on the rotor blades
of the toy. If the hub rotation does not reverse, the +
mechanical aoa of a powered nose up gyro becomes - wrt the
airflow in the retreating blade,

No. And this is where you are striking trouble, I suspect. The AoA on
the retreating blade does not go -ve. In fact, if anything, the
retreating blade has a higher angle of attack than the advancing blade
(but that is another discussion). the rotor disk is tilted back, yes,
but the blade is doing several hundred miles per hour. The apparent wind
is several hundred miles per hour *in the direction of travel* (plus a
downward component). The direction of the craft has an influence, but
not as significant as I think you think.

the flight path becomes
downward, wrt apparent airflow to the craft now nose down wrt
horizon, but still + in one sense, and - in another. In
autorotation, part of the rotor's flightpath must be - aoa wrt
the aparrent airflow on the blade.

I don't see what you mean. Unless you are thinking that the airflow
caused by craft movement is the apparent airflow. In an aeroplane this
would be the case, but an aeroplane's wings are not doing several
hundred miles an hour and creating their own apparent airflow.

Watch the toy fly again. Meditate some more. Grok the universe.
Be one with gravity.

Ooom...OOooommm...

Is it possible to explain this in some other way? I can't seem to
express it well. Perhaps a cartoon video is available somewhere?

This is possibly a flawed intuit, and I cannot yet grasp it all.
I am not a rotorhead, but have watched many autorotations from
the tower at a helo flight school, complete with commentary from
those "3 dimensional thinkers" that direct the traffic. As a
control tower radio tech, I have had some "unofficial" flight
training, er, perhaps I should say unofficial experience in fixed
wing and rotor, and understand aerodynamics to an extent in model
design and flight.

If I claim to be a little perlexed, at least you must respect my
honesty. In autorotation flight, part of the rotor disk must be
in - air aoa, yes or no?

My vote? No.

Cheers,

Phil
--
Definitions of a pilot - No. 1

The average pilot, despite the sometimes swaggering exterior,
is very much capable of such feelings as love, affection,
intimacy and caring.
These feelings just don't involve anybody else.
US Navy Times

The change in direction of the airflow between a nose down
attitude under power in a helo and a nose up, direction slightly
down for autorotation makes the mechanical aoa at the hub
different from the aoa of the airflow on the rotor.

Not sure what you mean by mechanical AoA at the hub, but the AoA of the
blades increases (more +ve), for the same pitch angle, with an upward
flowing airstream.

I think what they're refering to when they say "mechanical AoA" is blade
incidence.

Fixed wing aircraft generally have their wings set a some positive incidence
relative to the long axis (longitudinal) of the fuselage. The main reason for
this, as I understand it, is so the fuse will fly in a level attitude at cruise
while allowing the wing to fly at a positive AoA to produce the lift needed to
maintain flight. This positive incidence is fixed and cannot be changed or
adjusted by the pilot.

Helicopters, OTOH, adjust the incidence of their rotor blades through
collective and cyclic commands. In this case, the incidence is a measurement
of the angle of the rotor blade cord line relative to an imaginary line running
perpendicular (90 degrees to) the rotor mast. While changing the incidence of
the main rotor blades (through collective and cyclic commands) will change the
aerodynamic AoA, they are not the same thing. For example, if you could lower
the collective to a negative "incidence" (I'm not sure if full size helicopters
do this, I know that RC models do), the rotor blades would still be seeing a
positive AoA while in flight due to the steep descent angle.

FWIW,
Fly SAfe,
Steve R.

On 05 Aug 2003 04:43:37 GMT, (Rhodesst) wrote:

I wrote:

Not sure what you mean by mechanical AoA at the hub, but the AoA of the
blades increases (more +ve), for the same pitch angle, with an upward
flowing airstream.

I think what they're refering to when they say "mechanical AoA" is blade
incidence.

Yeah, I think you're right. Pitch angle (or angle of incidence).


Phil
--
Pfft...english! Who needs that? I'm never going to England.
Homer J. Simpson