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Why do gyros use tilt head rather than swashplate



 
 
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  #11  
Old July 25th 03, 02:28 PM
Rhodesst
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On Fri, 25 Jul 2003 11:57:53 GMT, Terry Spragg
wrote:

For the simplest possible reason:

An auto gyro is always flown in autorotation, with fixed plade
pitch set for 'gliding'. If you apply torque to an auto rotor,
you do not get lift, you get propelled groundward.


G'day Terry,

Are you saying that autogyro's have negative pitch on their rotor
blades? If so, you are wrong.

If that's not what your saying, I apologise. I have misunderstood your
meaning. Could you please explain this statement?

Cheers,


Phil


I was wondering about that myself!

Fly Safe,
Steve R.
  #13  
Old August 1st 03, 12:27 AM
PW
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"Rod Buck" wrote in message
...
In message , Terry Spragg
writes

An auto gyro is always flown in autorotation, with fixed plade
pitch set for 'gliding'. If you apply torque to an auto rotor,
you do not get lift, you get propelled groundward.


Alas, not true - an autorotating rotor does NOT normally have negative
blade pitch - it has positive pitch of 1-3 degrees, and therefore
generates lift by rotation.

Therefore, if you powered it by a torque from the hub, it would lift off
perfectly well.

It would NOT try to screw it's way into the ground, as it would if the
blade pitch was neggy.


Pitcairn gyros used jumpstart, where the blades are spun up to well
above normal RPM, in zero pitch, then the usual autorotational pitch of
1-3 degrees is applied (either by collective or by delta-three hinges,
which increase the pitch angle when hub torque disappears.)

You must, of course, declutch the hub drive as soon as you increase
pitch and take off - with no tail rotor or other countertorque means,
you'd spin real fine....


The delta-3 hinge is a ferocious beast - once you get above flight rpm,
you MUST take off, there is no way out - bit like lighting those Shuttle
SRB's - it's not a question of whether you go or not, it's just which
direction....

As soon as you declutch the motor, or reduce motor RPM, the hinges
increase the pitch, and off you go.....

The gyro leaps 1-200 ft in the air, and then the prop drive provides
forward motion to start normal autorotational gyro flight - the rotor
pitch is NOT reduced again to do this - it stays at the normal 1-3 deg.


If you apply
power, you are flying a helicopter, which must have some manual
control over pitch if both powered flight and autorotation is to
be possible.


Wrong again, friend.

It's perfectly possible to have a helo with a fixed pitch rotor, set to
the small positive angle for autorotation, and alter lift by changing
engine power, and thus rpm. (You can then use head gimbal as in gyro to
alter pitch cyclically for directional control).

(The rotor rpm would be quite a bit higher than normal for the same
lift)

However, the rotor momentum (flywheel effect) makes the control
extremely sluggish and impractical, compared to collective pitch
control.

However, one safety improvement would be that, as the rotor is always in
the low-angle suitable for autorotation, if the engine quits, a
freewheel device in the rotor drive chain would ensure you entered auto
painlessly.

--
Rod Buck


Rod,

You just answered soooo many of my questions. THANK YOU! Would you mind
if we talked a bit via e-mail?

Again, thanks

Phil Williamson
Oregon City, Or.


  #14  
Old August 1st 03, 11:22 AM
Rod Buck
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In message , Charlie+
writes
On Wed, 30 Jul 2003 21:06:07 +0100, Rod Buck
wrote as underneath my scribble :

Rod, as a matter of interest have you personally done one of these
takeoff jumps? Sounds exciting/scary!


Not done it personally - seen it in the flesh once, and on film lots of
times. Scared me sh*tless to WATCH it - never mind fly it!

Presumably the change over to propdrive is immediate on initiation so
there is some element of forward drive from the start of the leap?


Yup, dead right. My impression is that there is some sort of changeover
mechanism, whereby the motor drives either the rotor hub, to spin-up the
rotor to well above flight RPM, or the normal propeller to provide
inflight thrust as in a normal gyro.

So, when you get up to 150% flight RPM, you pull the lever, the hub is
declutched, and the prop engaged....

Is the direction of the leap unpredictable / local wind etc causing
major imbalances?
Also is there some drop in initial altitude until full gyro level
flight and control can be achieved? if so - how much swoop do you
estimate?
Charlie+

From what I've seen, you obviously line the gyro's nose into wind, (and
as in all aviation, a decent wind helps no end).

Got to be careful to get the cyclic central, so the thing leaps up
vertically (but with some forward thrust from the prop during the leap,
so the leap is actually forward, even if the rotor is horizontal)

Then, at the top of the leap, the rotor must be angled back to the
normal gyro angle to ensure upward airflow through the rotor (and drag
to balance the prop thrust) as you move into wind.

Where there is a wind of, say, 15mph or more, I saw no evidence of drop
or swoop at the top of the leap - it just flew off.

In nil-wind, I would anticipate there must be some drop, maybe 50ft.
Depends on the engine thrust, and how quickly it accelerates the gyro to
normal forward flightspeed. This is why a wind prevents drop, of course.
A 15mph wind means you've got at least 20mph airspeed at the top of the
leap - you must gain at least 5mph forward speed in the leap if the prop
is running.

This is enough to maintain level flight.

--
Rod Buck
  #15  
Old August 1st 03, 11:30 AM
Rod Buck
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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
  #16  
Old August 2nd 03, 06:02 AM
Ken Sandyeggo
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Let me jump in here a little. I got to ride with John Potter at
Paducah, KY last winter in an Air & Space. The prop is constantly
engaged. There is no transition from the engine powering the rotor
and then switching to the prop after the jump in this craft. The
Lycoming turns the prop just like any other prop in a pusher
configuration. The rotor is engaged via a multi-belt driven geared
transmission. The rotor spins up to somewhere over 300 rpm (while
depitched)if my memory isn't letting me too far down. The tires alone
keep the gyro from counter-rotating. One wheel was on a piece of ice
during one of the spinups and John had to move it onto a dry patch.

Once the proper rotor rpm was reached, John reached to the panel and
pressed a button. The blades instantly repitched and we shot straight
up like a rocket. I'm sure that the transmission disengages
simultaneously. The ship reaches maybe 30-40 feet and smoothly
transitions into forward flight and a climb mode. There is no loss of
altitude during the transition. Now I suppose someone could screw it
up and not have enough throttle on during the jump start, but the
engine rpm seemed to up there some to get the blades going, and the
jump happens very quickly. As I recall, full throttle is applied as
soon as she jumps. It's a very, very fast sequence. You're at the
top of that jump before you can blink.

We took off on the runway and then from the tarmac perpendicular to
the wind, and there didn't seem to be any problems.....no more than a
plane taking off in a crosswind. I felt no yawing or unsteadiness
from the crosswind jump and never experienced any dropping.

Now I'm going strictly by memory on the mechanics, but I believe I
have the basics correct.

Ken J. - Sandy Eggo



Rod Buck wrote in message ...
In message , Charlie+
writes
On Wed, 30 Jul 2003 21:06:07 +0100, Rod Buck
wrote as underneath my scribble :

Rod, as a matter of interest have you personally done one of these
takeoff jumps? Sounds exciting/scary!


Not done it personally - seen it in the flesh once, and on film lots of
times. Scared me sh*tless to WATCH it - never mind fly it!

Presumably the change over to propdrive is immediate on initiation so
there is some element of forward drive from the start of the leap?


Yup, dead right. My impression is that there is some sort of changeover
mechanism, whereby the motor drives either the rotor hub, to spin-up the
rotor to well above flight RPM, or the normal propeller to provide
inflight thrust as in a normal gyro.

So, when you get up to 150% flight RPM, you pull the lever, the hub is
declutched, and the prop engaged....

Is the direction of the leap unpredictable / local wind etc causing
major imbalances?
Also is there some drop in initial altitude until full gyro level
flight and control can be achieved? if so - how much swoop do you
estimate?
Charlie+

From what I've seen, you obviously line the gyro's nose into wind, (and
as in all aviation, a decent wind helps no end).

Got to be careful to get the cyclic central, so the thing leaps up
vertically (but with some forward thrust from the prop during the leap,
so the leap is actually forward, even if the rotor is horizontal)

Then, at the top of the leap, the rotor must be angled back to the
normal gyro angle to ensure upward airflow through the rotor (and drag
to balance the prop thrust) as you move into wind.

Where there is a wind of, say, 15mph or more, I saw no evidence of drop
or swoop at the top of the leap - it just flew off.

In nil-wind, I would anticipate there must be some drop, maybe 50ft.
Depends on the engine thrust, and how quickly it accelerates the gyro to
normal forward flightspeed. This is why a wind prevents drop, of course.
A 15mph wind means you've got at least 20mph airspeed at the top of the
leap - you must gain at least 5mph forward speed in the leap if the prop
is running.

This is enough to maintain level flight.

  #17  
Old August 3rd 03, 10:21 AM
Stephen Austin
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Let me jump in here a little. I got to ride with John Potter at
Paducah, KY last winter in an Air & Space.


I think the key word there is winter. Low density altitude is key.

Once the proper rotor rpm was reached, John reached to the panel and
pressed a button.


Hmm, well, the button on the panel is to engage the prerotator clutch. If
you'll remember, once that button is pushed the throttle is increased so the
engine is about 1500 RPM. At that point you begin pumping the hydraulic lever
to engage the clutch. You slowly pump the handle to maintain continous
pressure while at the same time working the throttle to maintain 800 RPM. It
is very much like engaging the clutch on an Enstrom. Once rotor RPM and engine
RPM marry the handle is quickly pumped until the engage button on the panel
pops out. This indicates the system is fully engaged. From then the RPM is
increased to 370 RPM.

The blades instantly repitched and we shot straight
up like a rocket. I'm sure that the transmission disengages
simultaneously.


The blades are depitched by pushing a button on the top of the throttle. Hehe,
it all happens so quickly that it is easy to lose track of what happens when.
That's why the depitch button is on the throttle. It would take too long to
push a button on the panel and then have to bring your hand back to the
throttle.

The ship reaches maybe 30-40 feet and smoothly
transitions into forward flight and a climb mode. There is no loss of
altitude during the transition.


Winter had a lot to do with that. Jump takeoff is unlikely above a 2000' DA.
No doubt pilot skill, gross weight, wind, etc. play a major part. But I do
know that when I've flown the 18A it was in the summer. Max jump height was
about fifteen feet and after the jump we sunk to about five feet before we got
ahead of the power curve and established a climb. I do remember also that if,
after the jump, the aircraft settles to the ground, power must be reduced and
the sequence started all over again.

BTW, for a jump, the rotor is spun to 370 RPM. Operating RPM green arc is from
200-320 RPM. It's all a bit tricky but quickly becomes second nature.
Regardless the 18A is a great flying ship.


Stephen Austin
Austin Ag Aviation
Charleston, Missouri
  #18  
Old August 3rd 03, 10:24 AM
Stephen Austin
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The only problem with the A&S was a severe limitation on the operation at
the local field because of noise restrictions - it sure was a loud bird.


Hmm, at Farrington Airpark? I think someone was pulling your leg. I've never
heard of any noise restrictions there. It's kind of in the boondocks anyway.
And, for that matter, there is a stock car track about one hundred yards from
the strip (the strip actually began life as a drag strip until Don bought it).
Anyway all those cars are one heck of a lot louder than an 18A.


Stephen Austin
Austin Ag Aviation
Charleston, Missouri
  #19  
Old August 3rd 03, 01:54 PM
Michael Pilla
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"Stephen Austin" wrote in message
...

The only problem with the A&S was a severe limitation on the operation at
the local field because of noise restrictions - it sure was a loud bird.


Hmm, at Farrington Airpark? I think someone was pulling your leg. I've

never
heard of any noise restrictions there. It's kind of in the boondocks

anyway.
And, for that matter, there is a stock car track about one hundred yards

from
the strip (the strip actually began life as a drag strip until Don bought

it).
Anyway all those cars are one heck of a lot louder than an 18A.


Stephen Austin
Austin Ag Aviation
Charleston, Missouri


Nope. My A&S 18A lesson was in NJ, but I forget the town. Even though the
airstrip was in the Western (less populated portion of the state - rural,
actually), the local residents were the highly paid, city-commuter types
(NYC) and, apparently both vocal and influential. The 18A clearly was
louder than the other planes (typical spam can range, plus a goodly amount
of low-powered Cubs, Champs, etc.) - it definitely stood out during ops.

Michael Pilla


  #20  
Old August 3rd 03, 05:24 PM
Stephen Austin
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NJ, but I forget the town. Even though the
airstrip was in the Western (less populated portion of the state - rural,
actually), the local residents were the highly paid, city-commuter types
(NYC) and, apparently both vocal and influential. The 18A clearly was
louder than the other planes (typical spam can range, plus a goodly amount
of low-powered Cubs, Champs, etc.) - it definitely stood out during ops.

Michael Pilla








Man, don't you hate that? Was it one of those deals where the airport was
there long before any of the homes?


Stephen Austin
Austin Ag Aviation
Charleston, Missouri
 




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