How does gyroscopic procession effect DC model autogyros, do you have to put your control inputs into the rotor 90 deg prior to your wanted movement. I kind of thought thats what made my Chippawa four go in. thanks for any info.

"geink",

>>>...do you have to put your control inputs into the rotor 90 deg prior to your wanted movement...<<<

No.

Geink...

Our good friend, Emilio Cabezas offered an explanation of how an autogyro rotor gets "tilted" when side-to-side or fore-aft inputs are applied. Steve is certainly right that the control inputs don't have to be applied 90 degrees ahead of the desired motion. But what does happen is as follows:

Assume that the rotor has two teetering blades. And that the rotor is spinning counter clockwise as viewed from above. Also consider that the blades are oriented exactly fore and aft. Now, apply tilt input to the left. The blade pointing rearward is twisted to increase its angle of attack and the blade pointing forward is twisted to decrease its angle of attack.

The rearward pointing blade, because it now has a greater angle of attack, is subjected to an upward force. But due to the gyroscopic nature of the spinning rotor, the blade doesn't move upward until it is 90 degrees farther around...namely on the right side of the disk and so the blade moves up and tilts the plane of rotation to the left.

The same argument can be used to explain the tilting of a rotor that has flapping blades.

Hope this is clear!! <G>

Bill

Hi Bill,
Thanks for the info on the gyro rotor, I understand now when you tilt the mast or rotor shaft , blade pitch is automatically changed 90 deg to your intended tilt. thanks.
Another question: what difference does it make if the rotor rotation is clockwise or counter clockwise , I dug out my org. rotor blades and plans for my Chippewa four and discovered that the plans call for clockwise rotation blades,and I had made the org blades counter clockwise ( those were the blades it flew with) . new blades I tried since then have all been clockwise,and they would spin up, but would not lift off .Thanks.

Earl.

Hi Earl...

Since about 1994 or '95 the fellows (gyronuts, that is) who have been getting together once or twice a year have debated the question of rotor rotation. There seems to be no difference. Other factors probably have more influence than direction of rotation.

If the gyro is to be hand lauched and the launcher is right handed, (holds the plane in his/her right hand to launch) then it's easy to give the rotor an initial spin with the left hand. It's easier to spin the near-side blade backward than forward....at least it is for me! <G>

The fact that the clockwise rotating blades spin up but don't produce lift may be caused by too much negative incidence of the blades. Try adding a layer or two of masking tape, or a 1/64 shim ahead of the mounting bolts. The rotor will probably be less easy to get spinning, but once it does, it should produce more lift as it goes into autorotation.

Bill

Hi ya'll,
I keep seeing info indicating rotor blades change angle of attack
during rotation and or change the angle when they articulate.
The articulate portion is hard to understand considering the
blades are mechanically attached at a given angle.
Raising the tip does not change the mechanics?
The blades can hardly (same reason) change their angle to the
rotor plane of rotation?
The rotor itself flys at an angle of attack to the line of flight.
There are four spectrums of rotation> forward, advancing, rearward and retreating.
It would seem the blades do see a change of angle to the line
of flight such as>
Advancing> rotor angle plus or minus blade incidence.
Forward> blade incidence only
Retreating> minus rotor angle and plus or minus blade incidence
Rearward> blade incidence only
Does any of this hold water and or make common sense?
If anyone has definative ideas would appreciate personal input,
OK?

Hal [email protected]

I just wanted to comment on CW or CCW rotation of the blades. I am using molded fiberglass blades for the comparison tests mounted on a converted single rotor SIG Senior Kadet . My first comparison test was performed on a gusty windy day and the blades that turn CW, (as viewed from the top) seem to perform better. I did more testing on a calm day and the direction of rotor rotation didn't seem to make any difference. However, each time I switched rotor direction, I did have to change both the rudder and head tilt trims slightly to make the gyro fly straight and level.

Phil

Hello Hal

I saw your post and just couldnt resist...

Angle of attack has nothing whatsoever to do with the fact that the incidence of the blades are fixed. Angle of attack is by definition the angle between the chordline and the 'relative airflow'. Relative airflow will constantly change with the speed that the blades are flapping as well as the change bought about by the aft tilt of the rotor and the advancing/retreating effects.

Foward flight is only one component of many that combine to give angle of attack

Angle of attack would only be a constant for a rotor if it was in still air and in a vertical descent.

Remember, incidence is fixed. Angle of Attack for an autogyro is constantly changing ( lets face it, it has to or the rotor disc would be unable to allow for lift differential).

Have I re-opened a whole can of worms ? Hee hee

Sean

This image shows the effects of flapping quite clearly.
You can see that the angle between the relative airflow and the chordline ( angle of atack ) is constantly changing despite being in the same rotational plane.

Sean

Sean,

I saw your post and just couldn't resist... ( me either)!!

Now you've got Hal throughly confused. He will probably stay up all night to try and figure that one out. I can say that now because I fly with Hal ever week from October through May in Florida. I'm now in Vermont for the summer so Hal can't step on my Gyro.


I agree with you're following statement:

"Angle of attack would only be a constant for a rotor if it was in still air and in a vertical descent. "

I have a lot of these "vertical descents" usually followed by a lot of repair.

Keep up the good work. I brought one gyro with me but haven't had time to re-assemble for the first flight in Vermont. Naturally it's got to stop raining first. (Maybe I'm in the UK )

Phil

Hi ya'll,
As Phil suggests you all have me thoroughly confused when
you say "angle of attack is constantly changing", how come?
I have a Gyro with the rotor set at 5 deg. pos to the line of
flight. With aerodynamics everything relates to line of flight?
My Gyro is perking along in level flight. doing that certainly the
angle of attack of EVERYTHING concerned is fixed?
Line of flight should be understood. Except in a maneuver such
as a loop. the line of flight is the path the aircraft is following.
The path does not have to be paralell to the horizon, it is there
in a climb or dive even if vertical!

Spindizzy's diagrams are misleading to me. Relative wind is indicated. Wind has nothing to do with angle of attack?? An
aircraft MOVES THROUGH air, air does not blow past it so to speak
Considering flight is in calm air.
The angle of an aircraft component (wing, rotor, tail, etc) can be
any degree to the LINE OF FLIGHT, what ever that angle may be
is the components angle of attack.
How any portion's angle of attack can change or be different is
a mystery to me that I would like to see explained.
The only instance I am aware of is a rotor using "Delta" blade
hinges. Raising a delta hinge does change the angle but this is
not a normal arrangement.
All input appreciated!

Hal [email protected]

Hello Folks

Ummm, no. Everything does not relate to line of flight where angle of attack is concerned. it does if you refer to fixed wing but when it comes to autogyros you can throw most of the aerodynamics for fixed wing in the trash <-- practicing my American

It would appear that this thread on precession has been hijacked

Sorry Hal, I didnt mean to be confusing. I think the big stumbling block here is what the definitions really are.

Using a rotor blade as an example :

Angle of incidence- This is mechanically fixed as you stated, it doesn’t change regardless of how much flapping or how fast it spins except for delta hinging which we will ignore for the sake of this description. Angle of incidence is the difference in angle between the chordline of the airfoil and the longitudinal axis, because this is a gyro the longitudinal axis must be at 90 degrees to the rotor mast.

Angle of attack- This is a constantly changing angle especially for an autogyro. The precise definition for angle of attack is 'the difference in angle between the chordline of the airfoil and the relative airflow'

I have made additions to the drawing to make things a little more clear.

An easier way to explain it is this, take yourself down to the lovely warm seas you have in Florida ( remembering to take plenty of sunblock) and wade out up to your shoulders in the sea.

Hold out your arm straight with your palm facing the seabed and spin round ! You will feel the water flowing equally over the top and bottom of your hand. Now as you spin move your arm up and
down and you will feel an increase in pressure on the top and bottom of your hand depending whether you are flapping up or down. This is because the relative waterflow is changing and therefore the angle of attack of your hand to the oncoming flow of water is also changing. Notice that your hands incidence did not change, only your hands angle to the flow of water changed due to the up and down movement of your arm.

If you stand still and simply move your arm up and down you would feel the flow as a pressure increase on the top and bottom of your hand respectively. Remember this flow is still there if you also spin round. You will see that angle of attack is a combination of values of both the vertical and horizontal flow.

Oh and Phil, I will have you know that it hasn’t rained here for over 3 hours !!! This weekend its going to be in the eighties with horrible humidity..........of course it will have to rain for 3 weeks after as payback

Much respect to you Hal

Sean

Hi Spin Dizzy<
Just to drag this out a little further. Hey, we have sharks in our
ocean!
An important factor is overlooked. The water (air) is NOT moving.
the hand or craft is moving thru calm air.
I assume you are discussing the advancing blade and /or retreating blade.
Also inferring that articulation changes angle of attack.
Paramount is that the angle of attack of EVERYTHING relates to
the line of flight which is a constant.
Considering a blade-wing whose neutral point in it;s articulation
range is say 4 deg. At that point a similarity would be a flat airplane wing. If the blade-wing articulates up or down a comparison would be the wing with either dihedral or anhedral.
We know the dihedraled or anhedaled wing mounted ths same
as a flat wing will have identical angle of attack, Because the tips
are higher or lower has no relavince to angle of attack.
All portions of a craft are relative to the line of flight and they
have no way of changing once set.
With a rotor the same is true. The advancing blade is set at one
AofA and the retreating is at another angle. But rhose are "fixed"
angles, they do not change.
Also consider what lifts a Gyro.
There are revolving blades which create lift. The lift which they
create is finalized as the rotor disc. Thus it is the disc which lifts
the craft and it is what should be considered in all respects.
What makes flight so very interesting is that as many study the
hows and whys the reasons visuallized can vary and usually the
very differences have merit.
If it was not, flight would be like a cup of soup?
Have fun!

Hal deBolt

Oh Dear, sorry Hal I wasn't trying to make you in to fish food.

I obviously totally failed to explain how angle of attack constantly changes on an autogyro.

I notice that you state that your rotor disc has a fixed angle of attack........I would prefer to use the term ' aft rake ' as it is the tilt back of the rotor shaft that gives you what you are describing. Even that angle of attack is not fixed, what about when you are flying very slowly at a high nose angle ???? Is the angle of attack not greater now ?

This is what I think you are getting me all confused with, I am talking about changes in angle of attack of the individual blades as they rotate whereas I think you are talking about the rotors as if they are a solid disc fixed to the fuselage like a wing. This is why I said to throw away the book on fixed wing aerodynamics, as the terminology for rotary wing is very specific to prevent just this type of confusion with angle of attack.

You mentioned that the air/water is not moving. It IS moving RELATIVE to your hand/the rotor blade. Makes no difference whether your blade is moving forward or if the air is moving due to it being a windy day, the effects are exactly the same which is why the term 'relative' is used.

On a fixed wing everything relates to line of flight because everything is facing the line of flight, on a gyro is the retreating blade facing the line of flight ? If the craft is moving forward at 20 knots and the tip speed velocity of the retreating blade is 70 knots how can you say that its angle of attack is fixed to the crafts direction ? The airflow for that blade is 50 knots in the opposite direction that the gyro is flying.

Picture that retreating blade, right now it is moving through the air at 50 knots in the opposite direction the gyro is flying( blade tip velocity minus gyro forward speed ). Right now its angle of attack is the angle between its chordline and its plane of rotation. As this blade is retreating and articulated it would be flapping down, this flapping down would cause the airflow to hit the blade more on the underneath than straight ahead....hey presto the blade has increased its angle of attack by flapping curing dysymmetry of lift .

If you hold your model in the breeze and don't let the rotors spin then the retreating blade aerodynamics are the same as for a fixed wing and the above does not apply. You have to forget everything ever written about fixed wing aerodynamics for a rotating wing, as line of flight is only 1 component of many that set the rules for what is going on with how airflow relates to the blades.

Phew !

Watch out for them sharks

Sean

Hal...

Sean's diagrams of angle of attack are correct.

Bill

Hi ya'll,
Soindizzy is probably right, we are confused and speaking about
different factors.
But, first a couple of wonders> It is said that the retyreating blade articulates downwards, what causes that? Advancing blade
rises because lift increases.
Retreating blade is supposedly flat, is flat in our flight observations. Very strong centrifugal force is generated by a rotor
For a blade to move downwards that strong force would have to
be overcome, correct? What force would do the trick?
There is no question that a blade can not change it's angle of
attack to the plane of rotation, that angle is fixed.
However, the blades act individually to create lift, It is the combo
of each of there lift which creates flight?
That combo is known as the rotor disc which has a center of lift and it is this center from which the craft hangs.
This makes the blades varying angle of attack only important in
that lift is created by each.
The craft flys because the rotor disc (culmination of all blade lift)
has an angle of attack to the line of flight, period!
What is the line of flight if the Gyro is in slow speed (say) with
the nose high? The line of flight is the path the craft is following
which does not relate to it's attitude
Observation of full scale rotor action has been mentioned which
brings an observation.
Full scale Autogyro blades are flexible. Choptors I believe are not
"Our" model blades are not flexible by comparison
It is said the "photos" show full scale blades flexing and perhaps
twisting so that their angle of incidence alters when they turn.
Again I am speaking of "our" rotors which may well differ from
what others use. Our blades are very stiff, it would take a very
strong effort to distort them.
Very good flight photos show the rotor rotation is pancake in
nature, no conning and very little evidence of articulation. Both
advancing and retreating blades appear to be on same plane.
All I have to say is what experience has taught and the bottom
line always is that Aerodynamics prevails for anything airborn.
Hey, Spindizzy> stay out of that Channel on Holloween, the
skeletons of those 109 pilots will grab you, if you are not careful!
Be good,

Hal debolt

Hello Hal

Once more unto the breach.......or something like that .

I quote >>But, first a couple of wonders> It is said that the retyreating blade articulates downwards, what causes that? <<

Simple, centripetal force. Advancing blade meets a faster airflow so its lift is greater than the centripetal force holding it down therefore it flaps up. The retreating blade meets a considerably lower airflow therefore its lift is less and centripetal force returns the blade to its original position ( flaps down ). Do not think that by saying the blade is flapping down that it goes below the horizontal. Flapping and coning can be fractions of a degree yet their effects are quite considerable. You say that your rotors spin virtually flat, they probably do but the amount it flaps is miniscule especially with a high rpm. As I said once before, you will not be able to see this flapping happening. The flapping could be as little as 1 degree but this is still enough to alter its angle of attack to alllow for lift disymmetry.

I have attached an image of my Huff&Puff in flight, you can quite clearly see it has a visible coning angle.

Quote >>There is no question that a blade can not change it's angle of attack to the plane of rotation, that angle is fixed. <<

Well, if the blade is flapping then its plane of rotation is changing, it has to. The axis of rotation doesn't change but the plane of rotation is constantly changing for a gyro in forward flight. I am not making this up honest !! I sit through lectures all about this stuff and watch demonstrations as well as have to study all sorts of books that explain it. The proof is that my own gyros fly and don't roll over like dead turkeys.

In USA, George Townson's book 'Autogiro, the windmill plane' gives a good description of what is going on. I ordered mine through Hannans Runway online. It also contains lots of great images and three views of all the Pitcairn and Kellet gyros.

Well, regardless of what exactly is going on up in that whirling maelstrom that is a gyro rotor, the fact is that it works. I am one of those poor unfortunate people that HAS to know how things work to sleep at night . First thing I did when i bought my first motorcycle was take it apart to see how it all worked !!

I will just happily agree to disagree with you on the aerodynamics and spend my time building those wonderful quick and dirty blades you designed.

>>stay out of that Channel on Holloween, the
skeletons of those 109 pilots will grab you, if you are not careful! <<

Do you have any idea how cold the channel is at halloween ?!
Who needs scary ghosts when you would have to be raving mad to go in the sea at that time of year.

Respectfully

Sean

P.S Angle of attack is also a different angle from root to tip on an autogyro in forward flight. You have a stall region near the hub ( high angle of attack) prgressing outward to a driving region and then on to the driven region at the tip which is at a low angle of attack relative to the root !!!!!!!! You may now shout 'HERETIC' and throw bottles of Budweiser at me.

Hi ya'll,
There has been some technical talk on this spot so thought
the following might be of interest and you might have a reply
that would be most welcome.
It is said " The air flows down through a coptor rotor and up
through an autogyro rotor" The heli portion is obvious with the
rotor being much like a propeller. however the Gyro portion seems
to defy common sense. Lets look at the factors envolved.
Would believe rotor blades are rotating wings?
Wings develope lift and as Gyro arranged also forward motion?
A wing is an airfoil and for a foil to develope lift the airstream
must pass it chordwise.
Airstream is the air which an aircraft passes through. the air
streams by. The craft is moving, not the air.
Thus the airstream is paralell to the line of flight.
The airstream passes the rotor in the same manner as it does
the wing of an airplane.
Can assume the blade airfoils see a chordwise airstream?
Thus the blade airfoils develope lift and forward motion
The sum of all blade lift equates to disc and rotor lift?

Bottom line would appear to be that an airfoil could not work if
the airstream was at a vertical angle to it Flowed upwards.

A rotor developes lift because the upper blade airfoil side is at
a low pressure and the lower side is at a high pressure.
The lower side seeing high pressure suggests the blades are
pushing air downwards, displacing it. Displaced areas are filled
with air from an opposite direction. It could be assumed that some
amount of air thus flows down through the rotor?

Any thoughts on the above? Would be nice to have any thing
explained in plain English, OK?

Hal deBolt [email protected]

I found this old thread in a search on gyroscopic precession. Regarding the first persons question, I see how it is not necessary to index the tilting hinge 90 degrees from the intended direction because from the way I understand it you are not tilting the rotating disk, you are causing it to try to tilt itself.

However, just to make sure I fully understand: Say you had a DC gyro and instead of rotor blades you had a solid disk of aluminum mounted instead, spinning ccw at 500 rpm. The autogyro is sitting in a swimming pool on floats. If you tried to tilt the head to the right suddenly, the precession would cause the nose to dip into the water. Correct?

Your example is correct for a solid disk. However the rotor is not solid so a different set of physical
principles apply. Gyroscopic precession does not really apply, resonance does.
A rotor blade acts like it is spring loaded, centrifugal force tends to pull it flat, other forces
tend to push it away from flat. Vibration analysis shows that when you push on something
with a spring it tends to keep going after you keep pushing. The distance that it keeps going is
the maximum displacement. The maximum displacement occurs after the maximum force. If
you hold a slinky in your hand and let it dangle, then push down and stop, the bottom of the slinky
keeps going after you stop. This illustrates how displacement is behind force. A kids paddle ball toy with
a paddle, rubber band and ball does the same thing. You hit the ball but the ball keeps pulling out (displaces)
way after the ball has left the paddle (the force has stopped).

In your rotor, if you tilt the shaft to the right, the blade that is at the rear gets tilted right, and therefore
negative. This creates a push down on the blade in the back. It displaces (flaps) down, but after the force
is gone. It turns out that the maximum displacement is 90 degrees later on the right side. The blade in the
front gets positive pitch, an up push and 90 degrees later flaps up on the left side. This is how right tilt creates
right rotor disk roll.
Your solid disk when tilted right will tip down in the front. A rotor when tilted right will tilt right.
mick

Interesting.. so why wouldn't it be acceptable to use Direct-Control to tilt the main mast on a model helicopter?

Maybe like this...

[link=http://i5.photobucket.com/albums/y177/dustyatticx/Tiltrotor.jpg]Tilting Rotor Drawing[/link]

Hello Kriegsmacht !! The WW II-era Flettner 282 Kolibri helicopter used twin two-bladed synchronized intermeshing rotors (similar to Georges Chaulet's Synchrogyro) and did have a rotor tilt system as you mention. It was very successful and stable. There was no tail rotor, and had a rudder (mounted directly behind the engine cooling fan's prop blast) to control yaw. I have long wondered why a model helicopter manufacturer has not produced a miniature of this system. It seems like it would be easier to produce than a swashplate/flybar system. Are you aware of this aircraft ?? Charlie Anderson

Otto,

No, I have never heard of the Flettner. Before reading this thread I just naturally assumed it was impractical or impossible to control a rotorcraft by actually tilting the hub or shaft the blades were mounted on.

Live and learn I guess.

Hi Kriegsmacht !!! Apparently the Flettner 282 Kolibri used lead-lag hinges for the blades, and combining lead-lag with contra-rotating rotors must have cancelled out gyroscopic forces, allowing for easy controllability. Sounds like a cool project for an enterprising individual to undertake !!! Happy Flying !!! Charlie Anderson

Direct cyclic control by mast tilting was an itermediate step
in rotorcraft development, like hand crank starting a car.
Once something came along the solved the problems with it
it disappeared.
The problem with mast tilt is very high forces are fed back
to the pilot, in this case the servos.
It seems there is a lot of resistance to a swashplate due
to the fact that it seems complicated, however on a full size
tilt head gyro of the thirties the gimbal that the head tilted on
was huge, big enough to carry the weight of the whole aircraft.
When the swasplate came along, which is just moving the
gimbal off the head and using it to control just the pitch of
the blades, it carried almost no load and could be much smaller
and lighter and cheaper.
The only reason that mast tilt survives today is the bensen gyro
copter where idea was a homebuilt gyrocopter that was as simple
as possible. This kind of construction doesn't scale well to model
sizes, thus the swashplate is still the best choice for model helicopters.
mick