Autogyro Question ?

[AndyOne]

Can anyone explain how the autogyro shown on this web site...

http://pustka.bei.t-online.de/Autogyro.htm

…can fly without any negative pitch on the blades.

Here is a translation of the tech data courtesy of Bablefish:

Tech, DATA Tragi:
Rotor diameter 950mm profile Clark Y 10%
engine speed 400 7,2V
transmission Aeronaut 3:1
propeller APC 11x7 slow
7 cells Sanyo 500 ACRE/Hecell 1000
weight 440g, of it rotor 70g
inclination rotor mast 8° to the rear
blade pitch angle 0°
attitude:roll +-6°, nod + - 10° linkage:
Steel wire 1,2mm
flying time 6/10min

Andy.

[omega blood]

It looks to me as it is using purely cyclic controll for movement, like a helo. I don't know what you mean by negative pitch. If you are refering to the 0 degree blade pitch lift is created (I belive) by tilting the rotor. I realy don't know a lot 'bout auto gyros but they are coool.

[AndyOne]

omega

All Autoguro designs I have seen except this one have made a point about negative pitch on the blades in order to create the correct lift.
I don't think the control method effects the essential aerodynamics of an autogyro.

Andy.

[HELModels]

AndyOne,
I dont know if this helps or not, but a Clark Y at 12% thickness still puts out some lift at an angle of attack of 0 degrees. Coefficient of lift hits zero at around -1.25 degrees. As for how, it looks like the mechanism for controlling the blades is how the angle of attack is altered, but it looks to me like the mechansim only allows side to side tilt of the rotor head. Maybe pitch is controlled by throttle only - rotor spins faster, more lift, throttle back less lift.

Whoops, just saw that the rotor head is "inclined 8 deg." that would put Coefficient of lift at around .8 - .9 at an angle of attack of 8 degrees. Use another airfoil and the incline would probably need to change.

[jcstalls]

Greetings,

Electric Autogyros have been an intrest of mine for a while, yet still have not built any yet. Few plans and parts.

The head on this unit is a DC head, or Direct Controll. Roll and pitch with both servos via mixing. This unit was designed and uses a SP400 no less. Like most AutoGyros, this one requires use of a mill for fabricating the head components. There is a short thread on this unit here somewhere started from the designer.

The head is also facinating in that the hinge, or flex joints for the blades are offset so that if one blade comes up higher in tracking than another, the incidence reduces to aid in keeping them on track.

Jared

[AndyOne]

I'm sure that the control is a simple weight shift system and no more, there are no control surfaces at the tail like other autos.
Some autos have helicopter like controls with collective and cyclic pitch control.

The flying principal, as far as I understand it, is like a helicopter constantly in auto rotation mode therefore it needs negative pitch on the blades but this one doesn’t have it so I'm not sure what’s going on. Other model autos have have similar flat bottom section aerofoil together with negative pitch.
My question is, is it really necessary.

Incidentally, does anyone know if a two-blade autogyro is feasible or would there be a practical problem with this set-up.

Andy.

[jcstalls]

Greetings,

Here is a nice link to many, many hours of reading. Looks like I will go dig up my Sparky plans and parts.

http://www.autogyro.com/index.html

The Clark Y seems very popular due to as it is fairly easy to carve, lift properties and strength. I do not remember seeing any two blade systems. Three is very prominent.

Jared

[AndyOne]

Jared,

Good stuff! plenty of good ideas.

Thanks,

Andy.

[max z]

…can fly without any negative pitch on the blades

The flex hinge line is offset by 15 deg. This would give the blade some negative incidence when it flexes up under load. Maybe just enough to keep it going?

Max.

[HELModels]

I read some parts of the autogyro.com link and apparently, helos are a special case in that they alter the incidence angle of the blades and push air down through blades(symmetrical foils better for that). Autogyros apparently tilt the entire rotor disc to change angle of attack(change direction of lift vector) and must produce lift like a fixed wing. That is why 8 degree tilt of mast head(positive alpha- positive lift).

[max z]

That is why 8 degree tilt of mast head(positive incidence- positive lift).
Sure, mast tilt will give you lift - until the rotor stops that is. It is the seemingly impossible combination of windmill effect to keep the rotor going, requiring airflow to go upwards through the plane of the rotorblades (and not downwards as with a helicopter), and creating a downward force: the required lift. That is where the negative incidence angle of the blades comes in, in this case the incidence angle meaning the angle relative to the plane of rotation.

Max.

[AndyOne]

Many thanks to Jared for pointing me to the autogyro.com web site, this explains most everything you would want to know about model autogyros.
The answer to the question of negative pitch is that you DON'T need it, but it helps to spin up the rotor.

Max,
Thanks for pointing out about the angled flex plate, it seems to increase the negative pitch with increasing rotor load although I'm not sure why it needs to work that way, I guess it's some sort of negative feed-back to stop flutter.

Andy.

[HELModels]

I realize the autogyro.com is a definitive source for autogyros and that you are satisfied that negative incidence is Not necessary. Here is something to consider, however.

Downwash of the preceding rotor blade, which might explain why 2 blades are used less. 3 blades would increase the frequency and coherence of flow with which a blade encounters the flow off the back of the preceding blade. Downwash would lower the angle of incidence.
A cambered airfoil has a more severe downwash angle off the back of the airfoil than a symmetrical blade of the same thickness and AOA. If airflow hits an airfoil at the right angle(like the Zero Lift Line angle), there can actually be some thrust forward and so the blade would want to keep advancing. The ZLL would be lower for a symmetrical foil, same thickness.

The trust angle and flow from the prop is probably more important to the first advancing blade to hit it than to the other blades because of the first blade training the flow.

Build one, run it up, and blow some smoke into it and it is probably easier to see than to say.