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Thread: Rotor Pitch angle...

  1. #1
    Registered User Bob Kelly's Avatar
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    Aug 2004
    N.Calif Mountians

    Rotor Pitch angle...

    Ok i got'a Question for you fella's out there....

    My Ultralight Gyrocopter has a Fixed Rotor pitch of 2 degrees....
    what would happen if i changed it to say 4 degrees ?

    i have thought about this for a while now and come up with a few theories...but no proof at all....

    1. the Rotor would be alot harder to turn .... due to the increased pitch
    but once up to flight speed it should do fine i would think.

    2. it SHOULD give more lift .... but be harder to push through the air...
    so top speed would be much slower.

    But what about engine failure ? would the glide path be affected much ? and in what way? i would think that the increased pitch would
    just swap hight above ground for lift at a given speed... fall below that speed and you decend faster....

    Helicopters usually flatten out their pitch when engines fail in order to auto-rotate.... Correct ? now what happens if they don't flatten them
    out all the way ? faster desent correct ?

    So what i'm wondering is there a point of no return on rotor pitch for auto-rotation ? or what ? will a steeper pitch turn my Rotor blades faster than they are intended to go ? will ground speed have to be 70 mph to take off ??? hehehehe

    I have a Pre-Rotator so spinning the extra load shouldn't be a problem
    and i have hurd of other Gyro-Owners setting their rotor-pitch to 6 degrees for better performance...
    I am realy courious just exactly What happens to the Rotors on a Auto-rotating arrangement when pitch is changed !!!

    Thanks for your Time


  2. #2
    Site Developer Jman's Avatar
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    Oct 2002
    Pacific NW, USA!
    Well, I can tell you what happens in autorotation for helicopters but I can't recommend what pitch angle you should use. I just plain don't know.
    Helicopters usually flatten out their pitch when engines fail in order to auto-rotate.... Correct ? now what happens if they don't flatten them
    out all the way ? faster descent correct ?
    If you do not reduce collective, the rotor speed will decrease rapidly and you will fall from the sky like a rock. If the collective is reduced, pitch angle will be reduced, which will lower the drag of the rotor system and allow the air moving up through the rotor system to maintain rotor RPM. At the termination of the Auto, when the pitch angle is increased you trade rotor RPM for lift and hopefully arrest your decent enough to effect a soft landing. So you can see, in helicopters at least, increasing pitch angle, decreases RPM. Now, some slowing of your decent rate (increased lift) can be accomplish by decelerating. This trades forward airspeed for lift without reducing Rotor RPM. This is done by tilting the rotor disc back (bringing the nose up) which causes the air to move more perpendicular to the plane of rotation. As soon as the decel has taken place however, you begin to drop vertically and must use a pitch change to use the stored energy in the form of RPM to land in a controlled manner.

    Now, for your problem - it seems to me that if you increase your pitch angle you will increase lift but will also increase drag. This means you will need the air to be moving faster through your rotor disc to keep the same Rotor RPM. I also imagine that how much your rotor disc is tilted in relation to the oncoming air can also affect Rotor RPM. So I guess that would mean that if you wanted to increase your pitch angle and still maintain your normal rotor RPM you would either have to increase forward airspeed or, if you didn't want to go faster, alter the angle the oncoming air enters your rotor disc. In both cases more power would be required and in the case of tilting the rotor disc, who knows what control issues would be introduced.

    Now, remember, I know VERY little about gyrocopters and have only a base knowledge of helicopters. Someone with more knowledge and ACTUAL experience, please respond. I may actually be making a fool of myself here

  3. #3
    Registered User Bob Kelly's Avatar
    Join Date
    Aug 2004
    N.Calif Mountians
    Just for your info... the Rotor head tilts up and down with the stick and right and left with the stick.... it is not fixed at any one angle....
    so increasing the angle of attack is almost automatic...
    however that is not all there is to the rotorhead.... above the berring
    on witch the rotor attaches is another pivit point that alows the rotor blades to travel up and down at the tips indipendent of the controls
    ....this is a "spinning disk -stability thing " that without it, the craft would roll off the supporting air and fall sideways into the ground.... if i got my understanding right anyway. it allows the blades to FLAP to a certain degree ...
    the only Pitch adjustment is on the Rotor-Hub-Bar where the rotor blades attach.... the bar is cut at an angle so as to alow some adjustment with 4 alen screws, then the whole thing is tightened down
    with 2 large 3/4" bolts that hold the blade on !

    I remember reading an artical in a magizine about a feller who was experimenting with pitch angle in his Gyro-copter, the only ill effects he knoticed was trying to get the Rotors up to flight speed.... take off was shorter than it had been before the change.... but other than that no real diference..... he desided to reduce the pitch back down to a lower angle in order to make the Spin-up easier ....

    I am thinking a shorter take-off distance is a good thing ! and if that can be accomplished by a adjustment ...why not do it ?
    i might experiment with it next year...... flying season is just about over here now.... time to winterize the o'l girl before the snow falls i guess
    ..... or perhaps make ski 's for it ! HAHAHA ! Burrrrr now that would be cold indeed !

    thanks fer the info !


  4. #4
    Registered User Othman's Avatar
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    Aug 2004
    Ottawa, ON, Canada

    I'm not sure if you are talking about increasing the blade pitch or the rotor disc tilt (angle of incidence).

    Either way, the angle of attack of the blades may end up exceeding the max angle of attack at some blade stations... at some flight conditions, resulting in localized blade stall. This would be anti-progressive, and may be dangerous if large areas of the blade begin to stall.

    You should continue researching what others have done for the same model gyro as yours (other gyros have different blade designs that may behave very different from yours), and make sure you get facts and not just hearsay evidence.

    You don't want to be the guinea pig without putting your design mods through a proper analysis first.

  5. #5
    Registered User Bob Kelly's Avatar
    Join Date
    Aug 2004
    N.Calif Mountians
    Othman my friend !

    I'm not sure if you are talking about increasing the blade pitch or the rotor disc tilt (angle of incidence).

    I am refureing to the blade Pitch angle itself not the rotor disk tilt angle.
    .... Any your right it is a touchie subject because alot of unseen things go on on the Rotordisk.

    Sense i have no intentions of becomeing another Gyrocopter Stistic I do alot of looking into it before i do anything to that aircraft.

    As you can see from my Avatar, it is not your usual looking Gyrocopter
    now ! ....i am alwayse looking to improve on it though.

    I guess the Question is Kind'a Mute because without enough info i will never change it from the recomended settings... but i am realy courious
    about the goings on up there ! hehehehe!

    For instance, i am toying with the idea of makeing a Helicopter and was thinking sense i have already made a tail rotor complete with adjustable pitch and all, why couldn't i use the rotor of the Gyrocopter just set at a steeper angle of attack of each Rotorblade ( pitch) .... and use the Throttle and Rotor RPM to control lift and decent.......
    ..... but here is the problem ... what about engine failure....?
    i guess with the pitch set steep enough to lift the craft from the ground you would loose ALL of its Autorotational ability all togather .... but i am not positive ! .... in effect you'ed probably fall like a stone as Jman put it ! ....

    the G1 Ultralight Helicopter uses this method.... useing 11 degrees pitch, but it also uses Weight-shift for steering too ! ... a far cry from optimum !

    if a person was to meld the 2 crafts togather he may have something worth having.... the entire control head of the gyro-copter with the blades set at a fixed pitch... And a tail Rotor for control of the turning of the craft.... the only draw backs i can see is the engine failure....
    even if i added a clutch to disenguage the engine... the Rotors would still
    loose enough lift so as to cause it to make a small crator in the ground...

    Any Sujestions on this line ?

    thanks fot the Info guys !


  6. #6
    Registered User
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    Aug 2004
    Big bad New Zealand
    2 degrees doesnt sound like a whole lot.
    why dont you crank it up to 4 or 5 degrees and ground run the thing? nothing wrong with that. Ground run will initally answer a lot of your questions.

    as far as an autorotation goes, there would be more drag on the blades as they move through the air, so if you didnt watch your airspeed closely your rotor rpm would bleed off quicker. again, add in a half a degree or so and go for a fly, pull your throttle back to idle when your at cruise speed and see what happens.

    talk to someone with a gyro, they'd be able to tell you alot more than we can.

    as far as helicopters go, when they enter autorotation they dont completly go to flat pitch, if they did, there wouldnt be any reaction force from the up moving airflow, so they wouldnt spin up. on the Iroquois, there is a worm screw that adjusts the amount of preload on the TT (tension/torsion) straps, and alters how much pitch the blades sit at when the collective is on the floor, thats how you adjust the autorotation rpm, no different to a gyro.

    hope this helps.

    p.s. if your gonna build a helicopter, it'd probably work without having a collective, but i personally wouldnt recommend it. the simplicity of the idea kicks ass, but i think that the cons would outweigh the pro's.
    Last edited by Leighton; October 8th, 2004 at 07:17 PM.

  7. #7
    Registered User Bob Kelly's Avatar
    Join Date
    Aug 2004
    N.Calif Mountians
    talking to someone that has one is kind'a hard sense i don't know of ANYONE that has a Gyro-copter ! HAHAHHA

    Yes , your right on that , adjust it and see what it does.... that is what the other feller did that i read about.... seams safe enough....

    And your correct on the Helicopters never flatening their pitch completely
    .... glad you cought that...

    and though i am still toying with the idea.... the more i think of it the less i like the idea of motorfailure brings you down like a stone !
    .... i guess i'll work on a simplier collective , and see if i can make it work that way.....
    .... i've came up with a darn easy collective for the tail rotor....
    i suppose if i just beefed it up Alot it would do the job....
    hehhehe it took about 3 weeks to make the tail rotor it'ed probably take a month to make the rotorhead..... but with the added security of being able to auto-rotate it would be well worth it........

    now as for weight goes...... 1/2" plate steel is heavy ! for an Ultralight
    that is a No,No..... but the day i let regulations take presidence over my own safty is the day i give it all up ! HAHHAAH !
    .... oh well, just thinkin' .... perhaps i'll build Learido Divinchie's Wood bird wings and attach a chainsaw motor to do the hard work ! HA !
    ..... but what i'ed realy like to see is a Aircraft paterned after a Dragon fly..... a honnest to goodness Ornithorpter.... or how ever ya spell it.

    ..... Thanks for the Input fells'a !


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