Different blades, different trim.

[asra]

If a gyro hangs nose down at say 10 degree's, and the trim set, then test flown and fine tunned.Why is it that when set up to fly with smaller lighter blades the machine requires more tension on the spring, compared to a lighter spring pressure with the heavier larger blades, Hhhhmmmmm ?Sam

[asra]

I've had a good think about this, and feel its got to do with the gyro's C of G moving rearward when fitted with the larger heavier blades.This is what I found when I fitted my new blades.Need some support & encouragement !!!!Sam.

[Tim]

Sam, given that everything else is equal then the tension required on the spring is directly proportional to the distance the rotor thrust vector is from the pitch pivot bolt.In a vertical descent the rotor thrust vector passes directly down the main bearing bolt and is the pre-set distance in the torque tube from the pitch pivot. Hence all blades should have the same spring tension.Once forward speed is introduced the rotor disc assumes a flap back angle which moves the thrust vector closer to the pitch pivot requiring lesser spring tension. The spring tension required from then on at a given airspeed will depend on the flap back angle, which will vary with the efficiency of every different set of blades.The number of degrees that the gyro hangs nose down will normally only affect the stick position, not the spring tension.Tim McClure

[asra]

quote:Originally posted by Tim Once forward speed is introduced the rotor disc assumes a flap back angle which moves the thrust vector closer to the pitch pivot requiring lesser spring tension. The spring tension required from then on at a given airspeed will depend on the flap back angle, which will vary with the efficiency of every different set of blades ???.Tim McClureHi Tim, Thanks for your reply. I have heard of the flap back angle, and am sure we have spoken about it on this forum before. I would greatly appriciate a detailled explanation as I still am struggling with it.My C of G answer was based on the VC of G balance test. When a heavier set of blades are placed onto a machine, it will now balance on its mains with a lesser tail down angle due to the C of G having moved rearward by the addition of the heavier blades. Sam.

[Tim]

Sam, the weight of the blades does not effect the hang test or the horizontal C of M, only the vertical C of M. In flight the gyro is supported by the centre of the blades no matter what weight they are. The different angle you refer to reflects the change in vertical C of M (pitch moment).The blow or flap back angle results from increase in lift on the incoming blade. This lift force at the RH side of the rotor disc reacts 90* later (precession or phase delay) and the lift happens at the front of the disc. While the gyro has forward speed, this "reaction" is always there meaning that the pilot has to use some forward stick to compensate. Thus in forward flight the rotor thrust vector is always slightly misaligned (rearward) of the rotorhead spindle. In some instances with high forward speed you can actually run out of forward stick and the gyro will climb.Tim McClure

[birdy]

Blades of differing efficiancy and radius will require different trimm tensions.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

Thanks Tim for that explaination, seems logical now that I understand it.It's amazing just how mechanically simple our machines are, but at the same time, just how many forces come into play in order for us to fly !!!!. Sam