Autogyro CofG locating?

[David Oatham]

I have recently completed my 'Revolver' autogyro, and the instructions were quite clear on how to locate the CofG. This seemed reasonably straight forward by hanging the model from the rotor spindle and then by the noseleg.  I understand that the motor thrustline must not be below the position of this, and in fact all seems OK.

My question is regarding how to find the CogG position on a tractor type autogyro rather than a pusher type.

I am in the process designing something along the lines of the 'Atom' but slightly larger.  Also is there a rule regarding the height of the mast in relation to the fuselage / stabiliser?  Any advice would be greatly appreciated.

 

All the best, Dave

 

[Big T]

Hi David, with a tractor we hang the model from the Jesus Nut in ready to fly condition with blades fitted and radio on. You then measure the angle of the thrust line and horizontal, this is known as the Hang Angle. With a 3 blade set up you are looking for a start point hang angle  of between 15 and 18 degrees as a starting point. Too shallow an angle is equivalent to a fixed wing model having too rearward a c of g. You also need to build in right motor thrust. The mast can be angled rearward to aid the hang angle. The longer the mast the more stable in flight but too long causes issues with roll stability. I would start with the Whippit dimensions and work from that. Blade size can be calculated by use of the spreadsheet. 

[David Oatham]

Thanks for your reply Big T. 

Yes I am aware of the hang angle as I have built an Atom in the past. 

The other factor that I'm unsure of is how much motor downthrust is required? 

I assume the right motor thrust is looking from behind, and as a 'dyed in the wool' fixed wing guy this always seems odd as fixed wing motors tend to have a few degrees of left thrust. Is the whippet plan available anywhere do you know?

Dave

[Richard Harris]

Dave,

 

As a rule I start off with 5-6 degrees of down thrust, this generally aligns the thrust line above the vertical CG on a tractor model and also aids spin up.

The effects of pitch instability are nowhere near as severe as that with a pusher as the moment arm between CG and  tail surfaces are longer in  comparison and dampen any offsets.

You can move mass around to suit (servos, ESC, lipos etc), alter mast height and thrustline.

If you are close in alignment but climb is to severe for your liking under power  just simply increase the  tailplane area.

Or if your not up for experimenting simply copy the dimensions of a proven design.

 

Edited April 22, 2022 by Richard Harris

[David Oatham]

Thanks Richard, that's all very helpful. I'm glad you mentioned the vertical CG as that was my initial query. How can you locate / estimate the vertical CG on a tractor model?

[Richard Harris]

Dave,

 

Unless you have a modern 3d drawing program that will give you an accurate CG position its a case of trail and error.

It's worth taking the time to read THIS, written by Jim Baxter back in the 90's. It gives you a good underpinning of what is needed in the designing of a tractor autogyro, it really needs re writing as it is out of date now but still good info. 

 

Rich

Edited April 22, 2022 by Richard Harris