Q: How do you calculate blade angle on a 3-bladed tail rotor?

[ticedoff8]

On the SOKO web site, there is a nice online tool to calculate tail rotor blade angles based on measuring from fixed points.
http://www.soko-heli-tools.com/en/ta...alculator.html
It gives very precise angles

Obviously, it is a formula that is based on a rectangle

Is there a similar formula for calculating blade angles for a 3-bladed tail rotor?

I would think the fact that the grips are at 120-degree points around the tail shaft vs 180-degree must factor into the calculation.
There is no right-angle at the blade grips on the 3-bladed tail.

[ticedoff8]

I started a conversation with Andi at SOKO Tools on this question.
I got this reply:

We investigated the calculations involved in a 3-blade tail the last couple of days and unfortunately its not looking good. The rotation of the edge of the blade is too complex for us to calculate. We even talked to a Math professor at the university. He said its possible to calculate it but only in the affine space. The math function to solve it though is not available on a simple web-server.

In short: We wont be able to calculate your pitch with the measurements you have done for us.

There may be though another alignment of the blades and taking different measurements where the math becomes way more easier. We will investigate in this direction and let you know if we have something promising...

Sorry for the bad news

I think they tried and I appreciate it.
If there is an update, I will add to this thread.

[AndrewJHawes]

You probably have many, albeit relatively low-tech options.
Use a traditional protractor-style pitch gauge with one of the 3 blades facing in a convenient orientation, such as perpendicular-upright, in relation to the tail boom as reference. Alternatively, place your digital pitch gauge onto the blade running parallel to the boom (in or out), and subtract from 90 degrees.
The real question probably should be why does it matter?
We need more pitch available to counteract the torque of the main rotor, and a little less the other way. but more importantly, we should have mechanically sound clearances, and ranges sympathetic to the type and specification of tail symmetry/tail-gyro configuration.
Andy

[hANDYman]

I have to agree with Andy above. Why do you need to know the pitch of the tail rotor? Simply set the gyro so that there is maximum throw either way just shy of any mechanical binding. This gives the gyro the maximum amount to play with as it tries to keep the tail in place.

But since you were curious, and since I like a challenge...

Given the measurements provided in the pictures of:

a = 64 mm
b = 100 mm (it looked like about 3-15/16 from bolt centre to leading edge corner)
c = 112.19 mm

The pitch angle is 28.85º. (28.85º actually comes out to 112.13 mm but I'd call that close enough.)

If you're really anal, then it's closer to 28.876º, which is 112.189 mm.