Caco's trial helicopter flight.....

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Cacophonix
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Re: Caco's trial helicopter flight.....

#341 Post by Cacophonix » Fri Mar 15, 2019 12:09 pm

For those who have ever wondered how the pitch controls on a helicopter rotorhead work then this video showing the stationary and the rotating swashplates with scissors links etc. is one of the best I have ever seen.

Simple for the helicopter LAME's, hard for the heli non-cognoscenti like me, to visualise

The chap who developed this digital simulation is a genius.
IMG_1885Control.jpg
IMG_1885Control.jpg (84.38 KiB) Viewed 633 times
http://www.helistart.com/pitchControl.aspx



Mechanical poetry in motion and a work of sheer beauty.

Caco

Seenenough

Re: Caco's trial helicopter flight.....

#342 Post by Seenenough » Sat Mar 16, 2019 9:02 pm

You also got to bring gyroscopic preession into it Caco.

An input action will only occur 90 degrees later

None the less the video is very good at showing how the parts all work together.

Seenenough

Re: Caco's trial helicopter flight.....

#343 Post by Seenenough » Sat Mar 16, 2019 9:20 pm

Caco-there are two publications that you have to have if you do not already have them.

Wagtendonk on the Principles of Helicopter Flying and the FAA Helicopter Flying Handbook that will explain just about everything there is o know about rotorwing flying in simple and easy to understand terms (which was an absolutely necessary for a dunce like me).

Cacophonix
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Re: Caco's trial helicopter flight.....

#344 Post by Cacophonix » Sun Mar 17, 2019 10:43 am

Seenenough wrote:
Sat Mar 16, 2019 9:02 pm
You also got to bring gyroscopic preession into it Caco.

An input action will only occur 90 degrees later

None the less the video is very good at showing how the parts all work together.

Yes, I haven't been able to find a video that shows the 90 degree phase difference correctly although it is discussed in the link above.

Effect of Cyclic Pitch on Rotor Disc Attitude
When the swash plate tilts in a certain direction, cyclic pitch is introduced. The question now is: how does tilting the swashplate in a particular direction influence rotor disc attitude (and its thrust vector)? We will, therefore, now examine the circumstances in which the swashplate is tilted forwards, and will assume that there is a counter clockwise rotating system (viewed from above). To assist our line of reasoning, we will first focus on 4 strategic points on the circle of rotation, and will examine the blade angles at these points. The points (A to D) are each 90 degrees apart, and the figure below sets out their locations. To understand what the blade angle is at these points, there is something else we need to know: the configuration of the control rod and links. In this example, we will assume that these are attached to the blades’ advancing site.

Heli flap.JPG

In these circumstances, the blade angles will be:
A : Negative (max amplitude)
B : zero
C : Positive (max amplitude)
D : zero

Note that the maximum amplitude is related to the angle at which the swashplate is tilted.
The figure shows the quadrants in which the blade angle (pitch) is positive and negative, and we will now look at the behaviour of the blades in each of them. If the angle is positive, it will rise. If the angle is negative, it will fall. Following this line of reasoning, it is clear that the blades will be at their highest position at point D, because they have travelled for 180 degrees in an upwards direction. Similarly, the blades will be at their lowest position at B. The altitude of the disc is, thus, sideward, meaning that this helicopter would move to the left if the swashplate is tilted forwards. This probably isn’t what you expected! Clearly, there is a 90 degree difference in the tilting direction between the swashplate and the rotordisc. This is known as the phase lag.

Note that there is another way of explaining this phenomenon, namely in terms of gyroscopic precession. Both have the same result, and so we won’t consider this latter explanation any further.

Phase lag
If it is not compensated for, the phase lag between the swashplate and rotordisc would make the life of the pilot very difficult. The way to overcome this problem is to connect the control inputs to the swashplate 90 degrees in advance of the blade to which they relate.

I have downloaded the FAA document which really is excellent. I will order the Wagtendonk (great name for an aviation related subject by the way).

Caco

bob2s
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Re: Caco's trial helicopter flight.....

#345 Post by bob2s » Sun Mar 17, 2019 11:38 pm

Caco, another book that is very good is, John Fays - The Helicopter and how it flies. This book was required reading for for all budding Sky Gods and Black Handers
when I started out.

Cacophonix
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Re: Caco's trial helicopter flight.....

#346 Post by Cacophonix » Mon Mar 18, 2019 7:04 pm

bob2s wrote:
Sun Mar 17, 2019 11:38 pm
Caco, another book that is very good is, John Fays - The Helicopter and how it flies. This book was required reading for for all budding Sky Gods and Black Handers
when I started out.
Will seek it out bob2s. Many thanks for the steer.

Caco

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