New 90 Head

12/28/08

Home
Up
3rd Bearing Upgrade
New 90 Head

 

Useful Upgrade Tips on radio controlled helicopter models. If you want to help me and provide upgrade tips, please Mail them to me.

bullet

New 90 head design.

Currently I am in the process of designing a complete new head. Amongst others the design goals are:
bulletBetter linearity in the head when putting in control inputs. Most heads on the marked are very non-linear and even asymmetric.
bulletVirtually no interaction between roll and nick.
bulletVirtually no interaction between pitch and roll and pitch and nick.
bulletVirtually no interaction between forward speed and roll trim.
bulletMore stable head. More stable in the hover, and more stable in fast forward flight.
bulletMore responsive head. More agile when roll or nick is provided to the head.
bulletProgrammable head such that people can choose their own setup with reference to e.g. Delta-3 offset, or Bell-Hiller mixing ratio, or Flybar Feedback, or Flybar Input or Cyclic Input or etc. This allows end-users that know what they are doing to easily tradeoff between certain aspects (e.g. stability v.s. load on servo's v.s. agility of the head v.s. etc.
bulletLighter design. Some designs are way over dimensioned and too heavy.

In order to do so I first designed a very conventional head as a starting point. At this point in time I have already fully designed a rather conventional head with very linear and symmetric response. This head is already better than most commercially available heads.

The reason for designing this conventional head first is to be able to only replace a small set of parts on the head first for testing. By doing so I can verify my mathematical models and simulations on the dynamics of the head and the interaction of design options. This helps to speedup the design (less prototypes required) for the final head and builds up confidence in the mathematical models.

To give you an idea of this process I have provided below some pictures of the head modeled in 3D, some animations of the control input simulations as well as some stress analysis.

Note that the head is a parameterized 3D model. This means that I can very easily change any parameter. I can use this to fit it to any helicopter I like with very little effort. I use the Raptor 90 as a test bed at this point in time.

 

bulletThe Conventional head

Here you have an exploded view of the conventional head (+- 130 parts modeled in 3D with actual measurements).

 

Some simple aspects that play a role in the design of a head to give you an idea on the design space (there are many more not listed here :-)):
bullet

Distance between blade bolts -> introduces more or less blade lead / lag and thus changes blade stability and servo load.

bullet

Bell / Hiller mixing ratio -> changes the feedback loop of the flybar, changes stability, roll rate and the amount of energy that goes in the flybar.

bullet

Flybar control input gain -> can buy you extra stability at the cost of additional power loss in the flybar system.

bullet

Delta-3 offset -> changes stability and agility. Changes stability in fast forward flight.

bullet

Airfoil of paddles -> changes aggressiveness of the flybar (stability).

bullet

Weight of paddles - changes stability in the flybar, changes the stability of the helicopter and the agility.

bullet

Position of paddles on the flybar (chord wise) -> changes stability of the flybar, load on servos.

bullet

Position of the paddles on the flybar (length wise) -> changes stability and agility at the cost of power loss in the flybar.

bullet

Stiffness of the flybar -> changes the aggressiveness of the flybar.

bullet

Length of the main blades -> changes tip speed of the blade, agility of the helicopter and the energy loss in the main blades.

bullet

Chord of the main blades -> changes the agility of the main blades and the energy loss in the main blades.

bullet

Airfoil of the main blades -> changes the lift and the stability and the efficiency of the blades at different angles of attack.

bullet

Weight of the main blades -> changes the agility, changes the stability, changes the autorotation and changes the loading of the engine.

bullet

Center of gravity of the main blades (length wise) -> changes the stability, agility and autorotation as well as the engine loading.

bullet

Center of gravity of the main blades (chord wise) -> changes the lead / lag and thus blade stability and servo load.

bullet

Torsion stiffness of the main blades -> changes the blade stability and the agility of the helicopter.

bullet

Bending stiffness of the main blades - changes the stability and agility of the helicopter.

bullet

Blade spindle dampers or flap dampers -> changes agility and stress on the head and helicopter.

These are just some of the aspects that play a role. I listed them as said just to give you an idea of the complexity of designing a new head. There are many interacting design decisions to be made.

 

The final head design is likely to have a flybar below the main blades. This simplifies the control layout considerably.

Here is the current conventional head:

 

Here you have it in 360 degrees view (takes a while to load):

 

Here you can see the pitch movement (takes a while to load):

If you want to see some more simulations, then please click here (Having them all on one page would make the loading too slow).

Here you can see some stress analysis pictures:

This first picture shows the absolute cumulative bending upwards due to roll input. The spindle is locked on the utmost left-hand side and the force applies on the bolt hole in this simulation.

Cumulative bending as a function of Roll input (mm).

In this simulation you can see the absolute cumulative stretch due to the centripetal force. Here you can see that the stretch starts from where the bladeholder becomes thinner (light blue part). Due to the "stretch upon stretch" you can see that just after the bolt hole there is the maximum displacement due to the strain.

Bladeholder stretch due to centripetal force of the main blade pulling (mm).

 

Here you can see the stress in the bladeholder. You can clearly see that the stress is the highest near the bolt hole and where the bearing sits to take the thrust load.

Stress in the bladeholder (mN/mm2).

 

Here you can see the strain in the bladholder due to the stress in the bladholder. The strain is the relative stretch due to the stress.

Strain in the bladeholder.

 

Here you can see again the strain in the bladeholder.

Strain in the bladeholder. Note that strain where the blade bolt pulls, and where the bearing rests inside the bladeholder.

 

Non manufactured parts

Some pictures of parts that I just modeled for fun, I will never manufacture these :-).

The paddle. I will not manufacture these since SAB already manufactures whatever paddles are required.

The links I used for modeling. I will not manufacture these since it is too expensive to create a mold for them. They do have an upgrade aspect to them though. I explicitly modeled them such that you can use them both sides. This design would require a very expensive mold though to manufacture them.

 

Home | 3rd Bearing Upgrade | New 90 Head

Just double click on any word on this page to get an explanation of the specific word!

Copyright 2003, 2004, 2005, 2006, 2007, 2008, 2009 by John Vugts. All rights reserved.
It is not allowed to copy or translate any content of this page for publication to any other media.
This site was last updated 12/28/08
For comments about this page e-mail: webfeedback@tiscali.nl