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PowerHelis . JR-Spektrum . Gyro Hobbies

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CAD - Engineering - Technical > What purpose(s) do the dampers serve?
 
 
gregj
Heliman
Location: Palos Verdes, Calif.

What purpose(s) do the dampers (little rubber rings around the feathering spindle) serve?

What would happen if I replaced the dampers with something rigid such as metal rings of the right size?

Do advanced pilots intentionally modify flight qualities of their helis by going with stiffer or softer dampers? What changes would one expect from stiffer or softer dampers?

If you were designing a helicopter, how would you decide if your dampeners were too stiff or too soft?

Do R/C helicopter designers use mathematical models, computer modeling and simulation etc. during the design process? If so, are the dampers mathematically represented in the models and are their properties optimized?

It seems like dampers are an important part of the design of an R/C helicopter; every helicopter I have seen has them, and occasionally the designers have devised somewhat ingenious, nonstandard schemes in order to incorporate dampers. (The Hornet is an example.)

Their effect can be seen very easily (always be safe of course): Hold a Hornet with one hand and spool up the motor, then move the cyclic stick. You can see that the rotor disk changes its plane of rotation pretty significantly even though you are not letting the fuselage move with it.

A few guesses:

- flapping in forward flight? (allow forward-moving blade to move upward and and backward-moving blade to move downward, to compensate for asymmetric lift and reduce necessity of forward cyclic inputs to achieve the same goal)

- dampen or delay movement of the rotor disk as it affects the fuselage of the helicopter? (I.e., if you input cyclic, it will make the rotor disk change its plane of rotation, and that might not immediately translate to changes in the fuselage's orientation because of the dampers) (Or, for example in a hover if a gust causes the rotor disk to change its plane of rotation, the delay might cause the fuse to stay level while the flybar, which stayed level anyway, inputs controls to bring the rotor disk back to level.)

- Lessen vibration induced by the main rotor on the rest of the helicopter?

Thanks,

Greg
05-12-2004 Over year old.
 
 
Doug
Elite Veteran
Location: Naples Florida....

The dampers provide "damping", that is they convert the movement of the spindle into heat. The rubber dampers provide both velocity damping (max resistance at max rate or movement which acts 90 degrees later in phase and displacement damping ("spring force" where max effort is at max displacement) The phase delay of the displacement damping is highly dependent on each setup and is responsible for the "Graininess" of control when the head is too stiff. A lot of damping does come from the blades and blade holders. at the end of the day it is to control the turning Torque against the mast as smoothly as possible. and yes the flapping is necessary to overcome the dissymmetry of lift caused by motion of the rotor through the air

First member of Member of Bearings Anonymous
05-13-2004 Over year old.
 
 
DTM
Senior Heliman
Location: Belgium

Quote 
Do R/C helicopter designers use mathematical models, computer modeling and simulation etc. during the design process? If so, are the dampers mathematically represented in the models and are their properties optimized?


My mathematical models is called Mathew

Seriously, the fysics are so complex that trail and error works much faster, also the damping of the head is not fixed data: it has a lot to do with flying style: 3D pilots often go for harder dampers because they transmit the controls better; the beginners benefit from a soft damping because it damps out unwanted disturbances like wind gusts. I found the damping on my Sceadu too soft because the heli wobbled during tictocs etc.

Quote 
What would happen if I replaced the dampers with something rigid such as metal rings of the right size?


You would end up with a very responsive and unstable helicopter, it has been done by the way, the blades take over the damping then.

Quote 
- flapping in forward flight? (allow forward-moving blade to move upward and and backward-moving blade to move downward, to compensate for asymmetric lift and reduce necessity of forward cyclic inputs to achieve the same goal)
Quote 
- dampen or delay movement of the rotor disk as it affects the fuselage of the helicopter


It does both: of cours the flapping is desirable whereas the damping is not (for 3D)
05-13-2004 Over year old.
 
 
K-Lo
Senior Heliman
Location: Monterrey Mexico

Dampling.

Ok, a simple explanation stolen from real helicopters.

The angle of attack (forward flight) of the rotor blade that is advancing is not the same angle of attack of the blace that is going towards the rear.


The blades change their angle of attack at a given position at unequal values depending on cyclic (aileron / elevator) control, this means that if you want to go forward the blade going towards the back has more angle of attack (piitch) than that one of the front to make the disc go forward (not considering gyroscopic perception).

This makes one of the blades to receive more drag than the other and thus moves at unequal speed than the one going forward.

At fast induced flow (air going thru the rotor on the horizontal axis) (aka: at fast speeds in forward flight) one blade produces more lift than the other, and this fenomenom is called dissimetry of lift, where there is not an equal lift produced by one blade with respect to the other.

The dampers allow this blade to flap back a little so that the one going forward does not decreases the speed.

All blades move at unequal velocity and at unequal angles of attack, and they (should) provide equal lift. The dampeners provide means of resolving the disimetry of lift. Airspeed in real helis is limited in great part by this factor, cause if you put too much pitch on one blade the critical angle of attack can be reached and causes the blade to stall (the one going towards the back.

I don't understand how this can translate to smaller rc helis, but this is (according to the faa) the right definition of fully articulated helicopters, and also semi rigid helicopters. In true helicopters the dampers are known as elastomeric dampers, and i don't know the whole physics behind this, but i am learning

I hope this "reduced" explanation helps understand the dampers.

Do what you can with what you have. Often is more than enough.
06-02-2004 Over year old.
HOMEPAGE  
 
 
Davo
Veteran
Location: London, UK

Quote 
one blade produces more lift than the other, and this fenomenom is called dissimetry of lift, where there is not an equal lift produced by one blade with respect to the other.... The dampers allow this blade to flap back a little so that the one going forward does not decreases the speed.


K-Lo this explanation is close but not quite accurate for full scale helicopters. To balance out the dissymmetry of lift the blades flap up and down (this is defined as flapping) thereby by changing the relative wind vector and hence the angle of attack of each blade. The blade on the advancing side climbs (therby reducing AoA and decreasing lift) whilst the blade on the retreating side falls. This is what balances the lift forces on each side.

However, this flapping up and down causes coriolis forces (think conservation of angular momentum - the CG of the blade is moving in and out slightly as the blade flaps up and down so it will want to speed up and slow down as a figure skater would if they move their arms in and out whilst spinning). Now this causes large stresses in the rotor hub unless you incorporate a lag hinge. The lag hinge allows the blade to 'lag' and lead' back and forward in order to relieve the stresses.

This make sense???

So the flapping would be wholly in the dampers, whilst the lag hinge effect in our helis must be partly made up of movement in the dampers and partly from the blades lagging in the blade grips???

06-02-2004 Over year old.
 
 
K-Lo
Senior Heliman
Location: Monterrey Mexico

It does makes

Perfect sense

Thanks for clearing out... i'l get it next time.


The coriolis effect is produced by coning of the blades right?

Do what you can with what you have. Often is more than enough.
06-02-2004 Over year old.
HOMEPAGE  
 
 
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CAD - Engineering - Technical > What purpose(s) do the dampers serve?
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