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Old Oct 11, 2009, 10:34 PM
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MSR Stability secret (?)

Aside from the 45 degree offset, if you have not noticed, the rotor blades diameter vs the length of the shaft ratio is pretty much different from the standard FP heli. That is;

- Shaft is longer and blades are shorter.

If you look at the profile of the MSR's head, it is way higher and more distant from the heli's COG point - which makes the heli's body a "pull-down" dangling pendulum. This configuration actually keeps the heli upright and stable during a handsfree hover.

But with so much distance from the COG to the cylic head, and with shorter blades, this configuration now makes the heli more restrictive to fore/aft/roll movements as the blades neet to drag the pendulum body to the intended the direction. How is this compensated ? Simply by increasing the pitch transmitted by the swash to the rotor blades which is done through a bell hiller head configuration and by simply making the heli lighter.

Update:

Here is a little diagram on the ratio;

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Old Oct 11, 2009, 10:36 PM
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Could the msr ratio be applied to the 4#3b?
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Old Oct 11, 2009, 10:44 PM
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i noticed this effect on two different 45 degree offset coaxials as one is exhibiting a more stable flight than the other considering they are both coaxial helicopters;

Coaxial with a shorter shaft;

Badminton Mania - 3 (0 min 31 sec)



And the 735 Set "Longer Coaxial Shaft" heli;


Badminton Mania - 1 (0 min 50 sec)


The 735 set body is practically a pendulum with the stabilizing agent (upper flybared rotor head) way distant from the heli's main body which makes them ultra stable handsfree hovering machines;

COG Balancing Handsfree demo on an indoor 3 channel helicopter (0 min 32 sec)
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Old Oct 11, 2009, 10:49 PM
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Quote:
Originally Posted by lazer155
Could the msr ratio be applied to the 4#3b?
That would mean using a longer shaft, longer servo linkages, and a higher anti rotation bracket.
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Old Oct 11, 2009, 11:00 PM
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The MSR head is heavily anticpated for stability as such;

- Main shaft is tilted to compensate for the tail rotor sideways translating tendency

- Length of blades computed and balanced for upright lift while maintaining blade diameter to shaft length ratio

- Body Weight + Lipo weight optimally set for that given ratio


.. that any mod you apply to this heli would require a lot of weight optimization and COG balancing. In short, the MSR would be a little sensitive on any configuration changes.


The flybar plus the pendulum body works hand in hand to keep that shaft upright. I bet that anything a miss on the flybar or its weight (due to mods) will automatically translate to a "Pendulum" like swing (has anyone seen this yet ?)
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Old Oct 11, 2009, 11:13 PM
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This also explains why coaxials are more stable than standard single rotor helis. Other than the fact that there is no tail rotor for the left-wise sideways drift (translating tendency), the shafts are longer (basically to accomodate the two rotors) which makes the body a dangling pendulum weight.
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Old Oct 11, 2009, 11:25 PM
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Raising the cg is seen quite often in cp helis. The lipo is placed as high as possible to bring the cog up closer to the rotor disk. Same thing can be seen in the manouvrability of top wing trainer planes as opposed to mid wing / low wing planes. Ultimately if the vertical cog could be right at the feathering shaft the heli would have really axial rolls and they could be fast but the heli would be very twitchy about the roll axis.

Perhaps the lipo could be moved higher in the msr for better response and less "coax" tendancy.

Amp
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Old Oct 12, 2009, 01:27 AM
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Also consider the blade design. The blades are fatter at the base and taper towards the tips. When spooled up it forms a cone shape. This would create a stabilizing effect.

Make a paper cone and drop it and watch the way it falls. This can also be seen when looking at the Gaui Ep200 blades vs the T250 blades. I noticed that the T250 blades have this taper design even though it is a CP heli.
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Old Oct 12, 2009, 03:30 AM
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Quote:
Originally Posted by Ampdraw
Raising the cg is seen quite often in cp helis. The lipo is placed as high as possible to bring the cog up closer to the rotor disk. Same thing can be seen in the manouvrability of top wing trainer planes as opposed to mid wing / low wing planes. Ultimately if the vertical cog could be right at the feathering shaft the heli would have really axial rolls and they could be fast but the heli would be very twitchy about the roll axis.

Perhaps the lipo could be moved higher in the msr for better response and less "coax" tendancy.

Amp

This is true. I had an experiment before on coaxials where i situated the weight in between the upper and lower blades. Although its a coaxial with the 45 degree offset, heli behaves like an FP and is extremely cyclics sensitive and less stable and requires more stick input to control.


http://www.rcgroups.com/forums/showthread.php?t=906720

Coaxial UFO Project (2 min 12 sec)


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Old Oct 12, 2009, 03:36 AM
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Quote:
Originally Posted by whilrybird
Also consider the blade design. The blades are fatter at the base and taper towards the tips. When spooled up it forms a cone shape. This would create a stabilizing effect.

Make a paper cone and drop it and watch the way it falls. This can also be seen when looking at the Gaui Ep200 blades vs the T250 blades. I noticed that the T250 blades have this taper design even though it is a CP heli.
From my understanding (Flash could also comment on this), the reason why the blades closer to the root are wider/fatter is that the angular velocity at this part is much slower than compared to close to the tip. As a result, the blade wash and lift force and air displacement is evenly distributed from root to tip.

Specially with plastic blades, if they have the same width across from root to tip, the part of the blade closer to the tip will displace more air than the part of the blade closer to the root causing the tip to bend up forming a conical shape which makes the heli less stable.
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Old Oct 12, 2009, 11:01 AM
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Quote:
Originally Posted by Ampdraw

Perhaps the lipo could be moved higher in the msr for better response and less "coax" tendancy.

Amp
Yes. Also another way is to lighten the battery but still, the best option is to move the battery closer to the rotor head.
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Old Oct 12, 2009, 05:16 PM
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Hmmm. on second thought moving the battery higher might cause the body to oscillate. One should try experimenting on this.
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Old Oct 15, 2009, 04:27 PM
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Quote:
Originally Posted by EQMOD
From my understanding (Flash could also comment on this), the reason why the blades closer to the root are wider/fatter is that the angular velocity at this part is much slower than compared to close to the tip. As a result, the blade wash and lift force and air displacement is evenly distributed from root to tip.

Specially with plastic blades, if they have the same width across from root to tip, the part of the blade closer to the tip will displace more air than the part of the blade closer to the root causing the tip to bend up forming a conical shape which makes the heli less stable.
To add..

Mon's understanding is correct. Lift is proportional to the square of the blades velocity, in other words a three fold increase in velocity will produce a nine fold increase in both lift and induced drag. As seen in the following equations where velocity is a squared function:

L = Cl 1/2 P V^2 S ( Total Lift )

and

D = Cd 1/2 p V^2 S ( Induced drag not total drag ).

where

L = Lift
D = Drag
P = Air density
V = Velocity (m/s not rpm)
S = Surface area
Cl = Lift coefficient
Cd = Drag coefficient

So from that we can say the outer portion of a blade travels faster than the inner. If a thrust curve is plotter for this lift figure spanwise, you would see most of the lift is produced at the blade tips. In order to even out the blades loading two methods are adopted and create a term called "washout". The first is to add twist to a rectangular blade and reduce it's angle of attack towards the tip therefore reducing lift towards the tip. The other is to create a larger surface area towards the root ( tapered blade ) increasing lift towards the root without twisting the blade. Something every hard to do on thin plastic blade - they would not simply hold there form during rotation and flatten out.



Coning on these helis is mainly due to the flexibility of the plastic blades, they are so weak they just cone up in the opposite direction to thrust and weight. Coning does add some stability by deflecting the downwash outwards in a wider cone. However it also reduces lift by shortening the radius of the blade and therefore it's disc area. The image below shows the principle in shortening the length but is for another rotor application, hence the text does not fit the question. But you can see radius X is shorter than radius Y due to a greater coning angle. Since a blades radius is proportional to it's velocity, lift is also dramatically effected, even by small changes in R.



I recently bought some mSR blades to study and came to the conclusion the the root area width is mainly for strength and not for additional aerodynamic lift.

hope that helps.

Flash

ps. nice diagrams mon.
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Old Oct 15, 2009, 06:47 PM
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Quote:
Originally Posted by CQE Flash
I recently bought some mSR blades to study and came to the conclusion the the root area width is mainly for strength and not for additional aerodynamic lift.
If measurements are done on the thrust of a rotating blade using the measured pitch, it will be discovered that at any RPM the lift is less than calculated.

The reason for this is inflow. The air sucked into the rotor disk is reducing the effective angle of attack.

The interesting thing is that the velocity of the incoming air at the root of the blades at lower pitch levels can actually be going in the wrong direction.

The diagram below illustrates the effective angle of attack for blades at a measured pitch of 10 degrees as a function of blade radius.

10 degrees is the measured pitch and the blue line is the effective angle of attack which has diminished considerably because of inflow.

It can be noted that the angle of attack at the root of this blade is very much smaller than at the tips. If the pitch is reduced further, the incoming air at the root could become outgoing with the root of the blade contributing negative thrust.

If the real life thrust distribution is plotted as a function of radius, it will be noted that it is not a linear distribution as a function of radius and there is very little contribution to total thrust near the blade roots.

The optimum lift-drag ratio for flat-bottomed airfoils occurs at about 5 degrees.

At angles of attack less than 5 degrees the lift-drag ratio decreases dramatically.

Although the mSR blades are curved and not flat-bottomed, it leads me to wonder how much improvement in flight time might result by trimmimg away a large portion of the blade root.
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Old Oct 15, 2009, 07:35 PM
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The coning does create stability, but the performance in handling and efficiency is sacrificed. Whether it was an intentional design or side effect of cheap plastic makes no difference, it contributes to stability. That was the original topic.
It might not contribute much, but I've never seen anyone install flat bottom wood blades to a coaxial. Maybe if someone tries this we can know what the effect is.
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