Aerodynamics

[BustedRaptor77]

Gyroscopic precession relates to the pulling of the rotor system with the controls. It is not related to dissymmetry of lift in the capacity you are describing.

Please read the section in your link on compensation to understand what I am referring to about articulation. In RC aircraft the contributing factors are the blade dampers, the rigidity of the blades, blade span moment, and the blade bolt tightness.

I would imagine if you reached VNE with a FBL model the FBL controller would make things worse not better. You would be simply inputting forward cyclic and the controller would be giving the compensation to correct the result would be an immediate roll.

I do agree with the pitching upwards the faster you are going just not in the capacity you are trying to piece it together.

[jolyboy]

"pulling on the rotor system with controls" creates dissymmetry of lift. Thats what controls the heli.

If you actually look at your helis you'll find:

Up elevator (nose up control input) increases the blade pitch on the advancing side of the rotor - creating more lift on the advancing side. This causes the nose of the heli to lift. Nose up in FFF happens because the advancing side is generating more lift as well. Not "effects about the roll axis" (unless maybe you are in a real heli in which case I would have no idea)

I agree that its possible for a heli experiencing RBS to roll in the direction of the stalling blades, but thats no what I'm trying to discuss.

Every person Ive spoken to about his (except you) has told me to make my heli nose heavy for FFF to avoid pitch-up problems. So I make my heli nose heavy. It really works, you should try it. I'm no heli mechanic, but it works for me.

[BustedRaptor77]

I am quite familiar with how a rotor system works. The term dissymmetry of lift is referring to a specific phenomenon. I have never heard it used to explain the actual left, right, nose up, nose down response of cyclic input is all.

I never stated to not shift the heli cg forward to stop the nose from coming up. This works.

What it would do is cause the heli to flip faster (as in without as long of an indication of the nose coming up). This is because the RBS is related to the rotor dynamics and not the CG.

All I was pointing out on the gyroscopic precession is this is not related to airspeed or lift. It has to do with what direction the aircraft will move in relation to where the swash is tilted and the connection to the rotor blades.

[jolyboy]

Originally Posted by BustedRaptor77

I'm sorry, but this is completely wrong. "Dissymmetry of lift" is compensated through blade articulation that is not controlled by a flybarless system, and it has effects about the roll axis.

I think half the problem is you are talking about full size helis and im talking about RC helis. This discussion is focused on RC helis. RC helis dont have articulation. Also RC helis cant hit VNE.

Let me expand on what I originally said.

Originally Posted by jolyboy

The FBL unit compensates for dissymmetry of lift

Dissymmetry of lift = nose-up in FFF
Nose-up = error signal generated my FBL controller
FBL controller compensates for error-signal by adding forward cyclic.

Therefore FBL unit has compensated for dissymmetry of lift.

BUT - its better to have a CG forward so the FBL unit does have to compensate as much. Dont ask me why, it just flies better like that.

Was I really completely wrong on this?

[BustedRaptor77]

Originally Posted by jolyboy

I think half the problem is you are talking about full size helis and im talking about RC helis. This discussion is focused on RC helis. RC helis dont have articulation.

Please explain what thrust bearings and the dampers do then.

[jolyboy]

Ok, correct, they have a simple form of that, but this does not explain how I was completely wrong

EDIT: even rigid head helis without articulation still get compensated by an FBL unit

[BustedRaptor77]

Originally Posted by jolyboy

Dissymmetry of lift = nose-up in FFF
Nose-up = error signal generated my FBL controller
FBL controller compensates for error-signal by adding forward cyclic.

Therefore FBL unit has compensated for dissymmetry of lift.

BUT - its better to have a CG forward so the FBL unit does have to compensate as much. Dont ask me why, it just flies better like that.

Was I really completely wrong on this?

You are not completely off. I am really not disagreeing with you. This is a thread in "Aerodynamics" a rotor system is a rotor system. The huge difference in RC to full scale has more to do with power to weight ratios. All of the physics are the same.

[jolyboy]

Ok cool.

[BustedRaptor77]

Originally Posted by jolyboy

Ok, correct, they have a simple form of that, but this does not explain how I was completely wrong

EDIT: even rigid head helis without articulation still get compensated by an FBL unit

You can call it simple but there is no difference between this and a Bell 206.

[BustedRaptor77]

Originally Posted by jolyboy

Ok cool.

[BustedRaptor77]

Since this thread moved to dissymmetry of lift, can one of you super genius people that was posting in the beginning of the thread explain why we don't see the effects of translational lift in our model helicopters? I have never gotten the shimmy coming back to a hover. Is this related to the wicked fast head speeds?

[Elios000]

Originally Posted by BustedRaptor77

Since this thread moved to dissymmetry of lift, can one of you super genius people that was posting in the beginning of the thread explain why we don't see the effects of translational lift in our model helicopters? I have never gotten the shimmy coming back to a hover. Is this related to the wicked fast head speeds?

we see the effect of it
ever fly in the wind? even just hovering in the wind you can see it

[BustedRaptor77]

Originally Posted by Elios000

we see the effect of it
ever fly in the wind? even just hovering in the wind you can see it

I'll have to pay closer attention. It is definitely not as abrupt I guess due to the size.

[Elios000]

Originally Posted by BustedRaptor77

I'll have to pay closer attention. It is definitely not as abrupt I guess due to the size.

what size are you flying i feel it for sure with my 500 and 600
and when i had my mCP X you could feel it in the wind
its also why fixed pitch helis fly like crap in the wind

[BustedRaptor77]

Originally Posted by Elios000

what size are you flying i feel it for sure with my 500 and 600
and when i had my mCP X you could feel it in the wind
its also why fixed pitch helis fly like crap in the wind

700 and 600 mostly. I have a couple of little blade helis I fiddle with.

[desertstalker]

Originally Posted by BustedRaptor77

I'll have to pay closer attention. It is definitely not as abrupt I guess due to the size.

You can definitely tell, try going from level forward flight to hover without touching collective. Heli will drop quite fast (many people refer to this as translational lift, not sure if it is the same thing or not).

You dont always get the little wobble in my experience, could be FBL can compensate for it, or the helis are so small and light it is nor really apparent?

[extrapilot]

Guys

The last few pages has a lot of convoluted/incorrect info.

Fore and aft CG vs pitch stability is the same with helis as with fixed wing. If you have CG ahead of the center of lift in forward flight, when the rotor experiences a transient period of positive lift (i.e. slight change of headwind, etc), that causes the chassis to pitch down. Since the chassis is fairly rigidly attached to the rotor, this action tends to pitch the rotor to a reduced angle, reducing lift. This holds for the reverse (i.e. negative lift countered by chassis pitch up).

Dissymmetry of lift is not generally considered to be offset by blade articulation (i.e. flap to equality). Flap to equality is a transient condition normally seen during changes in velocity, but helis dont cruise with high rotor cant angles relative to the chassis. The pilot compensates for it with cyclic.

Phase lag is not 90deg in our machines. That is the ongoing myth of rotors as gyroscopes, used for ages as a simple way to teach students why rigging angles are what they are. No one seems to question why the rigging angle is not 90deg in reality, or why it changes with blade Lock number or with hinge offset. The rotor flies to position, and swamps underlying gyro forces.

Dissymmetry of lift is not limited to left/right rotor hemispheres (i.e. advancing vs retreating blades). Another variant is fore/aft, which is a function of asymmetric inflow in horizontal flight. In conventional flight, the air pulled in axially through the top front of the rotor has not accelerated as much as the air that is pulled in at the top rear of the rotor. As a result, the AOA at the rear is reduced. While the advancing blade/retreating blade effect impacts pitch in FF, the inflow asymmetry impacts roll in FF (hence its common name, inflow roll). Pilots learn to deal with this without thinking- which is probably a reason it is non-intuitive.

On the shimmy question, that is normally attributed to blade vortex interaction. We are less likely to see this, because our blade vortex energy is very low due to our low rotor disc loading (which is generally about 10% of full scale).

Sorry to not break this out by thread, etc, but time is a bit short.

Regards,

[BustedRaptor77]

Originally Posted by extrapilot

Dissymmetry of lift is not generally considered to be offset by blade articulation (i.e. flap to equality). Flap to equality is a transient condition normally seen during changes in velocity, but helis dont cruise with high rotor cant angles relative to the chassis. The pilot compensates for it with cyclic.

This is a little confusing. Are the blades not undergoing transient velocity as they rotate 360 degrees around the mast? The speed of the blades are constant, if the velocity is not altered you would constantly have unequal amounts of lift. Is this what you are implying or am I reading it wrong?

If the effects of the blades bending, leading/lagging, and flapping are limited to periods of transient velocity relative to the entire aircraft you would only ever see a per rev vibration during transient periods (if the track was in at any one given airspeed). This is not what happens because the blades can fly out of position at different airspeeds.

[BustedRaptor77]

Originally Posted by extrapilot

On the shimmy question, that is normally attributed to blade vortex interaction. We are less likely to see this, because our blade vortex energy is very low due to our low rotor disc loading (which is generally about 10% of full scale).

Gottcha thanks.