Gazelle FM Minor Gripe

[Jester986]

I'm well aware of gyroscopic precession. What you're talking about though affects the tilt of the disk. I push cyclic forward in an American helicopter, pitch decreases at the 3oclock side of the rotor disk and increases on the 9oclock side of the rotor disk. The change in lift is instantaneous at the location of the change however it takes the 90 degrees for the upward displacement of the rotor blade to occur. This results in the blade dipping at the 12 oclock and rising at the six. This tilts the overall lift vector from the vertical to forward. However with RBS the retreating side has stopped making lift and dissymmetry of lift can not be compensated for. This causes the nose up you're talking about, but there is a violent roll to the left because there is no lift supporting the left side of the rotor disk. Again I'm talking about American style counter clockwise rotor systems, of course its the same but reversed for clockwise. I've had a 206 high hot and heavy in the mountains of Utah and got the onset of RBS. I know a pilot who should have died in a R22 because he had it at too high of an altitude and it flipped over on him. How he didn't get mast bumping I don't know... RBS gives you a lot of warning signs to avoid it with the nose up and roll that you have to chose to ignore and the buffeting so it rarely is a factor IRL. But high hot and heavy lower the speed it occurs at and makes its onset much more rapid.

 

 

 

 

 

 

The precession your talking about affects the direction of the lift vector, it does not affect where the lift occurs. If you have a high lift on the one side and none on the other it will tilt the lift vector back and pick up the nose, but the side without lift is dropping...

[Jester986]

In normal flight there is a stalled region off set from the center of the rotorhub to the retreating side. as forward speed increases RBS onset happens at the tip and moves inward. At that point it can be recovered or pushed through. When it is fully developed and the balance of lift can not be maintained by blade flapping you're going to have a violent roll.

Edited November 27, 2017 by Jester986

[Capn kamikaze]

NO, in RBS the rotor has not stopped producing lift, a portion of it has, meaning the centre of lift moves to the advancing side, and has the same effect as pulling back on the cyclic....

 

 

The side without lift acts like it does when you use cyclic, its force manifests 90 degrees around, at the tail.

 

Gyroscopic precession doesn't just work for applying a force, it works for removing one too...

[Capn kamikaze]

Let me put it another way, I fly R/C model helicopters, and I am quite good at "3D" flying, the way we set them up is that at centre stick on the collective you're at zero degrees pitch, you can flip them over on the spot because you can generate lift one one side of the disk and negative lift on the other, these manifest 90 degrees around, as a cyclic response, if it didn't work like that it would not be possible to do on the spot flips.

 

Removing a force is similar to adding a force in the opposite direction, if you add more lift on one side, and reducing it on the other you still end up with a 90 degree precessed response, the heli doesn't know if its from RBS or from a cyclic input.

 

When you have RBS and have an abrupt pitch up, that will then also cause a secondary roll, but that roll is NOT caused by the loss of lift as if you'd snapped a wing off a plane, but as a secondary precession effect that only manifests because the nose came up.

[Capn kamikaze]

In the case above with the CCW disk, you'd get a roll to the right, in the case of the Gazelle it should be to the left.

Edited November 27, 2017 by Cap'n kamikaze

[Jester986]

Sure man, I do this for a living but you know better. BTW I'm not arguing against the pitch up. Just that there is a significant roll. Go fly a helicopter up to VNE and let me know if you flip over before you die or not.

[Capn kamikaze]

... and I am arguing as to whether it is more significant than the pitch up, and as to the mechanism behind it, and I have studied aeronautical engineering.

 

Consider it from another angle, we can agree that the rotor disk is a gyroscope, like any other rotating mass, so any rotation it makes outside of its plane of rotation HAS to be caused by a force 90 degrees preceeding the observed motion, if the left hand side drops and the right hand side rises, then it is caused by a force, or pair of forces acting as a couple at the 12 and 6 positions.

 

Just because it has stalled already does not mean it breaks the laws of the conservation of angular momentum, and therefore must still be acting like a gyroscope, and it will do that until only one thing happens..... it stops rotating.

Edited November 27, 2017 by Cap'n kamikaze

[borchi_2b]

just to mention something important. Offsets in a fully articulated rotosystem like gazelle, bell 407, bo105 or any modern helicopter are not 90°. they are more likely less and something like 45° to 60°. It depends on the rotor design and the way it is articulated. Just wanted to mention that ;):pilotfly::joystick:

[Jester986]

Even in two bladed systems its not 90 degrees because gyroscopic precession is a simplification of what's actually going on which is phase lag. But for our intents and purposes it's close enough.

[Capn kamikaze]

just to mention something important. Offsets in a fully articulated rotosystem like gazelle, bell 407, bo105 or any modern helicopter are not 90°. they are more likely less and something like 45° to 60°. It depends on the rotor design and the way it is articulated. Just wanted to mention that ;):pilotfly::joystick:

 

That's swash linkage phasing offsets, or phase lag, which is a separate issue, but the gyroscopic precession of a rotating mass is always 90 degrees.

[borchi_2b]

hey, i just wanted to sound smart at least once :music_whistling::D:joystick::pilotfly::smartass:

 

Well my dad fly RC helos and I have flown them too in the past. Fact is, in basic they share the same physics, but in terms of rotor designs, and here it already starts with the airfoils and gos down to some differences in the rotohead itself, I would never compare a Rc helicopter with a big real pessenger helo, cause there are differences. for example the rotor dis rpm. just to mention it. The RC helo can produce way more lift in comparison and forces will act more violently in first place, so a real helicopter has a different momentum and a different inertia too that all makes it move different in details. Many details make huge ammount. So to speak abut it. I am happy to read that people conversate about helicopter flight in general.

[Capn kamikaze]

The fundamental physics are the same, and depending on the heli, scale, 3D, etc, you can have exactly the same type of rotorhead design, eg a rigid, or semi-rigid design, my first one a had a split spindle which was very similar to a fully articulated one.

 

You're right about the RPM, which mostly means lower coning angles, and more force pulling the blades out straight which reduces lead and lag angles to a much tighter range than full scale.

 

The only reason I mentioned RC was because of the fact you can set them up to have negative pitch, with zero in the middle, and that demonstrates that a force from the cyclic in a negative direction on one side of the disk will have the same effect as a positive force of the same magnitude on the other side of the disk, causing one side to precess up and the other down.

[Jester986]

Some crazy pilots who fly helicopters from ships in the South pacific have their mechanics rig them so full down collective is nrgative pitch so they can pin themselves to the deck in heavy seas... but what do I know ;)

[GunfighterSIX]

Ok, sorry if I came off as kind of a jerk in the last post but it's frustrating to see an excellent flight model picked apart by people without experience and having to explain things many times. With respect to Fragbum, I know you've done an R44 flight or two which gives you a much better understanding than most how helicopters handle, there are however a lot intricacies that will be missing.

 

 

104th initial complaint about holding left cyclic to hover has been addressed as translating tendency but there seems to be some confusion about what's causing it. Translating tendency results from the thrust of the tail rotor and it is in the direction of that thrust. I think everyone gets that part. But... CapnKamikaze talks about the position of the tailrotor causing a roll. This only affects helicopters with a tailrotor placed well above the CG. In the gazelle the fenestron is inline with the cg so you do get translating tendency but no roll from the tailrotor EXCEPT when in a nose low attitude. If you drop the nose you raise the tail bringing it above CG.

 

 

Capnkamikaze goes on to say that retreating blade stall will not induce a roll, and it absolutely does. I do not believe this is the roll you all are discussing though. RBS is compensated for by flapping. As the advancing blade flaps up it increases induced flow decreasing angle of attack and lift while the retreating blade flaps down decreasing induced flow increasing angle of attack and increasing lift. However the blades can only physically flap so far. When they reach this flapping limit retreating blade stall fully develops and the helicopter will pitch up and violently roll inverted.

 

 

The blow back and roll fragbum is talking about on initial acceleration is due to the rotor disk not evenly experiencing the effects of ETL over its entire surface. As I push the cyclic forward I trade some lift vector for speed and get a slight dip in altitude, requiring added collective, then as the tip of the rotor disk comes into ETL I get a right roll because only the front portion is experiencing increased lift that manifests 90 degrees later in the plane of rotation, requiring left cyclic, then as the whole rotor disk comes into etl this is when you get blowback, requiring forward cyclic. I can really go down the rabbit hole here but for every one thing you explain you have to go into two others....

 

 

I believe the roll you guys are talking about when adding collective is a result of the mixing unit. The mixing unit combines the actions of the cyclic and collective to send the movements to the swashplate. The MD500 and Bell 206 are the two helicopters I have the most experience flying. If you sit in one of these that is not running and pull up on the collective you will see the cyclic move forward and right. While flying we simply hold some pressure to stop that movement, but in a flightsim it can't move your joystick for you(some force feedback devices excluded) So you get a right roll that requires you to put in left cyclic.

 

 

I haven't tested the Gazelle since the most recent update, but its flight model really has been excellent. You will not get a desktop sim that will make it so you can get in a helicopter and fly, but this is the best approximation I have seen to include the sim at flight safety. If you have questions about helicopter aerodynamics, or concerns about the flight model please ask, but at some point we have to say polychop has done well and let them stop updating and move on to the next project. I have 2000 flight hours in helicopters.

 

 

Also disclaimer I'm not associated with polychop, I just think its an excellent product that includes 3 variants at one price with a rumored 4th in the works!

 

I am also an exprenced really world helicopter pilot, and like most of the helicopter pilots on this forum, I try not to mention in every other post like you do. I tested the Gazelle for Polychop early on and I think it is a beautifully model. But under no circumstance do I think the flight model is excellent. To much obvious and basic stuff is wrong with it. I applauded Polychops work, I think the model and systems are excellent but anyone with a little bit of helicopter time will tell you the flight model needs a lot of work.

[Jester986]

A very fair criticism gunfighter. I embarrass myself with it sometimes but I do think it lends me some credibility.

[Chic]

Even in two bladed systems its not 90 degrees because gyroscopic precession is a simplification of what's actually going on which is phase lag. But for our intents and purposes it's close enough.

 

 

 

Thank you Jester for getting this out there. It had to be said.

[FragBum]

hey, i just wanted to sound smart at least once :music_whistling::D:joystick::pilotfly::smartass:

 

Well my dad fly RC helos and I have flown them too in the past.

 

...

 

I am happy to read that people conversate about helicopter flight in general.

 

Yes at least I get to learn stuff. :thumbup:

 

What would be great is for the discussion to continue forward in a polite and constructive way, perhaps then we all could learn stuff.

 

I still think the Gazelle is fun to fly. :D

[FragBum]

 

...,

 

there are however a lot intricacies that will be missing.

 

 

I totally get that, hence why I am here, someone has to ask the dumb questions,.. Right? :D

[Holbeach]

Thanks for sharing.

 

TL; DR: Although you have the Gyro, SAS and auto-pilot turned off, it still looks like there is a SAS or something still working 'under the hood' as I don't think reducing the collective should induce a roll. It should rather require countering with pedal (yaw) which in turn has an effect on pitch (and much smaller (negligible ?) effect on roll). For now, I'd consider the flight model as WIP but improving.

 

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When you turn the SAS off, it still has an influence, but at a reduced rate. (it shouldn't but it does). When you turn the gyro off, all influence is removed. (It shouldn't work like that, but it does) and the helicopter is now in its natural state, with only pilot influence.

 

With regard to the autorotation, the peddles are disabled and can't be used and I deliberately killed the rotor energy on the descent, so it's not a standard autorotation. I had to keep the speed up for a skid landing to prevent a yaw on the flare. Not very pretty, but we all walked away.

(That part has nothing to do with the thread, but is only part of a larger video I'm doing).

 

..

[Capn kamikaze]

Jester, the reason why the roll is not directly caused by the retreating blade stall is the coning angle and the oncoming flow....

[Capn kamikaze]

Chic, that is a swash linkage geometry control issue, which is not relevant to what we're talking about, once you get to the upper part of the swash those linkages, or the hub or rotors don't "know" what has gone on below, it transfers the control inputs but still allows the rotation of the disk, and that leaves the problem as being simply an issue of angular momentum of the disk, in other words all that matters when talking about reactions of the disk to aerodynamic forces on it is that the response will be 90 degrees offset from where the force is applied to it.

 

Don't confuse control inputs from below the swash with aerodynamic inputs from the air that it's flying through, they have different effects, that's one of the first things you learn on an aeronautical engineering course.

[Chic]

Chic, that is a swash linkage geometry control issue, which is not relevant to what we're talking about, once you get to the upper part of the swash those linkages, or the hub or rotors don't "know" what has gone on below, it transfers the control inputs but still allows the rotation of the disk, and that leaves the problem as being simply an issue of angular momentum of the disk, in other words all that matters when talking about reactions of the disk to aerodynamic forces on it is that the response will be 90 degrees offset from where the force is applied to it.

 

Don't confuse control inputs from below the swash with aerodynamic inputs from the air that it's flying through, they have different effects, that's one of the first things you learn on an aeronautical engineering course.

 

 

 

Precisely!

I'm not confused.

Just took issue with your post #57; " we can agree that the rotor disk is a gyroscope". No, we cannot.

Edited November 28, 2017 by Chic

[Capn kamikaze]

What?

 

Sorry but the fact is it is a gyroscope, any spinning mass is, it's why precession applies to the problem.

[Capn kamikaze]

http://navybmr.com/study%20material/14008a/14008A_ch7.pdf

 

http://www.rotorandwing.com/2017/08/04/ray-prouty-rotor-gyro/

 

http://www.danubewings.com/gyroscopic-precession/

 

How you can really say it does not act like a gyroscope is frankly astounding.

[Chic]

OK