Gazelle FM Minor Gripe

[Capn kamikaze]

I can only tell you what I am seeing, as I said it is a light response, so your control set up may be hiding it a bit, but it definitely wants to pitch the nose back up, and you end up slowing down.

[Holbeach]

I can only tell you what I am seeing' date=' as I said it is a light response, so your control set up may be hiding it a bit, but it definitely wants to pitch the nose back up, and you end up slowing down.[/quote']

v 1.5.7

With the gyro/ap/sas off, the nose will not pitch up, unless you pull the cyclic back

 

Carry on straight and level with the cyclic and rudder centred and without trimming at around 80 knots and drop the collective without touching the cyclic/rudder.

The Gazelle will roll to the right until it is upside down and will remain in that state until you roll or loop upright.

Perform an autorotation landing because the engine will have stopped..

 

..

[Capn kamikaze]

When I have time, I'll do a video, what I am seeing is a low rate recovery from a nose down forward flight condition, the helicopter does not maintain its orientation relative to the flightpath unless you hold a little bit of forward stick in, once you release that, the nose comes up to level and your forward speed drops off, and if you leave the collective alone, you start to gain altitude.

[Capn kamikaze]

How fast are you going when you're trying this.

 

It's quite noticeable at about 100 knots, straight and level, let go of the cyclic, and the nose will definitely rise scrubbing your speed off.

[Holbeach]

When I have time' date=' I'll do a video, what I am seeing is a low rate recovery from a nose down forward flight condition, the helicopter does not maintain its orientation relative to the flightpath unless you hold a little bit of forward stick in, once you release that, the nose comes up to level and your forward speed drops off, and if you leave the collective alone, you start to gain altitude.[/quote']

 

Make sure you have the gyro off.

 

..

[Capn kamikaze]

Even with that on or off I am still seeing the nose rise if you release the cyclic.

 

The effect is greater the faster you're going.

[FragBum]

Even with that on or off I am still seeing the nose rise if you release the cyclic.

 

The effect is greater the faster you're going.

 

 

Okay what do you mean release the cyclic? I have a home brew cyclic if I let go of it it pretty much stays where it's left there is no centre detent or return spring. I get the nose up effect doing transition into forward flight in the Huey.

 

All for criticism I may well have it wrong here is a short video

 

pickup at about 10secs

Forward flight about 18secs

I see lift to about 30sec but not any nose up force and of course if I don't pull back on cyclic I start dropping altitude. To me and I am not a RL pilot that phase seems missing to me.

[Jester986]

People with no experience in helicopters conflating things they read online... I'll do a right up tomorrow.

[FragBum]

People with no experience in helicopters conflating things they read online... I'll do a right up tomorrow.

 

Looking forward to that.

[SnowTiger]

I am noticing this rolling effect (and slowing down dramatically but not sure about the nose up attitude) even when I'm in Altitude Hold. Speeding up and/or slowing down while in Altitude Hold (AH) and the rolling action really make it difficult to see the advantage of AH.

 

On the other had, my Fine Trim no longer seems to have ANY effect (unlike prior to the update patch). In DCS Options/Controls I can see my Hotas Trim Hat is recognized and works (as it has since the Gazelle was released). While in the cockpit with the Controls Manager (CM) visible, I can clearly see that the Trim Inputs are being registered on the CM. But there appears to be absolutely No Trimming Effects taking place.

Previously, I could and did fly primarily using this Trim for speeding up and slowing down.

Now it seems completely Useless and just difficult to fly. And the Altitude Hold just acts wonky now.

[Holbeach]

DCS, Gazelle, using collective to roll inverted.

 

Polychop have recently added this effect to the FM, whereby a rolling motion is introduced when the collective is used. This is what happens with the SAS and gyro turned off. No other controls are used to make the roll or remain inverted. An autorotate is performed with low rotor energy, without using the rudder and minimal use of the collective. Version 1.5.7.11762. (Note. The second part of this vid is only me practicing autorotation)

 

 

 

..

Edited November 28, 2017 by Holbeach
added, "and gyro"

[Jester986]

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!

Edited November 27, 2017 by Jester986

[SnowTiger]

I think most of my issue is a result of something completely different (re: Trim).

So I think I'll repost in Trim-Related Thread (or start a new one).

Thanks y'all.

[Chic]

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.

 

 

The Roll during this phase is due primarily to Transverse Flow Effect. The Blowback is the result of DOL. These effects are coincident with ETL.

Edited November 27, 2017 by Chic

[Jester986]

Agreed Chic, that's what I was talking about, I just neglected to use that term.

[Chic]

I thought so.

[borchi_2b]

@Jester: Thanks for sharing your thoughts. It was interesting to read your informations. At one point I got confused about the direction of the effects you described, cause it looks like you describe it for a counterclockwise rotatation. Am I right ?

[Capn kamikaze]

CapnKamikaze talks about the position of the tailrotor causing a roll. This only affects helicopters with a tailrotor placed well above the CG.

 

No, I said it causes a trim change in the roll axis due to a linear thrust component, not a roll, they are two different things, a roll is a continuous rotation about the roll axis, if the tail rotor is high it actually reduces the hovering angle, because it introduces a moment about the vertical axis opposing the linear one.

 

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.

 

Any roll there is a secondary gyroscopic reaction to the sudden nose up, the RBS cannot directly cause a roll, it will cause a nose up pitching moment, and that can cause a rolling moment..... see below.

 

Edited November 27, 2017 by Cap'n kamikaze

[Capn kamikaze]

http://www.pprune.org/rotorheads/296643-tail-rotor-position.html

[Jester986]

@Borochi, your right. I mixed it up when I was talking about transverse flow effect. I described it for a counter clockwise helicopter.

 

 

@Capn I apologize but I missed that in your statement that you were talking about trim. You are incorrect though in saying RBS doesn't cause a roll. Blowback does cause an pitch up because the rotor disc is still making lift on the advancing side and the nose, however the lack of lift on the right side of the helicopter causes it to act like a plane with only a left wing.

 

 

Page 11 https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch11.pdf

Edited November 27, 2017 by Jester986

[Ramsay]

Polychop have recently added this effect to the FM, whereby a rolling motion is introduced when the collective is used. This is what happens with the SAS turned off. No other controls are used to make the roll or remain inverted...

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.

 

Detail:

 

Ref: Flight Test of the Aerospatiale SA-342 Helicopter, Aug 1975, ADA016921

 

As the page is of poor quality, I've tried to clean it using the info from the text and other diagrams.

 

 

For the auto-rotation, ADA016921 says:

Normal type autorotative landings into a 5- to 8-knot headwind required pedal inputs which increased from 32 to 50 percent of total control displacement during the flare, and then

decreased from 50 to 10 percent of total control displacement as collective pitch was applied to arrest the sink rate and to complete the landings.

 

which again suggests the SAS or something is still active?

 

It'll be interesting to see how the DCS SA342 compares to ADA016921, once the FM is again signed off by the IRL gazelle pilots. For now, I'll continue to enjoy what we have (bugs and all).

Edited November 28, 2017 by Ramsay
Add trim change due to airspeed for completeness

[Capn kamikaze]

Jester, that roll is a secondary indirect effect, because the nose of the helicopter is being forced up, due to the extra lift on one side of the disk becoming dominant, that translates to a secondary effect, via gyroscopic precession.

 

The first effect, the nose up force, is caused by the asymmetry of lift, 90 degrees around, the rotation of the blades doesn't matter, either CW or CCW, it always appears as a nose up force, when that abruptly manifests, that motion also causes a further 90 degree shift, and a much smaller roll motion is observed.

 

Hypothetically if the nose never came up, the much less severe roll force would never manifest.

Edited November 27, 2017 by Cap'n kamikaze

[Jester986]

You have it backwards. The increase in lift on the advancing side precesess to the nose causing the pitch up. The lack of lift on the retreating side causes it to drop off. If I have no lift on one side of the aircraft that side is not flying.

[Capn kamikaze]

It doesn't work that way, the lack of lift on the retreating side causes the lift to drop off at the rear of the disk.

 

Think of it like how the cyclic control works, the control inputs are offset by 90 degrees, the front of the swash plate rising, the rear falling, consider a CW rotating two rotor disk, at zero collective pitch, you pull back on the stick, the front of the swash rises, the rear falls, the linkages translate that through 90 degrees, the rotor at the 9 oclock position pitchs up, and the one at the 3 oclock pitches down, now think about the positions 90 degrees around from that, the blade that was at 9 will rise to a high point at 12, and the one at 3 will drop to a low point at 6.

[Capn kamikaze]

Also the stalled region is towards the tip of the blades, it does not include the entire side of the disk.

 

What is happening is that the centre of lift is moving towards the advancing blades side of the disk, and it doesn't have to move far to cause a large nose up moment.