Bug - pedal turns do not effect cyclic stability with SAS off.

[Frusheen]

BUG TEMPLATE:

 

Description: From a stable hover performing pedal turns in either direction have no impact on the airframes attitude.

DCS Version: 2.0

Steam: No

Map: NTTR

SP/MP: SP

Reproducible: Yes

Step to Reproduce: Turn off SAS. Perform a pedal turn from an established hover. The helicopter attitude remains completely neutral with no requirement for any cyclic correction. No apparent interaction between changing heading by pedal input and aircraft attitude.

Screenshot/Video available: can be provided on request

Track Available: No

Mission File: No

Controllers: In signature

OS: Windows 10

CPU etc: in signature

Mods: No

Any Additional Information:

[Goggles]

BUG TEMPLATE:

 

Description: From a stable hover performing pedal turns in either direction have no impact on the airframes attitude.

DCS Version: 2.0

Steam: No

Map: NTTR

SP/MP: SP

Reproducible: Yes

Step to Reproduce: Turn off SAS. Perform a pedal turn from an established hover. The helicopter attitude remains completely neutral with no requirement for any cyclic correction. No apparent interaction between changing heading by pedal input and aircraft attitude.

Screenshot/Video available: can be provided on request

Track Available: No

Mission File: No

Controllers: In signature

OS: Windows 10

CPU etc: in signature

Mods: No

Any Additional Information:

 

Please elaborate what should happen with the cyclic when, in a stable hover without wind, a pedal turn is done, and why?

 

What about when turning into or out of a crosswind?

[Frusheen]

Please elaborate what should happen with the cyclic when, in a stable hover without wind, a pedal turn is done, and why?

 

What about when turning into or out of a crosswind?

 

The moment produced by the tail rotor to overcome torque causes a sideways force on the airframe which will make the helicopter want to drift sideways. The varying of thrust at the tail rotor to pivot the helicopter about the mast will vary this sideways force meaning greater or less cyclic deflection is needed to counteract it. The rigging of the main rotor can be set to counteract this but only for a given setting so some cyclic adjustment is needed to maintain position over the ground when varying tail rotor pitch. The effect is manifest on helicopters which have the tail rotor below the main rotor by the helicopter hovering with one skid lower than the other. Sudden stops in rotation about the main mast should see a cyclic deflection required to arrest the momentum of the airframe from following through.

 

In wind cyclic pressure into wind should increase when turning crosswind.

[Goggles]

I don't think that the attitude itself would change with application of pedal. What would happen with a change in pedal is a change in the sideways force vector on the helicopter, and that would cause the helicopter to start drifting sideways.

 

Of course, to stop the drift, it would be necessary to tilt the rotor disk in the opposite direction to compensate, if one would want to maintain position in hover and not start drifting, and that would require a small amout of sideways cyclic.

 

So the solution might be to apply an algorithm that applies a lateral force vector on the rotor system, as a function of the amount of fenestron thrust, as determined by pedal position.

 

It is then up to the pilot to apply enough cyclic in order to counteract this drift.

 

Then there is the case that the sideways force vector of the fenestron is lower in height than the counteracting vector of the rotor. But I would not expect a change in attitude (roll) unless the application of pedal is sudden and violent. This would be different to a sudden sideways wind gust that pushes against the airframe and makes it swing like a pendulum.

[0xDEADBEEF]

I don't think that the attitude itself would change with application of pedal.

 

Pedal input in any aircraft also induces a roll component, this is not exclusive to helicopters. But if you wanna try: go to your local airfield and try to get a dual-control ride in any helicopter you can get/afford. You will find just that, roll component by adding pedal input. Of course, the more input the more roll. if you barely apply pressure to the pedal you wont even notice your hand compensating for cyclic. If you go full pedal, you will notice a distinct compensation on cyclic very much necessary. Depending on your experience flying choppers your flight instructor may take over at this point ;)

 

On a Sidenote: the Bo105 induces as a right-roll component if you add collective ;)

[Goggles]

Pedal input in any aircraft also induces a roll component, this is not exclusive to helicopters.

 

Applying rudder only on an airplane in forward flight induces a skid, resulting in the upper wing having a greater angle of attack than the inside wing, resulting in a roll. The same would happen with a helicopter in cruise flight.

 

But we're talking about a helicopter in stationary flight, not forward flight. The comparison cannot apply because airplanes are not capable of stationary flight (unless it's its last flight).

 

If you go full pedal, you will notice a distinct compensation on cyclic very much necessary

 

 

My quote: "But I would not expect a change in attitude (roll) unless the application of pedal is sudden and violent."

 

On a Sidenote: the Bo105 induces as a right-roll component if you add collective

 

Is this in forward flight or stationary flight? What is the reason for this? Any relation to it having a fully rigid rotor system?

 

I'm not denying that cross-coupling exists. Only that the effect of applying moderate pedal is very minimal (not worth the effort of simulating) on roll when in a hover.

 

That being said, applying pedal in a hover changes the sideways force vector on the helicopter, resulting in sideways drift. Correcting that drift requires that the rotor disk be tilted by application of lateral cyclic, resulting is a slight roll. But the application of pedal in this case does not cause roll per se: only the necessary pilot reaction in order to prevent sideways drift.

 

Depending on your experience flying choppers your flight instructor may take over at this point

 

Any instructor would call the session quits over something like that.

 

The dynamics of helicopter flight are complex, with various forces interacting. These vectors have to be broken down into their various components in order to help the programmers with a high fidelity simulation. That's my goal here.

Edited November 7, 2016 by Goggles

[Frusheen]

No offence Goggles but I'm just reporting a bug in the flight model. I'll leave the programming to the developers and your further questions about helicopter flight dynamics to google.

[Goggles]

No offence Goggles but I'm just reporting a bug in the flight model. I'll leave the programming to the developers and your further questions about helicopter flight dynamics to google.

 

Fair enough, but is what you're reporting as a bug, a bug?

 

I'm saying that your's is a valid observation, but for different causes.

 

The flight model lacks many attributes. How to convince the developers to implement changes without understanding why or where you're coming from?

[Ramsay]

Fair enough, but is what you're reporting as a bug, a bug?

Frusheen's report is a valid report of incorrect flight behaviour.

 

I don't think that the attitude itself would change with application of pedal. What would happen with a change in pedal is a change in the sideways force vector on the helicopter, and that would cause the helicopter to start drifting sideways.

This would only be true for a helicopter where the tail rotor was aligned with the main rotor, perfectly, as in this diagram.

 

The Gazelle fenestron centre line is below the main rotor head, so causes a rolling moment, that has to be compensated for with the cyclic. As the anti-torque thrust is below the mast head and to the right, it causes a roll to the right and the right skid touches down first on the real helicopter.

 

On a Sidenote: the Bo105 induces as a right-roll component if you add collective

 

Is this in forward flight or stationary flight? What is the reason for this? Any relation to it having a fully rigid rotor system?

Not sure, I expected to find a offset tail rotor like the Gazelle but the Bo-105 tail rotor looks aligned with the mast head and both skids touch down together on landing, perhaps it is due to the tail rotor riding higher/lower than the mast head in certain flight regimes/speeds or twist in the flexible main rotor blades.

A Bo-105 rolling moment is present in extreme yaw manoeuvres (although in this case it's roll to the left).

 

Note: The behaviour is dynamic and depends on how each helicopter is arranged i.e. the UH-1H in forward flight is pitched down, raising the tail rotor so it aligns with the mast head, however when landing/ in a hover the tail rotor is below the mast head, inducing a roll moment and the left skid touches down first.

 

For the Gazelle - in forward flight/cruise it pitches down, raising the tail so the top of the fenestron duct is level with the mast head (anti-torque forces are also generated from the tail fin above the duct, so the centre of anti-torque moment is probably close to that point), however when landing or in a hover, the mast head is level with the top of the tail fin (which produces no/little anti-torque in those regimes) and because the fenestron is below the mast head it induces a rolling moment and the right skid touches down first.

Edited November 8, 2016 by Ramsay
Add a note about dynamic Huey and Gazelle regimes.

[Ramsay]

For info.

Pitch and Roll Due to Yaw

 

30. A brief investigation of the pitch and roll due to yaw characteristics was made at 60 KIAS by applying right and left directional control inputs while holding the cyclic stick fixed. The inputs varied from 7 to 18 percent of total control displacement, which was sufficient to yaw the aircraft beyond the sideslip deadband region discussed in paragraph 23. The SA-342 exhibited positive apparent dihedral effect in both directions, being significantly stronger to the left. The aircraft, in response to the pedal step inputs, began an immediate roll in the direction of the input, which assumed a steady rate within about 2 seconds. Extrapolated data for 1-inch inputs (approximately 25 percent of total control displacement) indicated roll rates of about 10° per second to the right and 15° per second to the left. The aircraft also exhibited a pitch due to yaw that was almost of the same magnitude as the roll due to yaw. A pitch-up was associated with right pedal step inputs, and a pitch-down was associated with left step inputs.

 

From page 21 (24 of the pdf) U.S. Army evaluation of the Aerospatiale SA-342 Helicopter, 1975, ADA016921.pdf

https://ntrl.ntis.gov/NTRL/dashboard/searchResults/titleDetail/ADA016921.xhtml

Edited November 8, 2016 by Ramsay

[Fredo_69]

For info.

 

 

From page 21 (24 of the pdf) U.S. Army evaluation of the Aerospatiale SA-342 Helicopter, 1975, ADA016921.pdf

https://ntrl.ntis.gov/NTRL/dashboard/searchResults/titleDetail/ADA016921.xhtml

Interesting reading that report, hopefully it can be of great help for Polychop when they work on the FM?!

[0xDEADBEEF]

For info.

 

 

From page 21 (24 of the pdf) U.S. Army evaluation of the Aerospatiale SA-342 Helicopter, 1975, ADA016921.pdf

https://ntrl.ntis.gov/NTRL/dashboard/searchResults/titleDetail/ADA016921.xhtml

 

There you go, very nice info! thanks! Would not even have expected it to be that much roll...

 

A Bo-105 rolling moment is present in extreme yaw manoeuvres (although in this case it's roll to the left).

 

I know that pilot, and honestly would suspect that heavy left-bank nose-up attitude during the spinclimb is intentional to make the maneuver more "fun" for his passenger. While pressing right pedal all the way would require left cyclic to compensate for it, if you compensate more than needed, this is what it would look like (center of rotation away from the rotorhead, he does not spin in place and is essentially flying backwards).

[0xDEADBEEF]

My quote: "But I would not expect a change in attitude (roll) unless the application of pedal is sudden and violent."

 

Yes, I disagree.

 

Is this in forward flight or stationary flight? What is the reason for this? Any relation to it having a fully rigid rotor system?

 

I do not know. I know this specifically because there was a crash where the pilot put the Helicopter into a 90°-bank right turn at low altitude, which he could not recover from due to this phenomenon. If you do a 90°-bank left hand turn you have enough right-stick-travel to roll out of the turn, if you do it right, you already need some left-stick-travel to compensate for the added collective that is needed to even perform such a turn and thus you end up having insufficient right-stick-travel to roll back out. Once you find yourself in that situation there is no way out. This was explained to me like this by a pilot who was part of the crash investigation team.

 

 

I'm not denying that cross-coupling exists. Only that the effect of applying moderate pedal is very minimal (not worth the effort of simulating) on roll when in a hover.

 

Well, it sounded like you do. I still absolutely disagree on "not worth simulating".

 

 

That being said, applying pedal in a hover changes the sideways force vector on the helicopter, resulting in sideways drift. Correcting that drift requires that the rotor disk be tilted by application of lateral cyclic, resulting is a slight roll. But the application of pedal in this case does not cause roll per se: only the necessary pilot reaction in order to prevent sideways drift.

 

So, what about the Tailrotor not being aligned in the same plane as the rotor disk? Especially on the gazelle it is quite a bit below that, and that alone would cause a roll component even with simple vector math.

 

Any instructor would call the session quits over something like that.

 

If you wanna try mastbumping a robinson then yes, he should not even have taken off with you. But an instructor quitting a session after the student looses control should not be an instructor in the first place imho ;)