Two questions I've been thinking about.

[Canada]

Here are two questions I've been throwing around in my brain over the months of regular visits to this forum.

 

1. Since there is no anti-torque rotor (tail rotor) to over- or under-compensate the main rotor torque (should there be just one), what mechanical effect does the anti-torque pedals have on a coaxial rotor machine? I've done some searches on this but most books and web sites only explain up to the MD 500 NOTAR system. Anything "fancier" is ignored.

 

And while in the same subject, how does a tandem rotor helicopter varies torque ?

 

2. Since we will operate the aircraft with USB controllers, we will have infinite amounts of force to apply on the (virtual) cyclic stick. So how can you simulate a complete hydraulic system failure ? What I mean is that flying an actual helicopter without "power steering" can be hard on your arms.

 

Thanks.

[AlphaOneSix]

1. The pedals actually modify the amount of torque produced by each of the main rotor blades. For example, if you push on the right pedal, the controls are moved in such a way that your counter-clockwise rotating blades slightly increase their torque, and your clockwise rotating blades slightly decrease their torque. There is no net change in total lift, but your torque effect has changed, resulting and the aircraft's nose turning to the right.

 

2. In an aircraft like the Ka-50, it will be impossible to control the aircraft without hydraulics. I can move the controls on an Mi-17 without hydraulics, but it takes just about all the strength I can muster. In flight, a complete hydraulic failure means that the aircraft is uncontrollable, period. In other words, a Ka-50 without "power steering" is going to crash. Only very light helicopters are flyable with a total loss of hydraulic power (some don't have hydraulics at all).

[britgliderpilot]

1. The pedals actually modify the amount of torque produced by each of the main rotor blades. For example, if you push on the right pedal, the controls are moved in such a way that your counter-clockwise rotating blades slightly increase their torque, and your clockwise rotating blades slightly decrease their torque. There is no net change in total lift, but your torque effect has changed, resulting and the aircraft's nose turning to the right.

 

Yup. "Differential collective" is the key term here.

 

You increase the pitch of one rotor disc while decreasing pitch on the other. Total upward thrust remains the same, but the torque imbalance between the counter-rotating discs causes a yawing moment. Magic.

 

2. In an aircraft like the Ka-50, it will be impossible to control the aircraft without hydraulics. I can move the controls on an Mi-17 without hydraulics, but it takes just about all the strength I can muster. In flight, a complete hydraulic failure means that the aircraft is uncontrollable, period. In other words, a Ka-50 without "power steering" is going to crash. Only very light helicopters are flyable with a total loss of hydraulic power (some don't have hydraulics at all).

 

I'm not sure that's strictly the case. IIRC, the controls aren't hydraulically boosted, the control linkage itself is hydraulic - so a failure removes all control.. Will check and get back to you.

[arneh]

You increase the pitch of one rotor disc while decreasing pitch on the other. Total upward thrust remains the same, but the torque imbalance between the counter-rotating discs causes a yawing moment.

 

A quick follow-up. What happens if you already have the collective at max or min, so that one of the rotors cannot change collective anymore? Does it just change it for the one rotor and let you lose or gain some total lift?

[RvETito]

A quick follow-up. What happens if you already have the collective at max or min, so that one of the rotors cannot change collective anymore? Does it just change it for the one rotor and let you lose or gain some total lift?

 

The controls are independant, having the collective at max or min doesn't affect the differential pitch control. Although collective and differential pitch use one and the same device (Collective and Differential Pitch Mechanism- CDPM) they don't interefere each other. The two rods that control the colelctive of each rotor via the upper and lower sliders are twisted with left and right threading respectively in the sleeve- with the collective you move the sleeve up-down and the two rods move together in the same direction, with the pedal you rotate the sleeve making one of the rods to 'twist' and other to 'untwist'- they move in opposite directions while the sleve itself is stationary. If the sleeve is in the max up or down position that doesn't disturb it to be rotated. The CDPM is installed on the bottom of the gearbox with the two rods coaxial inside the inner (upper rotor) shaft.

 

This is the rotor mast assmebly of a Ka-32 helicopter, it's a bit different from the Ka-50's but the CDPM is built at the same principle.

 

 

The CDPM- general view and schematic:

 

[arneh]

The controls are independant, having the collective at max or min doesn't affect the differential pitch control.

 

Ah, thanks, I think I got it. But to illustrate with an example, say the collective is at max which gives the blade an AOA of 20° (the number is just an example), and the pedals can then increase the AOA even beyond that (for one rotor), to e.g. 25°? And the other rotor then at 15°.

 

I don't know if this is the case for the Ka-50, but some helicopters can have negative AOA on the blades at lowest collective level. I.e. the blades actually produce a downforce instead of lift.

 

But I can imagine that would cause problems with differential collective. Negative lift should produce more torque than no lift (and in the same direction as torque from positive lift, since it still rotates in the same direction). So if the collective is in such a position that the blades produce no lift, then applying differential collective so that one produces negative and the other equal positive lift, then the torque from both would also cancel each other out, and we don't get the twist we wanted by pressing the pedals... If my musings are correct :)

 

But guess the Ka-50s rotors cannot do negative AOA? I bet it would cause all sorts of aerodynamic problems as well, with the downwash/upwash between the rotors interfering if one was creating positive and the other negative lift. Sounds like a bad idea :)

[RvETito]

No, negative blade AOA is a no-go :) Ka-50 is fast and highly maneuvarable helicopter (3-3.5 G). To achieve this it needs high rotor loading therefore the blades have shorter chord than the Ka-32 for example.

[VS461]

IIRC, the controls aren't hydraulically boosted, the control linkage itself is hydraulic - so a failure removes all control.. Will check and get back to you.

 

If the hydraulic pressure goes away, the boosters will become a direct link between the stick and the rotor blade rods, so the stick will start to circle bw your knees and crash 'em away: you cannot go out from the helo (on Mi-24 you ain't have ejection seats). On the other hand, the helo will make a very rapid kind of Pugachev cobra, then start breakdancing around its 6DOF, so you have no time to get out.

Bye!

[britgliderpilot]

If the hydraulic pressure goes away, the boosters will become a direct link between the stick and the rotor blade rods, so the stick will start to circle bw your knees and crash 'em away: you cannot go out from the helo (on Mi-24 you ain't have ejection seats). On the other hand, the helo will make a very rapid kind of Pugachev cobra, then start breakdancing around its 6DOF, so you have no time to get out.

Bye!

 

I rechecked it, and it appears the original suggestion was right - the controls are hydraulically boosted rather than a hydraulic linkage.

 

 

Your description of the resulting crash does sound very familiar to my experiences of battle damage in the BS betas, though :)

[Canada]

I have another question: in DCS, does the air displaced by an aircraft affects the the behavior of an other (nearby) aircraft ? For example, can two helicopters hover closely one on top of another as if they were alone ?

[EvilBivol-1]

I have another question: in DCS, does the air displaced by an aircraft affects the the behavior of an other (nearby) aircraft ? For example, can two helicopters hover closely one on top of another as if they were alone ?

Not in Black Shark. A more dynamic atmosphere model is of interest to ED though, so perhaps it's something we might see in future DCS modules.

[-sulan-]

Not in Black Shark. A more dynamic atmosphere model is of interest to ED though, so perhaps it's something we might see in future DCS modules.

 

Sweet! I hope so :)

Very interesting thread btw me thinks!

[A.S]

Nice Post Tito :thumbup:

[Canada]

Not in Black Shark. A more dynamic atmosphere model is of interest to ED though, so perhaps it's something we might see in future DCS modules.

 

Anybody got a time machine ?

[VMFA117_Poko]

Anybody got a time machine ?

Yupp ;) Beta testers have it. :D