Helicopters (coax/tandem) have no yaw control?

[UncleManuel]

Hi there!

 

I'm fiddeling around with the BGE and play with some helicopter designs. After some airplane designs with propellers I do understand the basic mechanics and aerodynamic forces of different deploy angles. So far I've not used the KAL-1000, I use the main throttle for the deploy angle. Even with the helicopters because the blades have full axis control and can switch between collective & cyclic mode.

But...

...the yaw control. It's basically non-existent with coaxial or tandem helicopter designs. All axis controls are activated with the helicopter blades and pitch/roll are working fine but yaw does nothing (reaction wheel is turned off). Do I miss a thing or is it simply not implemented in the KSP BGE game mechanics yet? 

[AHHans]

Well, aero control-surfaces in KSP work in the way that they can change their deflection, thus angle of attack, and (hopefully) the lift they generate. If you have two rotor discs rotating in the same plane, how can you generate a yaw torque by changing the lift of the rotor blades? (Short answer: you can't.)

What you can do is change the RPM of the two rotors so that one generates more torque than the other. The resulting uncompensated torque will rotate the craft in the yaw direction. I did this for the roll control (which is yaw in craft that have the control direction forward and not up) on my Basic Quadcopter.

One problem of that solution is that SAS (or the manual trim) will not use self-defined axis groups for control, see also this forum post.

[IkranMakto]

[Shadowmage]

19 hours ago, AHHans said:

Well, aero control-surfaces in KSP work in the way that they can change their deflection, thus angle of attack, and (hopefully) the lift they generate. If you have two rotor discs rotating in the same plane, how can you generate a yaw torque by changing the lift of the rotor blades? (Short answer: you can't.)

Yes, you can.  The coax (RC helo) that I've dealt with induce yaw by speeding up one set of blades while slowing the other (this could also be done with pitch/collective adjustments).  Basically, use your rotor disc to induce torque around the rotor axis, by unbalancing the torque between the two sets of rotors.  Quad-copters do something even cooler, speeding/slowing alternating sets of blades to induce yaw.

Now, tandems are a bit more difficult, but you can also do yaw on them (around one of three vertical axes) by adjusting the cyclic on their front and rear sets of blades in opposite directions to spin about the shared axis (or adjust only one, to spin about the axis of the opposite blade).  There was a Chinook operators manual link floating around on a similar forum post that explained how this is done on the big birds...(sadly, can't find the link now...)

Short answer: you can, it just isn't calculated (properly) by KSPs control modules.

[UncleManuel]

On 2/26/2020 at 5:07 PM, Shadowmage said:

There was a Chinook operators manual link floating around on a similar forum post that explained how this is done on the big birds...(sadly, can't find the link now...)

Here, on the bottom: 

But the CH-47 is tilting the whole head assembly to create horizontal forces. I guess in KSP you would have to use "roll" one rotor to one side and the other to the opposite side to create yaw torque.

With the coax design you would have to mix yaw inputs into the deploy angle of the rotors like with the real helicopters like the Kamov KA-50. They do "opposite AoA movement" for yaw inputs which means: one rotor has higher lift and the other less lift. Using engine rpm takes too long to react.

I guess with both designs it'll be a nightmare to program a KAL-1000 to actually work as intended... 

[AHHans]

12 hours ago, UncleManuel said:

But the CH-47 is tilting the whole head assembly to create horizontal forces.

Do you have proof that the CH-47 Chinook is indeed tilting the whole head assembly and not just using cyclic control to reach the same effect?

In KSP you would have to tilt the whole assembly because of the way cyclic control is implemented.

[Curveball Anders]

3 hours ago, AHHans said:

Do you have proof that the CH-47 Chinook is indeed tilting the whole head assembly and not just using cyclic control to reach the same effect?

In KSP you would have to tilt the whole assembly because of the way cyclic control is implemented.

From the linked "CH-47D FLIGHT CONTROLS STUDENT HANDOUT"

Yaw (directional pedals).

• Directional control is accomplished by differential lateral tilting of both rotor discs, which causes the aircraft to turn about its vertical axis.
• Left pedal input causes a left tilt to the forward rotor disc and a right tilt to the aft rotor disc.
• Right pedal input causes the forward rotor disc to tilt right and the aft rotor disc to tilt left.
• The coordinated use of lateral cyclic and directional pedals is necessary to maintain or change course heading in flight with Advanced Flight Control System (AFCS) off

To my fixed wing mind this sounds like that the CH-47D is "tilting" the rotor discs.
If that's done by actually tilting the disc or achieving the same result using cyclic is beyond my limited fixed wing mind

 

[AHHans]

1 hour ago, Curveball Anders said:

If that's done by actually tilting the disc or achieving the same result using cyclic is beyond my limited fixed wing mind

But that's my actual question! And a document that is aimed at teaching pilots how to use the craft is IMHO not the best technical reference. Especially if it shows that the craft has a swashplate at each rotor and doesn't say anything about a device that actually tilts the rotor head assembly. And I wouldn't be surprised if in helicopter-pilot speak "tilting the rotor disc" means "using cyclic blade-pitch control to direct the thrust of the rotor disc". (Actually I believe that most pilots don't really care how system actually works as long as it points the thrust where they want it.)

[Curveball Anders]

8 minutes ago, AHHans said:

But that's my actual question! And a document that is aimed at teaching pilots how to use the craft is IMHO not the best technical reference. Especially if it shows that the craft has a swashplate at each rotor and doesn't say anything about a device that actually tilts the rotor head assembly. And I wouldn't be surprised if in helicopter-pilot speak "tilting the rotor disc" means "using cyclic blade-pitch control to direct the thrust of the rotor disc". (Actually I believe that most pilots don't really care how system actually works as long as it points the thrust where they want it.)

I think (and this based on my fixed wing knowledge and completely random knowledge of rotorcraft).

The blades of a chopper isn't 100% stiff (as can be seen when a chopper is on the ground and the blades drops).

A Chinook doesn't actually tilt the rotor, but cyclic input tilts the "rotor dish" since the blades aren't stiff enough.

Ergo, using cyclic and tilting the rotor dish are both correct

Edited February 28, 2020 by Curveball Anders
gramaar

[AHHans]

1 hour ago, Curveball Anders said:

A Chinook doesn't actually tilt the rotor, but cyclic input tilts the "rotor dish" since the blades aren't stiff enough.

Ergo, using cyclic and tilting the rotor dish are both correct

I believe (hope?) that helicopter rotor disks don't tilt that much under the forces of the cyclic. (I think the main force is still "keeping the craft in the air" and that the control forces are just a minor variation of that.) But that's actually my point: I think that in contrast to what is claimed by some, there is no device that tilts the whole head assembly with the resulting kink in the drive shaft. (The more I think about this the more I come to the conclusion that helicopters are pure maintenance nightmares, even without moving around major load-bearing structures.)

Edited February 28, 2020 by AHHans

[Curveball Anders]

50 minutes ago, AHHans said:

 I think that in contrast to what is claimed by some, there is no device that tilts the whole head assembly with the resulting kink in the drive shaft.

I concur.

The thing comes down to the definition of a "rotor dish".
The head isn't tilting, but the blades aren't rigid so the "dish" flexes (under cyclic control).