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Yaw in high speed flight


sobek

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I have found that when you trim the shark for a hover (or cancel trim) and then transition into a fast forward airspeed, it tends to yaw hard to the left, so you have to trim with a lot of right rudder to keep the skid-indicator lined up. Anybody know what causes this? Is this some sort of inbalance between the rotors when flying at high airspeed? Just curious.

 

cheers

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I don't know what causes it but I do know if you get it flying in that configuration and you push hard right pedal trying to straighten things out the rotors will hit and you will crash:joystick:

Ask Jesus for Forgiveness before you takeoff :pilotfly:!

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Good question.

 

Perhaps this is caused by the coaxial rotor system?! I dont know...

 

But I have anther question to fly at high speeds:

 

When I trim the aircraft to the center of the skid indicator, I am drifting!

 

Why is that?

 

Does the downwind of the rotor takes affect to the skid indicator sensor?

DCS Rafale - please :thumbup:

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Good question.

 

Perhaps this is caused by the coaxial rotor system?! I dont know...

 

But I have anther question to fly at high speeds:

 

When I trim the aircraft to the center of the skid indicator, I am drifting!

 

Why is that?

 

Does the downwind of the rotor takes affect to the skid indicator sensor?

 

That is part of my question. ;)

Good, fast, cheap. Choose any two.

Come let's eat grandpa!

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Yep, that's a feature but except for the left yaw it speed brings up also right bank though the helicopter is much more stable in bank as it is in yaw so you notice the side slip mostly. Normal, because of the blades flappaing hence rotors' cones inclination.

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

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But why am I not flying straight at high speeds, even when the slip-ball is centered (no side wind)?

 

Example: acceleration to 250kph -> slipball wanders to the right -> i counter with right rudder -> ball is centered -> trim -> chopper is not flying straight :huh:

 

Or from a different perspective: When I'm SEEING that I'm flying straight by observing the ground or watching with external views, then the slipball is almost always off to one side, sometimes all the way to the right.


Edited by therion_prime

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I tend to ignore the ball and am only interested in the wind vains on the front. Would prefer a piece of string tho...that never fails:thumbup:

i9-9900K,Z390 Aorus Master, 32GB GSkill Trident F4-3600 DDR4, ROG Strix RTX 2080 Ti, Oculus Rift S. Thrustmaster Warthog T&S, TPR Pedals.

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But why am I not flying straight at high speeds, even when the slip-ball is centered (no side wind)?

 

Example: acceleration to 250kph -> slipball wanders to the right -> i counter with right rudder -> ball is centered -> trim -> chopper is not flying straight :huh:

 

Or from a different perspective: When I'm SEEING that I'm flying straight by observing the ground or watching with external views, then the slipball is almost always off to one side, sometimes all the way to the right.

 

Are you applying left cyclic as well? Because if you don't you won't go straight. On this screenshot I'm doing 300+ kph in level flight, hands free flight, ball is in the center, chopper is going straight. Look at what position the controls are trimmed.

 

Look, it's not like you should fly without side slip all time. Also, keep in mind that the ball doesn't show you the slip angle exactly, it gives the lateral acceleration magnitude. Basicly, as written in the flight manuals, the pilot should try to keep the ball in the center while flying and maneuvering and also for shooting. Note that when the ball is not in the middle your laser LOS is way off the center, same for the rockets and fixed gun pippers. So it's important for weapons precision quiet a lot.

ScreenShot_018.thumb.jpg.42a605d389b4d37bfeae58b63a9569f7.jpg


Edited by =RvE=Tito

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

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Are you applying left cyclic as well? Because if you don't you won't go straight. On this screenshot I'm doing 300+ kph in level flight, hands free flight, ball is in the center, chopper is going straight. Look at what position the controls are trimmed.

 

But you are still slipping over your left side, or aren't you?

Good, fast, cheap. Choose any two.

Come let's eat grandpa!

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Yes, slightly. But the key is that when I select either Vikhrs or rockets (gunpods) the LOS or pipper will be pretty much centered.

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

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Look, it's not like you should fly without side slip all time. Also, keep in mind that the ball doesn't show you the slip angle exactly, it gives the lateral acceleration magnitude.

 

Hmm. I have many years of flight simming experience - mostly props. I'm perfectly familiar with torque / sideslipping / rudder(trim). Its' just akward that the BS is behaving so different / akward. I always thought this is a bug or a "feature" of the AP dampeners that try to maintain a heading different to the longitudinal axis of the copter.

But on the other hand: I'm pretty new to helo sims, so what do I know? :D

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In order the AP to maintain heading (or track) you need to use the Enroute mode (DH-DT). The Pitch, Bank, Yaw channels by default continously provide stability augmentation in the 3 axis. After engaging one of the Enroute modes along with stabilization they fully control the helicopters trajectory. In manual mode, the Yaw channel will not maintain your heading, it just damps and stabilizes the chopper around the vertical axis.

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

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Hmm. I have many years of flight simming experience - mostly props. I'm perfectly familiar with torque / sideslipping / rudder(trim). Its' just akward that the BS is behaving so different / akward. I always thought this is a bug or a "feature" of the AP dampeners that try to maintain a heading different to the longitudinal axis of the copter.

But on the other hand: I'm pretty new to helo sims, so what do I know? :D

 

Well, you're not in Kansas anymore.... I don't know about the accuracy of EDs flight model throughout the BS envelope, but I can tell you that helicopter aerodynamics are only similar to a fixed wing when the rotors aren't turning.

 

Anyway on to the subject.

 

1) Regarding the yaw change during transition from hover to forward flight.

 

There are lots of reasons for that to happen. The horizontal and vertical stabiliser will be designed to generate a stabilising pitching and yaw moment at some nominal cruise speed. As you start flying forward, the stabiliser starts to generate this moment and the yaw pedal requirement changes.

 

In addition to that, translational lift and a couple of other effects of forward flight will adjust alter the efficiency of the rotor disks - since one disk is acting in the "shadow" of the other, this change is not the same for both.

 

So, between those effects it seems reasonable that there will be a change in both the torque of the main rotors and the aerodynamic stabilisation of the aircraft and so a change of pedal (all controls in fact) will be required.

 

Bear in mind the inherent instability of rotorcraft - when one control is changed all the others have to change as well.

 

2) Side slipping

 

In a single rotor helicopter like say an Apache, the tail rotor and main rotor forces are coupled. The net effect of these two is that in a hover the helicopter will hover with one wheel or skid lower than the other - if you hover and use cyclic to keep the helicopter in a level roll attitude, you start to sideslip across the ground (I should point out here that in reality some helicopters build in an offset to the main rotor mast or control rigging to aid control - most don't).

 

Now in a coaxial system like the KA 50 in principle there is no equivalent couple of forces. However in forward flight as we've been told, the lower disk is not generating as much torque as the upper one - I guess this is because the lower disk is working in the air already disturbed by the upper disk (at least to an extent depending on airspeed).

 

Anyway, the effect is that the disks are generating unequal amounts of lift on opposing sides of the aircraft (in forward flight, the upper disk generates most lift on the port side and the lower disk generates most lift on the starboard side). However the disks don't generate the same amount of lift so there is an imbalance leading to a roll unless the pilot corrects.

 

Just like the Apache, if the pilot flies exactly straight and level with the trim centered, the resulting couple will have the aircraft slip sideways. On the other hand, you can fly in trim but with a bit of bank to stop the drift or you can fly with a level roll attitude and slightly out of trim and that will stop the aircraft sideslipping too.

 

 

So, while EDs flight model may not be perfect, the interacting forces and effects are such that the ideal straight and level in trim flight is not necessarily something we should expect (or even try to achieve) - it isn't in real helicopters and while the KA 50 is co-ax in design I don't see why it should be any different.

 

r.


Edited by ryuzu
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Thanks for the very insightful explanation, ryuzu.

 

Especially this part:

 

"...that the ideal straight and level in trim flight is not necessarily something we should expect (or even try to achieve) - it isn't in real helicopters and while the KA 50 is co-ax in design I don't see why it should be any different."

I always tried to achieve a perfect "straight" flight like I was used to.

It seems that I'm not flying as bad as I had thought! :doh:

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More induced flow over the lower blades than translational. Fair enough.

 

I bet you've even modelled ground oscillations for this chopper. I'd better check that it has an Idiot Mode before I buy it so I can cope with flying the model.

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  • ED Team

2 ryuzu

 

Thank you for the correct explanation and for doing the job I must do :).

I only have to add that even if the rotor forces were equal THE MOMENTS of these forces would not be equal because of different arms of these forces.

If we fly forward the yawing moment arms are different too because of rotor shaft inclination.

So the coaxial rotor system can not be considered as a SYMMETRICAL. The right term would be "quasi-symmetrical" or "much more symmetrical than classic". :)

 

And by the way - lower rotor does have less thrust. It was done intentialy lowering the blade inclination angle to get both torques equal.


Edited by Yo-Yo

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

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Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Good post ryuzu, though one thing to clarify- coaxial helicopters have much more independant controls than classic. The crossconnections are down to bank-yaw like in airplanes.

 

The difference between the thrust of the rotors may reach 20% dependeing the flight regime, in a favor of the upper rotor. Like Yo-Yo said, given the bigger arm of the upper rotor to the center of gravity hence the right bank and side slip.

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

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I have a question pertaining to this. I posted a screenshot, but not sure it'll show enough. While flying above 200 or so, and trimmed out for level flight (even with the slight sideslip) I've noticed that my lower rotors are banked so hard over that almost any movement of the stick would make the blades hit the upper set.

However, I've seen on others while flying online, that their blades (while above 200) are perfectly straight or close to it. Am I doing something wrong here?

ka-50.JPG.62bb199821d3698b2f94190787ba19e9.JPG

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That from one side and from another- you don't see blades flapping on AI's either online or offline.

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

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