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Odd Flight Dynamics IGE while entering ETL


Buznee
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Not sure if anyone else has seen this phenomenon in the sim but I notice the aircraft behaves as if it is overly contrained when in ground effect and starting to enter into translational lift. By overly constrained I mean it almost looks like the aircraft moves or translates and doesn't rotate for a small portion of time during this regime of flight. It feels very unnatural. The aircraft also rises quite rapidly as if the ETL is almost a step function. If you'd like all try to get a replay that I can share with you guys. If you've ever flown Microsoft flight simulator and watched a replay you know how unnatural the replay looks, the overly-constrained effect I'm referring to is shown similarly through fsx replay. I see a similar temporary motion out of the aircraft during this condition. Once you pass through this regime the aircraft feels free and alive again.

 

Thoughts? :huh:

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I'm a total noob when trying to explain air dynamics, but, the word "transitional" is used for a reason.

 

In ground effect, you are literally floating on the compressed air underneath you (developed by rotor down-wash) when you start going forward and adding speed you are transitioning from a stable float to using the lift of the rotors and it will stumble (so to speak) until you fully leave the ground effect and are actually using the lift of the rotor blades.

 

In a twin rotor, as opposed to a tri-blade rotor, the effect, or stumble is more apparent. It could be that our Huey might be a little exaggerated in that "stumble", but maybe not. ---- sounds good to me.

 

Okay, somebody else's turn.

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You say that it feels 'unnatural'. Unnatural compared to what? A real Huey? There are real life pilots who can attest to the accuracy of the sim, so unless you have flown the Huey yourself and can really attest to its behaviour when entering FTL I will trust the simulated dynamic flight model that DCS boasts over any subjective 'feel'.

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I tried doing this last night, I didn't notice anything weird.

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Videos as promised.

 

As promised, here are the videos. Both internal and external views. Notice the abrupt jump with no change in collective. I understand the aircraft enters translational lift as well as transverse flow effect of the rotor but is it really that abrupt to cause you to jump in this manner uncommanded? It almost feels like you just got slingshotted up over 10 ft. It doesn't seem right. Overly exaggerated effect if anything. Almost like a step function in the power required. Its not consistent either, sometimes it won't do it, sometimes it will. Not sure if ground effect is playing a role in this.

 

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Working as intended. Its aerodynamics, pure and simple. At a certain airspeed the helicpter starts to 'fall off' the cushion of air that is called the ground effect, this is where the turbulance and shaking start. At another point shortly therafter the aircraft's rotor starts to act sort of as a fixed wing, in addition to the lift of the blades themselves the forward motion of the aircraft induces extra lift, which is why you see the jump.

 

You have to factor in other stuff into the equation, like windspeed and direction and weight/load of the helicopter itself.

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Does look a little funky in the video (like the transition should be smoother) but I've never noticed it in game. I'll try it tonight. Huey is still beta though so maybe the ground effect/translational lift interaction hasn't been tweaked properly yet.

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I have experienced similar jumps and also dips but haven't been able to exactly pin down when they occur as it doesn't happen always. But yeah, I think this is what is causing most of the grief with VRS as when it happens the other way around it can cause your vertical speed downwards to increase suddenly when slowing down.

 

Has anyone checked if the step function like behavior is seen also in the torque meter, as if you suddenly get a huge boost in efficiency as there's no sudden step in UH-1 cruise chart in ETL transition, just gradual decrease in required torque as speed increases.

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I've noticed this too, discussed it, and concluded that it is not a feature, but a bug that will hopefully be fixed for final release.

 

I think there is some kind of conflict in the calculations sometimes. Sometimes it doesn't happen and sometimes you get a kick up, down, to the left or to the right. (The inconsistency itself reveals that there is some kind of conflict/error)

 

There are no sudden torque/RPM/EGT changes as this happens.

 

BTW in the video it is not that bad and I understand that it can be explained as an AFM feature. But I've seen much worse in all different directions.


Edited by TurboHog

'Frett'

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  • 2 weeks later...

I fly helicopters for the US Army. The ETL effect in this sim is pretty dead on. I haven't flown a Huey, but I have flown a B206, and an H-60, and the effect is pretty much the same in all helicopters, as it is a basic aerodynamic principle.

 

I've never looked at it from the outside, but from the cockpit I find it very realistic. In fact, one of the things I hate about FSX helicopters is the fact that you have to increase power as you accelerate from a hover in order to climb. Based on environmental factors, and aircraft weight, you really shouldn't have to in most situations. The aircraft should start to climb all by itself without a change in power as it accelerates through 16-24 knots. Now, depending on PA, temp, how much weight you're carrying, or minimum climb requirements at the airfield you're operating at, you may have to make some adjustments, but for normal take-offs at low DAs and at the lower end of a helicopters gross weight, it shouldn't be a problem.

 

In fact, even if the helo is overloaded, it will begin to climb as it goes through ETL, but if you are too heavy, you'll descend back down, and possible contact the ground unless you get up enough speed. In those cases you would just adjust the cyclic to keep yourself in ground effect until you build enough speed to climb normally, but I digress to a totally different subject...

 

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Understood. Is it a smooth transition though? In the sim it's currently showing a jump and also it's very inconsistent. This only happens sometimes. The aircraft will seem to have a jump in thrust output and then it goes away again. Doesn't seem right. I don't see this happening in blackshark either. The blackshark shows a nice smooth transition into ETL. And by smooth I don't mean that there are no vibrations in the cockpit, but the fact the thrust output acts a continuous smooth function while entering and exiting ETL. This seems almost like a discontinuity in the model that is erratic.

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...I don't see this happening in blackshark either. The blackshark shows a nice smooth transition into ETL...
If I'm not wrong, you shouldn't feel ETL in Ka-50. It doesn't happen. Because she has no tail rotor.

 

Right?

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right (sort of - as you wont have to work your rudder because of the coaxial design) - and there is also a significant change how pronounced the vibration is while transitioning ETL when you have more than just 2 main-rotor-blades.

 

It's elementary to Understanding what happens to the down-wash and aerodynamic forces that change the effectiveness of your tail-rotor, and how this changes the whole behaviour of the FM, when transitioning ETL.

This knowledge makes it easy to counteract it with the anti-torque rudders and controlling this effect will become easy.

 

I suggest to make some I-net search about ETL .


Edited by PeterP

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Working as intended. Its aerodynamics, pure and simple. At a certain airspeed the helicpter starts to 'fall off' the cushion of air that is called the ground effect, this is where the turbulance and shaking start. At another point shortly therafter the aircraft's rotor starts to act sort of as a fixed wing, in addition to the lift of the blades themselves the forward motion of the aircraft induces extra lift, which is why you see the jump.

 

You have to factor in other stuff into the equation, like windspeed and direction and weight/load of the helicopter itself.

 

I think the aerodynamics are right in the game, but I also think your explanation is wrong. I would just like to mention, that because of the reasons it happens, is the reason it doesn't happen when moving forward. The "cushion" is no longer under the rotors as you are moving too fast for it to accumulate under the helicopter. Hence why it goes away. At least that's how the real aerodynamics work. Basically your rotors are always in cleaner air.

 

I fly helicopters for the US Army. The ETL effect in this sim is pretty dead on. I haven't flown a Huey, but I have flown a B206, and an H-60, and the effect is pretty much the same in all helicopters, as it is a basic aerodynamic principle.

 

I agree that its pretty close, I'm not sure of the speeds though, they seem like they need a bit of adjustment. It also seems that with an empty payload it will stick regardless. We had an issue the other day where the helicopter would not takeoff unless speed was given. There are bugs still with the flight model I think, and they are very difficult to reproduce. The helicopter that could not takeoff had the same load out and weight as the others but could not get off the ground. Definitely a bug, eventually it shot up like a rocket, but like I said, there do seem to be some quirks that need to be worked out still and its not always the same experience, when it should be.

 

I'm going to try and get the actual flight test data and compare. Probably have some thing to post next week.


Edited by pbishop
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This doesn't have anything directly to do with the OPs original question, but I hope to clear up a couple of misunderstandings.

 

IGE = in ground effect

ELT = effective transitional lift

 

In ground effect occurs when the helicopter is close to the ground. Downwash from the rotors can't escape so readily which produces a cushioning effect, increasing lift and reducing power requirements. As the helicopter starts to move forward, even at the same height, the cushion is left behind and the benefits from IGE reduce.

 

Effective transitional lift occurs when the helicopter moves out of a hover into smoother air (can be in any direction). The faster the relative wind, the less recirculation through the rotor disc, the stronger the effect and the greater the lift. This works for the tail rotor also, so one of the reason left pedal requirements reduce with speed.

 

If I'm not wrong, you shouldn't feel ETL in Ka-50. It doesn't happen. Because she has no tail rotor.

 

Right?

 

You'll get ELT in the Ka-50, as moving forward will still mean smoother air through the disc. It does appear to be less in DCS; I assume this is accurate but my knowledge isn't deep enough to know the reasons.

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Effective transitional lift occurs when the helicopter moves out of a hover into smoother air (can be in any direction). The faster the relative wind, the less recirculation through the rotor disc, the stronger the effect and the greater the lift . This works for the tail rotor also, so one of the reason left pedal requirements reduce with speed.

 

Your post is a good explanation. However, in red is not exactly accurate. To be 100% on this, its not the relative wind, its moving out of turbulent air caused by the rotors. The transition between the state of being in turbulent air to undisturbed air is the transitional part of the effective transitional lift. Relative wind has nothing to do with it. The less turbulent air, the greater the lift.


Edited by pbishop
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Actually if anything my first sentence wasn't quite right. The helicopter doesn't have to go anywhere to benefit from ETL. It just needs a horizontal air flow component through the rotor disc. Whether the helicopter flies strait ahead at 20kts in calm wind or hovers over a fixed point in a 20kts head wind, the effect is the same. Hopefully that clears up any misunderstanding.

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Actually if anything my first sentence wasn't quite right. The helicopter doesn't have to go anywhere to benefit from ETL. It just needs a horizontal air flow component through the rotor disc. Whether the helicopter flies strait ahead at 20kts in calm wind or hovers over a fixed point in a 20kts head wind, the effect is the same. Hopefully that clears up any misunderstanding.

 

Ah, indeed. Lol.

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I'm glad I'm not the only crazy here :) can the real Huey pilots and/or belsimtek developers weigh in on this?

Hi there,

 

Buznee, you´re not crazy. The flight dynamics are a little bit buggy. The helicopter does not make some of these movements in real life.

 

While still IGE(or better say very low height) and entering ETL sometimes the fuselage drifts to one side or up or down. I didn´t try at high heights, because I normally don´t enter ETL at higher heights.

 

I recorded two videos to demonstrate the problem.

Video 1 cockpitview and video 2 external view of the same situation.

Video 1 with comments.

 

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Video 1

 

Video 2

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Thank you thank you thank you! Your video shows it much more obvious than mine. It definitely tends to jump in different directions as well. Good job capturing that all. On your second video that odd flight dynamics start around 2:04 time stamp. Everyone watch carefully and you can see the discontinuity in the way the aircraft behaves as it jumps around.


Edited by Buznee
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Hi there,

 

Buznee, you´re not crazy. The flight dynamics are a little bit buggy. The helicopter does not make some of these movements in real life.

 

While still IGE(or better say very low height) and entering ETL sometimes the fuselage drifts to one side or up or down. I didn´t try at high heights, because I normally don´t enter ETL at higher heights.

 

I recorded two videos to demonstrate the problem.

Video 1 cockpitview and video 2 external view of the same situation.

Video 1 with comments.

 

URSS 250800Z 06010G15KT 9999 CAVOK 15/XX Q1013 NOSIG

 

DCS Ver 1.2.5.16184

 

iFoxRomeo

 

Video 1

 

Video 2

 

Is that behaviour reproducible when playing the .trk file? If so, you should definately post it here.

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