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In the PC game Arma 3, there is a helicopter-race game mode, in which the player tries to manipulate the helicopter to pass a serious of hoops as fast as possible in an AH-6.

In the game, if the player banks the helicopter then make a sudden and hard pull on the cyclic to make a turn while max collective is applied, the low rpm warning will sound, followed by a loss of power and a few seconds later loss of attitude control. The aircraft can be recovered by:

  • Lowering the collective
  • Releasing stick pressure
  • Leveling the aircraft

After which the rpm will increase back to normal again.

My guess explanation is as follows:

  1. When max collective is applied, the blades are working hard by cutting air at a large angle of attack
  2. When the aircraft is banked and aft cyclic is applied, the airflow relative to the aircraft is like when it is in a very high rate descend
  3. The critical angle of attack is exceed on the rotor blades, they stall and cease to produce lift
  4. Due to the greatly increased drag, the main rotor is unable to keep rpm up with its power, therefore rpm decreases
  5. As rpm decreases, less lift is produced and the stall worsens.
  6. By reducing collective, the drag of the blades is reduced, and rotor power is enough to keep rpm at normal range again.

Is my explanation correct? Can this happen in real life?

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    $\begingroup$ What you describe is completely unrealistic (but then again, I have never seen or heard of a PC simulator that accurately simulates helicopters in anything except the basic flight regimes). Therefore, the answer to "can this happen in real life" is no. $\endgroup$ – Simon Mar 7 '17 at 20:54
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    $\begingroup$ @Simon perhaps you can add your own answer then? $\endgroup$ – kevin Mar 7 '17 at 21:06
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    $\begingroup$ The only answer I could give is no, it's not realistic. If the question was "what would happen in real life if you used these control inputs" (with some more detail), that would be answerable. $\endgroup$ – Simon Mar 7 '17 at 22:06
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About High Energy Maneuvers in a Helicopter: not quite like fixed wing flying

... if the player banks the helicopter then make a sudden and hard pull on the cyclic to make a turn while max collective is applied, the low rpm warning will sound, followed by a loss of power and a few seconds later loss of attitude control.

I don't understand what you mean by "loss of power" here (engines are red lined and don't produce any more power?) ... but to proceed

The aircraft can be recovered by:(recovery steps)

While the recovery steps are rational, the point is to not wait that few seconds if you droop Nr. I am not sure if the simulation is trying to model retreating blade stall or not, but the response to droop in a max performance maneuver cannot be delayed.

With max collective and a maneuver that induces low NR you don't wait: you reduce the load on the rotor head. You do that because the engines aren't able to keep the rotor turning based on the load you've put on the system. If you keep the load on you just make the problem worse. (In that regard, the simulation is more or less faithful to real life ...)

My guess explanation is as follows:

1. When max collective is applied, the blades are working hard by cutting air at a large angle of attack
Yes. Higher angle of attack.
2. When the aircraft is banked and aft cyclic is applied, the airflow relative to the aircraft is like when it is in a very high rate descend
No. Not the same.
3. The critical angle of attack is exceed on the rotor blades, they stall and cease to produce lift
While technically true, the reduction of Rotor RPM (Nr) is your cue to Stop Doing That! (whatever "that" is) so that Nr does not decay further. Note that the retreating blade will tend to stall first, which is a different stall mode than a fixed wing aircraft stalling due to high AoA.
4. Due to the greatly increased drag, the main rotor is unable to keep rpm up with its power, therefore rpm decreases
It's the Engines that can't keep the rotors turning that is causing the Nr to droop ... and that's due to the loads you are putting on the drive system.
5. As rpm decreases, less lift is produced and the stall worsens.
Less lift is produced and you may approach the stall rpm (somewhere around 70-80% of rated Nr depending on the helicopter) if you don't do something to restore Nr. That means lessen the severity of the maneuver and stop overloading your engines' ability to keep the rotors turning as soon as the Nr droops.
6. By reducing collective, the drag of the blades is reduced, and rotor power is enough to keep rpm at normal range again.
No, the Engine Power is enough (or becomes enough) to keep the Nr in its proper range, but reducing collective is a key first step (as is leveling the wings) to retain control of your rotor system.
Is my explanation correct?
No
Can this happen in real life?
Flying a helicopter like it's an F-18 in a Blue Angels flight demonstration is a good way to kill yourself.

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In short: yes. Rotors and propeller blades are airfoils and will stall when they reach a critical angle of attack just like fixed wings.

Known in the business as rotor stalls, these can occur during all regimes of flight but can occur in extreme pitch up attitudes, usually encountered during takeoff and landing. A helicopter's Vne is also calculated as the maximum forward velocity which the helicopter can reach before the retreating rotor blades stall. Stalls are also common during autorotate even landings when inexperienced students apply hard aft input to the cyclic as they watch the ground rush toward them.

It would be reasonable to expect, therefore, that at high loading near maximum for the airframe and depending on the velocity vector of the air entering the rotor disk that a stall could happen in all or parts of the rotor disk. You would probably feel buffeting followed by rapid sinking or abrupt rolling or pitching moments at the onset of this.

https://www.aopa.org/news-and-media/all-news/2014/may/08/rotorocraft-rookie-helicopter-stalls

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    $\begingroup$ "apply hard aft input to the cyclic " I think you mean pull up on the collective? Pulling hard is never good but pulling aft cyclic at the bottom of the auto is required to convert speed into rotor RPM. $\endgroup$ – Simon Mar 7 '17 at 19:48
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    $\begingroup$ Please go over your answer again. I'm afraid that it, and the article you link to, have many problems. I'll have to down-vote if you can't get it fixed. $\endgroup$ – Simon Mar 7 '17 at 20:51
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    $\begingroup$ The only rotor stall that cab be recovered (ignoring the tail rotor which can stall like any other aerofoil) is a retreating blade stall. Stalls due to exceeding the critical angle are not recoverable and will lead to a crash. Stalls are not common in any regime, since they usually result in death and students are therefore trained from the first minute to not do the things which lead to stalls. Extreme pitch attitudes are not attained during take-offs and landings and are certainly not common in autorotations. The rookie mistake is to pull collective but the instructor will stop the stall. $\endgroup$ – Simon Mar 8 '17 at 6:19
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    $\begingroup$ The article you link to confuses lots of different things - see the comments further down the page. Sorry, but this answer is not correct in any way - most importantly that the answer to the OP question "is the explanation correct" is no. More detail is needed to answer is if asked as "what would happen" but the likely result is that rotor would stall resulting in a crash or a blade would strike the tail or cockpit, resulting in a crash. $\endgroup$ – Simon Mar 8 '17 at 6:22
  • $\begingroup$ Carlo, are you referring to powered or unpowered flight? $\endgroup$ – KorvinStarmast Mar 8 '17 at 19:26
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I will answer the question as written, but as I stated in a comment, it would be better posed as "what would happen if..."

The answer is "no" and "no".

In the game, if the player banks the helicopter then make a sudden and hard pull on the cyclic to make a turn while max collective is applied, the low rpm warning will sound, followed by a loss of power and a few seconds later loss of attitude control.

This is wrong. Therefore the recovery is wrong. Therefore your explanation is wrong. Therefore it could not happen in real life.

If you were to bank an AH6, then pull full collective, then pull back hard on the cyclic, you would die (assuming you are high enough to not survive the crash).

Without spending time thinking through all of the interactions, and since that is not the question asked I haven't done so, either the rotor will stall, which is not recoverable, and/or a blade will strike the tail or the top of the cockpit, which is also not recoverable. I suspect a rotor stall would occur first, which depending on a lot of factors might then result in a fuselage strike but one or the other will happen almost immediately.

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  • $\begingroup$ While "no" may technically answer the question, I think an explanation of why the explanation is wrong and/or more detail on what would happen to a real helicopter would be good to include. Unless you think it really needs (or already has?) its own question. $\endgroup$ – fooot Mar 8 '17 at 20:16
  • $\begingroup$ In my opinion, it needs it's own question. The portion I've highlighted, from which the rest of the question follows, will result in a crash. I'm not sure what else to say! $\endgroup$ – Simon Mar 8 '17 at 20:29

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