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Noob question: sudden roll to one side while diving


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This happens when I am performing a dive and the Hind suddenly rolls to one side, usually to the right. When I try to correct it by moving the cyclic to the left, the helicopter returns upright but the nose starts pointing upwards.

Why does this happen and how can I avoid it? I somehow never have this problem with Huey.

Sorry if this has been asked before.

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From page 57 of the Quick Start guide:

At faster airspeeds, e.g. above 150 kph, the helicopter will tend to roll right, so increasing left cyclic pressure will also be needed to maintain level flight.

Also, try to keep your airspeed below ~300 Kph (IIRC), as otherwise it may roll right/dive uncontrollably.

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It’s a retreating blade stall. You’re moving forward fast enough that the rotors moving backwards relative to your direction of travel are almost still in the air, so they stop generating lift and you roll over. Scrub speed when it starts to happen and it should even out. 

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Very different from fixed-wings and Huey. And this thing really tries to murder me whenever it gets the chance.

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I find the Hind can safely dive to right around 350 km/h in DCS. But that is the wall, after that, the right side of the rotor disk will stall and there is often little time to recover, though it can be done reliably if diligent. To recover, chop the collective to minimum and get the nose up. Chopping the collective will briefly unstall the blades or at least make them less badly stalled. Pulling up will dump speed and that's what must be done. As for the sudden uncontrolled pitch up when even slight pitch up is commanded, that I am unsure about. That parachuting effect is unique to the DCS hind as far as I can tell. Never noticed it in the other DCS choppers. Often a dive recovery for me involves pulling back ever so slightly to initiate the pitch up and then giving full forwards stick to keep it from going 90 degrees off axis of travel and stalling out due to VRS. It's certainly odd behavior, no clue if it's realistic or not but it does strike me as highly suspicious.

Outside of dive recovery, pitching up should be done gently at any kind of significant forward speed to avoid that parachuting effect. It's still pretty nimble and can definitely get around, just has that bizarre handling characteristic as modeled.

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10 hours ago, BeastyBaiter said:

I find the Hind can safely dive to right around 350 km/h in DCS. But that is the wall, after that, the right side of the rotor disk will stall and there is often little time to recover, though it can be done reliably if diligent. To recover, chop the collective to minimum and get the nose up. Chopping the collective will briefly unstall the blades or at least make them less badly stalled. Pulling up will dump speed and that's what must be done. As for the sudden uncontrolled pitch up when even slight pitch up is commanded, that I am unsure about. That parachuting effect is unique to the DCS hind as far as I can tell. Never noticed it in the other DCS choppers. Often a dive recovery for me involves pulling back ever so slightly to initiate the pitch up and then giving full forwards stick to keep it from going 90 degrees off axis of travel and stalling out due to VRS. It's certainly odd behavior, no clue if it's realistic or not but it does strike me as highly suspicious.

Outside of dive recovery, pitching up should be done gently at any kind of significant forward speed to avoid that parachuting effect. It's still pretty nimble and can definitely get around, just has that bizarre handling characteristic as modeled.

It’s called “mushing” and happens with all helicopters to some degree. To stop it happening the G limit is suppossed to decrease with speed from 1.8 below 200kmh to 1.2 above cruise. Though personally, as the below description indicates, based on gross weight you want to not exceed a certain AOA, with Hind at normal flight regimes weights and nearly full collective you never want to go higher then 1.8-2G 

 

however the aspect of the Hind that exaggerates the mushing is the wing, the wing is at 19 degrees incidence. Just a couple more degrees if that and it stalls. The wing is behind the CG/rotor mast, this means that as the wing adds lift it also creates a nose down moment.

so when the wing stalls from positive AOA, the loss of lift removes a nose down force that causes you to pitch up, if you watch the mushing happen in external F2 view, you will notice it always happens at positive AOA and never at neutral or negative AOA where the wing is not stalled 

from the Army fundamentals of flight “. Mushing is a temporary stall condition occurring in helicopters when rapid aft cyclic is applied at high forward airspeeds. Normally associated with dive recoveries, which result in a significant loss of altitude, this phenomenon can also occur in a steep turn resulting in an increased turn radius. Mushing results during high G-maneuvers when at high forward airspeeds aft cyclic is abruptly applied. This results in a change in the airflow pattern on the rotor exacerbated by total lift area reduction as a result of rotor disc coning. Instead of an induced flow down through the rotor system, an upflow is introduced which results in a stall condition on portions of the entire rotor system. While this is a temporary condition (because in due time the upflow will dissipate and the stall will abate), the situation may become critical during low altitude recoveries or when maneuvering engagements require precise, tight turning radii. High aircraft gross weight and high density altitude are conditions conducive to and can aggravate mushing.
1-148. Mushing can be recognized by the aircraft failing to respond immediately but continuing on the same flight path as before the application of aft cyclic. Slight feedback and mushiness may be felt in the controls. When mushing occurs, the tendency is to pull more aft cyclic which prolongs stall and increases recovery times. Make a forward cyclic adjustment to recover from the mushing condition. This reduces the induced flow, improves the resultant AOA, and reduces rotor disc coning which increases the total lift area of the disc. The pilot will immediately feel a change in direction of the aircraft and increased forward momentum as the cyclic is moved forward. To avoid mushing, the pilot must use smooth and progressive application of the aft cyclic during high G-maneuvers such as dive recoveries and tight turns.”

this is from this document, fundamentals of flight by the army https://irp.fas.org/doddir/army/fm3-04-203.pdf

 

while it mentions altitude pressure, as y increase altitude in the Hind your maximum speed decreases, and it becomes less and less of an issue. Despite this to stop it happening at higher altitudes the manual for Mi-24 has decreasing limits for roll angle and maneuvering at increased altitudes 


Edited by AeriaGloria
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