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Trim with slip indicator always nose left?


MstrCmdr

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1 hour ago, bradmick said:

Here's a quick diagram of the major forces and things going on with the aircraft. You have the main rotor torque effect which is counteracted by the tail rotor, not only that but we have a high set tail rotor too. You then have vertical fin lift which is slowly offloading the tail rotor. Both of these combined will work to cause a rolling moment in the aircraft which has to be counteracted by applying left cyclic. The left efab is also more streamlined than the right efab (so the pilots can more easily get into the crewstations). This causes a differential in lift between the two sides of the helicopter. There are gurney flaps on the stabilator that help to offset this a bit. Translating tendency (tendency of the helicopter to drift in the direction of tail rotor thrust, or to the right for the AH-64D) doesn't go away in forward flight. You're always going to be compensating for it. Not only that, but the main rotor torque also mixes into the equation also. 

 

Bottom line: It's complicated. And all of the forces diagramed out combine to cause the aircraft to fly with a slight yaw offset, even in no winds.

 

image.png

Thanks!.

A lot going on!  First impression is extend vertical fin lower with tail rotor closer to center. But no doubt several other considerations this will give other effect. A compromise of design for hover and max speed.

Now back to hover land training, I really sucks at this. It ends up with holding breath and white knuckles

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18 minutes ago, bradmick said:

So you’re saying I should go repaint the Sistine chapel, got it!

 

Id say it is right in line!

Personally, I am more of  Cassius Marcellus Coolidge art fan myself.

 

In all seriousness thanks for imparting your insight and knowledge of the Apache in this forum.  


Edited by RaisedByWolves
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4 hours ago, bradmick said:

The yaw offset (difference between the FPV and headtracker (nose of the helicopter)) is always there, even in a no wind situation. The yaw offset is currently approximately double what it should be for a given airspeed. At 40 knots the VV and FPV should be centered in the head tracker with a centered trim ball, at 70knots the the fligh path vector circle will enclose the right chevron of the head tracker with the trim ball centered, at 90 knots the tip of the 'left wing' of the flight path vector will be touching the right tip of the head tracker. This equates to roughly 0 degrees yaw offset at 40 knots, 5 degrees yaw offset at 70 knots and 6 or so degrees at 90 knots. At 150 knots the yaw offset is around 10 degrees. 

 

All of the above is in a no wind scenario. So the aircraft can absolutely be 'in trim' (aerodynamically, ball centered), but still have a small degree of sideslip. This isn't unique to the apaches, all helicopters fly with some degree of yaw offset. The difference is, you have symbology now to show you that it's happening, where as before it was a subconscious thing in the background you just accounted for.

 

So, bottom line: You will always have a yaw offset even in zero winds.

 

Might want to copy this post to the bug report section since a mod hasn’t commented yet. Dunno if they are investigating it or not?

 

 

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I find that even in calm winds in cruise the AH 64 always wants to fly nose left when trimmed according to the " trim ball" it always wants to fly nose left, or when centered on the Vector indicator, half a ball right.

 

is that correct behavior?

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11 minutes ago, razo+r said:

 

Thank you. simple answer: it is as is. the Boeing guys designed a <profanity>ty helo, it doesn't fly straight. the vertical stab can't compensate for TQ in forward flight, so the tail rotor has to do it and makes the chopper crab.

thank you.

 

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4 hours ago, frostycab said:

I think it's true of most helicopters. I've spent a lot of time in the Mi-8 over recent weeks and if you do fly in aerodynamic trim then it also has a noticeable crab,

I think not. I have spent almost 6000 hours in different real Helicopters as PIC and they do fly straight in calm air. maybe the problem seems exaggerated in AH 64 because you have the indicator ( vector) right in front of your nose (IHADSS)

 

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21 hours ago, S. Low said:

Might want to copy this post to the bug report section since a mod hasn’t commented yet. Dunno if they are investigating it or not?

If you find a solution- You are guarantied a job as a helicopter designer. 

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12 minutes ago, zero said:

If you find a solution- You are guarantied a job as a helicopter designer. 

Neither myself nor the actual SME are suggesting a “solution” to crabbing. Simply that the crabbing on the DCS Apache is possibly excessive (and I’m not trying to speak on his behalf. Read his post).

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On 3/23/2022 at 7:37 AM, bradmick said:

The yaw offset (difference between the FPV and headtracker (nose of the helicopter)) is always there, even in a no wind situation. The yaw offset is currently approximately double what it should be for a given airspeed. At 40 knots the VV and FPV should be centered in the head tracker with a centered trim ball, at 70knots the the fligh path vector circle will enclose the right chevron of the head tracker with the trim ball centered, at 90 knots the tip of the 'left wing' of the flight path vector will be touching the right tip of the head tracker. This equates to roughly 0 degrees yaw offset at 40 knots, 5 degrees yaw offset at 70 knots and 6 or so degrees at 90 knots. At 150 knots the yaw offset is around 10 degrees. 

 

All of the above is in a no wind scenario. So the aircraft can absolutely be 'in trim' (aerodynamically, ball centered), but still have a small degree of sideslip. This isn't unique to the apaches, all helicopters fly with some degree of yaw offset. The difference is, you have symbology now to show you that it's happening, where as before it was a subconscious thing in the background you just accounted for.

 

So, bottom line: You will always have a yaw offset even in zero winds.

Concur with the aerodynamic modeling requiring constant trim adjustments depending on airspeed. It is a real thing but the problem is not quite solved as the yaw is still in excess of reality. 

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  • 1 month later...

Yeah, I thought the fully accepted answer (as per the SMEs comments) was that currently the crab value is doubled vs the real aircraft. If you do a conditions mimic of any AH-64 HDU video you find on YouTube you'll see that that rule of thumb is roughly correct.

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  • ED Team
29 minutes ago, Scaley said:

Yeah, I thought the fully accepted answer (as per the SMEs comments) was that currently the crab value is doubled vs the real aircraft. If you do a conditions mimic of any AH-64 HDU video you find on YouTube you'll see that that rule of thumb is roughly correct.

I must be missing this, can you show me where the SME has said it is double?

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  • ED Team

Thanks

threads merged into bugs. 

Edit: We do already have a report open for checking the Yaw / crabbing 

thanks

 

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  • 3 weeks later...

The Apache IS supposed to fly with quite a noticeable crab, but it appears it is a bit too much and the discrepancy gets worse with speed. This might be the reason why.

If the apache's implementation of the slip indicator is anything like the huey's, the problem is that it does not take the aerodynamic effects of speed into account.

As the helicopter speeds up, the tail rotor gains power, and the tailfin applies its own antitorque force to the helicopter.

this should cause the ball to move right as you gain speed.

A simple test, turn off any flight assists you can, and put the apache into a hover, then without touching the collective or pedals, nose forward to gain speed and translational lift, the nose will probably move to the left, if the slip indicator doesn't move to the right in response to this, you have your answer.

As you gain even more speed than that, you're likely pulling more collective, and thus more torque, causing the ball to move even farther to the left, exaggerating the crabbing when you follow the ball.

In the case of our huey module, it causes it to fly with a crab when there should be none and destabilizes the entire aircraft.
In the case of our apache, it would appear that it causes an increasing error in the amount of crabbing that there is supposed to be.

The increase in left pedal requirement as you raise the collective is correct, but the problem is that you're being asked to push the pedals to the left of a position that was already too far to the left due to not going to the right first. Eventually the natural anti torque gains from the tailfin and tail rotor at high speeds are outweighed by the additional torque of a higher collective.

 

These likely aren't valid reference for a 64D, but here are some documents showing this effect on a 64A
 

apache2.png

Here is a document showing the control positions in trimmed low speed flight, you can see the directional control moving to the right as the helicopter gains speed, with a total travel of 5.6 inches, 2.8 inches from full left sees the pedals centered

apache1.png

In this document showing the control positions in trimmed level flight, here you can see the rightward pedal requirement continuing all the way to 90knots, even though the collective started being raised after 60knots.

apache3.png

And here you can see that in trim "slip indicator centered" the 64A flies with about 6 degrees of sideslip in the specified parameters at the top.


Edited by Tim_Fragmagnet
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  • 1 year later...

Has this been fixed? trying the Apache and also noticed a big crab angle.

Also what I don't get is, with a centered ball, the AoA probes on the side of the engines are not aligned with the fuselage (as they would be if the aircraft was flying straight), is this to be expected with a centered ball and "aerodynamic" trim? 

 

Thanks

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Ok, so is it "normal" even with this error that at 20kts and 40kts, with no wind, I have crab angle as shown below (centered ball and level)?

This looks very far from the numbers you mentioned

 

Quote

he yaw offset is currently approximately double what it should be for a given airspeed. At 40 knots the VV and FPV should be centered in the head tracker with a centered trim ball, at 70knots the the fligh path vector circle will enclose the right chevron of the head tracker with the trim ball centered, at 90 knots the tip of the 'left wing' of the flight path vector will be touching the right tip of the head tracker. This equates to roughly 0 degrees yaw offset at 40 knots, 5 degrees yaw offset at 70 knots and 6 or so degrees at 90 knots. At 150 knots the yaw offset is around 10 degrees. 

 

Digital Combat Simulator  Black Shark Screenshot 2023.07.14 - 17.09.36.26.png

Digital Combat Simulator  Black Shark Screenshot 2023.07.14 - 17.14.17.27.png


Edited by bkthunder

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No, what I said was the game is off by 2.4 times the real helicopter helicopters no wind sideslip angle. So whatever you get in game, it should be 2.4 times less what it is, this also varies with gross weight and density altitude.

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