Low blades stall AoA combined with high drag, making it impossible to fly using autorotation

[85th_Maverick]

I always felt that this helo has difficulties when it comes to blade stall which takes place rather quickly or at a quite low AoA which is also accompanied by an abrupt and uncontrollable pitch up whenever the blades pitch is at higher values compared to any other helo in DCS which don't have a great difference from one another. All other helos blades stall at a rather correct AoA, but these on the MI-24 stall at slightly above half the other helos. The stall AoA being a bit too small is one thing, but their drag is yet another thing.

In autorotation tests, even with just 10% fuel, no heat suppressors, 0% ammo and clean loadout, the helo just barely flies on a high slope at some 200km/h with very little rotor blades pitch (some 2-3 degrees mostly) while the rotor rpm is visibly very low for all of these autorotation conditions which should keep the rotor rpm near 100% or at least that's what happens on either MI-8, KA-50 or UH-1.

Although each helo has it's own autorotation performances, the differences should never be great between them or something is wrong somewhere.

For short, the small stall AoA and too high blades requires some attention.

Another thing (different topic maybe) is that below generator power rotor RPM, the controls trim does nothing anymore. Is this normal? Isn't the trim suppose to be mechanical like it happens on MI-8, KA-50 and so on, not requiring electrical power in order to work?

Thanks!

Edited September 25, 2022 by 85th_Maverick

[AeriaGloria]

5 hours ago, 85th_Maverick said:

I always felt that this helo has difficulties when it comes to blade stall which takes place rather quickly or at a quite low AoA which is also accompanied by an abrupt and uncontrollable pitch up whenever the blades pitch is at higher values compared to any other helo in DCS which don't have a great difference from one another. All other helos blades stall at a rather correct AoA, but these on the MI-24 stall at slightly above half the other helos. The stall AoA being a bit too small is one thing, but their drag is yet another thing.

In autorotation tests, even with just 10% fuel, no heat suppressors, 0% ammo and clean loadout, the helo just barely flies on a high slope at some 200km/h with very little rotor blades pitch (some 2-3 degrees mostly) while the rotor rpm is visibly very low for all of these autorotation conditions which should keep the rotor rpm near 100% or at least that's what happens on either MI-8, KA-50 or UH-1.

Although each helo has it's own autorotation performances, the differences should never be great between them or something is wrong somewhere.

For short, the small stall AoA and too high blades requires some attention.

Another thing (different topic maybe) is that below generator power rotor RPM, the controls trim does nothing anymore. Is this normal? Isn't the trim suppose to be mechanical like it happens on MI-8, KA-50 and so on, not requiring electrical power in order to work?

Thanks!

 

  Perhaps something is too much, but there are two reasons I know of for such behavior 

  The Mi-8 rotor blades have 8 degrees of twist/washout. This means that the tips can have a 8 degree lower AOA Then the root. 
 

  In our Mi-24P model, there is only 3 degrees of blade twist/washout. Which means that compared to Mi-8, the outer blades have an average of 5 degrees more AOA. 
 

In addition, at high speed you have the mushing that happens when you pull too much AOA and you have a rapid pitch up. This is warned about in the manual and you get a chart with different speeds and altitudes that tells you max G you can pull. If you disregard speed, it and load about 75-100% fuel it hits the numbers perfectly. And can pull more G at lower weights. 
 

I believe one reason this mushing is more severe In Mi-24 then other helos is other then the Mi-24s extreme speed, low blade twist/washout, but that the wing is behind the CG and has 19 degrees incidence. 
 
According to manual, wings begins to stall at about 20 degrees AOA. So basically any positive AOA compared to fuselage will begin wing stall. Since wings are behind CG and far below rotor disk, the lift and drag causes a huge amount of pitch down force on the airframe. Once the wing stalls, a lot of lift and drag it creates suddenly goes away, and pitch down force it created now becomes smaller.

  With the wings producing 20-25% lift at cruise, when they stall you not only have their pitch down force they create going away, but the rotor must now also carry the weight the wing was helping to lift. So you have a shift of the trim point due to the wing stalling causing a nose up, and increase of load on the rotor when they stall, a rotor with very little blade twist/washout. And since the wing is at 19 degrees incidence and stalls around 20 degrees, the behavior we see in the sim makes sense to me. 
 

The manual advises to decrease G with increase in speed but I disagree. I believe the manuals says that to increase margin of safety, as since the faster you go the wings have greater effect, and thus make the pitch up more violent. In my experience speed doesn’t change your max G, only how sudden the onset of blade stall. 
 

You can test this with combined arms and destroying the wings. Once you destroy the wings and fly it without them, the blade stall/mushing is JUST AS smooth as it is in Mi-8 or Ka-50. It’s quite amazing. 
 

Here is chart from the manual, at lower weights I view 2G as the upper limit. Any increase of altitude will make it worse. Once you know the ballpark G you can feel and hear it and ride the “edge.” 
 

The manual mentions similar countermeasures and preventatives as the army manual, reduce collective 1-3 degrees, full cyclic forward, level out to decrease altitude loss. It also recommends to build up to your target G over the course of 3 seconds, so that you have time to feel and react to a blade stall/mush. 

Edited September 25, 2022 by AeriaGloria

[randomTOTEN]

"impossible to fly using autorotation"

I can't find the motivation to do a break down of the faulty assumptions of the OP

 

LLLLLLLLLOOOOOOOOLLLLLLLLLL FIRST TRY.trk

[AeriaGloria]

2 hours ago, randomTOTEN said:

"impossible to fly using autorotation"

I can't find the motivation to do a break down of the faulty assumptions of the OP

 

LLLLLLLLLOOOOOOOOLLLLLLLLLL FIRST TRY.trk 900.54 kB · 0 downloads

Yes. It’s just pretty difficult compared to the nice long rotors of Mi-8. 

[randomTOTEN]

6 hours ago, AeriaGloria said:

Yes. It’s just pretty difficult compared to the nice long rotors of Mi-8. 

I feel like this is a substantial improvement from my last attempt (which was months ago). I think we've had improvements in the FM here.

[85th_Maverick]

16 hours ago, randomTOTEN said:

"impossible to fly using autorotation"

I can't find the motivation to do a break down of the faulty assumptions of the OP

 

LLLLLLLLLOOOOOOOOLLLLLLLLLL FIRST TRY.trk 900.54 kB · 0 downloads

So in your belief the MI-24 cannot use autorotation to land smoothly at all even when very light?!

[randomTOTEN]

8 minutes ago, 85th_Maverick said:

So in your belief the MI-24 cannot use autorotation to land smoothly at all even when very light?!

What?

[85th_Maverick]

On 9/25/2022 at 11:32 PM, AeriaGloria said:

  Perhaps something is too much, but there are two reasons I know of for such behavior 

  The Mi-8 rotor blades have 8 degrees of twist/washout. This means that the tips can have a 8 degree lower AOA Then the root. 
 

  In our Mi-24P model, there is only 3 degrees of blade twist/washout. Which means that compared to Mi-8, the outer blades have an average of 5 degrees more AOA. 
 

In addition, at high speed you have the mushing that happens when you pull too much AOA and you have a rapid pitch up. This is warned about in the manual and you get a chart with different speeds and altitudes that tells you max G you can pull. If you disregard speed, it and load about 75-100% fuel it hits the numbers perfectly. And can pull more G at lower weights. 
 

I believe one reason this mushing is more severe In Mi-24 then other helos is other then the Mi-24s extreme speed, low blade twist/washout, but that the wing is behind the CG and has 19 degrees incidence. 
 
According to manual, wings begins to stall at about 20 degrees AOA. So basically any positive AOA compared to fuselage will begin wing stall. Since wings are behind CG and far below rotor disk, the lift and drag causes a huge amount of pitch down force on the airframe. Once the wing stalls, a lot of lift and drag it creates suddenly goes away, and pitch down force it created now becomes smaller.

  With the wings producing 20-25% lift at cruise, when they stall you not only have their pitch down force they create going away, but the rotor must now also carry the weight the wing was helping to lift. So you have a shift of the trim point due to the wing stalling causing a nose up, and increase of load on the rotor when they stall, a rotor with very little blade twist/washout. And since the wing is at 19 degrees incidence and stalls around 20 degrees, the behavior we see in the sim makes sense to me. 
 

The manual advises to decrease G with increase in speed but I disagree. I believe the manuals says that to increase margin of safety, as since the faster you go the wings have greater effect, and thus make the pitch up more violent. In my experience speed doesn’t change your max G, only how sudden the onset of blade stall. 
 

You can test this with combined arms and destroying the wings. Once you destroy the wings and fly it without them, the blade stall/mushing is JUST AS smooth as it is in Mi-8 or Ka-50. It’s quite amazing. 
 

Here is chart from the manual, at lower weights I view 2G as the upper limit. Any increase of altitude will make it worse. Once you know the ballpark G you can feel and hear it and ride the “edge.” 
 

The manual mentions similar countermeasures and preventatives as the army manual, reduce collective 1-3 degrees, full cyclic forward, level out to decrease altitude loss. It also recommends to build up to your target G over the course of 3 seconds, so that you have time to feel and react to a blade stall/mush. 

 

Many thanks for putting this effort to answer about this topic. You've put a bit too much effort in what concerns the sudden pitch ups and I appreciate it, but when the wings on the helo stall (and basically any wing, fixed or rotary), the drag roughly doubles (more or less depending on design) the value it had at critical AoA, but it never drops as the lift does (the lift dropping to usually anywhere between 40% to 50% the max CL).

Copy that about the much lower washout which is the only thing that can actually help generate autorotation torque on the main rotor. Thanks, so it has 5 degrees lower than for the MI-8. But..., is the MI-24's maximum takeoff weight actual in sim performance (ability to lift off at X IAS and Y altitude) that much lower compared to the MI-8? Cause then either the MI-8 has a bit overrated performances on the rotor (lift to drag) or the MI-24 has it a bit lower as it seems. Again, this is how it seems during comparisons. At MTOW, the MI-8 has a much better ability to vertically liftoff and hold a given altitude compared to the MI-24 for the same conditions.

Many thanks!

[AeriaGloria]

4 hours ago, 85th_Maverick said:

Many thanks for putting this effort to answer about this topic. You've put a bit too much effort in what concerns the sudden pitch ups and I appreciate it, but when the wings on the helo stall (and basically any wing, fixed or rotary), the drag roughly doubles (more or less depending on design) the value it had at critical AoA, but it never drops as the lift does (the lift dropping to usually anywhere between 40% to 50% the max CL).

Copy that about the much lower washout which is the only thing that can actually help generate autorotation torque on the main rotor. Thanks, so it has 5 degrees lower than for the MI-8. But..., is the MI-24's maximum takeoff weight actual in sim performance (ability to lift off at X IAS and Y altitude) that much lower compared to the MI-8? Cause then either the MI-8 has a bit overrated performances on the rotor (lift to drag) or the MI-24 has it a bit lower as it seems. Again, this is how it seems during comparisons. At MTOW, the MI-8 has a much better ability to vertically liftoff and hold a given altitude compared to the MI-24 for the same conditions.

Many thanks!

Mi-8 is suppossed to have better MTOW performance. Especially at higher altitudes. The Mi-8 has a much larger diameter rotor, the Mi-24 has 8.5% more chord on each rotor to make up for this as well as 25% more RPM, but that does not completely make up for the loss of lift from the smaller diameter rotor. The difference of blade area is that Mi-8 has 39% more blade area then Mi-24! With more chord and less son, the aspect ratio and thus tip drag also is worse. 
 

If you use the max takeoff weight chart found in chucks guide it works perfectly, most times you will be limited to about 12,000 kg, but 11,500 kg is the “normal” limit for combat ops regardless of being able to hover at 12,000 kg in most conditions. 
 

  As for the wing producing 2x drag at stall it does not. I have a chart that I can’t share becuase it’s from a modern 2006 manual. It shows the relationship of AOA to lift and drag on the wing. It has a very odd relationship where lift decreases very little at stall but more plateaus, and drag increases at a relatively linear rate. If you can find info about 230 NACA wings with 20% thickness, which I can’t find any but curves of 23018 and 23021 wing show similar curves on most aerofoil sites. By drag dropping from stall I was referring to lift induced drag. 
 

  You can absolutely tell that it’s caused by the wings becuase when they’re shot off it has no “cobra” manuever from blade stall/mush. They definitly stall at the AOA the mushing happens at, I can’t see how it could possibly not be the wings stalling which changes both shifts trim point back and increases rotor load. 
 

These figures are for Mi-35M with less wing span, but wing airfoil is exactly the same. So I don’t know that would change Cy except perhaps increase it compared to Cx. Unfortunetly a lift/drag Vs AOA plot is not available for the wing before the Mi-35M shortened it. What should Atleast be accurate here is how the wing stalls, and perhaps it’s ratio of Cy to Cx

Less then 1 degree wing AOA,

0.0 Cy lift Coefficent 

0.05 cx drag coefficient 
 

5 degrees AOA:

0.2 Cy lift Coefficent

0. 05 cx drag coefficient

True Aerodynamic Force: 22.5 degrees back from vertical 

Average Coefficent: .0125 

10 degrees AOA:

0.45 Cy lift coefficient,

0.075 Cx drag Coefficent 

TAF: 15 degrees back from vertical 

Average Coefficent: 0.26
15 degrees AOA;

0.78 Cy lift Coefficent,

0.1 Cx drag Coefficent. Wing lift plateaus here and only decreases after this point 

TAF: 11.5 degrees back from vertical 

Average Coefficient: 0.44 

20 degree AOA:

0.74 Cy lift Coefficent,

0.15 Cx drag Coefficent 

TAF: 18 degrees back from vertical 

Avg Coefficent: 0.45

25 degree AOA (end of chart):

0.74 Cy lift Coefficent,

0.24 Cx drag Coefficent. 

TAF: 30 degrees back from vertical 

Avg Coefficent: 0.49

I always thought of it like, lift after stall might not drastically decrease but plateau, but one of the main functions of the wing on the Mi-24 when it comes to flying is stability. This was apparently one reason they went back to the original wide wing for the 2020 Mi-35P Phoenix, is becuase pilots complained that the smaller wing of Mi-24PN/Mi-35M made the helicopter more unstable in forward flight. Even in DCS maintaining a perfect pitch angle without wings is much more difficult then with wings! 
 

  Since the wing is behind the CG, as alpha increases and it’s lift increases, it’s increasing pitch down force fights the pitch up. This makes it more stable and you can really feel the effect if you fly with your wings shot off. But as you reach stall and the lift starts to plateau and decrease it changes the stability performance. After 15-20 degrees AOA, it loses its stabilizing relationship with increasing helicopter AOA. 

At 15 degrees AOA, the average aerodynamic force (TAF), is only 11.5 degrees back. Once you get to 25 degrees the average aerodynamic force is 30 degrees back! 

Edited September 28, 2022 by AeriaGloria

[85th_Maverick]

Copy that. So all in all it's the lower blades radius + the much lower blades washout (decreasing incidence towards the tips) and both reduce the aerodynamic autorotation of the main rotor as the AoA increases on the blades. The effect is there indeed, but I just thought that it's way too small to be true and now I understood why.

So, it's a very bad day for a HIND pilot to lose both engines and rely on autorotation for landing. He can have a landing but he must do it at a much higher speed and much better coordinated aft stick and collective at the right moments in order to reduce the vertical speed as much as possible before touchdown.

Thanks;)

[admiki]

Not sure if that is how it behaves in real life, but I use different technique to any other helicopter. In a Huey or Mi-8 you would pull back on the cyclic to flare, level off and only then start pulling collective.

I found out that you need to flare quite agresively to stop descent in a Hind (might be a function of needing to have collective all the way down, compared to some collective on Huey/Hip to prevent overspeeding them). Better option is to flare and pull some collective to stop descent, level off and then pull all of the collective you have left and pull it fast.

I can easily do a touchdown at <100 kph if with full fuel (SOP calls for ordnance jettison anyway), down to about 60 kph if light on fuel.

[85th_Maverick]

5 hours ago, admiki said:

...might be a function of needing to have collective all the way down, compared to some collective on Huey/Hip to prevent overspeeding them...

You don't overspeed due to collective up, but due to collective down + increase in AoA on the advancing blades.

[admiki]

7 hours ago, 85th_Maverick said:

You don't overspeed due to collective up, but due to collective down + increase in AoA on the advancing blades.

That's what I said, you need some collective up to prevent overspeeding rotor?

[85th_Maverick]

On 12/26/2022 at 9:01 AM, admiki said:

That's what I said, you need some collective up to prevent overspeeding rotor?

I believed you said that you need to have all the collective down as for preventing rotor overspeed due to that comparison. So, it's doable in the MI-24 as I've already showed in my track but it's just very difficult unless you have some lower weight and I was thinking that something might be wrong, but AeriaGloria better explained what goes on with the blades through his technically detailed reply.

[Tiger-II]

Mi-24 can auto like any other helicopter, but you must do a roll-on landing. No hovering!!! Just flare and roll it on.

On 12/26/2022 at 7:01 AM, admiki said:

That's what I said, you need some collective up to prevent overspeeding rotor?

Correct. Pitch for speed and collective to maintain healthy rotor RPM. Collective should be on the floor initially.

[AeriaGloria]

Mi-24 manual says collective only needs to be raised above bottom for autorotation above 1,000m. Above 1,000m altitude, it is allowed to increase pitch to 2-3 degrees.