Jump to content

Blade stall on the wrong side


Robin885

Recommended Posts

I'm pretty sure this has been mentionned before but I haven't managed to find any post about it. Currently the Mi-8's blade stall will force the helicopter to bank left, while I believe it should actually bank right, as the retreating blade on the right side of the rotor disk. 

 

Terminus, who does excellent Hip tutorials, covers this bug in his blade stall tutorial (I set the timestamp): 

 

 

  • Like 2
Link to comment
Share on other sites

I'm pretty sure this has been mentionned before but I haven't managed to find any post about it. Currently the Mi-8's blade stall will force the helicopter to bank left, while I believe it should actually bank right, as the retreating blade on the right side of the rotor disk. 
 
Terminus, who does excellent Hip tutorials, covers this bug in his blade stall tutorial (I set the timestamp): 
 
 
It goes to the right in the Hind. So we'll see.

Sent from my MAR-LX1A using Tapatalk

Link to comment
Share on other sites

  • 1 month later...
  • 3 weeks later...
  • 2 months later...
On 6/19/2021 at 9:55 PM, Robin885 said:

I'm pretty sure this has been mentionned before but I haven't managed to find any post about it. Currently the Mi-8's blade stall will force the helicopter to bank left, while I believe it should actually bank right, as the retreating blade on the right side of the rotor disk. 

 

Terminus, who does excellent Hip tutorials, covers this bug in his blade stall tutorial (I set the timestamp): 

 

 

As far as I now Hip is not prone to retreating blade stall in operating speeds. This means Vne=250kph under 1km of height under 11,1t weight. However I know about the accident where the factory crew with full brand new helicopter transporting it to customer. Weight about 12t. They flew about 280kph all time. After 45minutes the rotor head broke from mast and they crashed. 

I believe that Mi-8 woud sooner get catastrophic destruction of main rotor than retreating blade stall. Another theory is that advancing blade is in transsonic speed so lift decreases and drag increases. 

Link to comment
Share on other sites

  • 1 month later...
On 11/27/2021 at 1:35 PM, DeMonteur said:

I believe that Mi-8 woud sooner get catastrophic destruction of main rotor than retreating blade stall. Another theory is that advancing blade is in transsonic speed so lift decreases and drag increases. 

An ex aerodynamicist here! Yes, if the advancing blade would reach it's stall AoA in transonic sooner than the retreating blade, you should have a left rolling moment when the stall occurs, but even if the helo is very heavy, let's say it actually flies at maximum takeoff weight and at TAS (true airspeed) where the advancing blade would be well in transonic, the AoA still should be lower than for stall if you fly at 1G (Z axis). The same should normally apply for the retreating blade. Yes, the higher the speed, the more AoA the retreating blade needs in order to equalize left and right rotor disc lift, and as for the advancing blade's case which needs to lower it's AoA to maintain lift, I can guarantee you that it's still a good couple of degrees away from stall angle, again, if you maintain 1G and although it's critical AoA is indeed lower (usually about 50-60% of what it used to be in subsonic), it will still have some degrees margin from stall when the retreating blades are already there. At least up to 300km/h TAS, this should be a good fact. Well above that, yes you'll be more into transonic on the advancing blade, but..., it's AoA should also be needed some lower (yes, the 1G needed AoA lowering is indeed logarithmic, but still decreases a bit) and keep in mind that throughout the whole transonic airspeed region, the critical AoA doesn't vary much (it looks a bit like a bowl on the graph, but not a deep one) and it increases again in supersonic by some amount (but not as good as it was in subsonic). So, you should never be able to blade stall due to transonic in any case at 1G. The retreating blade on the other hand will actually stall above X amount of TAS at 1G and at an actual weight. So, I'm sorry to contradict you here, but before you ever get to increase your AoA in transonic up to reaching the stall value, your retreating blade AoA will always get to the stall AoA first. The higher the TAS, the greater the difference in actual AoA between the left and right side rotor disc. The drag that you mentioned is only influenced by the rotor disc aerodynamic performance and TAS/MACH and will affect the required torque to be applied on it from the gearbox, otherwise the drag has no influence regarding this subject.

Now, at speeds let's say..., of about 100 to 200km/h where most known helos don't have blades long enough or rpm high enough to ever reach transonic and the retreating blade AoA is a good margin away from the stall value, if you pull enough aircraft AoA (hence vertical Gs automatically) to have the retreating blades reach above critical value, you will always have the advancing blades below critical AoA. The retreating blades will go above their critical AoA well before the advancing ones do, hence a stall rolling moment towards the retreating blades will always occur.

If I've said something wrong, please correct me!

Regards! o7


Edited by 85th_Maverick
Small context corrections.
  • Like 1

Good knowledge and common sense make the absurd run for defense.

Flying has always been a great interest for mankind, yet learning everything about it brought the greatest challenge!

Link to comment
Share on other sites

  • 3 weeks later...
On 1/5/2022 at 4:39 PM, 85th_Maverick said:

Yes, if the advancing blade would reach it's stall AoA in transonic sooner than the retreating blade, you should have a left rolling moment when the stall occurs, but even if the helo is very heavy, let's say it actually flies at maximum takeoff weight and at TAS (true airspeed) where the advancing blade would be well in transonic, the AoA still should be lower than for stall if you fly at 1G (Z axis). The same should normally apply for the retreating blade. Yes, the higher the speed, the more AoA the retreating blade needs in order to equalize left and right rotor disc lift, and as for the advancing blade's case which needs to lower it's AoA to maintain lift, I can guarantee you that it's still a good couple of degrees away from stall angle, again, if you maintain 1G and although it's critical AoA is indeed lower (usually about 50-60% of what it used to be in subsonic), it will still have some degrees margin from stall when the retreating blades are already there. At least up to 300km/h TAS, this should be a good fact. Well above that, yes you'll be more into transonic on the advancing blade, but..., it's AoA should also be needed some lower (yes, the 1G needed AoA lowering is indeed logarithmic, but still decreases a bit) and keep in mind that throughout the whole transonic airspeed region, the critical AoA doesn't vary much (it looks a bit like a bowl on the graph, but not a deep one) and it increases again in supersonic by some amount (but not as good as it was in subsonic). So, you should never be able to blade stall due to transonic in any case at 1G. The retreating blade on the other hand will actually stall above X amount of TAS at 1G and at an actual weight. So, I'm sorry to contradict you here, but before you ever get to increase your AoA in transonic up to reaching the stall value, your retreating blade AoA will always get to the stall AoA first. The higher the TAS, the greater the difference in actual AoA between the left and right side rotor disc. The drag that you mentioned is only influenced by the rotor disc aerodynamic performance and TAS/MACH and will affect the required torque to be applied on it from the gearbox, otherwise the drag has no influence regarding this subject.

Now, at speeds let's say..., of about 100 to 200km/h where most known helos don't have blades long enough or rpm high enough to ever reach transonic and the retreating blade AoA is a good margin away from the stall value, if you pull enough aircraft AoA (hence vertical Gs automatically) to have the retreating blades reach above critical value, you will always have the advancing blades below critical AoA. The retreating blades will go above their critical AoA well before the advancing ones do, hence a stall rolling moment towards the retreating blades will always occur.

If I've said something wrong, please correct me!

Regards! o7

 

 Mi-8 Limitations proves your article. In standard flight it is way below retreating blade stall. But in higher piloting technique there are limitations like no movement with collective when in dive or steep climb or in transition from dive to climb.

Moreover there is one Mi-8 crash which proves my theory about catastrophic destruction of main rotor. New helicopter from factory crashes after about 45min of flight from factory to operator. it was fully loaded with its equipment. Investigation finds out that they fly about 280km/h which is 50km/h more than they are allowed by Flight Manual in their conditions.

Link to comment
Share on other sites

  • 3 months later...

Hi guys,I find something interesting about this.

In a US Army TM manual of Mi-17, it states that during a retreating blade stall,the aircraft will pitch up and roll right,which is pretty logical if it has a clockwise rotating main rotor. BUT,it's does NOT. Also in that manual it stats that the it has a counterclockwise rotating main rotor,just like the Huey.  (this is an aircraft older than 1986 I guess, right?)

QQ截图20220504150032.pngQQ图片20220504145837.pngQQ图片20220504145850.png

 

So,in a Mi17 with a counterclockwise rotating main rotor,the retreating side is on the left,but the Army TM says it will roll right in blade stall. Maybe it's correct for what we have in game now?

 

Kyoto Animation forever!

Link to comment
Share on other sites

^ Clearly a mistake in a manual. There never was a CCW rotor version of the Hip (or any other Mil type helicopter for that matter). CW rotors are "industry standard" in Russian designs (apart from coaxial ones of course).


Edited by Art-J
  • Like 2

i7 9700K @ stock speed, single GTX1070, 32 gigs of RAM, TH Warthog, MFG Crosswind, Win10.

Link to comment
Share on other sites

  • 3 months later...
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...