Autorotation aerodynamics explained?

[eV1Te]

Edit: Scroll down to Post #16 to see the explanation...

 

 

After being puzzled about how autorotation works on an aerodynamic level for a couple of days now and not finding any good literature on the internet explain this I'm asking here. :music_whistling:

 

http://www.vimeo.com/3131589 This is what I have noticed by looking in Ka-50 (also by readnig about autorotation) and what I have experienced during tests of home made rotors or fans, the rotation direction is in the oposite direction. (press HD to look at HD)

 

If I would make a rotor out of paper (I have tried this) and drop it from some height in my living room or if I blow into a fan for example, the direction that the rotor would want to spin in is quite easy to determine (look at the video above).

 

Although during autorotation of helicopters the rotors turn the opposite direction that one would think is most intuitive.

 

So my first guess was that when one have reduces the collective to 0 degrees of pitch the shape of the rotors makes them want to turn in the "wrong" direction.

 

To test this i started my Ka-50 and turned of the engines at 1000+ meters and increased the collective to 10+ degrees of pitch and of course the rotor RPM reduces as expected.... but only to about 50% of normal RPM... from my logic the rotor would want to stop completely and turn around into the opposite direction but this never happens.

 

So my guess is ether one of these:

 

• I have missed something embarrassingly simple.
  

 

• There is something more aerodynamically complex happening during autorotation that I have missed.
  

Edited February 9, 2009 by eV1Te

[kaiserb_uk]

Let me google that for you...

 

Here you go: http://en.wikipedia.org/wiki/Autorotation_(helicopter)

[miguez]

The link above is missing a ) at the end (didn't make it into the HTML). Click here.

[eV1Te]

Let me google that for you...

 

Here you go: http://en.wikipedia.org/wiki/Autorotation_(helicopter)

 

Thank you for linking to Wikipedia as if i haven't already checked that!

 

Second, that page doesn't explain anything covering the complex aerodynamics of autorotation which would explains the unintuitive rotation directing of the rotors during autorotation and nether does any other page I found!

[LIONPRIDE]

Thank you for linking to Wikipedia as if i haven't already checked that!

 

Second, that page doesn't explain anything covering the complex aerodynamics of autorotation which would explains the unintuitive rotation directing of the rotors during autorotation and nether does any other page I found!

 

 

wow ... a bit hostile

 

... and your trying to 'understand' this in such detail because ... ?

 

I'm quite sure that you've looked here:

http://www.cybercom.net/~copters/aero/autorotation.html

 

It's obvious why your experiment failed if you look toward the bottom - diagrams ... at least at some level.

Also, you're talking about a co-axial design, so I fail to see how you can use a diagram w/ one Rotor-blade represented.

 

 

-OUT-

 

 

.

Edited February 8, 2009 by LIONPRIDE

[crazysundog]

Here's my interpretation of Autorotation:

 

1st- Your rotor cone IS A WING. The movement of air over the rotor cone is what generates lift.

 

2nd- The purpose of lowering collective is to reduce pitch, which in effect reduces drag on the rotor, and thus maintains precious RPM. You use forward cyclic to glide to your landing spot, also conserving the use of antitorque/rudder pedals...as they will also degrade your stored energy.

 

3rd- When you are close to landing position, you slow and cushion your landing by using aft cyclic and raising collective to make use of your stored rotor RPM...

 

Unless a major catastrophe occurs, your rotor will not reverse its direction....ever.

Edited February 8, 2009 by crazysundog
SP!

[eV1Te]

wow ... a bit hostile

.

 

Sorry about that, didn't mean it in that way... :music_whistling:

 

I'm not sure if anyone have thought about it, but what I am trying to understand is;

If you blow into a fan from below the fan will start to rotate in one direction, but in during autorotation (both coaxial and singel rotor helicopters) the rotors rotates in the opposite direction to how this fan would rotate.

 

I have even made experiments at home that gives the results that is contradictory to how the rotors rotate in Ka-50 and also from what I have read from autorotation manuals.

 

In the Ka-50 this happens even if you increase your cyclic to a maximum value which lowers the rotor RPM but if you decrease the cyclic only by a small amount the RPM increases again, indicating that the forward aerodynamically force is larger than the drag even at high pitch angles.

 

My guess is that the phenomena that gives this unintuitive result is much more complex than one can easily explain and therefore left out from helicopter manuals which only explain how to operate the helicopter during engine failure.

 

The real question is why the resulting aerodynamic force in the driving region (explained in the link LIONPRIDE wrote) is pointed forwards in relative to that rotor and therefore maintaining rotor RPM. Even when one is not in forward flight which means only dropping straight down and still have a large pitch angle on the rotors?

 

 

I hope it made some sense to someone.. :thumbup:

Edited February 8, 2009 by eV1Te

[sobek]

The real question is why the resulting aerodynamic force in the driving region (explained in the link LIONPRIDE wrote) is pointed forwards in relative to that rotor and therefore maintaining rotor RPM. Even when one is not in forward flight which means only dropping straight down and still have a large pitch angle on the rotors?

 

The point is not to view the rotor as a simple propeller, but to view it as rotating airfoil. Basically it's the same as when you're gliding with a fixed wing AC. The AOA musn't be too high of course or you will bleed rotor rpm. Also, as i understood it, it would be next to impossible to get the rotor turning from a standstill if in vertical descent, because the whole disk would be stalling then.

[eV1Te]

The point is not to view the rotor as a simple propeller, but to view it as rotating airfoil. Basically it's the same as when you're gliding with a fixed wing AC. The AOA musn't be too high of course or you will bleed rotor rpm. Also, as i understood it, it would be next to impossible to get the rotor turning from a standstill if in vertical descent, because the whole disk would be stalling then.

 

This actually makes some sense... :thumbup:

 

But when gliding an airplane you must have a negative AOA or you would stall after some time since only the gravity can make you go forward, right?

 

But in the Ka-50 you can have an AoA that is grater than 10 degrees (10 degree pitch plus the fact that the air comes from below since you are falling) and still gain rotor RPM... this is what is confusing me...

[LongbowDriver]

There are three regions in a helicopter blade. The Driven region near the tip, the Driving region near the middle and the Stall region near the mast. The Driving region has a Total Aerodynamic Force vector that is inclined slightly forward of the axis of rotation and produces a continual acceleration force. This direction of thrush provides thrust which tends to accelerate the rotation of the blade. I know this explanation would be a lot better with a picture, but the even a symmetrical blade will have three distinct regions that will act in different ways during a power off condition. Let me know if you need more detail. Bottomline a fan blade is not a good example of a rotor blade.

[eV1Te]

There are three regions in a helicopter blade. The Driven region near the tip, the Driving region near the middle and the Stall region near the mast. The Driving region has a Total Aerodynamic Force vector that is inclined slightly forward of the axis of rotation and produces a continual acceleration force. This direction of thrush provides thrust which tends to accelerate the rotation of the blade. I know this explanation would be a lot better with a picture, but the even a symmetrical blade will have three distinct regions that will act in different ways during a power off condition. Let me know if you need more detail. Bottomline a fan blade is not a good example of a rotor blade.

 

Yes I have read that as well, but why is the Total Aerodynamic Force vector inclined forward even when AoA is positive? That is the question... :music_whistling:

 

 

From this picture it seems like the Lift vector is always perpendicular to the Wind vector, is this true and is it the same for an ordinary airplane?

[LongbowDriver]

The greater induced flow will change the AoA pushing that TAF just forward which in turn spins up the Nr. During normal flight the induced flow is less and the Taf is just behind or else the rotor would spool up on you during normal flight. Same thing happens in uncompensated cyclic turns. The change in the induced flow will spool up the rotor that's why you have to apply collective in rapid turns, it's not only coriolis you contend with in rapid lateral cyclic inputs.

[sobek]

Yes I have read that as well, but why is the Total Aerodynamic Force vector inclined forward even when AoA is positive? That is the question... :music_whistling:

 

Thus is the nature of an airfoil

 

see http://en.wikipedia.org/wiki/Airfoil

 

it even has the math bells and whistles, if you want to have a deeper look into the subject :)

[Frederf]

I think the best understanding of autorotation comes from examining aircraft that do it for a living, namely the autogyro.

 

http://www.start-flying.com/new%20site/how_gyros_work.htm

http://en.wikipedia.org/wiki/Autogyro

[Sunjah]

Let me google that for you...

 

Here you go: http://en.wikipedia.org/wiki/Autorotation_(helicopter)

Thank you for linking to Wikipedia as if i haven't already checked that!

 

Second, that page doesn't explain anything covering the complex aerodynamics of autorotation which would explains the unintuitive rotation directing of the rotors during autorotation and nether does any other page I found!

 

That was rather unfriendly of him. When someone asks a question, it doesn't mean their incapable of using a search engine. It means they are looking to see if the people that frequent these boards know the answer.

Edited February 9, 2009 by Sunjah

[eV1Te]

Thank you for many of your answers, I think the posts by sobek and also the link in the last post from Frederf (http://www.start-flying.com/new%20site/how_gyros_work.htm) made me understand the concept. That link points to the only page I have found that actually mentions the differences in rotational direction in autorotation and a slow moving rotor. :thumbup:

My summarized version of autorotation (please correct me if I'm wrong):

In the case of traditional old windmills, which had rotors that wore turned by the wind in such a way that the wind "deflected" the blades. This is how I first thought of autorotation, but; One discovered a long time ago that if one reduces the pitch angle to only a few degrees, the windmill can rotate in the opposite direction and instead of the blades being "deflected" by the wind they are instead pulled into the wind by the aerodynamically forces!

 

This is the same thing that makes it possible for sailing ships to sail into the wind direction for example. And also the same concept when an airplane glides, although in that case one usually have a negative pitch angle of the wings instead of a positive in the case of autogyros.

 

As seen in this picture the Lifting force is always perpendicularity to the wind direction, so when the helicopter descends, the wind is coming from below and the Lift force is therefor pointed upwards and to the front. This cancels out the drag force and therefore a constant RPM can be maintaned, even though the Pitch is in the opposite direction from old rotor models. In the middle section of the rotor where these conditions are more favorable the total horizontal force is tilted to the front and in the other sections tilted to the back so the resultant force of the entire rotor is zero (this is during a constant RPM).

 

 

But, however in the game DCS: Ka-50... :music_whistling:

Even if one increases the rotor pitch to the maximum angle of 15 degrees for the Ka-50 the rotor still maintains in autorotation mode. Although from some mathematics I made the entire rotor should be in stall during these conditions and therefore it should slow down over time and at the end stop entirely. But this never happens. Also in autorotations guides for other helicopters it is always mentioned that it is critical that one reduces the collective quickly after engine failure, but it doesn't seem to be the case with Ka-50?

 

Some mathematics for autorotation of the Ka-50 in vertical decent with full pitch on the rotors:

Engine vs Rotor RPM ratio for Ka-50 = 55.15

100% Rotor RPM (calculated from engine to rotor ratio) = 354 RPM = 5.90 Rotations per Second

Circumference of rotor is 45.4 meter

 

Rotor velocity at the tip at 50% rotor RPM (corresponds to autorotation with full 15 degree rotor pitch) = 45.4*5.9*0.5 = 134 m/s

Descend rate of Ka-50 (in autorotation with full 15 degree rotor pitch) = 12 m/s

Relative wind pitch = arctan(12/134) = 5.1 degrees

 

Angle of attack = 15 + 5.1 = 20.1 degrees

Which would mean that the tips is stalled, which in turn means every other part of the rotor is even more stalled. (Assuming that stall AoA is below 20 degrees for the Ka-50, since the stall AoA is usually around 15 degrees)

 

How come it is still possible to maintain a constant rotor RPM of the Ka-50 even during these conditions?

 

Edited February 9, 2009 by eV1Te

[eV1Te]

There have been many reads of this thread but no responces lately, maybe I killed the thread with the last post.. :music_whistling:

 

Is there someone that have the knowledge of how autorotation works in reality; If the pilot fails to lower the collective or even worse increases collective after an engine malfunction what would happen?

(In Ka-50 nothing more happens than that the rotor RPM goes down a bit but is easily increased by lowering the collective again, even though logics tells me that the rotor would stall...)

[Yo-Yo]

The answer is very simple.

Firstly, I can not understand where did you get the Ka-50 gear ratio. The value you mentioned is not correct and the nominal rotor rpm is about 310 prm.

 

Secondly, the stall region is not a black hole sucking energy. Part of stall region can produce acceleration moment as well because Lift/Drag in velocity axis is greater than 1 even in this region.

The point is that if you have the rotor spinned up in the proper direction you can not spin it backward. The equilibrium rpm decreases when the pitch increases.

 

P.S. The drawing above are not very accurate too because the velocity triangle must take in account not only inflow velocity but so called inductive velocity too. It has no sence to explain the main points of AR but is very impotrant to calculate the rotor blade dynamics.

[Yo-Yo]

There have been many reads of this thread but no responces lately, maybe I killed the thread with the last post.. :music_whistling:

 

Is there someone that have the knowledge of how autorotation works in reality; If the pilot fails to lower the collective or even worse increases collective after an engine malfunction what would happen?

(In Ka-50 nothing more happens than that the rotor RPM goes down a bit but is easily increased by lowering the collective again, even though logics tells me that the rotor would stall...)

 

Blade intersection becomes very possible as a result of such pilot's behavior.

The second thing that will happen is lack of rotor rpm for flaring.

[eV1Te]

The answer is very simple.

Firstly, I can not understand where did you get the Ka-50 gear ratio. The value you mentioned is not correct and the nominal rotor rpm is about 310 prm.

 

Secondly, the stall region is not a black hole sucking energy. Part of stall region can produce acceleration moment as well because Lift/Drag in velocity axis is greater than 1 even in this region.

The point is that if you have the rotor spinned up in the proper direction you can not spin it backward. The equilibrium rpm decreases when the pitch increases.

 

P.S. The drawing above are not very accurate too because the velocity triangle must take in account not only inflow velocity but so called inductive velocity too. It has no sence to explain the main points of AR but is very impotrant to calculate the rotor blade dynamics.

 

Thank you for a good explanation;

 

First, the gear ratio I found in this print posted somewhere else and since the the ratio also matched the value i found by slowing down the simulation and counting the number of turns each second, I assumed it was approximately correct. :)

 

 

 

The reason I tried to make some calculations was to only for me to try and understand the subject better. I couldn't find any information about what happens when you increase the pitch drastically, so that the entire blade has an AoA that is higher than the stall AoA.

 

The only thing I knew was that real pilots on other forums and web pages said that depending on the helicopter. You only have a matter of seconds to reduce collective, from it's normal flight position (so increasing collective would be worse) when engine failure happens, until large portions of the blade stalls and the rotor RPM reduces below an unrecoverable limit. Some even mentioning that helicopters have crashed with their rotors almost completely still. But of course one can not trust what you read on the internet therefore I asked here...

 

Edit: The phenomena I'm referring to seems to be called Low-RPM-Rotor stall. Reports from for example US Academy of Aviationsays that it is the cause of many helicopter crashes. One example from a report about the R-22 said: The rotorblades of the R22 have a maximum angle of attack of about 15 degrees. If that angle of attack is exceeded the rotor blades stall, lift is cancelled, and the aircraft falls out of the sky. It is also mentioned that not even the engine at full throttle can give the required thrust to increase the RPM if that threshold is reached (below 80% of operational RPM)

 

How come the Ka-50 is not affected by this Low-RPM-Rotor stall in the same extend as other helicopters?

Edited February 11, 2009 by eV1Te

[Yo-Yo]

Thank you for a good explanation;

 

First, the gear ratio I found in this print posted somewhere else and since the the ratio also matched the value i found by slowing down the simulation and counting the number of turns each second, I assumed it was approximately correct. :)

 

 

The schematics is for Ka-32. Its rotor rpm printed near the rotor... the engines are the same so the gearbox is not the same Ka-50 has.

 

The reason I tried to make some calculations was to only for me to try and understand the subject better. I couldn't find any information about what happens when you increase the pitch drastically, so that the entire blade has an AoA that is higher than the stall AoA.

Blade pitch is not whole blade AoA because a) the blade is twisted so the different elements have different pitches, b) AoA is formed by inflow vector, inductive velocity vector, helicopter body velocity vector, blade velocity due to rotation. These velocities are different at upper and lower rotor and along the blade so the AoAs are very different.

 

 

The only thing I knew was that real pilots on other forums and web pages said that depending on the helicopter. You only have a matter of seconds to reduce collective, from it's normal flight position (so increasing collective would be worse) when engine failure happens, until large portions of the blade stalls and the rotor RPM reduces below an unrecoverable limit. Some even mentioning that helicopters have crashed with their rotors almost completely still. But of course one can not trust what you read on the internet therefore I asked here...

 

If you stall the rotors in DCS you can not survive too... :)

Edit: The phenomena I'm referring to seems to be called Low-RPM-Rotor stall. Reports from for example US Academy of Aviationsays that it is the cause of many helicopter crashes. One example from a report about the R-22 said: The rotorblades of the R22 have a maximum angle of attack of about 15 degrees. If that angle of attack is exceeded the rotor blades stall, lift is cancelled, and the aircraft falls out of the sky. It is also mentioned that not even the engine at full throttle can give the required thrust to increase the RPM if that threshold is reached (below 80% of operational RPM).

And this fact has its explanation: when engine schaft is decelerated lower than the summit on the moment curve you can advance full throttle but rpm will not increase. The same thing: if you try climb a mountain at high gear you can do it easily at 90 kph but at 40 kph you will stall your car... :) Turboschaft suffers of the underrev too (power turbine stall or effectiveness loss).

 

How come the Ka-50 is not affected by this Low-RPM-Rotor stall in the same extend as other helicopters?

Just try to survive after you slow the rotor.

[eV1Te]

The schematics is for Ka-32. Its rotor rpm printed near the rotor... the engines are the same so the gearbox is not the same Ka-50 has.

 

I see (I noticed that the image file name had Ka-32 in it after you mentioned it :music_whistling:), what is the gearbox ratio for the Ka-50 then? When you say nominal RPM do you mean the RPM at 100%? (there is no mentioning of what the scale is of the rotor RPM indicator in the manual, as there is for the engine RPM indicator)

 

Blade pitch is not whole blade AoA because a) the blade is twisted so the different elements have different pitches, b) AoA is formed by inflow vector, inductive velocity vector, helicopter body velocity vector, blade velocity due to rotation. These velocities are different at upper and lower rotor and along the blade so the AoAs are very different.

 

That is true, I simplified this some what since the airspeed was zero etc. But you mean that it is not possible, even with full collective and the wind coming from below and with rotor RPM at 50%; for the stall region to cover the majority of the rotor on the Ka-50? (when i say stall I mean when the horizontal components of the drag vector is larger than the lift vector so that without changing the collective the rotor RPM will not stop decaying)

 

If you stall the rotors in DCS you can not survive too... :)

 

How do you stall the rotors in DCS during autorotation in such a way that you can't recover from it? (I haven't succeeded in entering any kind of unrecoverable state during autorotation)

 

And this fact has its explanation: when engine schaft is decelerated lower than the summit on the moment curve you can advance full throttle but rpm will not increase. The same thing: if you try climb a mountain at high gear you can do it easily at 90 kph but at 40 kph you will stall your car... :) Turboschaft suffers of the underrev too (power turbine stall or effectiveness loss).

 

I believe have stalled the engines at least one time during my flight in DCS... :music_whistling:

[Yo-Yo]

I see (I noticed that the image file name had Ka-32 in it after you mentioned it :music_whistling:), what is the gearbox ratio for the Ka-50 then? When you say nominal RPM do you mean the RPM at 100%? (there is no mentioning of what the scale is of the rotor RPM indicator in the manual, as there is for the engine RPM indicator)

No, it is 89% on indicator.

That is true, I simplified this some what since the airspeed was zero etc. But you mean that it is not possible, even with full collective and the wind coming from below and with rotor RPM at 50%; for the stall region to cover the majority of the rotor on the Ka-50? (when i say stall I mean when the horizontal components of the drag vector is larger than the lift vector so that without changing the collective the rotor RPM will not stop decaying)

Estimating velocity triangle you must take in account inflow deceleration (i.e. inductive velocity addition) and this is very significant value. So the total inflow angle will be less than you estimate. Please take in account that upper rotor works with decelerated by lower rotor inflow. The total moment from both rotors will be very different with the moment of single rotor. By the way, in single rotor different blade elements accelerate and decelerate the blade but moreover in coax. rotor system one rotor can accelerate while another one can decelerate rotating.

And as far as I remember Ka-50 airfoil has stall peak more than 15 deg.

 

 

 

How do you stall the rotors in DCS during autorotation in such a way that you can't recover from it? (I haven't succeeded in entering any kind of unrecoverable state during autorotation)

 

Presumed impossible in RL too. :)

I believe have stalled the engines at least one time during my flight in DCS... :music_whistling:

Edited February 12, 2009 by Yo-Yo

[eV1Te]

Thanks for your answers they have been helpful! Now I can go on and actually fly the damn thing :thumbup:

 

By the way...

Since you tend to answer many questions covering the aerodynamics of the Ka-50 on the forums, have you had any involvement in the creation of the physics engine in DCS? :music_whistling:

[Yo-Yo]

Thanks for your answers they have been helpful! Now I can go on and actually fly the damn thing :thumbup:

 

By the way...

Since you tend to answer many questions covering the aerodynamics of the Ka-50 on the forums, have you had any involvement in the creation of the physics engine in DCS? :music_whistling:

 

Physics engine - what do you mean? You can see some of my duties in the credit list in the manual... :)