Autorotation capabilities accurate?

[Garuda]

First of all you have to stop the spin that has developed the seconds from when the T/R failed untill you cut the fuel to the engine. Many factors affect the extend of this (speed, torque the time of failure, attitude of the heli to name some). This is done by rolling the aircraft to the direction of the spin (while at the same time you are autorotating, whatever this means (RPM,speed etc.)) and basicaly making the uncoordinated spin a coordinated turn. When you achieve this you slowly roll horizondal so that the spin/turn stops.

 

The second difference from a normal autorotation is that you do not have yaw control with whatever this means in trying to navigate the heli to a safe spot to land.

 

The first part is the most difficult to cope with. If more than a couple of seconds pass until the fuel to the engine is cut, the spin may have become uncontrollable even to an extend that you cannot control your arm (by the great centrifugal forces) to cut the fuel in RL. In the Sim this will not be an issue though...

 

Worst scenario is hovering at low altitude. Best scenario is having 80kts speed (tail fin will reduce the spin developed) and sufficient altitude (to stop the spin, autorotate and find a spot to land)

 

You wouldn't have to cut fuel to the engine though, just roll the throttle down and lower the collective to enter an auto. Doing both of these will reduce the torque effect and help keep you fairly straight. As long as you have airspeed and altitude already it's not much different than doing an auto for an engine failure, you just get to decide when to enter to auto. At least some helicopters are capable of limited forward flight if you keep your airspeed up and collective setting low to give you a little more time to get to an area that looks better for an auto rotation landing.

 

What would suck is if enough of your tail gets shot off that CG shifts forward enough that you are unable to keep the nose level or to perform a flare. You're just a lawn dart at that point.

[sondo214]

You are right. Cutting fuel is nessesary before touchdown. Not only you stop the engine that you don't need anymore, but if the auto is not as smooth as you would want, you risk a fuel leak and maybe a fire from the running engine. But I am very curious to know which heli can do what you describe? I mean "limited forward flight if you keep your airspeed up and collective setting low to give you a little more time to get to an area that looks better for an auto rotation landing"?

[Garuda]

You are right. Cutting fuel is nessesary before touchdown. Not only you stop the engine that you don't need anymore, but if the auto is not as smooth as you would want, you risk a fuel leak and maybe a fire from the running engine. But I am very curious to know which heli can do what you describe? I mean "limited forward flight if you keep your airspeed up and collective setting low to give you a little more time to get to an area that looks better for an auto rotation landing"?

 

I've only ever flown Robinson helicopters, so the closest I've gotten to flying something like a Huey would be an R66 which is similar in that it is a two bladed single engine turbine helicopter. The emergency procedure for loss of tail rotor thrust in forward flight says to immediately enter autorotation and perform an autorotation landing, but it also includes a note afterwards that says this:

 

"NOTE: When a suitable landing site is not available, the vertical stabilizers may permit limited controlled flight at low power settings and airspeeds above 70 KIAS; however, prior to reducing airspeed, re-enter full autorotation."

 

So, not a Huey but at least some helicopters can do it. I would think most helicopters with a decently sized vertical stabilizer would be capable of it if they were not too heavily loaded.

[sondo214]

Have no experience in robinson helis. Nice to know this detail! In my RL heli this is not possible. Maybe a huey pilot can tell us if this is possible in UH-1?

[HuggyBear]

According to the Flight Manual and Emerg Checklists the UH-1H can be flown with a failed tail rotor above 40 KIAS to reach a suitable area.

 

A loss of components/structure however is likely to seriously affect the CoG.

 

- Bear

Edited September 25, 2014 by HuggyBear
Capitalisation

[sondo214]

Nice info thanks a lot! To be honest I had tried that in dcs and saw it could be done. I just didn't know if that was realistic

[BitMaster]

At least in DCS I have successfully landed the Mi-8 many times without the complete tail section.

 

The spinning is actually not the biggest problem as it also has a few positive effects like stabilising you over ground ( I do this with R/C helis if they are completely off-trim, apply full rudder after lift off and then slowly reduce it and trim things along, may need some training and just an example but spinning has a very stabilising effect for the ( upward ) Z-Axis, use that effect to your benefit).

The bigger problem is the shift in CG backwards and the constant rocking back and forth from it.

once you got the nose under control, pay attention to collective and gently lower the Mi-8 with her known characteristics when descending with zero IAS. gentle, gentle..take your time.

 

It works like 4 out of 5 times once you did it a few times and if conditions are not completely against you.

 

 

How real is this ? I have no idea !

I know how R/C helis behave when they loose random parts and tail rotors but I usually get beaten if I tell those stories..haha.

 

 

Bit

[BitMaster]

I think, honestly, in RL it is the hardest to keep SA up while that thing goes UP & DOWN with the nose while spinning around itself faster and faster with every couple spins.

 

Your neck and head will have to fight this first and recalibrate, maybe RL pilots need less time compared to standard mortals like us but it is not built in for them either, like 2-4 secs to recover

SA and start fighting.

 

What a nasty scenario.... even if you survive you need one of those foam cushions for your neck for weeks LoL and might not be able to walk straight for days :pain:

[Dirty Rotten Flieger]

So I just bought the Huey yesterday and I was looking for the clutch so I could disconnect the engine from the rotor and try some autorotations... Then I found this thread and learned a lot.

 

I tried a few today and started to get the hang of it i think... Any way after a few attempts I realise that you can do autorotation recovery from a hover and land without ever gaining any horizontal velocity.

 

So I started to wonder what the minimum altitude I could successfully do an autorotation from a hover dropping straight down?

 

118ft !?! lol My hovering is very amateur but the landing is perfect!

 

I am more than a little surprised this is possible!

Minimum autoroataion altitude.trk

Edited November 2, 2014 by Dirty Rotten Flieger

[sondo214]

Well actualy autorotating without longitudinal speed is not recomended in the first place. Various factors may not allow you to flare in order to touch down safely. Weight, Air temperature, Altitude of the landing site, Atmospheric preasure to name few... The correct teqnique dictates that if you have sufficient altitude you should begin forward flight. But if you still want to try hover autorotating (in DCS because in RL you would eventually get killed...) you can find for yourself the min and max altitude this will work! But bear in mind that the numbers will change as the factors I mentioned in the begining change...

[Dirty Rotten Flieger]

I'm still getting my head around this. As the chopper accelerates toward the ground (straight down just to keep it simple) some of the downward acceleration is absorbed by the acceleration of the rotors slowing down the sink rate OR the chopper drops and at first the airflow turns the rotors spinning them up and then when you pull the collective up it slows the descent due to thrust downward from the rotors.

 

It seems the seconds case is what happens but then again if I do a straight drop from say 2000ft I lower the collective quite a lot but not all the way and it is enough to both accelerate the rotor and slow down the descent. Now if the rotors are turned into the airflow and are being turned by the airflow then there should be no thrust coming downward from them but they still slow the decent...

 

Seems counter intuitive but it does make sense too. The energy to accelerate the rotors has to come from somewhere and this leaves less energy to accelerate the whole chopper toward the ground...

 

Probably a combination of both effects with the crucial flare coming from real thrust from the E stored in the rotors

Edited November 2, 2014 by Dirty Rotten Flieger

[sondo214]

Part\Portion of the MR blades produce thrust downwards althought not engine powered, due to their twist and other complicated physics. I do not remember the numbers by heart but I belive the internet has a lot more theory if you like to search. Try adding some weight to the chopper and turning up the OAT in the mission editor and see what happens while hover autorotating

Edited November 2, 2014 by sondo214

[Raptor9]

The Huey has a high-inertia rotor system, meaning that the momentum of the blades themselves will cause it to maintain rotor RPM's at a higher ratio than say a Ka-50. Also, like sondo said, depending on weight and air density, airflow can be sufficient enough to leave slight collective pitch in the blades. it is possible, even with the engine off, to overspeed the rotors during an autorotational descent in certain conditions.

 

In real helicopters, maintainers and pilots work together during a track and balance to "tweak" the rotor blades so that, among other things, they autorotate a certain way. Different helicopters have ideal autorotational glide speeds, and the trick is to adjust the rotor trim tabs or wedges on the ground to get the rotor system autorotating almost perfectly at that speed. The pilot doing a maintenance autorotational check at said airspeed bottoms the collective and pulls the engine offline, the rotors shouldn't overspeed or underspeed. If they do either (especially underspeed), he returns to the parking ramp and has the maintainers make some adjustments. They'll swap out wedges or bend the trim tabs. Then he starts the engine again, and goes out and autos again.

 

Track and balances can be relatively quick, or they can be a painstaking process depending on a maintenance team's/pilot's proficiency, the helo's "personality", and a tad of luck.

 

Sorry, tangent complete... :)

Edited November 2, 2014 by Raptor9

[Dirty Rotten Flieger]

Thanks for the intel. I know very little about choppers. They scare me ... being so close to so much visible kinetic energy.. There is mysterious and unseen forces at play in all that rotation and whirling...

 

I was doing gliding autorotations 20% fuel at first then tried straight drops and just as you say the rotor will over speed if you push too far down on the collective.

 

You say any one chopper is set up differently to another but in general when you push the collective all the way down does the rotor blade actually tilt past horizontal . So if you had the engine at full power and pushed the collective all the way down would the disk produce thrust upward?

 

thanks again for your insights.

[Raptor9]

Most choppers the blades are rigged to slice through the air at flat pitch. There are a few exceptions however. I believe there are certain models of the Seahawk (Navalized variant of the UH-60 Blackhawk) that are rigged to produce slight downward thrust. The purpose in this case is to push slightly downward on the airframe during rough seas as the ship pitches and rolls.

 

The best way to explain the airflow and force vectors of the airfoil of a helicopter rotor in autorotation is the following link:

 

http://www.dynamicflight.com/aerodynamics/autos/

 

The main thing to take away from this is that in the driving region of the blade, the resultant relative wind strikes the blade at such an angle to bring the total aerodynamic force forward of the axis of rotation. A similar example is how certain sailboats are designed to sail into the wind.

 

Scroll down a little to see a graph of how the wind strikes a sail to push a boat forward, pretty much the same concept:

http://tampicoventures.com/Boats/Design_of_a_Sailboat.htm

Edited November 3, 2014 by Raptor9

[Dirty Rotten Flieger]

Thanks for the link... that diagram of the force vectors in autorotative descent is very helpful. It surprises me that the lift vector moves further forward of the axis of rotation as the angle of attack of the blade is increased. It explains how the rotor rpm goes up while it still gives lift.

 

I can't say I really understand why though. I would expect the lift vector to move rear of the axis of rotation as the angle of attack increases.

 

I jumped in the Ka50 armed with my new sense of vague misunderstanding, and found it easier to fly than the last time I flew it.

 

For example if I am flying fast and low and want to stop suddenly, I pitch up . I would have pulled up on the collective thinking that the trust of the disk would help slow me down. But this can sometimes cause a vortex ring if it is a violent enough manoeuvre .

 

So to day when I did the same maneuver instead I pushed the collective down it reminded me of an autorotation and there was no vortex ring vibration. Much more smooth manoeuvre . Probably a terrible idea but it seemed much better than what I was doing before.

 

What would a real pilot do with his collective in this situation? The Ka 50 seems to be impossible to over rev in an autorotation

[Raptor9]

The lift vector will always be perpendicular to the resultant relative wind, which is why when the air impacting the airfoil during upflow in autorotation, it's forward. If the total aerodynamic force (lift and drag) is forward, then the airfoil/rotor blade will be pulled forward and/or accelerate. In that link you can see that only a portion of the blade is being pulled forward, the driven region of the blade is being pulled back by drag. As long as enough driving force can overcome the driven region's drag, autorotation condition exists.

 

How much of the blade is either in stall, driving, or driven is dependent on airspeed and collective pitch. These two factors will determine at what angle the air is impacting the airfoil (resultant relative wind). That's why at higher speeds, even if descending, the rotors will slow down since the driven region will be much larger than the driving region.

 

The Ka-50 is harder to over rev in autorotation since it's a low inertia rotor system, the blades don't have nearly the weight/momentum of a Huey blade. With engine power it is possible, and that is why there's a switch on the collective of the Shark to switch to low turbine speed. It lowers the rotor rpm reference for rapid descents so the Ka-50's rotors don't overspeed.

 

The reason you enter vortex ring state by throwing the nose really high rapidly is because you're putting the vertical column of air (rotor wash) out in front of your chopper. This is usually what happens when someone is in a descent and hauls back on the stick, the rotors are now thrusting air in the direction of flight, and you can fly right into it...VRS at an angle, lol.

 

A proper level deceleration in a helo, any helo, would be to smoothly lower the collective as you pitch up, to keep from climbing. As you decelerate below your max endurance airspeed (Ka-50: ~120-130kmh, UH-1: ~60-65kts I think), you'll start to build up a slight descent and you'll need to start inching collective back in. As your airspeed gets to about 30kts/50kmh you'll want to get more aggressive with applying collective as you approach effective translational lift. This is where it's really easy to get into VRS if you don't keep that descent rate from building at the end.

 

One last tip, if you can avoid entering a hover out-of-ground-effect (OGE), it'll be beneficial. You'll require less power to stop and maintain a hover, and you'll be less prone to enter VRS. Sometimes it may be necessary, but it's not always.

Edited November 3, 2014 by Raptor9